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Carb#1 CV Boot question/running ice cold

  • Thread starter Thread starter Rocketman
  • Start date Start date
R

Rocketman

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During tonight's ride, I completely lost cylinder#1 on my '82 GS850L. Carbs have been completely rebuilt three times (please don't ask!). Question is...if cylinder #1 is dead cold after a long run, but heats right up when I put petcock in "prime" rather than "run", could a defective CV Carb boot do this? I pulled the top cover off, and the diaphram looks just like the rest; seated evenly in the groove, with the little tab lined up with the carb body, and kind of wrinkly like the others with no obvious tears or gaps. Doubt the petcock's at fault as all three other cylinders run great and hot. Could the starter plunger have anything to do with this problem? (God-I wish we had fuel injection in 1982; carbs blow!)
Thanks guys,
Larry
By the way-plenty of spark from plug #1-so not ignition.
 
Since it's easy to reach, take the drain screw off #1 and see if there's fuel in the float bowl. Or if you've already done this, what were the results?

The vacuum line that operates the petcock is on #2, so this seems pretty odd...
 
If you mean the boot between the carb and cylinder head, a broken or defective boot or o-ring (or missing o-ring) could cause the mixture to be so far off that it won't ignite.

More likely, that cylinder would first overheat quite badly as it sucked in air to make the mixture far too lean.

But my leading theory at the moment is some sort of fuel delivery problem in #1.

Is the petcock of reasonably recent vintage?

You could also have some sort of really wacky linkage or sync screw problem that's causing the #1 throttle plate to stay closed.

I'm sure other ideas will occur to me and others at random intervals...
 
I knew I could count on you to help. No, I have not yet dropped bowl #1 to check for fuel. as I'm just getting ready to sit down for dinner. The boot I'm referencing is the rubber diaphram around the slide and pin. Is this diaphram supposed to be glued into the recess in the top of the carb, or just rests in there and is compressed by the crab top when its screwed in? Petcock is bone stock original from 1982 never been touched or maybe never even been seen. However if it were defective, I'm thinking it would affect all 4 cylinders, not just #1. Fuel delivery is my guess too, but baffling right now. Maybe after a meal I can think with a mroe clear head. I'll let you know later if there was fuel in the bowl.
 
1 thing came to mind. The carbs have screens at the needle and seat and #1 might be a little clogged. I know the petcock runs a bit more fuel on prime so maybe that's helping the situation but I really wouldn't bank on it.
 
I was also thinking that unscrewing the bowl screw to check for fuel won't help me a lot, as when I turned it from run to prime, cylinder #1 kicked in and got hot pretty quickly. Any way to clean these screeens out without disassembling the carbs for a 4th time that you know of?
 
No my friend. Taking those badboys off the bike has to happen. But the cool part is is that you only have to remove the #1 fuel bowl. Take out the pin holding the float in place, unscrew the phillips screw holding the needle and seat assy and out it comes.
 
I can basically remove my air box and carbs with my eyes closed now. I just absolutley HATE doing it. Think I'll wait until tomorrow and see if any other ideas pop into my head. Supposed to bring that '98 BMW R1200 to the dealer tomorrow at 11:30 to have it checked out. Ahhhhh....fuel injection alone may make the purchase worthwhile. Can't really ride both bikes at once now, can I? Will fix the 'Zook fuel dilemma tomorrow or Saturday. Thanks guys. Feel free to add on if you come up with any other suggestions before disassembly.
 
If you really wanted to get wacky, you could always switch the diaphragms from one of the other carbs to #1 and see if the problem moves.
 
I was thinking of trying that earlier, but exactly how do you pull a diaphragm off? When I pulled the slide and needle out, it looks like the diaphragm is part of the slide...
 
Rocketman said:
I was thinking of trying that earlier, but exactly how do you pull a diaphragm off? When I pulled the slide and needle out, it looks like the diaphragm is part of the slide...
Click to expand...

