360 w/4bbl Holley stumbles & stalls w/ little bit o' gas, fine with WOT?

[Eternalfootman]

Okay, I'm really new to carbs, so learning how to tune this one has been a PITA. Here's the deal:

1985 Dodge Ram 350
5.9L stock
Holley 750 4bbl dual pumper

So yesterday, when it was a little colder out, the engine - even when warm, would stumble and sometimes even stall if I gave it just a little bit of gas. It idles fine, both in D and in P, but give it a touhc of gas and it just wants to die. Step on the accellerator, and it goes!

I have the choke rigged so that it is all the way open, since it has a manual choke adapter on it, but there's no hande in this truck. It was a little cooler yesterday, but I figured that once the engine was warmed up that wouldn't matter.

I have the jet screws set at 1.5 turns open, but I played with those a bit, and it didn't seem to matter what they were set at. I also have an MSD variable timing controller, and I played with ignition timing - I was able to make it worse, but not better. At normal, it is set at 10 deg BTDC at idle.

What am I missing?

Thanks!!!

Dave

[peg leg]

Sounds like the accelerator pump is not pumping at all, or very little. Check in the venturis for a healthy squirt (with the engine off), while operating the throttle shaft. Then check to see if it's linkage is connected, or mal-adjusted.

[rumblefish360]

There ya go. It's all in the total operation and tune of the unit. Recheck and check again. Make sure the squirters give gas the very instant the throttle is moved. Thats key.

[lilcuda]

Holleys have different accelerator pump cams. Each one has a different profile. Some provide a larger shot right off idle, others deliver a more even shot of fuel throughout the range of throttle position. Since your engine stumbles right off idle, but goes great if you stomp the throttle, it sounds like you need more pump shot right off idle. Buy a set of Holley pump cams and try different ones to see what works best with your combo. Change ONE thing at a time.

Lee
1966 Barracuda, 340 4 speed

[1973Swinger]

A Holley tuning manual might be a good investment for you. The more you know, the better off you are.

[aarracer]

Hi

I believe it doubtfull/ (but possible if it has been laying around and has the diaphrams dried up) that a double pumper would have both pumps failed?

Buy a vacuum guage and adjust the throttle plates at a good vacuum range and then try to adjust the mixture screws. Remember, the idle rpm's are adjusted at the mixture screws NOT the throttle plates!!! Write down all your settings as you go, so that you may refer back to what you have done!

In the holley manual you will find that first you have to get mixture control from the idle mixture screws. If you do not have this ability to control the idle setting from the mixture screws, you are not generating sufficient vacuum at the transfer slot to operate the engine correctly in the 0 - 3000 rpm range.
You should see a maximum of 0.040 of the transfer slot from underneath the carb. The throttle plates should NEVER be closed tight as this may damage the bore.

If you cannot get sufficient vacuum at idle to operate the idle mixture screws you must go into the metering block and open up the idle siphon/suction tubes (0.002 at a time until idle mixture control is gained)to provide the fuel to the mixture screws to get control here! I have had to go as far as 0.024 to get control with 292/509 cam, but it idles at 400rpm in gear (3" of vacuum/water) and run's through the rpm range totally smooth.

This is advanced tuning and if you are not profficient at these steps go find a carb with changable/adjustable air bleeds and this will solve this problem.

You should be able to operate the carb sufficiently without the accelerator pump connected and /or with it delayed as in a foot brake drag car.

If this is a carburetor issue!

Cheers

AARRACER

[Eternalfootman]

Thanks for all of the replies - I love this board! I did test the "squirt" last night and both sides were providing a good shot of fuel. However, I'm wondering if my problem really is the choke. It's been in the 60's here (upstate NY) for the last couple of days, and I'm thinking I needed the choke on, even once the engine had warmed up a bit. Does this sound crazy? It would explain why I was able to drive it a few days ago and it worked fine...

The reason why I say this is that I did some fiddling after checking the squirt, and by moving the choke lever a bit, I got a much smoother idle. I didn't want to drive it around, because now the choke lever isn't attached. So I'm going to put on the manual choke adapter I have and get a cable, and we'll see how she drives tonight.

Thanks!

Dave

[dgc333]

FWIW, a 750 double pumper is way to much carb for a stock 360. With some careful tuning you may get it to run acceptably but you will have a much more pleasant driving experience with a vacuum secondary carb in the 600 to 650 range.

