Strange part/full throttle stumble: 500ci Cadillac

[znuh]

Hello forum!

I have a pretty pervasive stumble that seems to defy logic, and I'm at the point where I could use your advice. I recently picked up a 1976 Cadillac Fleetwood Brougham, with the 500ci engine, 59,000 original miles. The Quadrajet is behaving in a manner that really has me confused.

First off, here's the carb's ID:

17056230
0796

From what I can tell, this is the carb that originally came with the car. Here are the symptoms:

Cold (45 degrees F) startup is fine, after pumping pedal twice. After starting with choke on (at fast idle), engine starts to stumble and miss within about a minute.

Touching the gas and getting the car to step off fast idle, we come back onto all cylinders. As car warms up, part throttle results in missing again, as if one plug's bad. Take your foot off = smooth idle.

Once fully warm:

1/4 throttle, things are fine, car runs as it should. 1/2 throttle (up to just before opening secondaries), car stumbles, stumbles, stumbles. Has backfired through carb once or twice. Then, surprisingly, catches up with itself and acceleration takes place as it should.

WOT: About two seconds of stumble, then it 'fights its way through' and acceleration is normal.

If this was a fuel starvation issue, in theory the car should just continue to starve itself, not reward me with acceleration once it 'fights through'. Even worse, taking the car for a long trip on the highway seems to cure it of PT and WOT problems. Drove for 45 miles, PT throttle issues are gone, WOT has what I'd call the 'normal' Quadrajet pause as the secondaries open.

Any help is greatly appreciated.

With best regards,

James F. Smith
Portland, Maine

[Cliff Ruggles]

Symptoms indicate that the carb is very lean and would enjoy some more fuel in the range you are seeing the issues.

I would also verify engine temperature, someone at some point may have put a low temperature thermostat into the engine which was a common practice for those back in the day.  The big Caddy engines often ran very hot as did many other huge Detroit iron engines from that period.

The factory was up against some pretty tight emission standards at that time, and those huge inefficient engines required some work to get thru emissions.  The q-jets used in 1975 and early 1976 were also horrible, using an "odd-ball" APT system and many used a second or auxiliary power piston and single jet/metering rod as well.

The engineers meant well with those carburetors, but the cross-drilled and interconnecting passages drilled in the carb body are unequal length, and they are in the bottom of the casting and prone to getting plugged up over the years with "varnish" from drying up cycles.

I actually avoid those units when selecting carburetors here to build custom carburetors from.

What I would recommend for what you are doing is to completely/correctly rebuild the carb, and do some very minor metering changes while you are in there, and leave access to the APT system for additional tuning without having to remove the airhorn once it is placed in service.  This new fuel requires a bit more of it anyhow, and even more with some of the really "low" compression engines built during the "smog" era.

Replace the electric choke and choke pull-off while you are there.  We stock all the parts needed to get the carb up and running correctly for what you are doing......Cliff

[Hillbillyenginering]

As always. Follow cliffs suggestions.
On older vehicles I only use chevron super and add seafoam to the mix.
The newest formula of fuel really sucks. They have removed so much from it. Napha is one of them that makes a big difference on engine performance.

[znuh]

Wow! Thank you Cliff and co. for the replies - not sure why my email ate the notification about this, but here we are.

I'm guessing then that the proper way to approach this is to 'fall back' to a better-designed 4MV from an Eldorado (1970-1972). Thank you both, I'll be getting in touch with Cliff to see about purchasing a proper carb for the car.

Best,

-James F. Smith

[Cliff Ruggles]

The earlier units are actually not a "better design".  The better design would be to use a 1976 or newer Cadillac Q-jet with the later design APT system.  Before doing anything I would spend some time with the original unit.  Most likely it just needs a complete/correct rebuilding, new choke pull-off and electric choke.  We stock rebuild kits, and the correct choke pull-off and electric chokes for that unit......Cliff