wont stay steady on high rpms

[Danzick]

i have an 04. and for it to run smooth wide open, i have it running around 7600 rpm. when i try to get it to rev 8 grand, it wont hold a steady rpm floored. i revs up down up down real fast wide open. any ideas? cleaned exhaust valves, atleast 125 psi in both cylinders, good reed boots, clutches look good, brand new blue/ purple primary spring. any ideas?

 

[cubby]

If it runs up down while under load you need a clutch kit and that will hold the rpm's steady for you.

 

[Danzick]

it does it on the flats, and sometimes under a load

 

[cubby]

What do you mean sometimes under load, if you are riding it in the snow then that means there is a load on the motor and that is what I was referring to. I thiunk you just need a clutch kit.

 

[Danzick]

i dont really consider the flats a load, and i mean like if i just pin it through some what deep snow it will do it if i try to raise my clickers to get to the 8000 rpm. but if im climbing a steep and deep hill it usually wont do it. and why wont the stock clutching keep my rpms steady?

 

[cubby]

What under load means is to not have it on a stand. It means pretty much be on it and riding the more you load it the harder the motor is working to keep up. Clutching is everything why would any company make something aftermarket if factory was best?

 

[Danzick]

im not arguing with you. i want a clutch kit for sure. and i called dynamo joe out on it, and he gave me an answer i would have never guessed. he said its a pipe surge. i dont have enough pin weight in my clutch to make my motor work hard and keep the pipe warm. he said i need to stack the hell outta the pin weight to get my motor working hard and that should take care of it. he was saying its from my spi head and pipe i have, but he said if i get it dialed in right that set up runs pretty good.

 

[BackCountryBob]

My stock pipe is ceramic coated but I added a short piece of clam shell at the front grill to give me more heat. I also have D^Js mountain clutch kit and it made my 04 run with the XPs.

BCB

 

[Dynamo^Joe]

This commentary is from my old website years ago. I was blessed to be able to get invited by Team-X skidoo to Manitoba testing before the Rev chassis was released, getting to work on their sno-x sleds doing clutching. Bringing some of the funky helix's up there, some of them would "pipe surge" i was trying to get a drag race helix to work in a sno-x engine and it would not work except in a drag race to the first corner and that's it. Tom Lawrence immediately says to me not knowing what was in the sled "that engine ain't got enuf flyweight"
and I thought to myself, its running 5 more grams than everyone else, but i was playing with a start angle 6 degree lower start than the rest of the sleds.

A guy comes out of the corner and you can hear it so loud "rah rah rah rah rah" and i just waved Brandon in to change the helix. So in the Manitoba example ended up going from a 50 start to a 56 start and that cleaned up the "rah rah's"
...after getting to talk with Tom and Yvon Petit(race engine tech) they explain to me about "pipe surge" so I hope my example below will reveal some nice information for you and hope it works out.

The Zoomies = Pipe Surge

The Zoomies is one Tough problem to figure out for tuners regardless of their experiences in clutching and especially tough when a novice tuner has encountered this. After seeing this pipe surge event and figuring it out, you can hear symptoms from other sleds and quickly figure out the problem.

Example:
A tuner builds a clutch setup on an engine that happens to have 125 lbs compression. With 17 grams installed on the primary lever, the sled performs very well, good acceleration and backshift response.
The tuner's buddy has the same sled however that engine has 155 lbs. They install the same parts in this 155 lbs sled and there are problems.
***NOTE - The tuner does not know the compression of Buddy's sled is 155 lbs.***

Buddy runs his sled along at 3/4 throttle and maintains 8000-8100 rpm. When throttle pressed 100%, the engine rpm drops to 7500 rpm. Reduce throttle to 4/5th and the engine wakes back up to 8000-8100 rpms. Throttle pressed 100%, the engine rpm drops to 7500 rpm; Let off the throttle slowly to about 4/5th~3/4 throttle and the engine wakes back up to 8000-8100 rpms.
What...the...hell...is...going on?!!! After many questions, the tuner eventually gets around to asking Buddy what is the engine compression? Buddy says "155", Tuner replies 155???...Wow!
The tuner having experience with high compression engines run on race fuel, knowing what an engine sounds like if it is not loaded enough, asks..."Did the engine make a sound like "RAH RAH RAH" in a very quick repetition, surging quickly?
Did you notice the tach needle bounced up as you held the throttle pressed 100%; the sled surges while trying to accelerate?
.....take note on one of the best elements that reveal pipe surge is the 4/5th throttle test. IF the rpms raise when going 4/5th throttle, and rpms lower and surge when 100% throttle, (this is a dynamite effect to be able to chase down a pipe surge problem). Then the flyweight is not heavy enough for the timing of the engine and the compression of the head to heat the exhaust pipe properly.

Buddy replies "You are explaining perfectly". Buddy being kind of a newbie to the sport has never experienced this condition of Pipe Surge. So I have to say the way it run, he must have thought..."What kind of junk setup did my tuner friend give me?" HAHAHAHAHA

The tuner states this condition was called "Pipe surge" otherwise known as "The Zoomies", "Rah Rah's" or "Wack off's".
Further explanation revealed that the testing was done with engines that had compression of 125 to 132 lbs and that they all took flyweight in the range of 16 to 18 total grams of pin weight.
The Zoomies are caused from the exhaust pipe not heating up quick enough to produce power at proper rpms.
Few ways to get rid of "The Zoomies".
1] Lower the engine compression.
2] Add flyweight to load the engine more.
3] Retard the timing to heat the exh. pipe quicker.
4] Larger start angle on the helix at the start of the shift.
Easiest is to opt for #2 [Add flyweight to load engine more, making the primary push harder]

Tuner's conversation to Buddy:
Add flyweight to 19 grams on the levers and check rpms from there. If you need to add more flyweight to get 8000, then do so. If 19 grams is too much, then less flyweight. Do a MPH check. Try to go wide open and safely achieve top speed to check the flyweight for your 8000 "tach" rpm. If you have too much flyweight, then the engine will start to reduce rpms while full throttle, the rpms will lower at/or near your highest speed. If so, just take a few 10ths of a gram out at a time.

The exercise:
Buddy trial the sled with 19 grams and still had the zoomies. HOWEVER, he lost less rpms.
He would run part throttle at 8000, and then under full throttle the engine would Zoomie at 7700-7800 rpms.
He returned and added 2 grams for a total of 21 grams of flyweight.

Buddy called his tuner pal at work yelling in his ear of how good the sled run. Keeping the clicker the same position all the way thru the exercises, he chose to keep adding the weight to load the engine.
The flyweight loaded the engine hard enough to heat the exhaust pipe while the engine accelerated, the pipe stays hot and the combustion chamber now heats up to make even more power. No more zoomies, now just kicking other sleds butts.

 

[Danzick]

ill let you know how it goes this weekend. my pin weight kit goes up to 15.5 grams i believe. ill see if thats enough. otherwise i might be buying a heavy kit. thanks for the replies. ill let you know.