Help after A3-152 rebuild needed

[Chrisf]

I just finnished up a rebuild with all new liners, pistons,rings,crank etc. The problem that I have is low compression on cyl 2 & 3. I have 400 on cyl 1 and 300 on the other two. I thought it was just a ring seating problem so I ran it for about 30 minutes but no change. It"s super hard to start and only runs one cyl 1 for several minutes before the others come online. Does anyone know for sure that all the cylinders should be at 400? I"m leaning toward buying a new head but am looking for other thoughts before I spend the money. Thanks for your help. Chris.

 

[Phil Tibbetts]

Did you grind the valves and seats and lap them in good? If you did check your valve clearance again.

 

[Chrisf]

I had the head milled and the valves checked but not ground or lapped. They didn't see a problem but it didn't presure checked either. Do you know what presure should be or at least if they should be the same?

 

[Anonymous-0]

I would not buy a new head, but I would make sure the valve recession or protrusion is within spec. Did they take the valves out? If they did and they did not grind the valves or seat them they might not seat as well as they did before. Do you have the valve specs or should I look?

 

[Phil Tibbetts]

I don't know off hand what the compression should be but I'm sure someone on here does. On the Perkins 236 diesel the compression ratio is 16 to 1. If you didn't at least lap in the valves to be sure they are seating I would start there. There shouldn't be 100 psi difference.

 

[Phil Tibbetts]

Just found my MF 35 manual and according to that the compression ratio is 17.4 to 1 on the 152 engine.

 

[Chris F]

I do have the specs. but am not sure what to do with them except send the head back out for seats and valves. Do you think that I can just buy lapping compound and spin them in a drill to lap them back in?

 

[Phil Tibbetts]

If you don't have a valve spring compressor to remove the springs from the valves you wont be able to lap them in. They are lapped in with a special tool that turns back and forth. It is not that expensive if you buy the hand version, it just has a suction cup on the end. But if you want a good job the seats and valves should ground be to reduce the contact area. All the specs are in the manual.

 

[Phil Tibbetts]

I for got to mention the valve guides may be worn also.

 

[DavidP,South Wales]

Hi Chris,Most things have been suggested but I have a couple more to add.
Is there any possibility of any rag or paper left in the inlet port of the head which would cause a restriction of air?
If the valves have been checked can you be certain that they have been replaced in their respective ports. Were the valve stems cleaned of any carbon build-up. It is quite likely that as the valve turns, any carbon present will prevent it from closing fully.
If the head face has been skimmed the valves should have been removed to do so. There is a possibility that some swarf remained in the head and is now stuck on the valve seat preventing it closing. (Had this happen).
Valves should always be lapped in and replaced in correct order. The lapping in motion MUST be back and forth with either fine or coarse paste and lubricant applied (not oil).
DavidP

 

[Chris F]

well I went the the store and bought the lapping compound. It did clean up the faces but after I put the head back on, it only raised the pressure 25 psi on 2 and 3. I'm thinking that the only thing left is the possibility of the flanges on the liners being too low. The specs say .004 but they measure ~.010. Anyone think that a leak between the liner flange and the head gasket is possible?

 

[Anonymous-0]

Can you rig up to make a cylinder leak down test? Did you take a feeler gauge and a straight edge across the face of the valves and then measure that distance from the deck of the head? Compare that reading to the other two cylinders. Without looking the specs for intake & exhaust valves may not be the same. Get your liner height in spec or you will have head gasket problems. Can the shop pull a vacuum test on the valves for you ?

 

[Joe Ferguson]

Leak test

We made an air hose adapter from an old injector put it in the place of injector put 120 PSI air pressure in it with piston at top firing position. Then if air escapes in exhaust then exhaust valve/seat is defective...if air escapes in intake manifold then intake valve/seat is defective...if air escapes by the oil filling cap then rings/sleeve/piston is defective....if air escapes (bubbles) in radiator then head gasket/liner position is defective then we repeat the test for each cylinder.. Tractor must be in gear and brakes applied to avoid engine turning during the test.

My 2 cent

Joe Ferguson

 

[Joe Ferguson]

Forgot to mention....
Same test may be done on a gas engine just make an air hose adapter from an old spark plug.

Joe Ferguson

 

[Chrisf]

To make a long story short, it was the valves. Now on to trying to get the precombustion caps to stop leaking!