Need GMC Engine Help!

[5Tractorguy]

So I've been having trouble with the GMC 351 V6 in my 5000. It's been lacking power incredibly badly going up hills, bogs down without any load on the truck bed. I can mat the accelerator uphill and there is no difference.

I've done everything to the engine except for the distributor cap. I've cleaned the cap contacts and checked the spark and it is good. Points are set at .016, timing is set at 5deg, new coil, valves have been adjusted, cylinder compression is 120psi, air cleaners clean, fuel pump is good, fuel filter is new, etc...

It has the "Handy Governor" thing on there too. Removed it and set the spring by the GM book (maybe remove it all together)?

So right now I'm leaning towards either timing or too much fuel, but I'm not sure.

Any ideas??? I'm using it to haul tractors to a show this weekend so I gotta figure out something!

[DPowers]

You been to this website?
http://6066gmcguy.com/performance.html

[Irishbleueyes]

Originally Posted by 5Tractorguy

So I've been having trouble with the GMC 351 V6 in my 5000. It's been lacking power incredibly badly going up hills, bogs down without any load on the truck bed. I can mat the accelerator uphill and there is no difference.

I've done everything to the engine except for the distributor cap. I've cleaned the cap contacts and checked the spark and it is good. Points are set at .016, timing is set at 5deg, new coil, valves have been adjusted, cylinder compression is 120psi, air cleaners clean, fuel pump is good, fuel filter is new, etc...

It has the "Handy Governor" thing on there too. Removed it and set the spring by the GM book (maybe remove it all together)?

So right now I'm leaning towards either timing or too much fuel, but I'm not sure.

Any ideas??? I'm using it to haul tractors to a show this weekend so I gotta figure out something!

Its hard to conclude when all that you've checked out is supposed to be correctly functioning. However something is still clearly wrong. If it won't pull under a load it's most likely a timing issue, either with the ignition or the camshaft. Or it's drowning in fuel but I think you would quickly identify that. Get a timing light and check the timing and see if it does advance or fluctuate badly up and down with rpm change. Let us know...

[Irishbleueyes]

You do have a vented gas cap right?

[padresag]

also check for vacuum leaks around intake and if v/advance is working
ron

[TJ's Chevy]

Originally Posted by 5Tractorguy

So I've been having trouble with the GMC 351 V6 in my 5000. It's been lacking power incredibly badly going up hills, bogs down without any load on the truck bed. I can mat the accelerator uphill and there is no difference.

I've done everything to the engine except for the distributor cap. I've cleaned the cap contacts and checked the spark and it is good. Points are set at .016, timing is set at 5deg, new coil, valves have been adjusted, cylinder compression is 120psi, air cleaners clean, fuel pump is good, fuel filter is new, etc...

It has the "Handy Governor" thing on there too. Removed it and set the spring by the GM book (maybe remove it all together)?

So right now I'm leaning towards either timing or too much fuel, but I'm not sure.

Any ideas??? I'm using it to haul tractors to a show this weekend so I gotta figure out something!

My GMC lost power going up a hill one day and it ended up being the fuel system vapor locking. The fuel was boiling in the filter. Had to move the line and wrap a cold rag around the metal filter. When did it start to loose power bud? I remember watching your videos and it ran like a top. All the specs you have are dead on. If you were to listen out the window does the engine spit, misfire, pop, and sputter out the exhaust under that load? If it Does your looking at either ignition or vapor lock. That 351 has plenty of torque to carry that beast up a hill no problem. And if you subscribe to the 6066 GMC site, Pete who is a 40 years expert on these could give you some great advice for sure. I've been there since I got my GMC. But I would start with fuel and ignition. Like mentioned, take a light and see if the engine is advancing and retarding. You might even try advancing the initial to 8 degrees and See if it helps. These engines don't like a ton of advance so don't believe those people who are gonna say bump it up to 15 initial. I had my 305 at 12 and that was way to much. 10 was the max for me.

[5Tractorguy]

Originally Posted by DPowers

You been to this website?
http://6066gmcguy.com/performance.html

Yep, might have to give it another read through...

Originally Posted by Irishbleueyes

Its hard to conclude when all that you've checked out is supposed to be correctly functioning. However something is still clearly wrong. If it won't pull under a load it's most likely a timing issue, either with the ignition or the camshaft. Or it's drowning in fuel but I think you would quickly identify that. Get a timing light and check the timing and see if it does advance or fluctuate badly up and down with rpm change. Let us know...

I'm going to give the timing another shot. I don't think the camshaft is out of time (though is has crossed my mind). It was a fire truck in it's previous life so it has low miles but who knows what for engine hours.

Originally Posted by Irishbleueyes

You do have a vented gas cap right?

I believe I do but I'm definitely going to check and make sure it's venting!

Originally Posted by padresag

also check for vacuum leaks around intake and if v/advance is working
ron

Checked! Everything is tight!

