Engine Knock/Power issue

[Kixwy2]

Hi everyone!

I am new here, and very glad I found this forum. I bought a 1974 GMC truck several months ago (my first!) and am loving being a truck owner! I have some pics at the welcome page for everyone. So, I knew the truck had some issues when I bought it, and am trying to get those sorted out.

Ever since I have owned the truck, it seems to have no real power on acceleration. It goes, but it doesn't zoom, like you need to when you get on the freeway in CA. I looked under the hood, and realized that with the vacuum advance hooked up, the distributor reversed direction. So I tore apart the distributor and discovered it had one weight, and it was reversed.

That got replaced.

I have also replaced the spark plugs, wires, cap, coil, rebuilt the carb, and plugged up the extra vacuum ports on the carb. Even though this is a 1974 350, not needing to pass smog, it's a CA truck and has emissions gear like the EGR valve. The EGR is busted so it's just sitting there plugged up until I replace it or block it.

At the moment, it's timed at 12 degrees BTDC, which is CA standard, but I get this bad engine knock when I try to climb a little incline or accelerate hard. If I let off the gas, it goes away. This is all well and good if I am driving in 5pm traffic, not so good if I need to get on the freeway.

Any ideas? What am I missing or doing wrong?

[marvxyz1]

Firstly, I'd check all of your vacuum lines. I had an issue years ago with my car motor making a heavy knocking sound (it was an Rambler) Took it to the garage to discuss getting the motor repaired. My brother in the mean time was checking things under the hood and found a vacuum line off. Hooked it up and the knocking was immediately cured. No need for a engine repair... marvxyz1

[swb85]

What is your total timing and what rpm is it all in by? Are you running vacuum advance? If so, how much? Sounds to me like your timing curve is way too aggressive. This can be adjusted with weights/springs inside the distributor; an adjustable vacuum advance can is a nice thing to have as well.

[Kixwy2]

Hi guys,

When I discovered the distributor was bad beyond my repair, I got a rebuilt one at Auto Zone. I did get an adjustable vacuum advance, but it doesn't seem to affect the vacuum in anyway. In theory, it should have 10 degrees of adjustable advance, but no matter how far I turn that little allen wrench, my timing isn't affected one bit.

Do you suppose the distributor weights in the rebuilt are off? I would think that one that came from a shop would be built to factory spec, but now I'm not so sure...

I did adjust the timing to 9 degrees BTDC this morning, and have been running with the vacuum advance plugged up to try to isolate the problem. I also replaced one other vacuum line that didn't get replaced with all the others a couple months ago - that's why I don't think it's a hose problem. All of them are basically new.

I'm stumped...

[454HO]

How are you setting the initial timing? Have you verified the actual TDC of the #1 piston matches the TDC mark on the harmonic balancer? Have you tried running some premium gas to see if the ping goes away?

[Kixwy2]

I have a timing analyzer that I am hooking up to the #1 spark plug wire, as well as the pos/neg battery nubs, and firing it at the balancer. But you are right - I haven't checked to see if it's true (because I don't know how. oops)

I have put in several additives in the oil, as well as an octane boost in the gas to see if it helps.

Since I have plugged the line that goes to the adjustable vacuum advance, I should have less overall advance. It's my understanding that a knock/ping indicates too much advance?

[89 crew]

Yes the pinging means too much advance, or too low of octane in the fuel. I got some bad gas one time that made a Cadillac with factory 8-1 compression ping on every hill!

[454HO]

Since this truck is fairly new to you, and it is nearly 35 years old, you cannot assume anything. Always be methodical and check everything.

I suggest you read thru this FAQ thread by Bill Lattimer.

The first and most important step is to verify actual TDC of #1 piston against the mark on the balancer, then he tells how to properly set up your timing curve on the distributer. Very good thread.

[Kixwy2]

Wow - great step by step process. I think I'll pull the distributor and check that just to be sure. I have a guy down the street that's a car nut, and he might be able to help me out with this. I'd have to order a curve kit, but I can at least make sure the #1 is firing correctly and check the distributor for function.

Thanks guys - I'll post more as I make progress.

[Pyrotechnic]

There are 2 different balancers that I know of for small blocks. One is the thick balancer that has the timing pointer built into the timing cover that you have to look straight down at, and the other is a thinner and lighter balancer that has the timing pointer that you look at from an angle.

I would think that rebuilt distributors would get tested on a machine for proper advance curve but you never know. I bought a MSD Streetfire HEI and I checked the total advance with a timing tape and it was pretty much perfect.

[Kixwy2]

Update - I picked up a remote starter switch, and pulled the distributor today to verify piston placement. When I looked at the weights inside, lo and behold, one of them was chipped and bent. Back it went to the auto store under a warrenty exchange. Stupid me, I should have looked at that when I bought the distributor the first time. Duh.

