1990 Johnson 20HP - J20CRESB - Tachometer Questions

[wcsd106]

Hello,

I am wondering if it is possible to connect a standard 7000RPM tachometer such as a Faria or equivalent to my 1990 Johnson 20HP Outboard. It is model J20CRESB. It has electric start, but I don't believe it has a rectifier and charging circuit; I could be wrong.

I have the OMC factory service manual for that engine as well, but don't have it with me at the moment as I am away from home and have been researching tachs.

I do have a tiny tach clone that I purchased on eBay and it works fairly well. The accuracy seems acceptable when compared to the tach on my bass boat.

The reason for this question is I am getting ready to go prop shopping and wanted to get a good idea of my WOT rpm to get a better feel for what pitch prop I should be looking at for my 1436 jon boat.

Thanks

 

[boobie]

Re: 1990 Johnson 20HP - J20CRESB - Tachometer Questions

Your mtr doesn't have a charging system so you'll have to stick with your tiny tach.

 

[wcsd106]

Re: 1990 Johnson 20HP - J20CRESB - Tachometer Questions

Thanks for the reply. That's kinda what I was figuring

 

[wcsd106]

Re: 1990 Johnson 20HP - J20CRESB - Tachometer Questions

Took it out today. For some reason I thought it had a 10x10 prop on it.. It's actually propped at 10x15. I am getting 4500RPM at WOT with only me in the boat. Didn't bring much gear with me today, no tacklebox, only two rods & reel. Speed measured by the GPS Essentials application on my phone was 23.5MPH.

 

[gm280]

Re: 1990 Johnson 20HP - J20CRESB - Tachometer Questions

Took it out today. For some reason I thought it had a 10x10 prop on it.. It's actually propped at 10x15. I am getting 4500RPM at WOT with only me in the boat. Didn't bring much gear with me today, no tacklebox, only two rods & reel. Speed measured by the GPS Essentials application on my phone was 23.5MPH.

Not having a shop manual for your particular engine I don't know the upper limit for your WOT RPMs. But it seems you are pretty close. You may even go to a little less pitch which will raise your upper RPMs and may even gain a little MPH speed as well. Hard to actually predict though with out knowing all the specs of your boat setup and weights. A lower pitch prop will give you a quicker hole shot and plane out quicker too even with more weight. Maybe a 10x13 would be a compromise... JMHO!

 

[wcsd106]

Re: 1990 Johnson 20HP - J20CRESB - Tachometer Questions

gm280, I do have the shop manual for my engine and the specified operating range is 4500-5500RPM at WOT. That being said, here is an update on my engine's performance:

Today I installed an intake from a 25HP Johnson outboard onto my engine. The intake had a much larger opening for that matched the carburetor on my engine. After installing the intake, I took the boat to the lake to make measurements.

I tried to set up my boat as closely as possible to the last run as far as equipment in the boat, weight, etc. This time, I am seeing between 4980 and 5040RPM at WOT with a max GPS speed of 28MPH. That is an increase of about 500RPM and 4-5MPH.

I would like, however, to get my WOT RPM closer to 5500 when I am alone in the boat so that when I do add another fisherman, tackle boxes, etc I can still expect to remain in my operating range.

Since I am running a 10x15 prop right now, approximately how much would it affect my WOT RPM to step down to a 10x13 prop?