Only way to get #3 Cylinder to run is putting my finger over vacuum port....

I have a 1982 Xj650. It was setup for a KN Air filter and rejetted. When I got this old project bike #4 cylinder wasn't running. So I cleaned the carbs throughly, but hot dipping the stripped assemblies and verifying they are clean with air gun. Polished the slide pistons and checked the diaphragms (checked operation of slides with low pressure shop air)

After reinstalling, cylinders 1, 2 and 4 run perfect. But Cylinder 3 will only run when you put your finger over the vacuum/low pressure port (As in picture).



It is not hooked up the air box, as the velocity stacks going to air box are very hard cracked and ruined.
Rechecked all the jets quadruple times, the cover is off the top air jets on all 4 carburetors (Missing from rejet job I bet.)
Pulled the carb boots, and they are crack free and resealed to the head with new gaskets.

Could the airbox being detached cause something like this?

 

bike should start and run with out air box
check to make sure your getting spark swap #2 and #3 sparkplug wires see if problem follows plug wire
then swap spark plugs same as above.

did you wet set fuel levels?

try opening up mixture screw a little more
covers not needed Yamaha did away with the use of covers.

did you bench sync carbs?


check air jets for correct position. small main jet goes closest to plate screw hole
check main and pilot fuel jets for correct position

 

One 'other' thing... check the tiny little o-ring and washer under the mixture screw spring are in good shape or not missing.

 

Going to guess that if you take the top hat off that carb the diaphragm is not sealed correctly. Just thinking it is not going up with normal pressure so I could be out of alignment with the top hat seal ring? Just a though.

 

also look for holes in diaphram ans seal with liquid electrical tape.

when you cover the air hole it forces the motor to draw more fuel through the jets . re vist the jets for proper location

 

I agree, check air jets to make sure they are in the correct place. That slot provides air to the underside of the diaphragm when it rises in addition to the air jets. By covering that port you are making the mixture richer.

 

Guys! Thanks for responding!

yes I wet set all the float levels to the method described in the Yamaha manual.

The air jets are all still in same spot as I had drawn on my diagrams. The air jets are also in the same position as the other 3 carburetors. The diaphragm are all in place, and no holes that I can see with a very bright LED light behind them. Also when I crank the throttle all 4 Vacuum Pistons are raising fine.

Could the carbuetors be that far out of sync to keep a cylinder from running? I did not alter the sync because the bike ran well on 1 ,2 and 3. Before hand.

Very frustrating right now. But it cant be very much wrong not letting that cylinder run.

 

Sync can definitely be out enough to do that.

Vacuum sync and a colourtune turns everyone into a pro!

How was the o-ring and washer under the mix screw?

 

All 4 are complete, no tips damaged, springs still there etc etc. Turned out 2 turns each.

 

I will bench sync tomorrow then, should i match all other cylinders to #3.

#3 Cylinder is currently between 15-20 INHG. (Not running)
Rest of the cylinders are around 5-10 INHG. (All are running)

So should i just adjust #3 to 5-10 inhg?

 

Definitely make them all the same.

 

Right so I need to service and plug the YICS system, is there any way to roughly set these without yics system plugged to enough to see if this resolves my problem?

 

you could start with a bench sync.
the general opinion here is you do not need to bother with the yics port when doing a running sync of the carbs.
Just do a running sync.

 

with the bench sync you match them all to #3 as it is the baseline carb. then do a running sync.

 

Alright. Just in from the garage. Bench synced the carbs. double checked all fuel levels and adjusted #3 fuel level a bit higher than the 4mm max spec (and rechecked all passages). Installed the works on the bike no difference.

Getting really desperate now, screwed in all carb pilot screws... screwed them out 3 turns each. Starting screwing out #3 all the way to approx 5 turns... maybe its more.

Current Header Temps

Bike idling at approximately 1100 RPM. #1 Cyl - 220DegF, #2 Cyl - 230degF, #3 184 Deg F, #4 - 125 Deg F.
I know the #4 cylinder is running a bit low, but I know from experience now, that the hottest a dead cylinder will run is 90DegF.
But #3 has temperature now, no fingers or tape over any air ports on carburetor.

Bike running at 3500 RPM. #1 Cyl - 230 DegF, #2 Cyl - 228degF, #3 203 Deg F, #4 - 160 Deg F.

I took a video of me measuring header temps if someone wants to verify my work for me. I also took at video of spark testing #3 with a 1/4" gap.

 

Should I benchsync and color tune now? What colortune part# do I need?

 

Did you ended up fixing the problem? I have the same issue...

 

from your thread it does sound like a vacuum leak at the throttle shaft seals