StingrayLust

So after a year after buying my compression test kit, I actually got to it. On the way I busted 2 spark plugs on the drivers side, dang those are hard to get to. Anyways, here's the results on a *cold* engine with *no* oil added to the cylinders:

#1 - 123
#2 - 122
#3 - 118
#4 - 115
#5 - 120
#6 - 112
#7 - 119
#8 - 117

All spark plugs were removed and throttle opened.

The reason for doing this, the engine doesn't run as smoothly as I think it should and I don't know the engines history before I bought it. At idle when I listen to the exhaust I hear what I call skips in the "beat" of the exhaust, an example video that may or may not help is here.

I've read elsewhere anywhere from 80 - 180 but what I want is them all to be about equal. Mine are all pretty close to 120 though #6 is a bit out of whack compared to the rest.

I'm wondering if my timing chain is getting a bit worn. I know you're supposed to do this with the engine warm, but I value my skin and those exhaust manifolds being hot I'm not taking a chance.

All spark plugs looked good for burn other than the ones I broke the ceramic.

cardo0

How many miles on car/engine? Use much oil?
Those numbers look low even for a cold engine. A good performance engine is more like 200 psi and 1971 cars still had high compression. If engine miles are low then i would suspect that compression tester gauge is out of calibration.

cardo0

drwet

I agree the numbers are a little low, but they are all within about 10% which is good. You should be able to get the engine to run reasonably well.

cardo0

Ok i listened to the video and after the car started to warm up some it sounded like chipmunks talking. Over the internet it sounded like the fan was hitting something but u say skipping? well if its missing then either fuel or ignition. If its the timing chain slipping the engine gets really stupid with backfires and shutters.

If u haven't even timed it with a new dizzy then my guess is the timing is way to retarted or advanced.

Good luck,
cardo0

scottyp99

Set the timing,

http://forums.corvetteforum.com/c3-t...long-post.html

then set the idle mixture, (I like to use a vacuum gauge) then let us know how it's running. Good luck,

Scott

StingrayLust

The odometer says about 50k and some previous owner has been inside at least the intake manifold as the oil cover was off as well as the wrong exhaust manifolds were on it when I got it. The block # and heads seem like they are correct for the car however.

This was the base vette, 350ci/270hp 8:5:1 compression.

StingrayLust

Yes the video isn't of the best quality, sound compression, etc. However I've listened to quite a few rebuilt 350/383 on youtube and elsewhere and they all sound like clockwork, no wavering of the sound. While I can't say my engine runs bad or even poorly, I think it could be better so I've started to poke around a bit to see if it's something simple (chain) or if it just needs a full rebuild. Maybe it's just my cheap distributor or a sticking valve.

I have no idea if the previous owner had changed the original timing gear that would have come with plastic teeth or not when they were in the motor. I don't think the chain has slipped a tooth as it doesn't run that bad.

It's a bank repo so I don't really try to contact the previous owner even though I believe I know who it is and he's on Facebook.

Saving money now for a 5-speed manual trans so can't really spend much on the engine right now. It will be a 383 someday, just trying to make it worthy until then.

johnnyjaws

should have atlest 150lbs for a good motor my vette is a 76 with 49000 miles and reads 150

terrys6t8roadster

you have good cause to be concerned with the timing chain. did mine last winter on a 69, no plastic left causing eratic running. there was more than a inch of slack on both sides of chain. you might only have portions of the plastic missing causing timing changes on a few cyls. having only a 10% variation on your compression should give you a smooth running engine. another thought is your dist, points? never could get rid of the intermittent misfires until I installed a complete MSD ign [probably over kill] problem gone. the 69 stock engine has pretronix pickup and runs sweet.

StingrayLust

I've tried to find some videos that show what a bad timing chain sounds like but haven't found any. I guess I really should just bite the bullet and replace it as it's a fairly cheap job to do and crosses off the list one more things that could be bad on this unknown engine. It does have the correct heads, I looked up the casting #'s last night. If I do replace the chain, may as well go ahead and replace the cam as that's not that much extra work and is one more thing I can make a known quantity on the engine.

When I first got the car, I replaced the points dist with a new HEI unit (yes a cheap one), new HEI wires and new NGK plugs gapped to about .045-.050.

However I wonder if I'm just hearing things that aren't there. In this video, https://vimeo.com/31930131, I think it's more clear the difference in sound when the dist vac is hooked up and when it's not.