Anyway, I verified TDC (that remote switch is bad ass!), pulled all the spark plugs, re set all the wires, and checked the timing again. I got it functioning better, but still not great. No knock and ping, but the BTDC is really low - 4 degrees with the vac advance plugged in and adjusted to it's lowest advance.

Another odd problem presented itself today: backfiring into the carb. I set the mixture screws to run leaner, but now when I open the throttle at a hard advance (not under load) the engine starts backfiring. The guy down the street helping me thinks I might have a sticky valve that isn't sending the spark properly.

Does that sound reasonable? It's a little over my head at the moment, but I figured with the Haynes manual and some good advice I could get a better handle on where I go from here...

[Ivan D.]

Initial timing is way too low - I used to run 8*BTDC and got bogging when I stomp on the throttle, went up to 13-14*BTDC and all is great now. Yers 4* with the vac advance plugged in means yer initial is probably on the retarded side, which among onther things will indeed cause backfires, and also if you're "lucky" you can see yer headers glow orange.

Go up to 10-12*BTDC with the vacuum can unplugged, then make sure yer centrifugal advance don't kick in before 800-900 rpms, and tune the idle at like 600-700 rpms to make sure you're running just initial timing. Then mess with weights and springs and vacuum cans.

[jimfulco]

Pinging at part-throttle under load is a characteristic of a carb that's calibrated for use with EGR, but used with a non-functioning EGR system. Fix the EGR, or make the carb run a little richer at part-throttle, and the ping should go away.

[James McClure]

Listen to Ivan & jimfulco and attack it like this. Disconnect vac adv actuator & plug the line not the actuator (can) set timeing at 12 deg. With a timing light watch your marks and SLOWLY accelerate eng. If timing goes up after your above 1000 RPM the mechanical advance is working fine, reconnect vac advance and run it. If it still pings your going to have to richen the fuel mixture a bit. If your not expirenced with quad's don't try it. If your buddy is he will know how to bring the metering rods up and replace the power valve spring with the purple one. If you can't find a purple power valve spring take the one you got, measure it and the stretch it 3/16" longer and put it back in. This will "pop" the metering rods up sooner and faster and increase fuel flow under acceleration. Mileage will suffer, but only a little. Good luck. jim

[Kixwy2]

Update - tried all the suggestions you posted here, but I am still having a firing problem. And the truck sounds like crap when I make the mixture any richer. It seems like my #3 spark does not fire when the timing is anything above 5*. I ran with it for a while at 8* and the vac advance unplugged, but I am still getting carb backfire.

I took it over to the carb shop that rebuilt the carb, and they fiddled with it for a while and got it soundng better, but it's still at 4*.

Think it's worth taking it to a shop for a "tune up" special? what the hell am I missing here?

[James McClure]

Kix: You have to "break" the system into two parts, ign and carburation. Backfireing is usually an indication of a lean mixture, but don't screw with that just yet. Have someone you trust sit in the truck and hold it in gear and load the engine just a bit. one way or another short out each spark plug. If you have an ign problem related to one cylinder, when you short it out, the engine operation won't change. All the other cylinders will cause a definate change because they are working. Investigate any cylider where the short out does not cause a change. If all the cylinders seem to cause an equal amount of eng speed drop when you short them out, the ign is probably OK. Next is carburation and by this I don't mean just the carb itself but the whole intake system. Look for vac leaks not just the obvious like vac hoses. How is the EGR valve? is it seated? Brake booster, is it leaking internally? PCV? How is the intake gasket where you can see it? (Quick test here is to CAREFULLY drip some fuel along the intake\cyl head junction and see if the idle changes) The toughest vac leak to find is a leak under the intake, but these leaks are not a random mis fire and will affect only 1 cylinder. (more if you have multiple area gasket failures) The simplest way to eliminate this as a problem is to pull the air cleaner pcv valve fresh air feed (R/S valve cover in the rear) Plug the hole in the valve cover. Pull the pcv valve out of the left valve cover and install a vac\fuel pressure guage. The gauge you use is critical. It MUST show vac and pressure on the same guage. Now start the engine and observe the guage It MUST show a pressure build up If it does, thats normal. if it shows a vacuum or very low pressure (under 2 pounds) there is something wrong. I had one car when I worked for GM that was a small block that never ran right. I chased this problem for 2 months before I figured it out. I checked and I tested and replaced all sorts of stuff, carb, distributor, EGR, pcv, all the vac hoses, power brake booster and it still ran like crap. I had to actually sit and think about what I had done and how the system worked before I came up with an answer and how to do a test to verify my conclusion. The test showed that the crank case was building a vacuum and not a pressure as you would expect. So wanting further varification, I took a propane torch and stuck it in the valve cover and turned it on (no flame, just propane) Within a few seconds the engine smoothed right out. I changed the intake gasket even though the one in it looked good when I pulled the intake off. I thought I had it, WRONG!! The SOB still ran rough so I re ran the test, same result. OK, replace intake manifold and still ran rough. At this point I'm ready to scrap the entire eng when I found the problem quite by accident. Valve covers were off and I spilled some carb cleaner on top of the left cyl head and it ran down the intake valve push rod for #3 cyl and the eng smoothed right out. I took more cleaner and carefully dripped it on the pushrod and every time it did the same thing. Come to find out there was a small casting fault in the cyl head and when the hole was drilled for the pushrod it cut into this gap and it sucked air into the intake runner. Long story I know, but just to prove a point. Move slowly and carefully and above all THINK. jim

