Best jet size, etc ?? 650 Holley 302 Ford crate

[trs900]

Would really like to hear from someone who has the same engine..I recently bought a cobra with 302 Ford Racing 340HP came with 650 Holley double pumper # 4777-7 stamped on horn with mechanical secondary. Car doesn't seem to run right almost 99% sure it is carb related, stumbles etc... fuel pressure is 4 lbs has a electric Carter fuel pump Part #06K20A & P61296 on sticker looks to have plenty of flow, plugs look to be proper color.
What size jets would you run in primary-secondary and what other mods to carb would you recommend such as power valve-whistles in bowls.. best rebuild kit Barry Grant?? Its been 20 years since I last worked on a Holley which was mounted on a supercharged 1985 Mustang GT.. I am a little rusty..
I am going to remove it and check it over so it would be great if I knew the best set up so I can get proper jets, etc.. Car is strictly used on street not a dedicated racer..

[Jerry Clayton]

I've found as a first step is to find out whats in the carb as to jets and power valves and then check to see what Holley put in it when they made it

As a general note on most street engines the proper drivable jets will fall in a range of numericlly being about egual to the carb cfm


ie 650 carb---aprox #65 jets, 700-750 carb---70-72 jets, 800 carb 78-80 jets

Make sure your power valve isn't blown and is rated a couple numbers below your engines vacume

The carbs haven't changed since you worked on them, still don't look down the carb while the engine is cranking!!!!

Jerry

[trs900]

The guy who owned it before said it was running perfect, could be the change in elevation, CT then Florida. Said he gained 35HP with 650 over a 600 on dyno.

[Jerry Clayton]

Most carb problems I see were caused by a dyno operation---max power does not equal drivability

[CobraEd]

I have a carbed 302. I am putting out about 400hp. I have a 650 DP same as yours. When I upgraded to a single plane manifold, I had to upgrade to 72s in the front and 78s in the back. My car runs awsome!!!! I don't remember what the jetting was before, but it was MUCH leaner. A single plane manifold requires measurably richer jetting than a dual plane. Plus, you can get away with a smaller carb on a single plane than a dual plane since all 4 venturies can feed each cylinder vs only 2 venturies on a dual plane. But, as I indicated, the jetting must be richer.

I also have the K&N Stubstack on the carb which adds andother 28 CFM and smooths the airflow. I didn't notice much improvement at high rpm when I added it, but throttle response and mid range were noticably improved.

.

[Jerry Clayton]

650 + 28 = 678 round up to 700 sort of fits my profile, huh?

The stub stack will significantly flow more at the lower carb openings, causing the carb boosters and venturis to act like considerable bigger carb

You can put a divider in the single plane manifold plenum area that will pick up the lower ranges response

Jerry

[wtm442]

If the plugs look OK, and the only thing that is wrong is the car stumbles, I would look at the squirter size, or a different squirter cam. But it all depends if the stumble is a lean condition or an over rich condition.

Get the RPM's above 3000 RPM in any gear and mash the gas pedal. If there is NO stumble, look at the squirter and squirter cam.

[FUNFER2]

Do you have a local business that has a chassis dyno ?
That is the BEST,...way of tuning your fuel curve timing ect.. rather than in your garage or on the street. It's well worth even traveling to find one.

[wtm442]

Based on Holley's carb numerical listing

http://www.holley.com/TechService/Library.asp

that carb originally had 67 primary and 73 secondary jets with a 65 power valve.

[FUNFER2]

The great thing with a chassis dyno (if the operator know's what he's doing)
you can make a full run, read the numbers and adjust the mixture at high rpm's then do a idle adjustment through the whole rpm range and you're set. It's well worth the money.

[wtm442]

Jerry
I agree with you about seat of pants driving conditions, but a dyno will tell you if the jets are too lean or too rich. It will also tell you if a low speed stumble is because of a too lean or too rich condition. Then after the jets are OK (and the power valve), then you can play with squirter's, etc.

I find that many people make 2 or more changes to a carb at the same time. Bad decision. Only make ONE adjustment and then see if problem is better or worse.

