This thing's going to get me killed...

OK XJers:

I'm at my wit's end, and I'm clueless about what to do next...

When I start out a ride, my bike runs perfectly -- smooth, crisp acceleration -- with absolutely no issues.

But by the time I get to the end of my 40-mile commute, it has developed a MAJOR bog between 3000-4500 RPM. It still idles fine, and runs perfectly over 5000 rpm, but between 3000-4500 rpm, when you twist the throttle you hear the airbox moan as the butterflies open, but the RPM climbs VERRRYYY slowly.

Not a good thing when trying to cross a highway or merge into traffic.

It doesn't miss, cough, or sputter, it just falls on it's face and acts like it's starved for gas.

If I try to hold it steady between 3000-4500, such as when easing along in slow traffic, it bucks and surges.

Once the tach hits 4500 rpm, it's like someone throws the fuel switch ON again, and the bike takes off like a scared rabbit.

ALSO, twice in recent weeks the bike has lost power while cruising down the interstate. One time it wouldn't go past 45 mph, the other about 60 mph. After a mile or so on the shoulder, it took off again and ran fine.

The carbs are spotless, and the tank, petcock screen, and in-line filter are all clean, and the petcock works flawlessly. The in-line filter also stays full of gas, so fuel flow apparently is not an issue.

Here is what I've tried so far, with no improvement:

* Run with petcock in "Prime" position.
* Seafoam treatment.
* Clean and blow-out carburetors (3X).
* Polish slides and bores (they clunk with authority).
* Re-set float levels.
* Adjust pilot screws (from 2 1/2 to 3 3/4 turns out, in small increments).
* Check valve clearances.
* Place aluminum foil heat shield under float bowls.

So, any ideas? How can it run perfectly when cold, then develop a bog between 3-4500 rpm once warmed up? (Ambient temperature seems not to matter.)

I know these carbs don't have accelerator pumps, but what allows extra gas for acceleration until the primary slides open? If vacuum controls the slides, etc., why would the particular RPM range matter, i.e. what magically happens at 4500 rpm to get the fuel going again?

This is my daily commuter, and I'm really not comfortable riding it in traffic anymore. Please help!!!

Herb

 

I'm wondering if it's a timing-related issue. I'm not sure how timing advance works on these bikes (yet), but I'm wondering if you're losing your advance after the bike warms up fully. I'd look at either the pick-up plate to see if the plate rotates or try a known-to-be-good ignition module. One other thing that just occurred to me is you may be sucking air when your throttle shaft is in a particular position. You would have to roll on the throttle until the engine Rpm’s fall into the 3000-4500 range. Note the rotation of the shaft. Then I'd check the shaft at that position for excessive side-to-side wobble. You may have a worn bushing that's allowing too much air by causing the engine to stumble.

 

Ignition advance is controlled by the TCI.

 

What you describe sounds like a Carb issue. You make is sound like the Emulsion Tubes are Colged at and neat the Bottom of their multi-ports.

You get the "Whooo" from pulling air in ... But no "Whaaaaa" from the Fuel being drawn in to the Cylinder.

Id be looking for"
Clogged Emulsion Tubes
Clogged (or wrong) Main Air Jets
Diaphragm leaks causing the Diaphragm not to fully raise the piston.
Top of Diaphragm incorrectly installed.
Top of Diaphragms folded-over causing vacuum leak.

Fuel Delivery issue related to Float Heights, Supply line, Petcock etc.

 

Thanks for the replies, guys...

Gamuru -- I don't think it's the throttle shafts, because 1) it doesn't matter how far you twist the throttle - a little or a lot - the bike bogs until RPM reaches 4500, and 2) it doesn't happen when the bike is cold.

Rick -- The carbs are pristine and all passages are open. Also, if something was clogged it should do it all the time, not just when hot.

Also, I don't think it could be the wrong jets, because this problem developed over time. It used to run fine with the same jets installed.

I've inspected the diaphragms and don't see any holes or tears, and they're not folded over. I've had them off and on 3 times now, with no change in the behavior.

So I'm back to the same question -- what would make it go lean at 3000-4500 rpm after it's warmed up, but doesn't happen when it's cold?

Herb

 

Linkage issue is one thing.
Sync way off is another.
Pilot Jets too Lean and Mixture Screws adjusted way to Lean.

Coming off Idle there wouldn't be enough supplement from the Pilot Mixture Screws to add to the Intake until the Main Jets kicked-in Full.

You might have a washer below the O-rings and are getting a false setting on the Pilot Mixtures.

Other than that. Air. Too much of it from someplace.

