Stumble at 2200 RPM, shuts down

gantzcb · Posted: April 6, 2009, 10:34 am Post subject: Stumble at 2200 RPM, shuts down

1979 XS650F
Ok at idle, starts ok, ok up to 2000/2200 then dies, get across to 2500 and runs like a scared rabitt. Here's where I am and what I know: intake leak at top of right carb and intake boot, and weeping head gasket front of right cylinder. Remounted carbs three times, still won't seal. Finessed the casting flash off the top of carb where it goes into the intake, New manifolds, but going to order another set. Running PAMCO with new advance, carbs rebuilt, mixture screws at 2.25 turns, did not mess with the float setting - measured correct, stock jetting/air boxes with K & N's, new mikuni jets, stock ehaust remounted with new gaskets at head and mufflers, valves and chain adjusted, did not replace the shaft seals (no air leaks testing here), diaphrams test ok - fall slowly when covering port to test.

So my questions are: Is the air leak the most likely problem? Does the weeping head gasket play into this? I'm going to run a cylinder leak down. What else am I missing or should I check for

Thanks,
Clyde

pamcopete

gantzcb

Could be stiff advance springs holding off the advance untill you hit 2500 RPM, then, suddenly, the advance kicks in and away you go.

If the advance is new from Mikes, the springs tend to be a little stiff. Check to see that max advance happens at 3000 RPM and that it progresses as you increase RPM between idle and 3000 RPM. To loosen springs, poke a small screw driver in between the coils of the springs, just a little bit at a time, and check the advance after each "adjustment".

Xumi

Pete has a great point.. definitely worth looking into. If that doesn't fix it...

Had a very similar symptom once on another bike - Check your slide
diaphragms for a pinhole and good seating. Take your diaphragms out (don't mix them tho) and stretch them ever so slightly and see if you can find a hole while holding them up to a bright light.

The problem, as it turns out, was that as I rolled on the throttle, as the vaccuum difference started to raise the slide, the pressure against the diaphragm would increase (normal), but that increase would stretch the diaphragm ever so slightly, thereby opening a 1mm split in my diaphragm, which would make it deflate, and the bike would run like crap as one cyl was getting it's full alotment of fuel and the other was starving (dying and backfiring).

At idle, everything was great. From about 1200 RPM until about 2000 RPM, the bike would run horribly, backfire, die completely. After 2000 RPM, enough pressure would build on the diaphragm that it would overcome the small hole and the slide would rise, pulling the needle with it, and the bike would run like a scalded cat.

It was troubleshooting this issue that I finally began to truly understand how the needle (and CV carbs in general) worked.

gantzcb · Posted: April 6, 2009, 1:11 pm Post subject: Re: Stumble at 2200 RPM, shuts down

Both important things to check. I'll report back as the repair proceeds.
Thanks
Clyde

gantzcb

Took one advance spring completely off, still the same. Have not checked the diaphrams. I kind of got better as a rode it. It's trashed two plugs in the left cylinder. Brought it back it to do some tweaking. Adjusted the timing chain hot and running as per XSJohn & 3M. Only way to do it! I had followed the books doing it cold, it was too tight. Off subject here. I was doing the adustments while the bike was on the kick stand, not the center stand. With the bike leaning left, the left exhaust started smoking like an old freighter. Back to the weeping head gasket out the left front. Actually heavy weeping (as I am now).
I'm thinking it is leaking from the head gasket into the left cylinder.
Looking for what's next.
Thanks,
Clyde

xsleo

Try retorqueing the head, it may stop the weeping and bring the compression on the left side up some, which will help it run a little better. It may be fouling the left plugs from the low compression. My 75 was way down on one side and that side fouled plugs.

Retiredgentleman

A compression test and/or leak down test would be a good thing to do.

gantzcb

Quote::

A compression test and/or leak down test would be a good thing to do.

Thanks RG,
Ran a compression test before the project started. On a wet test both were even as I remember. But it's time for another. Neighbor has a leak down tester. So that is in order also. My mind keeps tripping back to the day I purchased it. The bike ran but sputtered when reving. Not that I cared. The guy said "another guy told me one of the points was out of adjustment".

I'll get a compression ck tonight, and maybe the head retorqued. I don't give it too much hope. It would billow the smoke, you could clear it by reving, the in a short while it would billow again. The cylinder is filling with oil.
Clyde

gantzcb · Posted: May 15, 2009, 5:27 pm Post subject: Re: Stumble at 2200 RPM, shuts down

Compression good, no leakdown

Sunken float left carb. Soldered, filed did not take, sunken float the second time. New float on order.

tacoswild · Posted: May 15, 2009, 6:01 pm Post subject: Re: Stumble at 2200 RPM, shuts down

Yeah those can be tough to solder, the air inside the float heats up and wants to spit out a new hole through your nice wet solder. I work in an electronics shop and still found those things a bear. There's usually a vent hole in the middle of one side. Suck the solder off that with some wick to open it then you can take your time sealing around the rim without the air pressure problem. Let it cool completely after then be real quick when you seal the vent.

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