Ok, I've been doing some searches on here because I think my carb is out of tune. I'm really not sure what model number, but it is a Holley double pumper. It has no choke, and it has vaccum secondaires. Anyway, I was talking to my friend about my carb and he said my secondaries might not be opening. I don't know how to tell if they are or not. I really don't get a "pull" feeling at WOT. It just seems like a constant steady pull. He said I might be running only on 2 barrels. Any time I try to punch it at WOT I get bog. Even on the expressway going about 60 Mph and I get on it, it almost feels like it's bogging. I read on here that bogging is always caused by a lean condition. My fuel pressure is set around 6-7 psi.

I know very little about cabs so if someone could help with how I can find out if it is in fact the secondaries I would appreciate it. Also, what is the difference between mechanical and vaccum secondaries other than the obvious...which type is better?

 

Those 2 things are mutually exclusive. They don't exist on the same carb. Only manual secondary carbs are equipped with a secondary accelerator pump; vacuum secondary carbs have no need for one.

So which kind have you really got?

A vacuum secondary carb should in fact feel like a steady pull; the way those work, is that as flow through the primary venturi increases, the "signal" from it, which is called "venturi vacuum" and is a very different thing from manifold vacuum, increases; and the vacuum diaphragm on your vacuum secondaries is operated by a sample of the primary venturi vacuum. In other words, the secondaries do not open at all until the primaries reach a certain proportion of their total flow capacity.

"Bogging" in a vac sec carb is almost always the result of the secondaries opening too much too soon (at too low RPM).

If a manual secondary carb "bogs", then almost always, either the carb is too big, or the driver needs to learn how to drive the car...... which means, use less throttle until the RPMs build up; and apply it gradually instead of stomping it to the floor.

What is the list number of your carb?

 

Well, I know that it has two inlets for fuel, One on either end of the carb, and they are both hooked up to the supply line. A friend of mine told me it was a double pumper, but the guy I bought the car from said it has vaccum secondaires. He gave me a bunch of springs to go with it also. I'm not sure how to use the springs but he gave me a sheet and basically it's a trial and error thing with the springs. I have heard to go with the lightest spring.

Also, for the most part when I drive I gradually apply pressure to the throttle to gain RPM's rather than just floor it. Like I said, I'm new to carbs and don't really know how to tune them.

I'm not quite sure on the list number. I tried to find one but didn't really see one. I'll have to double check on that and I'll let you know.

 

http://www.holley.com/

You have vaccum secondaries if they guy gave you a bunch of springs.

and i believe going with the lightest spring makes them open up sooner.

 

The fuel inlets are not "pumps". Having 2 fuel inlets does not make it a "double pumper".

But if it uses springs, it's a vacuum secondary carb, not manual, and therefore not a double pumper; so at least we know that much.

"Hearing" that you should use the lightest spring, pretty much guarantees that wherever you "heard" that from, you should ignore in the future. That source is not to be trusted.

Using a too-light spring causes the secondaries to open too soon, and creates the bog and stumble and all that, as referred to above. The lightest spring is about 100% guaranteed to make any car run like hammered dog plop, on any motor below 450 CID and carb above 650 CFM. And most especially, if the driver is ignorant of how to drive the car properly, and just stomps the gas to the floor all the time. So avoid that.

Try the heaviest spring first. Should be black IIRC. That one is probably too heavy; but it should cure the bog. Then try successively lighter springs, until the bog returns; and go back up to the next heavier one than the one that creates the bog. If this is a typical 350 and a typical 750 CFM carb, you'll probably end up with the brown spring in it.

Then once you get that right, it's time to start really tuning the carb, and dialing it in to the engine's needs.

What list # is this carb?

 

Alright, I just went out to look at the carb and cannot find any numbers on it at all. The only one I did find was 6R 2762. I don't think this is the list number. I did not find any numbers on the side, or in the front or back. The only thing cast in it is "Holley"

Where else would I look?

 

Look at the place where they put the list #.

 

Incidentally, the carb in my pic, is a double pumper; note that it doesn't have the diaphragm to operate the secondaries, and note also the second accelerator pump (hence "double pumper")on the rear fuel bowl. You can't see the linkage that operates the secondaries in this pic because it's on the other side.

 

ok, I found the list number: 3310-2

It also had 2948 below that. Don't know if that number is important or not.

 

Pretty familiar carb.... I've had more than I can count on cars over the years, I think I still have a couple of them laying around here and there.

It's a 750 (or 780, depending on how the "ratings" are generated) vac sec carb. It was a factory carb; originally came on mid 60s 427s and 327s, then 302s and 396s and 350s later on. Kind of GM's universal hot-rod carb. It was a 3310 - nothing, then later a -1, then the -2, and so on. They're up to about -6 now, so yours is pretty old.... probably 25-30 years old.

The factory spring in it was the plain one. Try the brown spring.

 

Heh, you think yours is old.... I got a "3310 - nothing"

But yea, I agree with RB about the secondaries.

BTW - if he tells you to look down the carb and see if they are opening, ignore it. They won't open unless the engine is actually pulling the car. If you just yank them open in your driveway, they aren't gonna open.

 

Thought you might find this interesting. Its not the same carb, but a very similar one.

1970 Z-28 Holley carb

 

well, the guy I bought the car from said the engine was originally from a GTO. He didn't say what year but I'm sure it was from the 70's. There has been some upgrades to it, but right now I'm just focusing on the carb. Thanks for the site by the way...lots of info!

 

Ok, I tried playing with the springs a little bit. Just put in the black spring. The bog did seem to go away and the car seemed to have a little bit more low end power. The one that was in it was like a gold colored one, in my instructions is says the plain one, which I think that one is, is the standard and should open the secondaries at about 1920 RPM. My sheet also says that heavier cars need a stiffer spring as do cars with a restrictive air element. I have a K&N, not sure how restrictive it is. So I should work backwards until I feel the bog again correct?

update...I tried a bunch of springs progressively lighter. I actually can hear the secondaries open up now and I can feel more power coming from the engine. There doesn't seem to be as much bog as there used to be. There is one more spring that is a little lighter than the one I have in now. Should I try this one?

 

Just try them all and see what they do, use the one that seems to work best.