Technical 235 chevy compression problem ....

5 and 6 cylinder arent firing lost some power recently and idle got rough number 5 and 6 had very low compression 50 pounds . Pulled the head off and the gasket looks fine . Car wasnt smoking or burning oil . Can it be possible that the head gasket was bad woth no signs or leaking on the gasket on removal ? it is a 1960 235 chevy that was out in a 1950 chevy . Thanks for any insight

 

pictures of the gasket, and the block and head, might help us see what's going on?

 

Could be stuck valves, worn valve seats, or worn camshaft lobe. Compare the lift of cylinders 5 and 6 to the rest of the engine.


Edit: It's probably not a worn lobe. it that were the case it would most likely be smoking.

....Double Edit: If the head gasket were leaking you would most likely be burning antifreeze before you'd see any "blow by" from the gasket itself.

 

I did check lobes they move in a similar fashion compared to other cylinders

Here are some pictures of the head , block , and both sides of gaskets .

It is a later model felpro gasket i have no clue how or who installed it and if it was done properly but it did run good all last year up untill this spring

 

I'm not an expert. But every engine I've taken apart(4, so far) the cylinder walls are always somewhat clean. it looks like the walls are covered in carbon. This would indicate a very rich gas mixture. Perhaps cylinders #5 & 6 got washed down with gas and the rings don't seal anymore?

This is just a wild guess in the dark. Unless the camera lighting isn't right and making those cylinder walls look darker than in real life. Don't take my word as fact.

 

They do have a decent amount of carbon on them

 

Were the head bolts all tight?

This is a good mystery...

 

Yup they were tight . Possibly
Over tightened were very hard
To get off with a breaker bar

 

Take those heads, flip em upside down and with the spark plugs in fill the combustion chamber with water. if the heads hold water with no leakage then take an air compressor and blast air into the intake and exhaust ports. if you see any bubbles or disturbance of the water at all you'd have leaky valves. Though this is most likely not the case. it's a quick way to test valves.

Another thing it could be is cracked or excessively worn rings in those two cylinders only. to check for that pour some thin oil(even WD-40 will work) onto the pistons, just enough to see if you have leakage past the rings. if you do find that its leaking, do it on a cylinder you tested that had good compression and compare.

What do the spark plugs look like compared to the rest of the motor?

Loss of compression in cylinders 5 & 6 and the engine doesn't smoke at all? I'm starting to run out of ideas.

 

No smoke at all ..... I did check the valves for leaking they dont . Will check the rings later on tonight . There does seem to be a decent amount of wear on all these cylinders and they appear to be glazed over pretty good . But the good cylinders look the same as the bad ones .

 

There are only 3 places to lose compression and that is valves, rings, or blown gasket. Too bad you did not do a compression test after oiling the cylinder, if compression comes up it's the rings, if not it's the valves or a badly burned piston or blown gasket.

If the gasket is good and the valves check out ok as per Floogan's test then it can only be bad rings.

 

Yea that is the only thing i didnt get to do my dad pulled the head before i had a chamce to tell him lol

 

I tried just your air method and the valves leak i did not try with air pressure before . Thanks for the tip

 

What's the story with #5 intake valve and why is it wet ? #5 & 6 cylinder share the same intake runner. Possible crack in the intake runner at #5 or valve pocket. A leak down test before disassemble is the way to go in cases like this along with a pressure test of the cooling system to see if it holds pressure.

 

It was wet from water when i was checking for leaks

 

Did you or anyone else recently adjust the valves then this issue developed ? If so, valves were set to tight.

 

If you remove the oil pan, you should be able to remove the offending pistons to check the rings. I see that the valves have already tested bad, so this may not be necessary.


I'm not sure of the differences in a '60 chassis, but from club hotrod.com according to the 49 to 53 Chevy shop manual this is the procedure:

1. Remove bolts and lockwashers attaching steering idler and third arm and bracket assembly to frame front cross member and let assembly drop down. NOTE: Carefully note number of shims used between upper mounting bolt and cross member, so that same number may be installed when replacing assembly.
2. Drain oil and remove flywheel housing underpan, flywheel underpan extension and oil pan.

Install: Install oil pan using a new oil pan gasket. Tighten oil pan flange bolts to 6-71/2 ft. lbs and the oil pan corner bolts to 121/2 to 15 ft. lbs., refill with oil. Install flywheel underpan extension and flywheelhousing underpan. Install steering idler and third arm and bracket assembly as per instructions in section 3 of the manual.

You put all three bolts in finger tight. Tighten two lower to 30 ft lbs, remove the upper bolt and check space between front suspension cross member and bracket at upper mounting hole. Maximum allowable space is .008 inch. Install uper bolt and tighten to 65 ft lbs, but a torque wrench won't fit so you have to guess at it.

That's why some will raise the engine rather than drop the third arm.

 

Havent toutched the valves never adjusted them lol