Spark-check...Compression-check...Fuel-oh Yes...no ignition on #3/4

[sedelen]

Thanks, wouldn't that also affect the fuel flow on PRIME?

That's a definite maybe! I certainly believe it would, don't really know to what degree that vent hole relieves pressure and vacuum to the tank.
A tight fitting gas cap with a good seal and plugged vent (within the gas cap) will also create problems.
I guess you could disconnect your gas line to the petcock, place another hose on it to a gas can and put it in prime and see it it flows steadily for awhile with the gas cap on.
You don't have any in-line filters in your gas line do you? Those can be restrictive.

[sedelen]

Here's a pic of that filler cap vent hole in this thread, post number 5.

http://www.thegsresources.com/_forum/showthread.php?t=177161&highlight=vent+hole

[BigD_83]

Hey Steve,

I get what you are saying, but the reason I was asking about flow and venting in PRIME is that the bike was running just fine with the tank off the bike and set on PRIME.

To clarify, I was using the tank like an auxiliary, with it set on the bench and the petcock on PRIME, with an inline filter just before the carbs, and the vacuum line plugged. Now that the tank is back on the frame, I've removed the inline filter, reconnected the vacuum line and set the petcock to ON.

Then the stumbling began.

I'll have to have a go at it tonight, but the bike will continue to run at idle indefinitely (well, as long as I had patience to run it) so it's not acting like it is fuel starved at idle. Maybe I should put a temporary clear line on to see if there is something funny happening as I roll the throttle on? It could be that I don't have ADEQUATE flow for that initial acceleration.

First things first. I'll try it on PRIME when I get home tonight to see if that helps.

Oh yeah, and more reading about off-idle stumbling during breaks in my day.

[sedelen]

What type of petcock do you have a 3 position vacuum operated one, or the other?
You do have adequate fuel in the tank?
Have you seen your petcock filter? outside of filter element could be coated with something that might be restricting fuel.
I know I'm grasping at straws here, but you just never know.

[sedelen]

Yeah, try running it in Prime and see it flounders, then switch it to on and see if there's any difference.

[BigD_83]

petcock issues causing stumble?

What type of petcock do you have a 3 position vacuum operated one, or the other?
You do have adequate fuel in the tank?
Have you seen your petcock filter? outside of filter element could be coated with something that might be restricting fuel.
I know I'm grasping at straws here, but you just never know.

Three position; OEM from the looks of it.

I've had it apart once b/c turning it was like pulling Excalibur out of that rock, and it was clean, but there was definitely bits and pieces of fuel varnish scraping around the bottom of the tank. The petcock opens and closes under vacuum, at least with the amount of suction I could create manually.

The filter element is picking up a little bit of rust. I cleaned the tank b/c there was some rust on the fuel gauge float arm and up around the top inside of the tank, and I didn't do the best job of mitigating new rust as I didn't have fresh gas on hand to fill it.

This latest round of testing started with a full tank, and the gauge says I'm around 1/4 (it was dumping raw fuel through to the muffler earlier). The dip test (I'm the dip ) says that it is a little more than 1/4 full.

Question about that filter element: When I cleaned the tank I was able to remove it and put it back. The petcock froze up on me again and when I pulled it out, I wasn't able to remove the filter element. Should I have been able to do it the first time around or are they supposed to be epoxied in place?

Don't worry about throwing out desperate suggestions...I never would have thought something was amiss with those pilot jets b/c i was POSITIVE I had tightened them down. It's easy to overlook something simple.

[tom203]

It doesn't take much suction power to get fuel to flow nicely in on or res spot- less than sipping a drink up a straw- you might have a problem petcock, so just try it in prime spot. I bet the petcock filter was meant to stay fixed, since you wouldn't want it to wander off after a fillup. A liitle rust in tank shouldn't be a problem- mine is not perfect.
How does this thing cold start? mine needs full " choke", starts instantly, but rpms fluctuate wildly for 15 seconds or so, till it calms down and I back off choke to 1/4 spot.

