Misfire while idling

[Alphamazing]

After my rejet and lunchbox install, my bike seems to be misfiring while I'm sitting at a stop. I see the tach needle drop and here a slight "pop" sound. It doesn't happen constantly, but only every now and again. It's most notable when the engine is warmed up.

Current jetting is 22.5/65/137.5 with the air/fuel 3.5 turns out. Carbs are clean, float height is adjusted correctly. Oil level is fine. I use 87 octane gas. I tried a search, but everything involved stumbling aroun 2k RPM, rather than idle (1300RPM).

Any ideas? Should I just go to 89 octane gas in order to prevent the predetonation?

[coll0412]

DO you hear the pop under your seat(like a lean back fire up the carbs)

Is the motor good and hot when it does this?

My best guess is that the mixture screws might need to be turned out a bit

[scratch]

Yep, either that or valves (u knew I was going to say that).

[Alphamazing]

DO you hear the pop under your seat(like a lean back fire up the carbs)

Is the motor good and hot when it does this?

My best guess is that the mixture screws might need to be turned out a bit

I'm not going to turn the mix screws out any more. Last time I turned them past 3.5 turns out, one of them fell out. I think it comes from the front of the bike, more under the tank, thus my assumption that it is the engine.

Yep, either that or valves (u knew I was going to say that).

Bah, I meant to say I checked the valves recently and they were all within spec.

[coll0412]

Yeah the only thing that comes to mind is pre-combustion due to a really hot cylinder. Maybe you are running lean through the rest of the band and getting some hot cyilnder conditions, how do the plugs look?

How about a carb balance? If one cylinder is having to do alot of the load it gets hot hot hot. How warm is the ambient air when it happens, I mean is like 90+, I am not that familiar with texas weather other than its hot

[Alphamazing]

Yeah the only thing that comes to mind is pre-combustion due to a really hot cylinder. Maybe you are running lean through the rest of the band and getting some hot cyilnder conditions, how do the plugs look?

How about a carb balance? If one cylinder is having to do alot of the load it gets hot hot hot. How warm is the ambient air when it happens, I mean is like 90+, I am not that familiar with texas weather other than its hot

Carbs are balanced. Weather has been pretty fair here lately. Mid 80s to 90s or so. I don't think I'm running very lean through the rest of the power range, but haven't checked the plugs since I upped the jetting. Not super lean though, I know that much.

[coll0412]

Yeah maybe try a little higher octane and see if it  goes away, I would just be a little concerned since if it is pre-ignition that is muy mal.

[Alphamazing]

Yeah maybe try a little higher octane and see if it  goes away, I would just be a little concerned since if it is pre-ignition that is muy mal.

What could cause pre-ignition? I realize the heat may be a contributing factor, as it increases the pressure, but enough to cause predetonation? I've seen engines with 12:1 compression using 87 octane gas and not having a problem. Why would the little 9:1 have this issue?

[coll0412]

I am not sure, that is why if it was lean you develop hot spots within the cylinder that can do that, does it do it right when you roll on the throttle at all, or just when it is sitting there idleing

My guess is that it could be some bad plugs maybe?

[patrick]

You guys are on crack. There's no way in hell it's detonation. The cylinder is barely filling at idle. Detonation happens when the cylinder's jam packed and the whole compress->heat thing brings the temp of the charge up above the point where the mixture ignites. Think full throttle near the torque peak.

Range of adjustment of the fuel screws should be about 1 - 5 turns out. If the screws fall out when you turn them past 3.5 turns, the springs are collapsed. Stretch them out a bit to keep the fuel screws locked in place.

This is a good way to find the mixture screw adjustment. Read bullet 4.

http://factorypro.com/tech/carbtune,CV,high_rpm_engines.html

Checked for air leaks? Torqued the cylinder head bolts lately?

[Alphamazing]

This happens when I'm sitting there without my hand on the throttle at all. It sits there idling, then suddenly "pook" and the RPMs drop, then it picks back up again.

