superprism

Gents:I have a kohler 12.5 engine fron a john deere model stx38mower...It backfires and won.t start because the exhaust valve opens twice on the compression stroke.thereby expelling the gases out the exhaust........We took the engine apart and looked under the exhaust valve lifter,,,There we saw a pin sticking under the valve lifter that at that posotion it opens the exhaust the second time....We took the pan off and found the pin that supposed to present a flat at that position but when the tension is off on the small spring loaded fly wheel(at the base of the cam shaft,,) It shows the round part of the pin thereby lifting the exhaust for the second time....(all compression goes out the exhaust when the piston is at top compression mode...Can send a pic of the gear and the flywheel to whoever can explain why it's blowing out the exhaust Thanks tools21@optonline.net

wrench97

That is designed to release compression at low speeds to make the engine easier to crank(pull start with a rope) the springs and flywheel weights should open at running speeds to allow the valve to work normally.

Usually the camshaft and the ACM(Automatic Compression Release) are replaced as a unit.

superprism

Thanks for your reply,,we have determined that the a.c.r. was working ok (after taking the pan off) We now feel the reason for the backlash and no starting symptoms,might be due to the fact that too much gas is entering thereby causing the backlash,,and the expeling gases out of the exhaust...It may have something to due with the carb,,(which we should have looked at first,,,But the fact that it was expelling gases out the exhaust made us think the valves were sticking,,,We will check further,,Thanks again for your answer...This is on a 12.5 kohler engine on a stx38 john deere
Also we thought the backfiring was caused by the slightly sear key on the flywheel,(we will replace that also
Anybody know where to buy head gasket and shear pi for a kohler 12.5 engine on a john deere...?

wrench97

Local is usually faster but there are a few web sites> http://www.jackssmallengines.net/parts.asp

http://www.kohler-engine-parts.opeengines.com/

But you'll need the Engine model and possibly the serial number off the engine tag usually on the flywheel shroud.

Check the float in the carb if it is the brass type shake it if you hear sloshing it has fuel in it and is not floating if you can locate the hole it can be soldered up but be careful of fumes and make sure it is empty first , if it's the composite type and feels heavy is it saturated and needs replaced.

superprism

Wrench 97 Thanks for your input,,,we had to take the whole engine aprart,,,And even thou the shear pin looked good,,(we replaced it,,Of course also the head gasket,,,,) We got a tip from someone else who suggested maybe the engine oil was too heavy and we should use 10-30 oil (which we did,,,,,) Putting it all back together it,,After rechecking the carb ,,it started right up....However when we put the oil pan up and all together we discovered an oil leak,,It seems the little funny shaped "o" ring that is right under the oil pump gave out...It looked fine when we took it apart,,,Now we have ot remove the pan again and replace the (%^&&**&^^%) oring kit # am117024 in the john deere parts book...I don't know what the replacement number would be for the O ring kits because john deere has their own numbers,,and when you go to the local lawn fix it place they have to cross reference it...At least it starts now...Thanks again Superprism..This is kohler 12.5s engine series 1215..P.S
At least we know now what the acr is supposed to do Thanks to you...

superprism

To wrench 97 ,,I used the site for kohler parts and they have the replacement numbers for my kohler 12.5s.... engine 1215......Thanks i was able to use their numbers and order the gasket that is on the under side of the oil pump...part number 12-032-04-s 0- ring oil pump cover....(that's what i hope the leak was,,,,It's a funny shaped o ring,,,,,,Thanks again for your help...superprism

wrench97

Good to hear you found it useful.

superprism

Wrench 97 : i have just one more question regarding the kohler 12.5s -1215 engine...At the parts site you directed me to for this engine they list part # 22 12-422-07-s (claiming that there is a shim under the cam shaft (white .028),,,Also part # 12-422-08-s there is a shim under the camshaft (blue .03),,,,And then they go further on down listing different shims for different colors red,yellow,green,gray,and black......My question is on these shims,,How do i know what color designation applies to my engine 12.5s-1215? so i know what shim to use?
Thanks for your help superprism The parts are listed in the section where they show a diagram of the camshafts... My tractor is an john deere stx38

wrench97

What shim is there now?
To determine the thickness you need to measure the end play without a shim installed then subtract the spec clearance that will give you the required shim, the original shim installed now will most likely work unless it is damaged.
The spec clearance will be listed in the service manual.

superprism

Wrench 97,,You are working on a sunday???hats off to you,,,,,,Anyway what scares me is there is no shim (or none there when we took the pan off,,,I'm wondering is that (without any shim ) going to rest and wear on the aluminum base??????Plus i guess nobody knows why the color designation is there,,,Thanks again Superprism

wrench97

The color just tells you what shim it is at a glance without having to measure them.
You would have to measure it but most likely it was a thin shim that wore away or the clearance was tight enough it did not need one, or the last guy left it out:)

superprism

Wrench 97...First i want to congs you for your dedication to a job..(and on sunday) You are one great knowlegable person..
Anyway you hit the nail on the head ,,,,My mechanic had checked originally and found there was no space for a shim,,,,,Therefore as you say it didn't need any,,,,And by the way the leak that was apparent when we first put the oil pan on was because the seat where the o-ring on the oil pump cover had a slight broken off piece and the oil leaked out past the break,,,(fixed with metal seal) that's around part number 12-032-04-s.(or oil pump cover....Well it's all together and running fine thanks to your help....God bless superprism kohler engine 12.5s -1215 off of a john deere stx38 rider mower from 1900,s....

wrench97

Good to hear you have it up and running.