Compression test on a 360, result OK or not?

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put a new timing chain and valve stem seal on your to do list
check the springs when you do the seals
those that have sat open will have taken a"set"
 
Bouncing vacuum is interesting. Something there for sure. Bad intake gasket?
 
Video of the vacuum gage while idling in his first post here.
Looking at it, I don't think its a valve issue, the way it wanders up and down. feel free to disagree
Unhappy 360 (At idle)
 
Mattax said:
Video of the vacuum gage while idling in his first post here.
Looking at it, I don't think its a valve issue, the way it wanders up and down. feel free to disagree
Unhappy 360 (At idle)
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Yes, the reason I did the compression test was to make sure there wasn't any major problem with the engine causing this slight idle "misfire".

Got a new timing gun now, will check timing at idle and make a timing curve.

(I could always go to a workshop and have them look at it, but trying to this myself i learn A LOT and actually having fun in the same time)
 
AJ/FormS said:
Hey Stephan, do you know what cam is in it or the specs and what is your idlespeed and idle timing in neutral/Park..
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It´s never been rebuilt, so it should have these specs found on Hot Rod Magazine:

"The camshaft was a 268-276-44 bumpstick and was 1968 340 hardware (as were the H/D valvesprings, dampers, double roller timing chain and pushrods). Compression was pegged at 8.4:1. All LRTs used the 1974-early 1975 360hp pistons (PN 3780071).
Rated at 225 hp and 340 lb-ft of torque, the special 360 was coded EH1."

Idle speed is 750 in neutral 650 in drive.

I'll get back with idle timing and a curve.
 
AJ/FormS said:
A Sticking intake valve will allow the piston to push charge back into the intake causing a momentary increase in pressure.
Click to expand...
From the action of the needle,in the subsequently posted video, I agree with Mattax, who said
Mattax said:
Looking at it, I don't think its a valve issue, the way it wanders up and down.
Click to expand...

Have you got headers on that beast?
And if yes; are they properly sealed at the head, and at the collector, and in the merges?
 
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AJ/FormS said:
Have you got headers on that beast?
And if yes; are they properly sealed at the head, and at the collector, and in the merges?
Click to expand...

No, very stock exhaust manifold, new spark plug cables since picture
Exhaust manifold.jpg
.
 
What happens if you increase the in gear idle to 800 RPM and the base timing is set at 10* advanced? I do believe those are the factory settings. It has been awhile since I have been in a manual but I know the symptom you are describing can be caused by too low an idle setting.
Your compression is fine, you do not need a re ring, unless you have bottom end issues we are unaware of.
Remember, the fuel path through the carb is a lot different at idle than under load. Under load you are pulling fuel through the main metering system, at idle it is the idle circuits. By increasing RPM slightly you are allowing more of the idle slot to be exposed and allow more fuel to be drawn in.
Humor me, try the RPM increase. After using your timing light to see if you have spark scatter.
Hope this helps you. Good luck
 
I experienced a similar issue with my 340. Traced it back to a vacuum leak on the back seal of the intake manifold. Pulling the intake and applying good helping of RTV fixed it. Just a thought.

Be well,
Pat
 
Got some fair weather today to try some more.

Used my new timing gun, I would say timing is very stable at idle, now at 13 degrees with disconnected vacuum advance, might move an ½ degree if it moves at all.

Increaseing the idle speed a few hundred RPM does not affect the timing, stable at 13 degrees, putting gear into drive does not affect the timing either.

Tried to slowly block the carb in order to see if the idle smoothened out, but it got worse and was about to stall if I didn't remove my hand quick enough.

I see some fumes from the crankcase breater cap when EGR is blocked, so there is some amount of blow-by despite the compression test, is that the reason for the idle misfires?

Right now I have run out of ideas, is it ignition, air/fuel or mechanical related?

Here is a small video clearly showing the vibrations at idle:
 
Have you adjusted the idle mixture screws? Are they reactive? I mean does the idle slow down and seem like the engine wants to die if they are slowly screwed in?
 
SGBARRACUDA said:
Have you adjusted the idle mixture screws? Are they reactive? I mean does the idle slow down and seem like the engine wants to die if they are slowly screwed in?
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Oh yes, they do react, I use the vacuum-meter to adjust them as good as possible.

Adding a short video of the blow-by with blocked PCV as well:
 
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I'm sorry, but I can't hear a misfire. I do see the vibrations though. Did increasing the idle RPM reduce or make worse the mis-fires? What happens if you disconnect the EGR Vacuum hose? Can you manually depress the EGR valve? Are you suffering a power loss?
 
moparallen said:
I'm sorry, but I can't hear a misfire. I do see the vibrations though. Did increasing the idle RPM reduce or make worse the mis-fires? What happens if you disconnect the EGR Vacuum hose? Can you manually depress the EGR valve? Are you suffering a power loss?
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It´s me calling it misfires, maybe vibrations is a better word, but it´s not smooth at idle.

If I turn idle speed up towards 900 RPM most of it goes away, but it feels like driving with the choke step engaged, pulling a bit too hard and the need to adjust slow speed with the brake.

It almost stalls immediately if I pull the PCV hose, so I quickly cover the tube with my finger, PCV valve is new.
 
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The egr is normally close at idle. It should not have any effect at idle. Perhaps you have it hooked to the wrong port on the carb.
 
Sorry, I messed up EGR with PCV, the EGR valve is removed and blocked tight.

It´s a very simple PCV setup, looking just like the diagram under the hood:
Hoses diagram.jpg
 
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