running awful when warm

I am stumped on my bike. Keeping this post as short as I can, my bike starts right up, all 4 cylinders are running, goes good, real zippy.


Once she gets warm, after about 5 minutes, it jumps around between running real poor and running normal. But mostly running real poor. I almost think that sometimes it is running on 2 cylinders just because it seems to lose all power, and really bogs, not to mention that it will stall if at an idle.... but then it will idle normal for a bit..... and then idle horribly and not rev without bogging. So I am lost because it doesn't just run bad when it is warm, it bounces back and forth between good and bad. Any ideas? Can this be some sort of vapor lock or fuel starvation issue? And I really suspect that maybe at the bad times, only 2 cylinders are firing(since the loss in power)... but how would I check for that since all 4 fire when it is cold?


Finally, if it where the valve clearance or something, wouldn't it just run bad the whole time it is warm, not randomly?


thanks



~one last little comment that might help someone understand..... when it is warm, and if I have a moment it is running good, I will open the throttle evenly, and it will zip for 1-2 seconds and then almost toss me over the handle bars when as it second guesses itself and bogs, and then will zip... bog...zip... bog....


And I get backfires when going downhill or coasting to a stop in gear.

Post edited by: b4schroer, at: 2006/07/23 20:11

The classic symptom of valves out of tolerance (on valve-shim motors) is that it stalls at idle after a hot highway run. This is because the valves clearance reduces with wear. If it's out of tolerance, the clearance goes to zero when the valve heats up and expands. The valves, therefore, no longer seal when the motor is hot and it stalls at idle. At higher RPMs, it seems to run fine. As the temperature of the engine cools slightly, the valves seal again and it runs fine.

Normally, you only get the stall after a highway run because that is when the valves at at their hottest. If you are stalling out randomly and when riding slowly, the problem could be valves or several other things... carbs, coils, ignition etc.

If the valves haven't been checked in a while, you should start there, as well as a compression test etc. You want to make sure the engine and ignition is good before you start messing with the carbs.

Ok, so that is information I should have posted in the beggining. Compression test is awesome on all 4 cylinders, as far as an ignition problem goes, that is clearly part of my whole question... I suspect that occasionally I am not getting a spark... but it is only when it warm, and when I check for sparks... I have it on all 4... so how can I check when it is running and warm?


As far as carbs go, I completely just took them apart and re-built them for the most part.... is there something that I might have done wrong? Again, since it runs normal cold, it has to be heat related... and the carb temperature doesn't change that much. I mean, it becomes warm, but my no means hot. If it was say, float levels... wouldn't it run just as sketchy cold?

What color are your plugs? Do you have points or electronic ignition? Sometimes coils that are going bad can act up when hot, but I'm not sure how to test that. There are specs for coils in your manual, I would check that too.

Post edited by: OKC_Kent, at: 2006/07/23 23:24

Nothing so far in this thread has narrowed down the problem any. I suggest you fill a spray bottle with water and go for a ride. Stop when the bike starts showing symptoms of mis-behaving and spray the cylinders. Water vaporizes if the cylinders are firing and rolls off if not. IF repeat IF cylinders are not firing, knowing which cylinders helps narrow the problem. It does sound like you have a couple cylinders cut out during operation. Since your compression is good, the valve clearances are not likely the source of the problem as someone guessed before you made this info available. The valve clearance DO decrease a tad as the bike and metal warms and if they are marginal, the valves COULD be hanging open but if compression is great, I would check valve clearances but look to either the ignition system or fuel system as more likely. The spray bottle test will tell a lot.

See the page on valve clearances:
www.wgcarbs.com/269767.html

Maybe this will help... My bike was running really well with a set of UNI foam pods on it... I just put a set of Velocity Stacks on it (the kind with screens). I knew that this would lean it out because of increased airflow... I went out and did a little test run around several blocks and had the same symptoms that you described...

I was running a little rich on the main jet previously so I plan on bumping up the pilot jet one size since it seems to be most effected in the mid-range.

I say all that to suggest that maybe you're running a little lean, especially if you notice it popping through the carbs.

Cheers,
Claude

A compression test won't show if the valve clearances are out of tolerance. This is because by the time you can do the test, the engine has cooled too much. It would have to be done within seconds of running.

It would only show if the tolerance was so far off that the cylinders don't seal even when cold. But by then the bike won't run right even at startup.

I'm not suggesting that your valves are out of tolerance, but just that a compression test is not necessarily an indicator. If you want to know if the clearances are ok, you have to measure them.

Post edited by: loudhvx, at: 2006/07/24 13:44

b4schroer wrote:

so how can I check when it is running and warm?

Timing light?

wiredgeorge wrote:

Stop when the bike starts showing symptoms of mis-behaving and spray the cylinders. Water vaporizes if the cylinders are firing and rolls off if not.

The water will roll off even if they have been firing and are clearly very hot??..... seems to me that they will be too hot for that to show properly..??



One more question, if the bike is running lean, does that produce those symptoms even when cold? Because mine is fine cold. And I haven't changed anything, it ran fine last year, so why would I need to adjust jets and such?


Oh, and I have a Dyna S elec. Ignition

cold engines like a richer mix (choke)
hot engines like leaner mix (hot start on dirt bikes)
just for a test restrict the airbox a little (rag) and see what happens

cold engines like a richer mix (choke)
hot engines like leaner mix (hot start on dirt bikes)
just for a test restrict the airbox a little (rag) and see what happens

Heres something easy to try, unlock your gas cap so its just laying down on the filler neck, and go for a ride, if the problem goes away, check your gas tank vent in the cap its probally plugged up...I had the same kind of symtoms last year and it was the vent plugged with tank sealer......

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