Thread: new carb, runs rough in gear

Tweet

Ok, put a holley double pumper 4160 on my ford 460, it's in a 1986 F-350. Has an automatice tranny and is 2wd. Truck idles ok in park, but when i shift into gear(any gear) the idle becomes rough and chopper and almost feels like it wants to die....once you hit the gas to drive it smooths out and drives ok, till i let off the gas to brake and it becomes chopy again....hasn't died yet. Any suggestions? Is it a vacuum leak or carb adjustment?

huh ok, denny glad you found my post funny....maybe i'm stupid or alittle slow, but your reply made no sense to me. I'm not on this board claiming i know everything there is to know, that's why i'm here cause i wanna learn. I'm guessing you could have posted something that would help me, but opted for the flame post.......ok, fine hope you feel good now. I think my origanil post is serious....i put a new carb on and now it runs very rough in gear......never did it before with the carb that came off. That leads me to believe it might be the carb.

ok, my bad.....feeling sick today so my humor level is low. I just honestly didn't understand your reply.....came across to me like you were being an a-hole. BUt i do appericate all the tech and humor. As you can tell by my post numbers i'm still pretty fresh here.....just learning the way and feeling people out. Sorry for my rude post back. Now then....it's a new carb, no back fire, adjusted the idle so that it idles good, but once it's in gear the motor sounds like it's gonna die. Put it in park and it's good again. By going to a 750 cfm carb from a 600 cfm, would i need to adjust my timing some?? If so which way and way?

Jeepguy----Denny gets a little strange now and then, but is definitley not an A-hole. He is one of the most experienced and knowledgeable people on this board when it comes to automotive engines and diagnostics.
(and I think its a vacuum leak)

ok, i checked the vacuum readings, not sure which is which but when i plugged the guage on the vacuum outlet by the choke it read 18 and the one on the opposite side read 6. So i adjusted them to read 13 and 9 and that seemed to help some. Readjusted the idle and checked the float level which front and back read correct(just under the plug.)

You want to go by manifold vacuum, not ported (your lower number).

I think you're going the wrong way with your settings. You should be trying to get the highest vacuum readings when adjusting your idle mixture screws. Most everything I have read, and it's what I do too, is to start with them out a little further than normal and then tighten them up little by little until the vacuum just starts to drop off.

yeah, i thought about and readjusted them to where they were before. I'll get the manifold number and go by that and see what happens.

All of the holleys I have had always needed to be a little rich on the idle to get them to idle well under a load such as when you put it in gear. Might be something to experiement.

Ah just thought of something else. What is your vacuum at idle. If it's too low and the power valve on the carb is opening due to the low vacuum it will cause the engine to run rich which sounds like it could match your symptoms. Those stock power valves start to open at about 10" of vacuum, I think. Might be something to look into.

i'll look into it....if that's the case which way do i go on the new power valve open on higher vacuum or lower?

Run the mixture screws in till they LIGHTLY bottom out, then turn them out 2 full turns. With the engine running, turn each screw in a little(1/8-1/4 turn) untill the idle starts to "drop" off, then turn the screw back the other way till it picks up again, and to fatten it up a little just turn the screw out 1/4-1/2 a turn. This should get you in the ballpark of a good idle setting with no eye burning. If this doesn't help then I'm afraid you're gonna have to pull the radiator cap off. Pull the cap, back your ride out from under it, then pull a Dodge truck under the radiator cap, re-install cap, and drive off happy!

Are you running a big cam?

One with an advertised duration of 280-290 degrees will idle between 10-12" of vacuum. (Manifold vacuum, sourced from lower than the throttle blades anywhere on the intake manifold.)

Stock cams - and very mild ones on bigger engines such as yours - will idle at 18-19" of vacuum.

I believe late double pumpers come with idle screws at all four corners.

It sounds to an extent that the two bibs (outlets) you're checking have different sources.
One ported and one manifold.

If your idle is say 600 rpm and you're running a stock or mild cam - and the engine is in good shape - you should be getting 18-19" vacuum.
Use that source for all vacuum referenced idle and running adjustments.

Idling at 600 rpm and plugging a ported source - above the throttle blades (most times*) you should get zero and possibly 1-2" vacuum.
Spin the engine up to 1000-1200 rpm and you may show 5-10" of vacuum.

*Some Holleys have a manifold source bib higher than the throttle blades (right side).

Carter/Edelbrocks have two bibs at the front of the carb - near the base plate - and the driver side (in the USA) is manifold vacuum with the passenger side being ported vacuum.


I do have a question though; if you're running on the street why a double pumper?

Imho you'd be a lot better off with a vacuum secondary Holley or a Carter/Edelbrock with air flow demand secondaries.
Gas mileage is considerably improved over the double pumper as well.

I see guys pop double pumpers on moderately built street engines now and then, drive only on the street and in general make life tough for themselves.
By the same token, I see cars at the dragstrip that would be better off with vacuum secondary carbs....

ok, did some searching today and found that the carb is NOT a double pumper....my mistake. It just doesn't use the transfer tube to balance the front and rear bowls. The motor is stock, not a big cam....just stock. Ran the truck to work the last couple days and it's going through gas like it's water....and it diesels after i turn it off. Talking to a guy i know at work he says more than likely the problem lays in my vacuum lines. He thinks i have a bad vacuum leak somewhere.....so i'm gonna try and find all my vacuum ports make sure there plugged and also replace the gaskets between the manifold and carb. I didn't do that when i pulled the old one off......stupid i know. So i'll see what that does.