Compression issues with a 1972 302

776 Views 5 Replies 4 Participants Last post by Doublecanister, Jun 17, 2021

72' mustang notchback

Discussion Starter · Jun 15, 2021

Hello, I was hoping for some ideas on what might be happening with the compression. The motor is a fresh rebuild that sat for 2 years. While driving it, there is absolutely no power to where a mini van could beat it. I checked the compression and cylinders 2 and 3 are at 90 - 95psi, and cylinders 7 and 8 are at around 80psi while the others all sit above 115psi. I readjusted the valves and it changed the compression by 5 - 10 psi. The other odd part is that the carb doesnt addust with the adjustment screw, it stays idling and wont speed up or stall. I tried to spray ether on the carb to see if there was a gasket leak to which there didnt seem to be any. The only things I can think of is that the piston rings are cracked or that the head gasket is shot. Any ideas?
Thanks!

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crjackson

· #2 · Jun 15, 2021

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Grimbrand

· #3 · Jun 15, 2021

It is possible that you've got more than one problem going on with your engine. The first thing I'd do is check to see if all your valves are working. You can just pull a valve cover, and have a friend turn the key while you watch the rockers. If you've got a flat tappet cam, it's very common to lose lobes, thanks to no ZDDP in the oil anymore. If your valves aren't opening and closing properly, that'd account for it.

115 is pretty low, not to mention 80-90! However, you do have a smog motor, and unless you've got aftermarket heads on it, the low compression ratio might account for that.

With no compression, it's going to be hard to troubleshoot your carb woes, but one thing that can cause your idle screws to become useless is having your idle set screw too high. With the throttle blades open too far, you uncover the transfer slots (a transition circuit intended to help the carb get between idle and the main circuit boosters). Once that happens, your idle air bleed screws don't even matter.

If just two cylinders are really low on pressure and the rest just fine, that's usually a blown headgasket between two cylinders, but frankly, the numbers you've got don't point to head gasket woes. I can believe that two of them would have a leak (and they are right next to each other!) but four? Not likely. And the number's too high to really account for head gasket problems, at even 80-90 lbs.

It's got to be rings (or at least ring seal problems - could be pistons or cylinder walls too) or valvetrain problems, I think.

Welcome to AFM!

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72' mustang notchback

Discussion Starter · #4 · Jun 16, 2021

Grimbrand said:
It is possible that you've got more than one problem going on with your engine. The first thing I'd do is check to see if all your valves are working. You can just pull a valve cover, and have a friend turn the key while you watch the rockers. If you've got a flat tappet cam, it's very common to lose lobes, thanks to no ZDDP in the oil anymore. If your valves aren't opening and closing properly, that'd account for it.

115 is pretty low, not to mention 80-90! However, you do have a smog motor, and unless you've got aftermarket heads on it, the low compression ratio might account for that.

With no compression, it's going to be hard to troubleshoot your carb woes, but one thing that can cause your idle screws to become useless is having your idle set screw too high. With the throttle blades open too far, you uncover the transfer slots (a transition circuit intended to help the carb get between idle and the main circuit boosters). Once that happens, your idle air bleed screws don't even matter.

If just two cylinders are really low on pressure and the rest just fine, that's usually a blown headgasket between two cylinders, but frankly, the numbers you've got don't point to head gasket woes. I can believe that two of them would have a leak (and they are right next to each other!) but four? Not likely. And the number's too high to really account for head gasket problems, at even 80-90 lbs.

It's got to be rings (or at least ring seal problems - could be pistons or cylinder walls too) or valvetrain problems, I think.

Welcome to AFM!

Awesome! Ill look into thoes and hopefully I dont need to pull the motor. Thanks for your insight!

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Grimbrand

· #5 · Jun 16, 2021

72' mustang notchback said:
Awesome! Ill look into thoes and hopefully I dont need to pull the motor. Thanks for your insight!

I should mention - don't start the car with the valve covers off. Pull your center spark plug lead or something. Otherwise the rockers will fling oil all over the engine bay and make an unholy mess. =)

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Doublecanister

· #6 · Jun 17, 2021

Vacuum leaks can make carb adjustments near impossible to notice.
Found this out on my 66' Gt and my '86 LX

Note: and this can be a Messy trick as I found this out quite by accident:
My 86 Lx was running one day and mucking around under the hood I just happened to spray the carb linkage with Wd40, when the spray aerosol was "in action"
the car's idle changed, let off the spray, it changed again.
I was like "WTF"??
I ended up finding I had lost a vacuum cap on the throttle body by doing this, apparently the Wd40 spray sealed the leak temporarily. I was like wow that was a neat trick for finding that.
But.... It can be messy and is also flammable so if you do this be careful, mind what you're spraying and how hot it is. we don't want any flash fires.

Also the compression readings you're getting could be valve related too, I had a bad head/valve job on my 66's 289 and had to get them redone. Still not sure they are right. I've tried using a vacuum gauge to check it and help set the carb to the highest most stable needle reading but it's not steady, heck I've done this on 3 diff vehicles and none have been steady.
Next up is a aluminum head swap if I can ever get to it.

Be safe if you do the wd40 vac leak test, (or get a smoker vac leak tester, a buddy got one of these and we used it on my wifes 07' Toyota and found a hose disconnected
that was causing a check engine lite). It used mineral oil to make the smoke so you can visually check for vac leaks, pretty neat tool.

Good luck with it,

T

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