312/8 in heavy grass rpms drop by 1/2 happened twice

[jeff lary 171]

Hi everyone I have a question.
   I have a 1995 312/8 twice this summer while mowing tall grass both times 5"- 6" the mower went from lets say 3500 rpms to 1500 rpms like right now boom !
 I of course kick the clutch as fast  as I can disengage the pto and lift the deck. I look at the throttle lever it is in the wot position like it should be. I pull the throttle down to the idle position wait maybe 20 seconds and then throttle back up. I goes right back to 3500 rpms again and all is well ??     What do you suppose is happening I can only assume the heavy grass some how works the engine to the point that for whatever reason the governor does something funky and reduces my engine out put. I can tell you it is very un-nerving and it worries me some. I and my son have mowed lawn today and the grass was at least 4" tall and no issues at all. We have had many weeks of on and off rain showers and it makes it hard to get the lawns done when they should be. Thanks Jeff

[callum12 256]

perhaps the governor is sticking? as the engine is loaded up by the grass the governor doesn't move to open the butterfly. 
 

[jeff lary 171]

Maybe I never thought of it that way I will blow it off tomorrow and lube the pivot points , cant hurt ?

[doc724 924]

Spinning a deck takes lots of HP.  You did not say how big a deck you are running or if you are cutting the grass low.  42 inches is the maximum I would ever consider for a 12 HP machine,  When the grass is tall, raise the deck or take a smaller bite (width) of grass on each pass.  Your PTO clutch may be slipping, maybe needs an adjustment.  If your mower is caked with grass underneath, that does not help either as lifting efficiency drops.  Paint a horizontal stripe on the PTO bell, that way you will be able to see it slow down (or not) when you encounter tall grass

[oldredrider 2,547]

Feeding the deck tall grass at too high of a speed with dull blades produces the results you are getting.
As doc724 said, take smaller bites and make sure your deck is not caked with old dry grass clippings.

[jeff lary 171]

Yes I think you are correct I will def clean the deck sharpen the blades and take smaller bites when the grass is tall. The deck bty is 37" a 2 blade configuration . The PTO may need to be adjusted as well,... you do that by turning the trunnion on the PTO lever linkage , correct? Adjust it a little so that when you engage the PTO lever it kind of "cams over" (ie) " snaps" into the engaged position right?

[can whlvr 991]

sure sounds like a governor problem if it happens instantly usually it will bog down but not go to half rpm in a flash

[Forest Road 594]

37" deck and bogging down like that. I'd lean toward a governor issue.

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[ericj 1,576]

i had an 88 or 89 312-A that would just about die down to an idle then back fire and rev up again, finally traced it to a sticking valve.it would only do it when it was hot out or i got the tractor hot. but since it take less then an hour to mow the yard i was mowing it pretty much only did it when the temp was a little past warm. once the valve stem and guide was cleaned she runs like a champ 






eric j

[Dirtdog524 61]

If it is a sticking valve, try some Marvel Mystery oil in the gas tank. It might be considered snake oil but it really does make a difference

[jeff lary 171]

Ok hi all once again.
 Today I took the deck off to look her over. It had some caked on grass but not as much as I thought it would have had. I scraped it clean and got maybe a plastic grocery bag 3/4 full of grass. Then I took the blades off to sharpen them they were as blunt as a book binding. They were brand new a couple months ago so this was the first sharpening since then . I should have sharpened them after 1 month but oh well been busy with other things.
   I put a flap disk on the 4" grinder and shined up the under side very clean,.. then hit it with some denatured alcohol to get it prepped for the next step.
 Undercoating 3 coats of some 3m stuff I have had for 20 years or so. Then I put the deck back on the mower. Moved on to the air filter and cleaned it well, the pre filter was baddd. I coated it with light oil this spring and man it was nasty 1/4 deep in dusty grass and crud. So it took that and the filter off and cleaned both and then re-installed them with another shot of light oil for the pre filter. I hit all the carb linkages with degreaser and then carb cleaner. After that I went back an oiled all the linkage pivot points.
  Now it was time to look at the PTO adjustment trunnion. I am sure it was fine,.. I adjusted it last winter early and there is a nice " cam over / snap over " when the lever it moved into the engage position. Next will be a wait and see I guess. I know I was taking a full cut of very tall grass both times when the rpm loss happened. I knew I was pushing it and each time I was pretty sure it was caused by the tall grass but I have never had that happen in 20 years of mowing with it. So I guess time will tell.

Edited August 30, 2015 by jeff lary
spelling

[oldredrider 2,547]

Good luck. You're on the right track. 

[Racinbob 10,280]

Yes, it does sound like you're hot on the trail. I don't see it as a PTO issue. If it was slipping the engine wouldn't pull down. Your 12hp should handle that deck fine. I used 2 tractors with 48" deck to mow and one of them had an 11hp Briggs on it. My Dad also had a 48" on a Raider 12 and it did fine as well. I like the governor idea. Just the clean up and lube. I wouldn't take more serious measures until you try it out again.

[prondzy 3,867]

Sounds to me like your taking too big a pass when it's too tall. That is a big load even for a twin. Your maintenance of the deck should help and if you have been busy (aren't we all) mowing might not be on schedule like it used to be?

[jeff lary 171]

Yes that is the issue, the grass was too tall and I took too big a bite. The blades being too dull , in a hurry you name it. The first time I was mowing my sisters lawn for her . She has recently been diagnosed with MS. Her husband works long hours at night driving truck and between house work, their Horse, cooking meals and many hospital visits his lawn got away from them. By the time I got over to mow it, it was between 4 & 8 inches tall in various areas. While helping her and others in the neighborhood, ( older gentleman bought a used bush hog that really needed work that he did not have the tools to fix) my own lawn got too tall itself. Lesson learned got to do a better job keeping up on the home front.