Quadrajet Problem

[mrcaruso]

Got a 'new' carb for my 75 Chev truck & it had no spring under the power piston. Engine kept running out of fuel except for a constant working of the accelerator pump. Put a long spring under the piston and it runs better but still won't keep steady rpm unless a constant working of the accelerator pump. Any ideas?
Thanks.........................

[Wrenchbender Ret]

Check the float level. It may be too low. If someone put this "new" carb together without a spring under the power piston, who knows what else is wrong with it.

[Userone]

if the carb has been sitting long, you might have trash in the needle seat. mine is running like crap like you're describing because of sitting

[mrcaruso]

Thanks guys. When I had the carb apart for cleaning, I raised the float level to approximately 1/4 inch. It was very low, at least 1/2inch. I replaced the needle & seat when I put in the spring under the power piston. I don't even have a filter installed so that can't be the problem. I appreciate your input.

[OutlawDrifter]

sounds like it needs to come apart and be inspected. there are a lot of parts that can go missing from a q-jet and they still look complete when put together.

do you know any history on the carb at all? did it have a kit put in it, or was it just setting on a shelf? most of the time if a q-jet is drained properly after being removed and stored well, you can throw them on and they run just like they did when they came off.

[mrcaruso]

The carb came off a 78 Chev1 ton truck. The guy said he put marvel oil in it then put it on the shelf. I took it apart and put in some parts out of a kit, but I think I may have out thought myself and raised the float level too far. My next step will be to drop it back to the 1/2 inch mark. There are so many diff float level settings motor/trans fuel pump pressure combos from 65 or so on that it's tough to determine the correct setting. This motor is a 383 with a 30-30 hydraulic cam, so vacuum could be a hurdle with the power piston as well. I love a challenge! I will post the cure, if I find it!
Thanks again.

[OutlawDrifter]

yes, that cam will have poor vacuum for sure!

the carb would probably run fine on a milder setup.

[D.PASSMORE]

There is a spring kit available. It comes with different length springs. At idle that spring keeps pressure on the power piston as the vacuum pulls the piston and rods down into the jets, restricting fuel flow (part of the idle circuit). As the engine is revved, vacuum drops, piston and rods pull out of jets and more fuel enters the intake manifold.
If you are playing with Quadrajets, I would read up on them. IMO, best carb out there, but have to be tuned properly. Not hard if you understand them.

[mrcaruso]

Thanks, David. I haven't been into a Quad since my stock eliminator racing days and I'm now studying my Rochester book again! When I took this baby apart, it had NO spring under the power piston, which I found odd. I purchased a kit and some parts which included a spring kit. i put the longest spring under the power piston and it does run better but still seems to starve for fuel. I will find the problem and let you guys know what happens. Thanks to all of you for helping me out.

[OutlawDrifter]

cliff ruggles seems to be the man in the know when i comes to q-jets.

when i was rebuilding the q-jet for my 82 K5 i bought his book and ordered the parts from his website. prices were fair and i was able to get my carb tuned dead on with his suggestions.

[mrcaruso]

Thanks for the info.

[mechanicalman]

Originally Posted by mrcaruso

Thanks for the info.

This might help on some parts that might work better than what you have. Sounds like lean jets? You may also have a vacuum leak internal or external.


For 1975-85 305-454 Chevy/GMC trucks and cars with a hot air style choke. Compatible with EGR. Not for computer-controlled vehicles. Main Jet: .073; Metering Rods: Primary-50M, Secondary-DR. Pictured with electric choke does come with hot air choke.
•795 cfm with hot air choke #1903

What are the numbers off the carb? 1904 pretty much same carb but uses an electric choke.
http://www.carburetion.com/quadnumber.htm

This site can get you just about any darned carb part you want, reasonable, I've dealt with them before and had no problems.
http://www.carburetion.com/
http://www.carburetion.com/quadrajet.asp

Get the right repair kit for it, break it down, replace any missing parts and jets, metering rods, secondary hanger/rods, etc. for a 1903 or 1904 and that will be a good starting point. You may need to alter idle air bypass if you have a big cam, if you have problems adjusting your idle mixture. Secondary air valve adjustment also helps responsiveness. Also make sure the accel pump check ball is there, they usually come with a kit. Use the blue (Viton) accel pump seal if alcohol mix fuel is used in your area for longer life. And put a filter in it, you will likely be missing the filter springs so you can get one from carbs unlimited.

Look out for the old warped air horn (top of carb). You can actually straighten them with a mallet and some blocks of wood, but be careful you might want an old carb guy to do that for you if you need it. This happens as a result of over-tightened bolts holding the carb to the intake manifold, always use a nice supple gasket and don't over-tighten.

[mrcaruso]

I have replaced the normal parts from a kit. The normal openings in the base plate are plugged or hooked to the PCV checks. There may be a vacuum leak internally that I missed. When I replaced the gaskets I matched them the old so as not to make that mistake. One thing I did do was raise the float level to approx. 1/4 inch. It was over .400 and I assumed that it was too low. That doesn't equate to what I think is not enough fuel flow, but I am going to lower it back to where it was and see if that makes a difference. I really appreciate all the information regarding the parts, gaskets and other problems.

[Fitz]

Originally Posted by mrcaruso

I have replaced the normal parts from a kit. The normal openings in the base plate are plugged or hooked to the PCV checks. There may be a vacuum leak internally that I missed. When I replaced the gaskets I matched them the old so as not to make that mistake. One thing I did do was raise the float level to approx. 1/4 inch. It was over .400 and I assumed that it was too low. That doesn't equate to what I think is not enough fuel flow, but I am going to lower it back to where it was and see if that makes a difference. I really appreciate all the information regarding the parts, gaskets and other problems.

Quadrajets are the reason I run a Holley. Never could figure one out. Must be a genetic problem or something.

[Troy Sr]

One thing that I've found helped me in getting a quadrajet working on my boat was a magnifying glass with a led light and I looked into every hole in the carb looking for blockages. I found some that couldn't be seen with naked eyes.

[jlsanborn]

I have a couple great documents prepared by Lars Grimsrud on the q-jet. Tuning and troubleshooting. Should be pretty easy to find on the web, or if you PM me an email address I can send them to you. The file sizes exceed what the forum will allow to be uploaded.

[mrcaruso]

What I have done is installed a new float (.345 setting), eliminated one PVC valve, installed the shorter power valve spring and it runs much better then it did, however it stills seems to have a small vacuum leak. I disconnected the vacuum heater control line and the dist. advance and it made no difference. It did act like it was flooded a couple of times. I noticed that the fuel rods don't seem to go very far inside the jets, so the small part of the rods are still up in them which make appear that there would be no difference in the fuel going through them at idle or higher rpm.
If you are unable to send those files on the site, how can we get that done?
Thanks.........................

[jlsanborn]

Originally Posted by mrcaruso

If you are unable to send those files on the site, how can we get that done?
Thanks.........................

PM me your email address and I will send them to you that way.

[mrcaruso]

It's paulcaruso38@yahoo.com

[geezer#99]

I snagged it!! LOL!!
Well, some of it!!
http://www.digitalcorvettes.com/foru...ad.php?t=88376