Rebuilt engine with low compression?

[steppinthrax]

This is kind of an advanced topic

I just finished rebuilding a Kohler kt17 2 cyl tractor engine. This was with new rings, head gasket, lapped valves and stone honned cylinder walls.

Upon getting it all back together I did a initial compression test and was getting around 60 psi in one cyl and around 65 or so in the other (min specs for this engine should be around 90).

I started and ran the engine for a few minutes and I'm getting the same!!!!

Both cylinder walls were checked for roundness and the new rings were inserted into the bore. The ring gap was within the min/max. The walls were honned with a 23 degree cross marks.

I was told for a rebuilt engine, compression results don't mean shit. Just curious if anybody know this to be correct?

 

[steppinthrax]

Forgot to mention.

When running the engine for 10 mins or so. I did another compression check. This time I get 20psi or so on each cylinder on a HOT engine!!!!

Is 20psi even possible?

Anyway the engine sounds good, no oil burning, no bad sounds etc. It runs well...

 

[Slacker]

Try a different gage

 

[steppinthrax]

Try a different gage

Did that, bought a new compression gauge (actron) from the store and did that, same results.

 

[boomerang]

Compression rings installed upside down? Do you have the throttle wide open when you're doing your checks? Valve clearance set correctly? Is there a built in compression release?

 

[steppinthrax]

Compression rings installed upside down? Do you have the throttle wide open when you're doing your checks? Valve clearance set correctly? Is there a built in compression release?

Engine has no compression release. I checked the valve clearance and this engine uses no shims. So I checked the tappet to valve stem clearance and it was ok.

I pulled the pistons already once for different reasons before (after rings installed) rings were good.

I removed the entire carb and intake plenum just to ensure there was no intake obstruction and I'm getting the same.

I have a suspicion it might be leaky valves??? Or engine just needs to break in?

 

[RLGL]

What is the clearance between the pistons and cylinder walls? What is the spec? How much taper in the cylinders? Warped valve or bent stem?

 

[boomerang]

Your low compression reading on a hot engine points to valves. But, did you check the cylinder head mating surface for flatness?

You can squirt a bit of oil in the cylinders and recheck your compression. If it rises, it's rings. If it doesn't it's valves, valve timing or head gasket issues which could be related to warped heads.

I would rule out valve timing because your initial readings were far higher and close to one another. Your other option is to run it a while and see what readings you're getting after giving the components some time to get happy with each other. But once again, because it is dropping so dramatically when hot, that points to valves or problems that are head sealing related.

The motor may run well but under load it's going to poop out. I think you know that already.

 

[steppinthrax]

Your low compression reading on a hot engine points to valves. But, did you check the cylinder head mating surface for flatness?

You can squirt a bit of oil in the cylinders and recheck your compression. If it rises, it's rings. If it doesn't it's valves, valve timing or head gasket issues which could be related to warped heads.

I would rule out valve timing because your initial readings were far higher and close to one another. Your other option is to run it a while and see what readings you're getting after giving the components some time to get happy with each other. But once again, because it is dropping so dramatically when hot, that points to valves or problems that are head sealing related.

The motor may run well but under load it's going to poop out. I think you know that already.

Thanks,

That's my suspicion. I'm going to have to pull the barrels and see what's up.

If I do a real good lapp job (drill lapp) v.s. suction cup. Will that correct this?

I want to avoid taking it down to a machine shop...

When the engine was running I ran my hand around the head, no puffs of hair. I doubt it's the cylinder head.

 

[twinrider1]

You can also pressurize a cylinder (put it on TDC) and listen for where the leak is. If you hear it coming out of the exhaust, it's an exhaust valve. Hear it from the intake, it's an intake valve. Hear it from the oil filler tube, it's the rings. Do it for each cylinder.

That both cylinders are down is probably going to turn out to be relevant.

 

[boomerang]

That's my suspicion. I'm going to have to pull the barrels and see what's up. If I do a real good lapp job (drill lapp) v.s. suction cup. Will that correct this?

