Misfire at low revs

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marshall100 · #1 Post by **marshall100** » Fri Sep 14, 2012 7:24 pm

Hi, did an enduro last weekend. Managed to get third in my class but spent most of the race nursing a sick quad. At full revs and idle the bike was absolutely bang on the money. At low revs it was misfiring like I've never encountered. An easy enough problem to ride round as you'd just blip the throttle.

After two hours I finished the race and had a yellow fault light on and a slow flashing red. Understandable given the fact I'd parked up in the pits waiting to get back to my trailer. I've done a brief damage check and mechanically the bike is fine, there are no lights on the dash now either. Before I start looking at this misfire problem I'm thinking it's going to be one of the following:

Injector
Fuel Pump
Air filter loose
Clogged fuel filter
Flow rate (2400)

I suspect that it's the last thing on that list as someone following said the bike was stinking of rich fuel.

Is there a chance it's something else I should be looking at?

yamadjs08 · #2 Post by **yamadjs08** » Sat Sep 15, 2012 1:46 am

Sounds like its too rich on bottom end. Try leaning out the offset (lowering the number) and see if that helps at all.

MX Quad Dad · #3 Post by **MX Quad Dad** » Sun Sep 16, 2012 12:32 pm

I would add the TPS to your list. Not sure if you can check it with the D&M, but you should be able to check with a ohm meter. readings should inrease and decrease without jumps as you go through the throttle range

Kuma · #4 Post by **Kuma** » Sun Sep 16, 2012 3:59 pm

If nothing has changed except the way the engine runs and it used to have no problems, I would exhaust all the items that do change before you start trying to retune. like you mentioned injectors, fuel pump, fuel regulator, air filter, check all you connections all the way from the air filter to the head, look for cracke, I know this would be dause for a lean condition but there is always the possibility that there was an air leak and it was tuned for the leak? do a throttle cal, you should also be able to check the smoothness ot the TPS at the same time.
weren't you having problems with spark plugs? maybe you have a weak coil. hope you get it sorted out, it's good to hear about people on the old cannondales woopin' it up on the new quads

marshall100 · #5 Post by **marshall100** » Sun Sep 16, 2012 5:32 pm

Fiddled with the flow rate, and got nowhere. Moved the injector offset from 1.2 to 1.0 and the bike was alot happier and smoother through the revs. I'm still going to go through the air filter, throttle, and exhaust, just to be on the safe side. Does seem very odd that it's suddenly gone from being quite happy to quite annoyed.....

There's also a shonky connection somewhere, as the ignition wouldn't prime until I fiddled with some wires under the bodywork.

marshall100 · #6 Post by **marshall100** » Fri Sep 21, 2012 6:36 pm

I'm now not sure I've solved this. I swapped my quads round in the garage and while moving this the problem was back. Nowhere near as bad but still there. I'm guessing that it being a bit cold would make some difference?

Guess it's time to start looking for other issues?

marshall100 · #7 Post by **marshall100** » Sun Sep 23, 2012 12:40 pm

Thought I'd better whip out my spark plug, and the reason I use a Denso is perfectly clear, and all this from a plug that was still working!

I clearly have some fueling issues to resolve.....

wistech · #8 Post by **wistech** » Sun Sep 23, 2012 1:26 pm

Im impressed your quad still runs. Youve overheated the living snot out of it and have been running pig rich for god knows how long. Of course the richness might have kept it alive while on the verge of meltdown.
All the things on your list are not going to cause the fouling. I would hope you gutted your quick connects long ago but you need to tee in a pressure guage to make sure your not overpressurizing the fuel from an obstruction. Take the throttle body off (pull the coil wire) . Pressurize the fuel system and be sure you dont have an injector stuck on and leaking . Its rare but it has happened.
When was the last time it had a correct throttle recal? What mods and map are you running? When was the last time this engine had a teardown update and inspection> Whats the compresion? How are the cam lobes? Is the water pump impellar melted? Hows the flywheel?

How long have you been fouling plugs and did it just happen all the sudden or from day one?
Il say this again your spark plug is not your problem and your just destroying your engine continuing to band aid it with special spark plugs so lets figure the problem out.

marshall100 · #9 Post by **marshall100** » Sun Sep 23, 2012 1:52 pm

Engine was fully rebuilt in January of this year. It overheated (I'm fairly certain) due to a blocked rad one race ago, though thinking about it, running it rich won't help much either? It doesn't tend to foul plugs since it was re-built. Before the re-build it would happen quite alot, this year it's not fouled a plug at all. Water pump impellar is fine. I'll check the flywheel shortly. Did a throttle re-cal before my last race. Though the feedback from anyone following was that it was stinking rich.

marshall100 · #10 Post by **marshall100** » Sun Sep 23, 2012 2:50 pm

Checked the flywheel and it's not spun off the woodruff key.

Tell me about this pressure guage.

I think it's running a blaze map. Stock piston, boomers air intake, hmf exhaust, and the usual recommended updates to fuel tank and oil lines etc.