Help! Carburetor on 250 I-6

[Memaloose]

You can't go wrong with a manual choke. It's closed when pulled out and open when pushed in.
I have seen people, mostly young and not experienced, "forget" to push it in when the engine has warmed up resulting in fouled plugs. They're simple and they work.

 

[JohnTaurus]

I agree. I've used it on farm equipment and other old vehicles, so it's very familiar to me. That is why I said that I would prefer it to an electric choke etc. Same reason I don't want power windows (or power steering for that matter), haha!

Being that this truck had a factory generator, points distributor, manual brakes and steering, I kinda expected it to have a manual choke. I guess the cost of engineering/building/installing-at-production a manual choke to serve as a cheaper alternative wasn't worth the minuscule savings.

Edit: as I read my posted comment, I realized that some would consider EFI vastly more complicated than a carbureted system, and I just said I prefer a simple manual choke. That is really because I don't trust automatic chokes. I have had problems with them in the past. But, my cousins 1965 Ford cranked very nicely with the manual choke, every cold morning. It's no surprise to me that the choke is/probably the source of some of my issues.

Hope that clears it up. I can be a man of contradictions, like owning a Chevy, when the 6 other vehicles parked on this hill all have Blue Ovals. Haha!

 

[CorvairGeek]

I like the way you guys think! My intent was also to keep CFM and fuel flow more comparable to the original design for my unmodified 292 (though it does have the later HUGE outlet exhaust manifold) for maximum efficiency and drive-ability. I chose the odd, smaller 2V version of the GM TBI (3.1 truck only) for my 292 with the intent of running 305 injectors on my still born EFI project. Just short a pair of injectors and an electric fuel pump. The wiring harness & sensors are ready and even have newer sets of higher pressure GM plastic fuel lines. Have quite a few EEPROMS and Cal Paks so it should run ('427 / '395 ECMs). Just lost motivation and the will to do the tuning.

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[AuroraGirl]

I agree. I've used it on farm equipment and other old vehicles, so it's very familiar to me. That is why I said that I would prefer it to an electric choke etc. Same reason I don't want power windows (or power steering for that matter), haha!

Being that this truck had a factory generator, points distributor, manual brakes and steering, I kinda expected it to have a manual choke. I guess the cost of engineering/building/installing-at-production a manual choke to serve as a cheaper alternative wasn't worth the minuscule savings.

Edit: as I read my posted comment, I realized that some would consider EFI vastly more complicated than a carbureted system, and I just said I prefer a simple manual choke. That is really because I don't trust automatic chokes. I have had problems with them in the past. But, my cousins 1965 Ford cranked very nicely with the manual choke, every cold morning. It's no surprise to me that the choke is/probably the source of some of my issues.

Hope that clears it up. I can be a man of contradictions, like owning a Chevy, when the 6 other vehicles parked on this hill all have Blue Ovals. Haha!

im fairly certain that manual chokes were an option on these trucks IN SOME years for sure. Manual references them

 

[JohnTaurus]

im fairly certain that manual chokes were an option on these trucks IN SOME years for sure. Manual references them

Haha well that's even more disappointing, kinda. Nahh, its fine, I can add it for the duration of the time it is running with a carburetor, assuming my mechanic can get it right.

I will definitely talk to him about this, get his opinion. He builds hot rods and all kinds of custom vehicles, so I'm quite sure he can get an old Chevy tuned right, haha. Hopefully a new carburetor isn't required, but it's a possibility, I guess.

 

[JohnTaurus]

I like the way you guys think! My intent was also to keep CFM and fuel flow more comparable to the original design for my unmodified 292 (though it does have the later HUGE outlet exhaust manifold) for maximum efficiency and drive-ability. I chose the odd, smaller 2V version of the GM TBI (3.1 truck only) for my 292 with the intent of running 305 injectors on my still born EFI project. Just short a pair of injectors and an electric fuel pump. The wiring harness & sensors are ready and even have newer sets of higher pressure GM plastic fuel lines. Have quite a few EEPROMS and Cal Paks so it should run ('427 / '395 ECMs). Just lost motivation and the will to do the tuning.

You must be registered for see images attach

Sounds like an amazing setup, maybe time to get the fire back in your belly and get it going! Then you can do my truck for me, wink wink. Hey, should be a breeze after yours, eh?

Anyway, yeah. It's hard to get yourself out there to get started again, especially when the work is tedious and complicated! I know that feeling very well.

