XJ700 Lean Misfire begins when ridden at 55MPH when warm

Hello all. Hoping I have done enough homework prior to posting, and I did review forum topics seeing if there's anything there. Some are close, but...let me begin.

1985 Maxim rescued from a garage. Has stock exhaust and airbox. Bike looked like a commuter, totally unmolested, ridden and parked for whatever reason. Bike sat with gas in tank and carbs for 10 years.

Tank cleaned out OK, fuel tap soaked and rebuilt with new rubbers including the vacuum operated shut off. Carbs completely disassembled, dunked, washed, and all orifices picked and blown out with 140PSI compressed air. Carb boots at head OK, and on airbox side. Put it all back together, changed oil and new battery: voila! Bike ran quite good. Starts right up, smooth clean idle, revved out fine, no problems. Since then have put around 3000 miles on it, and during this time it began to develop a hesitation, surging, ultimately a lean misfire, only when ridden for a while at freeway speeds. That has now degenerated to a very lean misfire, I believe 2 cylinders down, only after riding for 5 minutes at 55MPH or so. Occasionally popping from the exhaust during the worst of it. If I putt around my town with speeds of 35MPH and lower, it runs PERFECT. For the first five minutes up to the lean misfire, it runs PERFECT. Starts right up with a smooth idle when cold. Absolutely no sign of hesitation or misfire. During the lean misfire, if you whack the throttle wide open and wait a beat, it will pick up the dead holes and run exactly like it should, and the lean misfire will slowly return when the throttle is leveled off again.

I have taken the carbs off and cleaned them, at least 5 times, maybe more. Always using brake/carb cleaners and high air pressure. Float level checked as shown in another post (have not tried the clear hose method yet). Spark plugs are that beautiful Hawaiian tan color, no signs of black/soot at all. Changing the fuel tap position between on, reserve and prime makes no difference. Clear fuel filter has fuel visible when the issue occurs. When issue at it's worst popping the gas cap open made no difference. The vent hoses from the carbs are not kinked and are routed good.

I understand the importance of syncing the carbs. That said, if it was popping at idle, or running with the lean misfire All The Time, I would most certainly head that direction. Except that it runs absolutely perfect, smooth idle, smooth inline 4 cylinder acceleration, up until the lean condition rears its head.

I plan on doing the clear fuel line to check the float level test when it's fully warmed up. I plan on changing the fuel hose from tap to carbs. I suppose I'll also pull those fricking carbs off again. What I'm looking for is if anyone has any other ideas that I haven't thought of? Thanks very much...

 

I cringe at the thought of you needing to pull the carbs off again. 5+ times is more than enough. You have checked many things, but not wet float level.
How did you determine it is lean misfire?
If it’s lean at speed I suspect low float levels.
I have done clear tube float check with carbs on bike.
Must jack bike to get carbs level.
The bike does not need to be warmed up. Just have the fuel supply on prime.

 

Lean due to clean plugs, not a trace of soot, and the backfire out the exhaust at almost closed throttle on decel is always a sure sign. Occasionally the riding misfire hangs around even after stopped, and I've shot the headpipes with an infrared temp gun top determine the cool cylinder, then with a tube on a can of carb spray selectively did a short blast in the dead hole carb airbox side and the engine smoothed out.

It has it's centerstand still so that will help level it. The screws on the bowls are all thankfully pointing the right way, it's a matter of putting the hoses on and checking. I want to do it when the thing is in it's misfire mode, cause it runs perfect when it's cold.

Any history of these FJ's boiling the gas? Vapor lock? The engine isn't blazing hot by any stretch of the imagination, but it's running like it's vapor locking.

 

Good diagnosing.
You need to get the carbs level and thus must find way to get front of bike far off ground. Must also be level side to side. I put boards under center stand (very hard to lift onto center stand on boards), then floor jack under front exhaust with board sandwiched to protect pipes. Without boards under center stand, the rear wheel touches ground before carbs are level. WARNING: The center stand locks back when used normally, but when the front is lifted too far, the center stand gets positioned in front of it’s centerline and can fold up. Watch rear tire to ground clearance, Do not lift the centerstand off ground. BE CAREFULL if you do it this way. (Made me nervous)

Fuel level spec on my Seca 750 is 3mm +- 1mm below carburetor mixing chamber body edge.

I have not seen issues with vapor lock. Besides you have diagnosed lean to one cylinder.
It maybe runs good cold because you have fuel enrichment engaged?
What are the pilot mixing screws set to and are they set close to same setting on all carbs? How did you determine settings?

 

If you get the float levels in spec' on the work stand, you should be good.

 

For myself, I did not want to pull carbs off bike just to find they were good level.
I checked on bike and found only carb # 4 to need adjustment. It appears to me to be the only carb that can have the bowl removed on bike, so I got lucky.
The wizard is correct. If the level is off you will be pulling the carbs off bike anyway.

 

Thanks for the leveling tip, I'll mind the centerstand pivot point! The pilot adjusters are not set to the same turn, but when I blew the carbs apart I counted in first before removing them, thinking factory setting was a good place to start when it was back together. And it is pretty good, as like
I said, it runs great within <2M after starting cold (in California mind you, so cold is in the 70'sF) until it's run at moderate consistent speeds. It does like a little choke when it's colder out, but can be fully shut off after 10 seconds or so.

