70 HP Johnson to 90 HP?

I have a mid 90s model 70 HP Johnson (3 cyl) and I am curious if it is possible to convert it to a 90 HP in the same way that smaller (9.9 into 15 HP?) OMC's are converted. I just repowered with this 70 HP from a 90 and the top end is just not quite enough for the boat (20' Hurricaine Deck Boat).

Re: 70 HP Johnson to 90 HP?

not possible...sell your 70 and get a 115.

Re: 70 HP Johnson to 90 HP?

A 70 is an inline 3 and a 90 is a V4.
No

Re: 70 HP Johnson to 90 HP?

Perhaps we can give you a little help with the 70 if you can get us your present max wot rpm and speed
and the prop size.

Re: 70 HP Johnson to 90 HP?

steelespike said:

Perhaps we can give you a little help with the 70 if you can get us your present max wot rpm and speed
and the prop size.

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That would be great!!I will repost this weekend.

I was afraid that the 3 CYL wouldn't convert to a 90 HP, but it was worth a shot LOL.

Thanks for the help. I will repost this weekend when I can run the boat.

Re: 70 HP Johnson to 90 HP?

a 70 on a 20' deck boat is NOT going to obtain any fast speeds. I would probably put a 150 on it if it were mine. assuming it is rated for that HP of course. and it should be. a 115 would probably do ok. but a 150 would bring that boat to life for sure. But I think any boat that will not do over 40 is way too slow. lots of other people are happy with 25-30. so you get whats right for you.

Re: 70 HP Johnson to 90 HP?

If you put the max hp on the boat that is allowed you would probably be happy. Otherwise the old rule of thumb was 80 % of the boats max hp. Anything under that, not happy.

Re: 70 HP Johnson to 90 HP?

boobie said:

If you put the max hp on the boat that is allowed you would probably be happy. Otherwise the old rule of thumb was 80 % of the boats max hp. Anything under that, not happy.

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The boat actually had a 150 on it at one time but everyone around here says it was over powered with that much engine. I was pretty satisfied with the 90 that was on it when I bought it but alas the oil pump went out and the warning horn wasn't functioning so it is no more. The 70 that is on it now was the only engine available here at the time and the costs of shipping one is to prohibitive so I will either tinker with this one and wait on one to come available or learn some patience LOL. It is rated for a 115 so the 90 was about the smallest using the 80% rule and there is a 90 hourse 4 stroke Honda down here not being used right now but I am looking for a center console needing power before I try to broker that deal......

How much difference can prop size and pitch make? I am not a mechanic by any stretch and I have never used anything other than what was on boats that I bought OR were put on new boats when they were rigged. The engine "sounds" as if the WOT RPM's are not as high as they should be, and in fact when the hull is clean it does do much better speed wise and the engine sounds like it is actually turning at higher RPM's (the tach doesn't work on the boat but I think I know what to do to make it work). It has been in the water for about 5 months now due to the trailer bunkers needing repair and the lack of lumber available here but I plan on taking it out of the water this weekend, scraping the hull and repainting. It may well be all that it needs to be pretty satisfactory. Ain't owning a boat grand???

Re: 70 HP Johnson to 90 HP?

Where is here?? outboard motors are not that scarce..sometimes you have to wait a little while to get the right one..but they come around pretty regular. A 150 would do nicely on that boat I am sure..but if it is rated for a 115, I would put a 115 on it. but if you were happy with the 90..a 90 would do fine..a 70 will not do well no matter what prop you use, it is just too small. If you are near Savannah GA ..I have a couple of 115s for sale.

Re: 70 HP Johnson to 90 HP?

SparkieBoat said:

Where is here?? outboard motors are not that scarce..sometimes you have to wait a little while to get the right one..but they come around pretty regular. A 150 would do nicely on that boat I am sure..but if it is rated for a 115, I would put a 115 on it. but if you were happy with the 90..a 90 would do fine..a 70 will not do well no matter what prop you use, it is just too small. If you are near Savannah GA ..I have a couple of 115s for sale.

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I live at Guanatanamo Bay, Cuba. I have thought about buying a used outboard and shipping it down but the shipping costs run around $1500 and the base tries to keep the number of outboards that are shipped down to a minimum because they have a load of OLD boats here that are really not seaworthy that people are patching together with bailing wire and bubble gum and it is only a matter of time before someone gets in serious trouble on one of them. Otherwise there would be hundreds of barrell boats (pontoon boats made out of 50 gallon drums) instead of the relatively few there are here. I appreciate your advice, I know of one 90 HP 4 stroke here that is not being used and one 100 HP 2 stroke that needs some minor work that I may have to look into.

Not to hi'jack my own thread but there is a nice cabin cruise here for sale that needs repowering and I have been mulling over the idea of buying a new I/O and shipping it down. Does anyone know a good source for quality used I/O (outdrives included)? I have found a place at Lake Lanier in North Georgia but they seem a little shady.

Re: 70 HP Johnson to 90 HP?

my advice would be to stick to outboards...older I/O seem to really give a lot of trouble and be high maintenance. but that is just my observation, I am sure there are some people who think they are the best.
Depending on the condition of it I would go for the 100 HP 2 stroke, especially if it is carbbed.