I have almost all of the parts required for another project, but I've never done the job it needs, so I'm very hesitant about getting started. Right now, it runs and moves around the yard, it's just missing coolant hoses, so I literally just run it long enough to move it from one spot to another when mowing the property. I really love that car, I've had it for years and I've always wanted one like it since I was a kid and they were new. This one just has almost 270k miles and it's well past the typical service life it was designed for, and a lot of the original secondary systems are showing their age (and lack of maintenance in some cases).

The whole project, not just including the timing cover/components, is kinda overwhelming me. I think it's best to pay someone to do what it needs. I just want to make sure the Chevy is up to the task of being a dependable daily, until that car is back on the road, at least. Then the EFI and overdrive manual transmission conversions can commence lol.

 

[1987 GMC Jimmy]

I like the way you guys think! My intent was also to keep CFM and fuel flow more comparable to the original design for my unmodified 292 (though it does have the later HUGE outlet exhaust manifold) for maximum efficiency and drive-ability. I chose the odd, smaller 2V version of the GM TBI (3.1 truck only) for my 292 with the intent of running 305 injectors on my still born EFI project. Just short a pair of injectors and an electric fuel pump. The wiring harness & sensors are ready and even have newer sets of higher pressure GM plastic fuel lines. Have quite a few EEPROMS and Cal Paks so it should run ('427 / '395 ECMs). Just lost motivation and the will to do the tuning.

You must be registered for see images attach

It looks like I’m in the minority for advocating a single barrel setup. Only reason I feel like that is worrying about creating an overcarb situation unless it’s commonly thought that a 1MV is undercarbing a stock 250. I don’t know enough about them to say. It’s still more than the stock carb, which gives you exactly the room you need to upgrade to HEI, maybe do an Offy style intake, headers, and a notch above stock cam. If you need a little more air coming in, just do a port and shave the throttle shaft. Plus, you only have one injector to worry with, which good TBI injectors are expensive for what they are. It also keeps the induction footprint down similar to the Monojet, and it’s one injector simpler, which is appropriate for a simple truck.

A 2.5/3.1 TBI unit might do the trick with his also, and then try the 3.1 injectors or bump it up to 4.3’s. I think experimentation with both would be key, and I’d accept the verdict. I was reading where someone did straight 4.3 stuff on a hot 292, and they had a bunch of issues, but that could have been incorrect PROM parameters also. I have a spare 7727 w/ PROM set up for a 3.1/TH125C if you need. I think I yanked it out of a ‘93 Grand Prix. Also, if you come across one of those W-Body three pin Coolant Level Sensors, I’d love one. I have a later two pin unit, but I haven’t gone through the earlier and later engine diagrams to see how to make the two pin one work and shut that stupid Low Coolant light off.

 

[1987 GMC Jimmy]

Sounds like an amazing setup, maybe time to get the fire back in your belly and get it going! Then you can do my truck for me, wink wink. Hey, should be a breeze after yours, eh?

Anyway, yeah. It's hard to get yourself out there to get started again, especially when the work is tedious and complicated! I know that feeling very well.

I have almost all of the parts required for another project, but I've never done the job it needs, so I'm very hesitant about getting started. Right now, it runs and moves around the yard, it's just missing coolant hoses, so I literally just run it long enough to move it from one spot to another when mowing the property. I really love that car, I've had it for years and I've always wanted one like it since I was a kid and they were new. This one just has almost 270k miles and it's well past the typical service life it was designed for, and a lot of the original secondary systems are showing their age (and lack of maintenance in some cases).

The whole project, not just including the timing cover/components, is kinda overwhelming me. I think it's best to pay someone to do what it needs. I just want to make sure the Chevy is up to the task of being a dependable daily, until that car is back on the road, at least. Then the EFI and overdrive manual transmission conversions can commence lol.

Jerry has more ongoing/prospective projects than 85% of the people on here. You might be waiting quite a while for him. I think a 250 or 292 mated to an NP833 would a unique combo if you could put it together. I know you said an NV earlier. But a TBI 250 with a four speed manual overdrive would be very unique.

