Help! with 360 issues

[vtrigger77]

Hello everyone, i have a 1969 belvedere with a 1973 360 in it. rebuilt heads, cylinder walls look great, new edelbrock carburator, mopar electronic ignition. The issue i am having is that the copression is low on all cylinders, none of them go above 90psi. what can i check next? This is my first post here so if i am doing something wrong please let me know. thank you in advance.

 

[Curiousyellow71]

Are you holding the carb butterflies open while you are doing your compression test?

Wore out timing chain or retarded cam timing can cause low compression.

 

[MWbirdLOVER70]

I assuming piston rings 90 degrees apart and not all lined up together

 

[pnora]

Is your gauge accurate? Run a leak down test and you will know exactly why.

 

[Project66]

Yeah, that seems really low. Since it's all of your cylinders, not just one or 2, I suggest that you check your test equipment (gauge, hose, etc.) and your process just to make sure that is an accurate reading. A couple of years ago, I was chasing my tail, wondering why my vacuum was so low. One day, after yet another fruitless search for a vacuum leak, I realized that my gauge was, well like the picture below. Don't be like me...

Has the car run since the rebuild?

 

[vtrigger77]

i checked my compression tester with my sequoia and it worked fine. the car has been driven a few times since the rebuild. the engine was rebuild about 5 years ago and driven around the block every once in a while before we got the car. when we got the car 2 years ago we found out the heads were cracked so i replaced the heads. i changed the carburetor and distributor. it does have a slight rough idle kind of like a small misfire.

 

[PRHeads]

About 35 years ago I worked on a friends freshly rebuilt 360.
Done at a local performance builder……”9.5cr….400hp”, 2.02 heads, Ede torker plus cam & torker 2, 1850 carb, headers.
It would crap up the plugs quickly, and was a dog.

During my messing with it I discovered the cranking pressure was 100-110.
I worked on the carb, recurved the distributor, checked/set the lifter preload(273 rockers), etc.

I got the driveability better, but it was still a dog.

He drove it like that for years, until he decided he wanted something closer to an actual 400hp.

At this point I was working at a machine shop myself.
He brought it up to where I was working.
After pulling the heads, the next thing I did was check the deck clearance and piston dish volume, chamber volume.
The block had never been decked, nor the heads milled.

It was barely over 7.5cr and that particular Ede cam has loads of advertised duration.
That combo resulted in the low cranking pressure.

If the op has a low cr build(under 8:1) and the cam is retarded……..you can end up with under 100psi on the gauge, without there being anything mechanically “wrong”.

 

[vtrigger77]

i just checked it again.
Test 1=90psi
Test 2 carb butterflies open = 100psi
Test 3 carb butterflies open + a squirt of oil = 120psi

 

[Dutchrebuild]

Run it

 

[Geoff 2]

Really hard to tell with the info given so far. A that was installed in the retarded position will reduce the cranking pressure. Engine performance might be normal for that combo. Have you driven a car, or been in a car, of similar weight/gearing & a 360 to compare?

 

[Geoff 2]

Sorry, omitted the word 'cam' in second sentence above.

 

[Xp29h]

I assuming piston rings 90 degrees apart and not all lined up together

I had that issue with a fresh rebuild 383 hp! Not by me!

When I bought the car, the p.o. had just rebuilt it. When I spoke with the machine shop, the first thing they said was, “we did the machining but did not install the pistons!” That led me down a huge expensive 512 stroker rabbit hole and tearing apart the 383 to find the rings all lined up!

Moral of the story, unless you do it yourself, you just never know!

Unfortunately, sounds like it’s time to dive in deep. Certainly can start with timing chain and cam. But if that doesn’t do it..

Good luck!

 

[RemCharger]

Maybe with the cracked heads, the PO cooked the rings.. or they aren't seated..
The psi coming up with a squirt of oil is a clue..

 

[WileE]

What is the piston height ? What cc heads are you running ? What duration cam & setup ? Many questions. Methinks it's time to get right down to the real nitty-gritty.

 

[vtrigger77]

What is the piston height ? What cc heads are you running ? What duration cam & setup ? Many questions. Methinks it's time to get right down to the real nitty-gritty.

That is what i am thinking. i don't know anything about this engine besides that is is a 1973 360. i was thinking if this engine is worth it or try and find a 383 and rebuild that.