Missing at freeway speeds

[CG]

My trucks (71K20 350 4spd) basically been sitting since August. Moved it from front yard to back last time I drove it. I mostly just use it as a truck, hauling junk, towing more junk lol. But ever since I got it a couple years ago there is a miss at freeway speeds. Nothing terrible but it sure makes me nervous so I was never far from home, and only took back roads.

I checked everything I could think of, even asked here on the board. Well back before I moved it to the back yard my son and I were sitting there staring at the engine with the air cleaner off. He noticed one of the vac plugs on the carb looked funny. He went to pull it off and the tip pulled loose. We found another larger one the same way. So I replaced them. Never bothered to test drive the truck (stuff came up right after that).

This weekend I got a wild hair and decided to take the truck for a spin (literally, it was stuck in the grass) 4 wheel drive took care of that. I drove all over. Up to Sultan, back to Everett and back home to Monroe. Zero problems at any speed. That has never happened since Ive owned it.

So I guess what I'm saying here is check your old vacuum plugs on the carb. They can cause weird stumbling problems at higher speed if they are cracked a little.

Next I need to figure out what that horrible clunking sound is when Im putting around (sounds like a drive shaft clunking). Im the second owner and the original owner told me he had the truck back at the dealers many times trying to figure out what the noise was and its never been figured out after almost 40 years on the road. Maybe it will go away when I switch it over to automatic?

Gotta love these old rigs =)



Mark

[LostMy65]

Thanks for the reminder.

[breeh]

Originally Posted by CG

Next I need to figure out what that horrible clunking sound is when Im putting around (sounds like a drive shaft clunking). Im the second owner and the original owner told me he had the truck back at the dealers many times trying to figure out what the noise was and its never been figured out after almost 40 years on the road.
Mark

Does it have a No Spin (Detroit locker) rearend in it? Those things clunk pretty good. The first one that I had I thought that something was wrong with it.

[Classic Heartbeat]

I agree with breeh. I had a possi in one of my past big block burbs and it use to clunk around corners. It could be universal joints to, but I would emagine you already checked that. WES

[CG]

Yeah it has a no spin detroit and that makes a different sound, sometimes much scarier when it bangs lol.

[Classic Bowties]

Originally Posted by CG

Next I need to figure out what that horrible clunking sound is when Im putting around (sounds like a drive shaft clunking). Im the second owner and the original owner told me he had the truck back at the dealers many times trying to figure out what the noise was and its never been figured out after almost 40 years on the road. Maybe it will go away when I switch it over to automatic?

Gotta love these old rigs =)



Mark

Hi Mark, I see you mention your K20 as being a 4 speed, I would check the splined coupler that joins the SM465 output shaft to the NP205 input shaft. I have had a few 4'wds over the years that had this steel coupler get worn out, all were 4 speeds. I think the auto TH350 to NP205 coupler is a longer lasting & better designed part due to it having fine splines joining trans to transfer. Good luck, Mel

[CG]

Originally Posted by Classic Bowties

Hi Mark, I see you mention your K20 as being a 4 speed, I would check the splined coupler that joins the SM465 output shaft to the NP205 input shaft. I have had a few 4'wds over the years that had this steel coupler get worn out, all were 4 speeds. I think the auto TH350 to NP205 coupler is a longer lasting & better designed part due to it having fine splines joining trans to transfer. Good luck, Mel

You could be right. The original owner told me though he had this problem from year one when he bought it new. But after all this time he could have his clunking problems mixed up.

I do have all the 4X4 turbo 350 parts for it now. Hopefully one of these days I will get motivated to do the swap.

[Classic Heartbeat]

I have a 205 / T-400 sitting here... WES

[CG]

Originally Posted by Classic Heartbeat

I have a 205 / T-400 sitting here... WES

Wes you know that I would love to have that set up. If money wasnt so tight I would be all over it. Need anything in trade? How bout a green suburban full length qtr =)

Just hang on to it for me till the economy turns around (wishful thinking)

[factorystock]

Originally Posted by CG

Yeah it has a no spin detroit and that makes a different sound, sometimes much scarier when it bangs lol.

No Spin 3/4 ton rear ends (round pumpkin style) are notorious for banging noises on sharp left turns.

[CG]

Originally Posted by factorystock

No Spin 3/4 ton rear ends (round pumpkin style) are notorious for banging noises on sharp left turns.

I had no experience with this style rear end when I got this truck. Original owner I got it from told me it had a detroit locker rear and they can be a little noisy.

I thought the whole rear end blew up first time it made that bang in a turn, scared the crap outta me. I was shocked when I was just able to keep driving and pull over to take a cautious peek under the truck

[Classic Heartbeat]

Wes you know that I would love to have that set up. If money wasnt so tight I would be all over it. Need anything in trade?

I would consider that 59/60 Impala dash hanging on your wall as a partial trade... WES

[CG]

Originally Posted by Classic Heartbeat

I would consider that 59/60 Impala dash hanging on your wall as a partial trade... WES

Works for me ... wonder what else I can dig up for the guy that has every truck part imaginable?