Checking RPM

[mhorozko]

Re educate me on the proper way of checking RPM. Isn't 3600 max? Wasn't there a special Tach because Kohler fired on every cycle? Thanx in Advance

 

[mhorozko]

Dah.... maybe I should have added K301 12hp

 

[jkoenig]

Kohlers are four stroke/cycle engines. They fire once every two revolutions.

I can think of a few tach choices:
1) Vibration tach (Sirometer) by Treysit sold by Briggs dealers and Tecumseh dealers once upon a time. It is the only Tecumseh product I have ever gotten along with. The wire vibrates, adjust the length of wire, and when the loop end stays still, read the rpm. Don Vogt could tell by the arc of the swing if the engine had balance gears or not. I have forgotten whether he said big swing meant balance gears, or small swing. $15 about 20 years ago...

2) Optical tach with reflective tape on something rotating.

3) Contact tach against something rotating.

4) Tach/Dwell meter connected to ignition system.

 

[Stu]

When I rebuilt my K301 (rebore 10 over, all new guts) I started with one of those units that hooks inline with the spark plug wire but I couldn’t get it to work well at all, so I got the cheapest optical one I could find, maybe $30 bucks, works great. I would go that route.

 

[mgonitzke]

I have all three of those, as well. Lately I find myself using the sirometer the most. There are some limitations to my optical tach (needs to be dark enough the sensor can see the reflective tape). I need to get a new rubber tip for the mechanical tach so I can actually use it.

 

[Ken Black]

the optical ive also found. how far from the moving part you are. changes it a bit...
i myself use the optical and i set to 3kRPM. that xtra 600 i found doesnt make big diff.. makes me feel like iam in safe zone also.
then i adjust my throttle cable so that the top of the leaver is 3k . or if unable. at least put a lil mark there on dash.
i guess when you put 800 bucks into a rebuild.. wanna make her last a lil longer hhaha

i do that to all my motors. no mater the tractor make. cub , mtd , ayp , cub/mtd so on .

 

[mgonitzke]

the optical ive also found. how far from the moving part you are. changes it a bit...
i myself use the optical and i set to 3kRPM. that xtra 600 i found doesnt make big diff.. makes me feel like iam in safe zone also.
then i adjust my throttle cable so that the top of the leaver is 3k . or if unable. at least put a lil mark there on dash.
i guess when you put 800 bucks into a rebuild.. wanna make her last a lil longer hhaha

i do that to all my motors. no mater the tractor make. cub , mtd , ayp , cub/mtd so on .

Set it to 3600...these engines are designed to operate under load at 3600 rpm. By running it at 3000 you are reducing the performance and making it work harder than if you set it at 3600, and it will not cool as well.

 

[Ken Black]

Set it to 3600...these engines are designed to operate under load at 3600 rpm. By running it at 3000 you are reducing the performance and making it work harder than if you set it at 3600, and it will not cool as well.

i would love to dyno them and see how much loss it is... not saying wrong or right.
i wouldnt have thunk 600 rpm be that big of diff on power lose. guess its not a 8 hole. hahah
as for cooling heck i dont run my cubbies unless its below 80/85 and not humid. growing up around 7 to 10 years old. i blew up 4 ... 321s. 4 cranks and 2 blocks on the old girl.. was a 68 432 gravley. boy was my grand dad a lil mad lol ..

 

[mgonitzke]

There is a torque curve for the K-series floating around out there. Half to 1 hp loss from 3000 to 3600 RPM, but cooling airflow is essentially proportional to RPM, so the engine is going to run hotter making 90-95% of rated power with a little over 80% of the cooling airflow at rated RPM. I remember hearing a story on here years ago, guy had a mildly warmed over fresh rebuild, let somebody else mow with it, who mowed at partial throttle because he thought it was easier on the engine, and ended up overheating it and taking the temper out of the rings.

Some of the later tractors even said "Always mow at full throttle" on the throttle handle. There is also a HUGE difference in performance in the mower decks at even 3500 vs. 3600 RPM. Any other engine driven attachment will also not perform as well because they were all designed to be operated at 3600 RPM.

 

[Greg Riutzel]

Isn't 3600 max? Wasn't there a special Tach because Kohler fired on every cycle?

3600 is book value for max OPERATING rpm. That is the specified RPM to use
I also use the dancing wire or Sirometer. Quick, easy, done; and you can use it on almost anything without special tools, scopes, or meters.

