• Hop-Up an Engine

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    Hopping-up your R/C Model Boat engine to get more power deals with general rules applicable to every motor. What we are looking for is more power and for that we increase the rotation and/or displacement. We are taking about modifications that you can do, if we have access to some equipment and tools and the skill to use them. What can be done depends on:
    • the type of construction of the engine if increasing displacement can be done;
    • the sturdiness of the engine how much hop-up the internal parts can stand;
    • the availability of parts - carbs, pipes etc;
    • how much of easy of maintenance and durability you are willing to lose;
    • how much are you willing to spend.

    Note: The modifications suggested are the ones we made n our engines. To make then you will need to tear down your engine almost completely. Learn how to do this and the special tools need on the Chapter:Disassembling an engine, on another part of this section.

    The modifications that present the best results are:
    • exchange the original carb for other one with bigger flow;
    • exchange the original exhaust system for a tuned pipe specially designed for your engine;
    • reduce the case volume;
    • increase the compression;
    • increase the timing of the exhaust and intake ports;
    • increase the ignition timing;
    • lighten of the moving parts;
    • balancing the moving parts;
    • tune up the passageways of mix and burned gases;
    • increase the displacement of the engine;

    Note: the numbers proposed came from mods made by us in our engines, from numbers of a few Tuners and from suggestions picked at Jim's Board.

    We shall go trough each of those items, trying to inform theirs pros and cons. To starting work, the first thing to do is checking all data of your engine, as it is now: compression, ignition timing, exhaust, intake and transfer timing. You will need a dial indicator, a degree rule, a timing lamp and a compression gauge to do this.


    The first and maybe the simplest of all modifications.

    Walbro 167A carb is the first option to upgrade a engine. It is the stock carb on Zenoah engines and on Homelite engines it fits right on. Also, can be used on almost all others with very little effort. If using the stock manifold you will need to match the manifold port to the bigger carb bore. Use a Dremel and a drum sander to match the holes taking care not to go trough the pressure line cast into the manifold and the passageways on it's face that connects this pressure line to the carb. Remember to line up the pressure hole of the engine with the gasket hole and the intake manifold. This pressure hole drives the diaphragm that feed the fuel from the tank. Some engine builders use a external pressure line instead. If so, a line runs from there to a fitting in the carb and there is no need to line the pressure hole of the engine with the one on the manifold. There are other carbs that can be used. Check the diameter of your original carb and look for a bigger one. Take care not to use too big a carb, otherwise you will end with a hard starting engine and with bad transition from idle to high RPM and, possibly, no idle at all.

    On stock Homelite engines, an economic option is alter the stock intake, to prepare it to a big bore carburetor. On the pics bellow, Jeff Thompson shows how:

    Ok to mod your intake to accept a big bore carb you need to use some JB weld to fill in all of the void holes in it.

    Also you need to use a screw with a piece of plastic tube or spacer on it so that you can fill in the area as shown for the mounting hole. I used a teflon spacer. After you put on the epoxy use a razor blade to smear off the excess.

    Then you have to drill out your carb & motor case and install a nipple in them so that the carb will work.

    The final assembling

    Tuned Pipe

    Put together with a bigger carb, this is the most effective and most simple hop-up you can do to your engine. A two-stroke engine relies onto the fresh charge to get rid of the burned fuel and, at the same time, uses the suction created by them to admit the new charge. A specially designed tuned pipe and a bigger carb will result in a substantial power gain. If you wish to make your own pipe, look at Designing a Tuned Pipe, on this technical chapter a formula to designing one. On the same chapter there is a link to a Java Program that designs the pipe as you fill the fields with your engine data. If you are skilled on how to soldering or know a body shop that will do the job for you, you can make your own pipe. 0,6 mm steel plates have the better resonance characteristics.

    There is a decision to make: dry or wet pipe. On a wet pipe the water from the water jacket exits trough the pipe, thus the pipe is cooler and can be enclosed with no problems. Another benefit: a wet pipe is shorter. If you look at the Technical segment - Design of a Tuned Pipe you will see that, at the formula, the sound speed in a dry pipe is 1700 feet/s and 1000 feet/s in a wet one. Charge density makes the difference. And, because of the water, they are quieter. There are two drawbacks, however: they produce less power than the dry one and whenever the engine stops water entering the cylinder can be a problem.