Yeah, I meant the whole assembly. Diaphragm/slide/needle. If it's shot, it's shot, I believe.
 
Cylinder # 1 has a bad attitude. But swapping slides might not be a bad idea. We'll see. Maybe just a trip to the mechanic and let him deal with my headaches.....
 
Rocketman said:
Cylinder # 1 has a bad attitude. But swapping slides might not be a bad idea. We'll see. Maybe just a trip to the mechanic and let him deal with my headaches.....
Click to expand...

A...a...MECHANIC? Wash your mouth out with soap! That's filthy talk for a GS owner, and we'll have none of that around here.

If you have found that rarest being of all, a competent motorcycle mechanic, keep his name a secret and keep him well supplied with donuts or whatever his preferred stimulant might be. I've never seen or heard of a decent mechanic who will work on older bikes, myself, but I suppose it's possible.

Anyway, you don't have to remove the float bowl or the carbs to check the gas flow in #1 -- just the drain bolt at the bottom of the bowl. Since you're getting flow sometimes, there will probably be gas in there, but you'll need to drain that and then get an idea if it's only a tiny trickle or if it's enough to sustain the engine.

With the engine off and the petcock set to "ON", you shouldn't get any flow. With vacuum applied to the vacuum line, you should get plenty of flow. It's coming through the small hole in the needle seat, so it won't be a gusher, but it should be enough to fill the bowl quickly. There shouldn't be any difference in the way it flows when it's "ON" and vacuum is applied and when it's on "PRIME". If there is a difference, replace the petcock.

If you're not getting much fuel flow either way, then you likely have some sort of obstruction that is probably sitting on the little screen on the #1 float needle seat.

The original petcock is 24 - 25 years old, and no matter what the mileage on your bike, it is almost certainly dead or it will die very soon at the worst possible time and leave you stranded in a bad neighborhood or dump a tank full of fuel all over your garage. The petcock may or may not be the cause of your current problem, but for your own safety it would be an excellent idea to replace it ASAP. Install a new one and forget about it for another 20 years.
 
RobGS850L said:
1 thing came to mind. The carbs have screens at the needle and seat and #1 might be a little clogged. I know the petcock runs a bit more fuel on prime so maybe that's helping the situation but I really wouldn't bank on it.
Click to expand...

Exactly what I was thinking, too.
 
Also, you've mentioned the choke plunger a couple of times. Is there something abnormal about it?

As long as it moves out when you move the choke lever the left and back in when you move it to the right, you should be OK. And moving the petcock to PRIME wouldn't affect the enrichment system at all.

Also, have you replaced the vacuum line betweeen the #2 carb and the petcock, or at least verified that it's 100% OK?

My leading theory is still some sort of mung partially blocking fuel flow in #1, possibly combined with a deteriorating vacuum diaphragm in the petcock.

Also, make sure you don't accidentally leave the petcock on prime overnight through all this. The float valves are not designed to be absolutely leakproof for long periods of time, and you could end up with a crankcase full of gas.
 
Checking the screens seems like an excellent idea and that will be one of the first things I do. I also have no problem replacing the petcock as a preventative measure, but again, if it were defective, or marginal, wouldn't that affect fuel flow to all carbs, not just #1? Also, I will check and/or replace the vacuum line between #2 carb & the petcock, but what carbs would be affected by a defective vacuum line, #1, #2 or BOTH because #2 is firing fine?
When I pull the drain plug and check for fuel flow with the petcock "on" or on "Prime" should the engine be running at that time in order to apply vacuum?
Lastly, do you know if an OEM petcock is still available for my '82 GS850L through Flat Out Motorcycles or other source. How complicated to install? Do I have to completely remove the tank, drain it, etc, and what other parts do I need other than just the petcock to do it complete? Any other washers, O-rings, etc?
Oh, how can I see the choke plunger moving up and down when I move the choke lever left and right? If I have the carb top off, can I see it move through the slide?

Thanks again.
 
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