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Runs Weak on No.1 Cylinder '80 GS1100

  • Thread starter Thread starter MiketheBike
  • Start date Start date
M

MiketheBike

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After rebuilding the carbs (twice) and checking the valve clearances and changing the cam cover gaskets (twice) I have this problem where No.1 cylinder runs weak at idle and low throttle openings. On start up it feels like its running on three cylinders. It smoothes out as it warms up but If I pull the plug lead on number 1 it hardly makes any difference while if I pull the plug lead on no.4 it stops the engine. I changed the plug in No.1 but no difference. After warming up for a while the engine will idle. Using an IR thermometer all the exhaust pipes will read around 100C except number 1 which is only 50C. To me it felt like unburnt fuel so maybe a tight valve clearance. Pulled the cam cover to check clearances and all are O.K. Put it back together and checked compression which were like 135, 125 125 118 (from 1 to 4). So doesn't seem like a valve issue - looked at ignition. Swapped plug leads between 1 and 4 and No.1 still runs weak. Swapped the coils over and No.1 still runs weak. So then I'm figuring its in the carb. I rebuilt these carbs meticulously checking the float levels were all in spec and fitted with new rubber manifolds. Figured that maybe needle and seat (fuel valve) in carb has failed, so tried running with the fuel tap off to see if things improved as the fuel level in the carb drops down. No improvement. When the bike is at full throttle it goes so strong that I can't believe that No.1 is running weak so it looks like something that is only a problem at idle and low throttle. Before I pull the carbs again looking for something that is odd about carb no.1 just thought I would ask to see if I am missing something. The plug looks blacker on No.1 than the others. I have already checked that choke is not sticking and there don't seem to be any air leaks. Have I forgotten to check anything?
 
Where did you set the mixture screws after your carb re-build? Many of us use the "highect RPM method" here, it makes a huge difference as to how the bike idles & responds when you just crack the throttle... if it's all stock, somewhere around 2 1/2 - 3 turns out is about right.

Once the idle is set correctly, then do a good carb sync?

That should fix it up hopefully...
 
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On CV carbs, the mixture screw controls a premix of fuel and air. Further out on the mixture screw, the more fuel is delivered to that particular cylinder. I would set the screw on number 1 half of what it is now as a start point.

First though, remove it and be sure the tip isnt bent or broke off. While its out, blow some air down the hole to back flush the passage a bit. Reassemble and see what it does. If it still is acting up, remove the carbs and be sure the floats arent ever so slightly dragging on the bowl gasket. They squish out a bit when tightened and this may effect the fuel level in that bowl.
 
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Not exactly so Tom....even if they arent perfectly synced, the cylinder should get hot like the others and not be sooting the plug. A good bench sync will be very close..close enough to not have the symptoms he is having.

Gauge syncing will get them perfect and make it much smoother across the board..stop that knocking from the cylinders fighting each other.

Timing is another issue that could be contributing to poor fuel ignition.
 
tkent02 said:
If you haven't synched the carburetors, everything else is meaningless.
Click to expand...

Yes you're right. I guess you have to go back & forth once anyway so it becomes a little like the chicken & the egg thing. It's gotta run sorta smooth to do the sync, and its gotta be sync'ed to run sorta smooth.
 
chuck hahn said:
Not exactly so Tom....even if they arent perfectly synced, the cylinder should get hot like the others and not be sooting the plug. A good bench sync will be very close..close enough to not have the symptoms he is having.
Click to expand...

He didn't mention doing any bench synch, I was thinking the #1 so far off it wasn't doing anything at idle, #4 doing it all. He did say it quit when #4 spark plug wire was pulled off.
 
He also never mentioned a few things that I would do to be sure theres spark.


1....Trim about 1/4 inch off the wire and rescrew the cap back on to ensure a fresh connection.

2...Remove the brass thing that grips the plug itself from insdie the boots and replace the resistors ( if so equipped ) with a brass or copper rod the same length.

3..Reverify plug gapping.

4...Hold the plug along the engine and visually verify its sparking.
 
Guys. Thanks for your input. I really appreciate it. In answer to some of the questions.
- Mixture screws. the advise on this varies a bit some say 1 1/2 some 2 1/2. In my case the best idle was achieved at 1 1/4 so I set all of them a bit fatter at 1 1/2. Throttle response is good. No dead spots.
- Figuring it felt rich, I did screw down the mixture screw slowly to almost nothing but it didn't seem to make any difference at any time. I replaced all the mixture screws with brand new ones, springs, washers and seals but I hadn't thought to recheck the tip didn't break somehow. I'll check that out.
-When I ganged up the carbs, I set the non adjustable one (No.3 I think) so that the carb butterfly was exactly 25 thou. clearance. I then set all the rest to exactly 25 thou.
- I verified the plug was sparking by eye and it looked slightly weaker than the rest. Tried a brand new plug out of the box. Changed leads between 4 and one and swapped coils but it was always no. 1 cylinder that would continue to run weak (exhaust not hot). This ignition system does not use a resistor in the cap (Dyna S ignition with Dyna coils). The wires are carbon fibre and the caps are not resistor types. Everything new.
-
 
P.S I will also check out if the floats are hanging up on the gasket. I have a bowl drain plug that is set up to do that and I should be able to get it plugged into the number 1 carb without pulling anything out.
 
I'd sync the carbs and go from there. You can't diagnose running problems on a bike until the maintenance is all current.
 
While you're in there, remove the plug caps (boots, ends) from cylinders one and four and measure their resistance. They should be somewhere around 5K+ ohms. If you find them above 10K, they're rubbish. Get new ones.

Also, measure the voltage at the coils. I don't think that is your issue, but it may exacerbate the condition if the plug cap is bad.
 
Hmmm. The air screw was fine. No Broken tip. The float wasn't hung up on the carb gasket. Gotta be a problem in that carb. I'll try to rip them out this weekend and let you know if I find anything.
 
Compression is good. Maybe too good. 135, 125, 125, 118 (1-4) Never got back to my place to work on it this weekend.
 
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