    Reducing the case volume

    On a non-valve 2 stroke, the fresh charge enters the engine bellow the piston and is rushed into the case, from where goes, trough the transfer ports, to the piston head, to be compressed. The smaller the volume of the case, the quickly and more completely the charge is transferred to the piston head. On Homelite engines this reduction is achieved very easily: the back of the case is removable and its thickness can be increased by screwing and gluing to it a piece of aluminum. Use a plate as thick as possible, taking care to not touch the crankshaft. Otherwise, you can remove material from the back of the crankcase, in which case the plate will go further into the case. Removing .125" (3 mm) is a good number. Warehouse Hobbies sells a back plate that does exactly this and more: it's also a motor mount and has a notch that unshroudes the transfer port that is critical on Homelites. Anyway, increasing the thickness of the rear plate or using a special one, look at the transfer port and work on it. On other engines, things can be a bit more complicated. You must check carefully the motor, looking for the way to reduce the case volume. Sometimes it can't be done, at least on a practical way.

    Jeff Thompson (who else) shows how to reduce the carter volume on a Homelite

    These are some pics of how you can stuff the the Homie using 1/8" Alum. I use a 8/32 screw to secure it and a little JB Weld.

    When you stuff the crankcase of Homelite engine, care have to be take with the transfer port. Dennis has a good advice on this:

    "The notch in the stuffing block-case should match the width and depth of the transfer port in the cyld. It should be as shallow as possible to maintain a small volume in the crankcase, but more important, have a good radius to smooth the flow of fuel coming out of the case and up the transfer. I would say min. 1/8 radius on the turns of your notch. Something else you could try, is putting a generous radius ( 1/4 ) on the edge of the crank counterweight ( con-rod side ). This will have little if any effect on eng. bal., but will help to get the flow over the crank and into the transfer port when the piston opens the transfers near BDC. I hope this helps some."

    Increasing the compression ratio

    The three already suggested hop-ups do not reduce too much the life of your engine. You can use them on any engine, provided, of course, that it is in good shape to start with. Increasing the CR, otherwise, should be done very carefully. The weed eater engines are not suitable to compression over 130/135 PSI. Check on Diagrams, on this section, the compression we found on a few motors we work with. Be aware that stock engines have compression on the 90-PSI region. An increase to 135 is most effective. Stay away from compression on the 180/190 range. They are reserved for special engines, made on purpose.

    To increase the compression of the engine, take material from the lower side of the cylinder, never from the case. On doing this remember that, at the same time, you will be reducing the timing of the exhaust because the cylinder will seat lower on relation to the piston. To compensate for this undesirable effect, take off material from the upper portion of the exhaust port. See bellow how to do this when we talk about increasing the timing of exhaust and intake.

    In some engines, mainly the Homelite, it could be necessary to take off material from the inside top of the cylinder squish band to prevent the piston touching the head of the cylinder. Troy's site shows how to do it.

    To take off material from the cylinder, fix it to a lathe. Adjust the cylinder with a dial indicator and, with a very sharp tool, cut the base of the cylinder. At each 1/64" thrown away remove the cylinder from the lathe, assembly it to the engine and check the compression. When it gets 130/135, STOP. Time consuming? Yes. Boring? Certainly. But it is the only way I know to be sure not to trash a good engine that you are working for the first time.

    Just to make things a little easier, if you are working with a stock Zenoah you can cut off 1/64" (0,5 mm) before checking the compression.

    After assembling the engine, check the clearance (this is a tip from Wayne Rathbun): Take a piece of solder run it through the plug hole in line with the crank not in line with the wrist pin turn the engine over and measure the thickness of the solder this will tell you how much clearance you have. I cut the cylinder till I have .018 to .020 head clearance.

    Increasing exhaust and intake timing

    Taking off material from the cylinder to increase compression makes it lower in relation to the piston. Because of this, the piston has to travel further into the cylinder to start opening the exhaust port, meaning that, for each stroke, the exhaust will be open for less time. To compensate for that, we must open the exhaust port, taking material off its upper part. Use a Dremel and a cutting tool to do this. If you took off 1/16" from the cylinder, take the same amount of the port. This should restore the original diagram of the engine. Go a little further, in small increments, to increase the exhaust beyond the original timing. How much is, again, attained by successive trials. Look the engine diagrams on the proper section. Don't try to emulate the Zenoahs and QuickDraws, unless you are working with one of those engines.

