73 sporttwin only one cylinder firing?

[Canwalleye]

Here is the testing I have done
-replaced plugs
-reset points
-took apart carb to look for obvious defects
-tested fuel pump by removing line from carb and pulling on rope pull
-tested spark
-pulled top spark plug wire while engine running. made no difference. pull bottom wire and it shuts down immediatly
-disconnected kill sw to see if it was grounding the one cylinder while running
-coils looked good

Next I am going to buy a spark plug tester, them I am going to switch coils. The plug sparks on the top cylinder when its out.

Anyone have any ideas?

 

[1946Zephyr]

Re: 73 sporttwin only one cylinder firing?

If the coils look "good" does this mean they were replaced recently? If so, did the spark plug leads get replaced too. It's okay to re-use spark plug leads with new coils, but the ends need to be trimmed to provide a new, fresh connection.

A rebuilt carb won't have any bearing on firing on one cylinder. I have seen motors with brand new carbs that fired on no cylinders. LOL

If you're firing on one cylinder, it's either:
A dirty points
B dead condensor
c bad spark lead connection
d bad spark plug boot connection
e points out of adjustment.
f bad coil

So, I would start by checking that the points are CLEAN and gapped properly. If that fails, then check the connections on the spark plug lead, at both ends. If that fails, swap the condensors and see if the spark issue swaps. If that fails, then swap coils.

 

[F_R]

Re: 73 sporttwin only one cylinder firing?

Here is the testing I have done
-replaced plugs
-reset points
-took apart carb to look for obvious defects
-tested fuel pump by removing line from carb and pulling on rope pull
-tested spark
-pulled top spark plug wire while engine running. made no difference. pull bottom wire and it shuts down immediatly
-disconnected kill sw to see if it was grounding the one cylinder while running
-coils looked good

Next I am going to buy a spark plug tester, them I am going to switch coils. The plug sparks on the top cylinder when its out.

Anyone have any ideas?

Why would you rip into the carburetor for a one-cylinder condition? The one carburetor feeds BOTH cylinders.

Never mind that, you say you have spark on the upper cylinder. How MUCH spark do you have?? Will it jump a 1/4" open air gap? If not, the coil or wire might be arcing out.

There are other reasons a motor might run on only one cylinder.

Blown head gasket

Water getting into the cylinder (exhaust gaskets)

Low or No compression on the missing cylinder. Scored piston or cylinder, broken or stuck rings, broken connecting rod, broken reed or trash stuck under a reed come to mind.

Have you swapped spark plugs from cylinder to cylinder? Just because it sparks in open air doesn't mean it sparks under compression. EDIT: Never mind, I notice you said you replaced the plugs.

 

[Canwalleye]

Re: 73 sporttwin only one cylinder firing?

I forgot to say that I did a compression test and got 70lbs on both cylinders. I took apart the carb to get a look at the reeds to see if the top one was dammaged. From what I could see it looked good. I am going to find a spark tester today and see if it jumps the 1/4inch.

Should I assume that if both cylinders read 70 lbs that they are ok?

 

[F_R]

Re: 73 sporttwin only one cylinder firing?

Yeah, good enough to run anyway.

Let me say that the most common reason for running on one is dirty breaker points. Take them apart and clean each contact individually. Polish them up, shiny bright. That may not fix it, but it certainly can't hurt.