Top End Noob

[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?

 

[MisterCinders]

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.

My concern on the vacuum synch is not the idle speed, but the spitting, popping and missing on No. 3. I figured that the resulting inconsistency in the idle would render a vacuum synch kind of useless.

The poor idle also makes tuning the mixture screws awfully difficult. I do have a Colortune, so I will give that a try, but I don't like my chances.

 

[MisterCinders]

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?

Yes, I plugged the vacuum line when the tank was off.

 

[MisterCinders]

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!

Yeah, I am backing off that theory as well, since it does fire, just not consistently, and when it was not firing, the plug was dry.

 

[MisterCinders]

OK - decision time.

My non-firing No. 3 cylinder appears to result from botched ignition timing. I had mixed up the timing approach for the old ignition (based on the F marks) with the Dyna S instructions (advance timing).

Protip: remember to twist the ROTOR (not the plate) into the advance position when static timing the Dyna.

Anyway, all four cylinders fire, but not consistently. The No. 3 pipe gets hot, but not as quickly as the remaining pipes. I proceeded to attack mixture and carb synching.

Using the Colortune (highest idle never works for me), she's a bit rich on cylinder 1. Fuel screw at 1 turn out, no position on the air screw would get the thing to fire blue. Yellow/orange all the way. So I needed to pull and reset the carbs, since nothing I have tried lets me turn those damn fuel screws while the carbs are on the bike. I didn't move on to the other cylinders, as I needed to overhaul the carbs again anyway.

Anyway, if only for practice, I then hooked up the Carbtune to see what the synch looked like. Not very good. The wonky No. 3 cylinder made it hard to get a steady idle, so it comes up out of synch. The other three looked so-so, though No. 1 was running a bit high. With the bad No. 3 idle, it was all but impossible to get adjustments to No. 1 straight. Again, already needing to strip the carbs, I decided to revisit the synch later.

I found something was gumming the slides, so I had to pull and tear down the carbs anyway and address some sticky throttle and cabling issues (thanks Ed for the sidebar).

Right now, the carbs are bathing in poison for another cleaning. I have back up o-rings in case any of the new ones (all from Barr, natch) got mangled and were gumming the slides.

Subsequent compression tests still show No. 3 to be out-of-step. Cold numbers remain in the 125/125/90-100/125.

Here is the impasse.

I cannot figure out if the No. 3 compression problem is (a) leakage around that plug insert; (b) some unknown problem with those valves despite cleaning and lapping them; or (c) a problem with the new rings not seating or coming out of alignment when I put them in (i.e., slipping out of 120 degree offset).

I have replacement base and head gaskets and a second head and valves that I can put on this bad boy. What I don't have is a leak-down tester to determine where the rabbit-hole ends. I am especially loathe to pull the jugs, since that requires dragging my friend back out to help reinstall the pistons. That puts timing off to schedule the visit, check against restraining orders, etc.

Had no luck finding a loaner tester at AutoZone, etc., I do have a compressor. Absent buying a leak down tester myself (like this one?), is there any other way to ferret out whether I need to pull the cylinder jugs?

Recall that my clumsy (too much oil) wet compression tests showed improved numbers on no. 3. Does that confirm that No. 3 has a piston/ring problem? Or could it just be a slow-learner on those new rings. With clean carbs and a better air/fuel mix, could a vacuum synch compensate for the compression difference at all?

I am also inclined to run the thing on some clean carbs and deferring any more top end stuff until later in the year. No matter what, I am favoring the idea of changing the head and valves, since that other head has clean plug threads and better looking valves AFAICT.