1971 C-10 knocking sound and not reving past 3000 RPM
 

I have a 1971 C-10 pickup and have recently found that the truck will not rev over 3000 RPM and when it gets to around 2800 RPM, an popping sound is made from under the hood. The motor is a 350 SBC with an Edelbrock 1406 carb and Edelbrock intake. Also the distributor has no points on it. Not quite sure what that means. Other than that the motor is stock, that I know of. The tranny in the truck is a TH400 which seems to be working fine. I have read that there can be many things causing the problems I have described with the 3 major ones being timing, spark plug gap, and compression. Right now I have the timing set at 12 degrees, a plug gap of .042 and the compression was between 9.1 to 8.5. I am not sure if that is too big of a difference. Also it appears that my motor is running way rich which I plan to fix going down the road as I don't have lots of money to spend. Maybe that has something to do with it? I don't know. Any help would be much appreciated because I am 15 and this is the first vehicle I have had to work on like this. Thanks.

Re: 1971 C-10 knocking sound and not reving past 3000 RPM
 

Welcome to the forum. There must be a couple of old gearheads in your area that would be willing to bring some tools over and give you a hand. You will get a bunch of ideas on what to check pretty soon. Do post a pic of your engine, air cleaner off, lots of good eyes out here.

Re: 1971 C-10 knocking sound and not reving past 3000 RPM
 

Great, Thank you. I will post some pictures of the motor as soon as I can

Re: 1971 C-10 knocking sound and not reving past 3000 RPM
 

Sounds like your timing chain and gears could be worn out and it jumped a few teeth. Had it happen myself a couple times and had the same symptoms.

Re: 1971 C-10 knocking sound and not reving past 3000 RPM
 

Do I need to replace the timing chain and gears in order to fix it, or is there another way to solve the problem? Thanks

Re: 1971 C-10 knocking sound and not reving past 3000 RPM
 

Is this a recent issue or did you buy the truck this way? I think more details are needed before you go change a timing chain. It would be good to do a compression test on each cylinder. There is a tool you can borrow/rent if you don't have one. That would help confirm a timing chain and general condition of the engine. Is the engine recently rebuilt or original or what? Need a bigger picture of how you got to here.

You need to make sure of the problem first, you need to get number one cylinder to top dead center on the compression stroke then check the rotor to see if it's pointed to the wire on the cap that goes to number one. If it's not then chances are your gears have slipped. Unless you've pulled the distributor recently and got it back in wrong or it's been turned. If the distributor has only been turned the rotor should still be pointed to where number one would normally be. To change the chain and gears you'll have to pull the water pump and timing chain cover, it's fairly simple if you take your time and should cost you less than $100.
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Re: 1971 C-10 knocking sound and not reving past 3000 RPM
 

I ran a compression test on the motor last night. It was ranging from between 8.5 to 1 and 9.1 to 1 compression. I am fairly sure the motor came this way. I have only had the truck for about 6 months and have been slowly fixing problems on the truck. My first priority was putting in all new rotors, ball joints, etc and I have just now (past couple weeks) moved on to the drivetrain issues. The people that I bought the truck from said the truck's motor was rebuilt about 80,000 miles ago. I cannot recall how long ago this was done. I have little confidence in the mechanical state of the motor being ok as there have been several things on the motor that make it look like the motor was just thrown together, wiring is a mess, vacuum hoses were on incorrectly on the carb, kickdown switch was not working correctly (is fixed now.) I also do not know if the motor is the original since it has a TH 400. I read that the only way to get a TH 400 in my truck was to have 454 Big Block in my truck (don't know if that is accurate) which leads me to believe either the motor is not the original or the tranny is not the original.

Re: 1971 C-10 knocking sound and not reving past 3000 RPM
 

Ok, thanks for telling me how to do that. Do I just pull the cap off of the distributer to see where the rotor is pointing? Trying to learn as much as I can. All of this help is very much appreciated

It's more involved than that. I would look up some YouTube videos on timing, it will help you understand how it works and help diagnosing the issue. As said above don't just assume it's the timing chain, it could be as simple as the distributor being loose and turning. The reason I mentioned timing is because you said you hear a popping sound and it won't rev to 3000rpm. Look up some videos on how timing works, it will help you understand. If you hear an actual knocking sound I would have someone look at it before you run the engine again.
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Re: 1971 C-10 knocking sound and not reving past 3000 RPM
 

I am pretty sure the distributor because I had done a fair amount of research on what the cause of the popping sound could be before I posted any sort of questions on the forum if it had already been answered. I read that the timing could be too advanced or too retarted (I can't remember which.) I read that a good starting point would be 12-16 degrees advanced. Before I messed with any of the timing on the motor I made sure to figure out what my original timing was, 9.5 degrees advanced for the initial timing. I then adjusted the initial timing (with vacuum advance disconnected) to 14 degrees advanced and drove the truck again to see how it performed. In the driveway (not moving, in park) the truck had no trouble revving past 3000 RPM, with no audible popping coming from the truck, however went I went to drive it, the popping sound came back so I adjusted the timing to 12 degrees advanced and tried it again, same problem as before. I made certain to tighten the hold down bolt on the distributor enough so that it would not move when I tried to turn it, had my dad use the timing like to make sure the timing didn't change while I was tighting it down. Thanks for the help and I will be sure to look up some videos on how the distributor works. I'll be sure to let you know what I find with how the rotor is lining up with the number 1 cylinder and I'll go from there. Thanks again