Pull the barrels? Why, because of clearance issues? It looks to me like removing the heads and the tappet covers will allow access to the valves, springs and keepers.

Lapping is fine if the seats and valves are in good shape to begin with. It's just a good thing to do. But it should be closely checked afterwards either by purely visual means, (gray rough looking band fully around the valve and around the circumference of the seat, or by using machinist's dye and making sure the dye is rubbed off once again, around the whole valve sealing surface and the circumference of the valve seat.

It's not a fix for worn seats, worn valves, valve guides or stems worn out of spec.

Also, I have to mention it, but I noticed that the specs for valve clearance are quite different for the intake and exhaust valves. Are they different from each other? Any chance they got swapped? Exhaust in the intake and vice versa? Some valve stems are filled with heat transfer material. Swapping them could lead to wildly changing clearances from cold to hot.

Lastly, the method to adjust valve clearance is to grind the end of the stem. That's something that needs to be done on the right equipment. In other words, you're never going to get it square and flat holding it against the side of a bench grinder wheel. What I'm saying is that you may need to take the barrel assemblies somewhere to get the valve clearances set correctly as it can be a bit of a trial and error process. As an example of what I'm saying, if you need to take .004 off and the first dusting across the end of the stem shows it be out of square to the stem, you have to clean it up with minimal stock removal, reinsert it to check the clearance and go from there.

 

[tracerbullet]

FWIW I agree w/ boomerang, I think it's in the head as well, specifically the valves. Sounds like they just aren't seating correctly. As was also mentioned I would try the pressure test check to be sure before you did any more work. With a little work you can find or fab a way to connect an air compressor through a spark plug hole. You might be looking at replacing or even installing rings in the head and / or replacing the valves.

It may not matter but I'm curious, if the valves aren't shimmed how are they adjusted? Are there hydraulic lifters?

 

[steppinthrax]

Pull the barrels? Why, because of clearance issues? It looks to me like removing the heads and the tappet covers will allow access to the valves, springs and keepers.

Lapping is fine if the seats and valves are in good shape to begin with. It's just a good thing to do. But it should be closely checked afterwards either by purely visual means, (gray rough looking band fully around the valve and around the circumference of the seat, or by using machinist's dye and making sure the dye is rubbed off once again, around the whole valve sealing surface and the circumference of the valve seat.

It's not a fix for worn seats, worn valves, valve guides or stems worn out of spec.

Also, I have to mention it, but I noticed that the specs for valve clearance are quite different for the intake and exhaust valves. Are they different from each other? Any chance they got swapped? Exhaust in the intake and vice versa? Some valve stems are filled with heat transfer material. Swapping them could lead to wildly changing clearances from cold to hot.

Lastly, the method to adjust valve clearance is to grind the end of the stem. That's something that needs to be done on the right equipment. In other words, you're never going to get it square and flat holding it against the side of a bench grinder wheel. What I'm saying is that you may need to take the barrel assemblies somewhere to get the valve clearances set correctly as it can be a bit of a trial and error process. As an example of what I'm saying, if you need to take .004 off and the first dusting across the end of the stem shows it be out of square to the stem, you have to clean it up with minimal stock removal, reinsert it to check the clearance and go from there.

Thanks

The valves are in the barrel. The barrels and valves are one piece unit. Two barrels for the 2-cyl engine.

The springs and keeps are in the barrels as well.

The SM for this particular engine simply mentions to use a file between the tappet and valve stem for adjustment. It even mentions lapping the valves.

 

[steppinthrax]

FWIW I agree w/ boomerang, I think it's in the head as well, specifically the valves. Sounds like they just aren't seating correctly. As was also mentioned I would try the pressure test check to be sure before you did any more work. With a little work you can find or fab a way to connect an air compressor through a spark plug hole. You might be looking at replacing or even installing rings in the head and / or replacing the valves.

It may not matter but I'm curious, if the valves aren't shimmed how are they adjusted? Are there hydraulic lifters?

