Stutter at 4100 rpm
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Joined: Jan 2013
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Stutter at 4100 rpm
Hi All,
I'm pretty new to all this. I have a stock 68 427/390 4spd.
Last year, I had my carb rebuilt by Lars and dialed in the timing per his doc. After that, it was like a new car performance wise. I still have a lingering issue though. At around 4100 rpm when in 2nd gear or higher, the motor stutters. Power falls right off, and it probably won't rev at all beyond 4900 or so.
My car has a stock distributor, with an electronic points replacement. It looks to have an aftermarket adjustable vacuum advance.
The car runs and drives great... up until 4100 rpm.
Any thoughts on what I should look at?
Thanks for all the help!
I'm pretty new to all this. I have a stock 68 427/390 4spd.
Last year, I had my carb rebuilt by Lars and dialed in the timing per his doc. After that, it was like a new car performance wise. I still have a lingering issue though. At around 4100 rpm when in 2nd gear or higher, the motor stutters. Power falls right off, and it probably won't rev at all beyond 4900 or so.
My car has a stock distributor, with an electronic points replacement. It looks to have an aftermarket adjustable vacuum advance.
The car runs and drives great... up until 4100 rpm.
Any thoughts on what I should look at?
Thanks for all the help!
Pro
Joined: Aug 2013
Posts: 591
Likes: 3
From: Monroe Washington
Old worn out vacuum line for the advance collapsing under pressure?
Add a vacuum canister to provide MORE vacuum when needed?
Shooting from the hip here... Just throwing out ideas... I don't have a big block and am not privy to your specific needs... Just thinking out loud.. ;-)
Add a vacuum canister to provide MORE vacuum when needed?
Shooting from the hip here... Just throwing out ideas... I don't have a big block and am not privy to your specific needs... Just thinking out loud.. ;-)
Thread Starter
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Joined: Jan 2013
Posts: 54
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Do symptoms like this sound timing related? Could it be getting too much vacuum advance? Or not enough?
I just don't know enough about this to recognize the symptoms.
Probably would be a good idea to change the plugs, probably hasn't been done in 8 years or so (before I had the car). Is there a particular recommended/popular brand of spark plug people use on these?
It does have a new MSD coil and wires.
I just don't know enough about this to recognize the symptoms.
Probably would be a good idea to change the plugs, probably hasn't been done in 8 years or so (before I had the car). Is there a particular recommended/popular brand of spark plug people use on these?
It does have a new MSD coil and wires.
Le Mans Master
Joined: May 2002
Posts: 5,086
Likes: 786
From: Lebanon Township New Jersey
Does this happen with wide open throttle?
If so, try it at part throttle to see what happens.
What happens in 1st gear at 4100?
If only at WOT, in any gear, probably fuel related. Could be a bad pump, float level in the carb too low, pickup sock issue in the tank, gas filter, etc.
If so, try it at part throttle to see what happens.
What happens in 1st gear at 4100?
If only at WOT, in any gear, probably fuel related. Could be a bad pump, float level in the carb too low, pickup sock issue in the tank, gas filter, etc.
Le Mans Master
Joined: Apr 2011
Posts: 6,284
Likes: 1,171
From: Hermosa
Since you have a carb built Lars, that then can be eliminated as a cause of the problem. You've got new wires and HEI? ignition. So it sounds like you've worked on that end.
I would go with fuel delivery to the carb, Like BK said tank sock or fuel pump.
Just re-read your post, so not HEI just electronic pick up of some sort? Did this problem exist prior to that electronic pick up or is that the way you purchased it?
I would go with fuel delivery to the carb, Like BK said tank sock or fuel pump.
Just re-read your post, so not HEI just electronic pick up of some sort? Did this problem exist prior to that electronic pick up or is that the way you purchased it?
Last edited by REELAV8R; May 13, 2014 at 04:30 PM.
Thread Starter
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Joined: Jan 2013
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In 1st gear, it usually pulls right through, maybe a minor stumble at 4900 or so.
It only stutters at WOT. At part throttle, it revs right up through.
I replaced the fuel filter and the lines in the engine bay a couple months ago, using the correct and too-expensive filter. The little filter in the carb was replaced about 8 months ago, when Lars rebuilt it.
The ignition is a Pertronix points eliminator kit, if that matters.
What should I expect to see for fuel pressure? Where should I take that reading?
It only stutters at WOT. At part throttle, it revs right up through.
I replaced the fuel filter and the lines in the engine bay a couple months ago, using the correct and too-expensive filter. The little filter in the carb was replaced about 8 months ago, when Lars rebuilt it.
The ignition is a Pertronix points eliminator kit, if that matters.
What should I expect to see for fuel pressure? Where should I take that reading?
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Tech Contributor
Joined: Aug 1999
Posts: 14,380
Likes: 6,391
From: At my Bar drinking and wrenching in Lafayette Colorado
Sounds like fuel starvation. You don't want to be running two filters. Remove the in-carb filter if you have an in-line. If the car still falls on its face, install a small electric boost pump back near the tank.
Lars
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For posterity, I figured I'd close the loop on this and outline how I fixed it:
1. I replaced the soft fuel line from body to fuel pump. I think it was collapsing at high RPM.
2. I replaced the fuel pump.
3. I removed the small fuel filter inside the carb. It was all crushed and ruined. I didn't replace it, as I have a new OE style fuel filter and hard lines.
4. I found that the distributor rotor wasn't tight, and could actually flutter a bit as it spun. Tightened that down.
5. I replaced the spark plugs. Old ones looked pretty good though.
I'm not sure which of these steps actually solved the problem, because I did them all at once, but the problem is solved after performing these steps.
1. I replaced the soft fuel line from body to fuel pump. I think it was collapsing at high RPM.
2. I replaced the fuel pump.
3. I removed the small fuel filter inside the carb. It was all crushed and ruined. I didn't replace it, as I have a new OE style fuel filter and hard lines.
4. I found that the distributor rotor wasn't tight, and could actually flutter a bit as it spun. Tightened that down.
5. I replaced the spark plugs. Old ones looked pretty good though.
I'm not sure which of these steps actually solved the problem, because I did them all at once, but the problem is solved after performing these steps.
Last edited by Fendermb4; Sep 12, 2014 at 11:18 AM.