[FUNFER2]

We had a guy come to the dyno that raced in the Pro-5 class with his Mistang. He decided just to stop by for fun before racing at Heartland Park for a national meet. He assumed his motor was dialed in. When he hit 4,000 rpm, he was already way,.....lean. We went up three jet settings, adjusted timing and he put out 900 HP. He was extremely happy for two reasons, 1- he would have melted his engine being that lean. 2- we found 32 more HP just in simple tuning. We made 5 full runs and cost him $160 (2 hrs.) for shop time. Said he has $30,000 in the motor. He kissed me on the forehead for saving his engine !

[Jerry Clayton]

I agree with that somewhat---when the jet size is set for max power, then the drivability issues need to be dealt with within that wide open mixture ratio

It usually comes down to float hights, accelerator pump cams and squirters, along with getting the car to idle without so much throttle tip in that the idle circuits are out of the picture

I also use a much lower rpm to chech acceration and driveability 1500 rpm is good place to start and do it in 2 or 3 or else you will be at such a high road speed that it gets dangerous

Jerry

[Jerry Clayton]

I meant 442 post not funfer2

There is no way to successfully duplicate driving on a chassis dyno--

Flame on

Jerry

[Jerry Clayton]

We go to different dynos with our customers--they are excellent for being able to jet/time for max hp at max rpm---

I challenge any of you or for that manner, any rep for a dyno company to present a repeatable plan to adjust a carb for driving conditions---A car is not driven at wot on the street-----

The part throttle/ part load drive around is the best practical method to solve drivability problems that are not wot related---however it has its risks being on the road instead of sitting tied down on a couple of rollers

If you only got $160 for 5 runs and 2 hours time you should consider another line of work

Jerry

[Jerry Clayton]

Guys

You can charge whomever you want whatever you want, I don't care

The dyno DOES NOT TELL YOU IF IT IS LEAN OR RICH---sparkplugs, O2 sensor,EGT tells you that---the dyno tells you hp/tq

Weather conditions-----unless you are having a very, very unusual day, the weather really won't affect the dyno portion but will effect the corrected numbers---------!!!!!!!!!

If the weather is way unusual, you shouldn't maybe be doing dyno pulls,,, go drink the beer and eat pizza

As for drag racers---yep the dyno can posibly help them for max power, where to shift, etc---but if you don't tow the car back to the pits, you will lose even more valuble data than the dyno results---plugs need to be checked every run for max performance, egt or O2 readings evaluated,etc

Cars that are on the TRACK can be tuned on a chassis dyno---one dyno we frequent is a Mustang for 1700hp and 200 mph or something like that--

I think that some of the dyno installations need more air supply as shortly the air is fouled and doesn't have enough O2, at least my eyes and nose hurts

And besides all that all the females wave at us out on the street ( Females Kevin!!!!)

Jerry

[Jerry Clayton]

Cobra Ed

I disagree---the main jets are Carb 101, the rest is the advanced studies

Jerry

[CobraEd]

Originally Posted by Jerry Clayton

Cobra Ed

I disagree---the main jets are Carb 101, the rest is the advanced studies

Jerry

Jerry,

All that stuff can be done with the car sitting parked in the garage with absolutely no tools or analysis. Selecting the right main jets requires the car to be at idle, . . . and under full load, . . . .and everything in between while being monitered with an oxygen sensor to measure fuel/air ratio, all while under load on a dyno. Also plugs must be analyzed. This is the essence of carb tuning for performance. What you stated is for grandpa to ensure that the care drives "nice" and will idle. It has nothing to do with maximizing performance.

Ed

.

[Jerry Clayton]

One other thing that these days effect tuning----the gasoline being used, we have had guys show up with 100+ race gas for the dyno and later try to run street pump gas

Uh oh golly gee

What ever happened

Jerry

[Jerry Clayton]

Two things Ed

The carb will idle without any jets in it

This was about idling and driving nice

An dyno is not an O2 sensor

Jerry