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Engine knock 71 350

 
Old 03-09-2019, 04:18 PM
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Beltran
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Default Engine knock 71 350

I got the vette running pretty good but I have this engine knock that is concerning... I am used to Mopars and this is my first GM so I need some advice.
I bought the vette as an incomplete frame off resto. The engine was gone through. 'Freshened' as it was described to me. New bearings, seals, guides, gaskets, but not a full rebuild. Starting it has to be done from in the car. Have to give it gas while turning the key, it won't start with a turn of the key outside the car. It wants to idle at 1300+ RPM. which according to the book is normal.

It does run rough sounding to me. A bit of rumble. It shakes a bit in the engine bay. but after it is warmed up it will idle at 1300 without issue. Is got a quadrajet so this doesn't surprise me. I understand these are notorious carbs. But this one was rebuilt well and is the original from the car.

Now at idle I hear this knocking sound. Using a stethoscope I believe the sound is coming from number 2. It is not 100% consistent. its about 75% of the rotations. I pulled the wire and the plug. Plug is brand new as the engine literally has maybe 3 hours on it. The wire is not new but with a timing light I can see it pulsing consistently. So it's trying...

The knock doesn't get louder when I step on the gas. In fact the roar of the engine and exhaust (stock) drowns it out. It is lively and responsive when stepping on the gas. On occasion it will backfire. It seems to stumble a bit too which accounts for the shaking and a loud burp on occasion from the exhaust.

The previous owner told me that he did not have the heads decked, just new seals and cleaned up. He believed it had a 'missfire'. However, I don't know what a misfire sounds like in a chevy engine. This has me concerned that it is a rod knock or something more serious. The only thing I question is that it doesn't get louder when you step on the gas. It seems to go away or at least get drowned out. Given that it backfires if I step on the gas, hard, too quickly I tend to believe this explanation. However I really don't know how to resolve it. If its not electrical then what else needs to be addressed here?

Thoughts, experiences appreciated.
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Old 03-09-2019, 05:50 PM
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An engine is an engine be it Mopar or GM so an engine knock will sound similar in both. Often a knock will disappear under load until the bearing is completely shot. You say the engine was freshened, did the bearing clearances get checked? Have you got any documentation to support what was done? What oil pressure do you have? Have you checked the timing, what is it set at? Lots of things to check I know, if it were me I would get the timing and carb sorted so that the engine is running properly, (easiest thing to do if you are in the USA is send the carb to Lars who is a forum member and have him rebuild it), if the knock was still there then you can drop the oil pan without pulling the engine to check the bearings or it will have to be pulled out.
1300 RPM is not a normal idle speed for a 350 when warmed up more like 800 - 900 maybe higher if a big cam is fitted. I would expect to see 1300 when the car is on the fast idle position when it is cold though. For most a couple of pumps on the pedal and turn the key and it should start straight away if all is in order, after the choke opens the idle should drop.
I have no experience of the Quadrajet but many on the forum have and it would appear that their notoriety is derived from people who don't understand how they work and having units that have been rebuilt incorrectly.
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Old 03-09-2019, 08:51 PM
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You don't say if yours an auto or stick.

regardless of make ... sometimes an automatic's flexplate will crack/split ... it happens ... and sometimes that'll make a knocking sound.

Sometimes you can spot it with dustcover removed ... sometimes not ... it can be a "challenge"
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Old 03-10-2019, 08:30 AM
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Fuel pump pushrod should always be checked before you decide it's a rod bearing knock.
Flexplates tend to act different when not loaded as compared to when it's dropped in gear and under load. Mine didn't knock, it was a clicking/clanging sound.
Good luck, hope it's one of the easier ones to fix.
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Old 03-10-2019, 10:59 AM
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Another thing to consider is your choke is sticking and your running rich, it could account for the rough running and the backfires. Warm up the engine so that the choke should be opened and look to see if the front butterfly is fully openned. Give the throttle a goose by hand and see if you get 2 squirts from the accelerator pump. Check you timing and lower your idle to 700 to 900 rpm and repeat. Then check your vacuum and Reset the idle screws as necessary.

Also check your tune on your distibuter, if your cap and rotor is worn you wont get as strong as spark as you need or if you have points you should also check that.

If the choke is not functioning or you cant get it to idle low then just ship it to Lars so you know its done properly. Going by the PO's word isnt always a good idea. You HAVE to know that your baseline is good, its like the foundation of your house. If the foundation is bad everything up to the chimney is going to be screwy.
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Old 03-10-2019, 11:04 AM
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So did you pull the plug wire on #2 with the engine running? Did the knock stop? I've found this method good for finding loose wrist pins and big end knocks. Are you sure your lifters are pumping up (if you have hydraulics)? Lifters sometimes make strange noises when they stick in the bore.