I think that is what I"m going to do. I'm going to get a adapter or something that hooks up to my compressor and put in around 150 psi or so and see where it goes. Is there some smoke or something I can put in as well?

Purple smoke or something?

 

[boomerang]

FWIW I agree w/ boomerang, I think it's in the head as well, specifically the valves. Sounds like they just aren't seating correctly. As was also mentioned I would try the pressure test check to be sure before you did any more work. With a little work you can find or fab a way to connect an air compressor through a spark plug hole. You might be looking at replacing or even installing rings in the head and / or replacing the valves.

It may not matter but I'm curious, if the valves aren't shimmed how are they adjusted? Are there hydraulic lifters?

The only thing about a leakage test is that I would expect ring leakage on an engine that only has a few minutes on it. So, if it did, I couldn't be certain it was telling me anything. A leaking valve is another story. That would fall under the guise of the proverbial smoking gun.

 

[boomerang]

Thanks

The valves are in the barrel. The barrels and valves are one piece unit. Two barrels for the 2-cyl engine.

The springs and keeps are in the barrels as well.

I understand that. I have looked briefly through the manual. There is no need to remove the barrels to remove the valves. Remove the heads, remove the valve covers and there they are. Compress the springs, remove the keepers and the valves are out. But, maybe there is something I have overlooked.

The SM for this particular engine simply mentions to use a file between the tappet and valve stem for adjustment. It even mentions lapping the valves.

Wow! Filing, huh? That's pretty interesting in a scary sort of way.

Anyway, best of luck.

Oh, and you don't need anywhere even close to 150 psi. Hopefully you have a regulator you can use to ramp up the pressure. It will leak at very low pressure. You'll only need to increase it to the point that you are certain you are hearing what you think you're hearing and to locate the source of the leak. Air out the intake, the exhaust in your case into the crankcase which I would ignore for the reasons I gave.

 

[steppinthrax]

Wow! Filing, huh? That's pretty interesting in a scary sort of way.

Anyway, best of luck.

Oh, and you don't need anywhere even close to 150 psi. Hopefully you have a regulator you can use to ramp up the pressure. It will leak at very low pressure. You'll only need to increase it to the point that you are certain you are hearing what you think you're hearing and to locate the source of the leak. Air out the intake, the exhaust in your case into the crankcase which I would ignore for the reasons I gave.

Thanks,

I see your bold-ed response. There is a special tool to remove valve keeps in engines without clear access to the keeps. It's a vise looking tool that I simply don't have. The way I've removed and installed the valves is use a crescent wrench and my finger to keep the valve close while I wedge it between the "spring washer" to push it down and the keeps would dislodge and fall off. To put it back I would have the head upside down and I would actually use a wrench again and drop the keeps though the "tiny hole" where the tappet goes through.

The keep would kind of fall into the insert and automatically "engage".

I don't know if you can picture what I"m talking about. Kind of a poor man's method. I will see if they sell this at autozone for rental but I doubt it...

 

[slag]

You can also pressurize a cylinder (put it on TDC) and listen for where the leak is. If you hear it coming out of the exhaust, it's an exhaust valve. Hear it from the intake, it's an intake valve. Hear it from the oil filler tube, it's the rings. Do it for each cylinder.

That both cylinders are down is probably going to turn out to be relevant.

This. Pressurize the cylinder with a pressure tool with the cylinder at TDC. If you hear or see air escaping at the oil fill (with the cap off), its a head gasket or ring issue. If you see or hear it at the intake or exhaust, its a valve issue.

 

[Bartman39]

This time I get 20psi or so on each cylinder on a HOT engine!!!!

Is 20psi even possible?

Uh I have never seen an engine run with this low of compression...? Are you sure it is that low...?

I just cannot see an engine running with this low of compression...?

 

[LTC8K6]

Uh I have never seen an engine run with this low of compression...? Are you sure it is that low...?

I just cannot see an engine running with this low of compression...?

Yeah, that's what, 1.5:1 compression ratio?

