Ever feel like you met your match??

[Toolmaker Tim]

Valves

With the compression loss and the back fire it sounds like a valve clearance issue. Checking them would rule it out. What George says makes sense.

[GLoweVA]

Could number 1 intake valve be too tight to get a flow through to the cylinder?

I was kind of thinking the same train of thought, but he said the compression numbers were all about the same.
If #1 intake was too tight, then I would think the compression numbers should be off(lower) on that one and not the others.
But then again, he said all his numbers were kinda lower, maybe if he got the compression numbers back up (valve shims, etc) then the problem might clear itself.
In other words, maybe #1 is just a little more off than the others and thus the problem shows up in #1. (kinda like a premature detonation thing, thus the back fire)

Just thinking out loud. Hope this helps.

[LoHo]

Could number 1 intake valve be too tight to get a flow through to the cylinder?

[BA80]

In my mind I do feel like it is a fuel issue. Just running out of fuel options. As to spraying carb cleaner in the vac port, isn't carb cleaner more volatile and therefore require less spark to ignite?

Be Friday before I can try anyway, sitting in Charlotte waiting to get to New Orleans for two days.

The difference is insignificant. It IS a valid test for lack of fuel. Used that exact method on JWs XJ550 this last weekend. It needs carb work we found out.

[DGXSER]

In my mind I do feel like it is a fuel issue. Just running out of fuel options. As to spraying carb cleaner in the vac port, isn't carb cleaner more volatile and therefore require less spark to ignite?

Be Friday before I can try anyway, sitting in Charlotte waiting to get to New Orleans for two days.

[BA80]

Pull the rubber cap off of the vacuum nipple on that cylinders carb boot, use a can of spray carb cleaner with a straw on it and stick the straw in the nipple.

Start the engine and let it idle. Squirt some cleaner in the boot with very short bursts and see if that cylinder will fire at idle then. If it does it will save you from swapping a bunch of parts for no reason because then you will realize that it's a carb issue. Which I'm confident it is because I just went through a similar scenario with my 79.

[DGXSER]

I will check battery when I get back Friday.

I am going to go through the carbs.....again. Maybe swap out pickup coils, even ignition coils. Of course those will be one at a time.

[bikerphil]

Maybe battery voltage is low at idle and causing a weak spark. You did just get back from a long trip and possibly the electrolyte boiled away from overcharging, JAT

[GLoweVA]

I'm racking my brain to try to think of anything.

Could the fuel level in #1 be too low?
Do the 81SH's have the same bowls as the 81H's? (ie the drain port to check the running level of the bowl)

Maybe there's something else going on here.
Why would #1's pilot screw come out and not the others?
Maybe there was a little backfire action going on in there that kinda forced that screw out in the first place. (it's not like gravity is going to help it out, so It had to get 'pushed' out)

Recap for my memory jog, New plugs, swapped #1 #4 and no change, no vac leaks, even compression across all 4 and an occasional pop from #1.
So, why is #1 not getting any fuel on idle, but seems to get it with more throttle (ie main jets)?

Question: Hypothetical, if the timing was off by one tooth, could the compression be a little lower and maybe the intake on #1 open just a little so that you get the pop?
Are the valve clearances okay on #1?

Sorry if this seems like a shotgun try, but I'm trying to think outside the box on this one.

Edit, just re-read the whole post and thought: what if you sucked some little debris into the idle screw hole (where the missing one was) and it's clogged the pilot circuit so bad that even though you were maticulous about cleaning them, you might not have cleared the pilot circuit all the way.

I know it sounds like I'm second guessing your cleaning ablility and I don't want to sound like that.

Hope my brain rant helps a little.

[DGXSER]

They are there, I think the spring in the kits I got was weak. This screw in there now is tight on it. I can feel the difference in turning the screw.

I checked for vacuum leaks by starting the bike and spraying the crap out of no 1 intake manifold, carb, and the vacuum cap with carb cleaner, no change.

[BA80]

I don't see any threads in there Don.

[BA80]

I've never heard of a mix screw falling out of a late carb either.

If you have compression and spark the only thing it can be is fuel. Something is preventing the fuel from getting from the bowl, through the pilot circuit,and out that little hole.

[DGXSER]

Thanks Greg, this is the newer style carbs, almost impossible to break the tips off, but here is the hole I found when I went to remove the carbs when I got home....



I always feel for the tip to come through when I bottom the screw out and then feel it disappear as I preset the mix screws.

[BA80]

Did you check to see if the tip of the missing idle mixture screw is broken off and stuck in the idle port? That would cut the fuel off at idle.

[DGXSER]

Thanks for the thoughts George, no octy, ditched that long ago.