Carb Guru’s

[Bdhand]

So do you guys know if it is a standard procedure to either drill out the jet or install a bigger one in the carburetor if you bore your engine when you rebuild it? I have been dealing with my engine running like crap for the last few weeks and finally talked to someone and they suggested drilling out the jet. Did it today and it worked! Previous owner rebuilt the engine and bored it over but kept the autolite 1100 stock.

 

[1970 Palmer]

Seems like you would want to check and confirm your tune up spec's before you start changing parts.

Initial timing?
Mechanical timing, is it working, how much total?
Vacuum timing, is it working, how much total?
Compression test?
Take a plug reading, are they all uniform?
How much vacuum at idle?
What is the float level?
Has the carb been adjusted, do you have any vacuum leaks?
When you adjusted the carb, are both mixture screws out the same number of turns?
Have you checked for fuel pump flow (volume) and pressure?

After you have the basis's in adjustment, you can easily determine if it's actually very lean by just holding the choke plate partially closed with your thumb at 2500 rpm. If it likes "less air", it will respond well to "more fuel". You can adjust both the idle and main circuits. Most were very lean from the factory by 1970 when new to meet emission standards. It worked better at higher altitude, but lean caused surging at low altitudes.

You need to figure out where it is lean and fatten it up in that area.

Drilling jets is a last resort, IMO. If you cannot find the desired sizes, at least use number drills to make your fine tune changes. Always a good practice to file off the jet number if you drill it to save the next guy working your engine. I was always told that the number on the jet was rated by fuel flow, not necessarily just the hole diameter. Jets with the same diameter hole and different jet numbers have a different flow. If you end up drilling, go in very small increments. The idle/low speed fuel mixture is in the emulsion tubes in the end of the booster.

I believe you will get a better result by diagnosing the problem, then focusing on that area. Are you even sure the jets are a original size (did someone already drill them?) after 45 to 50 years?

John

 

[ared77]

Sounds like he already did it.

 

[blubuckaroo]

So do you guys know if it is a standard procedure to either drill out the jet or install a bigger one in the carburetor if you bore your engine when you rebuild it? I have been dealing with my engine running like crap for the last few weeks and finally talked to someone and they suggested drilling out the jet. Did it today and it worked! Previous owner rebuilt the engine and bored it over but kept the autolite 1100 stock.

Boring the engine out has nothing to do with it. Increased displacement doesn't need a richer mixture.
Who knows where the carb came from. It could have come from a high elevation where smaller jets are common.

 

[jckkys]

Correct, the size of the jet is determined by the size of the venturi. It establishes the A/F ratio. This remains the same regardless of engine displacement. A larger displacement may require a larger carb. not a larger jet.

 

[sprdv1]

Correct, the size of the jet is determined by the size of the venturi. It establishes the A/F ratio. This remains the same regardless of engine displacement. A larger displacement may require a larger carb. not a larger jet.

X2.....

Invest the $$ and just go efi if you can afford it

 

[Pa PITT]

BDhand... So you did it today & it worked.
... TELL ME what all you did.
I'd have not thought what your suggesting would have helped but if it did .... THEN IT WORKED FOR YOU .

so what number jets did you have in it & what did you drill it out to.... As I recall the Jets are drill in 100th of an inch .. Meaning if you had 52's in your carb .. 53 would be the next size .. & 54 would have been where I'd have gone to.
We did this back in the days When I had hot rod chevys . 327's with AFB's on them.
But those had like 62's in the front & we always put 120's in the rears.
120's where as big as you could get.
..But I'd like to know what you used.

 

[Bdhand]

BDhand... So you did it today & it worked.
... TELL ME what all you did.
I'd have not thought what your suggesting would have helped but if it did .... THEN IT WORKED FOR YOU .

so what number jets did you have in it & what did you drill it out to.... As I recall the Jets are drill in 100th of an inch .. Meaning if you had 52's in your carb .. 53 would be the next size .. & 54 would have been where I'd have gone to.
We did this back in the days When I had hot rod chevys . 327's with AFB's on them.
But those had like 62's in the front & we always put 120's in the rears.
120's where as big as you could get.
..But I'd like to know what you used.

The number that was in there was like a 61 or 62 on the side of the jet and we drilled it out to 80 thousandths so we increased about 20 thousandths.

 

[Pa PITT]

holly crap that's not supposed to work. But if it did then . Yes you most likely needed more fuel.
... DO YOUR Eyes burn while standing behind it . IF YOUR IN THE GARAGE WITH IT do you smell too much fuel in the air.
... I'll say keep an eye on you fuel mileage .. If it goes down then you'll need a new set of Jets.
I'm going to say don't move but 2..100th at a time .
& It may have been 2...1000th's i'm talking about . I'm having to reach back 55 years.
But we bought those bit's from our local machine shop.

 

[jckkys]

You would have to check the A/F ratio with a wide band gauge to know what you really did and if it was a good idea. It's a little hard to believe that going from 0.062" to 0.080" is an improvement rather than causing a super rich mixture.

 

[Bdhand]

Well I can’t explain it but I am all ears for suggestions. All I can tell you is that is was running like crap barely would go down the road and we drilled the jet and now it is better. I am thinking about buying a Weber 32/36 carb from Classic Inlines anyway so if I do that the problem may totally go away!

 

[blubuckaroo]

Maybe your engine has a vacuum leak.

 

[Bdhand]

Not sure about that but I did add power disc breaks that requires vacuum so that could be a cause??

 

[blubuckaroo]

Not sure about that but I did add power disc breaks that requires vacuum so that could be a cause??

Yes, I've had junk vacuum boosters do that. Try pulling the hose and capping it off.
Also check all the other vacuum hoses.
This is one place that the one piece intake/cylinder head is a good thing. There can be no vacuum leak at the ports like some other heads.

 

[Bdhand]

Well if I did that then I wouldn’t have any breaks.

Maybe I should have told the whole story. Before I drilled the jet in the carb the engine would fire right up and run like a top sitting at idle. Only when I would take it down the road and put it under load is when it would run like crap. Now that the jet is drilled it doesn’t do that.

 

[armynavy17]

While I'm not familiar at all with early straight 6s, it very much sounds like it's not a carb issue to me. I'd double check initial, mechanical, and vacuum advance. Also check everywhere for vacuum leaks including the vacuum advance housing and your new brake booster.

 

[Bdhand]

New DUI distributor. Which by the way we swapped distributors and put the original in and it did they same thing so I put the new DUI back in. Timing has been checked both with a light and dial indicators and it is good. I can’t help it but I think it goes back to the carb since it stopped once we drilled the jet.

 

[gnpenning]

Well if I did that then I wouldn’t have any breaks.

Maybe I should have told the whole story. Before I drilled the jet in the carb the engine would fire right up and run like a top sitting at idle. Only when I would take it down the road and put it under load is when it would run like crap. Now that the jet is drilled it doesn’t do that.

You would still have brakes, just not vacuum assist. You are just testing, your test drive should be planned accordingly. Just long enough to verify no vacuum leaks with the booster. Drive smart.

 

[Rustytruck]

did you check fuel pressure and flow and did you check float level this all comes into play as the motor speeds up under load. now that you drilled the jets so oversize do you have any workable idle mixture screw adjustment?

 

[Bdhand]

Installed a new fuel pump. Adjusted the float as much as we could. All of this done before I drilled the jet out. Just ran it down the road again and it is running fine.