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Old 08-18-2008, 07:47 PM
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Carb Size

I've got a 1969 L88427 it's .60 over 13.1 stock intake (can't remember cam size) its got a 4spd and 4:11's. The carb is a 975 holley(i think) it's stock. Anyway it backfires in the midrange and it seems like it's starvin for fuel. What size carb should i get? I only put about 100miles a year on it and only drive it about 5miles at the most 1 way.....
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Old 08-18-2008, 09:42 PM
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with 13:1 what octane do you run. this motor certinaly will not run on pump gas.
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Old 08-19-2008, 01:21 AM
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What 406 said.........the original REAL L-88 came with 850 holley's so you're over carb'd as it is.
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Old 08-19-2008, 06:12 AM
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I run 110 race gas, and im sorry the carb i believe is a 925 holley. I'll get the number and post them the block # 3963512. Anyway i need more gas it seems like it's starving for fuel, im thinking about buying a demon carb and was wondering how big to go?
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Old 08-19-2008, 06:14 AM
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Quote:
Originally Posted by L88427BigBlock
I run 110 race gas, and im sorry the carb i believe is a 925 holley. I'll get the number and post them the block # 3963512. Anyway i need more gas it seems like it's starving for fuel, im thinking about buying a demon carb and was wondering how big to go?

A bigger carb does not equate to more fuel. Why not calibrate the one you have?
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Old 08-19-2008, 06:14 AM
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what kind of fuel pump are you using and do you know the fuel pressure?
are the floats set correctly?
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Old 08-19-2008, 07:52 AM
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Stock mechnical fuel pump, floats havent been touched and the carb worked ok until i had the heads polished...it just starts to miss at about 4000rpm? I've had the car for 12yrs and never touched the carb. Just wondering if i should buy a new one or mess with the old one? I put a new cam in it to i'll get the cam specs this afternoon.
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Old 08-19-2008, 08:18 AM
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What carb actually is it?

Have you increased the jet size in the carburetor yet? Porting the heads will usually slow down the air velocity which will require you to increase the jet size.
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Old 08-19-2008, 09:25 AM
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I'll get the numbers off the carb to day and post? The heads weren't ported just polished. Just seems like the motor is starvin for air...........
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Old 08-19-2008, 09:34 AM
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Is the 975cfm Holley vacuum or DP mach. secondary?
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Old 08-19-2008, 10:32 AM
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Quote:
Originally Posted by L88427BigBlock
I've got a 1969 L88427 it's .60 over 13.1 stock intake (can't remember cam size) its got a 4spd and 4:11's. The carb is a 975 holley(i think) it's stock. Anyway it backfires in the midrange and it seems like it's starvin for fuel. What size carb should i get? I only put about 100miles a year on it and only drive it about 5miles at the most 1 way.....
Driven how, on a race track?

100 miles a year isn't much, which means the car sits a lot. This isn't good for the fuel and the stuff it touches. Do you drain and purge the fuel system when it's not used or do you just park it? If the latter, even with fuel preservative like Seafoam or Stabil added, I'd suspect that the fuel in the tank and system goes "stale".

Fuel losses it's more volatile components to evaporation as it ages. The top end highly volatile components are necessary for light off of the heavier molecules. The fuel burns erratically when these are lost.

The other issues with old fuel are the formation of varnishes and fuel system component deterioration thru reaction with the solvency of the fuel's components and oxidation of parts by contact with organic acids forming in the aging fuel. This is worse with leaded fuels as the organic lead and bromine compunds breakdown with time as well forming really ichy sludge.

Could be that the fuel system just needs a good overhaul. You might look to the ignition system as well, this is another system that doesn't take to being parked very well. The many types of materials used in electrical systems encourages oxidation with just the slightest hint of moisture present. The Navy did a study some years ago of Mean Time Between Failures (MTBF) of electronic equipment being used ship board to that having been in coastal storage facilities then put into service. They found the MTBF of unused stores of the same age as that of the equipment in shipboard use wasn't noticeably different. That said, there is reason why the Naval Stores Center is in Barstow, which is a long way from salt water and really close to Death Valley.

Bogie
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Old 08-19-2008, 11:02 AM
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Mechanical secondary no vachum lines at all hooked up all.... It's drivin like it should foot on the floor, thats why i only drive it 100miles a year. I've never cleaned the carb i realize fuel varnishs after awhile i only keep 10gallons in it. I didn't think race fuel went bad as fast as unleaded gas? Im sure the carb needs to be cleaned i yank it off and check it. And the cam is a 310/320 intake 570/595lift 3500-10,000rpm.......thanks for your answers....
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Old 08-19-2008, 02:01 PM
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It was bad fuel it will run like garbage no matter the rpm you are running.
By the way, I'm not to sure about this but the standard carb they used on a L88 was a Holley 780cfm vacuum secondary. Some say 800cfm VS. (who cares) Chevy never used mach. secondary Holley carb. They did used 6-pack setup with outside two as vacuum secondary. The carb you are using is an aftermarket.
I got funny feeling it's the ignition system. Your distributer cap could be arcing over inside. Check your ignition wires, too.
There may be a defective accelerator pump. Check your fuel pump. hope this helps.
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Old 08-19-2008, 04:37 PM
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The best way to tell how your carb is running is by pulling a spark plug and reading it. If it's real light color, your probably too lean, if it's sooty and black, your too rich. Then I would adjust with the jets. As a rule, you want to start with either two numbers richer or leaner. Install the jets, run the engine and see what happens. Pull the plugs and read them again. Or find someone with a dyno... That carb should be fine for what you have. Check your timing and vacuum on the engine also. That makes a big difference. One more thing, check your power valve rating in the carb. Powervalves are notorious for getting blown out on Holley's. Powervalve are rated by vacuum of the engine. You might have to switch power valves to a 4.5 or??? I believe your carb has a 6.5 which means you should have at least 16 of vacuum.
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Old 08-19-2008, 05:08 PM
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Also you may want to take a look at the power valves if you have them. After stting they can stick and then create a lean condition on mid range transiton.

Robin
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