Top speed of 88cc

Junior681

Member
Just wondering for those with similar setups.

3 speed semi auto engine
16T front sprocket, stock rear

88 cc stage 2 kit with race head, carb and air filter from TB parts

HD clutch
HD oil pump
Aftermarket underbody exhaust

i am getting 70 kph (44 mph) on the flats.

gets there much faster than stock.

Just wondering how that compares, and if maybe I need some carb tuning or anything

Thanks
 

Junior681

Member
Ok, I figured it was kind of low. Any idea where I should start? Is this a carb issue? I am sure I can hear the hell out of it and get more top speed but that can be done on any motor.

any suggestions?
 

fatcaaat

Well-Known Member
what is the base engine, what tires you running, etc? need a lot more information. A stock CT70 bottom end with gearing of 16/35 on stock trailwing tires should get you in the 50's. Timing and tuning if not. Assume that the 88 kit you got was the correct one and the piston wasn't down in the hole a few mm at TDC.
 

Junior681

Member
Thanks. The engine is an 84, which from my understand of these engines is essentially an 82 engine carried over.

the tires are Michelin reggae (120/90/10)

the kit was sold me for the bike and I didn’t notice any spacing in the piston.

I tried tuning the carb today (which I am completely new to) as I found at WOT I would bog heavily and do nothing. I am researching when I can today and am learning that the pilot jet is about 5 turns out from seated, and the needle is in the 3rd position of the clip. I’ll take it apart a little more and see what jets I actually have in there but while I am at that, I am wondering if an improperly better carb will be the cause of that loss of top speed? Or am I looking at something else
 

69ST

Well-Known Member
Thanks. The engine is an 84, which from my understand of these engines is essentially an 82 engine carried over.

the tires are Michelin reggae (120/90/10)

the kit was sold me for the bike and I didn’t notice any spacing in the piston.

I tried tuning the carb today (which I am completely new to) as I found at WOT I would bog heavily and do nothing. I am researching when I can today and am learning that the pilot jet is about 5 turns out from seated, and the needle is in the 3rd position of the clip. I’ll take it apart a little more and see what jets I actually have in there but while I am at that, I am wondering if an improperly better carb will be the cause of that loss of top speed? Or am I looking at something else

I believe you are correct about the engine being the same as an 82. It's kind of an odd duck...long (12v) rod, but 6v breaker point ignition and 6v (big dome) head. The fact that you sourced the entire top end as a complete kit should have the piston matched to the rod (compression height) and the head combustion chamber. As long as you verified the outer "squish band" (above the top ringland) was level with the deck (top surface of the cylinder) at TDC...not down a few mm inside the bore, you should be okay. By all indications, the engine is fine.

As for carb tuning, there's a widespread misunderstanding that the "mixture screw" has anything to do with tuning above idle. It's actually the pilot airbleed adjusting screw and strictly an idle adjustment. It controls the amount of air entering the pilot (idle) circuit and its effects are all done by 1/8 throttle. Above that throttle opening, it's the main jet and jet needle height setting that control fuel delivery. If it's running lean...as you make it sound...try moving the jet needle C-clip two notches lower on the jet needle. That'll richen-up the mixture. If that improves things, you're moving in the right direction. If you get it all the way to the lowest groove and the engine is still running lean at WOT, you'll need to order the next size larger main jet and begin dialing-in the jet needle height from scratch...hint, start in the middle and see what you get.

Where you may have gone in the wrong direction is with those oversized tires. 120/90-10 tires are available cheap, making them very appealing. They are too big for the a CT70. One can live with reduced chain adjustment range. However, compared to the stock 4.00-10 Trailwings, they're at least 10% "taller" and that's compared to brand new `Wings. As they wear, Trailwings gradually lose close to 5% of their original rolling circumference. That's causing two issues: effective gearing is now ~10% taller than it would be with the correct tire size...and...your speedo input is that much slower than it was, affected indicated speed and odometer readings. Normally, all things the same, swapping a 16t C/S sprocket on a K1-later, makes the final drive ratio ~7% taller than stock. 88cc increases torque but 6-7% is the limit of what it can pull. Your effective final drive ratio (taller tires + larger C/S sprocket) is now ~15-20% taller than stock and there's not enough torque to pull it effectively, i.e. allowing the engine to reach peak hp rpm. You're overloading the engine and slowing down the indicated speed readings... bad combination, imo.
 

Junior681

Member
Very Informative as always!

so you’re telling me that all else equal, stock size tires will give me the extra speed on the top end??
 

69ST

Well-Known Member
Very Informative as always!

so you’re telling me that all else equal, stock size tires will give me the extra speed on the top end??
No...
Correct gearing will give you the best actual results. You can compensate, just takes a little math. Slap a measuring tape around the tire crown, I can take it from there. If the difference is, say, 11% we'd just need to figure out a sprocket combo that's ~5% shorter than stock, to get an effective final drive ratio that's 6% taller than stock. IOW, crankshaft revs per mile need to be within a narrow range.

What you can't change is the speedo input error. A mechanical speedo drive is geared for one size tire...i.e. revs per mile and can't be changed.

All of that having been said, returning to stock Trailwings...or another tire with the same rolling circumference...would restore the original speedometer input (IOW, it'd read the same as it did with the `Wings mounted) and, you'd simply run the original sized wheel sprocket and a 16t C/S sprocket. That'd remove not only any guesswork, you could leave the calculator alone. That's a known combo.

