Jet Size Question (running way to rich)

[shotsy]

Ok fellas I need some Help!! I got the car fired the other day and It's running so rich it feels like my eyes are going to burn out. I've got the timing set at 6 deg initial. (I know it's low) The car starts and idles about 700 rpm and sounds great but I'm spitting little black dots out of the exhaust. I bought the carb (holley 750 vac sec) from a guy who had it on a chebby 454 and he said that I would need to change the jet sizes to match the 351C. There in lies the problem, are jet sizes based on cubic inches? I've set the mixture screws to about one and one quarter turns out and re adjusted the idle to 700 and it still stinks bad? I did notice that the front sight plug on the carb is totaly full (supposed to be at the botom of the sight plug) and the rear bowl is set to about the middle of the sight plug. Would the floats being out of adjustment cause me to run so rich? All of the specs for the motor cam etc are in my signature, I'm at a loss Timing, jets, floats, all of the above?

 

[12sec67]

floats are too high for starters.

bring down the floats till you don't see them in the clear tabs untill you shake the car a little.

then go from there

 

[grego37]

dont try to adjust your carb until your timing is set.

except the floatlevel, adjust those with the car running, just to the bottom of the hole , where if you took your plug out gas would barely trickle out if you shook the car,

6 initial seems a little low.

do you know how much total timing you're shooting for?

Which type if distributer are you using?

what type of advance unit does it have?

You must lock in your timing first.

ps. Vote for Pedro

 

[shotsy]

dont try to adjust your carb until your timing is set.

except the floatlevel, adjust those with the car running, just to the bottom of the hole , where if you took your plug out gas would barely trickle out if you shook the car,

6 initial seems a little low.

do you know how much total timing you're shooting for?

Which type if distributer are you using?

what type of advance unit does it have?

You must lock in your timing first.

ps. Vote for Pedro

Everything I've read said that for a Cleve my initial timing should be around 12 and a total of 36 all in by 3 grand. I'm running a durraspark setup out of an 85ish 460 pickup with vacuum advance?

 

[grego37]

I got this off holley's website.

How to determine what size carb:
CID x max rpm divided by 3456 = CFM at 100% volumetric efficiency

example 351 x 6000rpm = 2106000 divided by 3456 = 609

and usually its only 80-85% of that answer depending upon what mods are done to your engine.

I think your carb is too big , based on this formula you should run a 600-650cfm. your answer may differ, depends upon how high you plan on reving it etc....
and I allways hear better off too small than too big.
call Holley see what they say.

 

[10secgoal]

Yes, if the float are too high it can cause the car to idle rich. The vacuum from the motor actually pulls the gas out of the carb, into the motor. the higher it is, the less vacuum it takes to draw the gas out of the carb.
-The carb theory is a place to start. But is usually off for high performance motors. I have a 302 and it runs like ass with a 650. So I had to put the 750 back on.
-The jets are sized by thousanths of an inch. It gives you an idea of where on the scale of fuel flow you are at.

 

[ddonaca351]

The 750 will be ok if you jet it right...

Set the floats as described above.

Check to see if the throtle stop screw is turned in too far, sometimes this allows the motor to pull fuel out of the off-idle transfer circuit, or even the primaries, instead of through the idle circuit like its supposed too.

NOTE!!!!! (the jets should have no effect on idle!!!!)

If it still idles like crap check the powervalve to see if its blown, that will make it run way too rich.

Dave-

 

[brianj5600]

Floats first. Advancing the timing should pick up the idle, allowing you to back off the idle speed screw. If you are getting into the tranfer circuit, that will help. Holley carbs are great, but you HAVE TO KNOW how to tune them. There are books on modifying holleys, which are well worth the investment. More horsepower, better drivability and more mpg are things we want. With out a good understanding about what is going on, you will never get it.

 

[Max Power]

I would:

1. Get the carb number and look the carb up on Holleys website. They will list the OE power valve and jet size. Buy them.

2. Set the timing for 10 advanced.

3. Remove the front float bowl, install the OE jets and power valve and adjust the floats down by eye.

4. Re-Install and fine tune the float levels as listed above, then tweak the timing for best idle vacuum and speed.

5. Test drive.

This should get you a lot closer. 750cfm is not too much carb for your motor. The formula above is a guideline, but does not factor in cams size etc. You should be fine with this size carb.

Sometimes the bast way to final tune a carb is to dyno it and read the exhaust to see where you are lan and rich. Doing the above should get you close.

 

[jasonn]

Maxpower and ddonaca351 are right. 750 is not too big, should work fine. Like they said, get your timing right first. Then get the stock jetting size and adjust your floats, this will give you a good starting point. Do not jet (mains) from motor to motor. The jetting is a RATIO of air/fuel. If you put the same carb on a 650c.i. motor and jet it just perfect, then put the same carb on a 400c.i. motor, it is STILL jetted correctly. The bigger motor will be pulling much more air therefore much more fuel at the set jets ratio. You may have to move up or down one, maybe two just because each motor needs something a bit different to run it's MAX power. I had my motor dyno'd so I know I am jetted to what the motor likes. But yet mine also seems to run rich. Same as yours, exhaust seems real sooty. Also cannot STAND to have it running in the garage and be able to be there for more than about 2 minutes before being gagged to death. But it sure runs awesome out of the shed so it doesn't bother me.

