Low RPM

from what i know most briggs engines are rated for 3600 rpm max

Actually, all engines designed for lawn tractors are supposed to run at 3600 rpm at wide-open-throttle. The implements that spin are all geared with this rpm in mind. Snow blowers have an ideal tip speed and mowing decks have a maximum tip speed mandated by law.

I wonder about this quite a bit. I have a tach on my tractor, one of the little digital ones. Its says, with my throttle lever in the WOT postion, that my engine is running at 2600 RPM (incidentally this is the peak of my engine torque band). If I go to the carb and push the throttle open to the stop, then the tach reads 3600. I asked my FIL who has been a mechaninc longer than I have been alive about this, he said leave it. He said that if I readjusted it to 3600, when the engine came under load, it would have no more throttle to open up for more RPMs. Thoughts?

If the engine is running at 3600 and it comes under a load that causes the RPM to drop to 2800 RPM, then the governor will cause the throttle plate to open wider to allow more air/fuel mixture into the cylinder. This is done to try and bring the RPM's back to 3600 and not above that figure. It's the governor that controls the carb's throttle plate, not the lever on the tractor's dash.

Most engines are designed to run above their peak torque band so that when they do come under heavy load and the rpm's drop a bit, they hit that peak torque point and that's what prevents the rpm from falling any further. As an Operator, you're supposed to be listening to your engine and adjusting your ground speed accordingly so that the engine can get back to 3600 RPM as quickly as possible. That way, your mower deck or snow blower will continue to operate at peak efficiency.

Caseguytoo said:

It's the governor that controls the carb's throttle plate, not the lever on the tractor's dash.

Click to expand...

Correct. The governor uses a pair of centrifugal weights that push against a static spring. The spring tries to hold the throttle open and the centrifugal force tries to close it. It is this static spring I adjusted to pull harder against the governor.

The throttle on the dash connects to yet another spring going to the governor.

Back on the farm as a teenager, my little brother decided to hotwire our Case VAC and take it to town. To make it go faster, he tied a string directly to the carb's throttle plate. Well... he rev'd that old Case so high that the weights in the governor blew out and it went full-time WOT. Being as it was hot wired, he couldn't simply shut off the engine with the key that was in my pocket. The steering on the old Case was so worn out he could hardly keep it on the road as it sped up and he had to throw it in neutral, stop the tractor, jump off, and pull the wire off the coil with the engine screaming at no load WOT the whole time.

And what did daddy say and do?

Daddy was away and I was left in charge hence why the key was in my pocket. I fixed the governor on the Case, and daddy was none the wiser. I had to beat a confession out of my little brother so he already paid for his indiscretion. I figured no point in both of us getting a beating.

LLigetfa said:

I had to beat a confession out of my little brother so he already paid for his indiscretion. I figured no point in both of us getting a beating.

Click to expand...

I'm putting you up for the "Humanitarian of the Year" award. :sidelaugh

Hey, that's how it worked in those days. It didn't pay to rat out a sibling. My older sisters also took beatings for some of my indiscretions.

Caseguytoo said:

If the engine is running at 3600 and it comes under a load that causes the RPM to drop to 2800 RPM, then the governor will cause the throttle plate to open wider to allow more air/fuel mixture into the cylinder. This is done to try and bring the RPM's back to 3600 and not above that figure. It's the governor that controls the carb's throttle plate, not the lever on the tractor's dash.

Most engines are designed to run above their peak torque band so that when they do come under heavy load and the rpm's drop a bit, they hit that peak torque point and that's what prevents the rpm from falling any further. As an Operator, you're supposed to be listening to your engine and adjusting your ground speed accordingly so that the engine can get back to 3600 RPM as quickly as possible. That way, your mower deck or snow blower will continue to operate at peak efficiency.

Click to expand...

Right, I got that. I guess my question is, if I adjust the govener to hold the throttle all the way open( at 3600), then when the engine comes under load, the throttle will already be all the way open. How will it adjust engine speed when it is already holding the throttle against the stop? When the engine comes under load does the throttle try to shut automatically? Should I go ahead and adjust govener on my 35 year old Tecumseh OH160?

Your governor will decide how far open the throttle plate must be to make the engine run at 3600 rpm. I'd be surprised if you can only get 3600 rpm out of that engine if the carb is totally wide open but maybe Tec deliberately kept the throttle bore to the size it is in order to prevent the engine from ever exceeding 3600 rpm.

If so, it wouldn't be a good engine choice for a "pulling tractor" since those guys routinely ask their engines to run above that level.

RPM is there to make mower decks spin at a tip speed of about 18,000 feet per minute because that's where cutting decks are most efficient. RPM and torque don't necessarily go hand in hand. But high torque is only needed when there is high loading. Often times the high engine speed combined with the flywheel action of all the rotating parts is enough to keep the engine rpm fairly even.

But when the loading gets to great, then your engine will rely upon its torque curve to make it through the tough spots. Whether the carb is fully open or not, the engine will recover and return to the operating rpm. You need to look up the specs for your particular Tec engine to see what the correct rpm is before you start making changes. There should be a spec number on the ID plate. If you publish that here, maybe one of the guys familiar with Tec engines can find out what you need to know.

Yeah the funny thing is, in the OH160 manual, the technicians manual, that is posted here, it says to refer to the manufacturer to set the RPM. Plenty of guy pull with suburbans, I don't know what they do to thier engines. I don't really have any trouble except a ragged cut somtimes. Pulls a plow and middlebuster like no ones business.

Ok, so just to post this somewhere. I adjust the RPMs on the tractor. I think with the throttle all the way open it would be at 3600, but it sounds so loud, I am leary to put it all the way up there. I followed the basic procedures in the Tec. engine manual in the ref library. It talks about a "standard" variable speed engine screw, and an "optional" fixed speed engine screw. Mine only had a screw in the optional position. I removed the screw (and counted the turns just in case 31.5) and put it in the "standard" postion. Then I just eyeballed where to put it, adjusted the throttle cable, and started it up. Rpms went from 2600 before to about 2800. Shut it all down readjusted the screw and throttle cable, started it up and it sounded like an angry hornet so I quickly cut the throttle back so it would not over rev. Played with the throttle a bit. It runs nice around 3200 and below. Above 3200 and it hunts real bad. Can't stablilize the RPMs. I don't really know how to fix that. Mowed the grass at 3200 Rpms and had a lot more power. The grass was tall from the rain and the mower would have stalled in the past from the dense clippings. Not now, just powered on through. Just gotta get the top end smoothed out and it should be running fine.

If the engine seems happy at 3200 and the deck plows right on through the heavy stuff, then there's no real need to jack the rpm another 400 revs.