Popping (Backfires?) out of carbs.....Final Tuning Problems!

I am in the last leg of this race, or so I think! I am doing the final vacuum sync and colortune mixture adjustments, but am having problems. I've done all of the necessaries; complete teardown, rebuild, and cleaning of carbs, shimmed valves, set float heights, etc.

Bike: 1982 650 Maxim 4-1 exhaust, pods, and larger jet sizes to accomodate mods.

The bike is running quite well, but I get some murmuring in the idle. It's not silk smooth like I think it should be, but then again maybe it won't with the mods. I am having small pops or backfires out of carbs #1 and #4 intermittently. It doesn't happen all the time, as I'm adjusting the air mixture screws it will go away, but seem to come back at random.

Does anyone know the possible reasons behind this? Too rich? Should I double check valve clearances? Float height may need adjusting?

 

Popping and/or backfiring are typical signs of a too-lean condition. What do your spark plugs tell ya? When you re-jetted did you do mains only or did you check the pilot and/or air jet too? If you only did the main then you might be lean on the pilot circuit. How many turns out are your pilot screws? Can you adjust them rich enough at idle to get a yellow burn with the ColorTune?

 

Re: Popping (Backfires?) out of carbs.....Final Tuning Probl

I re-jetted (fuel) to 116 for mains and 42 for pilots. I did not know that you might have to re-jet the air jets. When I am adjusting the pilot mixtures I can pretty easily get a orange color when adjusting those carbs, but I haven't tried to get a yellow color.

I did try to individually lift the enrichment circuit on #4 and it of course gave me an strong orange/yellow color which made the backfire go away.

There is one strange note I have. I went through and lifted each enrichment circuit valve individually and with cylinders #1 - #3 it would bog down, but with #4 it wouldn't affect the idling at all. It was like nothing happened.

 

Lanker is right, it's a too-lean condition, and too-lean a fuel mixture is very hard to ignite.........what happens is that un-burned fuel builds up in the lean cylinders and then all of a sudden (over a couple of compression stroke cycles) the cylinder gets rich enough to fire. Lifting the choke valve on the too-lean cylinder gets that cylinder firing properly (the lean condition is eliminated).

The question becomes: why is that cylinder lean? Most probably, an air leak or an internally clogged pilot fuel circuit passage (they're tiny) or a fuel level that is way to low....

 

Re: Popping (Backfires?) out of carbs.....Final Tuning Probl

Ahh! Now that I know it's a too lean condition that helps out.

Does the pilot mixture screw entirely control the amount of fuel coming from the pilot jet? In other words are there any other passages the pilot jet leads to other than the little hole in front of the butterfly valve?

I think I remember the trick of using propane to check for air leaks around the carbs. Does this work?

 

The pilot fuel circuit draws FUEL thru the pilot fuel jet at the bottom of the carb, and draws AIR thru the pilot air jet under the vac disphram, and the two passages intersect and then go to the mixture screw passage which has 3 small outlets in the top of the carb throat.

http://www.xj4ever.com/inside%20your%20carbs.pdf

 

Re: Popping (Backfires?) out of carbs.....Final Tuning Probl

Wow, the carburetor in a glory of detail! I've always examined it from the outside trying to guess where each passageway is leading to.

I didn't realize there were 3 ports in the carb throat from the pilot circuit. I will definitely be checking those tonight.

Thank you for posting that pdf. Downloaded & saved!

 

I'm currently having this issue on my bike. I need to get one of those ColorTune gizmos but I'm having a hard time finding one. From what I've read we're supposed to get a cobalt blue, not yellow.

Also, I think my carbs all have the standard jets in them but someone put on the 4 into 1 exhaust, should I rejet it?

 

Nope.

I think "Blue" is too Lean.
A Lean Mixture backfires.

ColorTuning is a valuable tool for setting IDLE.
But, we need the Bike to RUN, too.

Blue will make the Bike IDLE.
Very well.
But, ... at the instant the Throttles are opened ... there needs to be enough Fuel available to SUSTAIN Combustion while the Plant transitions from IDLE to OFF IDLE.

If the Mixture is Blue its not Rich enough to alleviate a hesitation.
A modicum of "Supplemental Richness" (just beyond Blue) does the trick.

It is NOT much more.
Blue ... plus the width of a Dime ... "Tweaked" out.