SRX 1108 Race Sled Buildup

Ryan I may have to test on Friday only!! Parts for work are being delivered Wed.. and job has to be done this week.. I'm call you tonight..
 
I'm getting ready to burn the sled. I am having the same issues of it falling off on rpm hugely after about 700-800'. It runs 135mph in that distance, and just stops making power. I have changed head design, stinger diameter, coils, fuel system, jetting, gearing, clutching, all with the same results. I know the issue is not in the intake system, and is not in the chassis. I am starting to lean towards electrical. I am going to throw in a new stator tonight, and head out again tomorrow, with my spare CDI as well. I am running out of options.

Any other ideas would be appreciated. I am getting aggrivated.
 
I was running 13.6:1 for a compression ratio. I am now running 14.5:1. Same results. I had a big talk with Peter Duncanson tonight, and he mentioned something for me to try, so i'm taking his advice, and doing it. I will post results tomorrow. I hope to god he's right. :)
 
Maybe at max rpm you could have engine harmonics that shake the carbs and foam the fuel and shut off the needle and seat. Your old dyno run show fuel flow drop. I'd zip tie a long steel bar across the mouth of all three carbs (even a 1/2 breaker bar) and run it. Try to add weight to the mouth of the carb to stop the shake , if it has any.

If that doesn't work, get a downloadable EGT with jackshaft speed sensor. Record a run with the rpm drop issue and post the play back graph on here. Maybe all of us looking at it will help find the problem. Sometimes on the big end the driven shifts out past 1:1. The jackshaft rpm will jump from just under crank speed, to 1000 rpm over engine speed. That will also give you the same issue you have now.
 
The old dyno run showed a fuel drop because his fuel meter was not working. I also added a pile of weight to the carbs last season.

I am running a Digitron, and I have a jackshaft sensor that I was running on my titanium jackshaft, until I spun the clutch on the shaft, and went back to a Yamaha jackshaft, so now the collar doesn't fit on the shaft, of course :)

The sled was geared for 158 previously, so I geared it down to 147 to see if there would be a change, and ended up with the same results. I truely believe the issue is not in the chassis or driveline. The motor just stops making power at about 800'.

I have two more options to try today.
 
toy doc may be correct. common problem w/ solid motor mount engines. also sometimes a softer rubber carb mounting boot corrects the frothing problem. a large crescent wrench also works great. you may have to try different locations for mounting the stabilizing mass but it sure sounds like that's the issue.

bob

"Maybe at max rpm you could have engine harmonics that shake the carbs and foam the fuel and shut off the needle and seat. Your old dyno run show fuel flow drop. I'd zip tie a long steel bar across the mouth of all three carbs (even a 1/2 breaker bar) and run it. Try to add weight to the mouth of the carb to stop the shake , if it has any."
 
Well, it's worth a try. The motor is mounted the same way as it was last year, with no issues, however, but I am willing to try anything at this point. If my other two options do not work, I will try this. I am heading out around 2:30 today.
 
valin said:
The old dyno run showed a fuel drop because his fuel meter was not working. I also added a pile of weight to the carbs last season.

I am running a Digitron, and I have a jackshaft sensor that I was running on my titanium jackshaft, until I spun the clutch on the shaft, and went back to a Yamaha jackshaft, so now the collar doesn't fit on the shaft, of course :)

The sled was geared for 158 previously, so I geared it down to 147 to see if there would be a change, and ended up with the same results. I truely believe the issue is not in the chassis or driveline. The motor just stops making power at about 800'.

I have two more options to try today.
Click to expand...

That sucks when you junk a Ti shaft! I'd make that collar work or get one that works so you can see what your driven is doing. Gearing from 158 to 147 you can still see that same issue, only at 700' and not 800'. I would have went the other way, gear for 168 and see if it takes longer to lay down. You may need a driven spring with more side pressure, or less helix on the big end, or both. Is it a roller driven?

If your sure it's not clutching. I'd install your NOS system next. Keep the NOS bottle turned off, use the fuel side only to inject fuel into the intake when it lays down. If rpm pick up, it's a fuel issue. If not you then know for sure it's not fuel issue.
 
I know that there was a bet this year, AND a bet last year, which is why you paid me my case of beer earlier. Doesn't matter anyways....

I'll bring over that case of cans one of these days.
 
Toydoc, turns out i pitched either a mag side crankseal, oil plug, or something on that side. I haven't looked into it yet, but it finally completely let go on that side. I caught it on the jackstand and didn't hurt the motor. I'm going to yank the motor tomorrow and figure it out. Gotta love it.

On a lighter note, I won both days with the trail sled again, in the 800 improved trail class.
 
Well, I pulled the motor. The cases are damaged on the mag side. I managed to either break a weld for the stroked crank pin, or I cracked the outer magside lobe, which resulted in some of the weld letting go. The weld got between the rod and the cases, and split it open on the bottom a few inches long, about 1mm wide, and split open a hole on the intake side approx. 2mm x 5mm long. There is no damage to the rod, the upper case half, or the cylinder or piston. I probably cracked it last season when I locked it up at one time, and the crank is probably out of phase as well. We'll see.

I'll get some pictures when I find my camera. My wife put it away somewhere, and I can't find it.
 
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