Inline-6 carb preferences?

[Oregoon]

'74 GMC 2WD LWB with a 250 and a 3OTT.

Running an HEI, totally desmogged and the stock Rochester. No emissions tests, so that's not a concern.

The truck is my daily driver, so fuel economy and performance are the goals. I haul a bit with it, though nothing outlandish. Lumber, hay, crates of vegetables. Mostly, I cruise around Portland with it. Top-end is not a concern. I chill in the right lane and prefer backroads.

Considering a header, new intake (if needed!), and a carb. Maybe a Carter YF with a manual choke? The Rochester is tempermental and I'd like better economy if I can attain it. Best I've got is 20, but usually I get about 15-17.

Open to any suggestion. Not sure the 4bbl route is really worth it. I'd prefer a 2bbl progressive, though I have really enjoyed the Carters I've owned on Ford-sixes. Very simple, very reliable.

Educate me!

[D13]

You will not do better than the Monojset unless you are willing to do manifolds. When you say 'tempermental', is it a choke issue, or part throttle tip-in? Chocke, go electric with a later carb, or manual. The other is usually accelerator pump or linkage or air leakage at the throttle shaft.
The intake manifold has a heat tube built in so it's very difficult to modify (any throat work breaks into the surrounding void and has to be blocked back off).
The 2bbl progressive from the 80's is a great carb but very finicky in set up, and direct fits only the integral head that was designed for it. You would have to take a 4bbl manifold and make a custom top plate for it.
2jet Rochesters require an adaptor or aftermarket manifold.
Langdon's Holley / Weber package is sweet but $$.
Same for the the 4bbl (Holley 390 MAX an a stocker) and you will not save enough gas to pay for it for about 10 years, but will get better overall drive-ability. Header is good, long tubes for low end torque are good, Langdon irons are good tool. Inliners International can help more. I suspect a cam change may give yu the biggest bang for the buck on a stock motor - there's a pretty decent cam, often called the 'marine' cam, available.

Remember, GM spent a lot of time tuning these for driving and hauling. Unless you want to spend more time at higher RPM, you're not going to get a lot better.

[Bigstevex4]

I went backwards have an 83 with the interracial intake exhaust. I couldn't get the Vari jet to run right save my life. and there are no adapters plates for any thing else looks at the Webbers they seem be choice of a lot 6 banger fans. Any way I bought an old mono jet made an adapter plate. runs better than ever. Far from a power house got rid of all emission junk (Off road use only) Got 1 vacuum line from carb to distributor. And I don't think you'll get any better mileage 17 is great 20 awesome. that's about what I get. I read were with aluminum intake you have have some kind of of heat or cant get gas to vaporize. Also have heard since most gas is injected these days its not refined to vaporize like it used to. I know mine runs so much better once its warmed up. But ime just back yard Mechanic. so take note.

[greg64]

My truck is an original 6 banger. Started out with a 250 and I now run a 292. I don't entirely like the Rochester either. I ran injection for years and liked it, but ran into some computer problems that haven't been solved yet. So right now, I'm running a 350 cfm 2bbl, non-progressive. It works, but isn't great either. It's too big at light throttle. I'm going to switch to a carter yf, which is calibrated for a 292, not a Ford. I think the progressive 2 bbl is probably good, but I had bad experiences with a Rochester on an s10 years ago, so I'm gun shy.

[ChevDog1]

I can tell you that I have a rochester 2jet on my integrated head 250, and I am not happy with it. Lots of tuning and rebuilding, and I still have a bog on part throttle tip in that I can't seem to fix. Right now I'm looking for a complete TBI from a 4.3 v6 to put on it, and just give up on the carb.

[greg64]

Chevdog, the TBI is the best route. Port fuel injection, which makes so much sense on a v8, doesn't work on the chevy sixes due to the siamesed intake ports.

[ChevDog1]

I found a video online from a guy who swapped the 4.3 tbi onto a jeep six. Looks pretty simple to do, just need to find the parts at a reasonable price...

[Oregoon]

Thanks for the replies! Carbs are such a sticky issue... So much depends on ignition and overall engine condition.

GM spec'd Carter YFs for years, and they're pretty good all in all. Annular fuel discharge, adjustable metering, very few moving parts...