    Attention: if working on a 25 cc Homelite, take care to not taking off the bridge that runs on the exhaust port. It prevents the ring from going into the port, trashing the ring and the cylinder. The 30 cc Homelite does not have this bridge.

    For the same reason that taking material at the base of the cylinder causes reduction on the exhaust timing, this increase the intake timing. When checking the exhaust, after doing the exhaust port, check the intake. It is just possible that the cylinder work has altered the intake porting to very adequate numbers, preventing any additional work on it.

    For intake, 140 degrees, for exhaust 160 degrees are the maximum numbers we are looking for. On Zenoah engines this numbers could be increased to:

    Intake: 140/145 degrees

    Exhaust: 165/170 degrees

    Transfer: 124/126 degrees

    When working with exhaust timing you will be, at the same time, altering the blow down. What is this? It is the difference, in rotational degrees of the crankshaft, between the opening start of the transfer port and the opening start of the exhaust port. It should be between 18 and 24 degrees. Less than 18 the exhaust opens too early and the fresh charge is mixed with the burnt one; more than 24 and you lost the scavenging effect of the fresh charge helping get rid of the burnt gases. Altering the blow down is done on the transfer ports, much in the same way you do with the exhaust one.

    Increasing the ignition timing

    When you increase the RPM of your engine you may need also increase you ignition timing. This is necessary because the complete burning of the charge takes a time to occur. When the engine is turning at high RPM the piston is ahead of the ideal point when the ignition occurs. To compensate for this, the timing where the spark occurs is advanced in relation to the theoretical point. When we hop-up our engine, the stock timing can be not enough. We may need some additional degrees of timing increase, to compensate for the higher rotation of the engine. Connect a automotive timing lamp to the plug cable. Take off the plug. Put the piston on TDC (use a dial indicator whenever possible) Mark the flywheel position in relation to the crank. Attach a degree wheel on the flywheel. Pull the starter a few times and read the advance. On standard engines it should be 22 - 27 deg BTDC.

    Increasing the timing is achieved by changing the flywheel position where are fixed the magnets that trigger the coil - relative to the engine shaft. Normally, the engine shaft is conical and the flywheel is locked against it with a keyway. To advance the ignition timing you must relocate the keyway groove on the flywheel with a file. Once again, file a little (left side as seen from front), assemble the flywheel and check the timing. We are looking for 24/27 degrees BTDC (Before Top Dead Center). Stay at the lower limit if the hop up of your engine is limited to carb and pipe, go beyond if you are using the others tips too. When you get the desirable timing put the keyway in the flywheel and use JB Weld to fill the space that now exists between the keyway and the original groove.

    But some other variables must be considered before altering the ignition timing:

    • low octane gas burns quickly - retard the ignition timing;
    • increasing C.R. rises the mixture temperature and speeds burning - retard ignition timing;
    • a very rich or very lean mixture burns slowly - retard ignition timing;
    • higher RPM increases fuel atomization - quicker burning, retard ignition (note that is opposed to what is said at the beginning of this paragraph, as the increased RPM would need advanced timing - a balance between those demands is what you need, and this is not a easy task);
    • generally speaking, any mod that results in increased power rises the mixture temperature at the head, that burns quickly - retard ignition/
    • altering the combustion chamber design - eliminating the squish band, for instance, demands ignition advance.

    All considered, when modifying engines, normally the ignition time is left at the stock point.

    Lighten the rotational parts

    The lighter the moving parts, the best: more RPM and less time to get there. The minus side is on the idle, that has to be increased to prevent the engine to stall. The most effective and the simplest is done on the flywheel. As the engine will be water cooled, the fins used to drive fresh air over it are now unnecessary and represent just additional weight and drag. Take them off, breaking them with pliers and using a file after wards. Attention: on some engines, Homelites for instance, the starter coil is fixed to one fin. This one should not be removed. After taking off the fins and filing the surface, use a wet sand. The smooth surface gives you less drag and better performance.

