Engine Mods Engine ModsBy Clinton Whitehouse
I would like to preface this by saying there are many ways to achieve the same ends, so just because you see an article with different advice, it doesn't mean either source is wrong. I would however suggest that you do everything according to one plan or the other and not to try to mix and match.
My current racer is an RD400 that is stock, except where I will note. The crank has been rebuilt with 1979 RD400 Daytona slotted rods and silver coated big end bearing from an early TZ250. You can put a TZ-G roller element outer main on the ignition side, but they are very expensive if they are even available now. I don't run one and don't think it is necessary. Since the clutch side gears are helical, (which place a side load on the bearing), you must run a stock ball- bearing main on the right side. And since the stock center main bearings are apparently indestructible, I run them.
I am running Yamaha Blaster pistons (from an ATV Quad). They are 66mm in diameter as opposed to the 64mm RD pistons. They have 5mm less deck height than stock Rd's so you will have to mill off the top or bottom on some combination thereof. Therefore this gives you the opportunity to raise the exhaust and transfer ports by taking less of the bottom and more off the top. It is a lot easier to lower the transfer ported than to raise them, as the top of the transfer ports angles is critical. I have included a diagram of my porting so refer to it to determine how much to take off of the top and bottom to get your ports to my specs. If you are running standard deck height pistons you can still raise your transfer ports more easily than grinding by adding a spacer of appropriate thickness under the cylinder and milling the top an equal amount. if you do this remember to add a base gasket (.020") on both side of the spacer) to you calculations. If you run blaster pistons you will have to grind a little off the bottom corners of the skirt for crank clearance.
I don't increase the diameter of the exhaust outlet in the cylinder. On the intake side I have enlarged the reed valve area to accept RZ350 reeds, but when I did so, I couldn't tell any different and in fact the best dyno run that bike did was with stock reed cages. I run Boyesen reeds in either kind of cage mainly because if a steel reed breaks it will ruin your head and piston at least. Unless you use new cages that have fresh rubber where the reed seats, steel reeds will fail sooner or later.
I have tried 34mm round slides and 29.5 mm bored stock carburators. There doesn't seem to be much advantage to either over stock 28 mm's. So I use stock carburators except for the jetting. I am running stock carburetor except for jetting. The pilot jet is #40, the slide is stock as is the needle. I run the needle middle position. I run the stock style (primary) needle jet but I increase it's size from a P-2 to a P-5. I run a 320 main jet most of the time, but I would start with a 340 main jet on a new bike. I have seem Dale Alexander's recommended carburetor modifications and they are way different. His recommendations are for an RD350 but I doubt that makes that much difference. I think this is an example of what I was saying at the beginning of this article. Since I know Dale's bikes worked really well, I am sure what he says will work great, but I have had really good luck too. This setup gives good throttle response at all racing RPM. I have raced with Dale and with Dale prepared bikes back in the early 80's, winning most races at Willow Springs, with Dale winning at Sears Point.
You should run the RD350 air boot and a large K&N air filter (Harly XR750 filter) as it will give better carburation and power than open carburetors or God forbid those awful K&N's pod filters. Also don't run hoses on the float vents or overflow tubes. This is important!
I am currently running Moto Carrera Milennium GP expansion chambers. They are the best pipes I have tried by far. They peak at about 9200 RPM, have a broad power band, and run well off power band. Ground clearance is even more important than power characteristics, and the Milennium pipes have more than adequate clearance. The worst side effect of peaky expansion chambers is they are usually very efficient in one small RPM band. Not only does this make the bike hard to ride, but it also means your compression and timing must be set for this RPM or you will have detonation. With pipes tuned for a broad power band you give up horsepower at peak, but off-peak the engine is far more efficient. Since you can run more compression, you really don't lose that much peak horsepower RPM anyway. We will discuss compression later. It is a waste of reliability and durability to rev a RD to 11,000 RPM. Sure you can get good horse power numbers, but you get a peaky, hard to ride, unreliable bike for your trouble. If you look at an RD transfer port you will see one reason high RPM power is limited. From the back of an RD transfer port to the side of the piston is only about 10 mm if you look at the same dimensions on a 125 motorcross it is about triple that. Getting back to exhaust system, if you are racing at a tight track like The Streets of Willow Springs you might be better off with the stock pipes as they make very controllable power with no real power band hit. If you run stock pipes be prepared to beat the hell out them to gain ground clearance. My stock pipes that I ran in AFM and ARRA Mod Prod in the early 80's were flattened to about half their original width without any apparent lose of horsepower. I used these pipes for the first three F RD races at the Streets of Willow Springs. I only changed to expansion chambers to race at the big track and even through it made more power with the chambers, I didn't do any better lap times at the Streets of Willow Springs. This is with very mild chambers, so I can only imagine how bad a really peaky set would be to ride. Anther reason to go for a low RPM, broad power band is the RD's fairly widely spaced gears. At most tracks there are going to be turns that make you wish for a TZ gearbox (G model or older). The taller the gearing (i.e., faster gearing) the more pronounced the problem will be. Speaking of gearing I use 16/35 for Willow Springs main track and Firebird, 14/37 for the Streets of Willow Springs, and 15/37 for Buttonwillow.
