Compression on Rebuilt Engine.

[AGrinthal]

Folks.. Not sure if I should be sick or not. spent the past year restoring my '52 cub. Looks beautiful. Went to give her first start attempt. Wouldn't start. Carb leaking gas.. So could have a float issue. so I will strip that down and check it out. She would ALMOST start ...out running the starter but then not catch.

However in the mean time I figured I would give a compression test. I had done the rebuild when she had two flat cylinders (more than likely hung up valves but I just wen through with the rebuild). I had a valve job done and the cylinders honed. The machinist said it was in "Spec" but next go around would require more than a hone. Ok.

I did my compression test (dry) and came out at about 6o psi for all four. I thought that was a bit low. I got a little sick when I read that "new" the readings should be in the 120 range. I wet tested them with 2 tablespoons of oil in each cylinder and they shot up (as expected) 100,128,110,90. Now here's my question. I haven't gotten this thing started yet.. so are these readings... (that I consider to be low) because the new rings haven't really had a chance to seat? They haven't gone anywhere yet. I hope that's the case.

I think she will run once I fix the flooding problem.. but I'm a bit worried about the pathetic compression readings.

Signed..
Worried in Andover..

[Eugene]

Wet compression readings are uneven, but engine will start and run on the compression readings.

Check your valve tappet clearance.

Suggest getting the engine to run, run or work the engine for a while. Then take compression readings again.

[twotone]

The low compression readings could be the result of engine flooding with gas and washing down the cylinder walls, not a real good thing for a new engine.

[John *.?-!.* cub owner]

On the next start, try it without choking. everyone of my cubs will fire up without choking, but you have to be ready with the choke to keep them running. If you choke it at the beginning it frequently floods, and gas runs out of the carb. As Eugene said, get it going before worrying about even compression. It will need to run a while and wear in before the readings really mean much, not to mention it needs the oil spread around good. Also, make sure throttle lever is about 1/4 or a little more forward, then back it down once engine starts. That sometiems helps with one that hits and kicks out.

[Hengy]

When I put new rings in Merlin several years ago, I had to pull start the tractor in order for it to start the first time. I'm sure that the compression was low because the rings hadn't had a chance to seat in. After pull starting it the first time, it would start at the first pull of the starter after that. The leaking from the carb, is it after you crank it for a while or does it just leak sitting there once you turn the gas on? It is normal for gas to weep from the hole in the bottom of the air intake on a "no start" situation. That is just unburned gas that is running back into the carb. If it is leaking just sitting there when you turn on the gas, it is a leaking needle valve or seat...either from the float being set incorrectly or from the valve needing cleaning.

Mike

[ricky racer]

What did the ring end gaps measure when you were fitting them?

[Steve Butram]

Check for Vacuum leaks at the carburetor and intake.

[AGrinthal]

Folks... thanks for the advice. I have found this forum priceless and I appreciate how everyone shares their knowledge. So to answer a few questions..

1. Don't know ring gaps from installation. I didn't think to check them. They all went in pretty easily (except one a buggered up and had to replace ... Bad Andy)

2. Haven't checked for vacuum leaks... I guess that would be some sort of "Audible Test" . I'm not noticing anything.

3. I did choke it on start up and she did flood. Now I had had that problem when I had gotten the tractor initially (I had it running twice before my tear down). When I went to buy it (estate sale). they couldn't get it started and it was flooding. I rebuilt the carb.. and was careful measuring the float clearances as that's usually the issues. She started up pretty easily after the rebuild. I think I still had drip issues though. It sounds; from what I'm reading here to be more or less normal. I had it running one other time before my tear down (no compression in two cylinders.. oil leaking out of torque tub on incline tube... I was planning a restore anyway). Return to present.... I haven't done anything since this post. I have a "Gas" drip hooked up to the carb line and I was going to drain it because I though the float wasn't seating and my gas (about a cup in my drip bottle) would just spill out on the my floor pan. I marked the bottle with a magic marker and the gas level dropped somewhat (See picture.. that's an ocean spray cranberry juice container). I would assume that it shouldn't move at ALL. So this indicates my float is not completely sealed. this change in fluid level is from Thursday night to Saturday morning ... or about 36 hours. Is this a leaky enough float to cause starting problems?

I did notice when I pulled plugs for compression test starting by the back of engine near dash.. that plug was MAYBE moist... not bone dry.. but certainly nothing unusual.. Now when I got to the first plug (Next to radiator). that puppy was very wet. I just thought that was odd that the cylinders were not all equal. But here again this is a fresh rebuilt engine that has not yet run.

4. I have the hood off (thus the drip) and the choke lever disconnected. I'm going to try starting without the choke. She was kicking over enough to outrun the starter but wouldn't start. ..then I had the flooding.

5. I would never have thought of pull starting ... and I'm a little confused as to why that may work (or be needed).


I am going to put the plugs back in and try starting it UNCHOKED. and see what happens. If my float is ok.. is there anything else I need to look at that would cause the flooding or is the flooding to be expected for an engine that doesn't start? Also after it floods do you have to pull the plugs and let it "dry out" or just let it sit a while, make your tweak and try again?

Thanks
Andy
Andover, NJ

[John *.?-!.* cub owner]

New rings need to have the gap set before installing. If the gap is too small, they will bind the pistons in the bore when it warms up. Too loose and they will have blow by. Not something you would be expected to know unless experience at rebuilding engines, but if you go to the pdf manuals at the top left of the page, and go to the service manuals, then the blue ribbon manuals
http://www.farmallcub.com/[ Sorry, direct links to manual section is not allowed. ]/index.p ... ls&order=0
you can read more about it. At this point in time I would probably get it running and see how it goes. if it dogs down as it warms up, you will need to pull the pistons and gap the rings. If that is needed, You will get plenty of help and advice on how by coming back to the site.

[AGrinthal]

John... I hope I luck out and all works well. I did NOT know that... And I've rebuilt other engines as well. (a '72 Mercury Capri... when I was in college many moons ago!) I will have to read up on it.
But just an update...
I did go back out just now and put the plugs and wires back in and DID NOT CHOKE her.. and she started right up... as did the massive oil leak in a Hole or plug to the right of the top of the oil filter. Looks like I missed something there (Not sure what ...looking through my manual. I shut her right down as first thing I wanted to see was oil pressure. Of course I put the oil in last Turky day (Cold shop.... so not much done over the winter).. and didn't save one of the oil cans.. I have to remember the grade I got...

But progress was made..

Thanks
Andy