    Again Jeff Thompson comes with a interesting idea on how to lighten the Homie flywheel - and the tip can be used on other engines also. Lets see what he tell us:

    Carlos here is 3 pictures for you. I came up with a better way to lighten up the flywheels on the Homelites. I take off the starter prawls & turn the flywheel down on my lathe then I use some stainless steal rod and weld it onto the starter prawls so that they will be still be spring-loaded. I have put a lot of thought into this and it works great! Hope you get the pictures and this idea might work on some of your different style motors.

    Bellow, a comparison between the stock flywheel at left and the one Jeff modified.

    Now, detail of the stainless steel rod welded to the starter prawl:

    Finally, the flywheel on the engine:

    Balance the rotating parts

    After lightening and even if you didn't do it, it's important to balance the flywheel. For doing this you will need a balancer. The Top Flight, used for props, is not adequate. The imams that maintains the shaft suspended are not strong enough to support the weight of the flywheel. The Robart is made in such a way that it can handle the flywheel and so, it is indicated. Mark the heavier side and toke off material with a drill. As usual, after each hole check the balancing. It will be acceptable when the flywheel, let free on the balancer, stop in a different position each time.

    Optimize the passages for fresh and burned gases

    Even on special prepared engines, the passageways let to be desired. On stock engines it's worst, the tolerances are away from the desirable. Use a Dremel and a cutting tool to smooth and deburing all the passageways intake, exhaust and transfer ports to eliminate steps, to match round passages to square ones and v.v. (that's the case of the exhaust port that matches to a square manifold), and to increase the angles of the passageways. Match the intake port with the manifold, smoothing the angles. Deburing the edges of the ports is important. Take your time and you will be rewarded with a performance increase without cons.

    Jeff Thompson is a unanimity concerning hoping-up Homelite engines. Bellow, a few pics he sent to me, that show a Homie cylinder after his magic touch. Let's see what he has to say about it:

    The pics of the case mods really boost the comp & put out mass amount's of torque, It would just about barrel roll my 50" Deep Vee. But I could not find a big enough prop to handle it. So I have that case sitting on the shelf.
    The other mods are of some of my cylinder work on what I do to the Homies & what have really worked for me.

    Here are some of my pics of how I port my Homelite's.

    Note: Homelites 25 cc engines have a bridge over the exhaust port as a guide to the ring. This bridge hampers the exhaust flow but just getting rid of it is a invitation to disaster. But it can be done, and JT - who else? - shows how.

    "Hi Carlos, Here is a picture of one of my 25cc Homie pistons that I have pinned the ring on. I use a .060 stainless steel Tig welding rod. I use JB weld epoxy on the inside of the piston to help secure it. You want to make sure that you put the pin in the piston so that it will be between the intake port & the transfer port. I recommend doing this after you do the cylinder mods. Then after you install the pin you must cut the ring to get it to fit in the cylinder. This way you can take out the bar in the exhaust port & mod the cylinder the same as you would on a 30cc. JT"

    Increasing the displacement

    This is one of the most effective alterations. Unfortunately, there are very few engines where it can be done. The 2 stroke engines that we normally use have the cylinder and head in one piece, that has to be changed for another one of bigger diameter to increase the displacement of the engine. Some engines, Homelite, for instance - were offered in 2 versions: 25 and 30 cc. The difference between them was the diameter of the cylinder (and the piston, of course). If this is your case, look for the bigger set. The installation is very simple and the result very positive. Some engines are offered with different displacements, according with the utilization: weed eater, chain saw, water pump, and so. Talk to the vendor and look for exchangeable parts.

    Water cooling

    Whenever you extract more power from your engine, the original cooling system is insufficient. Besides that, it was designed for an engine exposed to air and uses a enormous plastic case to direct the refrigeration air. So, an alteration of the stock system is mandatory. The refrigeration fins of the stock cylinder are toke off on a lathe, as shows the picture below. Attention: take only the fins above the exhaust port. After this, make the water jacket. If the engine has the spark plug perpendicular to the head, it's a much easier task. Make the jacket 5/64 to 1/8" (2 to 3 mm) bigger than the cylinder diameter. CAREFULLY tap two 5/32 (4mm) threads on the cylinder head, that will fix the jacket. Tap on the jacket 4 threads for plugs (in and out for the refrigeration water). Screw those pressure plugs used on nitro engines for pressuring the gas tank). Put a O ring just a bit larger than the water jacket between the jacket and the cylinder and fix it with screws.

