GM 1970 GMC K2500 Engine Operation Issues (1 Viewer)

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Nov 9, 2012
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Location
Olathe, KS, USA
We have a 1970 GMC 3/4T 4x4 350/4 sp /4WD (SM465 / NP205) with front lockout hubs and we are having engine operational issues (as well as driveline issues) but I want to address the engine stuff in THIS thread.

This truck lived its life as a construction truck and apparently had a flatbed on it for a large portion of the time, as it has weld spatter all over the roof and back window.

We have been working on this thing, getting it back into driving condition and have done the following:
NEW GM Crate 350 to match the stock factory 350
Rebuilt the SM465 4 speed (granny low transmission)
Rebuilt the transfer case (New Process NP 205)
Rebuilt the rear axle with new gears (4.11:1) and a new Detroit Locker, Dana 60
Rebuilt the front axle with new seals, Dana 44
Rebuilt front and rear driveshafts with all new yokes and U-Joints. (Inland Truck Parts)
Installed a Skyjacker 4" lift all-spring lift.

Engine:
GM Crate P/N 12681429
Gen 1 5.7L crate engines are excellent entry-level performance engines for older cars, trucks, and street rods. All of the parts in these engines are brand new. These Chevrolet Performance Goodwrench 5.7L/350 c.i.d. engines have approximately 8.5:1 compression with a horsepower rating of 249 hp and 304 ft.-lbs. of torque. These crate engines have a 4.00 in. bore and a 3.48 in. stroke with 4-bolt mains and 2-piece rear main seals.

Camshaft specifications are .390 in. intake lift and .410 in. exhaust lift. The open duration is 195 degrees intake and 202 degrees exhaust at .050 in. with a 112 degree lobe separation. The cylinder heads are cast iron with 1.94 in. intake valves and 1.50 in. exhaust valves, 76cc combustion chambers, and 7-bolt style exhaust flanges. These engines include painted valve covers, oil pans, and timing covers. They are drilled for both left hand and right hand dipsticks.

We have installed the following on this engine. All parts are NEW.
Carburetor: Holley 650 CFM Double Pumper, P/N 0-4777S, 4V, mechanical secondaries, manual choke
Fuel Pump: Holley Red, electric pump, 7 PSI, P/N 12-801-1
Intake Manifold: Weiand 8120WND 180° Dual-Plane aluminum
Distributor: MSD Street Fire HEI P/N 8362
Plugs: AC Delco R44T, gapped at 0.035"
Wires: MSD 31409 8.5 MMSuper Conductor
Stock Fan w/ thermal clutch
Stock water pump (aftermarket)
Stock PS pump (aftermarket)
Stock type alternator (aftermarket)
Exhaust Manifolds: Cast Iron Rams Horn (Dorman P/N 674-501)
Exhaust: Dual 2-1/4" with Flowmaster 42441 mufflers
Timing is set at 14° BTDC (I do not know total timing)

The problem:
Every time you accelerate, the engine coughs, then will finally recover and go. Only under VERY light acceleration will it NOT cough.
It is a very cold-blooded engine. It does this even after it has reached the 190° operating temp.

We have been chasing this since day one (right about a year now) and have been very frustrated. I'll bet we haven't even put 5000 miles on this in a year because it is such a pain to drive.

We paid a shop to go through and tune the carburetor and replace the squirters with larger squirters and to narrow the jets slightly from what the carb came with from the factory. My son did not document what we started with or where we are now, so I don't know that answer.

We just replaced the intake manifold gaskets with a set from Cometic and it seems to have improved the operation SLIGHTLY.

The timing is set based on where it feels the best (14° BTDC), as there are NO specs that came with the engine on what they SHOULD be set to. The specs for a 1978 Chevy 350 K20 state that it should be at 8°-12° BTDC, but it just does NOT like it there.

To me the cough FEELS like a timing retarded too much type response.

The accelerator pump on the carb has been changed twice with no difference.

Once it "catches it's air" it screams very well and the 4V really wakes up.

But at 2500 RPM, cruising in 3rd gear, it feels like a steady miss and its response when you step on it, is to cough, then go.

