Carb rebuild

[LEB Paul]

I have a holley 750 double pumper vacuum secondary on my basically stock 460, when driving it over heats and when I pulled a plug after it got pretty hot it looked as if it were running lean. Timing is good and it will sit at 170-180 at idle for as long as it sits. So we decided that the secondaries were probably not functioning properly.. so pulled the air cleaner blip the throttle or open it up hard.. no fuel squirting in the back of the carb.

I'm fairly sure it is not a vacuum issue, but I will go over the lines and check to make sure the un used ports on the carb are blocked off. Assuming it is not a vacuum issue, it appears it is time for a rebuild. Does anyone know where I can find a kit to convert it to mechanical secondary and a good over haul of the carb? I've not worked on carbs before so I don't know what I'm looking for when looking up the different rebuild kits on summit or whatever.

So to recap
-can someone show me where to find a kit to convert this carb to a mechanical secondary?
-and which rebuild kit would one suggest for this carb?

Thanks for any help

 

[FordF-SuperDuty]

I don't know much about carbs but I don know the Holleys are more complex than Edelbrocks.

 

[surewhynot]

If it's vacuum secondary, it's not a double pumper. Double pumpers are mechanical secondary carbs, with an accelerator pump on both bowls. With the vacuum secondaries, you won't be able to see fuel in the back barrels when you blip the throttle. It works off of demand only.

If the carb s in good shape, you may just need to tune and jet it.

 

[LEB Ben]

If it's vacuum secondary, it's not a double pumper. Double pumpers are mechanical secondary carbs, with an accelerator pump on both bowls. With the vacuum secondaries, you won't be able to see fuel in the back barrels when you blip the throttle. It works off of demand only.

If the carb s in good shape, you may just need to tune and jet it.

Agreed...but if you are going to swap, I think the bulk of what you'll need is a DP base plate and the corresponding bowl. However, any time I've seen this topic come up, it seems alot of guys say this swap will cause excessive bogging. I'm not really sure why or how, but that seems to be the general concensus.


On edit...you have PN so we know exactly what carb you have?

 

[LEB Paul]

My mistake, I thought it was called double pumper because it has two fuel lines going into it - it is a vacuum secondary holley 750. (See how much I know about carbs? ) The carb sat for a while before going on the truck (possibly two years) and looked kind of rough. Is there any way to tell if I'm getting fuel in the secondaries?

 

[LEB Ben]

Directly from the Holley FAQ's:

QUESTION I can rev the engine when it is in park and the secondaries will not open. Why is this?

ANSWER The secondaries will not open by free revving the engine. The engine needs to be under a load before they will open. If you are still uncertain if they are opening or not you can take a normal paperclip and clip it onto the secondary diaphram rod. You will then push it up against the bottom of the secondary diaphram housing, now you will need to go out and drive the vehicle. When you return you will be able to look at the position of the paperclip on the rod. If it is lower on the rod then you can tell the secondaries opened and how far they opened. This is useful in determining if you need a heavier or lighter secondary spring.

 

[LEB Paul]

I didn't know holley had a FAQ lol

Thanks Ben, I'll give that a shot after I go over all the vacuum lines and make sure they're good. Do they have a diagram to show which vacuum ports should be plugged and where they others should be getting vacuum from?

Am I right in assuming that the secondary diaphram rod is the vertical rod on the passenger side of the carb that goes into a square box looking thing? Sorry again for sounding like an idiot, but this is the first carb I've worked on.

 

[surewhynot]

Yup, you assume correct.

 

[LEB Ben]

Not a problem Paul...and from your description...yes I think we're talking the same rod. Holley has a great tech section with owner's manuals, diagrams, etc...so if you post up your PN (should be somewhere on the front driver's side of the carb)...I'l post up your manuals for you to make it a little easier.

 

[LEB Paul]

Cool, thanks Tony

I'll get the PN tomorrow Ben.

 

[LEB Ben]

Cool, thanks Tony

I'll get the PN tomorrow Ben.

I was about to say bite me, and fix your own damn carb...but now I see you acknowledged my post...so you're safe...haha.

