qjet help - idle & black smoke

Hello everyone. This is my first post in the Hamb.

Well, I rebuilt my qjet. I used one of the superkits available. new gaskets, new pump, .38 primary rods, 68 primary jets ( lean primaries), DA secondary rods. 5hg power piston spring, 0.400 float level. The engine is a complete rebuilt 307 with a mild cam, msd box and Hei, roller rockers, perfomer intake, 1" carb spacer.
The problem is when the engine is idling, black smoke is coming from the exhaust. Turning idle screws has no effect. I removed the power piston spring to leave the carb in the full lean state and the situation is the same.
Fuel pump is a new, mechanical, 80gph one.

These are some pics I took with my cel after a 5 minute run.




Can anyone enlighten me on this?

thanks.

 

Sounds like the plugs on the bottom of the main body casting are leaking. Just a guess, I use epoxy on any of them I have done.

 

You might check and see if the secondary butterflies are stuck open just a crack.
Pretty common and causes a flooding condition.
Larry T

 

I forgot to mention that I epoxied those brass plugs in order to prevent leaking.
I will check again the secondaries but I´m pretty sure they closed perfect. I will try to disconnect the rod and run only with the primaries.

Also with the black smoke the exhaust spits what I think is gas.

 

Well plugs. If you did not seal them after tanking the carb, they are leaking.

 

How is the gasket between the trottle body and the main body casting? If you epoxied the plugs they shouldn't be leaking (as you say you did). If it's giving you black smoke AND spiting liquid fuel I'm amazed it runs at all (are you sure it's not condensation?).

This only happens at idle? Have you checked for trash between the needle and seat? How is the seal between the seat and the main body casting (sorry, I'm spacing what kind it is). Kind of a reach I know, but if it's only at idle, a small leak at the needle and seat (or too much fuel pressure) would do that.

 

how much fuel pressure are you running?

 

Sounds like a float / fuel pressure issue to me.

There is a book by Cliff Ruggles that is an AWESOME Q-jet reference. Seems to explain stuff a bit better than the old Doug Roe book, IMO. I think if you get this book and read and understand how this carb works, you'll be on your way to fixing it.

The reason I say "get the book", is that it explians what every drilled hole in that casting is used for. I am wondering if maybe you have an incorrect gasket installed, or something like that. If that is the case, it will be impossible for us to help you over the internet.

Also... pull your needle / seat and pull a vacuum with a mity-vac. I've seen new seats out of the box that didn't seal worth a damn.

 

I have epoxied the well plugs. Don´t laugh at the color of the epoxy, it´s transparent epoxy...

 

did you install a new float?, i dont usely buy a kit unless i tear a gasket taking them apart, i just buy a new float, needle and seat, and a pump, i have had casting flash keep the secondary flaps open, the secondary flaps spring tenion set to weak keeping them from closing all the way, and the secondary needle hanger bracket tweeked.

 

It was a full rebuild, I installed a superkit from carburetors.com. New seat, new needle, new gaskets, new everything. This is frustrating...

 

thats the first brass float in a qjet i have seen, you know if it were me i'd go to the gm dealer and get a foam float. and like has been said try and check the seal on the needle, if you dont have a vaccum pump just use a hose and suck on the end.

 

That, and fuel pressure was my first thought.

I swear this sounds like a fuel leak...

~Jason

 

Ok guys, thanks for the replies. The black phenolic floats are known for sucking gas and getting heavier. The brass float is one of the upgrades recommended for this carb.

 

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GET that seat needle clip in the proper position, NOT in the float hole. This can cause the needle to stick/bind open. Belongs hooked from the ft...H

 

LOL!

Man, it's been a while - I didn't even catch that until I read this!!! But there it is, plain as day!

Good catch, man.

~Jason

 

GREAT call! I missed that!

Also check idle air bleeds and make sure they are not plugged. This would cause a rich idle too.

 

Qjets often don't like open hole spacers. Not to the extent you describe, but still can cause erratic idle.

Are you sure it's timed and sparking properly?

 

The float soldering job looks like a non-factory solder job.

Any chance the float is leaking and partially drowned?

Take if off and shake it to see if there's gas rattling around inside.

 

If the idle screws do nothing but aren't clogged up then you have the IDLE SPEED adjustment screw screwed in too fast and it's trying to idle on the mid range circuit.
Screw the idle mixture screws all the way in and then open them up 2-3 turns,(to start with) then turn the idle speed down (out, more closed) till its idling at or near 650 RPM. then fine tune the mix screws.
Ignition timing needs to be correct, (whatever "correct" might be for that setup) first, for this to work too.

Put a stock power piston spring back in it, first!

Also,
Is that an EGR valve I see in that first picture?
If so, make sure it's working or at least not stuck open.

 

My power piston got stuck in the "up" position from carbon, black smoke at idle, stumbling mid-range, OK WOT.

What to do?

Squirt some Brake-Kleen etc. down the bowl vent stack while bouncing the top of the piston up and down a few times. Cleared right up.

 

at idle the power piston should be all the way down, if you can push it down when it is idleing then the spring is to strong or you have a vaccum leak, i still say install a GM factory foam float. also if the power valve height adjustment screw was removed then that needs to be set correctly.

 

Thanks again for the replies, I won´t give up on this carb yet.

The brass float is perfectly sealed, I have an auto trans so the idle is always set to 650 rpm or so. Tonight I will check air bleeds and the needle but I´m pretty sure it was not stuck. The strange thing is that even without the power piston spring and obviously the piston in the full lean position it stills spurs black smoke from the exhaust and turning clockwise o anti clockwise the idle screws has no reaction on the idle at all.

Is it correct to have fuel on these places?

 

I do believe that is where the secondary emulsion tubes draw fuel from.

So, how much fuel pressure are you running?

 

Is the choke pulled off?

The wells you are pointing to in the picture are the acelerator pump wells for the secondarys. The tubes go into the wells, when your secondarys open it will drain the wells. Then there are two small orifices that feed the wells that slowly fill them back up.

Are you pushing the nylon retainer for the primary metering rod piston down into the hole? If that isn't seated in the hole, it will make the piston bind up, and not go down all the way.

Make sure the gaskets fit the carb too. There have been many gaskets for the air horn and base over the years. I always match them up to the old gaskets, but who is to say they were right.

Black smoke at idle means all of the fuel is'nt being burned, right? That could be caused by to much fuel, not enough air, weak spark, and I'm sure a few things I can't think of now off the top of my head. How did it run before you built the carb?
Jeff

 

If the motor runs erratic almost like it's got a cam, as well as rich, it might have an internal vac leak. WTF, right? I never heard of it before but I had one I could not fix with a kit. A local shop owner clued me in as soon as I described what it did. He claims there is some internal warping that causes an internal leak that you can't fix, and that leak somehow causes it to run way rich.

Made no sense to me but the replacement carb really did sound different at idle with the aircleaner off, compared to the bad carb....as far as vacuum noises.

I had that carb apart twice and could not see any gasket that was not sealing.