Re: 1971 C-10 knocking sound and not reving past 3000 RPM
 

Sorry for the typo at the beginning. I meant to say that I am pretty sure the distributor is not loose

Re: 1971 C-10 knocking sound and not reving past 3000 RPM
 

It could be an exhaust valve causing the "popping" noise. Is the popping coming up the carb. What do you mean when you say compression test comes up 8.5 through 9.5 to 1. All gauges I own are in PSI not compression ratio.

Re: 1971 C-10 knocking sound and not reving past 3000 RPM
 

My apologies. I was getting between 8.5 to 9.1 Kg/Cm^2. I think that is around 125 PSI. I assumed that was the compression ratio, but maybe that was an incorrect assumption. And yes, it sounds like the popping sound is coming up the carb.

Re: 1971 C-10 knocking sound and not reving past 3000 RPM
 

When you said you did a compression test it was 8.5 or 9.5, I think there is a misunderstanding or miscommunication here. To do a compression check you need a compression gauge and then you take out a spark plug and put the compression tester in the spark plug hole.Wit h the coil unhooked, you then crank the engine over with the starter for a couple of seconds. the gauge will then read compression in a number form like 125 psi. All of the numbers should be close to each other say6 the first was 125 then the rest should be between 100 and 140ish. You see if you have one that is real low say 60 or less it may not even be firing when its time too. The low compression will tell you if you have problems such as burned piston or broken piston stuck valves or other problems that could arise. Get a gauge and do the compression test then post up the results for all 8 cylinders then maybe we can better answer your question. Jim

Re: 1971 C-10 knocking sound and not reving past 3000 RPM
 

Steady popping up the carb usually means something in the valve train. Exhaust/compression needing a way out. Common causes are bent push rod, bad rocker arm, bent valve or flat cam lobe.

Re: 1971 C-10 knocking sound and not reving past 3000 RPM
 

Sorry for the miscommunication, I have run the compression test as you have described, removing 1 spark plug, putting the compression tester in the spark plug hole, cranked the motor over 5-6 times, then put the spark plug back in and repeated for the remainder of the cylinders. I will run the test again and record all the results in PSI to share as soon as I am able. Thanks for all the help.

Re: 1971 C-10 knocking sound and not reving past 3000 RPM
 

A wiped exhaust lobe will also cause it to pop out of the carb. If you pull the valve covers, you can watch the rockers compress the springs, while letting the engine idle. If any of them barely move compared to the others, bad cam lobe.

Re: 1971 C-10 knocking sound and not reving past 3000 RPM
 

I am guessing the cam may be going flat.

Re: 1971 C-10 knocking sound and not reving past 3000 RPM
 

heres an old mechanics trick my grandpa taught me about timing. drive the truck till about 30-45 MPH (just after it switches to gears) coast for a short distance and then accelerate quickly about 5 mph giving the engine load. if you hear pre ignition knock (sounds kind of like marbles in the engine) retard the timing a little bit and the entry again. if you don't hear a knock advance it little bit. retest till you hear the knock and the back it off just slightly. that will set your low end timing but for the high rpm advance you need to play with the springs and weights underneath the distributer cap. heavier weight/ lighter spring = timing advanced quicker. lighter weigh/ heavier spring = timing advance later on.

Re: 1971 C-10 knocking sound and not reving past 3000 RPM
 

I hope to be home soon, then I will start on all the suggestions on figuring out what the problem is. Thanks for all the help. I had no idea where to start and now I have too many things to do and not enough time, which is definitely a good thing. Hope to test compression tonight and if pressure seems to be ok, I will move on to the distributor location, timing chain, and then watch the valves move to see if the cam is going out. Will update as much as I can

Re: 1971 C-10 knocking sound and not reving past 3000 RPM
 

I just ran a compression test on the motor, here are the results.

#1 130 PSI
#3 130 PSI
#5 128 PSI
#7 140 PSI
#2 120 PSI
#4 130 PSI
#6 120 PSI
#8 120 PSI

Attached are some basic pictures of the motor. Also on the #8 Cylinder I noticed an unusual amount of black hard particles on the spark plug. I do not know what that is and have attached a photo of it. Thanks

Re: 1971 C-10 knocking sound and not reving past 3000 RPM
 

Just took off the driver side valve covers and let the engine idle while watching the rockers. It looks like the 4th valve isn't moving nearly as much as the others. I tried to attach a video but for some reason it will not upload. I have posted it to youtube. https://youtu.be/zLxG61Eh5jI

Re: 1971 C-10 knocking sound and not reving past 3000 RPM
 

If that's not a collapsed lifter, it's probably a flat cam lobe.

Re: 1971 C-10 knocking sound and not reving past 3000 RPM
 

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