Ok, so i've had hell trying to jet me carb, i've changed jets about 5 or 6 times, and i know i'm not doing it right. Right now, the carb is off the motor, has been for a bout a month, and its got 68/72 jets as of now, and they dont seem to be working out, and i'm sick of buy jets. Is there a way to calculate what size jets i need???? cause that would be really really cool. or maybe i should just get a dyno.... ehh.....ehhh.

 

Calculating jet size is not easy. One simulation program we use suggests jet size, but only if it has every engine parameter. Basically I believe it takes calculated BSFC and manifold vacuum and calculates jet sizes. IMO, it's only a reference.

How about trying this: Holley and Edelbrock both sell carbs specifically for certain engine combinations. Actually they are probably for hp levels. Look up what they put in their 750 for engines like yours. Start there. You'll be close. Better 5% rich than 5% lean any day.

I've seen a 950 cfm Holley, recommended for 550+ hp big block, factory jetted just about right when put on a 550+ hp small block. It was within a jet size or two.

 

You may already know this but as a reference if you don't have a power valve in the rear you'll need 8 to 10 jet sizes over the front as a starting point.I think it's imposible to get it right unless you jet for mph at the track, use a rear wheel dyno or wide band O2.My 750 DP came with 72 and 80.Last year I was jetting up my truck more and more thinking it was helping the SOP feel but when I went to the track I thought I would go up 2 jet sizes more in the rear for good measure and I promptly lost a bunch of mph and slowed from a 7.83 to 8.13 so I hope I was pig rich to start.If you want a jet kit the people at Autozone don't know it but they have a complete jet kit behind the counter.You have to tell them it looks like a little white tackle box with Holley on top.I think it was 30 bucks and comes with all jets from like 60 to 95.How do you like that Torco converter? I've heard mostly good stuff about them and the price is definitly right.

 

You didn't give much of a description of what you were trying to cure.... was it driveability or full throttle problems?

If you vehicle matches your signature with a 750 mechanical secondary carb on a 355 in a truck, you are marginally big to begin with due to the vehicle weight. If it hesitates, you may need a bigger accelerator pump to help this.

Unfortunately a book can be written (and has) on how to tune a carb.

Fuel flow is in directly related to the vacuum signal that a carb receives. This can vary be manifold type, carb spacer use, carb size in relation to your motor etc. Usually the carb will work out of the box for an initial run if the motor is normally aspirated.

Here is a starter setup.

Set float level.

Start vehicle, adjust idle speed & adjust mixture screws for best manifold vacuum. Now Check/Install a power valve 1.5-2" below idle vacuum (in gear) ..... for 12" or higher vacuum start with 8.5 power valve. Higher is not normally needed.

Example... with 8.5" vacuum use a 6.5 power valve.
This prevents opening at idle.

Now for power settings.
Start with stock jets. Get (3) sets of spares in increments of 2 sizes in either direction. Example for 68 jet, get 62's, 65's, 71's & 74's. Try richening all jets from base up 3 sizes & see if it runs better. If so, try this 1 more time.

Conversely, try the same method going leaner if richening made it run worse.

Obviously a chassis dyno makes it nice as you can calculate the jet size increase/decrease from the base air/fuel reading & get very close.

The reason to go in 3 steps is that most jets can vary 3% (typical holley jet) in either direction. 1 jet size is typically 4.5%.... therefore changing 1 size (worst case) can in reality not make a difference.

This is just a start, hope this helps.

Lonnie

 

I'll check for those jets at Autozone, or maybe i'll just order it along with other stuff from jegs. And the torco, well i haven't got to play it too much, but it seems nice. No posi and bad brakes dont let me stall too high either.