High speed crap out
Thread Starter
Safety Car
Joined: Sep 2005
Posts: 4,212
Likes: 643
From: Orange County Ca
High speed crap out**update**
My '69 390 hp BB runs pretty well. But lately I've developed a major problem.
If I accelerate softly, I can get over 4k rpm with it running smoothly on just the primaries.
But if I punch it and the q jet secondaries open at about 2500 rpm, it takes a major dump at about 4k rpm and won't go over 4.5k rpm. Lots of noise, back firing through the side pipes, bucking and snorting, etc.
I've changed the points and condenser, the cap looks good, the wires are fairly new and the plugs look good.
So, now I'm thinking it must be the fuel system.
What should I look at on the carb? Any linkages, shafts, etc that could use a good dose of carb cleaner?
Run some Techron through it?
The secondaries seem to open and I get a power rush from 2.5k rpm to 4k rpm.
Fuel starvation with my stock mechanical pump.? How would I check this?
What do you think??
If I accelerate softly, I can get over 4k rpm with it running smoothly on just the primaries.
But if I punch it and the q jet secondaries open at about 2500 rpm, it takes a major dump at about 4k rpm and won't go over 4.5k rpm. Lots of noise, back firing through the side pipes, bucking and snorting, etc.
I've changed the points and condenser, the cap looks good, the wires are fairly new and the plugs look good.
So, now I'm thinking it must be the fuel system.
What should I look at on the carb? Any linkages, shafts, etc that could use a good dose of carb cleaner?
Run some Techron through it?
The secondaries seem to open and I get a power rush from 2.5k rpm to 4k rpm.
Fuel starvation with my stock mechanical pump.? How would I check this?
What do you think??
Last edited by Sayfoo; Dec 21, 2020 at 06:35 PM.
Le Mans Master
Joined: Jun 2019
Posts: 7,640
Likes: 2,460
From: Indiana
Fuel pressure and volume test is how i diagnosed my pump.
its in the manual..ill look for volume but remember 7-8 psi
edit my 79 says 7-8.5 psi and one pint at 30 seconds..i had 7 psi but only 1/2 pint at 30 seconds..new pump 8 psi and 2 pints easy
post 69 if bored
https://www.corvetteforum.com/forums...on-road-4.html
its in the manual..ill look for volume but remember 7-8 psi
edit my 79 says 7-8.5 psi and one pint at 30 seconds..i had 7 psi but only 1/2 pint at 30 seconds..new pump 8 psi and 2 pints easy
post 69 if bored
https://www.corvetteforum.com/forums...on-road-4.html
Last edited by interpon; Nov 30, 2020 at 07:33 PM.
Is my vette stock?? HAHA
Joined: Oct 2015
Posts: 20,102
Likes: 9,229
From: Im not allowed to tell you
2020 C3 of the Year Finalist - Modified
2020 Corvette of the Year Finalist (performance mods)
2019 C3 of Year Winner (performance mods)
2016 C3 of Year Finalist
also, if you run mechanical and vacuum advance, then check and see if the advance is operating correctly. I had a distributor seize and it acted that way as well.
Tech Contributor
Joined: Aug 1999
Posts: 14,366
Likes: 6,346
From: At my Bar drinking and wrenching in Lafayette Colorado
You have a fuel supply problem. Start with changing the filter. If that does not correct the problem, install a fuel pressure line "T" at your carb inlet and run it to a fuel pressure gauge you can tape to the outside of your windshield while you drive the car. See what the pressure does when the problem occurs. You should never have less than 4psi pressure at the carb inlet. 6 is better.
The popping in your exhaust is indicative of a lean condition - your are leaning out and not getting fuel at high rpm under load.
Lars
The popping in your exhaust is indicative of a lean condition - your are leaning out and not getting fuel at high rpm under load.
Lars
Thread Starter
Safety Car
Joined: Sep 2005
Posts: 4,212
Likes: 643
From: Orange County Ca
OK, it appears to be a fuel problem, either clogged filters or low pressure. Should be easy to track down.
Any opinions on a Carter M4552 fuel pump, if needed? Appears to be a exact replacement, US made, etc. $51 at Napa.
Thanks for the help guys.
Any opinions on a Carter M4552 fuel pump, if needed? Appears to be a exact replacement, US made, etc. $51 at Napa.
Thanks for the help guys.
Le Mans Master
Joined: May 2002
Posts: 5,084
Likes: 786
From: Lebanon Township New Jersey
I don't know about the Carter pump but I'm using an AC Delco stock replacement from Rock Auto. Cheap and works great and no problem of excessive fuel pressure, with my Qjet carb.