I have had FANTASTIC results with this exact setup in my early years and that is why we have followed it.
I have always liked the Holley products, but I'm about ready to throw this carburetor in the trash.

A little extra crap to confuse the issue:
We started out with a cheap "stock" HEI distributor from O'Reilly's.
We replaced that TWICE because it became evident that was part of the problem. Then we went to the MSD distributor and have replaced that ONCE.
We replaced the spark plugs once because we went from Champion to AC Delco.
We replaced the plug wires from the house brand from O'Reilly's TWICE, then went to the MSD wires and that helped.
We discovered many bare wires in the wiring harness, so we replaced the wiring harness under the hood, both the engine harness and the headlight harness.
We have done the HEI Ignition wire change to eliminate the resistance wire for the HEI.
We have gone to an internally regulated alternator (two wire) in place of the original one-wire with an external regulator.

The only part of the electrical system that has NOT been replaced is the in-cab harness and the actual ignition switch.

Any idea on what / where we should be focusing our time and energy?
Vacuum leaks?
Ignition?
Carburetion?
Fuel Pressure?
Valve train?
Camshaft issues?

Just looking for ideas and reasoning behind it. We are frustrated, and since I don't drive it every day, I don't get to feel all the nuances.

I appreciate any and all help!
 
When you broke the engine in what type of oil did you use?
 
When you broke the engine in what type of oil did you use?


Std. Quaker State Dino Oil 5W30 per Mfr recommendations.
 
I would check the camshaft lobes then. There have been reports of hydraulic flat tappet being wiped put due to not enough zinc in the oil when doing a start up. A rule of thumb is to add a zinc additive if you have a new camshaft that hasn't been broken in and then add half a bottle every oil change.
 
I would check the camshaft lobes then. There have been reports of hydraulic flat tappet being wiped put due to not enough zinc in the oil when doing a start up. A rule of thumb is to add a zinc additive if you have a new camshaft that hasn't been broken in and then add half a bottle every oil change.

That was actually where my thoughts were going based on a past life experience. I was curious if anyone else here would think that.

We replaced the intake manifold with Cometic gaskets last weekend, and he has said it's running "So much better now"......

We'll keep checking and fiddling. If it IS the flat lobe on the cam, then it COULD be a warranty issue, because it is a BRAND NEW engine. It has less than 5000 miles on it.
 
I prefer to run Rislone nano prime in my 460 which has a hydraulic flat tappet. I hope it isn't a flat lobe. The only other thing I'd suggest is take the valve covers off and pay attention to how much each valve is opening and make sure they look the same. It may not be scientific but it's easier than pulling the intake to look at lifters.
 
I prefer to run Rislone nano prime in my 460 which has a hydraulic flat tappet. I hope it isn't a flat lobe. The only other thing I'd suggest is take the valve covers off and pay attention to how much each valve is opening and make sure they look the same. It may not be scientific but it's easier than pulling the intake to look at lifters.


The older engines (1968-1972 ish) has a casting problem on the SBC where the casting hole in the rear "wall" of the block meets the intake manifold, right next to the oil pressure sending unit, there was a regular flaw that the oil galley that feeds the lifter area for exhaust lobe #7, has a casting flaw that allows the oil pressure to blow through the oil galley and out the casting hole in the block and down the back of the block, between the intake manifold and the block.

I have had one of these engines in my old truck and I had to drill and tap the casting hole and threaded in an Allen screw with thread tape to seal it and stop it from leaking. It took me a heck of a while to find the problem. I finally found it with the intake manifold off, and operating the oil pump with an adapter and drill.

I have since heard of (3) more engines with this failure.
I had received the engine for free, then threw a ton of machining at it (bored, line bored, ported, polished, balanced) and then found this issue.

I still ended up with a #7 exhaust lobe going flat at 75K mile on that engine. That then led to a whole host of OTHER machine-shop related moronic issues and hydraulic-ing a cylinder TWICE, junking a set of ported and polished and matched heads, which eventually ended up in losing about 75 HP and selling the truck.
 

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