# seems the previous owner of my jet didnt know anything...



## JoshKeller (Jun 24, 2014)

bought a used boat with a 50/35 1994 johnson jet on it. motor runs good, however, it just feels like something is missing. no matter how i load the boat, it does 22 - 23 mph. Took a look at bigterps motor (same as mine), and he gets much better performance than mine. Well tonight it all came together. he let me borrow an impeller to compare/test, and it appears my motor has a 5 7/8 impeller installed instead of the medium. also explains why our impeller keys are different - mines much larger/taller, presumably to make up for the difference in the 3/4" driveshaft vs the recommended 5/8"

i assume this is horribly effecting my performance? that also likely explains why my motor turns 5700 rpm all day long and his turns 5200? 

next question, anyone have a used medium impeller/key for sale? thanks guys!


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## Ranchero50 (Jun 24, 2014)

Which flavor pump do you have? Looks like 5 7/8 or 6 1/8 were available depending on the model.

https://outboardjets.com/download/medium_series/Impeller,%20Liner%20and%20Driveshaft%20ID.pdf

Can you adjust the impeller to liner clearance within spec? I remember you said you had trouble with it when you were here. A 3/4" shaft impeller shouldn't come close to working on a 5/8" shaft as it wouldn't be concentric with the shaft with a taller key taking up the difference.

I wonder how much better your impeller would work without all the cavitation holes (in your pic)? If you can get your liner clearance close I suspect filling the holes with Bondo as a test will make a difference in how high the engine revs.


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## JoshKeller (Jun 24, 2014)

i believe the pump is AF. same as bigterps. his impeller is noticeably larger. we shimmed mine down with all 9 washers and it was still approx .04+. put his impeller in with no shims and its perfectly in spec with no washers. with the plastic sleeve installed, his impeller fits nice and snug - mine tends to have a bit of side to side play, and i actually sent him a video the day i started running it of some rattle at low idle speed.


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## Ranchero50 (Jun 25, 2014)

Stainless, Ouch...

https://www.wooldridgeboats.com/store/equipment/stainless-steel-four-blade-impeller/stainless-steel-four-blade-impeller-6-18

Or aluminum...

https://jetdoctor.net/928

I don't know if I could weld the lip and then turn it back down or not. I guess the biggest trick is knowing what the pump taper angle is. Then it's just a matter of making a thicker plastic spacer.


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## BigTerp (Jun 25, 2014)

Yeah, your pump should be AF. That rattle now makes sense. Didn't realize you had that much play between your shaft and impeller. The clearance between your current impeller and shaft was so large I'm surprised it planes as good as it does and you get over 20mph. I'd guess that's why your at 5700 rpm's also. If my impeller fits close to .020 I'd bet you'll notice some vast improvements in performance!!!

The key you need is #1705. I just ordered one from here yesterday, ironically, just to have as a spare. https://jetdoctor.net/3595 If I had known you needed one I could have ordered you one. They only charged me $5.25 shipped for the key. I orginally had ordered a key from Marineengine.com when i ordered parts for my foot, but they sent me the key you have, which is to big for my setup. Your free to use the old one I have, but it's pretty beat up. Worked fine for the past 8 months in my impeller though. I just wouldn't run it too shallow or for too long with that crappy key.


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## Lil' Blue Rude (Jun 25, 2014)

The 5 7/8 impeller couldn't be welded up and turned back down to a 6 1/8 because along with being turned down smaller the trailing edge of the impellers are cut back as you can see in the pictures. The bottom impeller(6 1/8) blades almost touch the top of the impeller were the top impeller( 5 7/8) is about 1/2 to 3/4 of an inch from touching the top.
Don't buy the wooldridge stainless because it will hurt your performance big time. I had a built 40/28 and I lost 6mph and the hole shot wasn't any better then my stainless 3 blade impeller https://jetdoctor.net/1120. Stainless are well worth the extra money. Don't buy a key until you get a impeller because they have 2 different keys and key ways in the impeller. One is a square keyway and the other is a half circle keyway. 
Hope this helps some.


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## JoshKeller (Jun 25, 2014)

so you are confirming my thoughts that my motor has the 5 7/8" impeller when it needs the 6 1/8"? i assume this is severely hurting my performance?


