The dreaded 6 k RPM stumble pick up wires good?

[markb80]

Yes I've had that happen before too with the small pods on 2 xs650s and a Suzuki GS I worked on for someone else. Suzuki's are bad about that issue, and the BS34 carbs on the GS's were not tuned well from the Suzuki factory.. But this bike has the stock air box. It sure feels like what you describe, but doesn't make sense with stock airbox. I am curious about butterfly seals (carbs) and choke seals too. Really weird. I have never had a weak spark issue with the bike, and I've swapped coils, ignitor boxes, plugs, caps, carb slides, battery, cleaned emulsion tubes etc etc. The pickup wires appear fine.

[markb80]

Ok, so I got out the big guns. I had an NOS (new old stock) complete timing plate assembly with pickups, wires vacuum actuator etc. All new, never saw the light of day new. I just got it out (should have opened a bottle of champaign to mark the occasion. I didn't think it would fix it, but what the heck. I had to cut the old timing plate off due to the permanent screw things to keep you from tweaking the timing. So I put it on, and it sure idles nice, played with timing a little. Then took off for test ride.... Same thing! But it sure runs nice when the throttle isn't all the way open.

Starting to think I have a goofy emulsion tube or main jet issue, or possibly butterfly seals or choke seals. I just soaked the carbs prior to this effort in a mild gas/yamaha carb soak solution. I've done that 50 times before, never had it mess anything up, as long as I removed diaphragms and floats. I put new o-rings in the mixture screws and cleaned emulsion tubes, new floats too.

Could be a fuel level thing I guess. When it cools off I'll retention the cam chain.

[TopCatGr58]

Hey Mark,

You may be on to something with the butterfly shaft seals. Even though you've done the soaking before with the Yamaha dip solution, it may be the cummulative affects of repeated soakings that may have finally damaged the seals enough to allow some leakage! Have you tried spraying carb cleaner or starting fluid, something around the carbs/seals to see if you can sense/hear any rpm changes?? Also, have you checked the vac. synch port caps...the rubber can harden and allow leakage even though they may seem snug on the port tube.

T.C.

[markb80]

I think I know what it is, but not 100% sure. I always do a quickie test on diaphragms with my thumb, without taking lids off. I hold my thumb firmly on the hot dog shaped opening on the air box side of carbs that goes to the carburetor upper bell chamber. I push up on the diaphragms, and make sure they stop on the way up, and hold pressure. I had one carb that doesn't quite hold pressure with the "thumb test", even with 2 very good sets of diaphragm/slides. It isn't a bad leak, just a little one. I believe it is the choke plunger seal. The thumb test will not work if choke is on, as that is a big leak, and no carbs hold pressure with choke on, but that is normal. But I have one carb (#3) that has a slight leak with the thumb test that seems to be coming from the choke plunger seal. So I have a good set of plungers and seals, but I have to split the rack apart to put them in. Maybe I'll send them to motorcyclecarbs.com for all new O-rings (fuel tees, butterfly seals and choke plungers) this fall/winter. It's not a big problem, sure runs nice aside from that. For now I'm going to tell myself that is the problem so I quit spending so much time on it. : D

[markb80]

Hey Mark,

You may be on to something with the butterfly shaft seals. Even though you've done the soaking before with the Yamaha dip solution, it may be the cummulative affects of repeated soakings that may have finally damaged the seals enough to allow some leakage! Have you tried spraying carb cleaner or starting fluid, something around the carbs/seals to see if you can sense/hear any rpm changes?? Also, have you checked the vac. synch port caps...the rubber can harden and allow leakage even though they may seem snug on the port tube.

T.C.

Yes, thanks, I checked all that. Carb boots are fine too. If I had a big air leak on the butterfly side of carbs, it should cause more problems at idle and low throttle opening. But I think I know what the cause is, not sure, see my next post above. A choke plunger seal leak could maybe cause more problems at WOT than other types of air leaks, because I think they must hold pressure for the diaphragms to stay up at low vacuum/Wide open throttle situations. Once the diaphragm falls, "It's fallen and it can't get up" because there isn't much vacuum with the throttle all the way open : D

[petejw]

hi mark,
if u block that hole off at the carb bell, the slide
should still slowly fall, theres a small hole in the slide.
are u running stock jets?
what have u set ur float heights at?
depending on ur throttle position around 6000rpm
is the transition from the mid circuit to the mains
and is at its richest.
have u removed the plugs to see what they look like?

