carb problems

[p-townchris]

Hey guys, got a problem with my 85 suburban. 350 4x4 no a\c has 235000 on lower end heads 2 years old(blew head gasket) Qjet 650.
it's burning rich can wipe black soot off the inside of the primaries and the secondaries.

Have Accel high voltage coil, 8 mm wires and the hottest plugs that I can put in the truck, get maybe 10mpg. Carb is less than a year old.It also has a slight back fire through tail pipe when I let off the gas at highway speeds 55/60.

Thanks Chris

[NEUMANNZZ]

what carb is it? when fully warm is the choke plate fully open?

[Old88]

make sure your vacuum advance is working
check your timing

[p-townchris]

its a rebuilt Rochester 4BBL Quadrajet 650CFM.

it came with a stock Rochester 750 but I tried to rebuild when I was in Georgia big mistake so I went to Summit and got this one.

Yes choke plates are open

[83K1500]

have you tuned the enrichment screws at all?

There should be a set of screws that adjust the fuel needles on the front at the base. The best way to do it is with a vacuum gauge.

Plug the vacuum gauge into the brake booster port.
Start the engine.
Start with one needle and turn clockwise (to close or CCW to open) until the vacuum starts to drop off then back off a little raise it back up.
Then move to the second adjuster until the vacuum begins to drop, back off and go to the first.
You just go back and forth adjusting one at a time until you get the most vacuum.

Do this with engine at temp and steady idle. You may find yourself adjusting the idle several times.

That is a basic tune without swapping jets or valves. And is the easiest way to get the best with what you have. If you still are running rich after this, you need different jets and or springs.

[Big Orange 83]

turn screws in to decrease fuel mixture, turn out to let more in..

[GreaseDog]

uh... that's what he just said.

[retired wrench]

Do they still use a synthetic float that can soak up gas and run lower than normal? RW

[GreaseDog]

actually, the brass floats seem to have a bit more problem with getting pinholes in them and filling with gas in my experiences.

[83K1500]

Originally Posted by GreaseDog

actually, the brass floats seem to have a bit more problem with getting pinholes in them and filling with gas in my experiences.

Got to love american manufacturing... Most the time unless it is a name brand carb, the first thing I do is go over the floats brazing anything not normal.

[p-townchris]

The enrichment screws came stock at 3 turns Tried 2 and 2 1/2.
Will try using a vacuum gage this week end to set carb this weekend.

thanks for the help

[83K1500]

Originally Posted by p-townchris

The enrichment screws came stock at 3 turns Tried 2 and 2 1/2.
Will try using a vacuum gage this week end to set carb this weekend.

thanks for the help

Stock settings on a carb are never perfect for anyones situation.
They are set to make sure the engine will run, that's all. All carbs are set up like that.
You still need to tune them no matter what.
Hell most carbs need re-tuned about every 3-5 months or every 10-12* of extended temperature change to be honest. I will not even get into altitude effects...

[bob308]

in 40 years of driving and working on trucks and cars. the best fix for a qjet is to put a holley on.

[GreaseDog]

shadetree.

[83K1500]

Originally Posted by p-townchris

Will try using a vacuum gage this week end to set carb this weekend.

thanks for the help

Just FYI, I did say the only way to do it was with a vacuum gauge.
There is no, turn drive, turn drive option.
You will have absolutely no clue how much vacuum it is pulling unless you have a gauge on it. And unless you have the screws way out of adjustment your not gonna hear the difference either.

Originally Posted by bob308

in 40 years of driving and working on trucks and cars. the best fix for a qjet is to put a holley on.

I know people have their preferences... but I tried a Holly on my engine, tried everything, would not tune for crap. Put on a Eddy, and it was set in 10 minutes.
But both of those statements are really beside the point, 'IF' a Q-Jet is set up and tuned properly, you will have the best option you could for a D/D economy, and power when need it.

[bob308]

the only problems with a holley is the person that puts it on. they pick the wrong size {too big} they don't know how to tune them. they pick the wrong type. they do all kinds of mods before they even try to run it.or they get a used woreout carb and try to use it.


i have a 450 cfm on my 327 in my 79 and it has run for years with no problems at all. i have tried qjets and they are the doggiest carb i have ever tried.

[83K1500]

Originally Posted by bob308

the only problems with a holley is the person that puts it on. they pick the wrong size {too big} they don't know how to tune them. they pick the wrong type. they do all kinds of mods before they even try to run it.or they get a used woreout carb and try to use it.


i have a 450 cfm on my 327 in my 79 and it has run for years with no problems at all. i have tried qjets and they are the doggiest carb i have ever tried.

I did nothing to the Holley that I did not do to the Eddy to attempt to tune it. Both are/were 650 with mechanical secondaries. The carb was brand new. I know how to size them and set them up.

Again, if and when a Q-jet is properly tuned and set up right, which is hard to do, it WILL give you the best for economy and power.

[BigOrangeChevy]

I don't know about you guys but I had a qjet on my 79 chevy truck and it ran alright. Never seemed to have what I thought it should have. Then I bought a Speed Demon and that truck became totally different. So much quicker acceleration and just ran
100x better. I do agree if the qjet is set up properly it will be great but I dont think you are going to beat the Demon.

[GreaseDog]

Originally Posted by bob308

the only problems with a holley is the person that puts it on. they pick the wrong size {too big} they don't know how to tune them. they pick the wrong type. they do all kinds of mods before they even try to run it.or they get a used woreout carb and try to use it.


i have a 450 cfm on my 327 in my 79 and it has run for years with no problems at all. i have tried qjets and they are the doggiest carb i have ever tried.

funny, just swap Quadrajet for Holley in this post, and you get pretty much every pro-Qjet guy's opinion.

i used to have a carb guru, he could work on any carb you put in front of him, didnt matter what it was. Holley based, AFB (Carter/Edelcrock), Rochester, whatever. i've driven a truck (my own) with a QJet built by him that was in a good state of tune, changed a couple of things at the track one time, and picked up nearly a full second on the ET. left the change, and actually noticed a milage increase as well. put a Holley on the same engine in my TA (1850-3 IIRC, single feed, vac secondary), and had him tune it, 2 weeks later, it wouldnt run for ****. pulled the QJet back off the shelf, bolted it on, no problems at all. never once had to retune it, which is Holley's major downfall, they're WAY too picky when it comes to weather. ended up pulling the QJet back off and putting it on another engine, and replacing it with a Eddy 1405, tuned with a wideband, and never could get it to put out as much power as the QJet.

[BigOrangeChevy]

I think it all depends on what you want to do with it. I hang around a guy that has 2 7200lb pulling trucks. One with a 511 and the other with a 468. I know that you go putting a qjet on one of those and its not gona do ****. If you just want a good driver than a qjet will work great. I like them because they are easy to tune and work on. Also, i thought that they only made qjets (atleast originals) in 750 and 800cfm sizes. I didnt think they had a 650cfm model!?