Engine tune-up problem: 1970 BB

[Markus 38206]

Hi together,

I have a serious problem with my rebuilt engine. Its an original LS5 engine which was rebuilt after the specs of the 1970 LS6 (including correct pistons, heads (big ports), cam, intake, valves, valvesprings,etc.). I'm using a Holley 3310 (750cfm) and the stock distributer with a Mallory unlite module inside and a Mallory coil for electronic ignitions ("stock-looking").

The problem: it's accelerating well but it starts to stammer at about 4000rpm AND IT TURNS NOT HIGHER. I would expect that above 4000rpms it should event start to really turn up. You experts out there what would you expect? Faulty ignition? Problem with the carb (not enough fuel?)In stand still it revs up over 6500rpm. Should I take a new carb (Holly avenger 770cfm or a new ignition with a bigger coil and a Hifire module?)

Thank you very much for your hints
Markus

[Chris H.]

Re: Engine tune-up problem: 1970 BB

Markus, has it always run like this since the rebuild?

I had similiar symptoms when I had a weak coil in my 396. Would half-throttle to redline, but would bog at 4000 under full throttle.

[Markus 38206]

Re: Engine tune-up problem: 1970 BB

Chris,

yes, the engine is just new rebuilt and it has never seen more than this 4000rpm under load. Could somethin be wrong with the supply of the coil? It's a Mallory MAL-29216 an I have connected it to the stock resitor wire. Should I take the hot 12V?

[Clem Z.]

Re: Engine tune-up problem: 1970 BB

look at the plugs after a full throttle run,shut down the engine while at speed while pushing in the clutch. if the plugs are dark you have ignition problem and if the plugs are white you have a fuel problem. i have also seen this caused by the heat riser valve not being able to open.

[Chris H.]

Re: Engine tune-up problem: 1970 BB

Markus, is it the same ignition system as before the rebuild? If so, did it run good before the rebuild?

Clem's idea is good.

[Markus 38206]

Re: Engine tune-up problem: 1970 BB

Chris,

no the engine did not run before the rebuilt and I have done the conversion to unlite system myself.

[Jim T.]

Re: Engine tune-up problem: 1970 BB

Mark one way to use 12 volts to the ignition, use appropiate size wire and connect one end to the terminal in the center of your fuse block that is identified as IGN. This connection will power the ignition using the ignition key.

[Markus 38206]

Re: Engine tune-up problem: 1970 BB

Thanks Jim,

I have also thought about this. What do you think, continous 12V is no problem for this coil?

[Markus 38206]

Problem solved! Engine tune-up problem: 1970 BB

The problem is solved. -> Not working alternator! Eventough the battery was good enough to start the hi-compression engine (1:11.2) the voltage at the coil was to low for proper operation above 4000rpm. Thanks for your help.