[markb80]

Leaky choke plunger seals causing TOO RICH mixture

The opening on the air cleaner side of the carbs leads to the chamber around the outside the slide diaphragm, not the inside. If it went to the inside, then air would pass thru the slide hole. Some air can pass around the polished slide surface and aluminum carb body, but they are made to fit fairly snug and with a film of gasoline on the slide, they hold pressure (somewhat)

But I have more news, and observations. I tried gapping all plugs at .020" as suggested in a forum tech article. The stumble did not change AT ALL. So I went down the street at WOT, made it stumble and pulled the plugs. Plug number 3 (carb 3 has leaky choke seal), was soaked with gasoline. So, I DON'T think the slides are falling now. Instead, the choke/enrichener leaks, causing a SUPER rich mixture when slides are all the way up.

I was a bit surprised, because a super rich mixture should cause black smoke. I didn't have black smoke. But I think the poorly atomized fuel from the enrichener just doused the plug tip quickly, causing the spark to die. One other plug was also doused in gas, but not as much as number 3.

So I have a MINT set of 82 XJ1100 carbs that I am going to swap in. They are the same except for jetting. Emulsion tube are the same X300 (or whatever the number is). So I'll swap in my jets (don't forget the air jet). I'll work on fixing the original set when I have time. The choke lever does not exist on the 82 XJ carbs, but those levers suck anyway, and I'll just use the XS choke rod and add a knob for manually pulling the choke. I would like to find a handlbar mounted choke lever. That would be sweet.

BTW. I got a spin on oil filter adapter with oil cooler barbs from the guy who makes them on the forum. It is nice, but the middle bolt is drilled off center. It would still work, but I'd like to buy a another bolt from him if he's out there reading this. I'll make another post for that.

So, it looks like bad choke plunger seals are causing the choke to activate at WOT, causing the stumble. Not 100 percent sure, but looks that way.

[jwhughes3]

XJ 550 has a handlebar choke cable lever built into headlight switch. Check ebay.

Should swap, you just gotta figure how to turn cable movement into pulling movement in your mount since the 550 choke plungers are arranged different.

John

[markb80]

I think I'll just use the XS11 shaft and put a knob on left side of carb rack. I could try to find some goofy skull knob or 8 ball knob. I could even use a clothespin or narrow chip clip to hold it open if needed. I generally only use the choke for 10 seconds or so.

[Big Beadle]

not to jack the thread but is TC the one who is making the SOFA with the oil cooler hook ups?..My cooler plate has a crack in it so I had to remove it. Now I am on the hunt for another. I do already have TC's sofa. Any help in the correct direction would be great!!

[markb80]

No problem, I'm interested too. Hopefully I have this stumble situation figured out.

[TopCatGr58]

Hey Mark,

The oil supply/filter spin on threaded pipe is bought as solid threaded steel rod. With my meager home hobbyist level equipment it's difficult to get the
2+" long hole drilled exactly centered. But it's not important to the actual function of the piece. The end that sticks out of the adapter is totally immersed in oil in the oil filter, the other sticks into the engine, the opening in the engine is larger than the threaded pipe, so it's not going to catch an edge and create any turbulance in the oil flow as it goes into the engine. There are several mm's worth of wall thickness in the pipe section, more than strong enough for the mounting tension of the nut/washer. SO with this hopefully you can see that it doesn't really need to be replaced with one that is more centrally drilled!

Beadle, I do make the Oil cooler/spin on type of adapter as well, and will allow you to actually have less plate thickness sticking down from the engine since the SOFCA plate combines both functions into a 1.35" thick plate vs. having two 1" thick cooler and then Sofa plates. I'm working on getting some more raw stock so I can get out in the garage and crank out some more, have several folks interested in getting both plain and cooler style. You can see the newer design cooler plate in the tech tips where the regular plate is described..SOFCA vs. SOFA.

We now return this thread back on topic!

T.C.

[petejw]

if ur going to use xj carbs, use the stock jets that
came with the carbs instead of using the xs jets.
if its flooding in 1 carb and the problem stays with
that cylinder after replacing the carbs u need to suss
out the spark plug/lead/cap isnt faulty or breaking down.

[trbig]

I agree with Pete. Your 80 motor is going to be identical internally as the XJ motor, so just use it's stock jetting. And yes.. if you just swap out that "Choke" lever and bar, you can use it like an XS choke. The XJ bar doesn't have the detents in it like the XS does. Either that, or get a left handlebar control from an XJ so you can have the lever for the choke up there VS the XS lever down on the carbs. No clue about wire compatability between the two controls though.

[crazy steve]

...And yes.. if you just swap out that "Choke" lever and bar, you can use it like an XS choke. The XJ bar doesn't have the detents in it like the XS does..

I wish.... Yes, you can swap for the XS choke rod but the XJ version of these carbs isn't drilled for (and doesn't have) the detent balls/springs that the XS carbs have....



... as shown here. You'll need to drill at least the #1 carb body for the ball/spring or there won't be anything to hold the choke 'on'...