Last edited by stumpshot; 03-10-2019 at 11:05 AM.
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Old 03-10-2019, 12:25 PM
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Sources of a perceived “knock” could be lifters ( more of a clackity clack noise) exhaust leak at the manifold ( more of a ticking type noise) piston slap, which would subside more as the engine warms up and finally a con rod knock, which in my experience gets worse if you load it up, like accelerating from a stop rapidly. Reving it in neutral won’t load the rod up enough to hear it unless it’s really bad.
could be whom ever freshened up the motor failed to properly tighten a rod cap.

if you step on the gas it hesitates backfires and accelerates , it’s likely your accelerator pump is not functioning and your getting a lean pop. If the choke is stuck on it would chug chug and then clean up with rpm.
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Old 03-10-2019, 12:53 PM
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Suggestion of a fuel pump is one that can really fool ya. Sound travels around that block and is tough to pinpoint

When its dark start it up and see if you find a wire arcing somewhere esp near the exh or head
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Old 03-10-2019, 01:10 PM
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it is a 4spd.
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Old 03-10-2019, 01:14 PM
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Stumpshot, I did not pull the wire.. but I thought about it. Do you think it would stop the knock if it was ignition related?

A lifter knock would be an option to consider. But with the stethoscope it sounds lower like its in the block. I will try again to better isolate the noise.
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Old 03-10-2019, 01:40 PM
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Reelav8r and all those with carb suggestions.

Thanks for the tips. The choke does work. I have watched it from cold to hot. It may be out of adjustment a bit but it does function.

Even with the engine warmed up I cannot get it to idle below 1100. It will stumble and stall. I am using a GM conversion module on the distributor to replace the points. Otherwise it is the stock distro. It helped run a little better but not all that much. (this was the first thing I tried)

The exhaust seems rich but that is my perception. I have no way to measure that.

I have tried to adjust the timing but it is in it's happy spot. Translation, pulled the vacuum advance but did not plug the hose. idle slows down. adjusted the distro to an advancement point I could see (on the balancer) and the engine was not very smooth. I turned it clockwise to advance it more and it smoothed out and accelerated. Mind you this is all between 1000 - 1300 rpm. I believe the timing mark is actually at the top of the cycle but it is hard to see due to the waterpump.

Other than the idle speed I have not touched the carburetor. Maybe I should figure out the idle mix adjustments. But this doesn't solve the knocking issue.. which is very concerning. I am worried that the more I run it the more damage I am doing. I really don't want to get into a full rebuild. bleh...

I cannot put it on the street. I don't have plates so putting under load would be a challenge. Oil pressure when the engine was hot was just below 35psi. between the two marks. Is that telling of anything? too low?

I was told it had a slightly hotter cam, but he had no documentation on it. The engine is powerful. He felt it was 330+ hp at least. So the Cam is probably an factor here.

Last edited by Beltran; 03-10-2019 at 01:51 PM.
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Old 03-10-2019, 02:01 PM
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You can add timing marks to the balancer. Put timing mark at 10 deg brc. Get whiteout and paint line at 0. Move that line to 10 deg and paint another line at 0. Now you have 20 before mark. Set that on the 12 to 14 mark at 3000 with vacuum advance unplugged and you see your 32 to 34 all in advance. The idle number can be anywhere from 8 to 18. Wont make a lot of diff.
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Old 03-10-2019, 05:20 PM
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If it won’t idle below 1100 I would say your timing is too retarded. At least one of the problems anyhow.

on the carb you can bottom the idle screws and then screw them out 2 1/2 turns as a starting point.

when you disconnect the vacuum advance you must plug the hose or you are introducing air into the mixture making it lean, so you won’t get an accurate timing or idle with the vacuum leak present.

loosen the distributor hold down and advance the timing until either the idle stops rising or it starts running rough. Then back down the idle with the idle screw and adjust the timing either up or down for best idle again , back down the idle, repeat.
Get it to where it will idle at 800 to 900 rpm. Now fiddle with the idle screws on the carb using a similar procedure. You may have to go back to the timing using this procedure until you get it to idle properly.

make sure the vac advance is actually working too. Just suck on the hose and observe the vac advance moving. Put your tongue over the end of the hose and observe that it does not move until you take your tongue off the end of the hose.

if you got a timing light you’ll be likely looking for about 15 to 17 degrees timing advance (assuming a somewhat larger cam than stock) with the vac advance disconnected and hose plugged and it will save you time vs the above procedure, assuming the balancer has not slipped and is not giving you bad info.

if the vac advance is connected to a ported source of vacuum ( no vacuum until off idle) then move it to a manifold vacuum source. ( full manifold vacuum even at idle)
Just be sure to plug any vacuum ports you disconnect from.
your oil oil pressure is normal.

the fuel pump rod knocking is also a very real possibility since your hearing the noise near #2 cylinder.

