How can I measure RPM?

[NorthwichTrader]

How can I set (or determine) the RPM for a J engine?

[Flyboy]

You could use an optical tacho. for about £15 on Ebay or Amazon . It is possible to use a cheap (£5) bike speedo/mileometer computer.

 

http://www.amazon.co.uk/s/?ie=UTF8&keywords=tachometers&tag=googhydr-21&index=aps&hvadid=25716757016&hvpos=1s1&hvexid=&hvnetw=g&hvrand=7398654116378531761&hvpone=&hvptwo=&hvqmt=b&hvdev=c&ref=pd_sl_mvcpzo76e_b

Edited August 31, 2014 by Flyboy

[Kwacker]

Stroboscope app for smartphone?

http://forum.xda-developers.com/showthread.php?t=2058467

Edited August 31, 2014 by Kwacker

[BEngo]

You can just about count the firing strokes at tick-over, but otherwise you need a hand-held tacho of some sort. The bottom limit is set by the small brass plate with a bolt through it on the governor which limits the downward travel of the speed lever- move this so the tickover is 225 rpm cold. The top limit is set by the upper brass plate- it should be possible to set this to limit speed to1000 rpm, if you are brave!

 

 

Top speed is also limited by the stop on the rack of the fuel pump between the pump and the governor.

 

N

[NorthwichTrader]

Thanks N & all,
On the subject of which, are you able to explain in laymans terms the relationship between the governor speed control, and the tension created by the fuel pump spring, as these are clearly interlinked? As you know, you can slow the engine to a stop, just by pulling the spring towards the fuel pump, in the same way you can bring it to a stand still by lowering the throttle control arm!
I'm guessing there is an optimum setting for these, I'll post up a you-tube link of the pump in action in just a while.

 

edit to add link:

Edited September 1, 2014 by NorthwichTrader

[mattlad]

I'm sure someone on here made a tachometer using a bicycle speedo..... Not sure who it was now but it might be worth doing a search.

[by'eck]

I bought a beautiful boxed precision mechanical rev counting device very cheaply on eBay. They were used among other things for measuring lathe cutting speeds and come with a variety of rubber faced drive adaptors.

 

Working like a stopwatch, after pressing the start button the dials start counting the revs over a fixed few seconds before they freeze with an rpm reading. I use the end of the injection pump to drive mine then double the figure.

 

The laser type triggered by reflective tape on flywheel are also available cheaply on eBay, although not so satisfying to use.

[jake_crew]

Indeed By'eck they are nice instrumenets, but need to be applied to the end of a shaft.

 

Many of the non-contacting optical jobbies only need to see a bit of somthing rotating.

[BEngo]

Thanks N & all,

On the subject of which, are you able to explain in laymans terms the relationship between the governor speed control, and the tension created by the fuel pump spring, as these are clearly interlinked? As you know, you can slow the engine to a stop, just by pulling the spring towards the fuel pump, in the same way you can bring it to a stand still by lowering the throttle control arm!

I'm guessing there is an optimum setting for these, I'll post up a you-tube link of the pump in action in just a while.

 

 

 

 

 

It's all about the relationship between the governor lever position, the amount of stretch on the spring and the posdition of the fuel pump rack, which connects the previous two items. The position of the rack controls the amount of fuel injected on each firing stroke. Fully forward is maximum fuel, and full chat, fully aft is no fuel injected and engine stops. The spring always pulls the fuel pump rack to the full power position.

 

The governor contains a gear-type pump which pumps diesel round and round inside the governor. Some of the diesel is bled off to operate a piston which pulls the fuel pump rack toward the closed position. The amount of diesel that is bled off to the piston is controlled by the governor speed lever.

 

When it's all working there is a balance of forces between the spring and the governor piston.

 

If the engine slows for any reason ( bladeful?) the gear pump produces less diesel and the governor piston is pulled forward by the spring. The fuel pump injects more diesel and the engine speeds up again. And vice versa if the engine speeds up.

 

 

If you move the governor lever down more diesel is bled off to the governor piston which pulls the fuel rack toward the closed position. The fuel pump injects less diesel and the engine slows down.

 

 

There are no adjustable parts inside the governor. The only adjustments you can make are the top speed and tick-over speed stops on the outside of the governor and the position of the max fuel stop on the fuel pump.

 

The stiffness (spring rate) of the spring and the initial tension of the pump spring are theoretically quite important, but again, neither of these are (easily) adjustable if the original brass spring hook is fitted.and practical experience indicates that, provided the spring is not actually slack, the whole combination is pretty stable, if inclined to hunt a bit.

