What does this screw do?

 

Plugged vac hole....or access hole the rebuilder drilled??

 

It's spring loaded and adjustable.

 

I'm using a Q jet that Cliff Ruggles rebuilt, he installed a adjustment screw there on mine so the primary metering rod height could be easily adjusted.

 

If it's something that Cliff put in there then just email him the pic and ask.

http://www.cliffshighperformance.com/parts.html

Phone: 740.397.2921

 

That is the first design APT (adjustable part throttle). This is a factory adjustment for the position of the primary metering rods in relation to the primary metering jets. In other words it dictates how far down into the jets the rods will set. Turning this screw in will richen the mixture by pulling the rods out of the jets. This is a fine adjustment.

NOTE: If the base gasket is too thick or too thin it can throw this adjustment off!

Generally they are sealed at the factory.

BigBlockk

Later.....

 

I thought this was the first design.

 

thank you. the carb runs very rich, backfiring on release of throttle at speed. what do you think of the thickness of the base gasket? It looks unuaslly thick to me, but i don't have a referance. C.

 

That is the aneroid from the short lived altitude compensating system. This system was introduced in 1975 but wasn't around in 1976. This is why the '75 and up passenger car 'Jets have that strange looking dome on the passenger side front corner of the air horn. Truck carbs didn't have
this system.

BigBlockk

Later.....

 

First of all, backfiring in the exhaust is a lean condition. Popping in the exhaust is caused by lean misfires being dumped into the exhaust system where they ignite. What do the plugs look like? Many don't realize just how sensitive carburetors are. It is quite possible to have a carburetor that idles pig rich but goes dead lean at low cruise. Especially if parts are mismatched. This is one of the big failings of reman carburetors.

To try to figure this out we need information. Now if memory serves, 1973 was the first year that GM used EGR in their induction systems. Carburetors designed to use EGR are calibrated leaner at cruise to compensate for the less dense intake charge. Not getting the relationship between EGR flow and carburetor calibration just right was a big reason why the mid '70's V8's would sometimes surge under light cruise conditions. If you block off the EGR system but don't recalibrate the carburetor this mess gets even worse. This is meant to illustrate how sensitive carburetors are. I don't know how your induction system is setup but you may be able to figure out some problems using the info above.

As far as base gaskets go, they are generally on the thick side. In your first post it looks like the cap has been removed from the adjusting screw for the APT. If this is the case it could be good or it could be bad. It is good because you can always adjust it to compensate for a too thick gasket. It could be bad because it indicates that someone has been there before and possibly gotten the adjustment really fouled up. I would take the base plate off and make sure the screw moves freely before trying to adjust it.

Also, if you don't have one already it would be a good idea to get Doug Roe's book and study this system. His book shows it very well.

BigBlockk

Later.....

 

Everything BigBlockk says is correct:

The adjustment screw in the middle of the throttle plate is the first-style APT adjustment screw. It adjusts the height of a tang in the throttle plate, and this tang adjusts the height of the power piston and therefore the part throttle mixture. In most cases, the screw under the steel cover is seized and cannot be adjusted, so you adjust the tang by bending it instead.

Popping in the exhaust is almost always a lean condition - not rich.

I can almost guarantee you that you have significant problems with that carb: Your carb is a commerically rebuilt carb, and I'll bet it has been modified internally to be a complete piece of junk. With almost certaintly, the power piston lower stop has been cut off, making the APT feature completely inoperative - your power piston is riding way too low causing the lean condition and popping, and it cannot be adjusted due to the power piston being destroyed. I'll also bet that the idle bleed air holes have been plugged off with lead plugs, the power piston spring is incorrect, the float level is wrong, it has the wrong float installed, and the jetting is off. That carb needs some significant work and parts.

 

Thanks, Lars.
Well holley said they will check out the 4175 for free. Hope they fix it. I did take the Quadra apart to get a look at all the "goodies" in there, cleaned up the rods a bit and put it back together. pulled the holley and put the quarda back on. no more miss fireing on decall, the power piston still looks complete, but internally the carb has a lot of wear. Nice paint though! runs very nice, but I am sure that the secondaries aren't opening al the way. and when you try to adjust the F/A mix, you either get a nice clean exhaust and the engine lugging or a face full of gasoline fumes and a smooth idel. I reallt don't think there's enough left of this one to rebuild, there's some kind of 'scaring' on one of the inner venturies, and I had to teflon tape the ATP screw, as I was afraid of a Vac leak, it was so loose. I'll go with the Holley when I get it back. the car is drivable, as long as the wind is blowing away from the front of the car- cough- cough. thanks Lars,, Chris.

 

The internal parts look good - I'm surprised. The power piston has not been cut, and the idle air passages haven't been plugged as I suspected. You should be able to set that carb uip just fine for your application.

 

Well, had an enjoyable cruise in the 'Vette this evening, thank you Lars. Leaned the F/A mix out very well and adjusted the APT lower, just a touch open. Runs alot cleaner and smooth. tiny occasional miss on the pass side bank, but i think it's plug or wire related. exhaust a bit cooler on that side, but it's clean. fun car to drive!