[chitown hustler]

Originally Posted by dgc333

FWIW, a 750 double pumper is way to much carb for a stock 360. With some careful tuning you may get it to run acceptably but you will have a much more pleasant driving experience with a vacuum secondary carb in the 600 to 650 range.

Yes indeed ! Especially installed in a 1 ton truck .

[rumblefish360]

And I'll make another hand up to agree with that.

I have a 750 on my '78 - 400 right now and it is a bit fussy till it's warmed up. I have not yet changed the squirter nozzle out.
Do not think that the 400 is a big power house engine. The rating for this B engine is low in '78. All I did was add headers and a chrome box.

[RacerHog]

Spark plug check

[JD451]

[QUOT

The reason why I say this is that I did some fiddling after checking the squirt, and by moving the choke lever a bit, I got a much smoother idle.

Dave[/QUOTE]

By this statement, it sounds like you may have vacuum leak, start checking for any loose, cracked or hoses that came off, this will cause stumbling & a lean condition.

[dartster73]

Do you have a vacuum advance hooked up? I just ran in circles blaming my carb when it turns out my vacuum advance tension was too easy. It made it stumble all over itself just off idle in gear. Idled fine and get past 1/8" throttle and it goes.

Quick check, unplug the vacuum advance line and go for a drive.

[dgc333]

The vacuum advance should not be doing anything at idle if it's connected to the timed or ported port, so it would not contribute to an off idle stumble.

[dartster73]

I put a tee on the line with a gauge, the moment it got "off idle" the ported vacuum got 10-12 in Hg and it advanced the timing about 10-15 degrees. Just above off idle the ported vacuum dropped and the motor smoothed out

I used a 3/32" allen and tightened (counterclockwise) the vacuum canister about 2 turns. Problem solved, the vacuum only advances at higher RPM and no load.

[dgc333]

Sounds like the primary throttle blades are open to far. You may want to open the secondaries a bit so you can close down the primaries. Or advance the ignition timing which will also let you turn don the primaries.

[turbododge]

I agree with dgc333, it sounds like the primaries are not in the correct spot in relation to the transfer slot, but they could also be too far closed, especially with a 750 on a 360. You want just a bit of the slot uncovered at idle. If you have no slot showing on the underside of the plates, or near 1/2 of it, you will need to readjust it. Then you leave the primaries alone and get you idle speed by adjusting the secondary plate stops.

[hotrod7043]

Better of get a 9801 TQ or a edelbrock avs, dps are for race cars and hot street cars.

[Eternalfootman]

It's been running pretty well the yesterday and today, but all of your suggestions are very good things to look at...

First off, brand new spark plugs are in as of last week (issue 1 taken care of).

Secondly, I put the 750 Holley on the car because my edelbrock 650 lean burn carb developed some serious problems, but I need the truck to go. So this carb is temporary. But I have a feeling the problem will follow me, even when I finish reuilding the Edelbrock carb.

I will check vacuum lines tomorrow, if I get the chance. Changed out the struts on the Subaru today, and the Volvo needs new brakes and some other work tomorrow, so I'm not sure if I'll get to the truck. But I'm going to get a vacuum guage (don't have one - been doing all this by "ear," if you know what I mean.) and some more hoses when I return the spring compressor I borrowed from Advance Auto.

I also noticed tonight that the vacuum hose from the brake booster is going to the rear of the manifold instead of to the carb. The vacuum line that it going to the carb where the booster line should be is going to the PCV valve. This could be my problem? (I just hooked up the Holley the same way the Edelbrock was set up.)

Hmmm... Lots to do! And thanks for all your help: Tuning a carb is like voodoo magic to me. Give me a laptop and MPFI any day of the week, but this stuff is a %&!@# mystery!

Dave

[Duster367]

If you go up on the squiter you may need a 50cc accelerator pump. I really think yoiur carb is too much for a stoc 360 a vaccun secondary 650 is up your alley.

[dodger1]

Re vacuum line locations, both the brake booster and pcv need manifold vacuum. Booster vac. location is normally at the rear of the intake. PCV is normally at the unported carb vacuum port. So I think you're OK with the locations of these. Agree with your carb tuning opinions, that's why I"m keeping my TQ