The availability of very high octane fuel has made the tuning of an air-cooled two stroke a lot easier. When I started racing RD's I used pump gas which was about 98 octane in 1979. I soon found out how important race gas is, the hard way. Even though the first motor I built in 1980 was almost stock, it was teetering right on the edge of destruction (detonation). When it was the slightest bit off on tuning or jetting, it blew up. I was soon clued in by my competition about 110 octane race gas. This moved the teetering on the edge point way up. Gas is now even better than the Trick 110 octane we used then. I am now using UNCAL 118 octane but any modern leaded race gas is pretty good. 118 octane is probably over-kill, but it is cheap insurance, and I haven't had any detonation marks on my pistons at all.
This leads to my next subject, how do you know if you have detonation? Well , if you have a hole in the middle of your piston, you had detonation, but hopefully this article will keep you from seeing problems that bad! The most obvious sign on an air-cooled bike (other than the aforementioned) is an audible knocking. It is a distinct tinkling noise, usually under full throttle that goes away when you let off of the throttle. This is usually because of too much ignition advance. While knocking is hard on bearings and can lead to a holed piston, the insidious killer is end gas detonation. It happens when the last bit of fuel air mixture (the part under the squish band) is heated by the flame front to the point that it explodes instead of burning. This is bad, very bad, and you won't hear it. Some signs of this are: (1) an area around the edge of the piston that appears sand blasted, (2) piston seizure, and (3) the edge of the piston on the exhaust side actually melted away. End gas detonation is cause by several factors of combustion: (1) excessive squish clearance, (2) lean mixture, (3) excessive compression, (4) too low octane fuel, (5) insufficient cooling, and (6) air leaks that cause a lean mixture.
The way to fight end gas detonation is to reduce the squish clearance as much as is possible. I have run as little as .022 inch clearance but recommend .028 as it is safer. Using .022 clearance you must be very careful that the head is centered over the piston. This is difficult since the RD head and cylinder are not doweled. You should order volume 4, number 1, of The Expansion Chamber from Doug at Moto Carrera to explain how to dowel your heads to your cylinder. It is very important to machine the head squish area to match the piston (so the squish clearance is .028 at all parts of the squish area). The stock heads in addition to having too much squish do not match the head squish clearance to the piston. The way to check clearance is to use .035 solder (available at Radio Shack). Place the solder so that it goes to the edge of both the exhaust and intake port edges. A good way to do this is to lower the piston about 5 mm below tdc. Then cut a piece of lead wire 1 1/2 times longer than the bore, and curve the ends pushing them against the cylinder walls. Put a small piece of duct tape in the center of the piston to hold the lead wire in place. Then turn the engine over until you have felt the resistance of the solder against the head, making certain you go past tdc. Reverse the direction of crank rotation, making certain you don't turn the engine to the point where you could snag the solder in the exhaust port.
Since the piston crown is curved you will need to take the average angle and machine the head to match, or make a custom cutting tool that matches the piston curvature. Taking the average angle is good enough in my experience, although I have done both. If you are using Blaster pistons the squish angle is eight degrees. I don't remember the average angle for RD pistons but I think it is around fourteen degrees. Using the tight squish allows you to run much higher compression than you would expect. Compression is the real secret to horsepower on an RD and is only possible with the proper combustion chamber design. Back in the early 80's when I ran the stock pipes to be legal in ARRA and AFM Mod Prod, I ran as much as 195 psi of cranking compression. I am running 155 psi now because I am using expansion chambers and have adequate horsepower, anyway.
I use Castrol A747 oil. It is expensive ($25.00 to $30.00 a liter) but I think it is well worth the added cost. Any good oil will probably work fine, but like good gas, it is cheap(?) insurance. I run a 24 to 1 gas to oil ratio. It is very difficult to seize a piston when using A747 in a 24 to 1 mixture.
Ignition systems are always a problem on RDs. The stock points won't stay in time very well and the stock rotor also weighs so much it compromises crank reliability. The rotor also tends to fail at high RPM (above 10,000 RPM, sometimes less). You can avoid the problem with points by using an electronic ignition, such as the Piranha, (available from Moto Carrera for $145.00), and Moto Carrera guarantees the ignition for three years. Please resist the temptation to remove the stock rotor and run total loss. Everyone that I know running a total loss ignition has had problems that ruined their race day. If you must run a stock ignition, (say the rules for your class require a stock ignition) run mild pipes to keep the RPM below 9500 RPM and you rotor should live. You should run car coils (ones made to run on point ignitions) with the stock resistors. The car coil should pull about 3 amps each. Check the timing after each race and each practice. When I was running AFM Mod Prod I had to run a stock ignition. Each time I timed the bike I would start it and re-check the timing. I am now running a TZ250 E model ignition. This system has a light rotor and requires no battery. The only problem is finding one. I think any TZ250 G model or older will work. The RZ350 also has a CDI but requires extensive machining to adapt it to an RD crank. Since the RZ rotor is heavy and the electronic advance/retard curve is wrong for the RD, I don't think it is worth the trouble, although I did run it with some success. The final option I know about is Pro Flow's CDI system. It looks good but it costs about $350.00.
Using these specifications my RD made 65 horsepower on a Dyno Jet 150. That was not a flash reading, either. It was the third run in quick succession.
I hope I haven't left too much out, but I am sure I didn't give enough details on some aspects. If you have any questions or comments you can contact me at my E-mail address: clinton@lightspeed.net.