A few carburetor problems

johnsstang04 · May 13, 2006

Well... I have an Autolite 4100 off a '66 in my '69 302. The engine is mostly stock except it has electronic ignition. I intended on driving this car a bit more now that gas is $3 and the 351 convert gets 15mpg. But this thing actually seems to do worse on mileage. I've got a few questions.. maybe someone can help.

-The car idles well if its rich, but will overheat at idle if I lean it out. Is this a carb, timing, or cooling problem?

-It had 52/71 jets when I originally bought it, I switched to 51/52. Could I use smaller jets, like 40s, in the primary?

-The car has a slight miss at idle so I think the carb is worn out and leaks vacuum. I'm guessing this is actually the only real problem with the carb... any ideas if it can be fixed?

I've always preferred the original Autolites, but I'm thinking of throwing on a new holley. I was looking at 390cfm and a 465cfm carbs. Any suggestions on which to use?
http://www.holley.com/0-8007.asp
http://www.holley.com/0-1848-1.asp

D.Hearne · May 13, 2006

You cannot just keep going smaller on jet sizes with a carb. This ain't fuel injection. There's a limit to how small the jets can be vs the venturi size. You will burn the pistons untill you get it so lean that it won't even start. You want better economy, get a smaller carb. If you do buy either of those Holleys, leave the jets as is. Holley chose the size jets they come with for a reason. About the smallest you can go is 2-4 sizes smaller, NO more. You want more power with what you have try advancing the timing. Running hot at idle is also an indication that the timing's retarded too much. Set the initial at about 12 degrees BTDC.

johnsstang04 · May 14, 2006

I know you can't keep changing the jet size.. there is a point where it becomes useless. The carb had jets that seemed way to large (52/71). I've owned many stock 4100's with primary jets in the 40's, but I didn't realize you would damage the engine with a two size change. In fact, I was under the impression its pratically impossible to notice a two size change. Does anyone know the correct jet size for the carb? The venturi diameter is quite large... in fact its the same as my hipo carb (1.12), so its probably for a big block of some sort. Also... I was buying the holley so I wouldn't have to screw with jet size... or anything else for that matter. I had the timing at 10 BTDC... I'll see if 12 makes a difference. Thanks for the help.

D.Hearne · May 14, 2006

As far as I know, there was only two size 4100's -- 480 cfm and 600. The 480 has 1.08 bores, the 600 is 1.12. I've never messed with the 2100 or 4100's enough to say what jets sizes were.

D.Hearne · May 14, 2006

I think I'd go with the 465. With a smaller carb, you get to a point where you're pulling so much vacuum, it tends to pull that much more fuel, negating any fuel economy you might be looking for. On my 331, I went with the repro 3x2 carb setup. It's got three 250 cfm 2 bbls. I had to jet them all richer to get the mixture right. They came with 58 jets. I ended up with 60's on the primary carb and 62's on the secondaries.

johnsstang04 · May 14, 2006

Thanks again.. its good to see people who know what they're talking about. I'll probably just put that 465cfm on it.

bnickel · May 14, 2006

have you checked with pony carbs to see what they can do for your carb? i remember reading about a newer version of the 1.08 4100 they came up with that uses sleeves in the primary bores to help with economy, they say it is almost like efi it works so well. i have no experience with this particular carb but it sounds pretty cool.

www.ponycarburetors.com

here is an article that has the carb i mentioned.

http://www.mustangandfords.com/techarticles/38381/

bnickel · May 14, 2006

have you checked with pony carbs to see what they can do for your carb? i remember reading about a newer version of the 1.08 4100 they came up with that uses sleeves in the primary bores to help with economy, they say it is almost like efi it works so well. i have no experience with this particular carb but it sounds pretty cool.

www.ponycarburetors.com

here is an article that has the carb i mentioned.

http://www.mustangandfords.com/techarticles/38381/

bnickel · May 14, 2006

have you checked with pony carbs to see what they can do for your carb? i remember reading about a newer version of the 1.08 4100 they came up with that uses sleeves in the primary bores to help with economy, they say it is almost like efi it works so well. i have no experience with this particular carb but it sounds pretty cool.

www.ponycarburetors.com

johnsstang04 · May 14, 2006

The new holley 465cfm costs $280, a rebuilding the 4100 is $250+. I found an edelbrock 500cfm with an electric choke (the holley is hot air) for $280. I can get the new carbs at manufacturers price, so figure around $220. I would like to get fairly good fuel economy without completely sacrificing performance. Has anyone used the holley 465 or the edelbrock 500?

D.Hearne · May 14, 2006

Which ever you try, get one with a manual choke and then tie it open, or remove it completely. Where you are, you don't need a choke.

A few carburetor problems

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