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GMC V6 and V12 Engines Engine repair and rebuilding

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  #1  
Old March 9th, 2017, 11:55 PM
5Tractorguy 5Tractorguy is offline
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Default 351 Questions

Howdy folks, new to the forum here.

I have a couple questions regarding the 351 v6 in my '61 5000 flat bed and was hoping I can get some answers here. It's a low millage (20,XXX) ex tanker truck.

To start, I've been having trouble with the engine in terms of power. Going up a moderate incline at 35mph unloaded, I'm either to the floor or have to downshift. Doesn't seem like there is any gumption in it when you step on the pedal... no "pull" I guess...

-The carburetor has been rebuilt once and cleaned out twice.
-I removed the Handi governor deal (no difference)
-Valves have been adjusted.
-New points, condenser, coil, plug wires, distributor cap and rotor, and spark plugs.
-I also pulled the camshaft cover to look at the lobes and everything seems okay, no unusual wear.
-Timed at 10deg.
-Compression was checked and is good. All close to 120psi
-I'm in the process of doing the intake manifold gaskets now.

I've been beating my head against the wall trying to figure out this power issue (and trying not to overthink it). Part of me thinks that the carburetor isn't correct and maybe it's starving the engine for fuel resulting in loss of power.
Running the numbers from my maintenance manual they don't match, whether it makes a difference....

Code #-
Book 23-120A / Carb 23-120

Bendix #-
Book 380972 / Carb 387272

Main metering jet
Book 0.057 / Carb has 56 stamped into the jet

Also heard of a Rochester 7045114 being used on a 351 too as another option.


Any help/thoughts would be greatly appreciated.

-Jake
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  #2  
Old March 10th, 2017, 04:11 AM
bigblockv6 bigblockv6 is online now
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Default Re: 351 Questions

Is the timing set right? Rochester 2bbl carbs were used on some V6 engines in 1967 due to a shortage of Bendix WW2 carbs because Chrysler also used these carbs on there 273 and 318 V8's.
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  #3  
Old March 10th, 2017, 12:35 PM
turbobill turbobill is offline
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Default Re: 351 Questions

Quote:
Originally Posted by bigblockv6 View Post
Is the timing set right? Rochester 2bbl carbs were used on some V6 engines in 1967 due to a shortage of Bendix WW2 carbs because Chrysler also used these carbs on there 273 and 318 V8's.
That might explain the Rochester 2bbl on one of my 305 engines.
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Old March 10th, 2017, 12:47 PM
5Tractorguy 5Tractorguy is offline
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Default Re: 351 Questions

Quote:
Originally Posted by bigblockv6 View Post
Is the timing set right? Rochester 2bbl carbs were used on some V6 engines in 1967 due to a shortage of Bendix WW2 carbs because Chrysler also used these carbs on there 273 and 318 V8's.
The book has it as 5deg for the factory setting. I've played around with it some between 5 and 12 and it seems like 10 gave it the most pickup without any knocking or noise. The farther I go below that the more it pops.
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  #5  
Old March 10th, 2017, 12:53 PM
turbobill turbobill is offline
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Default Re: 351 Questions

Exhaust restricted or plugged? Fuel delivery issue (quantity)? Is the throttle opening all the way?

Those old mechanical linkages do wear. I had a '69 International Fleetstar dumptruck with a 406 RD engine that suffered from a worn throttle linkage. It only opened half way.
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  #6  
Old March 10th, 2017, 01:34 PM
5Tractorguy 5Tractorguy is offline
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Default Re: 351 Questions

Quote:
Originally Posted by turbobill View Post
Exhaust restricted or plugged? Fuel delivery issue (quantity)? Is the throttle opening all the way?

Those old mechanical linkages do wear. I had a '69 International Fleetstar dumptruck with a 406 RD engine that suffered from a worn throttle linkage. It only opened half way.
Exhaust system is good, I pulled the muffler off once and it seems to run the same with or without. I have thought about fuel delivery. I know for a fact my fuel pump is good, I change the filter too about twice a year.

Linkages are good too, seem to have very little play. Stepping down fully on the pedal puts it wide open.
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  #7  
Old March 11th, 2017, 03:50 AM
bigblockv6 bigblockv6 is online now
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Default Re: 351 Questions

Quote:
Originally Posted by turbobill View Post
That might explain the Rochester 2bbl on one of my 305 engines.
So is your 305E a 1967? Also can you get the ID numbers off the carb that way I can confirm it is on original Rochester that came with the engine, all Rochester 2bbl carbs that came with the 67 V6 engines only had manual chokes. Why GMC didn't follow the route of the rest of the GM divisions and go with the Rochester 2G carb is really a mystery, it seems they were still stuck in the 50's with the ww2 carb when both Buick and Pontiac were using those carbs and amazingly GMC still hung on to the Bendix carbs up to 1974 when no other manufacturer had been using these carbs.
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Old March 11th, 2017, 12:52 PM
turbobill turbobill is offline
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Default Re: 351 Questions

I was told it was a '65. The story I got with the engine was that it came out of a 4x4 Suburban that was shipped from Arizona to be turned into a street rod. The transmission is a pre '67 as it had no transfer case attached (divorced units were not used after the '66 model year)

The body shop that gave me the engine/trans/front driving axle had completed the project years before and got tired of the remaining parts laying around.

When our latest round of winter weather ends, I'll see if I can dig to the engine to get the carb info.
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  #9  
Old March 11th, 2017, 01:40 PM
5Tractorguy 5Tractorguy is offline
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Default Re: 351 Questions

On the Stromberg WW the main metering jets, ones you can remove, how are those numbered? I have number 56 jets, does that mean .056 or something different?

I was going to order a couple sets of larger jets and give them a whirl but wanted to double check on how they are sized.
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  #10  
Old March 11th, 2017, 09:56 PM
snazzypig snazzypig is offline
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Default Re: 351 Questions

Hi Jake, Welcome to the club. I've seen some of your videos and you really do good work!
As to your question about jet numbering, all I know is that the sizes increase with the numbers, and some people have gone to larger jets with some success. Also, I have a Bendix/Stromberg catalog that shows your carb number 23-120 is correct for your 5000 series with the 351.
Concerning the loss of power, since you say the carb has been rebuilt before, I would look there first.
The power system, which includes the vacuum power piston, may not be working properly. Make sure the gaskets above and below the throttle body aren't blocking the small opening (vacuum channel) leading to the intake manifold.
Not trying to underestimate your ability, just some things I would check if it were me.
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