1979 engine really bad compression numbers (1 Viewer)

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Cruiser is a 1977, with a '79 engine. Rebuilt head installed ~ 9 yrs ago-all new valves/springs/guides, and OEM head gasket. Daily driver. Recently have had to re-adjust dizzy point gap more frequently to keep good vac. Engine started to slowly sound like a cylinder was misfiring. Installed new plugs/points/wires/cap/rotor. While the plugs were out, checked compression(all plugs out and very cold-55 degrees ) #3 cylinder was essentially zero(maybe 10 psi) #2 was ~ 75 psi. All others were > 130 psi. The engine gradually gets better vac after warming up for ~ 5 mins(19 inches--16 at startup)) and the misfire sound seems to go away. Exhaust pipe is fairly clean-no oily residue. Compression has not been checked hot.
Any ideas here from the group? Head gasket maybe?
 

3_puppies

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central american rig? US 79 would have been electronic distributor.

I was thinking head gasket also, but if it goes away then not seeing how it is a head gasket.

compression should be checked when warm, also throttle wide open.
if compression is low when hot/warm then you could do a leak down test to figure out if it is head, upper end or rings, lower end
 
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central american rig? US 79 would have been electronic distributor.

I was thinking head gasket also, but if it goes away then not seeing how it is a head gasket.

compression should be checked when warm, also throttle wide open.
if compression is low when hot/warm then you could do a leak down test to figure out if it is head, upper end or rings, lower end
Ca truck--Jun 77 build date--engine was replaced w/79 in 1985.(the teenagers thought this thing could run at 70 mph---it didn't like it at all)
Cold compression test is certainly worst case--but 4 cyls are ok(>125). Plug condition shows 5 cylinders all tan color. # 3 had black deposits(not bad, or wet).
Last valve clearance check was done 6 yrs ago-hot, engine running-no adj required.
You are making me think it's the lower end---
 
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Do the leak down test. If you hear air escaping from the carb, it's probably intake valves. If you hear it at the tailpipe, it's probably the exhaust valves. If you hear air escaping at the oil fill plug, probably time to hone the cylinders and re-ring. Either way, do a valve job to get it running right. A '79 2F running properly will do 75mph all day. You are looking for about 155 psi in all cylinders, but it will run with as little as 110. There is also the possibility that you have worn the lobes right off your cam. Always use a ZINC additive with each oil change to protect the lobes and lifters.
 

SNLC

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Do the leak down test. If you hear air escaping from the carb, it's probably intake valves. If you hear it at the tailpipe, it's probably the exhaust valves. If you hear air escaping at the oil fill plug, probably time to hone the cylinders and re-ring. Either way, do a valve job to get it running right. A '79 2F running properly will do 75mph all day. You are looking for about 155 psi in all cylinders, but it will run with as little as 110. There is also the possibility that you have worn the lobes right off your cam. Always use a ZINC additive with each oil change to protect the lobes and lifters.

This. ^

Don’t proceed without a leak down test. Valve could be stuck which causes the compression numbers you are seeing. Also burned exhaust valves are common on all Cruiser engines across the decades, an inline-six thing in my opinion but if you have a burned exhaust valve compression will be low.

Also tip: If a Cruiser has sat and you are getting it going, lube the cylinders down the spark plug holes before starting. Also turn it over by hand before starting.

We have seen a number of low mile Cruisers in my shop that show low-ish compression on at least one cylinder. These were known to not be ran for many months if not years at a time or just started up and let idle than shut down.

This is not good practice to keep the miles down on a low mile Cruiser or good for engine longevity. What happens is condensation forms in the cylinder walls. This causes rust, when you start it after sitting it causes wear in the piston rings at a minimum and results in lower compression numbers. The best thing you can do for an old engine is run it. I suggest 25-50 miles a month of varied driving putting a load on the engine. The best is back roads where you are getting up to 45-55mph with at least some slight hills. And down shift in those decents vs ride the brakes, this is good for your engine. Up shifting and down shifting through all the gears is a good thing for more than just the engine. If you have a good engine don’t be afraid to run it up to 3,000rpms once in awhile.

Cheers
 
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All good info--thank all of you folks for the suggestions--the last head I had was cracked like a spider web--even into the valve seats-the shop that checked the head told me "you must have been doing a lot of idling to cause all of this crud on the valves"--(two of them exhaust)---the suggestions sound perfectly reasonable--
So the first check is to do the leak-down--if that fails, then pull the head and do the valve thing(I'm thinking that a valve adjustment would be indicated if ALL(or most) of the cylinders showed lowered compression--ie normal wear should occur on all the valve connections somewhat equally--not on just one--if I am fortunate, the findings will be as suggested and the valves will show the tale---the cam wear possibility is scary---
Thank you to all who replied and offered suggestions--followup as soon as I have definitive data---

Gary
 
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So the first check is to do the leak-down--if that fails, then pull the head and do the valve thing

No..........the first step is to check the valve clearances. A rebuilt engine I bought had 30K on it, and the clearances were way off - some valves had zero clearance. This was not good for the engine. I adjusted the valves, and almost all the compression came back.
 