 

[LEB Paul]

hahah you posted while I was typing

 

[money_pit]

Agreed...but if you are going to swap, I think the bulk of what you'll need is a DP base plate and the corresponding bowl. However, any time I've seen this topic come up, it seems alot of guys say this swap will cause excessive bogging. I'm not really sure why or how, but that seems to be the general concensus.


On edit...you have PN so we know exactly what carb you have?

It's because there is no accelerator pump in the rear. It's like you suddenly got a ton of air put in and no fuel, until the secondary fuel system catches up it's just a big lean bog.

 

[LEB Ben]

It's because there is no accelerator pump in the rear. It's like you suddenly got a ton of air put in and no fuel, until the secondary fuel system catches up it's just a big lean bog.

Thanks for the info...never dealt with DP's, so I wasn't sure about the construction.

 

[LEB Paul]

Directly from the Holley FAQ's:

QUESTION I can rev the engine when it is in park and the secondaries will not open. Why is this?

ANSWER The secondaries will not open by free revving the engine. The engine needs to be under a load before they will open. If you are still uncertain if they are opening or not you can take a normal paperclip and clip it onto the secondary diaphram rod. You will then push it up against the bottom of the secondary diaphram housing, now you will need to go out and drive the vehicle. When you return you will be able to look at the position of the paperclip on the rod. If it is lower on the rod then you can tell the secondaries opened and how far they opened. This is useful in determining if you need a heavier or lighter secondary spring.

Okay, I did this and one time the paper clip fell off, other two times it didn't move at all after driving it around a bit. Here's a picture to make sure I was doing it right.

I also rerouted the vacuum lines as I had both lines coming off the carb and manifold vacuum all connected with a 4 way "t" and connected to the dizzy. Now it's just the one off the carb straight to the dizzy and I plugged the other two off. Seems to run a little smoother now.

Not a problem Paul...and from your description...yes I think we're talking the same rod. Holley has a great tech section with owner's manuals, diagrams, etc...so if you post up your PN (should be somewhere on the front driver's side of the carb)...I'l post up your manuals for you to make it a little easier.

3310-10 is the number you're looking for I think. There's about 30 different numbers stamped on it lol

 

[LEB Ben]

Here's the instruction manual for a 3310 (went through 11 or 12 revisions, but this should cover the basics):

http://www.holley.com/data/Products/Technical/199R9934-3.pdf

So from what you said, that means your secondaries are more than likely not working properly. Any chance of bad vacuum? Blown diaphragm? How about a wrong spring (you may need a lighter one)? One more test, disconnect the secondary linkage, and see if it acts the same way...but leave the linkage rod in place so you can get the secondaries to close in case they are working. Only other thing I could think of is possibly a check ball problem in the vacuum chamber. Beyond that, I'm stumped. But really, I think all of this will be solved with a complete rebuild kit.

 

[LEB Paul]

There is a noticeable difference in idle when I pull when the vacuum caps off, so I think that would indicate that there isn't a leak anywhere else? The vacuum advance is working too as far as I can tell.

 

[surewhynot]

The secondaries are only going to open when conditions demand them to. Did you get on it some while driving it around?

 

[LEB Ben]

Alright...then I think it's time for a rebuild. A kit should have the new vacuum diaphragm if needed, it should have two check balls (IIRC the bigger one is for the vacuum chamber) and it may even include springs...I think most guys run either the yellow or purple springs with their 3310's. Hope that gives you some sorta direction.

 

[LEB Paul]

The secondaries are only going to open when conditions demand them to. Did you get on it some while driving it around?

Yeah, WOT pretty much the whole time. Couple of burnouts. 4wd launches. I was making sure it would be using that fuel if it was opening.

Alright...then I think it's time for a rebuild. A kit should have the new vacuum diaphragm if needed, it should have two check balls (IIRC the bigger one is for the vacuum chamber) and it may even include springs...I think most guys run either the yellow or purple springs with their 3310's. Hope that gives you some sorta direction.

Looking at the diagrams in that pdf the big vacuum port on the back of the carb is labeled as brake booster vacuum - I have my pcv hooked up to this and brake booster vacuum coming off of the manifold, is this a problem?