Tune up question.

I was in the process of colortuning my carbs and I found that I could only get the range of yellow to blue with carb #3. The others were all blue but backing out the pilot screw wouldn't make it yellow like #3 did.

I pretty sure I have to pull the carb rack (again) and do some more cleaning. Sheesh, I thought I had cleaned them thoroughly already. So what should I look for? It starts easy and runs strong otherwise.

Thanks for the help.

 

have you vacuum-synched the carbs yet? #3 is the only carb that is directly controlled by the throttle. the rest are set with the sync screws.

mixture going richer when you back out the idle screw is perfectly normal -- you're reducing the amount of air that gets past the butterflies.

 

Thanks, stereomind and yes, it has been synched.

The problem I'm having is that the pilot screws on #1, #2, and #4 won't get the colortune to show yellow (or rich). Heck, I can back them almost all the way out and it's still blue. The only one that works right is #3. It will go from a nice blue to a nice yellow within about two turns of it's pilot screw.

 

ok, let's back up a bit. have the carbs been cleaned (properly)?

it sounds like #1, #2 and #4 are getting too much air, or not enough fuel. my bet is on the former -- lack of response to mixture screw input usually means that the carb is getting its mixture elsewhere, like from the pilot passages just behind the butterfly (which are not controllable) or even the mains.

have you checked for vacuum leaks? (with an unlit propane torch or carb cleaner spray)? in particular, around the throttle shaft areas. bad shaft seals are notorious for vacuum leaks, which would lead to the exact symptoms that you're describing.

what happens when you turn the mixture screw all the way in on one of those carbs? does that cylinder quit firing completely or does it stay blue-ish?

 

Well, I thought I cleaned them properly. Apparently not.

Also, I thought yellow meant too rich, not too lean. Wouldn't air getting in from a leak cause it to be yellow?

Anyway, the carbs have new shaft seals and I did the propane test - no changes in RPM. Turning the screws in on #1, #2, or #4 will eventually stop the cylinders from firing. They stay blue until then.

Oh yeah, and I anally adjusted the float levels. (in a manner of speaking)

 

gitbox - I had the exact same issue when tuning my bike; only one of the carbs would respond to adjustments on the pilot screw while the rest stayed blue throughout the entire range until I turned the screws all the way in and the cylinders would stop firing.

I went crazy taking the carbs off, cleaning them (AGAIN!) and putting them back on several times. After a bit of thinking and a hint from Rick, I swapped out my airbox to carb intake boots for the 1.5" plumbing fixtures (see thread here: http://xjbikes.com/Forums/viewtopic/t=18757.html ). Despite my bike passing the unlit propane torch check for air leaks, these new boots sealed off whatever extra air was getting to the carbs and allowed me to tune properly.

I would highly suggest swapping in new intake boots before you take apart your carbs.

 

Yellow flame means the mixture is too rich (too much fuel). Adding air to a rich mixture will lean it out, turning it various shades of blue, and then almost white (extreme lean) at which point it would likely misfire.

You mentioned that if you back out the idle screw, the offending cylinders would go yellow (rich). Have you tried colortuning them at that point, or would it not hold idle?

 

OK, I get it now. An air leak will make the color turn blue.

Stereomind:
I can't get the color to turn yellow - it's just blue all the way from not firing to a completely backed out pilot screw.

Mestnii:
Great idea. I tried it and it did not make a difference. Bummer.

Oh well, the rack's off now (again). I'll shoot some more cleaner through the holes and see what happens.

I'm still open to any more suggestions.

Thanks.

 

have you checked the boots from carb to engine? any small cracks?
if you pull the carbs apart and spray the pilot holes, does it spray out the other side?

 

Hooray!! Someone was using my thread as a helpful link! Very exciting.
So heres why my #2 colortune stayed yellow
After I replaced my intake boots with the plumbing fixtures some plastic shavings fell down into the airbox. When I started the bike it must have sucked a plastic shaving into my Pilot jet, thus clogging it and not allowing air to reach the cylinders. Making it rich and sluggish on acceleration. So i put seafoam in it and it pushed it out I guess. I'll be pulling the carbs for another clean up soon. But after I used seafoam she ran perfect.
Soooo, maybe a clogged jet??
-Chris

 

Thanks helmet and SLKid. Intakes are OK and my problem is a lean condition at idle.

The rack's off and the pilot fuel jets in bowl #1, #2 and #4 are noticeably narrower than the jet in bowl #3. I think that's the problem. I'll know as soon as I can clean them out properly.

Anybody know the drill size for the pilot fuel jet hole? Carb cleaner and guitar strings won't open them up any.

 

All i have used to ever clean carbs is compressed air, carb cleaner ( wd40, release all,) and if theres passages that are stuck or clogged let it sit in release all for 24 hours so it loosens up the crap and if it needs convincing i take a wire string off a scraper and use that to clean with.

there about .003 .004 thick and strong, you can get this off a barbecue scraper or any scraper with metal brissels... also blowing the holes and making sure theres an exit to the hole your blowing... : )

 

Are the numbers on the jets the same?

 

Thats what I was about to say. Each Jet has numbers on it. It tells you what size they are
EX:
XJ700 Main Jet: 107
Pilot: 35.6
Should be on the head of the jet
-Chris

 

My mistake. I was talking about the starter jet holes. I may have confused everybody by calling them pilot jets - even myself. Sorry.

Does the starter jet affect pilot mixture? The only difference I can find among the carbs is that the starter jet holes are smaller on #1, #2 and #4 - coincidentally the same carbs that I could not adjust properly. They are all clean now and the same size. I guess I'll find out if that makes any difference when I get all back together.

The pilot jet holes all look the same size and they all have a #40 on them. The pilot screws seemed kind of loose when I removed them. I should probably replace the o-rings. Everything else is the proper size according to the Haynes manual for an XJ750J.

Anyway, I broke the rack and each carb is now getting a nice ultrasound bath. If they aren't clean after this I'm going to lay them on the railroad tracks.

 

Sounds like a good idea! Be sure to clean out your enrichment circuits with carb cleaner!
http://xjbikes.com/Forums/viewtopic/t=8918/

Works wonders for your start up
-Chris