No joke on the misfire when it starts, the thing runs absolutely terrible, sounds like a sick puppy. WOT and about 2 seconds and it's a inline 4 again.

I'm leaving on a business trip this morning, so will not be able to update until Friday night at the soonest. Thanks for the thoughts and tips, exactly what I needed!

 

Are the carb slides moving free, and are the diaphrams in good shape with no holes? I did not see you mention about a fuel filter, I would be a good idea to install one if you hav'nt. My X had similar problems and clean fuel made a big difference.

 

It's starving for fuel , try running on prime ,fuel filter plugged , tank vent plugged

 

cracked or split in the vacuum line; vacuum line improperly substituted with wrong hose is collapsing under vac; but you already know the biggie:

Float Levels.

 

To answer some of the tips since my last post:
1. Vacuum line is good; at any rate the issue is unchanged when the fuel tap is on prime.
2. Fuel filter changed before last test ride.
3. When issue was occurring I opened the gas cap, no change.
4. Carb slides are free, diaphragms have no holes.

I just did the float level check with the clear hose. I did it cold, though I will do it again after it's doing the deed. Anyway, the float levels are all right about the same height as the photo in the forum post that shows where it should be. Carb #1 is a smidgen higher than the rest, just a smidge. Here's what was interesting though: the fuel rushed into the clear hose on the center two carbs, but both outer carbs the fuel trickled, took like 7-10 seconds to fill the hose to the float height, whereas the center two carbs took about 1 second. I suppose this could be due to junk in the port that the fuel passes through to come out the drain, but interesting that BOTH outer carbs did that?

So, I'll button her up, take her for a spin and get the misfire occurring, bring it back and shoot the headpipes with the thermal gun, compare the temps. With a lean misfire not sure what I'll see, might be super hot because of running lean vs. a dead hole just pumping air. So:
1. Ride it
2. Shoot it
3. Clear hose level check.

More to come.

 

OK, took her out. No choke start, smooth idle, warm enough to take throttle and ride away in <2 minutes. Ran flawless, perfect, awesome! After 6 min of around town, got out onto the first stretch, 70 MPH for about 2 miles. Stopped, made a right, slight stumble but OK after that, 1 mile at 55MPH. Stopped, took off, single cylinder misfire at 1/4 throttle, cleans up at higher speeds. Another mile, right turn, multi-misfire 1/4 accel through 4 gears to 40MPH, seemed OK at steady throttle. Stopped again, and from this point until home, mutli-misfire and rough idle. Got home, left idling, ran and got the thermal gun:
Cyl1- 122 degrees
Cyl2- 149 degrees
Cyl3- 122 degrees
Cyl4- 152 degrees.

Did hose trick on bowls:
Cyl1- filled hose in 32 seconds, good final height
Cyl2- filled hose in 1 second, good final height
Cyl3- filled hose in 1 second, good final height
Cyl4- filled hose in 24 seconds, good final height.

It's been sitting for 10 minutes, so I just went up and down the street: runs flawless, absolutely no misfire.

Hmmmm.............interested in your thoughts. Me, it's got to be a fuel flow problem, be it the fuel tap (inlet to the tap, since on prime the issue doesn't change), or 1 or 2 of the needles are restrictive, or there's an obstruction in the cross tubes/passages between the inner/outer carbs. Here's my plan:
1. Run it til it's misfiring, bring along needlenose and clear hose. Stop, put the clear hose on and set the tap to prime, see how the flow is. I just had the tap off, completely disassembled it (again), and I didn't find any problems, but who knows??
2. Going to take the carbs off. I can do this now in less than 5 minutes. Take off all bowls, carbs upright, connect the fuel hose from the tank to the inlet, set to prime and watch the fuel descent from the needles.
3. If that's OK, look at bowl drains on the outer carbs for junk obstructing the flow when the drain screw is screwed out.
4. If that's OK, then I'll start cursing.

More later...thanks for all the tips so far!

 

Just noticed I'm now a Gear Grinder. How nice.

 

Next update: This time misfire was at it's worst! Switching onto prime setting of fuel tap made no difference. Headpipe temps when I pulled up:
Cyl1- 121 degrees
Cyl2- 132 degrees
Cyl3- 128 degrees
Cyl4- 147 degrees
Pulled the fuel hose off, put clear hose on, turned tap to prime and fuel could not come out fast enough. Put clear hose onto #1 cylinder bowl drain, opened screw: took over 60 seconds to get to fuel bowl height! Off come the carbs.

 

You said you had been through them 5 times. This may not help but I found when mine did the same thing, the screens in the internal fuel vales were plugged and i had some gunk,for no better way to describe it, in my metal feed lines.

 

capy, you hit it. Took the bowls off with the carb set right side up, and the middle two barely had any fuel in them. The #1 that took so long to fill was of course full, since I had tested it with the clear line when I got back. Cylinder 4 was full, as that was the one that had a good heat signature on the head pipe. Pulled the seats for the needles, and look what I found:



I did scatter the carbs on at least 5 occasions, but never pulled the seats. I normally only deal with older Japanese bikes, Yamaha/Kawasaki/Suzuki enduros, and those don't have a screen on the inlet side of the seat, so I got caught napping! Will clean, re-test, and report back.

 

It's fixed! Amazing, and again I learn something new. Thank you for all the tips gentlemen, and I hope this topic finds another someone with the same problem!

 

Nice find Apex, my screens are looking ok... checking float levels now, good times!