Even a Model T was something like 4:1

 

[steppinthrax]

Guys. I was able to hook and air compressor to the engine cylinder with the valves completely closed.

Cylinder 1: The exhaust valve was leaking on one cylinder like a sieve. The intake OK. The exhaust leak was so bad I couldn't hear anything else.

Cylinder 2: Both valves were leaking but not so much. I did hear a slight hiss in the crankcase on this cylinder.

I pulled both heads and removed both valves. This time I did a real strong valve lapping job with a drill and suction cup. I lapped the valves until there was a strong wide band on each valve. I didn't take off much because I could still see a space between the tappet and end of the stem.

Unfortunately the new head gaskets fell apart upon taking off the head. I will need to order some new gaskets. I will as a test put some RTV between the head (temp) and do another test to see if the valves improved.

I guess another question: You should always hear some air leakage. Even in a new engine with new rings you should hear a slight hiss on the piston right????

I inspected the cylinder walls. I completely bottomed the pistons and ran my finger on the inside of the wall. No score marks or anything. THE HASH MARKS WERE STILL THERE!!!!! So I guess the cylinder honing I did was good.

Is it possible that the compression will increase when the hash marks disappear (glaze)?

Another weird thing is one cylinder is running real rich, while the other fine. It only has one carb and the intake branches off the carb. The plug on one cylinder was sooty and the piston face/valves where black/sooty... So I guess too much fuel is going in there. What would cause this??? If the carb is out of adjustment I would think both cylinders would run rich, right???

 

[Squeetard]

Guys. I was able to hook and air compressor to the engine cylinder with the valves completely closed.

Cylinder 1: The exhaust valve was leaking on one cylinder like a sieve. The intake OK. The exhaust leak was so bad I couldn't hear anything else.

Cylinder 2: Both valves were leaking but not so much. I did hear a slight hiss in the crankcase on this cylinder.

I pulled both heads and removed both valves. This time I did a real strong valve lapping job with a drill and suction cup. I lapped the valves until there was a strong wide band on each valve. I didn't take off much because I could still see a space between the tappet and end of the stem.

Unfortunately the new head gaskets fell apart upon taking off the head. I will need to order some new gaskets. I will as a test put some RTV between the head (temp) and do another test to see if the valves improved.

I guess another question: You should always hear some air leakage. Even in a new engine with new rings you should hear a slight hiss on the piston right????

I inspected the cylinder walls. I completely bottomed the pistons and ran my finger on the inside of the wall. No score marks or anything. THE HASH MARKS WERE STILL THERE!!!!! So I guess the cylinder honing I did was good.

Is it possible that the compression will increase when the hash marks disappear (glaze)?

Another weird thing is one cylinder is running real rich, while the other fine. It only has one carb and the intake branches off the carb. The plug on one cylinder was sooty and the piston face/valves where black/sooty... So I guess too much fuel is going in there. What would cause this??? If the carb is out of adjustment I would think both cylinders would run rich, right???

You found your issue, valves.

Yes, you should hear a slight hiss when running air to it, no engine seals perfectly. The difference betweeen air pressure applied and air pressure measured in the cylinder is the leak down.

Fix your valves before you think you are running rich. My guess is you were barely running at all. Causing soot and other issues.

 

[boomerang]

Guys. I was able to hook and air compressor to the engine cylinder with the valves completely closed.

Cylinder 1: The exhaust valve was leaking on one cylinder like a sieve. The intake OK. The exhaust leak was so bad I couldn't hear anything else.

Cylinder 2: Both valves were leaking but not so much. I did hear a slight hiss in the crankcase on this cylinder.

I pulled both heads and removed both valves. This time I did a real strong valve lapping job with a drill and suction cup. I lapped the valves until there was a strong wide band on each valve. I didn't take off much because I could still see a space between the tappet and end of the stem.

