Problems with idle and dead cyl method

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heroclixor

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I have a 83 xs650 with TCI and BS34 carbs and 26k miles. Bike sat for about 2 years until I bought it and rebuilt the carbs and put on pods, more open exhaust, and mikes green coil and wires. Now I'm trying to get it running and having some issues.

I went up to 137.5 on main and 45 pilot and set the mix screws to 2.75 turns out. Checked for air leaks with carb cleaner.
Set chain tension and valves.

Now here's where the weirdness comes in. The bike has trouble idling below 1500rpm even after manometer syncing the carbs. When I tried to use the dead cylinder method I had no trouble getting the right side to idle down to 400rpm or so, but the left doesn't want to go much below 1000 without stalling. The left plug is more black/oily while the right plug is almost clean.

Does this sound like an engine issue or a carb tuning issue?
 
Well, I guess it could be either. Looking at it from a carb issue point of view, you may not have thoroughly cleared and cleaned the pilot circuit on that left carb. Or maybe the float level is off on it, set too high. That could cause rich running and flood out or choke out that cylinder.
 
Well I'm pretty sure I cleaned the hell out of that carb and I did just check the float level yesterday. I'll try again just to be sure.

What on the engine side could be the problem? The oily left plug combined with the fact that it doesn't want to idle down on that side is a little suspicious to me.
 
If the left side comes on when you give it some throttle - then like 5 Twins says, pilot circuit.
My '77 has a TCI upgrade with the bs 34's that are stock save the needle is shimmed, free flowing exhaust with the Dunstall repros, idle mix 3 turns out and Unis. Had those carbs off a couple of times due to no idle on one cylinder......she'd kick in great with some throttle but no idle. Pilot circuit.
 
nj1639 said:
If the left side comes on when you give it some throttle - then like 5 Twins says, pilot circuit.
My '77 has a TCI upgrade with the bs 34's that are stock save the needle is shimmed, free flowing exhaust with the Dunstall repros, idle mix 3 turns out and Unis. Had those carbs off a couple of times due to no idle on one cylinder......she'd kick in great with some throttle but no idle. Pilot circuit.
Click to expand...


Sorry I wasn't really clear on that last post. It does idle on the left, but it doesn't want to go below 1000rpm on just the left cylinder. The right cylinder can go as low as 300-400 without much bogging. The overall idle also doesn't want to come down below 1500rpm without starting to stall.

I pulled the carbs again and rechecked the floats and pilot. Sprayed all the little holes out with carb cleaner again. Only odd thing I found was the size 45 mikesxs pilot jets seemed to have little bits of metal visible in the side holes. Like leftovers from the holes being drilled at the factory. I cleaned them out with a piece of wire so we'll see if it makes any difference tomorrow.
 
I see we have another quality reproduction MikesXS part there, lol. I only use genuine Mikuni jets for reasons like you've just discovered. Put some of those in there and the problem could very well disappear. Or, you might try swapping the pilots from carb to carb and see if the symptom (won't go below 1000 RPMs) follows the jet.
 
The diameter and spacing of the cross drilled holes is critical on pilot jets and emulsion tubes to proper fuel atomization.
Sounds like the no-name supplier of the other jets does not even tumble them, much less inspect them.
 
Yeah the holes weren't obstructed completely or anything, but I could see the shavings when I held the holes up to a light. Now I'm really curious to see if a few little shavings could make a bike run like crap.
 
Not sure what to think. Reinstalled cleaned up mikesxs 45 pilot and it still doesn't want to go below 1500 without dying on me. Installed original 42.5 pilot and the left side doesn't want to run reliably no matter what mix screw settings I try. I'm going to try ordering original mikuni 45 pilot from niche cycle and if that doesn't work I'm out of ideas on the carbs. Maybe something engine related on that left side... I'd really hate having to tear the engine apart.

EDIT: Rediscovered an old account. Haha
 
Last edited:
So I swapped out the mikesxs jets for real mikuni 45s and cleaned all the carb passages again. Unfortunately the problem remains. It still doesn't want to idle below 1500rpm and seems like even if I let it idle at 1500 for a while it starts to want to stall out.

Any ideas on what else I can check? I've replaced pretty much everything I can think of besides the motor or the carburetors at this point.
 
its either burning oil, or has a weak spark on that side. Just because the coil is new, doesn't mean its good. Swap the plugs and coil wires from side to side, and see if follows or remains on the left side.
 
Do a compression test and a leak down test. I was having a very similar problem, couldnt idle. Seemed to run ok once I got her rolling, but Idling was a disaster. Went through the carbs like 6 times.

Did a compression test...... NOT TOO GOOD.....

Low compression due to exhaust valve leak on both sides, carbon related....and a small amount of oil seeping into the right cyl from the head gasket.

No el work-o right-o . My Spanish is getting better :thumbsup:
 
Have the day off so decided to start messing with the bike early.

First, I checked the coil with a multimeter and double checked condition of plug wires. Both check out good.

Second, I found my compression gauge in the basement and did a compression test. I did it cold with both plugs out and throttle open. Left side came up 110 dry and 145-150 with a squirt of oil. Right side came up 125-130 dry and 150 with a squirt of oil.

According to my reading on this side 110-120 is about borderline so I guess that explains why that cylinder is still sort of running but not very well. I see a top end rebuild in my future haha should be fun.
 
You'll get better results testing a warm engine. If you've just started running the bike after it's "sit" time, the compression may come back up after you put a few hundred miles on it.
 
heroclixor said:
Have the day off so decided to start messing with the bike early.

First, I checked the coil with a multimeter and double checked condition of plug wires. Both check out good.

Second, I found my compression gauge in the basement and did a compression test. I did it cold with both plugs out and throttle open. Left side came up 110 dry and 145-150 with a squirt of oil. Right side came up 125-130 dry and 150 with a squirt of oil.

According to my reading on this side 110-120 is about borderline so I guess that explains why that cylinder is still sort of running but not very well. I see a top end rebuild in my future haha should be fun.
Click to expand...

first, resistance is a poor check for ignition coils. The coil can "track" internally, causing a reduction in output voltage on one tower. If you don't have a scope capable of measuring secondary voltage, swapping the coil wires is the most simple check.

second, compression can vary from one cylinder to the other by 10% without issue. So110 vs 125 is close to normal. I wouldn't suspect compression.

Is the black deposit on the plug dry and powdery or sticky? Powdery is charred fuel, due to weak spark. Sticky is oil entering the combustion chamber.
 
The stuff on the plug is definitely oily and sticky. I tried swapping the coil wires just now and I didn't notice any difference. I also found my little spark checking gizmo that plugs into the plug cap and I can definitely see a good spark there on both sides.

I just can't imagine what else could cause it to run like that.
 
well, oily and sticky is burning oil. Most likely from the intake valve. Very possible the valve guide is worn, allowing excessive consumption.
 

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