I just rebuilt an old Holley 735 CFM center pivot carb from a 1970 428CJ Mustang to use on the 460 in my 77 HighBoy. It has headers, a 70 timing chain and dizzy work. No other mods.

It runs great idling and full throttle is ok but down on power compared to the stock 600cfm but the mid range is really bad. It spits and backfires through the carb on mid throttle application and cruising. (And the milage is even worse then usual) I'm constantly feathering the pedal to keep it moving.

At idle there is little spits of fuel coming out of the vent tube on a regular basis.

I checked the float level and it seems fine, barely trickling out of the level plugs. I cranked the front float WAYYYY down and it seemed to run better but still not good in the mid range.


Any suggestions? Did I miss something? What do the symtoms suggest? I think its way rich but I'm not sure why. But I'm probably wrong. ANd I forgot the old rule of thumb about the smell. If it burns your eyes it's rich? This thing doesn't. Just smells like an old pre smog big block. It sounds good and crisp revving in neutral.


Please no comments on putting the stock carb back on, I had to use it on my other big block.

That one runs good.

 

The float level should be just below the site plugs while running. Remove the needle and seats, inspect for debris and reset the floats to just below the site hole while running.

 

check your power valve maybe smaller main jets when you say center pivot do you mean center squirter accelerator pump sprayer? A crab. like this is for drag racing and the secondaries open at the same time as the primaries it's like flushing the toilet way to much air all at ounce which drops port velocity to zero and the engine falls flat. The best carb. for a street driven trail rig is a vacuum secondary 600 to 750(list #3310) to help keep port velocity up and low end/midrange power up.

 

Is your vacuum advance connected? and if so, to ported or manifold vacuum? if manifold you may want to try ported.

 

Does it hesitate when you hit the gas? I think it just sounds like too much carb for the motor---I could be wrong.

 

The carb is a center pivot float, vacuum secondary 735CFM from a 428CJ.

I cleaned the carb with a fine tooth comb, but I'll check the seats. And reset the float level.

This is the first time I've ever adjusted the floats on a Holley. Is it normal for the gas to come out of the set screw with a LOT of force when you crack it open too far? It was like putting your thumb over the end of a water hose.
I figured out how to do it without getting gas all over by the way.

 

Yes that is normal if you crack them open to much. Back fires through the carb are caused by lean fuel mixtures or over advanced timing. It sounds like the carb is functioning properly other than mid range and may need to be richened by putting in larger jets. If this is the case jump 2 jets sizes at a time until you reach reasonable drivabilty then move 1 at a time for optimization.

 

Thanks. The primary jets are MUCH smaller then the secondaries. I wish I had written the sizes down, but If I remember right the primaries were in the high 30's and the secondaries were in the 60's.

Now all I need is some gasoline.

 

One other thing to check is the throttle shafts themselves, its normal for them to be a little loose in their bores but if you can wiggle them very much you may need new bushings around them. With the enginer shut off, open the throttle about 1/4 th or so and try wiggling it in that position, if its very loose it can be a pretty good vacuum leak. Also, make sure you have the correct jets in it also to start with and power valve. I'm assuming you checked the power valve as mentioned above, and if it has backfired you probably blew the new valve already. if thats the case you can get a kit to protect the power valve from blowouts from someone who handles Holley parts. But mainly, make sure you have the correct parts for that particular carb, someone in the past may have mix n matched parts between a couple carbs , it happens all the time with Holleys, just cause they look the same does not mean they are the same with carb parts. If you need, I can use the list number and find out what jets accelerator pump discharge squirter it came stock with and everything else or you can also if you know where to look or who to ask.

 

Excellent.

I checked the throttle shafts and they seemed good to me.

The Sorensen rebuild kit had two power valves in it, and neither had any numbers on them. Weird, there was no secondary diaphram, so I reused the old one. Surprisingly it worked. The spring look modified though, nipped off, and stretched. I thought a new spring and diaphram would have been nice to include in the kit. Diaphrams and springs available seperately?

I'll have the list numbers posted by the end of the day.

Thanks in advance.

 

Primary jets in my 750 are 72's.

 

id say your primarys need to be at least 62 ish and your seconds around 66 thats how my 670 was jetted on my 460 and she loved it.

 

On the air horn are the numbers;

D0ZF-9510-0
List-6128-1
2368


What do the numbers on the jets equate to? Decimal inches? They do seem like it. If so, the primaries are VERY small compared to what you guys are teling me. The primaries are about .039", and the secondaries are about .062".

I'll check them out tomorrow.

 

Definitely sounds lean. Interchange the primary jets with the secondarys, take it out for a spin, and see if it runs ok in midrange and that will answer your question. If that works then you can start sizing your jets from there.

 

If you put the 39's in the secondaries for the test do not floor it as will definatly cause a serious running issue. I'm going to guess that you will need about 67's in the primary and 72's in the second. Yes the #'s on the jets do correspond with dec inch equivelant with a few exceptions in the lower 60's range maybe a couple of others. But 39's are definatley to lean.