The Stovebolt Site Forums The Shop Area Engine Shop Need quick answer: electric or auto choke carb?

I have a chevy 250 inline motor in my '49 that currently has an auto choke carb, but I'm needing to replace it and I've found a rebuilt carb except it is an eletric choke carb. Which one am I better off having? And if I do buy the electric, how hard is that to hook up? Thanks for the help!
Jeremy

With no other knowledge, the hot air choke would be superior to the electric; and a manual choke is superior to either. The electric choke is controlled only by time. If your engine is not sufficiently warm to run when the clock says the choke goes off, then your engine may stall, and may not restart for however long it requires the choke to reset (generally several minutes to a couple of hours). The hot air choke is controlled by the actual temperature of the engine.

The electric choke is easy to hook up. Most require wiring the "hot" terminal on the choke to some constant voltage source that goes through the ignition switch.

Jon.

Thanks for the quick reply Carbking! How hard would it be to eliminate the electric and install my auto choke on the new carb? or even make the new one a manual since I still have the knob in the cab? Or is that just more trouble than it's worth?

Don't know. Probably difficult, maybe impossible. Depends on brand(s) of carburetor(s), types, styles, etc.

Converting to manual is probably not too bad; NAPA has a decent auto ->manual conversion kit.

Jon.

Actually, there is a simple way to control an electric choke, and it's exactly the way the car manufacturers do it. GM uses an oil pressure switch inline with the ignition voltage that interrupts the circuit to the choke in case the engine stalls. It's a 3-terminal switch that mounts just above the oil filter on small block V-8's. The two "normally open" terminals connect ignition voltage to the choke. The "normally closed" single terminal that grounds when the oil pressure goes away is the idiot light switch.

Ford electric chokes have a circuit that supplies AC voltage from the stator of the alternator. Anytime the alternator is not turning the voltage drops to zero, so choke heating stops. Since GM alternators do not have a stator terminal, it's best to use the oil pressure switch if you're using a GM alternator or a generator. The GM pressure switch also supplies voltage to the electric fuel pump on fuel injected engines, so it acts as a rollover switch, stopping pump operation in case the oil pump pickup isn't submerged in the oil pan.
Jerry

okay, that sounds a bit too complicated for my simple mind! I'd like to keep things as simple as possible for myself. Thanks for the help guys, I'm just gonna keep looking for a auto or manual choke carb then.
Any good places to buy rebuilt/ reman carbs?

Are you running the Rochester carb that was original equipment on the 250? If so, you can probably swap the choke cover from it to the new carb, assuming the new one is also the same brand and type of carb. The problem will be with getting hot air to it to make it release.

An electric choke heater is the simplest, and most reliable way to run an automatic choke. How difficult is it to run a pressure switch and two wires? An automatic choke is smarter than all drivers, and most mechanics. It works when it's needed, with no input from the driver, and doesn't forget to release itself when the engine warms up.
Jerry

It's a '68-'72 250 motor with rochester monojet carb and I assume it's original but not 100% sure. The one I have uses a coil that's in between the exhaust manifold and head and has a rod that connects to the choke linkage (right?). I was shopping for a direct replacement but can't seem to find any just like mine- they're mostly electric choke. The carb I have has a stripped out bolt and leaks gas everywhere and someone also tried to weld the linkage back together (after it must have broke somehow) but did an aweful job and so on.

See if you can find the GM part number on your carb. I believe I've got an original condition carb on a 250 that's all complete and rebuildable. I don't plan to use it anytime soon. If I decide to build this engine, it's going to get some go-fast modifications including a conversion to a 2GC Rochester 2-barrel. IM or email me the part number, or the engine ID number, and we might be able to solve your problem.
Jerry

This article somewhat long, but may aid in understanding:

http://www.thecarburetorshop.com/Automaticchokes.htm

Personally, I replace any style automatic choke on my vehicles in favor of manual chokes. Very functional, very reliable, but everyone to their own opinion.

Jon.

What Hotrod said.
Give your carb a good look see and find some numbers. Been around here listening for a while now, and from what I can see, the guy seems to know what he's talking about. But then again...he might just be like most of us and be full of it. Hang on Hotrod... I'm just kidding. Everything I've read over time from you has been solid, sound advice, and in todays world, thats a rarity.

Lot's of knowledge here.

Jerry,
Have you seen/heard of a 2GC used on a 235/261 and if so which one would be appropriate?

Jon,
Am I correct in that the small, early 2GC are 280 CFM and that a pair of them would be way too much for a 261 or worked 235?

I have a 2GC from a mid 70's Pontiac and a stock manifold laying around and I'm considering making a flange and trying it on a 261. Can someone give me a reason to kill that idea? Thanks.

Curt - there is a partial chart of CFM for various Rochester carburetors in the Doug Roe book; you need to know the internal venturi size (measure it) and then compare it to the chart in the book.

Remember that 2-barrels and 4-barrels are rated differently. See the article on our website for ratings conversions. Here is the link:

http://www.thecarburetorshop.com/Carbshop_carbsizesandCFM.htm

More information would be necessary before I could suggest if it were too much. In the "for what its worth" category, I have successfully run a 400 CFM Carter 4-barrel on a 195 CID Pontiac four, a 132 CID Toyota 4, and a 120 CID Ford 4. I also had a pair of 500 CFM 2-barrels on a 120 CID Datsun 4 which was factory carburetion for the engine! Much depends on the state of tune of the engine, the manifold and head design, AND how you plan to use the vehicle.

