Top End Noob

[mike_of_bbg]

I don't think blue loctite is your problem. Most of the contact between the insert and head should still be metal to metal.

[MisterCinders]

Guess I need to hold a seance or something. Cripes.

[MisterCinders]

Bumping with a "progress" report.

I never could get the No. cylinder to fire. Plugs looked dry, so I tore down the carbs and dipped them again. Sprayed out every passage, and they look completely clear. The compression in that cylinder is lower than the other three, so perhaps it is not generating enough vacuum to pull in fuel. There is gas in the floats, so it's getting to the carbs.

Otherwise, I still have my spark insulation theory, but dry plugs probably strike that out. At this point, my plan is:

Do one more compression test dry, then wet, to see if No. 3's problems are piston rings or valves. This will determine how far I go back into the beast. If the test show ring problems, I will double check that the rings did not get out of line (i.e., the gaps fell out of staggered position) when I reinstalled the pistons into the jugs.

Even if the tests show valve problems, that second cylinder head is cleaning up to be much better than my original (no spark plug thread insert, nice looking valves/seats), so it's going on the bike either way.

Just hoping that the compression problem is valves not rings. I have fresh base and head gaskets for each stage, just in case.

We'll see.

[niclpnut]

Have access to a compressor/leakdown tester? Think the leak down tester is available as a loaner tool from most autoparts stores.

Use the tool and isolate where your compression leak is coming from.

Nic

[MisterCinders]

Damn.

No leak down test, but cold compression on that No. 3 cylinder comes in as follows:

dry = 95 psi
wet = 150+ psi

So those rings must be out of whack UNLESS compression is supposed to jump under ordinary conditions, and the compression in that cylinder is light because of new rings.

Before I dive back into this sumbitch, is there anything I can/should try first with these numbers?

Although I did not re-test compression on the other three cylinders, recall that prior cold/dry tests showed about 120 psi at last test.

Any ideas?

[MisterCinders]

Bump? Really not trying to redo rings if there is another problem.

[mike_of_bbg]

How much oil did you put in? Should only be 1 tsp. 150 psi seems high. You need to seat those rings before you can get full compression. Take your next reading in a couple hundred miles of aggressive riding.

[MisterCinders]

No hard riding on three cylinders. No.3 won't fire.

[tkent02]

95 is enough compression for it to fire and run more or less normally, although with just one cylinder that low it might not idle smoothly. It has to be a spark or fuel problem. You could ground the spark plug to the engine with a jumper wire to test your blue locktite theory. It does not sound plausible to me, but it could be possible. Tried a new spark plug?

Most likely fuel isn't getting into the cylinder for some reason. Does it fire a few times if you drip a little gas down the spark plug hole? Is the choke feed passage in the float bowl itself blocked? How is the fuel screw adjustment? Try opening it up more? How is the throttle synchronization? Maybe this carburetor has the throttle completely closed? Perhaps a clogged pilot jet?
Broken off fuel screw tip stuck in the port?
I have not read this entire thread, may have missed a few details.

I wouldn't tear it apart to fix the compression problem without a try at running it in. You need to get it firing on all four.

[MisterCinders]

Is the choke feed passage in the float bowl itself blocked?

I pulled the carbs apart this weekend soaked them again, and then shined lights, sprayed and checked every passage and hole.

Main jet passage clear. Fuel jet passage clear. Float needle passage clear. Choke passage was also clear.

95 psi should pull fuel from the bowl, and there is fuel in that bowl. I also made sure that the vacuum line was plugged, since I am testing things on PRIME with the tank on the side.

This riddle is seriously working my nerves. I can tunnel back into the head/cylinders, but 95 psi should not obstruct that cylinder from firing. If I go back down the rabbit hole and determine that the problem was a carb or electrical issue, I cannot be responsible for the blind rage that will overtake me.

To recap:

Spark plug, spark wire, ignition and coil all check out.

Fuel in the bowl.

Carbs cleaned (the real way) multiple times.

New rubber o-rings for the intake boots, carbs etc.

Valves checked and within specs.

Cold/dry compression is 125/125/95/125. A wet test on No. 3 brings compression up significantly, but I think I added too much oil for the wet test, since I read "tsp. or so" as "tblsp. or so."