My Rochester has been quite good, though when my distributor loosened up and I had a pretty bad backfire, it seems to have taken my accelerator-pump or power-valve to screwy new places. I went through it after I had a problem, and all looked well. Evidently not... Not a lot of response, seems to surge at higher rpm, fuel economy is down around 12 mpg.

Time to go through it again.

I like the idea of a 2bbl. I had a Weber on a Jeep 258 once, and it worked really well. The YFs van be found on all sorts of stuff: Kaiser, Willys, GM, Ford, etc... They can be a pain in the hindquarters to jet properly, but once you get it where you want, it's a set it and forget it deal.

Strongly leaning that way, though my Rochester is in great shape and may just need some minor repair. Too bad I have to buy an entire carb kit to do it.

[ChevDog1]

If I can't find the tbi, I may end up with a weber. I've never had one, but have heard good things about them.

[MikeB]

The 250 six in my 82 has the dreaded integral head/manifold and the progressive 2bbl carb, with emissions as a focus. However I looked into an early head and manifold, and would probably have gone with a one-bbl based on comments here and elsewhere.

A 4bbl manifold with 390 Holley#0-8007 would have been sweet, but the carb alone costs around $460 -- at least 50% more than a comparable 600.

Anyway, I caved and am now building a 355 to use along with a TH350 I just went through. (OK, my buddy who's a GM trans tech did most of it.) That combo should work much better with my truck's 2.73 axle. What was GM thinking?

[greg64]

Mike, a 2.73 rear with a 250 doesn't sound right, at least for a 70s truck anyway. Mine is 3.42 and 3.73 was common too. If it really is factory, GM must have been desperate for fuel economy.

[ElGracho]

Originally Posted by greg64

Mike, a 2.73 rear with a 250 doesn't sound right, at least for a 70s truck anyway. Mine is 3.42 and 3.73 was common too. If it really is factory, GM must have been desperate for fuel economy.

My 1984 C10 has a 250 (integrated head, variable venturi 2 barrel), 3 speed saginaw and a 2.73 geared 10 bolt. Not the worst combo, but far from the best.

Best fuel economy I've seen is about 13.

5.3, 4L60E and 3.42 posi are in line to go in.

Just to keep with the thread, I'm on my second of these carbs on this truck and I just can't make them work as well as I'd like.

[MikeB]

Originally Posted by ElGracho

My 1984 C10 has a 250 (integrated head, variable venturi 2 barrel), 3 speed saginaw and a 2.73 geared 10 bolt. Not the worst combo, but far from the best.

Best fuel economy I've seen is about 13.

I read where GM rated that combo at 24 hwy in 1982.

It's not too bad starting off with the 3.50 1st gear ratio, but the RPM drop to second is substantial. And forget about 3rd until 40 mph. However some previous owner put 255-70 tires on the truck, so that doesn't help. Originals were today's equivalent of 205- or 215-75x15. Tiny little guys!

[Bigstevex4]

My 83 has 250 465 4 speed and really tall gears I bet that's what I have 273 better be flat ground our down hill when hit 4th I have taller tires also.

[Oregoon]

I attain most of my economy via an adjustment of my right foot. Sounds preachy, but it's really nothing more than a habit. I drive truck for a living, and due to the raft of CDL regulations these days, any ticket I get (in my personal rig, or in the semi) counts against me. Three moving violations in 5 years and I'm looking for a new job. Total BS, but at 41 years old, I've learned not to take a swing at what I can't hit...

While I am a college grad and aspire to better things, I'm proud of my trade and I love what I do. Hence, I drive my old pickup slowly and it really pays off at the pump.

[Bigstevex4]

It was freaky warm here in SD other day after months driving that pickup got on the bike my slow one 98 RK done some work so runs good feels like a rocket sled.

[Oregoon]

Originally Posted by Bigstevex4

My 83 has 250 465 4 speed and really tall gears I bet that's what I have 273 better be flat ground our down hill when hit 4th I have taller tires also.

I had a Ford with a 300-six, 5 speed, and 272 gears.

I used 4th on hills, and fifth or neutral down hills. On the freeway, I got excellent fuel economy, and it town I did quite well, too. Good mixture of torque and gears.