 

[Ken Black]

There is a torque curve for the K-series floating around out there. Half to 1 hp loss from 3000 to 3600 RPM, but cooling airflow is essentially proportional to RPM, so the engine is going to run hotter making 90-95% of rated power with a little over 80% of the cooling airflow at rated RPM. I remember hearing a story on here years ago, guy had a mildly warmed over fresh rebuild, let somebody else mow with it, who mowed at partial throttle because he thought it was easier on the engine, and ended up overheating it and taking the temper out of the rings.

Some of the later tractors even said "Always mow at full throttle" on the throttle handle. There is also a HUGE difference in performance in the mower decks at even 3500 vs. 3600 RPM. Any other engine driven attachment will also not perform as well because they were all designed to be operated at 3600 RPM.

ill have to read up more on it.... it makes sence but who knows ..
hahha my tiller still pushs me thru the garden even at 3k hahahha just sayin.. dang thing is a monster compared to the troy bilt. tho thats only the horse ..

 

[dfrisk]

Ohhh how I missed all these discussions about having to run your air cooled engine 3600 rpm or it will melt into a puddle of goo on the mower deck.
I had a fairly fresh rebuild on a K241 years ago. Only time I ran it wide open, 3600 or more was when I needed ALL the hp it could make. One time I ran it wide open and I still got it hot enough to detonate. But a used oil analysis showed no harm done, was the oil change I'd blown snow with all winter, my 10W-30 was really loaded with raw gasoline from being "Over-Cooled".
Anyhow, I pulled that engine after 1400 hours of mowing, plowing, blowing snow, pulling my lawn vac. The ONLY problem I had was during a wide open hard pull the exhaust valve would stick just a bit, and if I reduced the load it would cool the exh valve stem and contract the stem diameter and run normally. When I pulled the valves there was a thin layer of hard carbon built up on the valve stems that caused the sticking. If I didn't run the engine wide open the exh valve didn't stick as bad.
I was always going to put exhaust gas temp gauge, oil temp gauge, and cyl head temp gauge on my K321 with kinda a Killer Kohler rebuild. I know Dave Kirk did some tests on engine rpm vs oil temp years ago.
The 27 hp Kawasaki in my CC Tank zero turn has pressure lube, oil filter and finned flat plate oil cooler. And yes, I don't run it wide open normally either. The pulley ratios spin the blades MUCH faster than the old IH Cub Cadets ever did, the zero turn throws clippings 10-12 feet, at least two swaths away.

 

[kmcconaughey]

Here's the torque curve that Matt mentioned:

 

[Ken Black]

Here's the torque curve that Matt mentioned:

View attachment 147731

if iam reading this right.... 1/2 a horse power for 600 rpms... on the 301s

but i guess if you hit a brick wall at 100 or 130mph. not much diff in what happens to you lol

 

[Neil Mullins]

I am a WOT person. They are manufactured to run there. Air compressors, welding machines & loads of other power equipment that have used these Kohler engines have 2 positions 3600 RPM & off.

 

[snicklas]

Yep, same here. The only time that my equipment isn't a WOT is if I am just moving it around, or loading/unloading from a trailer. If it is working, it is WOT. That is how we have run all our Cubs.... and we have one with 1400+ hours on is, so I don't think it is shorting its life. Even in the winter plowing snow, it is WOT.... BRS works better that way. My new Cub ZTR says to run it at WOT when mowing... it even says to engage the deck at WOT. I have a Cub push mower, and it doesn't have an adjustable throttle. You squeeze the handle, and pull the rope and it starts and goes to WOT.... that is the only choice.

Also, contrary to popular belief, lugging an engine at low RPM's is harder on the engine than running on the governor. I know on the 6.9/7.3 IDI's that Ford used in the pickups (IH/Navistar) and the 5.9 Cummins that Dodge used, have no issue with them running pinned to the governor, all day, every day, but lugging them below 1500 RPM is very hard on them and can be considered abuse.... I know someone that had a 7.3 powered F-Superduty rollback, and with the gearing, if he was anywhere near the speed limit, it was on the ~3300 RPM governor.

 

[Neil Mullins]

In My opinion, where you get in trouble is pushing the engine beyond what it will do. 3600 RPM is a governed speed of the engine. The RPM’s don’t do any damage. The job of the governor on the engine is to maintain that engineered “speed”. Now you begin loading the engine, mower deck, snow thrower, etc. by the very task at hand. Overheating happens when there is more demand on the engine than it has HP/torque to handle. So now the governor has the carburetor pulled wide open & your engine RPM begins to drop below 3600 RPM. RPM drops you have less cooling air. In a nut shell you are asking an engine to do more than it can possibly do and at the same time reducing it’s cooling system