    Otherwise, if your engine has a slanted plug, it will be much more harder to do the job. Look at the pictures below, to see the water jacket during fabrication. Use brass for the water jacket, because you will need to solder a cooper plate on it, as the picture shows. The other procedures are just like the perpendicular plug engine.

    Always mount the outlet for the water in such a way you can see it on a running boat. If you intend to go racing, on the port side. Checking once and while the proper operation of the cooling system is of utmost importance.

    The cylinder, with fins removed on a lathe. Take care to not reach the exhaust port. The first fin must be kept, that's where the water jacket will be threaded.

    To take off the fins, you should fix the cylinder to the lathe. The best way is using a mandrel for this.

    The brass tube, already with the top closed and with the cut parallel to the spark plug hole. It's the work's hardest part. Measure twice, check again... or ask William to make it for you, he uses only the naked eye and I never saw him spoil a part.

    The water jacket, ready and fixed to the engine. The spark plug pressure against the flexible cooper does the sealing.

    The water jacket after Fernando's Treatment.

    Spark plugs and gaps:

    As always, there is no single rule about what to use. The information below resume what experienced boaters are using.


    • Spark plug: Autolite 425, NGK BMR 6A, Champion RDJ7Y, BOSH HS8E
    • Spark plug gap: .025 to .035" (0.6 a 0.9 mm)
    • Coil to flywheel gap: .010 a .12" ( 0.25 a 0.30 mm)


    • Spark plug: NGK BMR7, Champion RCJ7 or RCJ4
    • Spark plug gap: .012 a .020" (0.3 a 0.5 mm)
    • Coil to flywheel gap: .015" (0.4 mm)

    Modify The G260pum

    G260PUM Specifications:
    • Bore: 34mm
    • Stroke: 28mm
    • Displacement: 25.4cc
    • Exhaust Duration: 162 degrees
    • Transfer Duration: 125 degrees
    • Piston Port Duration: 142 degrees
    • Engine Class: G-1 (NAMBA) or LS27 (IMPBA)
    • Output: 3.1 HP @ 12,000 RPM with open exhaust, 4 HP @ 14,400 RPM with M&D designs Muffled Tuned Pipe

    Scott Schneider started a new post on Jim's Board about how to modify a Zenoah 260. The information posted was so good that I edited it somehow to keep it for everybody interested, and this has to be anyone interested in R/C boats.

    You DO NOT need to machine cylinder base to get proper squish clearance. With the piston hitting head the plug hole to piston dome was .610" and installed on rod assembled with NO gasket it is .625" - that's .015" clearance and we wanted @ .020 So I cut a Note pad paper gasket of @ .005 thickness and coated with Loctite 518 Anaerobic gasket sealant and assembled to a clearance .020".

    When lowering jug by .015 and raising exhaust timings and adjusting the exhaust timing accordingly the blowdown changes very little and does help the torque a bit. But by fooling around the transfers upper areas more damage to the powerband would happen because: There direction and entry angles are very critical to maintain a symmetrical flow pattern from both sides. As well the increases in flow volume in transfer runners makes up for the slightly less open duration by having the ports flow at a higher speed through there openings.

    On an unpiped motor the blowdown has to be low or it will never run properly. Raising the transfers is ultimately what is needed to get maximum power but that's not so easy on the G23/G260's. First thing that can be done on the 260 which wasn't possible on the G23 is to widen the exhaust port to the maximum possible without ring trapping occurring. That should be between 72 to 75% of the cylinder bore. After that it's a trial and error rising of the exhaust to get max power and still give drivability. Raising the exhaust reduces compression ratio but the increased blowdown will enable the pipe the pipe to have more of a supercharging effect that will tend to overcome the lowered CR.