And the screws didn't fall out when I turned them past 3.5, but they vibrated out after a while. It isn't worth the risk, IMO, and I am not dropping another $20 on another air/fuel screw assembly.

I'll check engine bolt tightness and the filter for leaks when I can. Throw out any more ideas as they come.

[coll0412]

Actually your on crack,

Have you ever heard of a lean back fire, hmm what could that be from, fuel igniting before the intake has closed, well how could that happen maybe a combination of a lean mixture with a high temp exposed a hot prortion of the cylinder and boom you have fire. It just depends on exactly when it wants to occur, if it happens early than when the plug fires(hence termed pre-ignition) then you get a stumble, if it does it when the intake valve is still open then you get a back fire through the carbs. I is not dentonation which iswhen you have two flame fronts hitting eachother and causeing the "ping" sound, which occurs after the plug has sparked.

The latter is more lilkely to occur under compression but if you mixture is to lean it will back up throught the carbs, trust me my bike used to do it. And it was not because of the valves but because the idle mixture was crazy mad lean and when the bike would get hot enought it would fire up through the carbs.

[coll0412]

Although patrick does have a good point about air leaks, if you have a leak on the manifold vacuum side your mixture will always be to lean, so you could get the same thing...maybe do a wd-40 test or check to make sure the boots are well seated on the carbs.

[coll0412]

Itis weird that you can only get 3.5 turns out and then they vibrate loose, hmm.. I am not sure about that

P.S. all my assumptions are that the poof he is hearing is a backfire up through the carb....there are other possible solutions which may include other factors...so sorry Patrick if I seemed a bit crabby on the response...no hard feelings intended....


And yes I could have put it all in one post but I am tired and want to goto bed

[Alphamazing]

Itis weird that you can only get 3.5 turns out and then they vibrate loose, hmm.. I am not sure about that


And yes I could have put it all in one post but I am tired and want to goto bed

Well, I had it at 4 turns out and that's when it vibrated loose. I'm not taking it past 3.5 for that reason.

[coll0412]

Maybe its rich and turn it in a bit

Now I have covered the entire spectrum of possibilties and can only be right:laugh:

[patrick]

so sorry Patrick if I seemed a bit crabby on the response...no hard feelings intended....

No worries. If I read everything on this board, I'd be crabby too. There's a lot of nonsense being thrown around, and it can be tricky to sift out the good info from the bad if you don't have some time in this kind of stuff. I pop in once in a while, read a little here and there, and try to dispell a few myths now and again. But I sure don't try to keep pace with everything on the board.

I see a lot of the same stuff new bikers have been saying and doing since forever. It's probably never going to change. Nice turn signals. What kind of oil should I use? I can't believe the bike shop ripped me off! I heard from this buddy of mine who used to work on a race team if you run toluene in the gas you can pick up a bunch of power. What jets should I run? Check out this pic of my footpeg! You don't know what you're talking about, my new fork springs are way better than yours. I want a new pipe, which one sounds the best and makes the most power? I just want to, you know, get the MOST from by bike. Dooode check out my new K&N filter! Blah blah blah...

Not everything here is crap though. There are a couple of people who definitely have a clue. But you can be fairly certain those people are not the ones with like 4,000 posts.

Patrick

[Alphamazing]

Not everything here is crap though. There are a couple of people who definitely have a clue. But you can be fairly certain those people are not the ones with like 4,000 posts.

So hey, way to be an @$$hole.

[patrick]

So hey, way to be an @$$hole.

Dude, you are the biggest f%$king know it all who really doesn't that I have ever seen anywhere on the internet. Once in a while I check the board just to read what kind of crap you're spouting so I can have a good laugh.

Patrick

[ajgs500]

Not everything here is crap though. There are a couple of people who definitely have a clue. But you can be fairly certain those people are not the ones with like 4,000 posts.

So hey, way to be an @$$hole.

Ditto