 

what i'am reading sounds like a rich mixture, don't know why this would happen
but if is a lean mixture, a little blip of choke should wake it up
if it's rich it'll get worse

 

How hot is the engine when it starts playing up. If you spit on the head does it sizzle. If so then I'd say lean. If not then definately rich.

Has the air cleaner been replaced lately? Is the bike all stock?

 

Rule of thumb.


Runs better cold = Too Rich

Runs better hot = Too lean.

 

Would be interesting to pull off when it happens, have a cup of coffee and then look at the plugs. Find out if they are fuel fouled.

I think Gamuru may be on to something with the timing. I used to have a pair of antique outboard motors where the timing advance was set manually. If you opened the throttle up but left the timing retarded you'd get that low bwaaaaaa sound, and little power. Could be the TCI is getting hot and not advancing properly. You could pop the cover off the pickups and hit it with a timing light to see if it advances as RPM goes up.

 

MiCarl:

Could you elaborate? I'm fairly well acquainted with the fuel and mechanical systems on my bike, but know very little about the electrical side.

I know there's a triggering unit on the left end of the crankshaft. Is that what sends RPM info to the TCI? Is there a mechanical or vacuum device that communicates with the TCI to control spark advance?

Thanks,
Herb

 

Under the timing cover are the pickup coils for the ignition and a wheel on the end of the crankshaft. The wheel has timing marks on it and a tab that the pickup coils sense as it comes past them.

Grab some bright red lipstick from the Mrs., girlfriend, sister .... and put a mark on that wheel. Put a timing light on one of the plug leads and shine it at that wheel. The strobe effect will make the mark appear stationary.

As you increase RPM the timing should advance and the mark should appear to move clockwise. If you do this test when you have your problem with power you can determine if the timing isn't advancing.

The advance is all electronic. Controlled by the TCI.

 

The timing not being advanced by the Module is a very, very remote possibility.

I think this is going to boil down to a Carb Issue. I have s sneaking suspicion that the Emulsion Tubes aren't totally cleaned. The bike is bogging-out because the fuel supply from the Main Jets is too much of a stream and not atomized with sufficient air.

That's where I'd go looking.

 

Thanks Rick, I got to answering the last question without considering the ramifications.

Checking advance is probably the last thing to try.

I'll place my money with Rick. You're getting too much fuel and it really shows when the engine gets good and hot.

 

If you look at this picture I swiped from www.factorypro.com of an emulsion tube...

*

what Rick is saying is that the holes may be clogged where the little blue arrows are pointing about halfway up the shaft. There should be two to three on one side--say the twelve o'clock position--that are drilled through to the 6 o'clock position. Then you'll have three to four more drilled at the 3 o'clock position through to the 9 o'clock position. Make sure all those holes are clear.

Inspect, clean, re-test and let us know your results.
(*I'm assuming you have Hitachi carbs. If not, your emulsion tubes may be different.)

 

I'm no carb expert, but I'm just going to throw out a question that I haven't seen yet.. Does it happen only under load (clutch out/riding) or do you notice it with the clutch in and just revving? If its only under load, I would say its a vacuum leak -- cracked/loose intake boot, blown intake gasket, loose boot-to-carb clamp, cracked/loose vacuum cap would be the next things to check if you're positive it's not a leak within the carb rack. If your bike has YICS, make sure the passage bolts on each side of the head are tight too.

Just my $0.02

 

Rick, Gamuru, and Taz:

Thanks for all of the good insights.

My bike has Mikunis, but I'm assuming what they call the "Main nozzles" (Item #14 in the image linked below) are the emulsion tubes.

http://demo.motorsportdealers.com/modul ... 487B9J&z=2

I'll pull them this weekend and clean all of the little holes.

And, Taz, yes, it does only happen under load, but I don't know of any possible vacuum leak, and I'm not sure why it would only show up when the engine's hot. But I will do the WD-40 test to check all of my manifold fittings, just in case.

Thanks guys!
Herb

 

Still no luck...

This weekend I

1) checked the emulsion tubes. They were already clean, but I sprayed and blew them out and verified that I could see daylight through all of the tiny ports.

2) re-set the float heights -- lowering them slightly. (I had previously raised them in one of my many attempts at figuring this problem out.)

3) re-synched the carbs, which were almost perfect already.

and

4) pulled the plugs. They looked perfect.

This morning on my commute my bike started, idled, and warmed-up perfectly, and ran like a scalded ape for the first 10 or 15 miles.

It then stumbled a couple of times on the interstate (right after I passed an 18-wheeler 8O ), and when I hit the stop and go traffic on this end, the same flat spot was there between 3-5000 rpm.

Oh well...

Herb

 

how about the vent in the gas cap ?