StingrayLust

I know last summer I checked my vacuum and timing and it was erratic. Maybe that was before I had my carb rebuilt, I honestly don't recall. However, today I hooked back up my vacuum gauge and timing gun and took some videos:

Here's the version where all (except PCV) vacuum hoses are disconnected giving about 14 degrees (advanced?) and about 16.5Hg in vacuum.





Here's the version where all (except PCV and distributor) vacuum hoses are disconnected giving about complete guess of about 20 degrees (advanced?) timing and about 18.5Hg in vacuum.


So maybe I'm just crazy and the sound I'm hearing just isn't there as the compression, vacuum, and timing all seem like they're ok.

Dino_'72

As you said, and I agree, that vacuum gauge looks good to me.

As stated by others, that compression value is low. Are you burning any oil?

'71s are a 8.5:1 compression motors from the factory. Too much cam will lower static compression. Any knowledge on the cam. Doesn't sound too big, plus vacuum reading indicates a smaller cam. Steady vacuum gauge reading indicates your guides are good.

Leak down test should be tried if squirting oil in the cylinders does not raise compression noticeably.

I went through a similar experience when I bought my '72. Engine rebuilt but No real knowledge of what was done. Pulled heads. To my surprise, forged dish pistons on the rebuild. Normally you would see flat tops. Cam was also a mystery. No markings.

A process of elimination. Good luck. Also, the fluttery sound to me on the exhaust is not really IndicAtive of anything, other than many low HP stock type motors with straight duels often have that flutter sound at idle.

StingrayLust

I'm not burning oil from what I can tell, at least not an amount enough to make it obvious.

I don't know anything about the cam or pistons, but the cam has to be pretty small as it doesn't have a lot of oomph or sound that mean.

Before I threw the spark plugs back in, I did splash some oil in the #6 cylinder and took another compression reading as that one was the lowest. The wet test bumped it up to somewhere in the 130's can't remember the exact number.

StingrayLust

One thing left that I'm trying to understand. In the two videos the only difference is that one has the distributor vacuum connected and the other one it's disconnected. I've read that for my engine, the distributor vacuum advance should be connected to the quadrajet ported vacuum connection which is the one on the drivers side? That means the vacuum port on the passenger side of the QJ goes to the fuel vapor can?

I think I've asked this before in that I couldn't remember if the engine idle should go up just by connect the distributor vacuum hose.

If those are the right connections, I was hooking my distributor vacuum hose up to the full vacuum connection instead of the ported one, hence making my idle speed go up.

Dino_'72

On the wet test, would be interesting to see if you get a similar bump on another cylinder. If you do, that would suggest more or less even wear. I've always performed a compression test with the engine just warm, not hot for the same reason, who needs burnt hands.

If you want some sort of baseline, I have a completely stock '72 base 350 (L48). Same motor as in the base '71. I have 170 compression straight across and 19" vacuum.

If your compressions reading go up by 10% when hot, that should be enough that the car should have decent power relatively speaking for a L48.

If your engine has some wear, your camshaft may also have some wear. This would in effect make it into a smaller cam and affect performance and possibly your compression readings, especially if an intake valve is affected.

Ported vs full vacuum. Many have different opinions. In general most people on the street with lower performance engines run full vacuum to the distributor. Now in '71 and '72, there was a emissions devise (near carb on passenger side) that was vacuum operated that affected vacuum to the distributor. I think they operate slightly different from 71 to 72 (TCS?), but generally accomplish the same thing, that is to retard timing at idle. Most of these have been discarded, and people run full vacuum from carb to distributor. Regardless, at full throttle, vacuum advance is not in effect either under ported or full vacuum. I think the way you have it operating right now is probably OK.

StingrayLust

I guess my issue was primarily at idle it didn't see to run well. After looking at the subject further, it seems views range wildly either always run full manifold vacuum or always run ported. I've read quite a few posts about the subject. I think that for *my* engine, the full vacuum at idle was giving it too much advance and that's why I thought it sounded a bit off.

I've got my dist vacuum now connected to the ported vacuum so that it runs better at idle in my view.

I'll just run the vapor canister connection to the other side where it's full manifold vacuum as I don't see another fuel vapor connection on the carb.



Here's a short video of it running at idle with ported vacuum:


I'm now satisfied enough with the engine and how it performs until I get it rebuilt in the future (5-speed conversion first).

augiedoggy

I have 150-155 on my old 74 l48 with 104,000 miles maybe your tester is off a bit?