dkraus71cutlass

I have an 69 455 currently running a demon 750 carb with a Mallory 6a high fire ignition, offenhauser intake,carter mechanical fuel pump, high compression pistons bored 30 over, pretty mild cam, and mostly stock heads.


My issue is the car will sputter when you give it gas on take off, but will immediately clear up through the gears until you get up to cruising speed say (60mph). Then has a slight skip in the rpm like a sputter. I know its not the carb accelerator pump because the carb is brand new and adjusted properly. Not sure if this issue is because of plug gap (currently .030) or possibly the plugs are not hot enough? Another thought is that its not getting enough fuel. The mechanical fuel pump that's on it is 30 gph thinking it might need a little more not sure? Debating on putting an electric fuel pump. Been fighting the prob for little bit. Let me know what you guys think.

oldcutlass

What distributor are you running and what are your dwell (if applicable) and timing settings?

dkraus71cutlass

running a Mallory unilite distributer with a vacuum advance. Not sure on the Dwell? Timing is pretty good me and my buddy put a timing light on it when we were tuning the carb. Couldn't tell you the exact degree off hand.

oldcutlass

No dwell on the Unilite its electronic. Check to make sure the accelerator pump is giving a squirt as soon as you move the linkage. Double check your timing. Also check your total timing with vacuum to make sure your not over 50 degrees.

dkraus71cutlass

ok gottcha, I don't think its in the carb I had the same problem with edelbrock carb previous to the demon that's on there now. Just put the demon on there last summer. Yea we can double check it again need to put on a new harmonic balancer anyway. (outer ring slipped) But we went through and found tdc and remarked it when we had the timing light on adjusting it with the carburetor.

shiftbyear

I've heard horror stories on new carbs, most seasoned engine builders disassemble new carbs and check for mistakes. also check the manufactures website, most offer tuning help for their carbs. good luck

Fun71

If it sputters from a dead stop then it is not likely the fuel pump as the float bowls should be filled.

dkraus71cutlass

Well I have an clear inline filter before the carb and it is maybe a quarter of the way full. I've never seen it all the way full witch is where I'm getting the weak fuel pump idea from

TripDeuces

Proper shooters in carb and pump cam?

droldsmorland

Read the plugs. What are the 3 timing events set to? Verify fuel pressure and vacuum at idle. Does the Unilite have a vacuum advance canister? If so check the wires on the hall effect plate. They can be chafed and grounding out when the advance moves the plate.
Is the engine properly grounded? The Unilite system WILL blow the module if the ground from the block to the battery is compromised. Wheres your battery? In the trunk? If so you need to ground the trunk cable to the frame, then put a cable from the engine block to the frame. #2 or larger cable on both. Clean and tight followed by battery post anti corrosion spray. Are you running the recommended Ohm resistor in the positive side of the coil?
My money is on the ground or lack of Mallory recommended resistance on the positive side.
Dont ask how I know this.

Fun71

The inline fuel filter rarely ever fills entirely so that is not an indication of a weak fuel pump/low fuel pressure. If you really think there is an issue with the fuel pump, put a gauge on it and actually measure the pressure instead of guessing.

TripDeuces

I have a Unilite and I know it only needs to be resisted down once. In other words the stock resistance wire. If you have the inline ceramic one installed also that is your problem. Remove it.

oldcutlass

He has it hooked to a 6A box, so no resistor wire.

4speed455

You might want to reconsider your fuel filter. It's not causing your problem but could cause a lot worse.

https://classicoldsmobile.com/forums...l-filters.html

droldsmorland

The transient voltages that float around seeking a ground where a good one is not present will go at the electronics first, each and every time. I've see transients take out the hall switch, the module, 6a and 6al boxes. Mostly the modules. They either go completely dead or they act up slowly as the OP states until death.
Pay attention the brown wire off the distributor. Make sure its clean tight and attached to the block not the firewall. Also verify the ground on the engine block to fire wall and battery to engine block is good. Cant hurt to run one from the frame to block either.

dkraus71cutlass

Ok guys thanks sounds like I may have a ground problem lol. Im.gonna go through and clean up all my grounds see if that helps.

jeure

Any progress on troubleshooting this? I have a 455 that is getting replaced, but it has exactly the same issue. Doesn't matter about fuel - have used a 750cfm carb as well as TBI injection. I am stumped, but if you have some insight from your hunting, please let me know as I plan on using this engine on a later build.

dkraus71cutlass

Well I found my ground from my block to the body was not attached, and I put new plugs in and double checked the gap before installing them. Running .40 gap instead of the stock .30 due to the hei mallory ignition. Seemed to clear it up a little from what I could tell in the shop, but haven't had a real chance to drive the car yet due to left over salt on the roads. Did just put an electric fan and a one wire higher output alternator on gonna play with the engine temp a little see if maybe I was running the car a little cold. Was running a 160 thermostat with a straight blade fan.

droldsmorland

If all else fails git er up to 120mph on 54a and blow through the curve past Esperanza Mansion ...bail out before it hits the water...lol
.040" gap is good. Did you verify that the ground from the distributor is there and has a clean connection? Did you inspect under the cap to see if the wires were rubbing on the "breaker" module plate when the vac advance moves?
What did the plugs look like?

dkraus71cutlass

Yea lol electrical fire has to put it out in the lake lol I did check the ground to the distributor that one looked pretty good as well. Spaced a little on checking the ones under cap, got a little wrapped up in getting the fan installed lol. The plugs that were in it looked pretty black BUT smelled like oil not gas I'm thinking that the valve guides might be weeping a smidge from sitting all winter. I'm gonna check the distributor connection when I put on the new harmonic balancer then prob drive it in the next week or so hopefully see how it runs.

dkraus71cutlass

Update! So I think I got it finally....took the car out this weekend for a little warm up stretched her legs a little. She is purring like a a fricken jungle cat lol. Between changing the gap to .040 and finding that ground to the body I think that did it. Dropped it into first and stomped it brought the dam thing around and faced it the other way in the road lol thanks guys for all the advice.

oldcutlass

Glad you got it.