Tach Pulse

[bobandpeni]

I posted a message on this topic last year and got some great results, Paul Moir gave a very detailed explaination of alternator and tachs. <br />Here are my troubles, I have a 16' aluminum crestliner with a 1969 Johnson 55hp outboard. It seems that with the prop I have I'm only turning 2500 rpm at full throttle. My marina, which sold me a 17 inch pitch prop, says I should be getting more than that so I probably have the pulse set wrong on the tach. I have it set to 6 pulse but the marina said it should be set to 5. Pauls explaination makes all the sense in the world (12 stators = 6 pulses). The prop seems a little steep to me but the marina said that I wouldn't even get on plane at 2500 rpm. Any Help?<br /><br />Thanks folks!

 

[phantoms]

Re: Tach Pulse

Everything I was told also pointed to 6 pulses, but with my faria ended up being 5 to read correctly.

 

[bobandpeni]

Re: Tach Pulse

Phantoms,<br /><br />Sorry for my ignorance but what's a Faria?<br /><br />Thanks.

 

[Basscat 1]

Re: Tach Pulse

Faria, is a brand name for a tach. They also make other instruments.<br />The most accurate way to figure out what your tach pulse should be set on is to pull your flywheel and count the number of poles on your stator.<br />

 

[bobandpeni]

Re: Tach Pulse

Basscat,<br />Thanks for the reply. My stator has 12 little coils which I guess means that I have a 12 pole stator and therefore my tach should be set to 6 pulses. Still, I can't ignore that my marina and now Phantoms on the previous post have both said that the tach should be set to 5 pulses.<br /><br />Thanks again!

 

[bobandpeni]

Re: Tach Pulse

Basscat,<br /><br />Thanks for the picture, it didn't come through the first time I got the message. I'm guessing that the picture shows an 8 pole stator and therefore a tach connected to the pictured engine would need to be set to 4 pulses.<br /><br />Like I said in the last post, my stator has 12 of those little coils but no coils that resemble the black coils in the picture. I'm still thinking that my tach should be set to 6 but doesn't explain why Phantoms tach had to be set to 5 to read correctly.<br /><br />Thanks!

 

[Basscat 1]

Re: Tach Pulse

The two black ones are the ignition charge coils, dont count them.

 

[bobandpeni]

Re: Tach Pulse

Basscat,<br /><br />I definitely have a 12 pole stator, the only assumption that I can make is that I should have my tach set to 6 pulses and that I'm maxing my motor out at about 2500 rpm with the prop I currently have.<br /><br />I'll be looking for a lower pitch prop now so I wonder just how low I need to go. Currently I'm at 17" pitch, perhaps 13" or so but I'd sure like to hear from someone with a similar setup.<br /><br />Thanks again,

 

[Basscat 1]

Re: Tach Pulse

I would work with the setup you have before changing props. Now that you know what the tach is suppose to be set on 6P, are you sure you are only turning 2500 rpms? How does the engine run at wot?

 

[bobandpeni]

Re: Tach Pulse

Basscat,<br />I had the carbs rebuilt, timing and compression checked last year just before putting it to sleep last year. I've checked that the throttles open up all the way and the engine smoothly accelerates from idle to wot. I'm open to suggestions as to what else to check.<br />The only other thing that I can think to check is to bring along an engine analyzer and verify the rpm with an inductive tach.<br />Thanks!

 

[Basscat 1]

Re: Tach Pulse

How fast does the boat go at WOT?<br />More importantly, does the engine sound like it is going faster than 2500 rpms?<br />How high is your engine on the transom? Can you raise it any higher? When trimmed down, the AV plate should be even with the keel of the boat.

 

[bobandpeni]

Re: Tach Pulse

Basscat,<br /><br />According to my speedometer I'm going about 28mph. I got 27mph with a 22" pitch prop and now 28mph out of a 17" pitch prop WOT.<br /><br />The engine is very quiet at WOT, so I would say that it does sound like it's going 2500rpm.<br /><br />I can't raise the engine any higher and the plate is about even with the keel.<br /><br />Is this any help?<br /><br />Thanks!

 

[Basscat 1]

Re: Tach Pulse

It sounds like you need to check your engine out further.<br />Dont worry about the tach for now if you are sure it should be on 6P.<br />Check compression, spark, fuel, air in that order.<br />When was the last time you done a compression check? Cylinders should be equal with 10% or less difference.<br />Make sure you are getting a good blue spark that should jump about 7/16 inch, a real snap.<br />If thats ok, then move to the carbs. When was the last time they were rebuilt? May have some trash in the jets.<br />Basically an engine tune up is in store.

 

[mjash]

Re: Tach Pulse

Assuming your gear ratio is 12:29 with a 15% slip(I am just throwing numbers in)with a 17 pitch, moving 28mph. Your rpm would be ~4900.<br /><br />@ 2500rpm you would be moving 14mph.<br /><br />Look here: www.rbbi.com/folders/prop/propcalc.htm<br /><br />Look at that tach again if I got the numbers right.

 

[bobandpeni]

Re: Tach Pulse

Mark,<br />I do have a 12:29 ratio and I ran the rbbi calculator and it does seem to verify that I'm getting WOT which my Johnson manual says should be 4500 to 5500 rpm. I have actually looked at my stator and verified that it has 12 poles. I have an OMC tach, is it at all possible that I should have the tach set to something other than 6 pulses? I'm guessing that the tach must be bad.<br />Thanks,

 

[mjash]

Re: Tach Pulse

I would think the problem is the tach. I hate to be the one to tell you to buy a new one, and that not be the problem. I am new to this myself. I would bet a dollar to a doughnut that @ 28mph you aint spinnin 2500rpm

 

[mjash]

Re: Tach Pulse

Also if we back-up and take a reality check. A 55 pushing a 16' 28mph, running smoothly. Thats not all that bad. Has to be an indication problem.

 

[bobandpeni]

Re: Tach Pulse

Mark,<br /><br />I had to run to the marina to get some oil today and the tach seems suspect to the guys there as well. Their suggestion: buy a new tach. They also seem to think that 28mph out of the 55 is pretty good, they think that 30 - 32 is possible with the right trim, prop etc., but 28 aint' all that bad. They also think that if I was dropping a cylinder it would be VERY obvious on a 3 cylinder.<br /><br />Thanks for all of the help, I believe a new tach will be on order shortly.<br /><br />Thanks everyone!

 

[Boilermaker]

Re: Tach Pulse

How about checking the Retifier before buying a new tach. I believe if half the rect. is shorted, it would cut his pulses in half.<br />Its easy to check with an electrical resistance meter, or VOM. <br /> I can't see him running 2500rpm @ 28mph.<br />Good luck<br />

 

[mjash]

Re: Tach Pulse

I think the rectifier would be ok. One side is for tach and the other is for charging the battery.