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Update: Checked compression again-this time engine hot(150 deg)--I only checked the two that were low(#2 and #3) and got the same numbers I did when cold.

For So Co.--the rebuilt head was installed around 2010. Valve clearances were set then(hot).
Fast forward to ~ 6 yrs ago--the valve clearances were checked again(hot) while I had the cover off to replace a leaking gasket. All clearances(except #1 and #4) were within spec--1&4 were too loose(exh) by about .002 thousandths(I would expect this due to wear)--Adjusted back to .008 per FSM--

Seems to me normal wear would cause the clearances to open up(as evidenced by the results found on #1 and #4), and result in the valves being closed(more) when they are supposed to be open. In the case of low compression, if the valves are open when they are supposed to be shut, I'm thinking maybe crud is under the seats, or the adjusting locknuts have loosened and the adjuster screw has tightened up--is either of these possible?
 
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This is interesting... how does one check brake booster function to ensure no valve burnout? (I presume they just burn out because a bad booster leaks air to the intake side, causes the mixture to go lean and get too hot)....
 
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This is interesting... how does one check brake booster function to ensure no valve burnout? (I presume they just burn out because a bad booster leaks air to the intake side, causes the mixture to go lean and get too hot)....
That is my understanding of how it happens. You could prob check the vacuum at the the small check valve connection that runs to the intake manifold. With engine off, remove the hose from the booster chk vlv, then hook up a cooling system vacuum tester to the check vlv port. Pull 12-14 in Hg vac on the port, and watch to see if the gauge reading is steady - if the vac decreases, it might indicate a leak in the booster diaphragm, or the chk vlv gasket.
 
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Here's a faster way to check your booster for leaks: Start and run the truck for a minute or two and shut if off - this puts vacuum in the booster. Wait 10 minutes and try to remove the brake booster check valve. It should generate a vacuum sucking sound (whoosh) as air enters the booster. This tests the booster, the check valve, and the grommet. If your brake system won't hold a vacuum for 10 minutes (arbitrary), you have a problem.
 
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Poked a video camera into #3 cylinder today--it's a cheapo camera-doesn't have recording capabilities, but I was able to see some images of around the valves and the piston top--
The intake vlv looks pretty clean--the exh is dirty, but I couldn't see any gobs of stuff on the half side of it that looked like crud on the seat(the other side I couldn't see may indeed be gunked up)--
The piston top, however is a different story-that looked like a volcano landscape--there was crud everywhere-black, and a lot of it white(which I think is a reflection of probe light)-
My assessment is that even if I did a valve adjust, it would not help with the trash that appears to in there--maybe I could probe it again and try to get pics of the screen display----
 

Dizzy

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My set-up is a bit sweeter than stock. The header is bolted in front of #1 and behind #6. With that, I can remove the intake manifold and look at the valves from the top without pulling the exhaust system. My header is the same thickness as the intake manifold ears, so I don't have to mess with special gaskets, or material to scrape.

With your camera, I could get a detailed look. If there is too much oil at the top of a valve, it might form a crust, and keep the valve from closing properly, thus creating extra space in the valve lash, then it would require your adjustment, but it still isn't closed, which is what the clearances are supposed to be measured from. If the exhaust valve is worn, it is badly pitted right at the seat, and, there is your leak. How old are the valve guide seals? I've worked on three heads, and they all had bad valve guide seals.

If your rings are shot, you would have oil being pushed thru the PCV valve, and into the intake. I once heard that a blown head gasket can cause an uncapped radiator to bubble. If there is water in the oil, then you got foam in the crankcase.

It is barely possible to incorrectly set valves if you are a bit distracted and TDC isn't verified by the distributor rotor, or you don't notice the general pattern of the cam at cylinder #4. I did that once, but I caught the mistake, but I could see how someone might also miss it.

I took a Craigslist head apart one time, and found that a valve was bent. I had to use an abrasive cutting wheel to get it out. I always wonder about combustion that happens on the intake or exhaust stroke, could it bend the valve?
 
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Cruiser is a 1977, with a '79 engine. Rebuilt head installed ~ 9 yrs ago-all new valves/springs/guides, and OEM head gasket. Daily driver. Recently have had to re-adjust dizzy point gap more frequently to keep good vac. Engine started to slowly sound like a cylinder was misfiring. Installed new plugs/points/wires/cap/rotor. While the plugs were out, checked compression(all plugs out and very cold-55 degrees ) #3 cylinder was essentially zero(maybe 10 psi) #2 was ~ 75 psi. All others were > 130 psi. The engine gradually gets better vac after warming up for ~ 5 mins(19 inches--16 at startup)) and the misfire sound seems to go away. Exhaust pipe is fairly clean-no oily residue. Compression has not been checked hot.
Any ideas here from the group? Head gasket maybe?
leakdown , leakdown test why ??? it will give you a good starting point for probs < rings, valves , head gaskets, get back to basics stop guessing and get back to auto shop 1
 

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