Cylinder Compression Am I Screwed

[ruckin69]

Truck has been running like crap so it was suggested that I checked the compression of the cylinder heads. It is a 71 stock 302 original in the truck and here's what it read. Is it time for a new motor or is there a less expensive route? Thanks.

1- 95 then 100 psi
2-140 psi
3- 130 psi
4- 140 psi
5- 125 psi
6 - 130 psi
7- 140 psi
8- 140 psi


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[blubuckaroo]

How did the plugs look?

 

[ruckin69]

Here are 4

Plus these were the plugs. They were not the right ones for the truck. That's a whole another issue. Finding the right plugs was also a nightmare.

 

[blubuckaroo]

Those look pretty good. Did #1 look different?
A number of things can lower the compression on a particular cyl. Could be a valve adjustment, leaky valve, leaky cylinder head gasket, or rings not seating.
I'd try a wet cylinder compression test to verify the rings are ok.

 

[B RON CO]

Hi, first I'll say that even though a couple of cylinders are a little low it is probably not the end of the world.
When new, you would get @ 100,000 to 125,000 miles before valve job time.
But now engines go much farther.
Crappy performance could very well be worn points and condenser, bad wires, plugs, filters, timing, etc.
I would get the correct tune up parts and see if it is better.
You can't work on the carb until the ignition is in top shape.
Good luck

 

[cs_88]

You could rotate the low cylinder over to the compression stroke and then put some compressed air into it and see where the air goes.

 

[ruckin69]

I'm not going to mention his name but there's someone that is very good at working on Broncos and he said the motor needs to be replaced with those 2 low cylinder readings. I will do it but I am not happy about it. Do you think this motor is worth salvaging or should I start from scratch and dropped 5 to 7k? I just put a brand new distributor cap and rotor on brand new plugs and harmonic balancer. Still running like crap.

I will straight up say that I am not a mechanic and have very little experience with motors. They are way over my head. I have a Holley 374 Barrel on the truck as well. Do you think sending this for timing would work? I do not use this truck very often but do you plan on keeping it. Decisions decisions.

 

[blubuckaroo]

Ruckin69,
If this really is an all original '71 engine, then it's likely your loosing valves because those weren't made to run on modern unleaded fuel.

 

[Steve83]

A compression test is only valid if it's done right. Ford's method is in the caption of this diagram:


(phone app link)

 

[cldonley]

I would not replace a motor just based on compression results. Those don't even look that bad. As said above, make sure you're doing the compression check correctly. Then make sure you've corrected anything that could be making the engine "run like crap", like bad points, rotor, cap, wires, plugs, vacuum leaks, timing, etc. CS_88 has a good point. That's the only way we do compression checks on aircraft reciprocating engines, and when you have a low cylinder you either hear air coming out the carb (intake valve leaking) out the exhaust (exhaust valve) or through the oil filler cap (rings). It may be as simple as running something through the carb throat to remove carbon deposits so a valve will seat. In aircraft we 'stake the valve' by removing the valve cover, removing the rocker arm from the offending valve and massaging the valve stem (using a brass punch and a hammer, strike the valve stem and allow the valve to "slam shut" to knock carbon deposits off). That would be a heck of a lot cheaper than replacing a motor.

 

[Broncobowsher]

Does it burn excessive oil?
yes, the motor is shot
no, keep running it. While not perfect there are a lot worse on the road.

 

[DirtDonk]

Do you have another thread on this poor running situation? If so, are you posting up those numbers over there too, so anyone involved with the prior diagnosis can see what the test results were and maybe add more info to what has already been discussed?

Those don't look like an engine in need of an immediate rebuild to me either. If this engine has not been driven much over the last few years, you might even increase compression by driving it regularly.
Granted, at some point it's time to fix it. But just how bad is it running and as asked, does it burn oil, stall out, spit back, or do any other nastiness? Or just a little rough running?

Paul

 

[ruckin69]

Thank you for the input. It's not burning oil nor backfiring. Okay I will start doing some more homework and take it to a qualified mechanic who knows these older engines. The main situation is that it stalls while driving when I let my foot off the peddle. It runs a little rough but it's not terrible at idle in park. I'll take it to a shop and let you guys know. Hopefully it just needs timing or a carb rebuilt. I don't take this truck off road and really just drive it leisurely when it's a nice day so I'd really like to not have to go to the extremes of replacing the motor if I don't have to.

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[cs_88]

It shouldn't stall with low compression in one cylinder.

 

[ruckin69]

Okay so here's a video of the motor running. I'm actually took it because of a wobbling harmonic balancer but anyway maybe this will answer some questions maybe it won't. Thanks for all of your help. That rattling was the fan shroud.

https://youtu.be/Btase9Z0TJk

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[DirtDonk]

Well now that's an annoying sound! Didn't sound like it went completely away when you put pressure on the shroud either though. You sure that's all it is, and not something else in addition?

The lower pulley sure is wobbling like you said. Not the worst I've seen, but as you figured, the smooth running of the upper two pulleys is what you want them all to look like.
Could not see in the vid, but can you tell for sure it's the damper ring and not just the pulley? Be nice if all you had to do was replace the pulley, but then again the dampers are easier to source than pulleys sometimes!

For the "crappy" running you were talking about, have you ever done a quick tune up yourself? Simple things like replacing the plugs (you're already at that point, so no trouble there it seems) and plug wires and a quickie turn of the idle air screws here and there?

And have you checked for vacuum leaks with some fluid yet? Lots of crappy running engines simply have a hose that popped off of of a fitting, or a cap to prevent such things has deteriorated to the point it's no longer sealing.
VERY common indeed.

Good luck.

Paul

 

[blubuckaroo]

Sounds pretty good to me, except for the rattle.
Like Paul said, tune it up and run it for a while.

Lucky guy! You've got a lube pit!

 

[ruckin69]

Sounds pretty good to me, except for the rattle.

Like Paul said, tune it up and run it for a while.



Lucky guy! You've got a lube pit!

I will keep you posted
I am going to get it to an old timer was knows these and see what he says and then give you an update. I was wondering who was going to notice the pit. Yeah that seconds as a bunker for the end of days LOL.
Molon Labe

 

[DirtDonk]

You can also remove the fan belt(s) to see if any annoying rattles go away. Or stay...
At least that proves it's something with the accessories, or eliminates them as a possible culprit.

Paul

 

[Dirtroadjunkie]

Did it slowly begin to run bad, or did it suddenly show up. You mentioned that you only drive it once in awhile. If it's running bad after sitting for the winter I would guess the fuel got the carb.