400SBC/300A choking questions
Posted: Sun Jul 03, 2016 3:04 am
Hi all,
I stopped in here a few years ago with questions about convertor mounting, and five years later I’ve actually got the truck going! Well on petrol anyway.
Brief history, I’ve got a 1972 GMC K2500 (3/4ton 4X4) that I’ve rebuilt over the last (many) years. Stock 350 was replaced (before I got the truck) with a 400 from a mid 70’s truck with an age appropriate Quadrajet on it. The motor sat for many years, and the only work I did on it was external. As far as I know the motor is basically stock, miles unknown, 330817 block, 462624 heads (second worst Chevy ever made?!) with a Performer Air Gap manifold, and Hedman full length headers.
I completely rebuilt the Quadrajet, and have worked through a few issues with it but it is running pretty good. From the get go I’ve run the 300A and 10” filter on it figuring I’ve got to tune the carb with the propane deal on anyway. I’ve done several thousand k’s and wanted to get the last bugs out of the carb, before running lines for the propane and getting that sorted.
I have installed an Innovate o2 sensor and have been keeping an eye on that. It cruises (100kph/2800rpm) almost spot on in the mid/high 14’s. The issue I had to deal with is that under moderate to heavy acceleration (2800rpm - 3300rpm) o2 dropped to low 12’s then fell to 10 and the motor actually started ‘stuttering’. I took the 300A off and gave the carb a couple of tweaks and took the truck for a run with my daughter in tow to watch the o2 gauge while I stayed between the fences! Funny thing was the drive was to see if my tweaks had done anything, but the revelation was it was cured! Cured by removing the mixer…… Put it back on and my ‘carb’ problems returned.
So yeah. I guess my question is along the lines of, if I can’t get enough air in when running petrol, it’s not going to be any better on gas? If that is a correct statement, then where to from here? I want to retain dual fuel. My books say a 425 is only flows 6% more than a 50&70. In real life do you think a 425 would be adequate? If no, what are some practical options? I have just finished rebuilding a VFF30, and Model L to run this deal. I've had 'experts' tell me I should be good to 5000rpm with this set up, but real world driving says otherwise. Just checked the Impco book, and it says 400CID doing 4000rpm equals 392cfm, so both 300A and 425 'should' work? Am I missing anything really obvious?
Sorry about the long post, but hard to convey without the details.
I stopped in here a few years ago with questions about convertor mounting, and five years later I’ve actually got the truck going! Well on petrol anyway.
Brief history, I’ve got a 1972 GMC K2500 (3/4ton 4X4) that I’ve rebuilt over the last (many) years. Stock 350 was replaced (before I got the truck) with a 400 from a mid 70’s truck with an age appropriate Quadrajet on it. The motor sat for many years, and the only work I did on it was external. As far as I know the motor is basically stock, miles unknown, 330817 block, 462624 heads (second worst Chevy ever made?!) with a Performer Air Gap manifold, and Hedman full length headers.
I completely rebuilt the Quadrajet, and have worked through a few issues with it but it is running pretty good. From the get go I’ve run the 300A and 10” filter on it figuring I’ve got to tune the carb with the propane deal on anyway. I’ve done several thousand k’s and wanted to get the last bugs out of the carb, before running lines for the propane and getting that sorted.
I have installed an Innovate o2 sensor and have been keeping an eye on that. It cruises (100kph/2800rpm) almost spot on in the mid/high 14’s. The issue I had to deal with is that under moderate to heavy acceleration (2800rpm - 3300rpm) o2 dropped to low 12’s then fell to 10 and the motor actually started ‘stuttering’. I took the 300A off and gave the carb a couple of tweaks and took the truck for a run with my daughter in tow to watch the o2 gauge while I stayed between the fences! Funny thing was the drive was to see if my tweaks had done anything, but the revelation was it was cured! Cured by removing the mixer…… Put it back on and my ‘carb’ problems returned.
So yeah. I guess my question is along the lines of, if I can’t get enough air in when running petrol, it’s not going to be any better on gas? If that is a correct statement, then where to from here? I want to retain dual fuel. My books say a 425 is only flows 6% more than a 50&70. In real life do you think a 425 would be adequate? If no, what are some practical options? I have just finished rebuilding a VFF30, and Model L to run this deal. I've had 'experts' tell me I should be good to 5000rpm with this set up, but real world driving says otherwise. Just checked the Impco book, and it says 400CID doing 4000rpm equals 392cfm, so both 300A and 425 'should' work? Am I missing anything really obvious?
Sorry about the long post, but hard to convey without the details.