deadeyejedi

hey guys had a chance to get some drive time with the newly revamped 76 455 motor .it idles and runs great until i really get on it .it starts to stumble and pop slightly at around 4300 rpms .i know thats nearing the rpm limit for this particular motor but wondering if they're were any suggestions i could try to eliminate this.the motor has redone eheads 2.075 intake 1.710 exhausts valves dick miller cam 42-000-5 which has a lift of.476 int. .481ex. and duration of 218int. 224ex.at.050 on a 112 lobe separation.edelbrock aluminum intake,factory quadra-jet and repro wz dual exhaust .right now the timing is set at 20 degrees btdc.any thoughts?

oldcutlass

The rpms should go up to 5k with no issues. You need to see if its ignition or fuel.

Fun71

Oldcutlass is spot on.
I am curious if the valve springs were replaced when the cam was installed. Old, weak valve springs will let the valves float above some RPM (I had that happen on my first rebuild and new springs cured the problem). I had reduced voltage to the coil on my old points setup and had similar symptoms.

deadeyejedi

Yes new comp cams dual springs and the car has its factory electronic ignition . I'm going to try advancing the timing tomorrow.the fuel system was redone with new fuel lines , inline filter and a fresh carb rebuild was running perfectly before the upgrades. I'm leaning more towards the ignition end of it .thanks for the replies . I'll post my progress ,if any .

MDchanic

HEI (1976 motor) should be able to handle that speed without a problem, so it could be a bad module.

Has the HEI been updated with springs and weights to give you a timing curve that better suits the new cam?

- Eric

deadeyejedi

no eric i havent done anything to the hei system.is there a particular set of springs and wieghts i should try? or is that a trial and error thing ?

cutlassefi

Try putting another module in it. And springs and weights won't make it misfire in the upper rpm range unless you have too much total timing. You may want to check that too.

MDchanic

First, let me say that you should follow Mark's advice, and change the module before you mess with the springs. He's forgotten more about timing than I will ever know.

Second, you should disconnect the vacuum advance, then check and see whether you have about 35° of mechanical advance by 2,500 to 3,000 RPM (the exact number will be a trial and error setting, depending on your particular engine), with no further advance after that. If your advance comes in much sooner (light springs or heavy weights) or much later (heavy springs or light weights), then you would benefit from using a distributor tuning kit (from one of the big companies, like Accel or Moroso) to adjust your advance curve.

- Eric

deadeyejedi

Would the new mods cause my old module to break down ? Is there as better than stock module ?ive read 38 degrees total timing is good number is that correct . Sorry for the barrage of question

MDchanic

No. They just go bad.


The exact amount of timing advance needed for a given engine under a given set of circumstances (RPM, load, temperature) is dictated by how rapidly the flame front propagates from the spark plug through the combustion chamber, which is dictated by combustion chamber design, compression, fuel quality and octane, valve timing, etc.
38° BTDC may be fine for your engine, or it may be a bit advanced.
I would confirm the correct timing through trial and error.
Mark could probably get you to a pretty close number just by hearing how your engine was built.

- Eric

joe_padavano

I'll also chime in that this sounds like an ignition problem. Could be module, coil, bad plug wires, bad contact button in the cap, carbon track in the cap, breakdown in the rotor insulation (THAT was a fun one to find...), insufficient voltage to the HEI, or a number of other potential causes.

cjsdad

What type of spark plugs are you using? Use the best plugs you can afford! I had a SBC once that would not run right at high RPM no mater what I did. Did a full tune-up as soon as I bought it, plugs (AC Delco), wires, points, condenser, cap, rotor, carb, timing, the whole enchilada. Ran like crap above 3K RPM. Replaced the carb, same thing. Replaced the distributor with an HEI/ACCEL Super Coil, same thing, replaced the heads and cam, same thing! I was sitting on the floor muttering to myself and my loving wife asks if I had changed the plugs. "First thing I did", I replied so she says, "Try replacing them again, they aren't expensive are they?" I put in new plugs and it ran like it was supposed to. AC Rapid Fire plugs did the trick for me. A high performance engine deserves high performance plugs!

oldcutlass

Are you running aluminum heads? If so your total timing may be closer to 34.

cjsdad

gs72

X2 on the spark plugs. Have had the same experience.

