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Old 03-05-2021, 10:36 PM   #1
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74 gasser bbc 366

So I have a 1974 carpenter bus. It has a gasoline big block chevy 366. This motor doesnít want to run well for me I am thinking it is a stuck intake valve because while itís running it spits and backfires up through the intake. I am perfectly confident mechanically myself so I could, but I am not excited about taking the heads off. The other thing is the distributor is stuck in the block. I tried prying with every bit of might I have. I am thinking there is a pin for gear on the distributor shaft that has somehow walked one direction or the other and now it is preventing the distributor from moving more than about a half inch or so up out of the block.

So...

I just found out that big block and small block chevys share the same Bell-housing pattern and that the cylinder bore of the 366 is actually smaller than a 350. Also that the 366 is pretty underpowered. Itís a great big full-size bus but it isnít not nice enough to justify a diesel swap. I am thinking about swapping in a good running EFI probably throttle body injected small block 350. Any objections?
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Old 03-05-2021, 10:44 PM   #2
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Location: Bay area
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Year: 1995
Coachwork: Collins
Chassis: Bantam
Engine: International T444
Rated Cap: 16?
Maybe go early 90’s TBI 454, the mounts should line up. With an RV cam and free flowing exhaust it should reasonably pull you down the road. You can get that engine, harness, and ecm, for less than 1k easily.
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Old 03-05-2021, 11:02 PM   #3
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Depending on the length and GVWR, I don't know if I would go small-block. A shorter model (5-6 window) might be okay with a 350, but anything larger and I think I'd want a big-block.

I had suggested a big-block gas swap to another member here that felt their 8.2 Detroit was dangerously slow. My old man happens to have an '83 Pace Arrow with a perfectly good 454 big-block that runs great and drove fine, but the roof has gone bad and he'd like to sell it for $1500 for parts. 65k miles. The other member seems to have decided to stick with what they have for now, presumably until they have a better budget.

It has been verified as having the TH-475 (10,000 lb GVWR) and according to research, would do the job in an appropriately-sized bus. Pickups / SUVs of this model year were rated at 230 hp / 360 ft/lb with this engine, and motorhomes got a slightly more powerful setup. Depending on GVWR, it may be perfectly fine as-is, but 275-300 hp and 400 ft/lb of torque are completely doable with a few upgrades like a larger carb or beefier cam before installation. If you're close to or in Virginia and are interested, shoot me a PM. You can hear it run before making a decision.
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Old 03-06-2021, 12:21 AM   #4
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Backfire typically indicates the timing is off. Sure a 350 chevy won't be any worse than the 366. A 383 stroker might be better as it has more torque, can upgrade any 350 rebuild it for $1k-ish.
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Old 03-06-2021, 11:00 AM   #5
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Join Date: Feb 2019
Location: New Hampshire
Posts: 1,054
Year: 1990
Coachwork: Thomas 4 window w/lift
Chassis: G30~Chevy cutaway
Engine: 5.7/350 Chevy Vortec
Rated Cap: Just me and my "stuff"?
Spark plugs will tell you a lot.
To go to more accurate stage of evaluating your rough running a compression test will indicate an internal issue, and identify the affected cylinder.
Then you can pop off the valve cover on the affected bank and after unplugging the coil electrical power, crank over the engine to potentially see any valvetrain malfunction.

You could also keep the coil powered and run the engine briefly without the valve cover, to ID any valvetrain issue. Minor oil loss will occur.
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