 

[JohnTaurus]

It looks like I’m in the minority for advocating a single barrel setup. Only reason I feel like that is worrying about creating an overcarb situation unless it’s commonly thought that a 1MV is undercarbing a stock 250. I don’t know enough about them to say. It’s still more than the stock carb, which gives you exactly the room you need to upgrade to HEI, maybe do an Offy style intake, headers, and a notch above stock cam. If you need a little more air coming in, just do a port and shave the throttle shaft. Plus, you only have one injector to worry with, which good TBI injectors are expensive for what they are. It also keeps the induction footprint down similar to the Monojet, and it’s one injector simpler, which is appropriate for a simple truck.

A 2.5/3.1 TBI unit might do the trick with his also, and then try the 3.1 injectors or bump it up to 4.3’s. I think experimentation with both would be key, and I’d accept the verdict. I was reading where someone did straight 4.3 stuff on a hot 292, and they had a bunch of issues, but that could have been incorrect PROM parameters also. I have a spare 7727 w/ PROM set up for a 3.1/TH125C if you need. I think I yanked it out of a ‘93 Grand Prix. Also, if you come across one of those W-Body three pin Coolant Level Sensors, I’d love one. I have a later two pin unit, but I haven’t gone through the earlier and later engine diagrams to see how to make the two pin one work and shut that stupid Low Coolant light off.

I'm totally open to suggestions here, I promise. I'm definitely going to ask my mechanic his opinion on all of this, too.

How much searching online have you done for your coolant sensor?

 

[JohnTaurus]

Jerry has more ongoing/prospective projects than 85% of the people on here. You might be waiting quite a while for him. I think a 250 or 292 mated to an NP833 would a unique combo if you could put it together. I know you said an NV earlier. But a TBI 250 with a four speed manual overdrive would be very unique.

I bet THAT is the one that was in my uncle's old Chevy 3500. It was the first year of the 90s style for the heavy duty version (this replacing the updated square body trucks they were selling). It had an extra low gear, and an overdrive, if I remember correctly.

I was just telling my best friend that if I could find that transmission, I would have an excellent setup! I had no idea the name of it until you just posted it. Its gotta be the same one. I bet its much heavier duty than the NV3500.

 

[SirRobyn0]

I’ve softened on them considerably where I’d seek out a carbed vehicle over even a TBI.

I don't know if you know my background but I run an auto repair shop, and I've been at it for far to long. This is just my opinion, but in general I'd seek out a carbed vehicle over TBI, but I'd seek out a multiport injection over both, if it was available factory on the vehicle I wanted. TBI is really the bottom of the barrel in my mind, the reason being is that they weren't as well refined as carb, or MPI. TBI often had a limited data stream, and some systems the computers wouldn't set a code for nothing which left the tech guessing or manually testing sensors. With that said GM's TBI and TBI diagnostic system was better I think than Ford or Chryslers, and I can see how it could appeal to someone that wants to get away from carb, and obviously there are aftermarket TBI setups that are good too. To me the biggest downside to carb is added maintenance, and potential shortening of engine life if the engine is run out of tune for long periods. For me at least a well tuned carb engine is fun to drive. With that said I wouldn't refuse to buy a vehicle based on the fuel delivery system I just have preferences.

If I go strictly off of reliability without getting overly complicated I'd say the MPI engines in the 90's and even in to 2000's were the best ever built. Honestly I'm not a big fan of todays cars, or trucks, and I work on them.

One of the things I like about the squares is they are so easy to work on, but I also think the biggest problem square owners are faced with is if you don't do your own work, is finding someone today that is good at fixing carburetors.

 

[1987 GMC Jimmy]

I bet THAT is the one that was in my uncle's old Chevy 3500. It was the first year of the 90s style for the heavy duty version (this replacing the updated square body trucks they were selling). It had an extra low gear, and an overdrive, if I remember correctly.

I was just telling my best friend that if I could find that transmission, I would have an excellent setup! I had no idea the name of it until you just posted it. Its gotta be the same one. I bet its much heavier duty than the NV3500.

Actually, you’re probably thinking of an SM465. It’s technically a four speed, but one of those is “Granny Low” with three street gears, no overdrive. The NP833 is somewhere between the Saginaw Three and the SM465 in toughness, and the 465 is tougher than an NV3500. The 833s were more common in Dodges, I think, stylized as A833. Still a good transmission, though. I’d love to get a square with one.

