Fighting an ignition, carb, fuel issue

[LOOMER]

just had motor rebuilt 79 2f. putting it all back together but cant get it to smooth out. it acts as it missing.

what ive tried:
Fresh carb rebuild
A new coil
Another dizzy
Replaced igniter with Gmc igniter just for test
Cap and rotor
Spark plug wires
Spark plugs
Checked pvc
Vaccum is good
No air filter


Even though it seems as a timing issue i think i will try fuel filter next.

Any input appreciated.
Thx.

 

[907FJ]

just had motor rebuilt 79 2f. putting it all back together but cant get it to smooth out. it acts as it missing.

what ive tried:
Fresh carb rebuild
A new coil
Another dizzy
Replaced igniter with Gmc igniter just for test
Cap and rotor
Spark plug wires
Spark plugs
Checked pvc
Vaccum is good
No air filter


Even though it seems as a timing issue i think i will try fuel filter next.

Any input appreciated.
Thx.

Since you are checking fuel, I assume you have fuel in the sight window of the carb? The filter may be clogged or intermittently clogged if you have cleaned it before. This could be due to crud in the fuel tank. A quick inspection of the filter should rule this out if it is clean. If it is clean, this could be a fuel pump issue. Is the pump stock or electric?
Double check each spark plug wire for a tight fit. Is it missing but also able to rev high, or does it not want to rev past a certain level? could need to retard the timing a bit.

 

[LOOMER]

Mechanical fuelp pump and fuel half way up in sight glass. timing seems dead on except for the miss.
It will rev up fine jst sputters a bit

 

[Pin_Head]

Stop guessing and do some checking. Start with a compression check and report back the numbers.

 

[907FJ]

Have you tried adjusting the air / fuel mixture a bit?

 

[edwjmcgrath]

Check the compression as pinhead suggests, also put a timing light on each plug wire and see if the miss corresponds to a missing flash. This will tell you if something is wrong with the rebuild or if there's something wrong with ignition.

 

[LOOMER]

Compression numbers are 151ish across the board.

ive put the timing light on ea. Wire. good there.

ruling out fuel filter as there is constant fuel in sight glass. is there somewhere i should blow out in the carb.

Iam not really throwing parts at it as ive tested everything to my capabilities. but swapped some parts around that ive had or that or cheap.

I am stumped.

 

[edwjmcgrath]

That's all good. When is it missing/sputtering? At idle, coming off idle, high speed, under load?

 

[LOOMER]

At idle it has a miss. Iam still leaning towards fuel but I don't know why or where.

 

[Pin_Head]

Missing at idle is almost certainly not an ignition problem. Ignition problems typically show up at high load or high RPM or are all the time.

Probably insufficient idle fuel or a manifold vacuum leak.
Spray some propane gas from an unlit propane torch into the air horn and see if the miss goes away. If so, it confirms a lean mix.

 

[LOOMER]

Sometimes it is the stupid things. i am giving up for the night but am leaning towards non vented fuel cap. i only hope i could be so lucky. went to put more gas in it and could barely get cap to bust loose.

Put fuel in it and left cap off ran alot smoother. going to restart carb adj tom. i am keeping fingers crossed it could be something so stupid.

 

[Pin_Head]

Probably not and if so, it is probably coincidence.

 

[LOOMER]

Case closed.

i feel like an idiot. had a air leak. the secondary butterfly stuck open. pulled carb off trimmed up the gasket and it now opens and closes freely.

Thanks for all your help.

 

[Pin_Head]

They should all be that cheap and easy to fix.