 

On my 77 wing it would do the same thing, run great than all of a sudden boog out, I finally figured out it was the gas cap, it wasn't venting so it would create a vacuume and would slow the fuel. I could accelerate out of it and it would be fine but at that certain rpm range would have no power. Took the cap off and you could hear the vacuume release from the tank, and then it would be fine.
Don't know if thats it or not, but definetly worth a try, once it starts acting up, take the gas cap off, put it back on and see if problem is still there.

 

Polock and Gearhead:

The gas cap venting question was one of my original thoughts, as that would explain a lean condition after running for a while. But even after stopping and filling up with gas, the problem continues, so I don't think that's an issue.

To answer some earlier questions, the bike is bone stock, has approximately 17,000 miles on it, and the air filter is in good shape.

Something I have noticed is that, previously, the fuel level in my in-line fuel filter would sometimes draw down. Since this problem showed up, that doesn't happen anymore.

That seems backwards, I know.

What I keep visualizing is that something must be keeping the float bowls from refilling all the way, so that I have a dead spot in the transition off idle until the primaries kick in and suck gas up out of the bowl.

I've checked the float levels off of the bike, and they are spot-on. Is there anything else that would keep them from remaining full when the bike is running?

Thanks again, guys, for your time and ideas.

Herb

 

Here is something that helped me out. First a question though. Did you recently switch grades of fuel or switch to a different gas station? My 700 does, or did the same thing your bike does if I run low and mid grade gas in it. With the good stuff it does not. As an added bonus it gets better mileage with the good stuff in it also. Hope this helps.

 

Full fuel tank won't cure a problem with the cap not venting. In fact, problem should pop up faster because there is less air in the tank.

Check float levels on the bike with engine running. I got drastically different results that way than I did on the bench, allmost 1/2 inch lower.

 

while it's doing that bog thing, touch the choke and see what happens
could something be collecting on the screens on top of the petcock ?
we better find this soon or we'll be checking tire pressures and tail lights

 

MiCarl -- You're right, of course, about the full fuel tank creating a vacuum quicker. But I don't hear any "whoosh" when I crack the cap, and the bog returned immediately after taking off, before i would think it had time to create a vacuum. Also, I've drained the tank on the bench with the cap on, so I THINK it's venting OK.

Polock -- I tried the choke on the way home during the bog, and it didn't seem to have any effect?!? I slowly pushed it all the way ON, and nothing changed. It didn't help or hurt. Do you think that means there isn't enough fuel in the bowls for the "enrichment circuit" to have any effect?

BTW, the choke functions perfectly for my morning warm-up, so it's not a problem with the enrichment apparatus itself.

I think tomorrow, just for kicks, I'll remove the inline filter and run a straight line from the tank to the carbs, to see if my filter is creating some sort of problem. My tank is very clean, so I think I'll risk it in the name of science.

The mystery continues...

 

Don't do that ...

It is possible that the filter IS the problem ... if you look in and see more AIR than FUEL.

The Bubble wants to get out of there. It can't go to the Carbs. It MUST rise.
In doing so it limits the FLOW through the filter. Sort-of like a Vapor Lock situation.

Move the tank to such a position that the Bubble in the Filter can Rise. Put the Petcock on PRIME and let the Gas Filter fill. Either ... "Walk" the Bubble up into the Tank with the Petcock on PRIME ... or, remove the FILTER -- for a moment -- and PRIME ~~> THE FILTER.

Fill it up and put it back on.
You might very well have had one of the most obvious snags looking you in the face and we didn't figure it out until you threatened to remove the filter exposing Clean Carbs to foreign matter.

Leave the filter on there.
Just make sure its FILLED with Gas!!!

 

Well, the fuel filter wasn't the problem.

Back to square 79...


Herb

 

could the air jets have gotten swapped around ?
did this develop slowly or over night ?
needles are seated in the slides right ?
throttle shaft seals ?
this is really a test and were all flunking ?

 

Fellow XJers:

PROBLEM SOLVED!!!!!!!!

It was the pick-up coils.

After pulling the carbs for the umpteenth time, and verifying that everything with them was absolutely, positively spot-on, I decided it must be some kind of heat-related ignition problem.

I swapped the TCI unit first, with no change.

I then bought a junk bike last week for parts. It was supposed to have been running when it was parked (outside) 10 years ago, so I held out hope that the pick-up unit was OK.

I removed the pick-up unit Saturday, and spliced it into my harness yesterday. (The harness on the donor bike had "issues".)

My Seca fired right up last night after the transplant, and I cruised on into work this morning with absolutely no stumbling, hesitation, or stalling, for the first time in months! Eureka!!!

I think it's going to be a good day, even if it is Monday!

Thanks for the advice and support-
Herb

 

A non-carb related problem solved...
Good to hear your joy.
Lets go Tigers....that is if your an LSU fan