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## dhoganjr (Jun 26, 2014)

If it is the wrong impeller there is a larger than normal gap allowing water around the impeller and liner. This will not load the pump properly allowing it spin higher rpm's than it should. It will drastically hurt performance, speed and holeshot.


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## Lil' Blue Rude (Jun 26, 2014)

JoshKeller said:


> so you are confirming my thoughts that my motor has the 5 7/8" impeller when it needs the 6 1/8"? i assume this is severely hurting my performance?


Yep and yep.


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## JoshKeller (Jun 29, 2014)

tried it out yesteday morning with the correct impeller. had 3 of us in the boat weighing approx 675 lbs, and hole shot was pretty good, still ran 22 mph. tried it out yesterday evening with just me and kayla, approx 325 lbs, and it jumps on plane, turns 5400 rpm vs 5750 and did 24 mph. at this point, i believe its the hull thats limiting speed, but the increased fuel economy (ran up 17 miles from hancock ramp on 4 gallons with 3 people vs 6 gallons with 2 people and smaller impeller) and holeshot will likely pay for a new impeller after a few months.


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## jerseyjimk (Jun 29, 2014)

you did 17 miles on 4 gallons of gas,thats great.There must be something wrong with my motor.i have a yamaha 4 stroke 60/40 and I get no where near that.does anybody else get performance like that? I just used Google earth for my distance,81/2 miles up river and 81/2 miles back.i used 10 gallons of gas.wow that sucks ass.


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## dhoganjr (Jun 30, 2014)

[url=https://www.tinboats.net/forum/viewtopic.php?p=357596#p357596 said:


> JoshKeller » 29 Jun 2014, 10:12[/url]"]tried it out yesteday morning with the correct impeller. had 3 of us in the boat weighing approx 675 lbs, and hole shot was pretty good, still ran 22 mph. tried it out yesterday evening with just me and kayla, approx 325 lbs, and it jumps on plane, turns 5400 rpm vs 5750 and did 24 mph. at this point, i believe its the hull thats limiting speed, but the increased fuel economy (ran up 17 miles from hancock ramp on 4 gallons with 3 people vs 6 gallons with 2 people and smaller impeller) and holeshot will likely pay for a new impeller after a few months.



Glad to here you got it figured out, sounds like it is performing more like it should now.


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## dhoganjr (Jun 30, 2014)

[url=https://www.tinboats.net/forum/viewtopic.php?p=357616#p357616 said:


> jerseyjimk » 29 Jun 2014, 15:58[/url]"]you did 17 miles on 4 gallons of gas,thats great.There must be something wrong with my motor.i have a yamaha 4 stroke 60/40 and I get no where near that.does anybody else get performance like that? I just used Google earth for my distance,81/2 miles up river and 81/2 miles back.i used 10 gallons of gas.wow that sucks ass.



That only figures out to about 1.7 mpg. When I ran the Mercury 115/80 four stroke on mine I could run 60 miles on 12 gallons or about 5 mpg. With the 250/175 I ran 84 miles on 17 gallons at about 4.9 mpg, if I stay out of it I think I could break 5 mpg pretty easy. 

With both that is not running WOT all the time, mostly cruising about 4000 rpms. I know with the smaller hp engines you don't have much you can let off, but just a little will get a little better fuel milage if you're trying to conserve fuel. I would think you would be getting better than you are though.


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## lowe1648 (Jun 30, 2014)

[url=https://www.tinboats.net/forum/viewtopic.php?p=357616#p357616 said:


> jerseyjimk » Yesterday, 15:58[/url]"]you did 17 miles on 4 gallons of gas,thats great.There must be something wrong with my motor.i have a yamaha 4 stroke 60/40 and I get no where near that.does anybody else get performance like that? I just used Google earth for my distance,81/2 miles up river and 81/2 miles back.i used 10 gallons of gas.wow that sucks ass.



I did a 170 mile trip this spring and used 24 gallons with a 2 stroke 40/28 Yamaha.


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## Ranchero50 (Jun 30, 2014)

I had to check the distance with Google maps but I run 4.5 miles each way, did it twice up and back 40+ MPH on 5-6 gallons of fuel with my 85 hp Rotax 2 stroke. So 3-3.5 mpg WOT...


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## semojetman (Jul 2, 2014)

I can run just over 65 miles on 12 gallons.
Mercury 60/40 4 stroke pushing a rigged 1856 blazer ss


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