What about the valve seats? You could show full contact around the circumference of the valve with only have contact at one spot on the seat. IMO, the seats need to be recut and the valves ground or replaced. You already said you lapped the valves once. Valve lapping is done mostly to verify that there is good sealing between the valve and the seat. It is not intended to take the place of a valve job which as I said earlier entails recutting the seats and grinding or replacing the valves. If it is hard to tell if there is full contact around the seat and lacking any machinist's dye, take a black marker and use it on the valve seat. Put in the valve, press down while turning it one quarter turn. The residue from the marker should be removed or at the very least disturbed.

Unfortunately the new head gaskets fell apart upon taking off the head. I will need to order some new gaskets. I will as a test put some RTV between the head (temp) and do another test to see if the valves improved.

Head gaskets are not reusable. The test is not a bad idea for the reasons I gave above but the cleanup will be a bear. You could completely bypass this by making sure the valves are truly seating by using the procedures I outlined in my earlier post and that I have described in more detail here. Honestly, the RTV thing is kinda weird.

I guess another question: You should always hear some air leakage. Even in a new engine with new rings you should hear a slight hiss on the piston right????

Yes. Even if you have perfect ring sealing, you still have gaps in the two compression rings.

I inspected the cylinder walls. I completely bottomed the pistons and ran my finger on the inside of the wall. No score marks or anything. THE HASH MARKS WERE STILL THERE!!!!! So I guess the cylinder honing I did was good.

Is it possible that the compression will increase when the hash marks disappear (glaze)?

Some motors with 100,000 miles or even many more can be disassembled and the crosshatch will still show. The crosshatch is for better break in of the rings and for oil retention on the cylinder walls for long ring life. The crosshatch itself has little to do with compression from the perspective of it needing to disappear.

Another weird thing is one cylinder is running real rich, while the other fine. It only has one carb and the intake branches off the carb. The plug on one cylinder was sooty and the piston face/valves where black/sooty... So I guess too much fuel is going in there. What would cause this??? If the carb is out of adjustment I would think both cylinders would run rich, right???

You already got good advice on this. Get the compression up to snuff, get it running well and more than likely this problem will be taken care of.

 

[steppinthrax]

What about the valve seats? You could show full contact around the circumference of the valve with only have contact at one spot on the seat. IMO, the seats need to be recut and the valves ground or replaced. You already said you lapped the valves once. Valve lapping is done mostly to verify that there is good sealing between the valve and the seat. It is not intended to take the place of a valve job which as I said earlier entails recutting the seats and grinding or replacing the valves. If it is hard to tell if there is full contact around the seat and lacking any machinist's dye, take a black marker and use it on the valve seat. Put in the valve, press down while turning it one quarter turn. The residue from the marker should be removed or at the very least disturbed.

Head gaskets are not reusable. The test is not a bad idea for the reasons I gave above but the cleanup will be a bear. You could completely bypass this by making sure the valves are truly seating by using the procedures I outlined in my earlier post and that I have described in more detail here. Honestly, the RTV thing is kinda weird.

Yes. Even if you have perfect ring sealing, you still have gaps in the two compression rings.

Some motors with 100,000 miles or even many more can be disassembled and the crosshatch will still show. The crosshatch is for better break in of the rings and for oil retention on the cylinder walls for long ring life. The crosshatch itself has little to do with compression from the perspective of it needing to disappear.

You already got good advice on this. Get the compression up to snuff, get it running well and more than likely this problem will be taken care of.

Thanks,

I don't have any Prussian Blue with me. Just to be clear on the procedure. I take a black sharpie and do both the seat and the valve. I spin it a few times and it the seat is making contact I should see a ring (OF NO INK) on both the valve and the seat??

Yeah I know the RTV is weird, just that I will have to order these gaskets, wait for them and all this shit. I won't run it with RTV just want to abs verify.

This is a simple cylinder head (aluminum billet style). There are not coolant passages. They sell (all metal) head gaskets that have a history of being re-used. People would spray some copper around them and slap them back on. The one's I had were the fiber type with a very thin metal foil core (came in a gasket pack!!!)

 

[jaha2000]

I would only use the marking on one side, either the valve or seat.