Qjet fuel drainback information

[Ricko1966]

Well I personally haven't had fuel drain back problems with a qjet,but turns out that there are reasons and fixes for the problem. I did not know this but when GM went to return lines on the fuel pumps the fuel pump was no longer a positive stop,at the same time GM changed the fuel filter, filter numbers are NAPA 23051 and 23052 depending on short or long filter. Anyway these filters have a check valve in them,it wasn't designed as a drainback valve but as a rollover valve because an upside-down return line car would run fuel out of the tank through the carb . By coincidence it also stops fuel gravity feeding the other direction. Also if you use a needle and seat without windows the fuel can't enter down at the seat level,thirdly if you leave the clip off the needle when doing a rebuild,the needle will fall into the seat when there is no fuel pressure,instead of being hung off the seat by the float. Fuel evaporation is still going to be a problem,seems there only has to be an 1/8 th inch drop in float bowl level to neuter the accelerator pump. I hope this info helps anyone with hard start problems.

 

[Velder]

I did not know all that. I just finished rebuilding my quad, and sealing the lead plugs under the fuel bowl, but I put the little clip back on the needle. Can you leave it off?
When you say that an 1/8in drop neuters the accelerator pump, do you mean that it will not give it's little shot of fuel to the engine?
I am trying to learn as much about these carbs as possible, I have yet to try tuning it, so any information is greatly appreciated.

 

[Ricko1966]

An 1/8 of an inch drop supposedly only allows for 1 shot of the accelerator pump if your car starts on one,great. Since you've already built it and it's back together I wouldn't go back in to xhange anything,but yes, you can leave the clip off,then gravity makes the needle a check valve,and if you use a needle and seat that doesn't have windows in it fuel bowl level can't drop as far.If you're running a 2 line factory pump you shouldn't have a problem anyway. It's the 3 line pumps that have a problem,and the correct factory fuel filter should prevent it Anyway. Is your qjet original to your truck? Is your engine stock?

 

[Snoots]

I did not know all that. I just finished rebuilding my quad, and sealing the lead plugs under the fuel bowl, but I put the little clip back on the needle. Can you leave it off?
When you say that an 1/8in drop neuters the accelerator pump, do you mean that it will not give it's little shot of fuel to the engine?
I am trying to learn as much about these carbs as possible, I have yet to try tuning it, so any information is greatly appreciated.

Get Cliff Ruggles book on Q-Jets. Well worth the money.

 

[Ricko1966]

Get Cliff Ruggles book on Q-Jets. Well worth the money.

I have the Ruggles book,didnt see that info in it. Maybe I wasn't paying enough attention. The best qjet knowledge I've found is from an aerospace engineer on the Corvette forums as a matter of fact Ruggles gives the man a nod by saying,Lars uses my products. Google. Lars,Corvette,Quadrajet look his stuff over,you will find the best Qjet tuning guidelines in the world.

(PDF) How to Tune a Q-Jet - DOKUMEN.TIPS

How to Tune a Q-Jet by Lars Grimsrud SVE Automotive Restoration Musclecar, Collector & Exotic Auto Repair & Restoration Broomfield, CO Rev. New 10-4-01 This tech…

dokumen.tips

 

[Velder]

Yes my truck is all factory original. It was purchased new by my Grandfather, and only has around 60k miles on it. I am just wrapping up a complete restoration, and have yet to take it for a spin.
Before the teardown it had very long hard starts if it sat for more than overnight. Then it would just die at around 30 mph. I am hoping that those problems will be fixed.
I have looked at Cliff Ruggles book but have not bought it yet. would it cover the later model quads?

 

[Ricko1966]

@Velder you need to give us more info,year of truck,which engine. Browse that pdf link I put on post 5,really good information there.

 

[75gmck25]

If you do an online search you can find the check valve as a separate part, and fit it into any stock filter.

The built-in Quadrajet filter has somehow gotten maligned and screwed up by many shade tree mechanics. They get the filter in backwards, leave out the spring, cross thread the housing, or create other issues, and finally just pull out the filter and add an external one. Its really a simple device that is easy to maintain, and it works.

However, due to the age of these trucks I still recommend adding an external pre-filter to catch most of the gunk before it gets to the Quadrajet. Pick up one with 3/8” fittings and add it in a convenient spot.

 

[Ricko1966]

A new Qjet filter with the valve is about 5.00,10.00 with a new spring and sealing washers,why search for a valve separately on the internet. Adding an external filter in most cases ends up with the nice OEM steel hardline replaced with a rubber hose 4 clamps and a plastic or glass filter. Or the O.E. line cut 2 short pieces of hose 4 clamps and a filter. I'd rather have my O.E. hard line intact and change my factory fuel filter regularly. I've personally witnessed 3 engine fires started by leaking fuel hoses.[/QUOTE]

 

[Velder]

Thanks for all the info.
My truck is an 85 C10 with the 305 V8 engine. my carb is the M4MED. It doesn't have the power piston found on later models but it does have the Dual Capacity Pump Solenoid on top. can any one tell me what that actually does?
Like I said, at this point I think I have fixed all my major issues, and am just trying to tune, but I'm hoping to take it for a test drive next week, so we will see.
@Ricko1966, sorry to have hijacked your thread, probably should have started my own, but thanks for the help.