1990 FXRS-SP Stock motor except mikuni hs40 carb and supertrapp exhaust. Bike has about 13350 miles. Sat for years before I took ownership. Have been riding it for about two years after getting running again. Was running fine then started running rough and popping, loss of power. Traced problem to rear cylinder not firing. Replaced entire ignition system - new stator, rotor, voltage regulator, cam sensor, ignition module, wire harness, coil plug wires and plugs, still not firing rear cylinder. This is a dual fire system. Tried swapping plug wires at plug end only still no rear cylinder. Plug is wet with gas when removed. Checked for leaks on intake manifold and exhaust ports with brake clean, no change to engine rpm. Tried pulling plugs and looking for spark when cranking. Both show spark but rear looks to be lower level. Tried another set of plugs no change. Bike will run but runs like crap on one cylinder and don’t want to cause other issues like washing fuel into crankcase due to blow by on rings, etc. I am an old ho trodder and know to drill on spark fuel air for an engine to run. I have tried adjusting timing and no improvement. I have tried three separate brand new coils one a 5 ohm and two 3 ohm. No change. How can one cylinder fire and not the other on a dual fire coil?

new ignition module is a TwinTec with adjustable curves - tried multiple settings. Definitely set to dual fire mode.

bike has been down for a few months due to this issue.

thanks
,

 

Time, (actually past time) for a compression and/or leak down test, Chief.

Both plugs out and throttle wide open when turning engine over with starter on the compression test.

 

You sound like you have some mechanical experience working on it so.

If you haven't pulled the plugs to do a compression and leak down test (which is a very good thing to do as mentioned ) The tests can tell you and or us a lot.
Spray some brake clean between the head and cylinder (jug) with it running.
Kinda like cking for a intake vacuum leak except spray were the head gasket is.

WP

 

Just ran compression test. Both cylinders at 125lbs and hold the compression for several minutes without loss until I press the bleed valve on the test gauge, so cylinders and top end would appear good mechanically. Purchased a simple inline spark plug test light and tested both plugs while running. This shows same result for both cylinders with engine running. Again new plugs and old plugs give same results. Rear exhaust pipe and cylinder gets warm but never hot. Front exhaust pipe and cylinder get hot as expected. I have done the brake clean spray test on the intake and exhaust areas of the cylinders and no changes. Per your suggestion I sprayed brake clean around the head gasket on both cylinders and again no change.

I can start engine off front cylinder only (rear plug wire disconnected) and runs the same. Engine will not start off rear cylinder alone.

I actually expected the compression test to show some kind of problem - so very surprised the test is good.

About ready to just try to find a replacement engine (not sure I want that much work) or sell the bike and move to something newer. Problem is I really like the bike. One of the best handling bikes I have ever ridden/owned.

 

Good looking bike, I'd say keep it if you like it. I half expected you'd find something there myself. Those numbers I wouldn't call good, but certainly acceptable enough it should run just fine.

Just typed (and deleted) a long reply only to go back and see it has a Mikuni. Been thru those identical symptoms before and if it were a CV, I could say with full confidence and a money back guarantee to replace the starter valve, manifold o-rings and grommet and that'd fix it.

Being it's not, I'd still wager an intake and/or carb problem. I've no experience with those carbs but the enricher is basically the same design and just because you can't pinpoint an intake leak, does not mean it doesn't have one.

 

I would definitely like to see higher compression - say north of 150, but since both are same that is what was good about the test results. Hard to see as a carb issue x fuel definitely getting into cylinder, but I may buy an intake side leak and will continue to chase that. I did replace intake seals and exhaust gaskets a few months ago thinking it was intake leaks. Unfortunately it didn’t help, but was still worth doing.

 

Even though you replaced the plug wires, maybe a bad plug wire?

 

Yes, 150 is in the ballpark of totally healthy, stock pistons, etc. but being equal, rule out that as the issue.

Those intake seals can be tricky. If there's any distortion of the flanges like twist or warped the least bit, they will drive you crazy trying to chase problems that don't present themselves as an intake leak. If the manifold spigots at the head ends are worn down on the very ends, that will let them leak. If the bung where the grommet sits is out of round more than .010 that is a problem. Also, the manifold absolutely must be dead center between the heads and perpendicular with the heads or they will leak.

With an aftermarket carb and air filter assembly, it's really easy to get the whole assembly out of alignment when bolting the a/c to the heads or if tightening the flanges with the back plate loosely bolted to the heads and carb. You need to get the manifold right FIRST, then bolt the back pate to the carb and eyeball align all that as you push it on, then shim/modify whatever you need to for the head mounting tabs, brackets or whatever it uses.

I'm embarrassed to go on like this but mine was doing the exact same thing with a CV, which are much more unforgiving about intake leaks. Start, run 15 seconds, rear cyl go flat then dead miss. I'd rev, clear out and run fine when warm. No cold spit or cough, good mileage, no spark knock, nothing. But the starter valve was (apparently) leaking by just a tick and (I assume) the front head was sucking a tiny bit of air. That made the front mix right but flooded the rear at idle speed when cold. Replaced starter valve, some better, then re-sealed manifold, all good after that.

So be meticulous about positioning/sealing the manifold. If in doubt about the flanges, they are only $8 each from a dealer. A magnetic base dial angle finder is a cheap tool to reference the manifold angle to the heads and easy enough to eyeball measure the gap between the shoulder of the manifold in relation to the flanges from the left side as you tighten them down equally. If that doesn't do it, something's off in the carb. Logically, with stock coil, equal compression it has to be on the intake side of things.... don't forget to check the vacuum hose to the VOES for a leak.

This is my last mini-novel of the day, I promise

 

When mine did that it was the coil. I swapped the plug wires at the coil and the spark moved with the wire.

 

Swapping plug wires makes no difference, cold or warmed up makes no difference. I will reseat the intake again with another set of new seals and I will get new flanges to ensure they are true and not warped in any degree. The entire design of the intake is amazingly poor in my opinion, they should have used a solid flanged edge with a solid seal similar with to the way a V8 intake works. They could adjust the gasket/seal thickness as needed, but I am not a engine designer, just an old hot rod guy that rides and wrenches when necessary.

I appreciate all of the advice and will follow up after reseating / resealing the intake.