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.

 

[KnuckleBuster]

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.

Velder, the DCP system merely reduces the accelerator pump shot volume after the engine warms up. It was supposed to reduce emissions. Seems like it was only used for a couple of years. It was pretty much trouble free, as far as I'm concerned. The only parts I ever replaced were the occasional switch or connector that had been accidentally broken, usually, in part at least, due to heat deterioration.

 

[KnuckleBuster]

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]

Gotta agree with Ricko on this one. I would NOT cut a steel line to add an inline filter. Once cut, you'll be hard pressed to find another if you decide to return it to factory. A new, properly installed filter at the carburetor will do the job.

By the way, someone mentioned Cliff Ruggles' book as a good book on Qjet's. I agree and I own one. I also have a Qjet book authored by Doug Roe (also excellent!). Between the 2 of them I can usually find any info I'm looking for on Qjet's. I have several GM/Rochester publications, too. Each source offers something that the others do not. I just can't get enough Qjet info.

 

[Matt69olds]

The fuel draining myth is just that, a myth. Think about it, for fuel to drain, fuel will have to go uphill, past the needle, out the filter housing, and then drain.

What’s happening is a result of todays fuels. Gas made 30 years ago had additives for carbureted vehicles to prevent fuel evaporation. The vast majority of cars/trucks on the road today have closed fuel systems, electric pumps, it’s not cost effective for oil refineries to add the additives for a tiny percentage of buyers.

Either live with the long cranking, or install an electric pump. There are several fuel pump controllers that will run the pump for a couple seconds when you turn on the ignition, restart the pimp once the engine is running, and then shut off the pump when it no longer detects a tach signal. Don’t wire the pump to an ignition ounce without some kind of safety switch. If the engine dies and no one is there to shut off the pump, or your in an accident and are unable to shut off the pump, bad things can happen.

 

[Ricko1966]

The fuel draining myth is just that, a myth. Think about it, for fuel to drain, fuel will have to go uphill, past the needle, out the filter housing, and then drain.

What’s happening is a result of todays fuels. Gas made 30 years ago had additives for carbureted vehicles to prevent fuel evaporation. The vast majority of cars/trucks on the road today have closed fuel systems, electric pumps, it’s not cost effective for oil refineries to add the additives for a tiny percentage of buyers.

Either live with the long cranking, or install an electric pump. There are several fuel pump controllers that will run the pump for a couple seconds when you turn on the ignition, restart the pimp once the engine is running, and then shut off the pump when it no longer detects a tach signal. Don’t wire the pump to an ignition ounce without some kind of safety switch. If the engine dies and no one is there to shut off the pump, or your in an accident and are unable to shut off the pump, bad things can happen.

If they replace the factory needle and seat with a windowed needle and seat it changes the game,personally It wouldn't be my choice,but they come in the kits and most people don't know the difference.

 

[SirRobyn0]

On the drain back issue, however you want to look at it myth or not doesn't matter. But my two cents, lets suppose the welch plugs in a Q-jet leak a tiny bit, so the truck sits overnight and the float bowl is dry, you crank it over and within a revolution of the engine there is going to be at least some fuel pumped into the float bowl, which gets sucked into the engine and the truck fires up first try and within seconds of cranking. Now lets suppose all the fuel drains back out of the lines and the lines are bone dry by morning, you get in it, pump the gas hit the key and she fires on the first try because there is gas in float bowl, and before that gas is burned up the fuel pump, if it's healthy and the socks in the tank aren't creating a restriction brings fuel up right away and the driver never even knows it's happened.

My square never sits for more than a few days and it always fires on the first try. So lets take my 77 Cadillac, it can sometimes sit for 6 months. I'm sure the float bowl is dry and the lines are dry or mostly dry by then just from evaporation. Guess what. She'll almost always fire on the first try, admittedly sometimes I'll have to stop cranking and give her a second pump to get it to fire up.

My point, regardless of why fuel might not be present in the lines or in the float bowl when a carburetored engine is cold started, if the fuel system is healthy it'll still fire off in short order. So for that reason I disagree with the electric pump, as it's not necessary. A weak fuel pump can take forever to prime if it needs to, and dirty clogged socks in the fuel tank can slow the process even with a healthy pump. Keep the maintenance up and she'll fire up right. Don't and she won't.

Still @Ricko1966 gives good info, I knew most of it, but did not really know the intended reasons for the check valve I just knew it was there.