The hardcore math...stock gearing for any of these bikes is ~4.77mph/1000rpm, in top gear. An 88cc (52mm bore) makes enough torque to pull up to ~5.05mph/1000rpm. You now have somewhere between 5.49 - 5.68mph/1000rpm. The numeric differences might seem small, they aren't. Small-displacement and limited torque engines need to be able to rev to make full power. I'm guessing that your tune should reach its peak in the 9000-10000rpm range and not really fall-off until closer to 11,000rpm.

44mph, as you're now geared, takes ~7800rpm...if the speedometer is close. Bone-stock, a CT70 engine doesn't peak until 8000rpm, and that's a 3-speed; the 4-speeds can rev beyond 10,000. If gearing is holding the revs down below 8000rpm, the engine is being overloaded. The awful part of this is that we don't really know if that's an accurate indicated speed, nor how far it differs with those big shoes on the bike. You could, conceivably, be topping-out at 48-50mph with the engine revving closer to 8700rpm...which is still too low to let it reach its full potential. Multiple variables make this a guessing game. 88cc can make decent power but everything, including the final drive gearing must be dialed-in, there's not much room for error.
 

Junior681

Member
Thanks! You are definitely informed on this stuff these numbers are hard on the brain!

I just measured the tires they are 59 inches around at the crown.

I’m excited to see your magic!
 

69ST

Well-Known Member
Yeah, I know how the math can cause migraines. Going back to 15t on the countershaft will get to within 2-3% of where you want to be. CS sprockets are cheap, easy to change out...wheel sprockets, not so much. The perfectionist in me would want 2 wheel sprockets to test, +1t and +2t larger than what the bike came with but, the exact sizes aren't always available...and at 25-30 bucks a pop, they add up quickly. Best guess, 1t smaller up front combined with 1t larger at the wheel should dial-in the gearing within about 1%; that's close enough to be considered "perfection".

First thing I'd do, however, is swap the 16t cog for 15t...see what that does. If you're a lighter rider and don't have much in the way of grades where you ride, that may work well enough for you...a workable solution on the cheap, quick too.

One final note, 15t is the smallest sprocket I'd want to run. Below that size, chain life is shortened.
 
Hi Racer, Curious to know any weaknesses with an 88cc hot cam 4 speed engine build, VM20 Mikuni, K&N filter and a Tb performance pipe. 15/38T sprockets and Maxxis M6024 120-90-10 tires. Thank you in advance!!!
 
Yeah, I know how the math can cause migraines. Going back to 15t on the countershaft will get to within 2-3% of where you want to be. CS sprockets are cheap, easy to change out...wheel sprockets, not so much. The perfectionist in me would want 2 wheel sprockets to test, +1t and +2t larger than what the bike came with but, the exact sizes aren't always available...and at 25-30 bucks a pop, they add up quickly. Best guess, 1t smaller up front combined with 1t larger at the wheel should dial-in the gearing within about 1%; that's close enough to be considered "perfection".

First thing I'd do, however, is swap the 16t cog for 15t...see what that does. If you're a lighter rider and don't have much in the way of grades where you ride, that may work well enough for you...a workable solution on the cheap, quick too.

One final note, 15t is the smallest sprocket I'd want to run. Below that size, chain life is shortened.


Hi Racer, Curious to know any weaknesses with an 88cc hot cam 4 speed engine build, VM20 Mikuni, K&N filter and a Tb performance pipe. 15/38T sprockets and Maxxis M6024 120-90-10 tires. Thank you in advance!!!
 

69ST

Well-Known Member
Hi Racer, Curious to know any weaknesses with an 88cc hot cam 4 speed engine build, VM20 Mikuni, K&N filter and a Tb performance pipe. 15/38T sprockets and Maxxis M6024 120-90-10 tires. Thank you in advance!!!
Lack of torque is the only "weakness". It is certainly possible to pull 7hp+ from 88cc. Hottest tune I've run across, with this displacement, could just touch 60mph...which corresponds to more like 8hp. It takes efficient breathing at 11,000rpm to flow 8hp worth of air volume. That means working through the gears to keep the engine "on the boil"...a.k.a. on the cam. For fans of screamer tunes and an athletic riding style, such a machine could be a lot of grins. The Achilles heel will be grades and stiff headwinds, where speed may be limited to what can be reached in 3rd at max rpm.

Without knowing the primary drive and 4th gear ratios, I can't comment intelligently on where you might start with sprocket combos. Without knowing the circumference to the tire, I'd have to guess 5-12% larger circumference...a range equivalent to 2-4 sprocket teeth difference. That's too many unknown variables.

The number you're aiming for is ~5mph/1000rpm, in top gear. There's a little "wiggle room" in that, depending upon the cam selected. The more rpm/horsepower-biased the cam, the less "wiggle room", as torque is traded for hp. If you're talking about the now-iconic "fast road" cam, the engine may be able to pull as much as 5.2mph/1000rpm, on the flat. If you have an accurate tachometer and a reasonably accurate speedometer, you can quickly figure out what's needed, vs what is installed. The final arbiter, as always, is the seat-of-the-pants dyno. The math just helps shorten the path to peak SOTP numbers.

Fitted with a HV oil pump, dipstick thermometer, oil cooler, if needed and with the carb optimally jetted, the weakest link in this "chain" is the stock clutch basket. It becomes a liability at 11,000rpm. The rotating & reciprocating assembly can easily handle north of 12,000rpm. The rod/stroke ratio is 2.52:1, as I recall...that's like an F1 engine parameter. IOW, as long as you keep heat under control (oil, combustion chamber), these engines are damned-near bulletproof.
 
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