Engine runs only on three cylinders, No. 3 pipe is cold. Plug looks dry, though, so something is blocking the fuel.

If that wonky compression result means I botched the gaps out of 120 degrees (perhaps when wrestling the pistons back into the jugs?), so be it. I will go back in to fix that.

If the No. 3 cylinder's demon is that plug insert, so be it. I have a replacement head and valves that I am cleaning and lapping now. That wet test showed improved compression, though. I figured that a compression loss at the plug threads would (like a bad valve) show consistently low compression.

[MisterCinders]

95 is enough compression for it to fire and run more or less normally, although with just one cylinder that low it might not idle smoothly. It has to be a spark or fuel problem. You could ground the spark plug to the engine with a jumper wire to test your blue locktite theory. It does not sound plausible to me, but it could be possible. Tried a new spark plug?

Most likely fuel isn't getting into the cylinder for some reason. Does it fire a few times if you drip a little gas down the spark plug hole? Is the choke feed passage in the float bowl itself blocked? How is the fuel screw adjustment? Try opening it up more? How is the throttle synchronization? Maybe this carburetor has the throttle completely closed? Perhaps a clogged pilot jet?
Broken off fuel screw tip stuck in the port?
I have not read this entire thread, may have missed a few details.

I wouldn't tear it apart to fix the compression problem without a try at running it in. You need to get it firing on all four.

Not sure I understand the "jumper wire" idea you have. The dry plug suggests that my loctite theory sucks. I have swapped plugs and wires to make sure those are good. Changing plugs and/or wire from 2 to 3 does not change the result (i.e., the problem does not migrate from 3 to 2).

The carbs are bench synched (since I cannot get an idle to do the vacuum synch), but it's a pretty good bench synch. Also, even turning up the idle does not prompt the No. 3 cylinder to fire. If the throttle slide were out of synch, it should catch as the idle screw opened up.

I set the fuel screws at 1 turn out. I can try backing that No. 3 screw a bit more.

Pilot jet was clean. No broken tips in the pilot fuel passage. I cleaned the hell out all passages, but I will check the choke passage again, to be certain.

I have not tried putting a bit of fuel directly into the cylinder yet. Will test that tonight.

[MisterCinders]

checked out the No. 3 carb, and it is clean and clear. Backed out the pilot fuel screw an extra half turn. Re-installed the carbs and fired her up.

The No. 3 cylinder fires some, but the engine stutters and pops a bit, and that cylinder is missing a fair amount. The exhaust pops pretty frequently, but the No. 3 header at least gets a little warm.

Oddly enough, while it would not run without choke a day ago, it hates the choke now. Almost immediately, I have to kill the choke and the idle smoothes out a touch and levels out.

It also seems to need a healthy rpm for idle. After it smoothed out at 2500 RPM, I tried to back off the idle screw to bring it down to 1000-1500. Soon after it struggled and petered out. Then it resisted starting, with or without choke. Only when I dialed up the idle screw would it start back up, nice and strong. Rinse repeat.

I may try taking it for a couple short rides to see if some usage does anything to smooth out these kinks. Maybe that compression will level out as the rings get seated in.

Without a stable idle, I cannot use my shiny new Carbtune to tighten up the synch. Given the inconsistent compression (i.e. lower compression in No. 3), it will be pretty tricky to synch the carbs anyway, I suppose.

[mike_of_bbg]

Don't mothball the Carbtune yet. I'd sync at whatever speed I could. That could be a big contributing factor to your issues. You might find that if you can sync @ 2500, then you can bring your idle down further and sync at a lower speed. Most people sync around 2k anyway (though I've read where people sync much higher for highway smoothness) Bench sync is very imprecise - at idle those butterflies are almost completely closed; it's just to get you in the ballpark. At tkent02 mentioned, your #3 butterfly might be fully closed and not letting any air in.

[mike_of_bbg]

On the "jumper wire". If you don't think you're getting ground at the #3 plug because of the insert (I find that questionable, but let's just say), run a wire from the ground of #4 and touch it to the ground on #3. See if #3 comes alive. Wear gloves!

[mike_of_bbg]

Oh, and now that we're talking carbs I see this is a 78, which should mean VM carbs and that your petcock vacuum tap is on #3. You do have that plugged when your tank is off, right?