 

[Max Power]

Threadjack:

Hey jason, I see you are in Rochester. I am in the process of bidding on a woodgrain dash from somebody in Rochester that has a number of Mustang parts. Wouldn't be you, would it?

 

[jasonn]

Threadjack:

Hey jason, I see you are in Rochester. I am in the process of bidding on a woodgrain dash from somebody in Rochester that has a number of Mustang parts. Wouldn't be you, would it?

No but I probably would know him!

 

[brianj5600]

If there was only one perfect size jet for every combo, there would not be changable jets. Different air cleaners or no air cleaner changes the jetting. If you are fixing the idle, as said above, jets won't matter. You could pull out all 4 main jets or plug them, it should not affect idle. There is atleast 5 ways to alter idle air/fuel ratio. The carb has no idea how big the motor is that it is bolted to, only how much air is being pulled through it. Jetting has no relation to ci other than bigger motors are capable of moving more air, but a smog 454 from the mid 70s won't move as much air as a well thought out small block.

 

[shotsy]

If there was only one perfect size jet for every combo, there would not be changable jets. Different air cleaners or no air cleaner changes the jetting. If you are fixing the idle, as said above, jets won't matter. You could pull out all 4 main jets or plug them, it should not affect idle. There is atleast 5 ways to alter idle air/fuel ratio. The carb has no idea how big the motor is that it is bolted to, only how much air is being pulled through it. Jetting has no relation to ci other than bigger motors are capable of moving more air, but a smog 454 from the mid 70s won't move as much air as a well thought out small block.

Lots of good advice from all of you, I'm planning on tearing into it in the morning, I think the jets that are in it are 69's and the fella that I bought it from gave me a set of 64, said thats what was in it stock. I guess that I'll put the 64's in and then do all of the above steps, and let ya know how it turns out. Glad to hear that the 750 isn't too big!! A carb change isnt in the budget at the moment. Here's a pic of the for-mentioned beast it's got some sort of tricked out center section with no choke, got it for 100 bucks? did I get a deal? All I know is that it is a modified 4160?

 

[brianj5600]

I think you did ok. Looks like a proform mainbody. Holley makes a kit to put a metering block on the rear, so you can change out jets. Screw in air bleeds are cool, you can lean the idle by enlarging the idle air bleed. I think with your combo it won't come to that.

 

[ddonaca351]

Oh crap... Vacuum secondaries, you are set. The VS will compensate for the motor being slightly small anyhow. NO PROBLEM.

Do as above, but if he gave you the stock 64s, throw them in anyhow. The chances are that Holleys settings (with millions of hours of experience) are probably closer than our cross-country guess.

I agree 100% with the buy a book idea. If you have never tuned a Holley before you could really fugger it up. (they work different than most other carbs) So be carefull if some rodchester or edelbrock "expert" offers to help....

(carb wise)
You will probably just have to adjust the:
-floats
-idle mixture
-idle speed
-accelerator pump cam
and maybe...
-secondary spring (determines when the 2nd-aries open)
-a jet size or two

*pretty much* in that order. Resist the temptation to jump to the jets too soon, usually the problem is elsewhere, and you are just covering the problem up by changing the jets. (IMHO)

Dave-

 

[ddonaca351]

Oh as long as Im on my soap-box...



take that metal line running to both float bowls and throw that fugger in the garbage... or cut it half way to the rear bowl and put a small length of rubber line on it. I swear in 20+ years I have seen like 2 of those damn things that DIDN'T leak.... (guess the teflon tape helps)

Anyhow, just thought I would throw that out there, feel free to ignore this portion of my advice.


Dave-

 

[69Rcode_Mach1]

That 4160 looks very similar to the 4150HP's if you can get it jetted right and still a little rich it may require you to change the idle air bleed jets on the top, but usually from my experience you don't need to touch those unless you got an engine that really flies which it looks like you have.

 

[shotsy]

It all started well,

I adjusted the floats and in the process I pulled the coil wire off so I could turn the motor over and fill the float bowls back up, This took about 5 minutes to get right and I paid no attention th the coil arc-ing back to itself. Well the floats are now right but the damn duraspark module is now fried Guess that the spark fed back to the ignition box through the negative conection on top of the coil So now I've got no spark and cant fire the motor. Guess that this should fall under my previous post as I am still dumb, I shoulda left the coil wire on and just pulled it offa the cap. Another 20 bucks at Autozone here I come!!

UPDATE:::

New duraspark module in, Floats adjusted, Initial Timing Set at 12 deg. BTDC, Rich/lean screws adjusted out 1 full turn from bottom, (gave me the highest manifold vac at 7 inches) and have the idle set to 700 rpm. and the thing still smells rich but no black splatters out of the exhaust now , guess that the next step is to pull the front bowl and put the 64 jets in? Sound good?

 

[10secgoal]

I wouldn't worry so much about the smell. Unless it is burning yours eyes real bad. Change the jets depending on how it drives. If you can ease into and out of the throttle without hesitation, it's gravy if the timing is set right. If it stumbles, look into total timing before spending money on jets.