The $20 mystery Q-Jet (back to looking at it again)

[Longroof79]

Does your '85 truck carb have the capped off mixture screws? I have an '85 truck carb apart on my bench. I had to drill and extract the caps in order to access the mixture screws which were recessed into the base plate. It does look like it was leaned out, but has 75 main jets, 48K primary metering rods, EF secondary metering rods and an H-hanger.
It was allegedly for a big block. I'm sure it will need modifications to fatten it up some. Although the main jets may be a little too rich for an average small block.

 

[Bonnewagon]

I once did up that kind of carb for a friend's '85 Suburban. Some dork jammed an "LT1 ?" cam into an otherwise stock 350 motor and it refused to co-operate. I think the original jets were #71's or so. The biggest jets I had were #75 and they were too lean. I had to drill out some common #71 jets to .078" and drill the idle discharge ports too just to get the motor happy.

 

[Longroof79]

Mark,
Isn't there a modification that involves drilling out the idle mixture ports to allow a richer mixture?
Also, who's a good source for drill bits for enlarging jets?

 

[pagrunt]

I would use the body with the number removed for parts and use them on the other quadrajets you have to rebuild.

Sad thing is that is the only Q-Jet I have for now. Atleast I'll have it for that purpose. The more I think about it I'm liking my Holley more & more.

Does your '85 truck carb have the capped off mixture screws? I have an '85 truck carb apart on my bench. I had to drill and extract the caps in order to access the mixture screws which were recessed into the base plate. It does look like it was leaned out, but has 75 main jets, 48K primary metering rods, EF secondary metering rods and an H-hanger.
It was allegedly for a big block. I'm sure it will need modifications to fatten it up some. Although the main jets may be a little too rich for an average small block.

It doesn't have the plugs. Oddly the base plate does look like a pre plug piece. Until it is opened up I don't know if mine has the same set up as yours.

 

[Longroof79]

Sad thing is that is the only Q-Jet I have for now. Atleast I'll have it for that purpose. The more I think about it I'm liking my Holley more & more.


It doesn't have the plugs. Oddly the base plate does look like a pre plug piece. Until it is opened up I don't know if mine has the same set up as yours.

I'll have to post a picture of it. I thought if I ever decide use this carb, I may consider replacing the baseplate with an earlier type. Right now it's pretty much an experimental /parts carb.
I have a '75 Chevy Q-jet on it now that was originally from a 400 sbc.

 

[Local Hero]

I've gone through three other carbs on the 350 in my wagon before retorting to a freebie box of old Q-Jets my Dad gave me when he was cleaning out his old inventory. In there was one that looked to be the cleanest and most complete. I researched the numbers and it corresponded to an early 80s GMC Truck with a 454.

I got a kit for it anyway, did a straight up rebuild on it, and it's been on the wagon since last October. The car never ran better.

 

[Bonnewagon]

Jack, the pre-emissions hole size is .095. Before drilling anything measure yours with a drill bit and see what it is for a baseline. Some are extremely small and meant for super lean emissions. I have found that opening up those discharge ports even a little can solve a crummy idle situation with no other modifications. Plus, if it was not the problem, you can still tune for a smooth idle by simply turning the screws back to a lean mixture. As for a drill bit set that is complete and cheap the 115 piece Cobalt set that harbor Freight sells is pretty hard to beat. I can usually snag it for under $100 and it is drill-through-anything COBALT man!

 

[pontiacgp]

the first thing I do is open the idle holes to .090 before I tweak anything else.

 

[Bonnewagon]

I should also mention that drilling jets is a frowned upon practice left over from the '50's- '60's. The carefully machined entrance angle of the jet is compromised by hand drilling. That's all good and well but I figure that when you can't get a big enough jet for what you need then drill it is. The biggest I've ever seen was a #75 that came in a Pontiac 455 Q-jet. I've needed to drill to .078" and even .080" to get bad acting motors to comply. Just use more common small size jets for the sacrifice so you don't waste your large ones.

 

[pontiacgp]

I have built some quadrajets for circle track cars in the enduro and v8 bombers. I remove the power piston and spring along with the two metering rods and run 68 jets. The carb runs great and you'd think unless you have the foot to the floor it would flood but it doesn't . I know some guys who ran quadrajets without the main rod set up in their street cars.