Runs on 3 cylinders until higher RPM

Anyone got any clue as to what I could check? It'll run on 3 cylinders and then when under load accelerating, it feels as though the fourth cylinder will kick on. After short rides around the block, #4 is SLIGHTLY warm, I'm assuming from when it kicks in during hard acceleration. Otherwise, the 3 other cylinders seem to all be firing. They aren't tuned at all, but I figured I need to get this #4 cylinder figured out first.

I have had the carbs off 3 or 4 times, and #4 looks no different from any of the others. Pilot Jet is clear, Main Jet is clear... they're clean. I can't imagine a carb would be dirty enough to completely plug fuel flow from reaching the cylinder and I wouldn't be able to tell after taking it off 3 times to double and triple check it. I also bought a brand new battery, spec'd with my owner's manual. Brand new set of spark plugs fixed a no-start problem I was having before hand.

I ground the plug to the engine head and it sparks every time. I swapped 4 and 1 plug wires and 4 still had the problem. All signs point to a fuel issue, but as I said, the carbs all look the same. Emulsion tubes are clear, I took special care to ensure that. Clunk tested, bench synced and float level spec'd (with gas).

Help, gents! I'd greatly appreciate it!

 

Pull that #4 plug after its been running rough. If its black, you need to tune it leaner on that carb.

 

It's not THAT rich. Even if it were rich, it would still fire. I mean, I have it 2 and a half turns out on the pilot screw. Fine tuning it later, I understand. But having it so rich that it won't fire?

Definitely a good thought but I've pulled the plug numerous times after running it to try to figure this out, and it's definitely dark in color, but not you're-so-rich-it-won't-fire black.

 

Check to see if that plug is firing. Put s new plug on the wire and lay it on the cylinder head, turn it over and see if you have spark. DONT TOUCH IT , it will hurt. Possible problem with ignition coil? Also do a compression test to see if all cylinders have good compression.

 

swap the plugs around, maybe it's just a funky plug

 

I'm sorry, but did you even ready my post? I think I said, in the exact same words you used, that I've already done exactly what you just posted. Please read more carefully.

Ignition coil problems would result in mirror problems in cylinders 1 and 4. There are only 2 coils.

Compression test? Why?

Already have swapped plugs around. Good thought given my description! I guess this must be a carb dissection again. Anyone live near Columbia, know their stuff, and want to pin point my less than obvious mistake?

 

I'm sorry, but did you even ready my post? I think I said, in the exact same words you used, that I've already done exactly what you just posted. Please read more carefully.

Ignition coil problems would result in mirror problems in cylinders 1 and 4. There are only 2 coils.

Compression test? Why?

Already have swapped plugs around. Good thought given my description, Polock! I guess this must be a carb dissection again. Anyone live near Columbia, know their stuff, and want to pin point my less than obvious mistake?

 

Compression check would be a good start. If anything looks out of wack, I'd check and reset valve clearances as needed.

Sounds like you did a fuel level check and sync on the bench. Have you re-checked float levels and synced the carbs on the bike?

I take it, it still has a YICS head/motor?

 

Hey Pipe Dreams...keep asking...I am watching the posts. I appear to have the same problem. Carbs clean, floats set, compression good. I did a running vacuum sync and all vacuums equalled out.

Then I moved to color tune...#3 cylinder is so lean there is no fire. I open the pilot and it will fire rich occasionally and then stop firing. All the while I think I can see the spark arcing in the color tune every time.

I will get after this problem again tomorrow night but if you have gained any insights, let me know.

 

Have you considered that maybe you are just feeling the powerband kick in on your bike?

Maybe you just need to finish tuning your carbs. On my bike, I need to turn the #1 pilot screw out, a bit more than the others, to get it to fire consistently. If I don't turn it out a little more, that pipe will just be warm while the other three are hot.

 

Yeah, I'll let you know. I'm beginning to suspect a faulty spark plug wire. My plan of attack is to try to to tune the cylinder radically different, pull the carbs and take ONE MORE look to make sure that everything looks okay in there, and then if it is still giving me trouble swapping the plug wire and doing that whole modification.

If I remember correctly, it's kind of arduous. Oh well...

 

personally i'd do 15 plug wires rather than pull the carbs, maybe 20

 

I know there is a link on this site somewhere about doing the job. Know where I can find it? I'll keep looking.

 

This might be what you are looking for
http://xjbikes.com/Forums/viewtopic/t=11221.html

 

Before you replace wires, you can hold the plug against the engine and see the spark. I swapped another coil (with different wires) and the condition is the same. So problem is not spark.

I cleaned pilot mixture screw and the port into the Main & Pilot air jets. Still no change. The color tune still shows occasional fire at idle. You can still hear the engine smooth out on those occasions when it fires.

As I let it idle, the vac gauge for the #3 cyl will occasionally lose some vacuum. In fact, I was feeding fuel into the vacuum port with a syringe and boom...a little fuel flashed up to the syringe.

This is sounding like the intake valve is open too long. Maybe getting some exploding gas back in the intake. Compression on #3 is 145ish but the others are 150. Bet I have a valve lash issue. Anybody seen this?

 

That's along the lines of what I had on my GPz. In my case it was because of bent valves, with #3 being REALLY bent, and not sealing at all. Probably damaged due to an over-rev at some point during a prior administration.

A compression check on mine showed like 10psi on #3. I then pulled the cams and put some compressed air into the spark plug holes. From #3 I could hear and smell air being blown back through the carburetor.

Sorry if I missed it, but have you run a compression check?

Hopefully, in your case, it's just a shimming issue, or maybe some carbon buildup on the seat, and not anything more serious.

Cheers,
Paul

 

Hey Paul - thanks for the reply. Please go to XJ Chat "Rejetting xj 750 Carbs". I listed the symptons there. Interesting comment about bent valve and carbon build-up. When you look at the other post, could you comment on how I might clean a carbon build-up on the valve without pulling the head.

 

Greetings Pipe Dreams...
I went to work this weekend and can't say for sure which item fixed the problem. Thought I might share with you and encourage you to keep after it. Here we go:
1. Shimmed the valves. All 4 intakes were 0.10 mm or less.
2. Used sil-glyde lubricant on the intake manifold flanges. Also coated the inside of the vacuum caps.
3. "Painted" the intake boots with spray on rubber coat. (the stuff used to coat tool handles). I used 8 coats of the spray rubber for some tiny cracks showing in the exterior rubber.
4. Checked timing. (dead on)
5. Adjusted pilot mixtures till idle speed increased and then turned 1/4 out.

All that seems to have cured the low speed miss. Oh, yes, paint the valve cover and buff the ribs for a good visual when you roar off into the sunset.
Cheers

 

Ready to polish another valve cover???

 

Hey...polishing aluminum is quite a workout. I must say...still feel inadequate every time I look at Fitz's valve cover job. But in the true American spirit...I aspire to greater achievements and could always practice a little more on your valve cover Mr. Robert!!!