Running out of breath at 5500 rpm

427Swede

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Jan 6, 2011
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East coast of Sweden
The 427 is running nice and strong, up until 5500rpm where it starts to misfire. Or maybe not misfire, it is more like it is running out of breath.
The Rochester carbuerator is functioning absolutely flawless below 5500rpm without any hesitation or tendencies to bog. It is totally renovated to factory 427/390hp specification with the exception that it has been compensated for todays low quality fuels with alcohol in them.

I'm thinking that my engine probably needs more fuel at higher rpm:s. The carb was built before I decided to upgrade the engine with a comp XE 268H cam and I probably should have done something about the carb.
A friend of mine drove the car yesterday with me sitting at the passenger seat and thats the first time I could actually feel the engine starting to pull like crazy at 5000rpm untill it slows down after 5500...

I have a Carter mec. fuel pump so that should not be the problem or? Could the very heavy pump shaft begin to develop a "float" condition at higher rpms?

Engine: 427 0,030 over, 9,5:1measured comp. Original, restored oval port heads. 268H Comp cam with matching comp springs and lifters. Comp roller rockers. Original alu intake. MSD pro billet dist and 6al 2 box set to 6500rpm. (A bit too high...)
 
I'll relate a story- personal experience with carb sizes on 2 different engines-.
First one, a 350- 9:1 compression, GM 3927140 cam and matching springs, 74cc heads with 2.02 intakes. I had access to 3 carbs- all Holley- a 650, a 750 and an 850. All mechanical secondary double pumpers. I started with the 650- off idle and street manners were outstanding, easy to get rolling, excellent response. It just would quit pulling over 5000RPM.
Next was the 850.. Absolute DOG around town on the street, nothing off idle, PITA to get rolling. Get it up around 2500 RPM and it would start waking up and pulling- and it would pull hard until I quit- 7500 RPM.
The 750 was fair on the street, little soft down low, but not horrible. Once the RPM got up to 2500, it pulled good up to 7000, then you could feel it nose over a little.

The other engine was a crate L-88 427- right off GM's parts counter in 1971. The 650 was crazy off idle and up to about 5000, then nothing. The 750 was a little worse down low but not bad, and picked up the top RPM to 6500. The 850 - still soft down low, but above idle it did nothing BUT pull. And I've seen the tach on the upper stop over 7000 and still pulling hard.
 
Thank's for your input!

Based on what you say my 750 cfm Rochester should be enough, cfm wise, for my engine then.
Then the stumble is probably related to fuel starvation above 5-5500rpm and it should be posibe to increase the fuel amount by increasing the jet size.
I will have to talk to the carb guy about this.
 
It could be jet size, but also could be a volume pressure problem. Make sure your fuel pump is keeping the carb filled. Disconnect the line at the carb, catch the fuel and see how much it pumps. And check the pressure. Most carbs like 5-7 PSI.
 
Verify that the rev limiter is set to 6500 as you say and not 5500....

Your 427 is pretty much rebuilt o stock except the ignition box and the cam - the cam shouln't prevent it from going past 5500 .... Do you still have a stock dizzy or can you run it without the box ?
 
I was running out of fuel with my Q-Jet, went with a Mighty Demon 750 annular, made it better but didn't solve the problem. Upgraded the fuel pump from a Carter Strip to a Holley HP (required frame modification). Solved the problem. (LM2 confirmed)
 
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Stick a vacuum gauge on it and see what the reading is at WOT. If you still any significant form of vacuum in the intake you are running out of breath.

I would check if other parts are up to running in the higher rpm range.
 
Verify that the rev limiter is set to 6500 as you say and not 5500....

Your 427 is pretty much rebuilt o stock except the ignition box and the cam - the cam shouln't prevent it from going past 5500 .... Do you still have a stock dizzy or can you run it without the box ?

Good tip to check the rev limit setting. I´m pretty sure it is at 6500rpm but I will check this.
The complete ign. system is a brand new MSD with 6Al2 box so this part of the system should be ok.
As you say, the cam is the major modification execpt from a little more compression and a mild porting job on the heads.
 
It could be jet size, but also could be a volume pressure problem. Make sure your fuel pump is keeping the carb filled. Disconnect the line at the carb, catch the fuel and see how much it pumps. And check the pressure. Most carbs like 5-7 PSI.

I will try to get hold of a fuel pressure clock and connect it between the carb/pump.
The pump is a Carter, I do not know the specification of the pump since it was the previous owner that bought it. Should have checked for numbers on it during assembly...
 
Jetting won't change the curve much, that's more the air bleeds. If it's fine at 4500 and not 5500, it's not the jets.

If it is actually draining the bowls fully, it will buck. Run it through 4 gears at WOT and see if you can get it to buck at the top of 4th gear.

I have my bored out Carter strip pump if you're interested (7-8 actual idle psi, as opposed to the 5-6 with the Carter street pump) It was fine until I went past 430whp. So prob good for 500 engine hp. Only a couple thousand miles on it.
 
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Then again, if it consistently runs out of steam at 5500, whether a short blast in 1st gear, or running through all 4 gears, then it's prob not running out of fuel.

More like a fuel curve issue or electrical. And unless the car has always done this, I'd put my money on electrical.
 
Also, you would know if you hit the rev limiter. My MSD rev limiter isn't subtle.

A lot of people seem to have issues with those MSD boxes, ask Brutal64. Dont rule it out. Id check your spark plug wires to see if anything melted to the headers or cracked too.
 
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Thank's enkeivette for your input!
At this point I will not rule out anything. Been there, done that...
Have not had time checking the ign. box but will do.
 
easiest method to eliminate some of these potential problems is running the engine with the stock dizzy setup.... do you have a stock distributor or can you "borrow" one ??
 
Yes I have the old complete ignition system. As you say, a test would eliminate the ignition or, confirm it to be the root cause. If that would be the case, what a depressing result... A complete brand new US made system not working...

Checked the rev limit setting and for sure, it was set at 6,5k rpm.

Did a little test tonight. I set the rev limit to 4k rpm and did a test run. The engine started sputtering at 4k. Set it to 4.5k rpm and got sputtering at 4.5k rpm
Repeated this with 500rpm increments and got consistent results all the way up to 5.5k rpm where I had to stop. Setting the rev limit at 6.5k rpm did not change the behaviour. Still got sputtering at 5.5k and the engine will not go past 5.5k.
The sensation of the "sputtering" is more violent at 5.5 than at lower rpm:s where the rev limit function clearly can be felt setting in. The rev limit sensation has a much softer feel to it.
 
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Here is a short movie from one of the test runs, rolling start.
I went to just a little above 5k rpm in this run so there is no significant sound of the sputter or misfire. I think you can hear a little bit of it when changing from 2:nd to 3:d.
After 4:th gear I did not continue to keep the pedal "to the metal", we were already going fast...
Anyway, you can hear that the engine is running nice without any bogs or hesitation through the rpm:s.

http://www.youtube.com/watch?v=kjQtC2mfY_U&feature=youtu.be
 
Engine sounds good in the video, it also looks like Sweden is quiet nice this time of year :D

I've wanted one of these ignition boxes for a while, main reason is that I want a rev limiter.... Your experience does not make me feel very good about the box, I've "heard" many prefer the Mallory boxes over MSD but then I sort of feel that all this stuff is made in the same low cost factory next to Magnavox TVs....

Maybe it's the distributor? It has cntrifugal advance, right? Check under the cap......
 
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