Carb upgrade for 250 I6 in a C50

[mcarlo86]

We have a C50 truck that has a flat bed on it that we use to haul scrap, dirt, hay, etc. Originally equipped with a 292, my Dad replaced it when it was in need of an overhaul (burning oil/blow by) with a 250 he had found out of a C10 that had recently been overhauled. He got the 250 for $200, which was a lot cheaper than trying to overhaul the 292 at the time. He used the intake, exhaust manifold, and carb off the 292. The truck is really geared low, so the 250 seems to handle the higher rpm's a bit better than the 292 did and we haven't seen really any noticeable power difference since the swap. I was thinking about upgrading to a newer/bigger carburetor and was wondering if anyone had experience with a 2 bbl or maybe even a small (500 cfm) 4 bbl on a 250? I was looking at Clifford Performance's website and they have cool stuff on there, but kind of pricey. I was hoping to find an adaptor to put a 2 or 4 barrel on the current intake if possible?

 

[Ricko1966]

Here's the problem with using an adapter to put a 2bbl on a 1 bbl manifold the 1bbl manifold has a throttle opening about the same size as the 1bbl butterflies so your manifold becomes a restriction.Also adapting a 1bbl to a 2 bbl manifold your basically going to have an open plenum below the 2 bbl so your going to lose low end, and probably will be stupid rich at idle.The cam in the 250 doesn't need anymore air and fuel than your 1bbl can provide anyway.I would leave it alone or blow through turbo the 1bbl.But dollars and sense choice is leave it alone.

 

[1987 GMC Jimmy]

Here's the problem with using an adapter to put a 2bbl on a 1 bbl manifold the 1bbl manifold has a throttle opening about the same size as the 1bbl butterflies so your manifold becomes a restriction.Also adapting a 1bbl to a 2 bbl manifold your basically going to have an open plenum below the 2 bbl so your going to lose low end, and probably will be stupid rich at idle.The cam in the 250 doesn't need anymore air and fuel than your 1bbl can provide anyway.I would leave it alone or blow through turbo the 1bbl.But dollars and sense choice is leave it alone.

This.

 

[devin bianco]

Maybe instead of messing with intake you could try more performance oriented headers? That might be an easy way to add some hp

just a thought I would wait for somebody to confirm that’s a good idea before you take my word lol

 

[AuroraGirl]

800 cfm 4barrel quadrajet.

dont ask how, i havent gotten that far.

 

[Shorty81]

Here's the problem with using an adapter to put a 2bbl on a 1 bbl manifold the 1bbl manifold has a throttle opening about the same size as the 1bbl butterflies so your manifold becomes a restriction.Also adapting a 1bbl to a 2 bbl manifold your basically going to have an open plenum below the 2 bbl so your going to lose low end, and probably will be stupid rich at idle.The cam in the 250 doesn't need anymore air and fuel than your 1bbl can provide anyway.I would leave it alone or blow through turbo the 1bbl.But dollars and sense choice is leave it alone.

X2

 

[mcarlo86]

Thanks for the input. The carb needs to be rebuilt, so was debating on fixing it or replacing it. Sounds like I would be best off just going through it and rebuilding it.

 

[mrburitto]

The exhaust, intake, and heads are all restrictions on the 250. Upgrading one doesnt do a whole lot. You really need to upgrade the exhaust to a true dual setup before you will see any gains. But if you want to go to a 2bbl, do the Holley 390. You can buy one that is tuned specifically for the 250 from Tom at 12bolt.com

Tom is the authority on i6 250s. he has done it all, and documented most of it. you can find everything you need on his site.

 

[DoubleDingo]

Thanks for the input. The carb needs to be rebuilt, so was debating on fixing it or replacing it. Sounds like I would be best off just going through it and rebuilding it.

^This^

 

[DoubleDingo]

The exhaust, intake, and heads are all restrictions on the 250. Upgrading one doesnt do a whole lot. You really need to upgrade the exhaust to a true dual setup before you will see any gains. But if you want to go to a 2bbl, do the Holley 390. You can buy one that is tuned specifically for the 250 from Tom at 12bolt.com

Tom is the authority on i6 250s. he has done it all, and documented most of it. you can find everything you need on his site.

Tom's Holley is a three barrel. It runs on one barrel for low speed, then when higher rpm power is needed the secondaries kick in.

 

[DoubleDingo]

Your 1BBL is already a little oversized for a 250, but you've been running it without issue, so just rebuild it. Really the only way to make these engines breathe better is to do the lump kit with bigger valves, dual exhaust, and a cam. Or turbo.

 

[MikeB]

Don't forget to optimize ignition timing and limit exhaust restrictions, such as a muffler that necks down from, let's say, a 2-1/4" pipe to 1.75" internally. According to David Vizard, a good bang for the buck is a performance muffler that's one size bigger than the exhaust pipe, using adapters on both ends. And try to use OEM-type mandrel-bent pipes, instead of muffler shop "crush" bends.