 

[AuroraGirl]

It looks like I’m in the minority for advocating a single barrel setup. Only reason I feel like that is worrying about creating an overcarb situation unless it’s commonly thought that a 1MV is undercarbing a stock 250. I don’t know enough about them to say. It’s still more than the stock carb, which gives you exactly the room you need to upgrade to HEI, maybe do an Offy style intake, headers, and a notch above stock cam. If you need a little more air coming in, just do a port and shave the throttle shaft. Plus, you only have one injector to worry with, which good TBI injectors are expensive for what they are. It also keeps the induction footprint down similar to the Monojet, and it’s one injector simpler, which is appropriate for a simple truck.

A 2.5/3.1 TBI unit might do the trick with his also, and then try the 3.1 injectors or bump it up to 4.3’s. I think experimentation with both would be key, and I’d accept the verdict. I was reading where someone did straight 4.3 stuff on a hot 292, and they had a bunch of issues, but that could have been incorrect PROM parameters also. I have a spare 7727 w/ PROM set up for a 3.1/TH125C if you need. I think I yanked it out of a ‘93 Grand Prix. Also, if you come across one of those W-Body three pin Coolant Level Sensors, I’d love one. I have a later two pin unit, but I haven’t gone through the earlier and later engine diagrams to see how to make the two pin one work and shut that stupid Low Coolant light off.

is a 1988 lesabre or a 1990 olds 88 have a three pin level sensor. I have parts from them but i never checked if i had that. 3800

 

[1987 GMC Jimmy]

I don't know if you know my background but I run an auto repair shop, and I've been at it for far to long. This is just my opinion, but in general I'd seek out a carbed vehicle over TBI, but I'd seek out a multiport injection over both, if it was available factory on the vehicle I wanted. TBI is really the bottom of the barrel in my mind, the reason being is that they weren't as well refined as carb, or MPI. TBI often had a limited data stream, and some systems the computers wouldn't set a code for nothing which left the tech guessing or manually testing sensors. With that said GM's TBI and TBI diagnostic system was better I think than Ford or Chryslers, and I can see how it could appeal to someone that wants to get away from carb, and obviously there are aftermarket TBI setups that are good too. To me the biggest downside to carb is added maintenance, and potential shortening of engine life if the engine is run out of tune for long periods. For me at least a well tuned carb engine is fun to drive. With that said I wouldn't refuse to buy a vehicle based on the fuel delivery system I just have preferences.

If I go strictly off of reliability without getting overly complicated I'd say the MPI engines in the 90's and even in to 2000's were the best ever built. Honestly I'm not a big fan of todays cars, or trucks, and I work on them.

One of the things I like about the squares is they are so easy to work on, but I also think the biggest problem square owners are faced with is if you don't do your own work, is finding someone today that is good at fixing carburetors.

Agree 100% with the caveat on MPFI being my experience with GM 60* V6 injectors, they can be flaky once they get to a certain age, and I’ve also heard bad things about the Gen I spider system, but that once they were upgraded to the Gen III, LS-based style, they were perfect.

My Jimmy will set a code for any little discrepancy, which I’ve always appreciated, but the CCC setup in my Caprice will happily ignore some issues in plain sight, AND I don’t have a serial data pin to hook up my laptop. I do have a 35 year old scanner that can give me 75% of that data, though. I know OBD 1 can be fickle like that. The 128 baud rate on the older ECMs is cartoonish looking at what’s out there today, but if you have a good command on how they work, what they’ll do when they’re messed up, and how to analyze the relatively slipshod data, you can be happy with them on a personal level.

I think TBI is pretty simple, definitely beats the spaghetti you saw in the trucks from 81-86, and I don’t mind having to do the extra work having to manually test sensors like you said. It would get old on multiple vehicles like that, not to mention working on hundreds of them, but mine’s been good to me; even when it’s broken, it’ll tell me things to lead me to the problem. You’ll almost certainly never make the power that you can with a carb/HEI setup, and making TBI more powerful comes with a time/money investment, but even in spite of its limitations, you can work around the inherent shortcomings like the heads, the lame cams, and a partially data blind ECM.

 

[JohnTaurus]

Actually, you’re probably thinking of an SM465. It’s technically a four speed, but one of those is “Granny Low” with three street gears, no overdrive. The NP833 is somewhere between the Saginaw Three and the SM465 in toughness, and the 465 is tougher than an NV3500. The 833s were more common in Dodges, I think, stylized as A833. Still a good transmission, though. I’d love to get a square with one.

Yes, you're probably right. I think it is the transmission you are talking about, and I was thinking about something else. I looked up that transmission and found a shifter that looked exactly like the one in that old dually. I was wrong about the granny low.