 

N

[BEngo]

Indeed By'eck they are nice instrumenets, but need to be applied to the end of a shaft.

 

Many of the non-contacting optical jobbies only need to see a bit of somthing rotating.

 

 

Mine has a special wheel for cutting speeds, with a particular diameter as part of the kit. This is applied to the outside of a shaft and produces surface feet per minute of the outside of the shaft as a direct reading.

 

N

[NorthwichTrader]

Many thanks, N, as always!

Theoretically speaking, I could disconnect the spring and set the governor to an approximate tickover position, then pull the spring to a tension that achieves a tickover rate! Equally, I could set the spring to a reasonable tension, and then possibly move the governor arm to a position that achieves a tickover rate.

Would I be right in thinking that, as long as the spring isn't slack, and creating sufficient 'stability' to the forces, it doesn't really matter 'theoretically' which way you achieve it, as long as everything is about in the right place, namely, tickover is somewhere sensible on the arc of the governor arm?

[Jessydog]

Best way is to get or borrow a held held Tacho .there should be a small hole in the end of the crankshaft where the Fly wheel is attached .set at 225rpm as per hand book, for a J3 anything less and you could have problems with Lubrication.

[Jessydog]

Dont you just set the stop on the govener arm lower for idle and upper for max revs.? dont set idle below 225 as this can cause problems with Lubrication,Thats why kelvin set it as this speed.

[BEngo]

Many thanks, N, as always!

Theoretically speaking, I could disconnect the spring and set the governor to an approximate tickover position, then pull the spring to a tension that achieves a tickover rate! Equally, I could set the spring to a reasonable tension, and then possibly move the governor arm to a position that achieves a tickover rate.

Would I be right in thinking that, as long as the spring isn't slack, and creating sufficient 'stability' to the forces, it doesn't really matter 'theoretically' which way you achieve it, as long as everything is about in the right place, namely, tickover is somewhere sensible on the arc of the governor arm?

 

 

I'm largely with Jessydog on this, but that might be dependent on your set up. As standard there is a non-adjustable brass rod with a hook, screwed into the timing gear case, the spring, the pump rack, a piece of chain and the governor rod.

 

Often the brass rod has been replaced by a piece of studding and the chain by something else- a metal strip or piece of wire. If you want I can measure the location of the spring hook and the length of the bit of chain. Neither the rack length nor the governor rod are likely to have been changed.

 

N

[NorthwichTrader]

 

 

I'm largely with Jessydog on this, but that might be dependent on your set up. As standard there is a non-adjustable brass rod with a hook, screwed into the timing gear case, the spring, the pump rack, a piece of chain and the governor rod.

 

Often the brass rod has been replaced by a piece of studding and the chain by something else- a metal strip or piece of wire. If you want I can measure the location of the spring hook and the length of the bit of chain. Neither the rack length nor the governor rod are likely to have been changed.

 

N

Thanks, N, I do have the brass spring-hook, AND an original spring, but had to insert a piece of wire between the spring and the brass hook to stop the engine racing!

I'll take some photos and measurements, and post them up, maybe the chain's a different length, or something!

Many thanks again!

Incidentally, I can easily stop the engine by pushing the spring a very small amount towards the fuel pump when it's on tickover of 220ish!

[dave moore]

Stuart,

 

Why bother? I've never had a tachometer on Ressie, prefer to use my ears and instinct when choosing throttle settings. Your call of course, not trying to tell you what to do, I note a tendency among many boaters these days to value technology above common sense, but then I suppose I'm a bit of an old fart in that respect. What exactly would you do with the given readings, assuming they were accurate?

 

Cheers

 

Dave

[NorthwichTrader]

I guess it's not really knowing what the baseline is, without an initial first measurement! I've heard a good number of Ks around Saul Junction, and it would be easy to disappoint yourself, and confuse your senses, trying to draw a comparison!

If I had a pound for every time a K owner has said to me 'it's ticking over a bit fast, isn't it?' I'd be able to buy myself a top-notch inverter!

[dave moore]

Yours sounds absolutely fine to me. Years ago I tried dropping the bottom stop to slow tick over, only to be rewarded by repeated stalls. I doubt you'll ever use full chat except in emergency or deep water for a short while. Relax and enjoy, you'll soon be happy by ear. Is are a slower beast altogether!

 

Dave

[NorthwichTrader]

Reassuring stuff, Dave, thank-you! Will buy you a beer at the Bell next time you're down!