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Discussion Forum

Engine trouble???

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05-21-2001 20:29:03

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Last fall I put new pistons, rings and liners in my smta. I also had the valves ground and some of the seats were a bit narrow. The tractor does not seem to have the power it should. I ran a compression test on all cylinders with it cond and they showed 60 pounds, even in all cylinders. With the tractor warmed up but not to operating temp all cylinders were at 90 pounds plus or minus 2 pounds. I haven't really had a chance to work the tractor hard, I am wondering if maybe the rings haven't seated. It probably has 20 hours on it since the overhaul mostly light work with snow blade and harrow. What do you think? I don't have a job to put it on a hard pull for an extended time, thinking about taking it to a shop and running it on a dyno for a couple hours. Any suggestions appreciated.

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05-23-2001 06:58:27

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 Re: engine trouble??? in reply to SMTA Man, 05-21-2001 20:29:03  

compression is way too low. are you sure you put in the right pistons?

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05-22-2001 12:16:44

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 Re: engine trouble??? in reply to SMTA Man, 05-21-2001 20:29:03  
Perhaps you can get access to a deferential pressure gauge. This tool allows you supply a known pressure from a compressor to the cylinder which is displayed on one gauge a second gauge measures the difference between input pressure (80 PSI for aircraft engines) and the pressure that the cylinder is capable of maintaining. The difference between input and cylinder pressure allows you to compare the cylinders. On aircraft engines the allowable difference is most often 20 PSI maximum with an average diffence of 10PSI for all cylinders.
This test requires that the piston in the cylinder being checked be at TDC. A TDC indicator is handy. Any pressure being lost will be past the piston rings or valve seats,or. You will be able to hear which direction escaping air is going. Sorry for the lengthy post.

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05-22-2001 09:57:09

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 Re: engine trouble??? in reply to SMTA Man, 05-21-2001 20:29:03  
I don`t think the narrow valve set is the trouble, try this before pulling head. Hold a piece of paper over the exhaust pipe or muffler, if the paper tries to suck in the exhaust then just maybe you have a valve problem. If not you need to put the tractor on a dyno at a tractor show or shop and check the horsepower, to know for sure. Do this with the motor at idle speed.
Rick Wisehart.

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05-22-2001 08:01:13

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 Re: engine trouble??? in reply to SMTA Man, 05-21-2001 20:29:03  
I have been down this road so many times on tractors with live hydraulic pumps. The cam gear's center breaks out or the nut comes loose and / or the keyway wears out. and slows the cam timing and bleeds off the compression. My advice is to pull the hydraulic pump and see if the cam gear is loose.

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Bob M

05-22-2001 08:21:38

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 Re: Re: engine trouble??? in reply to Jim, 05-22-2001 08:01:13  
That's exactly what happened to my '53 Super M. The nut unscrewed and dropped off the camshaft allowing the cam gear to slowly "flop" itself to death inside the gear cover. Never checked the compression, but the motor ran noticably better after a new cam gear was installed.

Also if the cam timing is off - even by one tooth - compression will be affected. Unfortunately there's no way to check cam timing without removing the entire front end of the tractor. Also you need a 2-1/2" socket to get the fan pulley off. It's a lot more than a "2 beer" job, so exhaust all other possibilities first!!

Punch below for a few pix of my cam gear replacement exercise...

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The Red

05-22-2001 03:50:16

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 Re: engine trouble??? in reply to SMTA Man, 05-21-2001 20:29:03  
That compression is way too low. It should be more in the 130 psi range. Re-torque the head bolts and then reset the clearances on the valves and see if that makes a difference. Also inspect the spark plugs to see if you are burning oil by chance.

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05-22-2001 06:04:08

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 Re: Re: engine trouble??? in reply to The Red, 05-22-2001 03:50:16  
Red, Get a Deere
Ha! ---- now that I have your attention, make sure the compression gauge is accurate they tend to get dropped by "borrowers" and make sure that the throttle is open as it will make a difference in the reading also it will take sometimes 40 or so hrs. to "seat the rings" depending on if
they are chrome plated.Try this first,also remove all plugs at once not one at a time, luck!

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05-22-2001 06:41:15

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 Re: Re: Re: engine trouble??? in reply to George, 05-22-2001 06:04:08  
Thanks for the help, retorqued head bolts once, also rechecked valve clearence. I am wondering if the narrow valve seats may cause a problem. Thing is it is consistant across the cylinders. Compression tester is new and hasn't had a chance to get dropped yet. Thinking as you say the rings may not be seated yet.

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The Red

05-22-2001 06:48:31

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 Re: Re: Re: Re: engine trouble??? in reply to SMTA Man, 05-22-2001 06:41:15  
Yep the narrow valve seats might be part of the culprit. If you don't see an improvement over the next 20 hours or so, I would recommend you replace the bad seats. At least you are getting good practice at installing and removing the head......

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05-22-2001 18:23:39

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 Re: Re: Re: Re: Re: engine trouble??? in reply to The Red, 05-22-2001 06:48:31  
do not worry about the narrow valve seats, on hi dollor motors like in racing they are very thin , it will help it flow better.i think with a deegre wheel and a dile indicator you will find out if the cam has retarted it self by coming loose. i have seen them wen the key-way becomes longated ,wen you deegree in a cam like in totla power you some times only of - set the key way by .010 to .015 deegres.

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05-22-2001 20:23:46

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 Re: Re: Re: Re: Re: Re: engine trouble??? in reply to jakee, 05-22-2001 18:23:39  
Thanks for all the suggestions, think you may be right about the cam gear. I have noticed that at times it appears to push air down into the top of the carb instead of creating suction, especially when starting. Will check it out and report back. Thanks again.

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