RPM - Am I imagining things?

jshwhite · May 27, 2012

I've got a Kubota L3710. Last time I was out bush hogging I thought the tachometer was reading incorrect RPMs - seemed to be showing higher than it was really running at. I thought I must have been imagining it. Well, I was bush hogging this afternoon and noticed it again. A fair amount of seat time will teach one the "sound" of the relative RPM, and it just sounds like it is running a little lower than it is showing. Is that possible?

Is that possible? Is there a simple/cheap way I can test it? I'm assuming it is a mechanical tach right? If it is off, what could be causing it?

Thanks!

JimRB · May 28, 2012

If you come by my house we can try my mechanical tach to your PTO shaft.
Shop for a wide range of tachometers, from non-contact tachometers, panel mount tachometer, laser and strobe tachometers. From manufactureres like Monarch, Electromatic, Checkline, SKF, and more!
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I have one like this by Biddle
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jshwhite · May 28, 2012

JimRB said:
If you come by my house we can try my mechanical tach to your PTO shaft.

Thanks for the offer, but I'm afraid it might be a bit far

Thanks for the links. I just ordered the one from Amazon. I have needed one for a long time, but I had no idea they were available so cheap. I'll have it in a couple days and I'll get some answers

So, if it is incorrect, what could be some causes?

jshwhite · May 30, 2012

I haven't received the tach yet (tracking shows I should get it tomorrow), but in the meantime this problem has become more frequent and more obvious.

One thing that has helped it become obvious is that it is intermittent. For example, today it was running great while bushhogging and stopped, disengaged the pto, and throttled down. A minute or two later I throttled back up and it's reading 2500 RPM (540pto) but sounds like about 2100rpm. I throttled all the way up to 2900rpm for it to sound like 2500 rpm. I headed to the shop and by the time I got there (several times throttling down and up to go through gates) it was normal

Does anyone have any idea what could be causing this. My first thoughts are something dirty and sticking, or maybe the tach drive slipping?

Thanks.

SPYDERLK · May 30, 2012

jshwhite said:
I haven't received the tach yet (tracking shows I should get it tomorrow), but in the meantime this problem has become more frequent and more obvious.

One thing that has helped it become obvious is that it is intermittent. For example, today it was running great while bushhogging and stopped, disengaged the pto, and throttled down. A minute or two later I throttled back up and it's reading 2500 RPM (540pto) but sounds like about 2100rpm. I throttled all the way up to 2900rpm for it to sound like 2500 rpm. I headed to the shop and by the time I got there (several times throttling down and up to go through gates) it was normal

Does anyone have any idea what could be causing this. My first thoughts are something dirty and sticking, or maybe the tach drive slipping?

Thanks.

Wide open should always be the same rpm. Is it?
larry

BeezFun · May 30, 2012

jshwhite said:
al
Does anyone have any idea what could be causing this. My first thoughts are something dirty and sticking, or maybe the tach drive slipping?

Could the slip clutch on your bush hog be slipping? If it is the hog will be turning at a lower speed and will not sound as loud to you, leading you to believe the engine is running slower. One easy way to tell is (carefully) touch the clutch after you've used it for a bit. It shouldn't be any warmer than the rest of the bush hog.

jshwhite · May 30, 2012

SPYDERLK said:
Wide open should always be the same rpm. Is it?
larry

I don't run it wide open very often, but as best I remember wide open is about 2800 rpm. Today it showed 2900-3000 rpm (and sounded like 2600).

BeezFun said:
Could the slip clutch on your bush hog be slipping? If it is the hog will be turning at a lower speed and will not sound as loud to you, leading you to believe the engine is running slower. One easy way to tell is (carefully) touch the clutch after you've used it for a bit. It shouldn't be any warmer than the rest of the bush hog.

That's funny you mention that. Yesterday, for the first time several years, I had a problem with the clutch slipping. I tightened it up though, so no problems there. However, it is definitely the engine I'm hearing, because it has done it a few times without a load, or even without any implement attached.

I did pull the dash enough to get my hand behind it and check where the cable attaches. It was a little loose, so I threaded it finger tight - didn't make any difference.

