Compression Low- 1cyl

[jdeignan]

Hi, if you squirt tranny fluid in the cylinder
Good luck

Ok so this worked.. except now that cylinder is putting out 180psi.. just leave it be?

Thanks for the help

 

[pcf_mark]

Awesome! It is over achieving now because the ATF is super sealing the rings.

 

[jdeignan]

I've seen some modern cars trail a stream behind them I would not chase that as a symptom of anything.

When I my stock engine was carbureted i was pulling 19" steady in neutral. I had 16 degrees of timing but that made the exhaust smell nasty backing it down to 12-14 is better.

I warmed it up, loosened up the dist and tried to see what I could do to the vac reading with timing.
Results: If I move in either direction, vac drops to about 10 and the truck wants to die. Think I may just be stuck with 15.

 

[DirtDonk]

I wouldn't say that's exactly normal behavior. Can't pinpoint just why, but I don't think it should react that way.

Paul

 

[DirtDonk]

This is with the vacuum advance disconnected? And where are you taking the vacuum reading from again?

And did you mention what type of distributor?

Paul

 

[jdeignan]

This is with the vacuum advance disconnected? And where are you taking the vacuum reading from again?

And did you mention what type of distributor?

Paul

vac advance was hooked up as depicted. I took my vac reading from the drivers side port (circled). Dist is MSD made.

https://ibb.co/byyZ9w

 

[jdeignan]

This is with the vacuum advance disconnected? And where are you taking the vacuum reading from again?

And did you mention what type of distributor?

Paul

What about the PCV setup? Think that could be the source of low vac signal?
I know very little about how that system works- is the valve specific to the vehicle? Cause the one installed is an off-the-shelf NAPA part
EDIT: No change in vac signal with PCV removed, C4 tranny vac line removed, or distributor port swapped. Runs at 13-15inches while hot. Hmm.

 

[DirtDonk]

Ok, looks like it's all correct. Must be the particular cam then. Not sure why it would go up and then back down through such a narrow range of ignition timing, but again, that's likely the characteristics of the cam.

Yes it can make a difference in air/fuel mix with what PCV valve you use, but most of the ones for a 351W from the early '70's, or from the late '90's would work. Something out of a mid to late seventies might let a little too much air in and lean out the mix because it was designed for low-po smog engines.
That's what I found with my 302 at least. Your results may vary, but you'd notice an overly lean condition with a carburetor by it kind of bogging down just off-idle.
It will idle fine, and cruise fine, but initial acceleration is weak.
Doesn't sound like that's the case here. Plus a steady vacuum reading is still a good sign.

Last thing i would do then would be to attach your gauge to a known-good stock engine of any type. The newer the better I would think, but even an old V8 with stock cam would be ok as long as it's in great shape.
You're just looking for numbers to compare to. If you see 22 on your gauge with the other engine, I would say your gauge is accurate.
If you see something like 17 or 18 however (or even 15 for that matter) then I would suspect your gauge is reading low.

Probably worth verifying this. Could save you some hair pulling!

Paul

 

[B RON CO]

Hi, my next guess is now your valves (probably), and rings, and everything else is loosening up. What oil do you use? I like Castrol GTX 10W 40. Did you try one of the oil or gas additives?
I would change the oil now if you haven't done it recently, and again in about six months. I change my oil every spring, but only put 2-3,000 miles a year.
If you only drive short trips, like a grocery getter, that could be part of the problem. The engine needs to come up to operating temperature to burn the by products of combustion out of the engine. That is why the thermostat is important. A stuck or missing thermostat and your engine will run cold and sludge can form.
I'm glad it runs ok now, and maybe you will see steady improvement. Good luck

 

[jdeignan]

Last thing i would do then would be to attach your gauge to a known-good stock engine of any type. l

Unfortunately, I hooked the gauge up
to my 07 Acura and it reads a steady 20inHg.. great for Acura, but not great for hair pulling.

 

[pcf_mark]

I warmed it up, loosened up the dist and tried to see what I could do to the vac reading with timing.
Results: If I move in either direction, vac drops to about 10 and the truck wants to die. Think I may just be stuck with 15.

