XSively Frustrating

[DGXSER]

Wokring on another memebrs bike, issue is a low end miss. Idles nice with an occasional pop or sputter but over all smooth idle. Synchs up nicely, but as it is idling every now and then all four will drop off to about nothing on the Carbtune.

Running about 2k turn the throttle and it starts missing, more throttle you give it the more it misses. Adding choke does not help. As it gets up to maybe 3k or so it seems to lessen the miss, but may just be running faster so it is less noticable.

So far, we tried colortuning, synching, no change. Checked the pick-up coils, when I tugged on the white wire of the lower coil the bike died. So we swapped pick-up coils with a known good set. Now the coil wires have no effect, but it still has the same miss.

So we pulled the carbs off. I found one diaphram had a pin hole which I put RTV over, I found another where about half the seam where the main diaphram meets the lip was able to see through it, RTVd it also. Also noted the diaphram in no 4 was soaked with gas, like it dripped off of it. And that no 4 mix screw was black at the tip.

Checked the pilot jet, main jet emulsion tubes all looked fine. Still the same mis under load in low RPM.

Any suggestions are welcomed.

[Yard Dogg]

Have you checked the timing advance? How do the intake boots look, are they sealed well? How does it sound when cold? Timing chain maybe? How many miles? Sorry about the 21 questions, just brainstorming.

[James England]

Personally, I would replace the diaphragms and not do the RTV thing. I seem to remember this has been discussed before and some people advise it and others don't. The damaged lip sounds beyond RTV anyway. I really do think the only way to eliminate the possible cause of a problem is to repair things properly... then one can 100% rule that part out......

Have you checked the float levels? Cleaned/renewed the needle valves and checked the hemispherical filters?

Have you actually done a full carb strip and clean? Or just the jets and emulsion tubes you mentioned?

Is it possible that this is an electrical problem, I wonder? I did have an irritating low rev miss, always when going round a left hand bend....... when I fitted green Dyna coils and removed the ballast resistor, it disappeared forever....

[TopCatGr58]

Hey DGXSer,

What year/model bike is it?? James just stole my thoughts with his fast post! I was going to ask if it had OEM coils and was a model with a ballast resistor? Folks have reported low rpm misses that were otherwise diagnosed and ruled out as to fuel problems, and once they installed Hi Output coils, the miss went away. At low rpm, the ALT isn't cranking much power, and with the early models running THRU the ballast, the coils are getting only ~9 volts anyways while running. You could try bypassing the ballast resistor for a few minutes and see if the miss goes away, since the coils will be getting the full 12 volts...IF they are getting 12 volts to the wires before the BR, and so it will be making a stronger spark. Then if miss is gone, you know what to pursue, either connection cleaning, spark plug wire/caps resistance testing, OR installing new Hi Output coils!

T.C.

[James England]

Hey DGXSer,

What year/model bike is it?? James just stole my thoughts with his fast post! I was going to ask if it had OEM coils and was a model with a ballast resistor? Folks have reported low rpm misses that were otherwise diagnosed and ruled out as to fuel problems, and once they installed Hi Output coils, the miss went away. At low rpm, the ALT isn't cranking much power, and with the early models running THRU the ballast, the coils are getting only ~9 volts anyways while running. You could try bypassing the ballast resistor for a few minutes and see if the miss goes away, since the coils will be getting the full 12 volts...IF they are getting 12 volts to the wires before the BR, and so it will be making a stronger spark. Then if miss is gone, you know what to pursue, either connection cleaning, spark plug wire/caps resistance testing, OR installing new Hi Output coils!

T.C.

Ole lightning fingers, that's me..... Another thing.... plugs and HT leads/suppressor caps..... are they in good condition? When I put my Dyna coils on, I also put iridium plugs in and bypassed the resistors in the HT caps (since iridium plugs have a built-in resistor). The little irritating 'cough' that my bike had was there from the day I bought it. It was annoying but not enough to strip the carbs for...;. more like a hiccup. Once I did the coils, leads and plugs, it stopped completely. That was before I did the full carb strip and clean. So, I'd been thinking carb strip/clean to cure the cough and couldn't have been more wrong.

[DGXSER]

Sorry, it is an SG, 85,000 miles with the original owner.

about two years ago DBeardslee put on a new cam chain, Accel coils with the BR removed. New leads and NGK plug caps.