Originally Posted by TJ's Chevy

My GMC lost power going up a hill one day and it ended up being the fuel system vapor locking. The fuel was boiling in the filter. Had to move the line and wrap a cold rag around the metal filter. When did it start to loose power bud? I remember watching your videos and it ran like a top. All the specs you have are dead on. If you were to listen out the window does the engine spit, misfire, pop, and sputter out the exhaust under that load? If it Does your looking at either ignition or vapor lock. That 351 has plenty of torque to carry that beast up a hill no problem. And if you subscribe to the 6066 GMC site, Pete who is a 40 years expert on these could give you some great advice for sure. I've been there since I got my GMC. But I would start with fuel and ignition. Like mentioned, take a light and see if the engine is advancing and retarding. You might even try advancing the initial to 8 degrees and See if it helps. These engines don't like a ton of advance so don't believe those people who are gonna say bump it up to 15 initial. I had my 305 at 12 and that was way to much. 10 was the max for me.

I'm really going to have to check the fuel system. I have a feeling that old gas cap might be plugged.
Loose power?? Since the day I first drove it across town last year. I was real boggy and I figured with all the work I've done to it it would go away.
It's gotten a lot better since then but the engine still face plants on a hill. It does run nice aside from that, if I listen under a load the exhaust is even toned, no funny noises or sputtering. Down hill it backfires and spits.
Although on occasion coming up a hill it will skip a few beats, like someone shut the ignition off for a second then turned it back on. So maybe it's lacking fuel?

I ordered a new distributor cap this AM, going to pick it up tomorrow morning and see what happens. And adjust that timing some.

I'll let you guys know my findings....

[TJ's Chevy]

Originally Posted by 5Tractorguy

Yep, might have to give it another read through...



I'm going to give the timing another shot. I don't think the camshaft is out of time (though is has crossed my mind). It was a fire truck in it's previous life so it has low miles but who knows what for engine hours.



I believe I do but I'm definitely going to check and make sure it's venting!



Checked! Everything is tight!



I'm really going to have to check the fuel system. I have a feeling that old gas cap might be plugged.
Loose power?? Since the day I first drove it across town last year. I was real boggy and I figured with all the work I've done to it it would go away.
It's gotten a lot better since then but the engine still face plants on a hill. It does run nice aside from that, if I listen under a load the exhaust is even toned, no funny noises or sputtering. Down hill it backfires and spits.
Although on occasion coming up a hill it will skip a few beats, like someone shut the ignition off for a second then turned it back on. So maybe it's lacking fuel?

I ordered a new distributor cap this AM, going to pick it up tomorrow morning and see what happens. And adjust that timing some.

I'll let you guys know my findings....

Backfiring down hill is usually just excess fuel exploding that the engine is not burning. Unless you have some exhaust valves to tight. When I converted to the MSD box and Pertronix it doesn't back fire at all...and if it does you can Barely tell. If it misses a few beats under load I would say that's ignition or a lazy valve spring. Did you install new points? If you were to look down the throat of the carb and pump the gas does it give a nice shot of fuel down the carb? Might be time for a 4 barrel mod buddy!

[5Tractorguy]

Originally Posted by TJ's Chevy

Backfiring down hill is usually just excess fuel exploding that the engine is not burning. Unless you have some exhaust valves to tight. When I converted to the MSD box and Pertronix it doesn't back fire at all...and if it does you can Barely tell. If it misses a few beats under load I would say that's ignition or a lazy valve spring. Did you install new points? If you were to look down the throat of the carb and pump the gas does it give a nice shot of fuel down the carb? Might be time for a 4 barrel mod buddy!

Yep, did the points/cond too. Oh it hogs the gas out pretty good when you pump it! Yeah, I can see it now..... hot rod tractor hauler!!

[padresag]

the bolts at the intake to the heads can be tight but that doesn't mean that they can't leak. just spray a little wd40 around the base of the manifold and see or hear what happens
ron

[TJ's Chevy]

Originally Posted by 5Tractorguy

Yep, did the points/cond too. Oh it hogs the gas out pretty good when you pump it! Yeah, I can see it now..... hot rod tractor hauler!!

lol! I say Turbo time then! Have you pulled a plug to see if that will tell you something? Even with the 4 barrel mine has been burning very clean. Even had started burning a little carbon off the piston. But now they are shiny due to the major service. haha

[5Tractorguy]

Checked for manifold and vacuum leaks with carb cleaner, everything is tight. Pulled all the plugs this morning and did another compression test. All cylinders are between 115 and 125psi. Reset points to .020 according to my GM Shop Manual. Lubed up the mechanical advance a bit and installed the new distributor cap.

Adjusted the timing to about 12deg and he woke right up! I tried around 8 and I still had a dead spot when I revved it. At 12 the dead spot is mostly gone and I have way more power on the hills. Might try to bring it up a hair more..... Maybe someone removed the distributor at one point and reinstalled it a tooth off... who knows.

Here's some shots for today. Getting everything loaded up