1971 carb idle mixture limiters--original color

[Jack H.]

Re: 1971 carb idle mixture limiters--original color

Russ is correct. What's shown in the pix is the Bay City 2-Bbl version of the limiter. It's service PN was 7043684. I managed to find a pair of these and they too are rust/red in color.

On what constitutes proof, well since the service replacement caps had a GM PN and the literature distinctly calls them out as being red in color, that would suggest there was another PN or a form of purchasing distinction for those of a different color. Note the caps are NOT called out in the AIM which says they shipped installed on the carb from Rochester. So, somewhere in the Rochester drawing system must be the call-out for the different PN that produced the black color, eh? What was available through service was 7042801 (two in a package, red, for Qjet), 7043684 (two in a package, red, for 2CV) and 7043677 (individually packed, red, for MonoJet).

Joe Lucia commented that there were troubles with these and I can believe it. Since the current '70-72 book contains the Dealer Service Bulletins, thumb through them and you'll find one that tells service techs they CAN remove and re-adjust the idle speed and mixture to solve an owner's complaint regarding poor idle.

But, it also tells techs they were supposed to use a CO meter to verify the new mixture adjustment also met emissions standards and they were supposed to replace the limiter cap after making the fresh adjustment just like the call out to do so if a major carb overhaul was performed...

I don't think there were DOT emissions cops on patrol back then, but these caps were GM's way to comply with the law. These plastic caps are tricky buggers. They snap on and virtually self-destruct upon removal. That means the injection mold that produced them was NOT simple/cheap... I don't know how you could have pulled the feat off without building a tool with a collapsible core!

I searched for a long time to see if these were supplied to GM vs. Rochester making them. I finally gave up on finding a viable purchase source....

[Joe L.]

Re: 1971 carb idle mixture limiters--original color

Russ is correct. What's shown in the pix is the Bay City 2-Bbl version of the limiter. It's service PN was 7043684. I managed to find a pair of these and they too are rust/red in color.

On what constitutes proof, well since the service replacement caps had a GM PN and the literature distinctly calls them out as being red in color, that would suggest there was another PN or a form of purchasing distinction for those of a different color. Note the caps are NOT called out in the AIM which says they shipped installed on the carb from Rochester. So, somewhere in the Rochester drawing system must be the call-out for the different PN that produced the black color, eh? What was available through service was 7042801 (two in a package, red, for Qjet), 7043684 (two in a package, red, for 2CV) and 7043677 (individually packed, red, for MonoJet).

Joe Lucia commented that there were troubles with these and I can believe it. Since the current '70-72 book contains the Dealer Service Bulletins, thumb through them and you'll find one that tells service techs they CAN remove and re-adjust the idle speed and mixture to solve an owner's complaint regarding poor idle.

But, it also tells techs they were supposed to use a CO meter to verify the new mixture adjustment also met emissions standards and they were supposed to replace the limiter cap after making the fresh adjustment just like the call out to do so if a major carb overhaul was performed...

I don't think there were DOT emissions cops on patrol back then, but these caps were GM's way to comply with the law. These plastic caps are tricky buggers. They snap on and virtually self-destruct upon removal. That means the injection mold that produced them was NOT simple/cheap... I don't know how you could have pulled the feat off without building a tool with a collapsible core!

I searched for a long time to see if these were supplied to GM vs. Rochester making them. I finally gave up on finding a viable purchase source....

Jack------


There's no doubt that the black limiter caps had a GM part number. The problem is that I highly suspect it was a PRODUCTION-only part number and never available in SERVICE. So, finding the part number would be an academic exercise and would not be the least bit useful in trying to obtain that part number.

The limiter caps were definitely designed to self-destruct upon removal. In fact, that was part of the rationale behind them and absent self-destruction they would have been useless. In other words, if someone could remove them, adjust the idle mixture, and then re-install them, there would really be no purpose to having them there in the first place. The whole point was these were installed to limit the range of idle mixture adjustment and, if that intent was to be defeated, they (the government) wanted there to be clear evidence that had occurred (i.e. no caps or different colored caps).

So, if there was ever a part that GM wanted to have as PRODUCTION-only, these black caps were IT. Probably in the same class as rosette rivets.

[Edward J.]

Re: 1971 carb idle mixture limiters--original color

Kevin, your car is likely one of a hand full of C3's in the NCRS that has these caps. you would will be hard pressed to find many cars with these caps, as the first tune ups were done they where removed to smooth out a lumpy idle. and the caps were brittle and broke in pieces.