Question on Carb

[Retro Rich]

Hi All,

Trying to figure out what the correct carb was for mine and make a decision...1984 GMC High Sierra 2500, 350 V8. Bought it a couple weeks ago, the previous owners brother in law replaced the original Quadrajet with one that looks like it had been sitting in a molasses vat for the past 10 years. Tried to clean it, I believe the internals are just roached and it has so much seepage doesn't appears worth saving without a major rebuild I don't have the time or patience for. Choke linkage is all screwed up, and they decided to drill into the throttle linkage and connect the TV cable for the 700R4 just by destroying the cable and sticking it in there.

I believe that mine would have come with a Hot Air Choke and be a M4MC model quadrajet...should be 750 cfm....does that sound correct? I can't really find anything in that cfm range on the shelf around here, and don't want to get a 600 cfm Eddy or Holly as I plan on building the engine at some point in the next year or so for more hp/torque and don't want to be short on air and fuel down the road and have a $300 carb I will have no use for (not sure if making it a built 355 or so or going 383 stroker).

I found some good looking reman/rebuild ones on ebay for around $220-250, and the M4MC type I believe I need....thoughts?

Thanks!

 

[chengny]

I'm not much on carburetors but my 1986 K2500 had a M4ME as stock - I believe it was just the basic 750 CFM.

You are most likely setup for electric choke. Is there a stray "light blue" colored wire running along the RH valve cover with the alternator wiring? If so that is the power to the choke heater.

 

[Retro Rich]

Thanks chengny...no blue wires or anything that looks like a lead for an electric choke on this one, and has some hard lines/tubing running from the intake manifold that I believe should be hooked up to a hot air styled choke...if I am thinking correctly.

 

[chengny]

If there are 2 tubes coming out of that small rectangular plate on the RH side of the intake manifold - I would say "Yep", you've got a hot air choke setup.

I had one of those little plates in my hand the other day. Looks like a double pigtail tube on the hidden side. Figured that it was to pick up waste heat from the exhaust gases.

I was too lazy to try to figure out how that heat was intended to get up and into the carburetor to open the choke.

Just more tubing, huh?

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But still don't know what makes the heat flow through? Is there a slight suction into the carb throat?


I am surprised that the original (1984) carburetor used hot air - and not battery voltage - to operate.

Maybe the PO put a new intake manifold on and the hot air setup came as a package deal?


Oh and please don't ask about the electric choke in addition to the hot air in the image- it's not mine.


Here is a 1984 C/K engine compartment wiring diagram showing an electric choke assist:

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[farmerchris]

i go to www.carbjunkey.com for my quadrajet parts.they have rebuild service for your carb.also have a electric choke conversion that make life a lot easier.i order my rebuild kits from them as they are alcohol resistant for the fuels now days .they also have a eBay store you can go through.

 

[Retro Rich]

Thanks guys...yeh, it's that plate with the metal tubing coming out of the manifold. Supposed to grab heat out of the manifold and move the choke around with the internal spring as the engine warms up but appears that wouldn't be as reliable as electric that I was used to in the 80's. I am going to go ahead and convert one over to an electric choke and cap that off on the manifold after thinking about it last night...should be more reliable for choke purposes and make life easier. Thanks for that wiring diagram chengny, that is very handy and will come in useful as I am getting mine back in proper running order.

 

[chengny]

Here's some more detail:

You will use the 2 prong oil pressure sender to power the electric choke assist. That sender is really just an on/off switch that closes when oil pressure reaches 10 psi. It also serves to illuminate the warning (idiot) light if you don't have gauges.

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The other one - single terminal - is a sensor (0 - 60# variable resistance to drive your oil pressure gauge).


A better dwg of the choke heater circuit:

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The pink/white 350 wire is hot from the fuse block
The dk blue 931 feeds the choke light on the dash
The lt blue78 powers the electric choke.

All the wiring is most likely still there. The plug for the 2 prong switch is big and has a locking tab to hold it in place:

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Be aware that when this circuit is fully functional, since the choke light on the dash is controlled by the oil pressure switch, it is effectively a low oil pressure warning alarm.