Hi!

Ive got a 302 from 1987 which is fitted with a Holley 3310-3 750cfm sized carb. From what I've read this carb is a bit big for the 302 which as far as i know is in stock configuration.

Ive been thinking about a rebuild of the carb in general, but fitting a rebuilt smaller carb on the engine might be the right thing to do. Im not doing anything crazy with the car, using it as a daily that has seen the strip 1 time so far.
Im thinking about getting a used smaller carb, like the 600 or 650, what do you think? Any advices?

Thanks!

 

Even a 500cfm is all thats needed. Wouldn't go over 600cfm.

 

If you cannot afford another carb, keep the 750. It's not too big, you can tune the secondaries with a stiffer spring, keeping them closed for 90% of the time. The primary side is equal to a 500 cfm Holley 2 bbl. Is it a little big for a 302 ? Yea, but it will work fine

 

If thats an option then ill look into changing the spring for the secondaries.

The PO installed a electric fuel pump and seem to have put the engine driven one on a loop. Im looking into removing the electrical pump because its both knocking badly, and i don't know if it properly regulates the pressure. If it puts to large amount of pressure, the flat chambers would fill up completely and cause the engine to run rich?

Is the engine driven pump completely shot if its been running dry?

Thanks for the help guys!

 

Very few electric pumps can be run without a regulator, and you're right about the pressure over coming the floats in the fuel bowls, but it won't just run rich, it'll completely flood the engine with fuel, as the fuel will fill the bowls and come out of the open tubes sticking straight up out of the carb's top. At best, it'll go into the engine and kill it, at worst, it'll spill out all over the top of the motor and catch fire. If the engine driven pump is sealed up, (the fuel tubes closed off) it should be fine.

 

Running a mechanical pump with no fuel in it will result in the pull rod and diaphragm travelling their full stroke at every camshaft revolution, causing more rapid wear. The good news is that even if the original mechanical fuel pump is no good, a new one is cheap and readily available. Plus mechanical pumps are simple and very reliable.

Which makes one wonder why someone would replace it with an electric unit.

 

Yea im also wondering why the PO put an electrical pump in it. I understand that the engine driven pump suffers excessive wear when its run dry, so ill have to check it out and see if it still works.
I don't like the electric one, with the loud ticking noise it makes when it builds pressure.

The engine runs good, no flooding or anything. There's just a tendency to get a gas smell in the car making me think its running rich.

My next step would be to try and check out the engine driven fuel pump, change the secondary spring in the carb, get some new jet's and do some tuning of the carb.
From what i can tell its running good, but a bit rich from reading the plugs (carbon black).

Thanks guys!

 

Cause it takes less power from the engine to run the electric pump. And there's virtually nothing to wear out. Mr Gasket 12S is all that's needed to feed up to 450HP with three carbs at WOT. No regulator needed.

 

If you're going to rejet the carb, go down 4 sizes (numbers) at a time. Going up or down two sizes often shows no results when reading the plugs.

 

Okey, will do! Those little things are expensive, 8 bucks a pair for those tiny things :P

Do you think its worth the effort of changing to the engine fuel pump or just leave the electrical one installed? They aren't supposed to knock, are they?
Im going to take a closer look at the electrical one tomorrow and try to see if its regulated somehow, and maybe open up and look at the float levels when the pump is running.

Thanks!

 

If you retain the electric fuel pump, check the output pressure to insure it's within limits. If not, and you don't want to use the stock mechanical pump, install a regulator to correct the pressure.

Don't change more than one thing at a time. If you change fuel pump or pressure, then run with that setup for a while before changing carburetor jets or other parts. This way you can tell what changes result from the changes made to the fuel pump or pressure.

 

Many electrics are loud, mounting them on rubber isolators will help some. The Mr Gasket 12S is just loud enough to hear before you crank the engine, but only with the drivers door open. It's got enough flow to feed most carbed motors @ 35 gals per hour, you'd have to get less than 2 mpg @60 MPH to use it all. I suspect those pumps that are loud, are so because they've got way more flow capacity than anyone would ever need and the pump is being choked severely, making it loud.

 

Yes, changing many things at once can make it hard to diagnose and correct anything thats not working.


This one is loud when it builds up pressure, and then it calms down a bit. But all of a sudden it can start sounding a lot again, making you hear it when in the car at idle speed.

 

Dug into this today, and found the following after opening the secondary housing. Is it supposed to look like this? All the pictures I've seen of the diaphragm is that it looks different from this.



According to Holley there's supposed to be a black spring already in the housing, and when i tried the "bag zip trick", it didn't appear to move at all.
Figured id check the housing anyway.

Will i need a new diaphragm?
Thanks!

 

Ye[ that's what it looks like. And there's a spring that rides on top of the diaphram too. Holley sells a kit with about 7 different sec springs so you can tailor the sec opening to the motor's needs. The black spring is the heaviest of all, on a 302, you'll never see the secondaries open with that spring. In a 600(on a 302), it'll only open at around 5000-6000 rpms.