[cartman]

I think you guys are forgetting a few things although you are correct in the things you have checked so far. Now it's time to go further. Backfiring through the carb leads me to believe one of two things ( due to the fact you have already checked the several other things that I would have) It is probably a worn out camshaft ( most likely) or a plugged exhaust ( more likely if it still has a catalitic converter on it) both will cause backfiring through the carb and low power! you need top do 2 things first take a vacume reading on the engine should be close to 15 IN , then a compression test of all cylinders. camshafts use to be common on old sm blocks of the 70's/early 80's GM had a problem with soft cams. one way to check the cam is to run it with the valve covers off and check rocker arm movement, usually you will see a few that barley move compared to the rest, Hence worn lobe. hope this helps.

[cartman]

Also both will cause "ping" / detoniation, because the engine cannot exhaust itself, also causing high piston cylinder temps.

[James McClure]

Kix: cartman is right also, VERY right and I thank him for rattling my cage and wakeing me up a bit. I can't even begin to count the number of cams I have replaced over the soft cam issue GM had. For a while, thats all I did!! The test was, vehicle in gear, mat the throttle and see if it backfired (not just one backfire but a continous pop,pop,pop,ect.) I did so many I had it down to 2 1\2 hrs. How could I forget that? Not bad when book time was almost 4 hrs. OH, by the way. The vac reading cartman refers to should read "vac reading should be 15 inches and STEADY". If the needle is bouncing and the engine does the pop,pop,pop thing procede as he said. jim

[Kixwy2]

Those sound like GREAT ideas! Given the condition of the other peripherals on the car, the engine appears to be original... no rebuild. Obiviously can't verify yet. I had been tossing around the idea of replacing the valve covers and even the heads to see if that solved the problem. This seems like a much better way to go about it, and much cheaper than buying a long block! Plus I like to know how it works so I know how to fix it - don't want to be one of those guys that's always running to the shops to get my problem fixed.

I'll see if I can get my buddy to help me out again to go through some of these tests. I hope we can figure out this problem. I'd love to move on to the more fun stuff like DRIVING the damn thing...

[cartman]

Please keep us posted on your results, always interested so see what happens! I personally would do a compression test before I spent a ton on aftermarket performance items for it. If the short block it getting tired, theres no sense throwing $ at it.

[Kixwy2]

Update:

Cartman, you were so right!

James, I did your propane test and the vacuum test and both came back clear, so no leaking. The vacuum was dead on at 15...

I took the truck to a buddy's shop and we pulled the heads and the valve covers. My #6 exhaust valve is stuck open, and the #2 valve stem is jammed. We did a bunch of tests and dicovered that the cam lobe to #6 is bad at best, and who knows what other shavings are in the engine.

I am weighing the pros and cons of a long block, and I think I'm going to have to shell out the cash for a new motor. I could fix the valve stems and replace the camshaft, but that doesn't mean I'm not going to have other motor issues... plus that's a ton of work. This sucks.

I am still having carb backfiring issues, and I can only surmise that it comes from the bad valves and the stuck intake. Dammit!!

[gchemist]

Time for a GM crate!!

[cartman]

Originally Posted by gchemist

Time for a GM crate!!

Sorry to hear about the bad news, I thought there was a valve train/camshaft issue there the way you were describing it. suprised you got a good vacum reading even with the issues you had. Just goes to show that even with all the tests some times exploratiory surgery is needed to really diagnose the problem. I don't know what kind of money you have to spend, but you could always but a cheap cam and a set of used heads to get you going for now while you get a nice motor put together. Or but a crate motor. There are a lot of options, rember that small block chevys are the cheapest to build, and most plentyfull engines ever produced.

[PhilaTruck]

Hey there Kix: Something I noticed in your post #6- Don't connect your ground to the battery post, whether you're grounding jumper cables or a timing light. Sparks over the battery could easily ignite and explode the battery gas. I always clip to the frame or some other similar place, just not the battery.

Welcome to the board- it sounds like you have a good attitude toward learning how to do your own stuff. I was in a similar place a few years ago, now I've done loads of mods/fixes including building a motor, with a LOT of board help. But I'll always be a hobbyist, lots of guys here do/have done this for a living.

-Todd