Carb questions

[BigPinoyHunk]

2) Should I look at investing in a better carb? I've read very mixed reviews on the 4300 (it was rebuilt ~10 or 12 years ago and only has about 50 miles on the rebuild).

I would consider a nice Holley. If you move between high and low altitude and stay at either for a period of time, the jets are easy to exchange on the Holley.

mike

I'm really considering going to an aftermarket carb, when funds allow. I figure that rejetting is cheap enough that I can get away with doing that for the time being. I'm not going to drive much below 6,000ft elevation, so I should be okay.

I'll keep you guys posted, maybe this weekend I'll be able to do it?

be caution when you do other stuff to the car that'll require different sets of jets. i went from 86 jets to 82 jets in the primaries because i changed my headers from 2 inch to 1.75 tubes. the 86 jets in my new headers was just just too rich and it would make my eyes water and smog up the house with the interior door closed as well. with the 82 jets its perfect.

my friend at work on his 69 GTO, he said he had to go UP 1-2 jet sizes just adding in his MSD dizzy and modulator box.

 

[c9zx]

If you ignore other factors, humidity and temperature, the jet size should decrease one number size for every 2000 feet of altitude gain.

Chuck

 

[sm3570]

If you ignore other factors, humidity and temperature, the jet size should decrease one number size for every 2000 feet of altitude gain.

Chuck

So we're talking about 3 jet sizes to get to the correct size for the altitude. I'm thinking of buying a kit so I don't have to make a bunch of trips to the parts store. That way I can experiment to get the correct setup.

 

[c9zx]

If you ignore other factors, humidity and temperature, the jet size should decrease one number size for every 2000 feet of altitude gain.

Chuck

So we're talking about 3 jet sizes to get to the correct size for the altitude. I'm thinking of buying a kit so I don't have to make a bunch of trips to the parts store. That way I can experiment to get the correct setup.

I went back and read your original post saying you have a Ford 4300 carb. The info I gave you is for a Holley. It may be true for Autolite 4300. I just don't remember if the Autolite jets used the same flow/numbering system as Holley. Finding jets could be a challenge. The only sources I know of is possibly Pony Carbs or other rebuilder or find a stock or Super Stock Racer running the Autolite. Do you know if it is a 4300A (near square bore 600 cfm) or 4300D (spread bore 715 cfm)?

Chuck

 

[sm3570]

It's not the 4300D, so I guess it's a 4300A? I've never heard it referred to as a 4300A, just 4300.

 

[c9zx]

It's not the 4300D, so I guess it's a 4300A? I've never heard it referred to as a 4300A, just 4300.

I'll check some local sources here tomorrow to see what is available, if any thing.

Chuck

 

[sm3570]

Thanks Chuck, that would be awesome. I'm going to call the local shops and see if they have anything. Maybe I'll just tune it for now, I'm thinking about saving up and just getting a Holley or Edelbrock if I can't get the Autolite tuned the way I want it. Have to wait and see!

 

[sm3570]

Well, the bad news is that the local parts houses don't carry ANYTHING for the 4300. The good news is that I don't think I'll have to rejet the carb. I rough-tuned it last night (by sound) and it's running GREAT! I'm going to get my hands on a digital tach and finish the tune, but I'm really encouraged by this.

 

[c9zx]

I found nothing available here either, other than an entire carb. I'm glad it looks like you won't need to try to track down parts.

Chuck

 

[sm3570]

I found nothing available here either, other than an entire carb. I'm glad it looks like you won't need to try to track down parts.

Chuck

It really surprises me that with a carb that was produced for 20+ years there is very little support. It speaks volumes for the design I suppose...I'll probably replace it at some point b/c I'm not 100 % happy with it. It has a slight fuel leak and there is a slight hesitation when I goose it.

 

[davechesnut]

all the fords that were sold by dealerships in denver metro area had specific jetting for the alltitude. i remember i had a chevelle which ran great in town here but when i drove up to dillon it wouldnt get out of its own way up there. once i got back to denver all was well again. ive read that one advantage of higher alltitude is the ability to run higher compression with no detonation. i had a buddy with a 340 duster with 11:1 compression and he ran junk octane 7-11 gas and never had an issue. i havent been brave enough to test this myself however!

 

[sm3570]

I've heard the same thing about running pump gas with no pinging. I'll keep everyone updated once I get a chance to fine-tune the carb.

 

[c9zx]

I found nothing available here either, other than an entire carb. I'm glad it looks like you won't need to try to track down parts.

Chuck

It really surprises me that with a carb that was produced for 20+ years there is very little support. It speaks volumes for the design I suppose...I'll probably replace it at some point b/c I'm not 100 % happy with it. It has a slight fuel leak and there is a slight hesitation when I goose it.

Steve, I'm sure there are a few Stock and Super Stock racers some where how have figured out how to make them work well. For most racers it was/is not worth the effort. They were notorious for the the accelerator pump "piston" (it is a neoprene cupped washer) going bad. I saw some actually wear out the "bore" of the pump. The pump has 3 settings, adjustable by changing the pivot pin location in one of three holes available. You might want to look at at these two items to see if it makes any difference.

Chuck