1979 Midas SBC 350 Rochester Qudrajet Fast Idle

[antonmnster]

Helping out a friend who bought a nice old and slightly moldy RV. Slapped an alternator on it tonight, which was such a delight compared to my usual n54 BMW stuff. When I started it up I noticed a pretty high idle, probably 1400 rpm.

I popped off the doghouse and looked around the carb. I haven't worked on anything like this in over 25 years. The one thing I did observe is that the throttle sat above the idle set screw by a couple millimeters. If I pulled the throttle closed, it would calm down, but only maybe to 1000 rpm, still pretty fast.

Could this be as simple as cleaning the carb up a bit? It seemed more mechanical than anything else, simply that the throttle isn't closing. I didn't get into vacuum lines or anything like that yet.

I have to say it's amazing how smooth it runs. I've been driving BMWs for 20 years, but that SBC just sounds so sweet.

 

[bmwtechguy]

Add or replace with a proper (just strong enough) throttle return spring so the adj screw rests on the stop each time.

 

[Johnny2Bad]

Does it calm down after a few minutes at fast idle if you leave it alone? What about if you blip the throttle (which should cause a little catch on the linkage to open)? If so, normal behaviour.

 

[antonmnster]

Good idea. But it seemed like I had to use a couple pounds of torque to close it. Could something be binding? The linkages all had slack

 

[antonmnster]

I thought it could be a fast cold idle. But I let it warm up and gave it a few blips by foot and by hand. No effect.

 

[spasm3]

there should be a choke adjustment that looks like a black dial with 3 screws around it. Itmay have too much thermostatic choke. Its really best to set those in cold weather with the engine/carb cold. You may can ease it back a bit.

I am assuming you tried turning the idle screw first. And as suggested above, you need good return springs on everything.

 

[Johnny2Bad]

Originally Posted By: antonmnster
Good idea. But it seemed like I had to use a couple pounds of torque to close it. Could something be binding? The linkages all had slack


Yes, something could be binding. You could try a little WD-40 over the mechanical linkages and where they attach to the carb body. Could also be a vacuum issue, as the high idle is sometimes vacuum actuated.

Might be a replacement carb not specific to this application. A Q-Jet is a Q-Jet, but the details about things like linkages and where / how they are attached to the carb body differ, so there are different part #'s depending on application.

Quadrajets are great carbs and offer good fuel economy if you can keep your foot out of it, but if the big bores are opening it's going to drink some serious gas. If you can avoid that, they are better than conventional 4-barrels gas-wise, but sometimes in a heavy application that is not easy to do.

You can tweak the linkages to modify the fuel taken up by the accelerator pump and delay the opening of the big bores, and get even better than stock gas consumption. There are books by HP Press that detail every little thing you can do to a Q-Jet and it is worth the effort. Carbs are not complex once you understand the concept of operation, and a book is sometimes necessary to get there.

 

[BigD1]

Hold the choke plate all the way open after it warms up, and see if that slows it down.

I had some chokes to fail over the years, and I would just disconnect it, and then wire the choke plate wide open until I could fix it right.

 

[Trav]

IIRC There is a fast idle adjusting screw on the right side of the carb, it sounds to me like it might have a bad choke pull off, check it with a hand held vacuum pump. Common problem with vacuum pull offs, some Q jets had 2 pull offs.

 

[ARCOgraphite]

Also make sure the carb base isn't loose, all vacuum lines are connected to the correct port and connected to the manifold, no lines (hoses/fittings) are cracked, PCV is connected and is the correct aperture ... Like TRAV says the choke should be pulled off the high idle cam. Make sire secondary are closed.

Now make sure the throttle cable (bracket) is adjusted correctly, and look in the drivers footwell to see if every thing is ok in the cabin.

 

[Trav]

Now you mention that these things had throttle shaft wear issues also, check for excess play in the throttle shaft it will cause a vacuum leak, a slight amount can be tweaked out but one with more play will require shaft bushings.

 

[Olas]

Holley and/or Jenvey will sell you a beautiful set of ITBs & associated bits to solve the problem and improve power/torque/mpg/drivability.

 

[bubbatime]

Isnt there an electric plunger that comes out for fast idle?

I dont know how much mileage an RV would have, but the quadrajet is notorious for wearing out the bushing that hold the throttle shaft. All rebuilt quadrajets will have new bushings. If there is play in the throttle shaft, it will suck air into the carb.

 

[antonmnster]

Thanks for the advise everyone. I'm young enough that this the first carb I've actually worked on

I think I have a pretty decent understanding of the fundamentals and what to look for. The funny thing is I remember my Dad doing pretty much the same operation on his '78 G30 work van when I was probably 12. I remember him mumbling something about how much better it was than the mid-70s Audi he had that used a coolant-fed choke... something about it never starting warm.

 

[The_Eric]

The choke pull offs that I've fiddled with would cause the engine to run rough and pig rich when they failed because the choke plate is almost completely closed.

This makes me think that the pull off is working, but the choke coil isn't. It is either worn out and not heating or its power/heat source has failed. This would cause a perpetual high idle since the coil will never fully relax so it will never move off of the fast idle cam.

 

[Number_35]

Originally Posted By: bubbatime
Isnt there an electric plunger that comes out for fast idle?

I'm not sure about the fast idle, but I remember an electric solenoid for anti-dieseling on a 70s Rochester Monojet on a 250 Chev Biscayne. When the ignition was turned off, the solenoid lost power and the plunger collapsed, almost cutting off the flow of fuel at idle. The idle speed was set with the plunger energized (extended). I think the idle was set by turning the hex head on the end of the plunger shaft. In contrast, I think the fast idle was set off a stepped cam.

 

[Trav]

Yes the fast idle was a stepped cam, the CCC Q jets had a electronic power valve on the primaries to prevent cold stumbling, other had bowl level metering, all sorts of weird stuff.
I worked on them a lot back then along with Ford VV (variable Venturi) carb, in fact I still have the OE Ford tool set for the VV.

 

[spasm3]

Does it look like this?