Hello all,

I have a Double Pumper 650 (non-4 corner adjustment) that is constantly giving me fits. Its got a fresh rebuild kit on it, and i have yet to have the thing run properly.

It will not hold a steady idle and vacuum levels are much lower than with my 600cfm double pumper.

I've adjusted the idle slot adjustments to what seems to be the appropriate amount of space.
70 primary and 72 secondary jets are installed
Primary and secondary floats are set properly, just a bit of fuel spilling out when the truck is rocked side to side.

I'll have a youtube link up shortly to show exactly what its been doing.

 

are you running a choke?

 

Both carbs have a choke, but no cable installed. I've only needed to use the choke in sub zero temps. The engine was warmish for testing both carbs.

 

alright most guys I know mill the choke off on the double pumpers, are you positive nothing in the carb is sticking? I know it may seem high but the guys I know have there Idle set 1000rpm and they don't idle good under that.

 

here is a video

 

I have not milled down the choke horn, as much as i would like to. The truck normally idles between 600 and 1000rpm depending on how warm the engine is. Normal vacuum is a solid 10 to 13 in/hg.

I do not run a vacuum advance. It just seemed to respond better without it when tuning the 600.

 

the only thing I can think of is maybe the one of the gaskets in the carb went bad and is causing a vacuum leak, I really don't have a whole lot of experience when It comes to carbs. btw the way your truck is pretty sweet.

 

I just rebuilt a Holley 600 single that had been set on the shelf for 20+ years. My 400ci runs 13-14 hg. I first had similar issues. Found a lot of good info here: Holley Tune, Part 1
Neighbor asked me to try to slowly close the choke and try to keep the engine running with the throttle at the same time. I found that I very nearly could. The carb was getting air from somewhere els. Therefore I ran a vacuum leak smoke test and found that my carb base plate was warped. I was able buy a new one and now I’m in good shape. I hope this helps.

 

Sounds like a vacuum leak. Check the throttle body gasket between the throttle body and main carb body. Make sure the correct gasket was used when the carb was rebuilt. The correct gasket for the 650 DP (0-4777) is Holley # 108-5.

 

You've gotta have a vac leak or other problem. Does your truck have an aftermarket cam in it? If so, than a lower vacuum reading may be normal. If not, you have to pick up 8" of vacuum from somewhere. Once you do, I would suggest giving the vacuum advance a try. You will likely improve operating temps, throttle response, idling and part throttle performance.

 

Im assuming its a bad gasket or vacuum leak inside the carb. I really hope that its not the metering rods, as those are tricky to change.

What really stumps me is the fact that it runs fantastic with the 600. When i bolt on the 650 it sounds like its got a miss, randomly throttles up and down, and is much louder. It seems like the carb is completely bypassing the idle circuit.


The truck does have a rather lumpy cam in it, for sure not an RV type. It doesn't come alive until after about 2,700 rpm.

The main body to base plate gasket is a good point. Ill pull it apart and check those to be sure.

 

What brand of carb? Since you are talking about a double pumper and jets I would assume this is likely a Holley? Each work in their own different ways and how to try and get them working correctly.

First things first, get a timing light on the engine and make sure the timing is exactly where it needs to be. It is amazing how just being off a little bit with the distributor can make "carb" related issues.

Second, get a vacuum gauge to dial in the idle screws. Yes you can do it by ear and the sound of the engine but the only way to really get it perfectly right is with a vacuum gauge. A stockish type cam will get you probably get you 20"+ of vacuum. As you get more and more aggressive cams the idle vacuum will get lower and lower.

Your power valves (if holley) should be half the value of the vacuum reading at idle.

After all of this you need to just put around town and never let the secondaries open up. Pull a few plugs and see if the primaries are rich, lean, just right. You then need to adjust your primary jets. Then you need to work on the secondary jet sizing.

Then there are the accelerator pump cams on a holley. These cause the slight hesitation from idle or near idle when you hit the gas. There is a slew of these to try if you have this hesitation issue, but only address this after everything else is dialed in.