Oh, I didn't realize you were the OP.
Oh, I didn't realize you were the OP.
Last edited by BKbroiler; Dec 1, 2020 at 10:13 AM.
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Tech Contributor
Joined: Aug 1999
Posts: 14,366
Likes: 6,346
From: At my Bar drinking and wrenching in Lafayette Colorado
But check or replace your fuel filter first - it's the most likely problem. If you're running the stock in-line filter, make sure you don't have a second filter installed in the carb. Only one or the other should be used. If you have the in-line, and you can blow through it with no problem, remove any in-carb filter that may be installed. Be sure to lubricate the fuel inlet nut fitting threads, and don't over-tighten it...
Also check all of your rubber fuel line hose sections: The hoses running between the frame rail and the fuel pump are often kinked or restricted as they go through the "S" shape. Same thing with the hoses between the fuel tank and the frame rails - make sure no hose is kinked or collapsed - it's a common problem.
Lars
Last edited by lars; Dec 1, 2020 at 10:54 AM.
Is my vette stock?? HAHA
Joined: Oct 2015
Posts: 20,102
Likes: 9,229
From: Im not allowed to tell you
2020 C3 of the Year Finalist - Modified
2020 Corvette of the Year Finalist (performance mods)
2019 C3 of Year Winner (performance mods)
2016 C3 of Year Finalist
Uld stay away from CVarter. I had threee fail on after the other at the hinge pin. Its not a through pin, its just swaged in. I ended up with an Edelbrock. Never had a problem since.
Team Owner
Joined: Apr 1999
Posts: 21,953
Likes: 1,444
From: Reno Nevada
2024 C3 of the Year Finalist- Modified
I had a friend call me about his Vette bucking , snorting, and popping. His car had been sitting for 8 months in a hot garage and never started. I took the top off the carb and it was gummed up with old dried varnish gas. It couldn't hardly flow through the jets. Thorough cleaning in a solvent tank, new inline filter. I also drained the tank and put in 5 gallons of local 91.
Thread Starter
Safety Car
Joined: Sep 2005
Posts: 4,212
Likes: 643
From: Orange County Ca
Looking ahead, if needed, I have a nos filter that is at least 30yrs old and possibly 50 yrs old. Any ideas if it's better to use it rather than a new repop?
https://www.ebay.com/itm/69-72-CORVE...G/330766126686
I do have both filters installed. Does it ever happen that the canister filter disintegrates?? Could it be a big problem without the inlet filter??
Carter pump $42 @ Rockauto + shipping.
$46 @ Napa with a AAA 10% discount, and pick up locally. No shipping charges.
RR, did you have this same model go bad?
I wonder if they're all made the same?
I like the fact that this is a exact replacement, looks the same, and US made.
https://www.ebay.com/itm/69-72-CORVE...G/330766126686
I do have both filters installed. Does it ever happen that the canister filter disintegrates?? Could it be a big problem without the inlet filter??
Carter pump $42 @ Rockauto + shipping.
$46 @ Napa with a AAA 10% discount, and pick up locally. No shipping charges.
RR, did you have this same model go bad?
I wonder if they're all made the same?
I like the fact that this is a exact replacement, looks the same, and US made.
Tech Contributor
Joined: Aug 1999
Posts: 14,366
Likes: 6,346
From: At my Bar drinking and wrenching in Lafayette Colorado
Dr. Detroit
Joined: Mar 2012
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From: New Braunfels Texas
Jebby
Thread Starter
Safety Car
Joined: Sep 2005
Posts: 4,212
Likes: 643
From: Orange County Ca
I changed out the canister in line filter--OEM for '69 BB-- and removed the filter from the carb inlet. I can see how the carb filter could be restrictive. It has the spring loaded anti drain back valve.
Anyway, the car starts and runs very well on the primary jets. It also runs very well at wot but does still start to miss a little at about 4500+ rpm. It is a 90% improvement at wot.
So, I think the fuel filter problem has been solved and I just need to fine tune the engine to get rid of that last little bit of high speed miss.
Thanks everyone for your help.
Anyway, the car starts and runs very well on the primary jets. It also runs very well at wot but does still start to miss a little at about 4500+ rpm. It is a 90% improvement at wot.
So, I think the fuel filter problem has been solved and I just need to fine tune the engine to get rid of that last little bit of high speed miss.
Thanks everyone for your help.
