Ugh. My new Shovelhead not firing on rear cylinder?

[Superman27e]

I just picked this 79 Shovelhead up 2 days ago. Owner said carb needed to be cleaned (mikuni). Got it home and started it with starter fluid (bad mistake). The next day I actually cleaned the carbs and put fresh gas in it. I rode it for about 30 mins straight with no issues. Parked it for the night. I tried to ride it to work and got half way there (15 mins) and it started to misfire. Sounded like it was bogging down, so me, thinking it was the carb and tank and hoping it wasn’t missing, took it home. I cleaned the carb, tank and got new plugs. Started it up, and it sounded the same. I felt the exhausts and the exhaust to the front cylinder was hot, the rear was not.

This bike Is not stock
93 ci
Big bore jugs
SS piston rings
Duel plug SS heads
Single fire ignition
Andrews cam
Solid lifters
Roller rockers


Things I’ve done so far
Cleaned carb and tank
Put known working front plug wires on rear and rear plug wires on front. Same deal. Front cylinder exhaust hot, rear is not.

Any suggestions?

Edit: Forgot to mention that as it was bogging on the way to work, it was intermittently run fine again for at most 2 seconds. It this a few times before never running on both cylinders since then. The carb also pops a bit as well.

[47str8leg]

Sounds electrical. is it points or electronic ignition ? Kickstarter still on it. Just asking before giving you some general troubleshooting .

[JBinNC]

Your symptoms are of an ignition module going bad.
But, one thought: when replacing the plugs, are you replacing all four?

Jim

[Superman27e]

Electronic ignition and yes, Kickstarter still on it.

[Superman27e]

Your symptoms are of an ignition module going bad.
But, one thought: when replacing the plugs, are you replacing all four?

Jim

Yep. I replaced all four.

[Superman27e]

I just found out that one of the coils has no fire on both coil wires, but the other coil has fire on both coil wires.

[JBinNC]

Good first step would be a compression test (throttle fully open).

Jim

[JBinNC]

I just found out that one of the coils has no fire on both coil wires, but the other coil has fire on both coil wires.

Does each coil serve one cylinder, or is it set up as a wasted spark system with wires from each coil going to each cylinder?

Jim

[Superman27e]

Does each coil serve one cylinder, or is it set up as a wasted spark system with wires from each coil going to each cylinder?

Jim

They both share a cylinder. Which makes me think it would still run on both cylinders, but maybe I’m wrong?

[JBinNC]

They both share a cylinder. Which makes me think it would still run on both cylinders, but maybe I’m wrong?

If each coil has a wire to a front and a rear plug, it should run just fine on one coil. What brand and type of ignition is it, can you post some pics? Channeling Tattooo here.

Jim

[Superman27e]

I’m not sure if the brand, but it’s a single fire ignition.
I’ll have to get the pics tomorrow when I take the ignition cover off.

[47str8leg]

Not a dual plug guy but it doesn't sound right to me that each coil is serving both cylinders. Obviously it worked ( at least in the short term ) but when you bought it did he say how long the ignition system was on there ? If not , it could be fucked up from the get go and burned out a coil. Why only one cylinder would be affected on both coils I honestly don't know unless both coils are now burned out in the same fashion .If you can borrow or have another coil I would disconnect the dual coil setup temporarily and run it off one coil.

[JBinNC]

Using a wasted spark system for a dual plug bike is the most efficient way, but rarely done with the single fire systems that are popular today. There are reasons for this that involve a lot of physics. Suffice it to say that fuel bikes, where ignition is PARAMOUNT, used dual, waste spark systems for many years, and many still do.

Jim

[47str8leg]

Using a wasted spark system for a dual plug bike is the most efficient way, but rarely done with the single fire systems that are popular today. There are reasons for this that involve a lot of physics. Suffice it to say that fuel bikes, where ignition is PARAMOUNT, used dual, waste spark systems for many years, and many still do.

Jim

I thought that's what I might here and it makes perfect sense in drag racing . To isolate a possible coil problem couldn't he just then hook the ignition wires up the way they are in the diagram and see if either cylinder is cutting out ? If it still is ( which I suspect) he can then reverse them and see if the opposite cylinder cuts out ?

Edit . If one of the coils is not firing it tells me the problem is electrical . It might still be able to run on one but isn't because a wire might be getting shorted to the coils themselves or loose jumper between the two coils depending which coil got fed first.
Being that Im a points guy I guess I'd check battery voltage for the fuck of it. Don't know how that effects an electronic system .

[Superman27e]

I thought that's what I might here and it makes perfect sense in drag racing . To isolate a possible coil problem couldn't he just then hook the ignition wires up the way they are in the diagram and see if either cylinder is cutting out ? If it still is ( which I suspect) he can then reverse them and see if the opposite cylinder cuts out ?

Edit . If one of the coils is not firing it tells me the problem is electrical . It might still be able to run on one but isn't because a wire might be getting shorted to the coils themselves or loose jumper between the two coils depending which coil got fed first.
Being that Im a points guy I guess I'd check battery voltage for the fuck of it. Don't know how that effects an electronic system .

Hmm, I didn’t think about the battery. Could a weak battery cause a weak spark in only one coil?

I talked to the PO last night and I believe he said he has had it on there for 5 years. He also said he still had the old points system along with a bunch of other stuff if I wanted to come pick them up. I forgot to get it when I purchased the bike a few days ago. Only problem is that it’s 2 hours away.