Hello, New to the forum but not to bikes. I've had over a dozen bikes over the years and always do the work my self from full engine rebuilds to simple oil changes. My current situation in a head scratcher so thought I should ask the people that know that most about this model. It's a 1978 Shovelhead with no crazy mods other than the dual plugs per head setup but the second set of plugs have not been hooked up for over a year. Bike was running fine but had an issue with the horn and right blinker so swapped out the horn and signal relay and they started working fine. When I started her back up I noticed an odd idle and that's when I figured out the rear cylinder wasn't firing. I went through the basics and made sure there was spark, compression, and fuel. The compression was only a few psi difference from the front, fuel was flowing and there was spark. The spark seemed a little weak so figured that must be it since under compression a weak spark can equal no spark. Had been thinking about switching to digital ignition and getting rid of the points so I went ahead and did it. Just finished about 20 minutes ago and the spark is way stronger so thought I would be all set. Started her up and still no rear fire. Is there a Shovelhead secret I'm missing out on lol? Any help anyone can provide will be greatly appreciated. Thank you for reading my thread

 

its more then likely a

bad sparkplug
ignition wire has gone away
the coil has also gone away

getting spark with out compression is far different then with compression < the compression load applied to the ignition system as a whole is challenged and reacts differently when something is a weak link

 

Thanks for the reply but like I mentioned I just finished installing a full digital ignition system to fix the spark issue. Even used a pulse ignition checker to confirm spark strength.

 

try swapping plug wires or plugs or coils to see if the problem moves to the front cylinder. that'll tell you what the problem is

 

I swapped everything but it didn't help and the compression is practically the same in both. I know it's getting gas but I even sprayed starter fluid in it and still nothing. I don't know how the valves could randomly get that far out of time to not even get a little pop when it was running before I worked on the horn and blinker
but I'm going to take the pushrods out and make sure everything's adjusted right.

 

you did not say the valve train was making noise - you said the compression test confirmed they were almost the same the two cylinders - why would you then take apart the pushrods

your missing the issue - and you said you have owned this sort of machine for quite some time - remove the carb and intake manifold and re seal and gasket all of it - a cracked o ring can cause this -- but in your explain the symptom went from one cylinder to neither

you sprayed starting fluid - why would you do that a back fire would have lit the bike up in flames

stop over thinking this - what is the ignition you installed and how did you find the front cylinder was the one you set the timing on - being 180 out the engine will not start and might not do anything in a re start no sound no kick back nothing some do but I have seen some don't

post up some info

 

Compression is few psi difference because when Ya tested it,one cylinder was hot,the other cold.( My guess)
Single fire set up? Brand?
What Id do is use some rubber handled pliers...hold the the plug wire cap barely off the spark plug,
(While its running) see if that makes it fire..
Check the intake manifold for leaks also.

 

check your ground wires...

 

Been out of town with family so haven't been able to to do anything. Disregard my pushrod comment, that was me "thinking out loud" and it didn't make any sense after I reread what I posted. The issue is just with the rear cylinder never with the front. Not sure what I said that made it seem like it went from one cylinder to neither but it's definitely the rear cylinder. I will try real sealing the intake. The ignition is a Ultima programmable unit but that's not the issue because I installed it after the problem started. It was something I had been wanting to do so I just did it since I was already taking things apart looking for the rear cylinder issue. I have owned various vtwins but most of my motor wrenching experience over the years has been on 4 cylinder bikes and with everything that I've tried so far I'd have the 4 cylinder running fine by now. I guess that would be why I'm over thinking it. Ill try resealing the intake and go from there

 

I'm thinking along the theory of johnjzjz you may have timed to the wrong mark on the fly wheel. I know this problem was present before the change of ignition but you may have cured it with parts , then reintroduced it by timing to the wrong mark.