idle speed backfire

[thumperjsa]

Got around to attempting to sync my carbs today. I used a sync I borrowed from Doc (Thanx). My pre-sync with bread ties must have been pretty close because I barely had to move the screws to get everything balanced.

After I synced, I decided to adjust the idle mixture screws. They were all about 6-7 turns out. I moved them all to 1.5 turns out. Now though it seems I have developed a backfire issue when I idle down from speed. For example, going about 60 in 5th and back off the throttle all the way. It is not bad but noticable. I also notice an occasional backfire at idle when waiting for a signal light to change.

Could it be because of the idle mixture screws are not set right? If so, is there a way to do it with out a color tune? Should I just set the screws to about where they were before? Is it the idle mixture at all that could be causing the backfiring.

One item of note. Ever since the pre-sync of my carbs, off the line response and 4k rpm is response is much better.

[Ken Talbot]

To set the pilot screws without a colortune, get it warmed up and bring the idle down with the idle adjustment screw to somewhere around 600 or 700 rpm - well below your normal idle setting.

Working one screw at a time, back the screw slowly out a couple of turns while listening carefully for any change in engine speed. You should hear the speed increase very slightly then begin to fall off. Turn the screw in carefully, again listening for any slight change in speed. Adjust past the sweet spot, then come back to it. Do this a few times until you're sure youve got thata screw at its perfect setting.

If the idle speed has come up at all, back it off again until you're back down to 600 or so. Repeat this process for each of the four screws, all the while keeping the idle speed as low as the bike will run with.

Check the sync again, then run through the four pilots again. One last look at the sync and you should be in fine tune! Take the idle speed back up to 1000 or so depending on your personal preference, and you're done.

I have found this method to work as well or better than using a colortune, and it is lot quicker.

Final tip - if you watch your tach closely while you're doing this, you'll notice the needle move ever so slightly as you reach and go past the sweet spot.

[prometheus578]

Lots of factors involved. Yeah, you're running lean now. Aftermarket exhaust? Free flowing air cleaner? Must be a reason why the pilot was backed out so far. Immediate fix would be to back out the pilot screws a half turn at a time and test ride till the popping stops. Next bigger pilot jet might be in order.
Need to watch the plug color. Best indicator of how the engine's running. Throw in a new set and see how they color. You're looking for a nice tan. Bone white is too lean, and obviously, black and sooty is too rich.
'zample: I put on a set of 4 into 2 exhaust. Plugs went bone white. Changed the main jet from stock 137.5 to 140's. Increase in power, but plugs still white. Saturday, pulled the carbs and put in 142.5 mains, and reset the pilots back down to 1 1/4 turns out. Haven't checked the plugs yet to see how they're burnin', but have started popping like you due to the leanness of the pilot circuit. If the plug color looks good from highway speeds, then I've finally got the right size o' main jet. I would probably then go up one size on the pilot jet, or just fiddle with the pilot screws till she runs right at idle, etc.

For example, going about 60 in 5th and back off the throttle all the way.

When you back off like that, you're shutting the carbs back down to the pilot circuit. Get a larger jet for that, or play with the screws.

Could it be because of the idle mixture screws are not set right? If so, is there a way to do it with out a color tune?

Get an accurate tachometer.(the one on the bike isn't sensitive enough) Put the bike on the center stand and let it warm up. Have a large fan blowing across the pipes so they don't overheat and turn blue. One carb at a time, mess with the pilot screw. Turn it in till the engine RPM drops(lean) Counting the turns, back it out. RPM will start to rise... stop counting when the RPM doesn't rise anymore and the engine starts to falter(too rich) Turn the screw back in half the number of turns that you turned it out. You then should be right in the middle of too lean and too rich.
Not all carbs may have the same setting due to internal differences: cyl's #2 and #3 run hotter than #'s 1 and 4 due to cooling airflow, etc.
Turn the bike off, and count the number of turns out on each carb for future reference. Test ride and enjoy.
Carb Guru's... did I miss anything?

[prometheus578]

Had to have a cigarette

Oh, Thanks, Ken
I leave the room to get a smoke before proofreading and submitting, and you beat me to the punch.
HHmm.. might be time to quit (But I doubt it!)

[Ken Talbot]

LOL! I don't proofread or smoke.

(Being a moderator, I can go back in and fix my typos long after I post the note)

[thumperjsa]

Thanx guys. I used the shortcut of Ken's method. Back down the idle till about 600 rpm. Play with one at a time for what I feel is the sweet spot, adjusting down the RPM each time. Not sure how balanced it is becaus it seemed like I turned the adjustment on carb 1 more than the others.

I never got back to adjusting the carb sync. Starting to get dark and I wanted to see how it was riding before dinner.

The adjustment I did cut back the back firing dramatically. Still wants to let out a few farts when I drop from high rpm to idle in gear on the road, but not as bad. RPM is a lot more steady now too. Does not seem to be bouncing on the tac like it was before.

When I get a chance, I will check the sync again to see if I can get the old girl from pushing out gas pockets out the back end. She is running much better than when I was paying a wrencher at a shop from doing it.

Only two more problems to figuire out now.
1) tapping sound near the front end. Only happens in gear, not in nuetral. I think it may be fooling me because it could be an exhaust leak. One of my bolts on the #3 cylinder broke so I only have one bolt holding it down. The other bolt is holding it on fine. I need to take it to a welder and have them tack on a bolt to it so I can remove it. Some of the stud is out of the head but not enough to grip on too.

2) First gear only, when I take off fast, at about 6k it feels like something misses and I get an instant drop and return of power to the rear wheel. No RPM increase when it happens or I guess I would think it was the clutch from all the tourqe. But I am not sure.

[mro]

hey thumperjsa

When I got my 80G it was doing the same thing (back-fire) as you described. Would blow off the vac lines to pet cocks. Not being MC mechanic wound up with poilt needels out just over 6 turns. Runs great all rpm's with out back fires.

Have been planning to take the carbs off for a proper cleaning as was sugested on anouther thread but have been short on time.


Should you find anything in particular that solves your problem please post it.

Thanks

mro
newbe

[prometheus578]

another thought

Thumperjsa,
QUOTE]. I think it may be fooling me because it could be an exhaust leak[/QUOTE]
Loose exhaust can, by allowing air into the system, also cause popping(backfiring).[

[marksalerno]

Idle speed backfire

Thanks Ket T

My son and I rebuilt the carbs on a 78 that had been sitting around for two years.

Cleaned and had to replace one idle screw.

The bike started right up, syncronized the carbs and it started back fireing at idle.

Used your idle scree adjustment method.

#1 worked great and the backfireing stoped. #2 idle adjustment did not seem to make a diferance no matter what the position.

I am going to try to readjust all tomorrow.

Any comments or sugestions.

[Ken Talbot]

Getting no change when adjusting screw #2 can mean three things:

A - the screw tip is broken off in the carb throat, or
B - the fuel passages that this screw controls are plugged somewhere in the body of the carb, or
C - the tip is broken off and the passageis plugged

Hope for A, but be prepared for C.