[sedelen]

Tank sound pretty good.
Petcock fuel filters coming off? not a big deal in my book as there's a piece
of the gasket (at least on my rubber O ring gasket, some don't have this flat piece which joins the two sides of the O ring) that keeps the pluggable (easily detaching) filter in place.
Also I have a petcock that I'be been unable to remove the filter.
If my petcock lever sticks I just spray around the lever with WD-40, there's a rubber seal behind that selector lever that will dry out on you and make it harder to turn.

[BigD_83]

Tom...

Startup is beautiful. After priming, with full choke it just takes a whisper on the starter button and it fires right up.

At the moment it revs up a little too high, I shut it down before it hits 5000rpm and I have to modulate the rpm with the choke. After about 15-30sec I can shut it off and it'll idle happily around 1200-1500 rpm

[tom203]

Sounds like normal startup! I'm pondering the stumbling issue.

[BigD_83]

I'm still thinking fuel starvation, but I'm not sure how to prove it...it's pouring rain, I don't have the bike insured right now, and it's very difficult to keep it puttering around the city at 2500 rpm to do a plug chop.

One thing I haven't tried is just cranking the main idle speed adjuster to the stumble point. If it runs nicely there (no stumble), then it would suggest that it is the rate of change in the opening of the butterflies that is causing the problem, which would be more indicative of a vacuum leak/inconsistent vacuum/mixture ratio problem rather than a frank lack of fuel.

How high SHOULD the rpms go on these 650s with full choke, at startup?

[bccap]

I don't usually use full choke. But I just tried it, 3800 rpm, on my bike.

[BigD_83]

I don't usually use full choke. But I just tried it, 3800 rpm, on my bike.

Thanks, Garth...

When I got home tonight I flipped the petcock over to PRIME and fired it up. First time it scared me a bit so I killed it. I then left it on full choke to see where the RPMs got to. The final number was 4200. Boy, that seems awfully fast for a bike that hasn't warmed up yet.

[BigD_83]

Replicating a problem...

Don't you hate it when you try to replicate a problem and can't do it?

That was ALMOST the case tonight. After starting the bike up tonight on PRIME and letting it run there for awhilie, I switched to ON again, and had no problems running it up and down through to about 1/2 throttle (6000 rpm?) It remained that way for a few minutes (up to maybe 5) at which point the stumble came back at very small throttle turns (less than 1/8th). EDIT-I did not take it to full throttle.

The engine died, and I fired it right back up again with no choke but on PRIME and it started like a trooper. No complaints whatsoever...after a short time, I switched it back to ON for a second time, and this time around I couldn't get it to bog or stumble at all.

I ran it for a minute or so at 3000rpm and chopped the engine. The plugs were all a little sooty with the exception of #2, which was a little on the lean (whitish) side. Cleaned them up, put them back in the bike, and it started up again with no hesitation. Again, I couldn't reproduce the stumble.

It would be presumptuous of me to call this a victory, but I'm finally to the point where I'm not worried the engine is going to cut out on me at lower rpms. I'm ready to take it the street on the next dry spell to see how it performs through the various throttle positions/rpms and gears.

If all goes well, I can start thinking about another oil and filter change, replacing the brake lines and pads, and getting all of the luggage racks mounted.

[tom203]

Thanks, Garth...

When I got home tonight I flipped the petcock over to PRIME and fired it up. First time it scared me a bit so I killed it. I then left it on full choke to see where the RPMs got to. The final number was 4200. Boy, that seems awfully fast for a bike that hasn't warmed up yet.

I don't let it go over 3000 and try to keep it under 2000, though it's very erratic for 15 seconds- it's better to wait for oil to get pumping then let it scream.
Anyways,sounds better, so maybe you just need to run it awhile to refresh its memory. keep your eye on the petcock, take a peek at #2 plug after a ride.