I did spray the entire intake area and all of the carbs with Starter fluid as well as carb cleaner and got no response from the engine at all. I did notice the first time we synched the carbs a few weeks ago that one vac plug was cracking so we bought new.

It has Irridium plugs all the way across. I did notice last week as we were working on it that the no 3 plug was covered with baked on ash, and we replaced the plug, did not see that this time.

I have to shamefully admit I did not check the float levels when I had the carbs out and stripped down. I basically did a complete removal of all but the floats and needles as all four do not appear to be low on fuel, I did pull all the jets, float bowls, and slides with needles and emulsion tubes, also the air jets. it idles reasonable fine with an occasional cough or sputter.

I do now wonder about float levels, although at the same time as the cam chain and coils work DBeardslee also worked on the carbs and I am 1000% certain he would have set them correctly (the man is more maticulous and detailed than I ever hope to be. and I often consider myself to be OCD about such things.)

But something is definitely giving this bike a bad cough at low RPM under power. Even if you let off and just let it kind of idle along at 2k or so at speed, it sounds fine. As soon as you open the throttle it starts sputtering and the more throttle the worse it gets. But, upon pulling the biek itno the garage from running it a mile ro two in the miss range, not letting it idle, all four pipes appear to be the same temp with water spray on them.

[bikerphil]

The baked on ash on a spark plug is just a sign of worn valve seals. Maybe got a tank of bad gas, JAT. You sure they are Accel coils in there and not the Mikes XS junk? Did you try running the bike on prime?

[DGXSER]

Phil, thanks on the plug read issue. It does smoke a bit on hard decel so valve seals may be needed.

I am not 100% certain on the coils, they may be the Mikes XS green coils which are supposed to hold up much better than the black ones. I would need to talk to the owner or DBeardslee who did the swap, I know he is a fan of the Mikes XS green coils. He had the black ones form Mike XS and they failed on him in a year or two and went to the green ones on his bike.

It does feel like an electrical failure though, I was even thinking TCI today. But those seem like one of those works or don't kind of boxes, not so intermittent.

[bikerphil]

Yep, if it's those green coils, I would suspect that to be the problem. Most owners don't shell out the $$ for Accels, I have them though but they came with the bike. If he really has the Accels I would look elsewhere for the problem, they never seem to go bad, at least to my knowledge.

[petejw]

hey don, do u know if the valve clearances
are set correctly,
also float height which has already been suggested,
did u check the off idle ports arent blocked
when the carbs were off. jat

[Gallaecus]

hey Don
Im sure you already thought of this, but if you had another rack of carbs in good order, at least you could narrow it down to the carbs or not.

[DGXSER]

Pete, I know the valves were set correctly when the cam chain was done, couple years, maybe 5-6k miles ago IIRC. Compression seems fine, and I would suspect that to be something that would effect performance across the RPM range. But definitely worth the effort to try.

Steven, I did think of that, even thought of pulling mine off my bike to try on it. I also have a rack of the 78-79 carbs sittign in the garage, however, the brass tube from the carb body for the enrichment circuit in one was bent. (PO decided to put an 80 float bowl on, bent the tube to make it line up with the hole on the 80 float bowl....kind of.) That and I have yet to get them on a bike and running properly, which is why I have not offered them for sale. So I do not count them as a known good set of carbs..yet.

All good thoughts, and greatly appreciated. I will probably not have a chance to dig into any of this for a week or so, but will let you all know what I find. By all means I welcome other ideas.

[DGXSER]

Some added info straight from the owner....Mikes green coils,5600 miles ago.
This has been acting up for a year or so but just started to get very annoying lately and harder to ignore.
I cranked it coming up the hill at your place Don to about 7k and looked like I did blow some blue smoke looking back in the mirror,possibly right side.
floats set at same time as coils and cam chain,all work done @79400, bike now has 85,000 miles.

[BA80]

Dragon Tamer had a similar problem with an XJ650 last spring. He kept losing vacuum also. It turned out to be a cracked carb. JAT

[petejw]

will it miss in neutral with the bike on the center stand?
if so was thinking u could remove a plug lead at a time
to find what cylinder is missing and u could go from there.