mystery skip

L

larry harding

Senior Member
first off i just turned 81 and have tinkered with small engines, hit and miss , motorcycle and car engines since very young. i don't claim to be any kind of an expert, but i'm not without some experience.
here is my delima... i built a speedster a couple of years ago and wanted a hotter engine. here's what i, working with
standered "a" block .060 over new rings, valves (oversize intakes), modern guides and adjustable tappets
line bored with inserted rods and mains
burtz 6.5 head
reds headers
12 v. coil with internal resistor
riley intake with two holley 94's(new kits) currently 45 main jets, plugs instead of economiser jets
brierly cam, next one above the late b cam rough idle normal but not a dead skip i hope.
75 psi compression on all cylinders
the problem is 3 and 4 are in a dead skip at idle, but seem to cut in at higher rpms, and feels really fast at high rpms.
i've tried different plugs. dist. cap and rotor button, nothing cures the low speed skip. i'm sure there is something really dumb that i'm missing, but i'm really stumped at present.
 
You possibly answered your own question. You installed a hotter cam hoping it wouldn’t skip but now it does skip. I would ask Jim (He’s on here and will probably see this post).
 
3 and 4 also have a different Holley. So it could be something there. Test for a vacuum leak first. Try moving the Holley's front to back and back to front to see if the skip follows.
 
The measured compression pressure depends on many factors, the primary one being the compression ratio. But the cam grind can have an effect too. Depending on when the intake valve closes some pressure is lost at the very low rpm during a compression test.
 
The engine should be warmed up and the carburetor should be locked in the full throttle position in order to get a more accurate compression gauge reading. I have also read that only one spark plug at a time should be removed and that cylinder checked instead of removing all four plugs and running the test. The idea behind that for the Model A engine is the shared "Siamese" intake ports in the block. 'Dunno about that, but I do one plug and one cylinder at a time anyway.
If I ever get the energy, I might try a compression check with the engine cold, the carburetor throttle position left at the idle and all the spark plugs removed. I wonder what kind of comparison will result versus testing the traditional way? May 5-10psi??? More???
Marshall
 
i checked the compression a while back and was getting 100 psi on each cylinder . at idle i.m only using the front carb and progresive linkage to open the rear. ive tried switching carbs, even completly pluging off one , no help. i've had the manifold off and back again... new gaskets, so intake leak not likely. no matter what i've tried 3 and 4 are dead at idle. i may try contacting mr brierly next week and see if he has any ideas.
 
with two carbs on separate runners you may not be able to run progressive successfully. the open plenum that connects them may not be good enough at low speed to compensate.

those carbs may have to open simultaneously and be close to balanced or same specs. like a twin carb MG

lets see a picture of your setup
 
I would look for a vacuum leak at the rear port, sometimes the header will keep the gasket from sealing a port. My 1R cam shouldn't cause such a problem, I've run that cam a lot, with no problems. Make sure the header isn't touching the intake manifold. Good luck!
 
Hmm. Firing order 1-2-4-3 wired correctly? Points opening/closing correctly on every lobe of the distributor cam? Spray carb cleaner around the rear intake gasket area to see if it is leaking.
A higher performance camshaft does not cause a skip on the same one or two cylinders. It will cause an uneven idle and all cylinders will be affected equally and randomly.
Compression is properly checked with a warm engine, all spark plugs removed and throttles wide open.
Sometimes these deals make us pull our remaining hair out! Gotta keep after it logically though. :)
 
Since the OP said that they kick in at higher rims, what old ugly is saying makes sense. Hook up the carbs simultaneously instead of progressively could very well solve the problem.
 
o.k. guys, a case of dumb a... has reared its head. i took the carbs apart and found i had used the wrong gasket on the economiser plug. i'm going to make new gaskets from some nylon rod i have. the gaskets i found in the kits were wrong and were probably letting a lot of gass flood the manifold at idle speed. ( i was putting out black smoke at idle.) i'm going to do some other work then try the carbs again in a few days....wish me luck!
 
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