Technical SBC Knock and Power Loss

Hello everyone,

I'm at my wits end over here. I've got a 282 bored .060 over and built very mildly with a stock cam. Did a top end rebuild on it 2years ago and swappedout the heads to the 601 castings from a 305. Had a Tarantula intake on it then swapped to a cracked(didnt know initially) performer. Started having misfire issues. Swapped back to Tarantula and cast iron original. I jave a Qjet on it.

It runs in idle, not super smooth, but the bigger issue comes when I put it in gear. It develops a solid knock and loss of power. I can accelerate, but not like it regularly would. I've tried making sure the valve lash was not an issue, higher octane gas, tuned it multiple times. I live in an apt complex, so working on her gets dicey. I'm pulling the oil pan this week to check things out, but have no"real" idea what I'm looking for. Just stabs in the dark. I've heard it could be a bent valve, broken piston rings, bad rod bearings. Pulling the heads isn't really what I wanna do in my parking lot, but.... Any ideas? The knock doesn't sound like a muffled bell, more like a crisp knocking as I accelerate under load. I've heard it could also be flex plate or trans too. So many options and so little money...ha! I appreciate the wisdom this board brings and really love the advice I get from here. Thank you.

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Before tearing it apart, I'd at least do a compression test on it. That will tell you if you have a cylinder or valve problem and doesn't take much time or work. Sun Electric had a motto; Test, Don't Guess.

 

282?

 

Before getting too carried away, if you don't have a stethoscope take a long screwdriver or a wooden dowel and listen around to see if you can isolate where the noise is coming from. Also pull the spark plus wires one at a time while running and see if when you drop a cylinder the noise changes or goes away.

 

After swapping the heads , what was your compression ratio ?

 

How did it run before the changes? If okay, that would eliminate the bottom end. I’d lean toward the simple stuff first, verify you spark plug wires are routed correctly on each plug and the firing order on the cap. Then check / set initial timing. 8-10 BTDC would be a decent start.

 

It ran ok, but I made the initial swap to improve my fuel economy. The Tarantula is really way too much intake for such a small bore motor. The compression did go up, but im not exactly sure how much. I'll get a tester from autozone and find out where I'm at.

The problem with trying to "drop" a cylinder while is running is that the noise really only happens under load, so if have to have it completely lifted to do that.

I'll let you guys know what I find out.

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Running in up on jack stands won't generate a load.

 

Still using the "282" balancer or did you swap it out for a later one?

 

Pay particularly close attention to your mufflers. SERIOUSLY!! My buddy had a pickup with a302 Ford and dual exhaust, had what sounded just like a rod knock but only running down the road under load....sounded behind the cab though. We took the bed off of the truck, I stood on the frame rails and down the road we went...one muffler was kind of pulsing so I put my foot on it and I he noise stopped. Double wall muffler with the inner skin corroded through...sometimes they collapse and cause a partial blockage hurting power as well. Check your manifold vacuum when it's running poorly, if it's low that's an indication of blocked exhaust...or other issues.

 

Maybe post a video so we can hear it too. It might be easier to pinpoint that way.

 

I'm trying to get it to a place where I can diagnose either tomorrow or Wednesday. Theexhaust system is brand new, so it not that. I'm going to compression test the cylinders and then go from there. I'll get a video of it running the day I do the test.

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Compression test will show you the cranking pressure , or lack thereof , it will
Not tell you your compression ratio. A knock under load is usually either a rod bearing or a cracked piston or loose wrist pin .

 

Hook up a vacuum gauge first and for most and see what’s going on
Cheapest and best diagnostic tool in your tool box.

then try a comp test and leak down.

many vacuum leaks ? Ya say it pretty much runs like crap from start up.

Choke coming off?

Valves set correctly? ( ya said you TRIED to set them correctly)

base timing on, push rods and lifters etc not damaged? Cam ok cam bearings ok ?

exhsust leaks ?

buy a stethoscope cheap investment to find exactly where the noise is coming ftom

 

as Vandenplas says!

 

When you're checking the idle vacuum, make sure the gauge is connected to a full manifold vacuum source. A port on the intake manifold if possible, or carburetor port that picks up its vacuum below the throttle blades. Ideally, you should also seal off any accessory vacuum ports like the vacuum advance, choke pull-off, vacuum modulator or power brake booster.

 

sounds like a flex plate issue

 

56 to 58 cc is what I have seen on the internet.

 

282?

 

While this chart is very good , it should be pointed out that it is applicable only to most stock or nearly stock engine configurations , throw in a hot cam with lots of duration & overlap & the chart looses validity .

 

2old2fast thats right. and every engine has a hot cam in it people say! HaHa

 

0.060" over 276?

 

282 @ .060 overse as a 292. I'll hook my vacuum gauge up today and stethoscope the motor as well may try and run it with valve covers off to eyeball anything wrong with the lifters.

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Will I be able to check loose wrist pin and cracked piston from below with the engine in if I drop the oil pan?

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A wrist pin knock will lessen, or go away, when you pull a plug wire while running. Unless very bad, you typically won't see a loose wrist pin from the bottom. You might see a cracked piston depending on where, and how bad, it cracked.

 

283 chevrolet? another simple thing to check is make sure #5 and #7 plug wires are not crossed. brother had a olds motor, it would idle fine but under a load it knocked and rattled. took me a minute to find it.

 

Ok, vacuum checked and it vibrates between 10-15 psi. Surges a little bit intermittently.

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Only under a load?? Sounds like "spark knock" or "pinging" to me....

 

Have you pulled valve covers to make sure you don't have a sticky valve or broken spring?

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Pulled the covers just now. Waiting for my friend so I get a second pair of hands and eyes to look it over.

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