I've only got 2 Holley rebuilds and just a smidge of tuning under my belt so I just wanted to bounce some ideas off of you guys who have more experience with carburetors.

My stock 429 Thunderjet has a Holley 600 with vacuum secondaries. The only issue I have with it is that the throttle shafts tend to leak fuel and drip onto the manifold. Other than that, it starts and runs pretty well.

My dad gave me a rebuilt Edelbrock 750 (#9907) with mechanical secondaries and I have some concerns...

-There is no provision for the C6's kickdown bar.

-Mechanical vs vacuum secondaries?

-There doesnt seem to be any way to adjust when the secondaries kick in...

-750cfm vs 600cfm? Stock carb was an Autolite 4100 - not sure of the stock flow. Google reveals 480 up to 600...

Any thoughts would would be appreciated!

 

here is some answers i can help with

- you will probably have to get an adapter from Edelbrock or the aftermarket (Lokar comes to mind) to adapt the kick down lever to the carb. lokar also makes new kickdown cables with adapters to go away from the factory rod system. helpful if transplanting engine into something with tight spaces.
-depending on what you are doing with the engine, each have their benefits and drawbacks. for a strictly street driven truck vacuum is the way to go. the secondaries will only kick in when there is a load on the engine (like towing, climbing hill, etc) and won't open at normal conditions (like cruising down the hi way under no load). mechanical is great if you are racing or such. just mash the throttle and the secondaries are open. not great for towing or street cruising as you have to mash the throttle to open them and they are abrupt. also depending on gearing and such, at hiway speeds they can always be open or partially open.
- usually the adjustment is made by bending the linkage between the front and rear plates. not a lot of adjustment can be made with this style of carb.
- for a stock engine that doesn't have much or anyone upgrades and probably will never see over 4500rpm (the power band on the smog motors usually falls off after 4000rpm anyways) a 600cfm carb is more then adequate. a 750 would be a good choice for a warmed up engine that has upgraded cam, intake and exhaust and can use the better breathing at the top end. for a really nice engine that has a lot of upgrades and is pushing closer to 500 hp a 850 would be minimum needed. but a 600 for a stock engine is more then adequate.

hope this helped some.

rgds
Mike

 

+1 on the above!

You can get away without running the downshift rod altogether, it just means having to manually downshift a gear when you want some get up and go.

A 600cfm carb will be just fine for any street use. I have to laugh at how many people I've seen with severely oversized carburetors and then wondering why their fuel economy and driveability are so poor (of course a high-RPM cam is often the primary culprit here as well) during "normal" driving.

You may need to rebush the throttle shaft bores if you have fuel leaking out - that's not normal and may also be due to some other problem inside the carb.

 

Wrong on the Edelbrock, the secondaries have mechanical linkage for the throttle valves but they do not control air flow thru the secondaries and you do NOT adjust them. The "air valve" in the top of the secondaries is what controls secondary opening for air and fuel and air flow. That is spring controlled and is adjustable on the air valve shaft on the top of the carb. It works pretty much just like the vacuum secondaries on the Holley do.

 

If your Holley (or any carb for that matter) is leaking fuel out the throttle shaft, there's an internal problem causing it to dump too much fuel that needs to be addressed. If your throttle shaft is worn, you'll get a vacuum leak, but you still shouldn't get fuel coming out there. Fuel is dripping in the throttle bores, puddling up on the throttle plates, and leaking out.

 

Thanks to all for the replies. I chose to switch back to the holley for now due to the kickdown rod issue.

As for the throttle shaft leakage, the excessive fuel issue makes sense. I do believe there is something wrong with the carb:

When I bought the car, there was fuel in the crankcase and badly fouled spark plugs. After a tune-up and a carb rebuild, it ran very rich. So rich in fact that it would start and run with both idle screws fully closed! After a second rebuild, I had a friend of the family who builds race cars go through the carb and he said it looked pretty good.

Finally I drilled a 1/8" hole in each of the primary throttle plates which helped considerably. I was now able to use the idle screws get a healthy 22inHg at idle and smooth operation at any rpm. Obviously this is some sort of band-aid fix since it still leaks at the secondary throttle shafts (though not as profusely) and will start easily with no choke at all.

Since the Holley starts easy with no choke, yet the Edel does not, I suspect its still running rich. Maybe there is a crack in the secondary metering block or maybe the carb body? I couldnt find anything visually. I'll eventually switch to the edelbrock 9907 and then later to a brand new #1406 Edelbrock 600 with an auto choke.

 

From what you've described, you definitely have an issue with that carb - either you've got a float setting too high, a sunken float, and/or some kind of gasket/seal issue which is causing excessive fuel to be leaking into your venturis. Definitely monitor your plugs and oil - you don't want raw fuel in the oil for a number of reasons.

 

Try a fuel pressure regulator set for 4 lbs for your fuel seep.

 

i had a similiar problem with the Ford Holley i used also. what i finally tracked it downt to was the transfer tube for the accelerator pump that goes between the main body and the metering plate has two o-rings on it and gets sandwiched between them. when installing it one of the orings got pinched and wasn't sealing and letting fuel drip into the carb. caused all kinds of issues till i finally noticed it after the 2nd or 3rd tear down.

might not be your problem but something to look at just incase.

rgds
Mike