Phantom Fault

[Bombasstic]

I have been battling a fault for a good while now and am finally at my wits end.
I bought my 1981 GL1100 in November 2023. It came with a fault on #4 cylinder.
It kept fouling the plug with dry black soot (indicating too rich mix). Compression test is good. Valves are good and tappets set.
I stripped and cleaned the carbs a good few times. Replaced Needle and Seat. Pilot screw. Air cut off on all 4 carbs + Accelerator pump on #3.
I thought I had it waxed.
I tuned the air/fuel mixes - Vaccuum synced the carbs at idle and also at 2500 rpm - fitted new plugs. She purrs at idle and for the first 3 or 4 Km's she sounds and pulls beautifully . . . then she starts with a single shudder, then a few small misses, then when I get to a slight incline and give some gas she really starts missing quite badly. I have tried pulling plug wires along the road to identify the culprit cylinder/s but to no avail.
On inspection at home, I find #1, #3, and #4 plugs badly soiled with black soot. #2 looks OKish. The right exhaust is a little more soot fouled than the left.
Now the thing is . . . I let the engine cool down, clean and refit the plugs . . . and I'm back to square 1. She purrs at idle but when I take her for a ride the same thing happens as before. FRUSTRATING to say the least.
I have tested the coils and they test perfectly (WHEN COLD). I have read that coil faults can show themselves when they get hot - but how does one test this? . . . and can carbs / fuel heat up to the extent of changing the mix to rich?
I have also been advised to change to a hotter plug (D7ea) to burn off the soot but I would imagine this to be trying to clear the symtem without solving the cause.
PLEASE HELP

[the_big_h]

Badly sooted up spark plugs is due to running too rich (not missing spark).
A couple things come to mind:
- is it an original equipment fuel pump? Did somebody fit a different fuel pump to it, that puts out too much pressure, overpowering the floats and stuffing fuel into the engine?
- Are the carburetor vent pipes clear and open, and not blocked? If the vents are not open, the bowls could get pressurized, again pushing too much fuel into the engine.
- Are the fuel metering needles original? Some rebuild kits may come with replacement needles, that may or may not be correct.

[Bombasstic]

the_big_h
Thanks for your reply.
Being 42 years old anything is possible. The fuel pump output can be tested and I can check on the metering needles but not sure about the vent pipes you mention. Please clarify.
Your input is appreciated.

[the_big_h]

I believe part number 9 is the vent tube, part number 10 is the fuel tube.
This diagram just show it connecting one carb to the next, but at some point the vent must be open to atmosphere.
I've not had the carbs on my 1200 out, but on my nighthawk the vents were all plumbed together, and the end left open. If somebody plugged it, or connected it to someplace it shouldn't go, it would be a problem.

[Bombasstic]

the_big_h, THANK YOU. I was just about to pop my coils into the oven so I could test them while hot when I decided to re-read your post.
Part 10 is in fact the vent tube but you hit the nail right on the head. The socket for the vent tube on #1 is blocked solid. #2, #3 and #4 are also clogged to various degrees. MY OWN STUPIDITY.
After all the cleaning etc. that I did, on assembly I found the tubes to be slightly loose in the sockets and as I live out in the sticks and nowhere near an "O" ring supplier, I thought it wise to wipe a little silicone gasket maker on the old "O" rings . . . . BAD MOVE.
I will go through the cleaning process once again and then put it all together once I have found the correct "O" rings (with no gasket maket).
THANK YOU once again. I will post the outcome asap but I've no doubt it will be possitive.

[Bombasstic]

As per the pic I've got carbs #1 and #3 flowing "round the corner" . . . but #2 and #4 not.
Please advise
Thanks

[OldguyGlen]

Been a while since I was deep into the carbs.. But I don't think there are "vent tubes". In your pic the larger tube is the main fuel crossover. The smaller tube is the accelerator pump fuel link. The accelerator pump on #3 feeds all 4 carbs. The accelerator pump fuel flows thru these smaller tubes and exits in each carb at the "round the corner" sintered brass posts. The tiny exit nozzles are hidden on the back side of these posts, and are easily plugged. Clean them with a "j" hook bent fine wire (and a mirror to help visualize what you're trying to access). All 4 should squirt with each pump of the throttle. There are some other threads that discuss this, some with pictures.

[the_big_h]

And make sure that the port you labelled "to top of float bowl" is clear.
Sorry if I steered you wrong on external vent pipes.

[Bombasstic]

Thanks guys - I have got a better understanding of the working of the accelerator pump and will get #2 and #4 pumping. I just hope this will prove to be the cause of my "sooted" plugs. I somehow done think it will solve the issue but I live in hope.
Will keep you up to date.
Thanks

[jeffie]

I used to have the same symptoms on my 84 1200 and it was the resistors in the spark plug caps. I would take them out and wipe the ends with a clean dry cloth and the bike would run great.

[Bombasstic]

Thanks all . . . problem solved.
Needle jets were badly worn causing very rich mix. Fitted new ones. Perfect.