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

[grannyknot]

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

[Diseazd]

Sounds like low compression.......do a compression check. I bent some valves once (over revved) and it wouldn't run under 2500 rpm's.

[madkaw]

Vacuum leak?

[grannyknot]

cyl #1-215lbs 2-200lbs 3-210lbs 4-210lbs 5-215lbs 6-215lbs

that's on an ice cold engine

[grannyknot]

Steve, I don't know where a leak could be, intake manifold gasket is new and bolts are tight, rubber seals from manifold to carbs are new and bolts are tight.

There is one vacuum take off on the #5 intake manifold tube going to the brake booster and the hoses are new but I'll clamp off that hose and try to start it.

[olzed]

Check the firing order/ plug wiring. Sounds as though you may have a couple of wires mixed

[Zed Head]

Do you have to have it floored to get it to start or just floor it after it kicks over? Does it sound right at 4000 RPM or does it run rough?

Backfiring is often from a lean mixture (as suggested re vacuum leaks). I'm not familiar with SK/OER carbs, but if there's some way to increase the "choke" it might offer a clue. Maybe your mixture is just off at low air flow.

You could also have someone standing over the carb with starter fluid or carb cleaner (the petro-based stuff not the new "green" stuff) or a small cup of gasoline to feed in while you start it. If it's mixture based they should be able to keep it running by dribbling or spraying in more fuel. Of course, a backfire on to a cup of gasoline wouldn't be good. Just pour a little in, back away and see if it stays running longer.

Brainstorming. How about a bad or disconnected brake booster?

Edited April 20, 2013 by Zed Head

[grannyknot]

olzed, checked again, ran my finger along each spark plug wire 153624.

Zed, no I don't have to floor it to start it, no pedal at all also no choke just like the last Z it was in. When I do pull the choke in the engine won't start.

It starts beautifully right away with no pedal and no choke.

It sounds great at 4000 rpm, no hicups no misses.

The SK/OER triples are basically Weber clones with a few added features that make them good for the racing crowd.

I also clamped off the brake booster hose with no effect .

I'm staring at this sucker wracking my brain as to what is different now from last fall. During the restoration I have made some electrical changes,

new 60amp alternator from MSA as well as their custom adapter plug.

Also, the side marker conversion, parking light upgrade and headlight relay upgrade but I cant see how this would affect the engine.

Even if I somehow screwed up the alternator wiring the engine would continue to run just using the battery wouldn't it? And yes the battery is new as well.

Edited April 20, 2013 by grannyknot

[grannyknot]

Just thinking out loud here, air, fuel and spark are all you need to make an engine run. Lots of air, K&N filters with no obstructions. Fuel, even if the new fuel pump wasn't working and it is, the float bowls are full, I can see it with the jet cover off so there would be enough fuel for a 1/2 min. or so.

Spark, it starts and runs at high rpm, could there be some condition where the spark is less at low rpm or after the key is released?

Hope I'm not chasing my tail.

[madkaw]

Have you actually measured voltage?

[grannyknot]

No I haven't but I'm not sure how I would do it for 2 reasons,

I'm sitting on the floor boards with my foot on the pedal but even if I had some one else to do that I'm not sure where I would start measuring with a multimeter in the seconds I have before she dies. These are the problems of an electronic deslexic working by himself.

Steve, I might be able to remote start the engine and clothes peg the throttle open enough to keep it going, were exactly would I measure the voltage while it's running?

Edited April 20, 2013 by grannyknot

[Zed Head]

Still not clear what happens between the engine starting and how it gets to 4000 RPM.

Sounds like you turn the key with no throttle applied, it starts, then will die unless you give it full throttle. I know that just a small amount of throttle with no load will get 4000 RPM easily. You're saying that you have the throttle to the floor and all you get is 4000 RPM? Do you have an air filter attached? Something's weird with the facts of the problem.

If you want to measure voltage, maybe you can set the meter on the cowl where you can see it through the windshield. Clamp the probes on to the battery posts and start the engine. Above 12.6 means the alternator is charging.