# Tecumseh RPM Range - What's Normal?



## Sign216

I just tried out my new Briggs + Stratton vibration tachometer, and it's great!
Used it on a '94 Tecumseh flathead Snowking engine. Here's what I got.

Idle 2,200 rpm

High 3,900 rpm

How are those numbers? Does my machine need adjustment?


Here's a pic of the sirometer (vibration tachometer)


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## Dauntae

If that's accurate you may want to slow it down, Tecumseh has a great engine but they don't like much above 3600, There known to break the piston shaft and ventilate the case when it happens.


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## classiccat

3900 is way too high. a tecumseh should never exceed 3600. That idle is about right.

This happens when tecs run low on oil &/or are over-revved (broken connecting rod):


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## RedOctobyr

I dunno, I think some Seafoam ought to fix up that blown connecting rod  

Yeah, take it down to 3600, or whatever the manual says. The manual may also have a suggested idle RPM. I believe the idle speed is selected to keep it from stalling, but also to provide enough circulation of the oil. 

How is the vibratach to use? I was thinking about getting one, but ended up buying a tachometer/hour meter instead.


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## Sign216

3,900 is too high? Ok, good to know.
Idle of 2,200 sounded high to me. But if you guys say that's normal, then I'm ok with it.
A working range of 2,200 to 3,600 isn't much. 

The vibratach is used by holding the unit firmly against the machine, and then turning the dial. Turning the dial feeds the wire in/out, and when the engine vibration matches the harmonic frequency of the wire length, the end of the wire vibrates at it's maximum. You then read the dial.
It sounds hard, but in actual use it's amazingly simple and easy. 

I thought about mounting magnets to the bottom of the unit, to free one hand, but I think the engine shakes too hard for this.


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## Stang

I recently bought one of those vibratachs. Really, really cool tool. Surprisingly easy to use


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## Sign216

Stang said:


> I recently bought one of those vibratachs. Really, really cool tool. Surprisingly easy to use


Hey, the vibratech (and this forum) stopped me from over-revving my engine.


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## JLawrence08648

I was shown the VibraTach by a John Deere dealer in 1979. He loved it and used it all the time.

I'd like to see someone compare that using a light tach to see how accurate it is. If it's off 100 rpms at 3,600 that could be a devastating difference to a Tecumseh engine.


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## justintendo

not saying the sirometer is inaccurate..but you cold just use a digital single wire tach that wraps around a spark plug lead. simple to use permanently or just temporary for testing. they are about 12 dollars from many places including ebay..


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## 1132le

Sign216 said:


> I just tried out my new Briggs + Stratton vibration tachometer, and it's great!
> Used it on a '94 Tecumseh flathead Snowking engine. Here's what I got.
> 
> Idle 2,200 rpm
> 
> High 3,900 rpm
> 
> How are those numbers? Does my machine need adjustment?
> 
> 
> Here's a pic of the sirometer (vibration tachometer)
> 
> .



How long have you been runnng it @ 3900 ? years?
if so maybe they throw rods from bad maintenance/ low oil not being run above 3600 +/- 150


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## RedOctobyr

1132le said:


> How long have you been runnng it @ 3900 ? years?
> if so maybe they throw rods from bad maintenance/ low oil not being run above 3600 +/- 150


Interesting point. It would be nice to be able to double-check the sirometer with another device, just to help be sure that the 3900 was actually 3900.


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## Sign216

1132le said:


> How long have you been runnng it @ 3900 ? years?
> if so maybe they throw rods from bad maintenance/ low oil not being run above 3600 +/- 150


I've been running it at that speed (3,900 rpm) for 3 to 6 years. Around then is when I last adjusted the carb. 
I change the oil once a year, either right before or after the season, and use a high quality oil.

Oddly, my maintenance book says in *Bold* to never use 10w-40 oil. They want 10w-30, or possibly single weights depending on the temperature. 

Is there a reference that lists 3,600 as the max running speed, or is this just common experience?
Edit: A search did show that 3,600 is the factory recommended running speed.


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## Sign216

The rpm measurement was done after I cut open and modified the exhaust, to redirect the fumes toward the front. Venting to the side was too close to me.

It's possible that the free flowing exhaust makes the engine run faster, than orig designed.


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## RedOctobyr

On a non-governed engine, I'd expect you might see an RPM change due to the exhaust work. But since these are governed, it shouldn't really matter. 

The governor is trying to keep the same RPM, regardless of the load, by sensing how fast the engine is spinning, and setting the throttle accordingly.


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## classiccat

Sign216 said:


> I've been running it at that speed (3,900 rpm) for 3 to 6 years. Around then is when I last adjusted the carb.
> I change the oil once a year, either right before or after the season, and use a high quality oil.
> 
> Oddly, my maintenance book says in *Bold* to never use 10w-40 oil. They want 10w-30, or possibly single weights depending on the temperature.
> 
> Is there a reference that lists 3,600 as the max running speed, or is this just common experience?
> Edit: A search did show that 3,600 is the factory recommended running speed.


IIRC, the stock Tecumseh valve springs start to float around that RPM you're running; they're pretty light duty.


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## Sign216

classiccat said:


> IIRC, the stock Tecumseh valve springs start to float around that RPM you're running; they're pretty light duty.


Ok, I'll back off the rpm to 3,600. 

I'm getting the sense that for performance, I should increase the torque at stock rpms. Instead of pushing rpms higher.


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## classiccat

Sign216 said:


> Ok, I'll back off the rpm to 3,600.
> 
> I'm getting the sense that for performance, I should increase the torque at stock rpms. Instead of pushing rpms higher.


I have hardline tachs installed on my blowers... I'll compare my sirometer to their readings the next time the next time I head out this afternoon.

i like having them permanently installed so that I can observe the RPM drop under load.


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## RIT333

Sign216 said:


> I'm getting the sense that for performance, I should increase the torque at stock rpms. Instead of pushing rpms higher.


How do you do that ?


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## Sign216

RIT333 said:


> How do you do that ?


Yes, that's the $64 question. On motorcycle forums they say adding a big bore kit increases torque more than hp. A big bore kit is usually new cylinders and pistons, which keeps the stroke the same, but increases the bore.


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## 1132le

RIT333 said:


> How do you do that ?


You dont

Buy a 13 hp blower


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