Last edited by REELAV8R; 03-10-2019 at 05:35 PM.
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Old 03-10-2019, 05:56 PM
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I tried many of the things you all suggest and I video'd the results. They are uploading to youtube now. it will be a bit.

That said, I discovered a few things. The number 2 wire was completely grounded out somewhere in the path. I got pickup with the timing light at the distro, but not at the spark plug. So I got a temp wire from an old set and now I have that firing. This obviously smoothed out the engine a bit. I found that #5 is intermittent so I will get a replacement set of wires.

The oil pressure started out COLD at 35psi. Then goes down from there. Once warm it goes below the 1st line a little. This is when I started hearing the knocking. In the videos I took you will be able to hear the knocking sound, it comes through loud and clear. But it does sound like it is from around the fuel pump. What the heck causes that? It's a new pump and it is obviously working, so an issue with the pushrod?

I got it tuned a little better. Idle now at 1000-1100 but it fluctuates.. it's not a smooth engine. Not sure that will improve until I change those wires. I did adjust the idle mixture and timing to get it where it is. The choke is adjusted properly. The timing ended up pretty much where I had it before.

Last edited by Beltran; 03-10-2019 at 05:57 PM.
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Old 03-10-2019, 06:44 PM
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Ok , ya 35 cold is pretty low. It could be the oil, or a problem with a Bearing somewhere. I got an old f150 that would knock softly at idle and had very low oil pressure. Started using Mobil one 0W40 and both problems went away.

sounds like you’ll be needing a new set of wires. If those ones are bad then it’s likely the others are not far behind.

how much vacuum is this engine making?

Last edited by REELAV8R; 03-10-2019 at 06:47 PM.
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Old 03-10-2019, 06:51 PM
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Ok first video. Pre-tuning, engine warm enough to idle on its own without me babying the peddle.

2nd video engine warmed up. You can hear the knocking sound. Pulling the #2 wire makes no difference. The #2 wire is changed out with a temp wire. It helps the engine run a bit better since it wasn't firing that cylinder at all!

3rd video. Best I can get it to run right now. Will order new wires? Anyone able to recommend some that will fit the wire loom properly, or should I just custom make them? oil pressure has me concerned here.
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Old 03-10-2019, 07:10 PM
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Its making enough vacuum so that everything works well. The entire vehicle is vacuum operated. Lights pup up fine. Vacuum is strong. I will have to check that.
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Old 03-10-2019, 07:25 PM
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Sure sounds like that carb has some issues.

The kncking sounds to frequent to be either be either a rod or a fuel pump. Cam turns half the speed of the crank.

easy stuff is a loose pully, one can hope. Worse is a main bearing issue.

I usually associate main bearing sound with more of a rapid grumbling rumbling kind of sound though, yours sound like something is loose or something hitting something.

Last edited by REELAV8R; 03-10-2019 at 07:30 PM.
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Old 03-10-2019, 07:49 PM
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Oh there is a thought... playing back the video I counted about 3 knocks per second, roughly.

at 1000 rpm, that would be 16.667 revolutions per second (rps). so that knock isn't happening at the speed of the crank or the speed of the cam (half of the crank) Lets just say its closer to half of the cam speed which would be 4 rps. Does that make sense?

The valves go half the speed of the cam correct? intake or exhaust? take your pick. Pushrods would also be in that group. Anything else?


On further thought. If the knock only happened on the down stroke of the #2 piston. 4 stroke engine, 16.67 rps. that would be 16 / 4 or 4 times per second. That is also close to the 3... so it could be a connecting rod. I don't think I am going to feel better about this until I drop the oil pan and check those rods...

Last edited by Beltran; 03-10-2019 at 07:55 PM.
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Old 03-10-2019, 08:01 PM
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Rod knock does seem most likely, probably because of the high idle that it sounds too rapid.
yep, eventually stuff is gonna have to come apart to confirm or deny the cause.

at a regular idle it’s a steady knock, knock ,knock.

yours sounds more like knockity knock, knockity knock, more than one cause?

Last edited by REELAV8R; 03-10-2019 at 08:07 PM.
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