2000 Johnson 25 hp jet

[rktman]

Greetings,
Just bought a 2000 Johnson 3 cyl 25 jet with a 16x48 blazer.
I like the combination but it needs just a little more power.
Is it worth trying to upgrade carbs to a 30 or 35 hp series?

Also, I noticed after a long period of running followed by shutting it off, the motor starts fine but will not develop power over idle. After a few minutes it picks up again with no trouble. I'm thinking its a fuel prime issue but wasn't sure if it is a sign of something else?

Thanks.

 

[Pappy]

If it is the 3-cylinder looper with the offset cylinders then the carburetors will do you no good. They are the same venturi size.
Not to mention that you probably already have the 35hp powerhead on that jet unit. find the correct outboard lower unit and then
you will have the performance you are looking for.

That engine was somewhat prone to heat soak. It should have two pumps on it as it is.
Richen each carburetor one full turn. Report back on the results.
Do not run anything over 89 octane fuel. Higher levels of aeromatics in the higher octane fuel can sometimes
aggrevate a heat soak situation.

 

[muddywaders]

Pappy what is "heat soak"? can octane rating effect idle charactristics?

 

[Pappy]

I can get writers cramp trying to explain heat soak....................
Basically, remember in your science classes that a liquid will boil at a lower temperature at a higher altitude? Keep that in the back of your mind.
In todays alcohol extended fuels there are more aeromatics than we had years ago. These aeromatics serve several purposes (not going into those) but for this just remember that an aeromatic will be the first substances to flash off or turn from a liquid into a vapor.
Now, when an engine is run for a while and shut off a lot of things occur.
Everything under the cowling starts coming to the same temperature, as the block normalizes the colder parts absorb that heat. The gasoline is getting hot!
On a restart after 20-30 minute or so the gas in the fuel bowls is quite hot. The aeromatics are starting to flash off faster when hot.
When started, in order to get the fuel from the carburetor bowl to the airstream back of the venture a vacuum has to occur. Go back to my first sentence.
When this hot fuel is pulled up through the idle passages via vacuum the higher ends of this fuel will basically boil off and turn into vapor which the engine will not accelerate on. Normally the idle circuit will function to a point but when the throttle blades start to open and the carb transitions to both idle and off idle circuits liquid fuel never makes it up the tubes in the quantity needed to support the running engine. It is boiling off and vaporizing.
Things that can be done are to richen the engine and lose running quality, live with it, or learn to touch the choke or primer as needed until enough cooler fuel is introduced into the carb bowls to allow normal engine operation.
Most of the engines that are seen in this forum are not prone to heat soak. Multi-cylinder, multi carburetor, higher horsepower will suffer the worst.
Buying fuel minus alcohol will minimize but not eliminate heat soak.

 

[rktman]

Thanks for the explanation.
I ran 91 octane when I had the issues, so that might have been part of the problem.
Also I spoke to the previous owner and he already put 35 hp carbs on it (with reported minimal power increase).
I suppose I either wait to have it rebuilt and bored or upgrade to a Tohatsu.

 

[Pappy]

I have both sets of carbs. They have identical venturi sizes.
The difference in the 3 cylinder looper blocks was displacement. The 25hp was 500CC and the 35 was 565CC.
At this point, if he did change carbs on you this needs to be verified and jets changed to stock. This may be your issue.
Take your model number in to a dealer and have them check jet sizes on the parts list. Verify against what is in your carburetor.