Jon.

I've been tinkering on carburetors since the early 1960's, first around the family shop, then as a technician and independent shop owner. I've done quite a bit of racing, with gas and alcohol-fueled engines, and I've developed some pretty firm opinions along the way. Some of them might even be correct!

Most people put WAY too much carburetor on their engines, then try to deal with stumbles, backfires, and a host of other problems that can be traced directly to having a too-slow velocity through the carb venturi(s), resulting in lots of air, and virtually no fuel flow. The venturi effect speeds up the airflow through the smallesrt diameter of the carb body (the venturi) and lower the pressure enough to allow atmospheric pressure to force fuel through the jets and into the airstream. Slow the venturi velocity down by putting too big a venturi (or too many of them) in the airstream, and you get LEAN in a hurry. The only fix is lots more cubic inch displacement, or lots less carburetor.

I'm planning to use the Rochester 2-barrel that came stock on the 265 and 283 engines, not the bigger-bore model that was used on the 289 Pontiac and others. The 250 will have a major bump in compression by using 307 V-8 flattop pistons, an .050" head milling job, and a pretty radical hydraulic cam. I might go a little bigger on valve size, too. It will idle at around 1200 RPM, and have a top speed of 6,000 RPM or so. That works out to just over 450 CFM, which the 2-BBL should supply with no problems, and I don't have to mess with multiple carburetors!
Jerry

That is pretty much what carbking's article says:


The first step (and the very more important step) is to determine WHICH carburetors to use. We would suggest for a basically stock 235 or 261 for street (normal driving, cruises, an occasional trip to the drag strip) that MATCHED TAGGED 216 CARBS ARE USED! For the primary strip vehicle that sees some street use, MATCHED TAGGED 235 CARBS MAY BE USED. For the race only vehicle (not licensed, no street use) MATCHED TAGGED 261 OR 292 CARBS SHOULD BE USED! For a basically stock 292, MATCHED TAGGED 235 CARBS SHOULD BE USED. For optimal results, choose the carburetors to fit the airflow requirement of the engine, and then modify (if necessary) the intake mounting flanges to accommodate the proper carburetors. If the manifold requires modification, the better way is to have a heli-arc welder fill the holes, and then redrill and retap new holes at the proper position. Mixing throttle bodies with bowls to “cheat” on the above is apt to meet with unusual results.


This information leaves me confused as to why are all the repop intakes made for the 235 carbs? And the linkage for them? I think it's nearly impossible for heat/stress not to affect the "floating" linkage. Sorry to wander off topic here and thanks for replies. If a 2G would sit and run nicely sideways on a stovebolt why are there not examples of it somewhere? Any welder with enough skill to make a split exhaust would have no trouble welding on an intake flange and if it worked well you think it would have caught on. I have yet to see a 2G on a stovebolt motor.

"This information leaves me confused as to why are all the repop intakes made for the 235 carbs?"

Curt - very simple, the repops are repops of what was available 50 years ago WITH NO EXTRA EXPENSE IN TOOLING OR MACHINING!

Jerry is absolutely correct about air velocity. The reason the Carter 400 worked so well on the small engines is that is it is an on-demand secondary. The primaries are smaller than most 2 barrels.

Now, back to your question. The early multiple carb manifolds were bought by RACERS (both track and street). These guys ALL had big cams, more compression, headers, etc. Hot rodding with a dogmatic transmission was unheard of, until Jim Hall proved it could work with the Chaparrel in the early 1960's. In other words, no one used multiple carbs for looks. Performance was the reason. A high idle was perfectly acceptable if the engine made lots of power.

Today, lots of guys want looks, but care little or nothing about power; so they are trying to use racing parts and then don't like the results on a stock engine.

And please, no one take offense at my next statement, as I don't mean any, but a statement of my opinion.

Hotrodders 50 years ago were much more experienced than those of today. These guys grew up in wrecking yards and garages. They could weld, they could modify chassis to make a certain shock or bigger brakes on a heavier differential fit. They could rebuild carburetors and parts were readily available at the local parts house. If they messed something up, a trip to the local wrecking yard and a few bucks, and they were back in business.

These guys had no where near the sophisticated tools of today, but they could make due pretty well with what they had. Example: I was taught to synchronize multiple carburetors by using a small hose, and listening to the volume and pitch of the airflow. Two carbs were (and still are) easy. Three carbs were not bad. Running 6 simultaneously was a handfull. Stethascopes (sp) were the realm of the "big boys" with big bucks; and I had worked on multiple carbs for 10 years before I could afford a Uni-Syn.

Summing up, these guys were glad to get a manifold they didn't have to fab from scratch, and relocating holes was not a problem.

I would like for the repops to come with no drilling, but I have suggested it to no avail. Perhaps if enough buyers were to do so, it might happen, but I doubt it.

Jon.

Carbking, I am 64 years old and thoroughly agree with you. I grew up in a small town in Texas where almost every secluded road with a straight stretch had a quarter marked off. Our cars weren't particularly pretty, but as long as the girls would go for a ride, it was ok. Multiple carbs meant an occasional fire under the hood, but oh well. Blowing the doors off a 283 or a 312 with an inline 270 or 302 was an accomplishment. But then came the muscle cars and it all changed. Rich kids talked their father into buying them a car that would show me tail lights the night it came off the show room. I still say I enjoy the smell of burning rubber and fuel, not money.