Idles 650rpm in water, 2300 on muffs

[pjsteelejr]

I have a 1985 150hp Johnson. Rebuilt the carbs and went for a spin everything worked fine. When got home a put on muffs to flush motor idle speed was about 2300rpm. Any sugestions on why this would happen.

 

[jimmbo]

Re: Idles 650rpm in water, 2300 on muffs

JUst a guess, less backpressure. I think those engines had an automatic timing advance to provide a fast idle while the engine was warming up. Normally in the water with the restrictive idle relief the engine didn't speed up quite thqt much.

 

[dajohnson53]

Re: Idles 650rpm in water, 2300 on muffs

Probably others will have comments and suggestions, but my experience is similar, although not as great. When I run the motor on the hose, it will turn much higher rpms than when in the water (therefore I have to adjust the idle down). I believe this is because there is very little or no back pressure on exhaust while on the hose vs. in the water. In fact, if the idle is set to be right on the hose, I have to adjust it upwards when I put it in the water or it's too low and will quit.

 

[pjsteelejr]

Re: Idles 650rpm in water, 2300 on muffs

Yep that is what is going on. I am trying to find a happy medium. Do you think I could have a dead cyl and could that cause it?

 

[Bradster941]

Re: Idles 650rpm in water, 2300 on muffs

Not really, what you want to check is back lash and idle positions of the butterlies.<br />i.e. remove the silencer cover and make sure all carbs. butterflies are closeing completly and evenly.<br />If not, you will need to adjust the linkage or the back lash.

 

[pjsteelejr]

Re: Idles 650rpm in water, 2300 on muffs

OK will check this weekend.

 

[pjsteelejr]

Re: Idles 650rpm in water, 2300 on muffs

The butterflies look like they are all in the same position and the linkages don't have any slack that I can see. Anything else I need to look for?

 

[angus63]

Re: Idles 650rpm in water, 2300 on muffs

Fill a barrel with water up to the same height the engine is at when launched and idle the motor in the barrel instead of the muffs to see if backpressure is giving you such a big differential.

 

[uncyboo]

Re: Idles 650rpm in water, 2300 on muffs

Stupid question........Will the water in the barrel create the same amount of back-pressure as the lake?

 

[lark2004]

Re: Idles 650rpm in water, 2300 on muffs

basiclly, it is the depth pressure that we are tring to achieve, hence angus's suggetion to fill barrel to same height as engine would be in when on the water.