720 Factory Pipe tuning

[speedskier]

I just installed a factory pipe on my hx and am having a real hard time tuning it.
The motor is always bogging down really bad. I changed the needle and seats out from 2.3 to 2.0 and i have 80g/s. Popoff is now 20 psi. But i still have the same bogging problem. I adjusted the low speeds by sitting the ski on the trailer and adjusting them untill i had a high sustainable rpm. When i take the ski off the trailer and then begin to idle out then apply the throttle i get a semi quick takeoff but can only get about 3300rpms. It doesnt matter how much throttle i give it. Full throttle is 3450rpms. Then when i let go of the throttle and apply it again the ski acclerates slowly to 3300 again. If i try again the ski wont go past idle. It completly bogs down and wont go anywhere. The only way to remedy this situation is to shut off the gas for a few seconds then gun it and turn the gas back on. Following this procedure i can get up on plane but as soon as i come back down I can not get the engine to past 3300 again with out shutting off the gas. Is my popoff still too low? I have 145 mains and 80 pilots. Why does it accelerate ok off the line originally but never get above 3300? This doesnt make much sense to me at all. The only other thing i can think of is maybe the ecwi is adjusted wrong. But then why does it load up so bad?
I should also mention i have prok flame arrestors.
Any help is appreciated
thanks

[Bill O'Neal]

From your description, I'd say the pop off is too high, not too low.

What impellor do you have ?

[speedskier]

Pop off is too high? how so? i thought that if it loads up all the time pop off is low or that the n/s o-ring is leaking and that its not poping off just letting fuel by or that the n/s is worn and its dribbling. If its too high what do you suggest I should try next?

I have a solas xo impellar, Im thinking that it may be too much pitch. The only other impellar that i have right now is a solas concord14/22. Do youthink i should try that instead?

Could it be something non-carb related, like maybe the rotary valve timing is off or im getting a weak spark or something? I replaced the spark plugs and the boots because originally the boots where kind of looose, but it did nothing to help my problem.

Thanks for you help.

[Bill O'Neal]

Yes, in some senerios you'd be right, but there are other ways these carbs can fool even the best of us.

For instance, if the jetting is too much, it could be sucking too much fuel and causing the bog that you are trying to fix with pop-off pressures and needle and seat changes. Conversley, if the pop-off is way out of line for the stronger carb signal, that could cause the bog.

Then there is the senerio where you think it is bogging from too much fuel, but in reality it is starving for fuel (and won't rev up) because the pop-off pressures are too high.

If you have followed my advice to others around here in the past, you'd know I don't care much for 720's with Factory Pipes and OEM carbs. Not only are they a bear to tune, but I have seen too many siezures even after they are "tuned great" and have run fine for a month or two, even longer. I honestly think that 720 oem carbs are marginal at best, with even an aftermarket air filter as the only other mod, but add a pipe and bingo, you are in a place I don't want to be.

My advice has always been to use Mikuni 44's or 46's on an aftermarket manifold, not only for the additional ease of tuning, but for the strength that the aftermarket manifold has over the oem (can you say, CRACKED) manifolds.

The Factory pipe is a very high performance pipe on a 720. It changes the motors charcteristics drastically with lots more case and clyinder pressures and signal strengths. It can make a 720 very fast and powerful, but it can also make it a totally unreliable POS if not built right using the correct parts to compliment the pipe.

There are two ways to do it. Either follow FPP instructions to the tee ( no filters, no high compression heads, their reccomended jetting), or you do it the way successful racers do it, use pre-mixed race fuel, lower pitched and more agressive impellors, case braces (different than carb braces), aftermarket manifolds and carbs, the right aftermarket head with the right squish clearances, and enough ignition to fire the fuel.

Feeding it with those oem carbs is a crapshoot at best, IMO. As soon as you get it running good with the jetting combo, you run out of spark to fire the extra fuel with the VERY weak oem 720 ignition.

[Bill O'Neal]

I use MSD total loss whenever possible. If not, I use a Micro Touch Rev limiter and I gap the plugs to .019 for the weak 720 oem spark.

Some guys like to backdate the ignitions to 1994 XP ignitions, stators and magnetos for their extra power for a stronger spark.

[speedskier]

Hey Bill,

Thanks for the info, I understand what youre saying, I guess I was under the wrong impression that the stock carbs could be used safely. I guess I'm currently in the market for some 44's. What kind have you had best luck with? Mikuni's?

After trying several different jets i found my original specs to be way way rich and not even close. Kinda pisses me off because I purchased the jet kit from a reputable company, but i guess every ski is different ???and they cant tune it from miles away. Anyhow motor runs strong now, great accleration with no hesitation or bog and after some full throttle plug chops the plugs look good. Ended up wtih 170 mains and 75 pilots. But still i suppose i'd be best off with the 44's and an aftermarket manifold based on what youre saying.

I've also heard that some racers back in the day were making some modifications to the pipe that were makin detonation less likely. I heard they bored the end of the expansion chamber and that affected the sonic wave somehow? Have you heard of this?

Again thanks for help.

[speedskier]

I see from your original post you said mikunis

Also, what ignition system do you reccomend, i noticed on msd's website they dont make an enhancer for the 720. Is the only other option a total loss ignition? or is there something else?