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Indy 500 Shift Rpms And Tach Accuracy

1192 Views 0 Replies 1 Participant Last post by  Beaxch
I'm going to throw out a bunch of thoughts...any comments on any one of them is appreciated.

I've been really scratching my head over this '92 500 SKS clutching and I know there are a lot of you who have ridden these engines. I have the engine totally stock as far as I know but it was rebuilt by the previous owner, the primary clutch set up for my altitude range per the clutching chart, and the stock red secondary spring and what I believe is the regular 36 degree helix. Basically, when you jam on the throttle from a stop or low speed it wants to rev too high, like 8300+ RPM. The target rpm is supposed to be 7800+/- 200RPM, so I really don't want to see over 8000. Anyway, as the sled speed increases, the RPMs come down to around 7800-8000 on the trail. Then on hill climbs, I might see 8000-8100 all the way up the hill the first time, and then 7500 and falling on the same hill doing basically the same thing the next time, like if it shifts out too far before the hill it won't backshift enough while climbing. I have already disassembled the secondary and used solvent on it and cleaned it up nice and replaced the ramp buttons. It does sound like the job for a multi-angle helix, like it just simply needs to shift faster at first, but it does also seem weirdly inconsistent at times. In the primary the weights pivot freely and the little rollers seem to roll fine. The primary spring is new, the sheaves are clean, and I don't know anything else about primary clutches. I can say with certainty that the clutches are running surprisingly cool and I don't believe it is slipping at all. I'm using an ultimax 3 belt and I have the deflection set right, the engagement seems firm and not wishy-washy.

By comparison, when I jam the throttle on the Vmax it will hit somewhere between 7800-8100RPM (target range is 7700-8300) and will never go out of that range at WOT under any circumstances. It also has a straight helix (39 degree) but the engine does have a wider power band so maybe that makes it easier to clutch, or so I've heard. At any rate, the sled is totally worry-free, it just goes fast when you juice it and never does anything weird. That's what I'm trying to get to with this SKS. Right now, seeing 500+ RPM over target peak on this SKS makes me worry.

On a related note, how accurate are these tachs in general and what is a good way to test my tach against reality? I wonder if I'm not overrevving at all because the power seems really good at 8300 RPM and pretty poor at 7600, and at 8300+ I should be past my power band. Perhaps I'm not at 8300 at all. I've heard tell that these tachs are 200 RPM too high in most cases, and if that's true then I'm not as far off as I thought.

I'm also wondering if I just change the gearing ratio to like 2.2-2.3 as I have done with the Vmax that the problem will solve itself, the theory being that the clutch has to do less work (dealing with less forces) and therefore works better. I don't know if that's true but I know that re-gearing the Vmax was a really good move so maybe that's the answer. It gave the sled more low-end and just generally more consistent performance. Right now that's exactly what this sled is lacking: low end and consistency. I just don't want to spend more money than I have to so if anyone has any comments feel free...the primary use for this sled is powder and the track has been upgraded to 136" x 1.75" (from 133x1") so it's pushing more rubber than stock, I don't know if it has been regeared already or not, but I'm going to find out. The stock gearing is not for hill climbing I'm sure. SLP recommended a 38/34 helix with a pretty heavy spring to go with the stock primary setup, but I only want to go that $$$ route if I really have to. At this point who knows if a different helix will really be the answer. The red spring could be worn out so I'm going to try that next, hopefully the horrible local dealership has one in stock.

So long story short...has anyone had success with the 500 and stock helix? If so, any suggestions on what might be wrong with my setup? Or is this kind of performance actually typical?
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