engine miss above 3000 RPM - 1275 MG engine

E

Erickz

Well-known member
My 1275 engine has a miss above 3000 rpm. Engine has 12:1 compression, non vacum advance distributor using a Pertronix coil (3 ohm) and a Pertronix LU-148 (non-advance) sensor. Plugs are NGK BR10EIX gapped at 0.025". Valve lash and timing (timing was set at 28BTDC)was done by the engine builder. Carb is a Weber 45DCOE recently cleaned. Car will start and idle (usual lumpy idle of a high lift cam) but will develop a miss as we rev it. Have replaced the cap, plugs, plug wires, recleaned the carb. We are not running an MSD box nor are we using an alternator.
Bit of a head scratcher. Any suggestions???
 
Try a different rotor button. If no change, try a different battery. If kill switch installed, by pass it temporarily.

Assume fuel pressure is good.
 
Check for stuck valves.
put a timing light on each cyl, and look for spark scatter
spray fuel in the inlets and see if it changes
 
Check grounds. Especially if you have a toggle ignition switch. Check ign switch for loose contacts/connectors. Had similar issue with FF.

James
 
Check over the ignition, but something wrong with the valves can also seem like a igintion miss.

I had a rocker sieze onto the rocker shaft, and it seemed like a miss. It would idle ok but didn't run very strong, it would work ok at lower RPM but worse at higher RPM, it would close slowly.

It could be a simple ignition issue or something else. I had a toggle switch for the ignition go bad, but it didn't run at all when that happened.
 
I assume that the 28 degrees of spark advance was set at a high engine rpm.

Test suggestion: Replace the electronic sensor in the distributor with a set of mechanical points,
and replace the coil with a Lucas sport coil (no ballast resistor).

I have seen many engines have a problem with your ignition combination, and most of the time it is due to the coil having a resistance that is too low, causing the transistor driver in the electronic control to flow too much current, thereby causing a
significant voltage drop across the transistor (not saturating), resulting in low spark voltage.

This might be the cause of your problem.

Charlie T.
 
We just measured the voltage at the battery (12.53vdc) and at the coil positive terminal (11.2vdc) - wow! How did that happen?
 
Disconnect the neg coil wire and measure again.
test with a jumper wire. make sure that you have a resistor if it is supposed to have one.

Timing light can also show a stuck valve. . Shine it on the valves running and you can see the defective action (as long as most are correct.)
The light should stay fixed on the front wheel. make 2 TDC marks. one for each pair.
 
It lives!! The venturi in the forward carb chamber (Weber 45DCOE9) was installed incorrectly - it was inverted. I would like to give credit to the guys at Pierce Manifold for coming up with the correct diagnosis.
Thanks to all for your help and interest.
 
You must log in or register to reply here.
Back
Top