Top End Noob

[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.