Engine will only run above 4000 RPM. Carbs or timing?

[bartsscooterservice]

L28 engine with triple SK/OER carbs, running beautifully last fall.

Transplanted the engine into 1970 240z using the same ignition setup.

Have not altered timing or carb adjustment. I did adjust the valves but only a couple were out.

When I start the engine I have to floor it to keep it going, if I let rpms get under 4000 it dies with back firing through the carbs,

new RX7 fuel pump working fine.

Help please.

Thanks,

Chris

back firing through carb, can be running to rich or to lean

ignition timing off

leaking valves ( but not assuming that )

[EuroDat]

I dont know much about SK/OER carbs, but it sounds like a mixture problem.

Like Zed Head said, with no load it should get close to red line easy with a little amount of fuel.

Thinking out of the box: could it be a blocked mufler? Causing exhaust backpressure. A rag of something that fell down the pipe and now blocking the mufler. Although this would probably let you idle but not rev so high.

Chas

[colinc]

L28 engine with triple SK/OER carbs, running beautifully last fall.

Transplanted the engine into 1970 240z using the same ignition setup.

Have not altered timing or carb adjustment. I did adjust the valves but only a couple were out.

When I start the engine I have to floor it to keep it going, if I let rpms get under 4000 it dies with back firing through the carbs,

new RX7 fuel pump working fine.

Help please.

Thanks,

Chris

Chris,

You may be able to get some "hands on" help with this by posting on OntarioZcar. I'm sure someone with the required knowledge would be willing to stop by.

Can't wait to see the car out!

[Travel'n Man]

I would have to assume your triples have a jet set up (much like my Mikuni's).

What is that setup?

[grannyknot]

Well I have tried alot of things today, none of them improved the situation.

Checked the muffler for obstruction, none. Took the carbs idle mixture screws back to base setting 3/4 up from the seat, set the static timing at 10*, no difference, tried the timing at 5* 8* 10* 12* 15* 18*, won't run.

I re-wired the HEI an alternate way as per Blue's article GM HEI retrofit, no difference. The plugs were fouled with gas at one point so removed them, blew out the cylinders, burned off the plugs.

I think I'm going to order a new HEI module & coil and start there.

Zed, I can even keep the engine going at full throttle anymore just starts and dies almost immediately.

Travel'n Man, idle jets are 60F8, main jets 120 (1.2mm), air jets have no markings,pump jet.40mm

Thanks for the help guys, process of elimination.

Chris

[Zed Head]

I think I'm going to order a new HEI module & coil and start there.

Chris

Worth a shot. Your initial wiring mistake with the extra ground might have damaged it. I damaged one when I forgot to put the one and two plug wires back on and started the engine. I turned the engine off immediately but afterward, the engine would only start with starting fluid, even when hot. I think that the module was damaged enough by stray spark or something I don't understand, to where it wouldn't generate a strong spark with the voltage available during starting. It would run just fine, it just wouldn't start. A new module and I was back to normal.

Anyway, I almost mentioned it before but your symptoms were different. Who knows.

[SteveJ]

If you have an ohmmeter, measure resistance from the positive terminal to the negative terminal and then from the center post to the negative terminal and post the results.

[grannyknot]

Zed, I hope that's all it is.

[beermanpete]

It sounds like you could have way too much fuel. Float level too high, jetting way too rich, etc. Try disabling the fuel pump and run it until the float bowls get low on fuel. If it will idle for a short time before it runs out of fuel it is from too much fuel.

[madkaw]

Still like to know if the alternator is putting out voltage

[grannyknot]

Pete, the float level hasn't been changed since since it ran like a top last fall, fuel pressure is at 3.5. But that is worth a try.

Steve, as soon as I can get the engine to run more than couple of seconds

I'll know if the alternator is doing it's job.

Chris

[madkaw]

You can test for exciter voltage with ignition on. Got to have 12 volts at the alternator to make voltage. Maybe your alt swap is not wired correctly