25 hp Evinrude jet- no power

[mckinney004]

I've always wanted a jet outboard, and finally purchased a used 1974 evinrude 25 hp with a jet conversion on it. I know jets lose about 30% of their power after the conversion, leaving this motor actually in the 15-20 hp range. I normally run an 18 hp mercury on this boat, pushing it about 25 with two people and a tackle box. That being said, I expected similar results with this jet.

Today I took it for a test run. Absolutely disappointed. I honestly think a 9.9 would outrun me. The boat is a 1448 fairly light weight riveted boat. We don't have much big rock here to put a hole in the boat, mostly just shallow gravel bars, so I'm not too worried about the thinner material.

I really think I should be getting substantially more power than I am. Any suggestions? I don't have it raised quite as high as it needs to be. Today was a test run to see how it performed and ran, so I haven't adjusted the transom yet, but I had the motor within 2 inches of where it needs to be permanently mounted. I really don't see that causing me to run half as fast as I expected. Almost the entire jet housing was out of the water, so really the only extra drag was from the foot of the jet... not that much area. I've seen some videos of these newer 20 HP merc jets on 14 ft boats and they scream.

 

[racerone]

Is the motor running on both cylinders ?----Compression test done ?

 

[sutor623]

Yes, a 25 converted to jet should SCREAM on a 1448 riveted jon.

To give reference, I had a Mercury 20 jet tiller (25hp powerhead) so in reality an 18 jet just like what you have, on a 1648 welded jon boat with wooden floors, trolling motor, 2 batteries, 2 people and 10 gallon livewell, and that boat would hop right on plane and cruise about 17MPH. With just me and no crap on board, that boat would move at 22mph. This was a heavy welded mod V jon boat. I would imagine on a light load youd be 22-25mph in that boat.

Too much weight on the boat and also un-even weight distribution in a jet boat will also KILL performance.

All that being said, 2" is HUGE when we talk about jets. Normally ?” height can make or break you. Something else you need to look at is the impeller and liner. If the jet impeller is damaged or not shimmed properly it will really kill your performance.

First off, like racer said check for spark on both cylinders and compression. Then pull the grates out of the foot and check impeller to liner clearance. Should be between 200ths-300ths of an inch with the feeler gauge. If all that tests well lift her up to the right height and try again.

 

[mckinney004]

Update.

I raised the transom and was eventually able to get 15mph on gps. I kept tinkering with the height and it has to be absolutely perfect to get on plane. After running 15 mph, I made about a 1/8" adjustment and went right back to 8-10 mph. Frustrating, but it gave me hope after getting on plane and running 15 mph a few times. Still not as fast as I hoped, but I can deal with it. I haven't done a compression test, but might try. I didn't have a feeler gauge, but the impeller is definitely close to the liner. It looks to be on its last level of adjustment though.

Sutor I looked at putting that exact motor on this boat, but went with this one for now to get my feet wet.

Does anyone have any advice on a good place to find info on setting up a jet? Height, trim, etc. haven't come across anything yet with a setup guideline besides start with the leading edge flush with the bottom of the boat and go from there. Ive very familiar with those processes for prop motors, but I can see that these are a totally different temperamental beast...

 

[sutor623]

If you have a gauge, check compression, please!

Here ya go bud. This is from www.outboardjets.com. They are the folks that make these jet lowers on 99.9% of all outboard jets that you will see. I almost guarantee it has outboardjets stamped on the starboard side of the jet lower.

http://outboardjets.com/jet-kit-misc-instructions/

Give me the model number of that motor so I can see the displacement of the block etc., and what is the stamp/serial on the side of the jet foot? I want to see what model jet lower you have.

Lets see some pics of your boat bottom, the engine, and a view from the side with a straightedge running from the keel to the foot.

Only way to tell if it is on its last adjustment is by counting the shims (washers) over or under the jet impeller. There are 8 total. Once you have all 8 on the TOP side of the impeller, it is time for a new liner or impeller. It may LOOK close, but we are talking about hundreths of an inch here. It is well worth the time and money to get a feeler gauge and adjust this gap. (For simplicity, some guys shim the impeller until it touches the liner, and then move one or two more washers down to set the clearance right.



A great site that you can join is called m e a n c h i c k e n.net. Those boys are all about some jets over there. Also t i n b o a t s.net. I am rotus623 on those sites.

 

[flyingscott]

15 MPH is a good speed for that set-up. That motor is a 22 cube cross flow power head rated motor, excellent motor has great reliability. Your 18 hp is more than likely prop rated hp as I am going to guess it is an 80s model it is also 25 CID looper block. Your 18 hp would probably be faster than the 25 omc if they both had props on it. You cannot compare a newer merc to your motor it's not a fair comparison the merc is a newer design, and jet loss is usually a little more than a 30% loss usually closer to 40.

As a powerhead rated motor the 25 is probably closer to 20 at the prop now take another 30% off and you are closer to 15 hp maybe less. The speed of your boat puts you right about how fast a a 15 hp would push it.

 

[sutor623]

As a powerhead rated motor the 25 is probably closer to 20 at the prop now take another 30% off and you are closer to 15 hp maybe less. The speed of your boat puts you right about how fast a a 15 hp would push it.

Yup that's where I was headed. Just wanted to get some serial # info but seems like you got your CID info straight!!

The 16' welded boat with wooden floors that I posted a picture of is likely 2X the weight of the 14' riveted boat that Mckinney has. We are talking 300lb boat, 125lb. motor, 50lbs of floor, 10 gallon livewell (80 lbs) 6 gallons of gas (36lbs.) 50lb. battery, 50lbs of gear with anchors, coolers etc, and 350lbs. of people. We are talking 1040lbs. or so.

What is your best estimate of weight of boat, motor, you and gear Mckinney? Try and be liberal with your estimate

I still think he just needs to check the impeller for excessive wear or rounded leading edges, check the clearance to the liner, and straighten out and clean up the intake grates if they are bent or smashed up. This will make a big difference.

Can't say that I disagree with Scott, as I don't think many often can, but I would hope to be able to squeeze another couple MPH out of that rig with some weight stripping/distributing and fine tuning.

 

[flyingscott]

Absolutely maximize the set-up and make sure the motor is good and healthy with a compression tent. And do what sutor says about the lower unit.