I can't wait to get my tach tomorrow and see how far it is off. I'll post what I find out.

boomer1025 · May 30, 2012

jshwhite said:
I haven't received the tach yet (tracking shows I should get it tomorrow), but in the meantime this problem has become more frequent and more obvious.

One thing that has helped it become obvious is that it is intermittent. For example, today it was running great while bushhogging and stopped, disengaged the pto, and throttled down. A minute or two later I throttled back up and it's reading 2500 RPM (540pto) but sounds like about 2100rpm. I throttled all the way up to 2900rpm for it to sound like 2500 rpm. I headed to the shop and by the time I got there (several times throttling down and up to go through gates) it was normal

Does anyone have any idea what could be causing this. My first thoughts are something dirty and sticking, or maybe the tach drive slipping?

Thanks.

I would say the tach drive cable is dry and needs lubed. Pull the cable, clean it off with some WD 40, then grease it with a light weight grease

jshwhite · May 30, 2012

boomer1025 said:
I would say the tach drive cable is dry and needs lubed. Pull the cable, clean it off with some WD 40, then grease it with a light weight grease

OK, thanks for the suggestion. I'll give it a try.

art · May 31, 2012

To get behind the dash WD-40 would go down the cable easily. Normally they might jump a bit when this is happening.

big bubba · May 31, 2012

graphite lube is also an option, but don't mix w/grease!

jshwhite · Jun 3, 2012

Got a chance to check the rpm with my new tach - what a great little tool. Glad I got it.

I was right - it's running about 200-300 rpm faster than the dash shows.

I pulled the cable and lubed it, although I don't think it needed it. It was spinning freely. Put it back in and it's doing the same thing.

So, now what. It's got to either be the drive or the dash. Any ideas? I don't really feel like throwing parts at it, as they cost $$

so I'd like to have a good reason to replace something.

kuboman · Jun 3, 2012

Likely is the tachometer head that is the problem. If you can get it out sometimes a drop of oil on the input drive helps otherwise is will be the mechanism inside that is worn or sticky. Taking them apart usually results in breakage and you end up buying new anyway.:laughing:

jshwhite · Jun 3, 2012

kuboman said:
Likely is the tachometer head that is the problem. If you can get it out sometimes a drop of oil on the input drive helps otherwise is will be the mechanism inside that is worn or sticky. Taking them apart usually results in breakage and you end up buying new anyway.:laughing:

Thanks, I'll give it a shot. If it breaks I guess that's the part I have to replace

jshwhite · Jun 4, 2012

Oiled the drive and still no luck. Think it's time for a new drive?

jshwhite · Jun 15, 2012

I just replaced the tach drive (on the engine) and still no go - reading 300 over what it's running. Any suggestions? I REALLY don't want to replace the dash.

Blkvoodoo · Jun 16, 2012

jshwhite said:
I just replaced the tach drive (on the engine) and still no go - reading 300 over what it's running. Any suggestions? I REALLY don't want to replace the dash.

I'd check other tractors too see how accurate they are. A couple hundred rpm one way or the other isn't really an issue, I'd be surprised if it wasn't some what a " designed in feature " too keep operators from over speeding the engine

Is the rpm discrepancy constant thru the rpm range ? Or just at full throttle ?

Bluegill2 · Jun 16, 2012

It's usually always the tachometer and rarely the other end.

I go more by how my engine sound and never run full throttle

SPYDERLK · Jun 16, 2012

jshwhite said:
Got a chance to check the rpm with my new tach - what a great little tool. Glad I got it.

I was right - it's running about 200-300 rpm faster than the dash shows.

I pulled the cable and lubed it, although I don't think it needed it. It was spinning freely. Put it back in and it's doing the same thing.

So, now what. It's got to either be the drive or the dash. Any ideas? I don't really feel like throwing parts at it, as they cost $$ so I'd like to have a good reason to replace something.

Have you verified that it is intermittent? ... If so it may be vibration related - a load and rpm function. Try damping the tach body w your hand when the rpm offset occurs.
larry