402 vacuum question

[franko72]

I just bought a rebuilt 4MV carb for my 72 402 engine. It seems to be sluggish when coming out of the hole.I attached a vac. guage to my intake that I have been leaving attached as I drive. the vac. is about 15 to 19 inches when it is at full temp. When I rev the motor the vac. will sometimes fall to 14 inches then slowly come back up. I adjusted the valves and it improved slightly.do I need a valve job?The motor was supposedly rebuilt a few years ago. It has great oil presure ,doesnt knock or ping. runs strong when you are on the road. just doesnt burn the tires when at a stand still .need advice

[grayw0lf]

Sorry I can't actually answer your question, but ai can perhaps shed a little light.

It is very normal for your vacuum to drop when you hit the gas. You have the higest vacuum at idle.

The best thing to do it you think your engine/heads are old & worn is to use a compression gauge. This will tell you what shape your piston rings are in & tell you if you are in dire need of a valve job.

Hope this helps a little.

John O>.

[FRENCHBLUE72]

Sound's like it has good vaccum maybe it has tall gear's in the rear end causing it not to but=rn the tire's off the line..

[franko72]

the rear end has a 3:54 dana .Do you think that the gear ratio could be keeping me from burning the tires out of the hole?It has a 400 turbo trans.too

[GMC Jim]

I would check engine ignition timing...Partial Specs for 1972 402 eng:--Initial advance= 8 degrees @ 600 RPM; with distributor vacuum disconnected and plugged:--centifugal advance should be = 10-13 degrees @ 2000 RPM. with distributor vacuum connected:-- total centrifugal & vacuum advance should be 28-35 degrees. (Specs are per GM 1971-80 Tune-Up Specifications; Manual SD-100

['70_402]

I have that same prob with my 402.....runs like a banshee when you get above 15-1800 rpms but just don't cut it at low end.... so i have tried eveything in the book and i have come to the conclusion that i need a bigger cam.....just not breathing well low end.... so i am putting that in as soon as i save up the $$$$

[TsgtE662]

My 70 GMC 402 was stumbling off the line until it caught and took off like it was in a hurry to get to the end of the road. I was told by several friends the accelerator pump was bad in the carb. So I rebuilt the carb ( 4MV 7040202 QJet) and now it is real touchy off the line. Had to get used to the gas pedal again. It'll spin'em easily now. And I have the 3.07 rear. I don't have a tach for it so I don't know what rpms it is runnin at idle or any other time. I noticed there isn't very much vacuum suction at idle from the vac adv port on the carb until I raise the rpms up. But I have no complaints on how it runs while driving. It'll roast'em on demand pretty much untill the tires get traction.
I don't have a vac gauge either so I can't help ya much there either. All I do know is it is running great since I rebuilt my carb. And the timing is on or near 8 degrees. And the engine is stock too and not rebuilt and showing a little over 66,xxx miles.

[No3Fan4Ever]

Tsgt662 how much work is involved in rebuilding the stock carb?
I'm having alot of difficulties starting mine after it sits fro a while.....

[TsgtE662]

Rebuilding the QJet was easy. I have never rebuilt one before and I was alittle apprehensive about trying it. But how else was I to learn?
I got the kit and bought 3 cans of carb cleaner. Then set out on the patio for about 3 hours taking it apart and then scrubbing every part of the carb with the cleaner and brass wire brush. I wasn't in any hurry and took my time at it. It was obvious why it ran crappy as I took it apart. The rubber seal for the accelerator pump was tore. I replaced all of the parts inside and put it back together. Pay attention to the directions with the kit, I also have a Rochester Carb book and read alittle of that too as I went. There are a couple of parts you will reuse and shouldn't use the cleaner on it says. For no experience at rebuilding carbs this was really easy I thought. I am even thinking that if I ever find any like at the salvageyards I may grab them and rebuild them on my own to get more experience at it. And maybe sell them.
With the cost of a new one/rebuilt one I would recommend rebuilding it yourself. Be careful when disassembling the carb as a couple of small spring are in there where the float is. In that area of it. I didn't see where the spring came from so I called a mechanic friend of mine to come over and help me get it back together. Then I felt stupid cause it was really simple.
I won't hesitate to do another one now though.
Have fun.
Mine is a little hard to start at first when its cold, but the choke theremostatic heat coil thing on the intake needs to be replaced yet. Other than that it runs great when warmed up.

[No3Fan4Ever]

Originally Posted by TsgtE662

Rebuilding the QJet was easy. I have never rebuilt one before and I was alittle apprehensive about trying it. But how else was I to learn?
I got the kit and bought 3 cans of carb cleaner. Then set out on the patio for about 3 hours taking it apart and then scrubbing every part of the carb with the cleaner and brass wire brush. I wasn't in any hurry and took my time at it. It was obvious why it ran crappy as I took it apart. The rubber seal for the accelerator pump was tore. I replaced all of the parts inside and put it back together. Pay attention to the directions with the kit, I also have a Rochester Carb book and read alittle of that too as I went. There are a couple of parts you will reuse and shouldn't use the cleaner on it says. For no experience at rebuilding carbs this was really easy I thought. I am even thinking that if I ever find any like at the salvageyards I may grab them and rebuild them on my own to get more experience at it. And maybe sell them.
With the cost of a new one/rebuilt one I would recommend rebuilding it yourself. Be careful when disassembling the carb as a couple of small spring are in there where the float is. In that area of it. I didn't see where the spring came from so I called a mechanic friend of mine to come over and help me get it back together. Then I felt stupid cause it was really simple.
I won't hesitate to do another one now though.
Have fun.
Mine is a little hard to start at first when its cold, but the choke theremostatic heat coil thing on the intake needs to be replaced yet. Other than that it runs great when warmed up.

Thanks for the inspiration, I might just try to do mine myself;
if I screw it up there is a pretty good carb shop in town that can fix it