Backfiring and throttle runs away

[mmaddix]

My 79 xs1100f is backfiring and while it is sitting there idling it just starts idling higher and higher like i'm giving it gas but i'm not. then i cut it off and start it back up and it does the same thing a few min later. Please help. I think I might need to fix or replace the carbs or something.

[shovelhead]

check the condition of the carb to engine boots for cracks/clamps tight, and make sure you dont have any open vac. ports/broken/cracked vac hoses putting additional air into the intake sys after the carbs creating a lean condition...be sure to check the condition of the vac hose that goes to the pick up coil advance assy also.

[GARTHXS]

Now the fun begins. Read the FAQs and I might be coerced into taking a look for myself I'm here in SD too

[TADracer]

At first blush it soundes like you need your carbs syncronized.

[DGXSER]

Yep, first thing to check is synchronization of the carbs.

Seems alot of folks think that the bread tie "bench" synch is synching the carbs, not lumping you in that group, just saying it is common theory. In reality, the synch is vacuum based, and each cylinder draws a little different vacuum based upon valve condition, spark plug, exhaust condition, what type of exhaust, and other factors. So you need to adjust the throttle plate open position to reach the same vacuum level on all four cylinders. If one pulls more vacuum than the others, it will run the engine idle up, which changes vacuum on the others which will pull the engine idle up and this continues and you see the revs climb.

No meaning to preach to you, just inform those who need the info that may read this post.

[mmaddix]

You guys are awesome. I was going to sync the carbs with the sync tool I have but I didn't know if syncing them would fix the throttle reving up. I love this site.

[cywelchjr]

In addition, part of the sync process is adjusting the idle mixture. The symptom you describe is common for a set of carbs that have idle mixture too lean and the idle speed set high to compensate when cold. These will tend to increase in speed as they warm up. There are two things you can start with, an idle mixture adjustment and adjusting the idle speed down as it warms up (which you need to do anyways to do a valid idle mixture adjustment) as you can't adjust the idle mixture until the engine is fully warmed up. Once this is done, then you need to sync the carbs with some sort of vacuum gauge device. There are several methods of this, ranging from homemade gauges made with plastic hose and transmission fluid to store bought units costing will over $200, but some of the best you can use will cost you less than $200, and decent units will cost well under $100 for a 4 gauge set.

Many make the mistake of thinking as was mentioned that the bread tie sync is having the job done, and that setting the idle mixture consists of setting the mixture screws to a certain number of turns out. If this was the case, they wouldn't have the screws, cause everything could be a fixed proper size and never have to worry about adjusting it, but everything as DGXSER said, must be adjusted to the individual engine, which can range from those starting points being right on, to them being quite a ways off.

[mmaddix]

Alright I synced the carbs with my vacuum gauge sync tool but I know the idle mixture screws need to be adjusted. After searching for a little while I found but want to make sure to put them back to their stock position I put them all the way in(without breaking the tips off) then turn them out 1 1/4 turns correct? And once I do adjust the idle mixture screws do I need to re sync them?

[cywelchjr]

Alright I synced the carbs with my vacuum gauge sync tool but I know the idle mixture screws need to be adjusted. After searching for a little while I found but want to make sure to put them back to their stock position I put them all the way in(without breaking the tips off) then turn them out 1 1/4 turns correct? And once I do adjust the idle mixture screws do I need to re sync them?

If it's popping and spitting the they very likely need to be turned out more so doing what your suggesting would likely be counterproductive as that would probably make it more lean making it worse. You really need to perform the idle mixture adjustment which is part of the sync process, as if you haven't done the idle mixture you haven't really done a sync, only half of one.