Carb jet size??? HP differences?

kentuckydiesel

Was just looking through the parts manual for the '63-'81 HD Carts. Noticed that they offered four different main jet sizes for the carbs. Lists .037, .039, .041, and .043.
Service manual says they are for "different operating conditions". What do you guys think? Altitude changes or tuning for power requirements?

Has anyone swapped or enlarged jets and/or modified intakes/exhausts to get more power and/or RPMs?

Thanks,
Phillip

BGW

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Ron Benedict

I finally found the .039 size at tillotson and drilled one out to .041. I can not tell any difference in performance. I believe that a larger carb would help if you could find one that would fit. The hl series have a different bolt pattern from the hr which was used on snowmobiles.I read an old post somewhere that a hl360 would bolt up but the throttle lever is on the wrong side of the carb and it doesn't have a choke. I have just about given up on doing wheel stands. Ron

Whitespider

The main jet size will make little, if any, power/performance difference in these engines, they're mostly for altitude and temperature tuning. The limiting factor is air flow... the little carburetors will only let so much air in, and the little outlets on the mufflers will only let so much combustion gas out. Opening the jet will only cause them to run rich (because they can't get more air to add to the fuel increase). This is all by design; these engines are not designed to run at high(er) RPM,s... that's why they can run at the 80:1 oil mix ratio. If you start running them faster you'll destroy them.

kentuckydiesel

So they aren't worth bothering with? I have never met a well made engine that I couldn't get to make 1.5-2 times stock power before they loose their cool. Is this the first?

Thanks,
Phillip

Whitespider

Unless you're have an altitude tuning problem, changing jet size won't make squat difference with the existing carb and exhaust (been there, done that). In fact, changing the "low" speed mixture screw has an effect on the high speed; you "can" make it too rich at high speed using that screw alone... so how could a jet change possibly help?? Take a look at the reed valve area on the engine case vs. the intake area of the carb, then compare the cylinder exhaust port to the muffler outlet... those restrictions are gonna' allow only so much "flow". And another thing to consider, there ain't any timing advance system on them... the more you increase RPM, ignition timing will become retarded.

But never say never... I suppose you could build a different intake manifold, use a larger carb, open the exhaust, increase the oil-to-fuel ratio, change spark plug heat range, screw around with timing... pray something don't melt-down during all the tuning. And then hope the piston, rings, rod, crank and bearings are up to the increased load. But when, or if, it all goes sour... you'll just have to remind yourself that it was originally intended to put-around a golf course at short duration run times (i.e., hole to hole), and it was you who desired to take it past the design limits.

tew45

If you do increase the power, use it on the low end only. Higher RPM will reduce engine life drastically. This engine is not designed for high RPM. If you want more power an engine change is necessary. Just remember another factor is speeds over 20MPH in a three wheeler is a lot more dangerous than a 4 wheeler.

kentuckydiesel

I've got a 4-wheel cart.

-Phillip

tew45

If you want more speed trade your cart engine for a more powerful engine and increase wheel size.

Ron Benedict

If you find a way to get more low end power please let me know. I use my cart to shuttle up and down a steep driveway. Thanks Ron

kentuckydiesel

If low end is all you need, you should be able to use a narrower drive belt. Basically a gearing change, so you'd loose top end.

-Phillip