    In terms of transfer open duration most all of the HIGH horsepower 2 cycles have @ 120 to 124 degree duration and have there transfers set up with as much area as cylinder walls will allow and this includes delivery ports under exhaust window! As the area total of all transfers increases, the speed in the port window decreases and there in lies the problem with high transfer durations. With out an explosive high velocity blast coming out of transfer port window you cannot control the short circuit tendencies of the residual exhaust and fresh charge from mixing. In short the loop scavenging is weak and the cylinder will be lazy to replace old burn fuel/air with fresh fuel/air. Confused yet? Now take an engine with low transfers but a Lot of them, you can maintain a very high discharge velocity through transfer windows therefore having a very strong loop scavenge effect. This very strong transfer port discharge is very helpful when big port/ long duration exhaust timing is used. Pretty Deep but I think it does make a valid point to not raise transfer timings when we are gaining more area in the current engines

    The bridge between transfer runners should not be overly short or the ports will flow at different rates. When the piston is at B.D.C. the bridge is at the level of the piston pin.
    Sharpened on intake side of bridge only. If you cut the piston dome you will ruin the squish band and will get less power because the fuel in the cut areas will not burn with the main charge in combustion chamber, And will be the first to discharge in to exhaust system where it will burn and only add to the operating temp of exhaust and not for the benefit of more power

    I STRONGLY suggest leaving the upper edges of transfers alone! With 4 ports now the 231/260 ports flow better than the older 230 2 port cylinder did even with high transfer timings.

    Our experience has been that the best approach is to leave the transfer timing alone, adjust the exhaust timing to suit your application, and don't worry too much about what the resulting blowdown angle is, as it basically comes out in the wash and ends up being basically where it needs to be anyway, when coupled to any decent tuned pipe.

    I widened intake port @ 3/32" each side and leveled off roof section on top right next to ring compression stub, net result is a lot bigger intake. Next remove half the length of the transfer port runners and reshape, Make sure that you maintain enough length to support ring at B.D.C. Net result a lot more transfer area. Last profile the outside edge of piston next to pin boss's to enhance flow along sides of piston.
    If you have a milling machine you can drill two 3/16"holes in piston just under ring that line up with the transfer runner closest exhaust that will give a little extra boost. Skirt piston @ .040 to give @ 140 to 150 degree intake timing. Match up the intake manifold and be sure to open carb mount hole to .625" and blend in.

    As to exhaust timings, @ 174 to *178 degrees work very well and will make great power for either monos or hydros.
    Keeping mind the more conservative timing will have better torque and the latter better revs.

    It's blueprinting work and flow improvement that will do the trick, so take it apart, fit it back together on one half of crankcase then the other side, follow the path of air/fuel and remove lumps and bumps, match case to cylinder joints, blend in corners and remove excess casting material that blocks the flow. You can even grind away edge of transfer passage on case and blend in around bearings Make sure that both sides are the same shape and finish.

    Don't polish your work! ! A soft matt finish like glass-beading gives is best. Scotch bright works fairly well.

    I have no way to measure H.P. but it is stronger that a full mod g230pum.

    Note: Some piston seizures are happening and a cure is to undercut piston Diameter @ .003 to .005 on the top 1/4" or so.

    The stock piston weighs at 28.8 grams The piston has lot of material on the inside by the ring area. You can take out their and the top crown some.

    Some builders have been taking the material from the piston crown approaching the 25 QD style, flat top, and then cutting the head to match. This combined with trimming the skirt sides and hollowing out the inside of the piston will give you a very light piston

    On the wristpin sides (both), I matched them the width to the transfers and took all the way up to the wristpin boss, then profiled to direct the flow "up" and at a slight angle (to the left) if piston is held with dome facing up.
    Deburr, deburr, deburr.......

    Absolutely DO NOT!!!!! grind or even polish rod. They are a case hardened forging and when you grind or polish them you change the grain structure of the metal and will cause an unwanted failure. Trust Me ! ! Don't touch the rod.

    There is not much to be gained by doing work to the rod, but there are a lot of risks, such as abusive machining stresses, grit in the big-end bearing, etc. Personally I would spend time elsewhere.

    One last word: hopping-up an engine means - always - reducing it's life when compared to the same engine in stock form. You may minimize this taking good care of your motor, mainly using first grade oil at the adequate ratio. Besides, some engines withstand better than other to modifications and high RPM. One reason for this - maybe most obvious - is the construction type of the engine. Bellow, the pic of a Homelite crankshaft cantilever type. It is only supported by bearings at one side, the other spins freely into the crankcase. This makes the crankshaft flex under high RPM, drastically limiting those RPM and engine life.

    Now, a pic of a Husqvarna crankshaft. It is easy to see the difference: both sides runs on bearings and so it is less prone to flex.

    Carlos Andrade