Rough running after warm up?

[jboughto70]

I have a 79 XS100SF that runs roughly (or doesn't even run) after warming up. It over revs sometimes and spits and sputters with terrible throttle response after warming up. I've rebuilt the carbs, lapped the valves and done some other top end work but still have the same problem. The left side is blowing a lot of smoke but it looks white rather than blue or black! I'm thinking it might be one of my coils but the resistances all checked out while it was cold. I can't even adjust the carbs yet! Upon initial startup it was idling around 1000RPM but then went out of control, running at over 4000RPM. The throttle was not sticking at the time. I'd rather not replace these expensive coils until I know this is the problem. Any ideas?

[pgg]

How do the sparkplugs look?

[Ken Talbot]

Whip the carbs off again, set the pilot screws to 2 turns, and pre-sync the carbs following this procedure . Put it all back together making sure there are no vacuum leaks and the throttle cable is operating freely. This should get you pretty close to where you need to be.

[Duster]

FYI

I fought the same thing with my 79SF and did what Ken said and rode it alot but it would still sometime run ruff when hot. Sea foam helped but one day I changed the float on one carb I wasn't happy with the way the plug looked. Now it is perfict!! Yea
Duster

[randy]

Worst case, this could be a head gasket issue. Try a warm compression test. Or, spraying a bit of WD-40 at the head gasket area and see if there's either a change in RPM or a noticeable pressure leak.

Randy

[jboughto70]

I don't think this is a carb problem. I will try the leak test but I'm pretty sure its an electrical problem. I don't understand the high revving and once in a while there is a backfire (can't tell if its intake or exhaust though). Any more tips would be welcome. Thanks for the inputs.

[pgg]

Check the plugs to see which cylinders are playing up for a start.

[TheDjost]

Sounds as if it's running lean. Try giving it just a bit of choke and see how it runs.

-Justin

[jboughto70]

I've now synched my carbs and adjusted the idle but the RPM's drop very slowly after it has been revved. When I rev it over 4,000 it won't slow down, throttle cable is not stuck and vacuum advance is not stuck. Pulling the choke out slowed it down and it would idle again! What the ##$#@! The last time however, pulling the choke out increased the RPM and the only way to slow it down was to shut it off. What now?

[TheDjost]

Now I'm sure it's running lean. Just keep turning out the pilot screws half a turn at a time until you get good throttle response. Run the bike with a healthy dose of carb cleaner gas additive for a couple of tankfulls and recheck the idle screw setting. There may still be a bit of varnish left in the pilot jet circuit.

-Justin

[TheDjost]

Let me elaborate:

The lean condition keeps the engine from idling properly. When you give it some choke, this compensates for the lean condition. however, if the engine is revving too fast, the vacum will be too high and will pull too much fuel (and some air) in through the choke circuit, making it rev even faster. You need to give it the extra fuel it needs, without opening the choke circuit. This is done using the pilot screws.

Don't worry too much if you're way out of spec for the moment, since you are still in the process of final tuning the carbs, all that you are looking for is a running bike. After a couple of tanks of gas, you may need to close the screws a bit.

You will know when you are on the money when you get good throttle response: the revs will stop hanging when you release the throttle, and just drop right back to idle. You may also need to play with the main idle speed screw(the one that holds the butterflies open) to get the idle to stabilise at the right spot.

-Justin