Usually when you set timing to highest idle vacuum you end up with too much timing. It should improve vacuum because the idle speed increases then stop improving. Are you saying when you advance it the idle does not go up? In the other direction it will retard so that would slow the idle down reducing vacuum.

You may also be chasing a non problem because 15" is pretty fine for a non-stock cam. When you say erratic vacuum what do you mean exactly? Floating back and forth between 14-15-14-15 or jumping 12-15-11-16 very quickly?

 

[jdeignan]

Are you saying when you advance it the idle does not go up? In the other direction it will retard so that would slow the idle down reducing vacuum.

You may also be chasing a non problem

Yeah, I’m saying with the truck warm, I am unable to twist the dist either direction without a resulting loss of vac signal and power.

I agree the 15” isnt a huge problem and is likely because of the cam at this point. It was steady when I tuned the A/F Idle Mix- now I have some
jitter. It bounces anywhere between 13-15 consistently.
I figured maybe I had a vacuum leak which is why I unplugged everything from the carb to see if I could isolate it- nothing. So unless the leak is at the manifold, I think I am chasing a non issue

 

[pcf_mark]

Do me one last test then I would let it rest too. Put the timing light on the number one cylinder and just point it at your hand. It should be super steady even if the engine has a roudy cam the plugs fire. Repeat for each wire. If they are all steady it is either cam or carb but most likely cam. If they are not all steady start looking at your wires, cap, rotor, etc for an ignition problem. My Pertronix faded away - it would dance around and miss fire here and there and you could see it with the light.

 

[jdeignan]

Do me one last test

Mark, I will let you know when I have some time to troubleshoot... I went out to start it.. started for a few seconds and stumbled a bunch.. died- dark puff of smoke. Now it cranks but wont turn over! Feels like an ignition issue that I’ll need a few days before I can handle it. Thanks for the advice! Joe

 

[jdeignan]

Mark,
Got it running again... she is pulling 13-14inches. needle bounces between the two numbers consistently. I have it idling hot at 650, 550 in drive.
I put the timing light on all wires and they all flashed at a steady rate.

 

[jdeignan]

I replaced the plugs with autolites gapped at .42.
Heres what the old ones looked like.. one plug in particular looked interesting.
https://ibb.co/hSVoBb
https://ibb.co/kf8dcG

 

[oldiron]

Unless im off, a consistently bouncing needle can be indicative of a valve sealing issue.

 

[pcf_mark]

If it bounces between 13 and 14 consistently I would call that cam. The plugs look pretty good to me. Just check the runner for that really light one. Is there a vacuum device connected to it anywhere? A vacuum tree or port? Power brakes connection? It looks a little white so maybe it is running lean. If it is lean it could be a vacuum leak.

 

[jdeignan]

If it bounces between 13 and 14 consistently I would call that cam. The plugs look pretty good to me. Just check the runner for that really light one. Is there a vacuum device connected to it anywhere? A vacuum tree or port? Power brakes connection? It looks a little white so maybe it is running lean. If it is lean it could be a vacuum leak.

Check the runner? Can you elaborate?
For devices, they are all attached to the Edelbrock 1406.
All manifold ports are plugged for now. (going to port my C4 vacuum to the manifold next)
I even unplugged all of the devices (PCV, advance, C4 vac line) from the carb and saw no change in vac signal. So if there is a vac leak, its the manifold seal.
Right?

 

[DirtDonk]

You got it. By that he meant that whatever cylinder that came out of (let's say it was #8) you find the intake manifold runner going to that cylinder and see if there are any items plumped into it that might be changing the mixture.
Things such as the vacuum tree (usually in #4) for multiple vacuum hoses, or a single port for say the vacuum brake booster (could be #8) or if all of the vacuum lines are plumbed to a common plenum area instead.

Since you have the intake ports plugged off and the hoses run to the carburetor, that rules that out.
Often people run the PCV valve to a single intake runner, which is a big mistake. It's like a big (but controlled) vacuum leak in only one cylinder. When it's hooked to the common area under the carburetor though, it's spread out among all the cylinders so does not cause any issue.

By the way, nothing wrong with plumbing low-use vacuum lines to an individual cylinder, just like the factories do it. Things like your C4 modulator, or a brake booster hose, or whatever else. All those are ok. That way you don't run out of available ports on your carburetor.

Paul