TripDeuces

My vote is less than 12v to the HEI. 4300-4500rpm is where they magically drop off without sufficient voltage.
I've never seen a module 'half work'. Practically every electrical component I've seen go bad just drops out completely, go no go.

cjsdad

Just to clarify one more thing, does it cut out at higher RPM or does it cut out at full throttle? If it is strictly an RPM thing it is more than likely an ignition problem. If it happens at full throttle no matter what RPM then it points more towards a fuel problem.

deadeyejedi

So would it be pure coincidence it ran great before the mods ? I will check the voltage .

deadeyejedi

Cj I can stomp it to the floor and it pulls great up to 4300 rpms or so then starts to faulter.

oldcutlass

Are your rockers adjustable and too tight?

deadeyejedi

I did check them Eric I think it was around .040 lifter preload( would have to check my notes on that) I am using my factory push rods and rockers

oldcutlass

You can run a temporary jumper from the battery to the distributor and see if it persists to rule out voltage. In order to shut the engine off you will have to remove the jumper after your done testing.

jcdynamic88

something i have seen with similar results is the pick up tube/tubes for the secondaries (q-jet)magically fall off into the secondary wells starving it for fuel under wot.

seansolds

With the revamp, did you add an ignition box? They make 4200-4400 rev-limiters for MSD or Mallory ignition.

deadeyejedi

No ignition box . I did work on it tonight advanced the timing reset the idle and adjusted the secondary air flaps . Seems to be running better cranked it up to 5200 rpms in low (turbo 400 )no break down . Hit second and started to break down around 4800 rpms but at 95 mph didn't want to see what it would do wound out in drivev. Think it needs to be driven and fine tuned . I'll keep you guys posted and appreciate the input

deadeyejedi

do have a question regardind my distributor .its seems be maxed out as far as being able to advance it .when i turned it to advance the timing last night it came to a dead stop ?am i missing something?

oldcutlass

I don't understand what your trying to say. It stopped rotating or its hitting something?

deadeyejedi

it feels like its hitting something .i made the adjustment from the drivers side, maybe the vacuum can is hitting the firewall ?im at 20 btdc so probly no need to go much farther .

deadeyejedi

thanks for responding eric .i went out and checked with a flash light and its the vac port on the vacuum advance can bumping the firewall .if i bend it slightly it should clear .

MDchanic

If you need more advance, you can just remove and re-stab it one more gear tooth over.

- Eric

cjsdad

^ This is the best way to fix the problem. You don't want to drive around with the distributor rubbing against the firewall. If the motor mounts become weak and soft or break you could damage it and you would be stuck.

deadeyejedi

Good advice ! This would mean realigning spark plug wires as well ?

cutlassefi

I'd leave it till you actually know what's wrong. Too little timing won't make it misfire, it'll just be lazy. Too much timing however will make it misfire.
You all are putting too much emphasis on this. I believe you have other issues.

Jmo.

deadeyejedi

I am off work tomorrow so I'll run the gamut on testing .

Kennybill

It could be multiple problems. Sounds like you made some headway with the ignition but it could be running out of fuel. Change your fuel filter. I'd sort out the timing then look to your carburetor/fuel system. Change any older rubber fuel hoses.

TripDeuces

What color is the wire supplying power to the HEI and what does it look like?

deadeyejedi

Not sure of the wire color it's all original wiring . connectors and wiring are in good shape

oldcutlass

Is this happening at wot (floored) or part throttle an accelerating?

deadeyejedi

Floored and when I'm shifting manually . If I just floor it and let the automatic shift on its own the rpms won't get high enough to cause this issue .

MDchanic

If it's all original wiring, then the HEI is not getting adequate voltage.

You will need to bypass the resistor wire to run a full 12V to the HEI.

Eric mentioned jumping the HEI to the battery as a troubleshooting tip in an earlier post - this is one reason why.

- Eric