Auto Decomp Mod 426 Missing at higher RPM's

5 replies to this topic
  • redright

Posted May 16, 2005 - 10:05 AM


I did the mod bike, rode fine at first. Now I'm having a problem with it missing at higher RPM's. I have tried jetting this thing all over the map. Seems to get worse the richer I jet it but is still there when I go leaner. Any suggestions? Could it be just coincidental and something else could be causing it?

  • grayracer513

Posted May 16, 2005 - 10:33 AM


It is almost certainly not related directly to the use of the '03 cam, but experience has taught me that the first place to look is at the things I messed with when something I did results in an unexpected problem.

If it's an irregular miss, I would look at the ignition system, checking for loosened connections or damaged wires around the cam cover, or the coil wire and plug boot. Check the plug itself.

If it's a regular miss, you might double check your cam timing. The 450 cam sets up differently than the stock 426 cam because the marks were intended to be used with a different cylinder head casting. Time the intake as per the manual, then position the exhaust so that there are 14 pins between the 12:00 marks on the two cam sprockets. Here's a photo of the setup done right courtesy of Satch922:

If you had the carb apart, look to what you worked on there.

  • redright

Posted May 16, 2005 - 11:28 AM


Thanks for the reply Gray, I have double checked the cam timing it is at 14 pins. I have done the coil on plug mod as well (could this be it?), guess I'll try a new coil. I did put in a new plug. I have never fully cleaned the carb, only pulled the bottom off. I'll clean it tonight.

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  • grayracer513

Posted May 16, 2005 - 03:21 PM


If you did the coil mod between now and the last time it ran right, you might look into the possibilty of reversed coil polarity. I don't know if it matters in this case, but it does in some. And, of course if it was a used coil, there's a chance it failed or was bad when you got it.

  • redright

Posted May 16, 2005 - 08:49 PM


Coil polarity is good, I replaced with a new coil (havn't riden it yet). I have the carb apart now (to clean it) but ran into a problem with the gaskets when I took off the section where the float is. The gasket appears to be glued in and when I took it off it came loose, won't lay back down. Is there glue for this?

  • coulterk

Posted May 17, 2005 - 01:03 PM


I had a similar problem as i was going up jump faces it was spluttering. I had adjusted float level the day before so I thought that I had it to high, readjusted after first race but still had same problem in second race. Next I thought it might be the plug, but it was fine then I noticed that the orange wire going to the coil was sitting between the 2 top coils of the rear spring so every time the suspension was loaded it crushed the wire and shorted out the ignition. I cabled tied the wire back out of the way and problem was solved.

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