Yamaha 4hp restoration and derestrict to 5hp

Lee1 · 29 March 2018, 16:30

Happy Easter all!
With the poor weather forecast all weekend I've decided to finish a last few projects before the season starts and my spare SIB outboard is first up. . .
I picked up a 4hp yam with a good 160 compression but a cracked steering mount and perished steering bushes a while ago so I decided to get started on it and was lucky enough to find another yam for sale in pieces in a box, this one was a 5hp and a latter model so I have a good steering mount, a 5hp carb and a later hood than the 4hp and with the 5hp badge

the leg where the steering mount goes on the 4hp was corroded where salt water has around the steering rubbers so that's been wire brushed cleaned and primed today as well as the 2 props primed for painting, I now have a spare prop and cdi coil pack that I will pack in the SIB kit

so in theory my new back up 5hp really is a back up lol will post up more pics when the project is fully finished. . . .

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beerbelly · 29 March 2018, 19:34

don't suppose you want to sell the spare transom screws do you iv a 5hp with bent ones without the flappy part you use to turn them

Lee1 · 29 March 2018, 19:53

Will have a look to see if I have the 4 I'm not sure I do. . . .

Lee1 · 29 March 2018, 20:00

Your in luck there is one with the handle and one without both thread bars are good condition, I take it you mean the transom screw thumb clamp yes, these are the non bolt type so I will have to fold the end in to get the disc off to remove it so you will have to repress it back on the disc once you fit it. Any good
. if so message me thanks

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Davie · 29 March 2018, 20:13

You can't just fit the carb from a 5 onto the 4 on this model for more power.

Lee1 · 29 March 2018, 20:31

I've checked the carbs, the 4 intake is a lot smaller than the 5 I've not run it with the 5 carb yet but will do as soon as I've finished the overall restoration. The carbs are exactly the same? I will pull the 5 carb off and pair it with the 4 for some pics

Are you saying the powerhead is different?

Lee1 · 29 March 2018, 20:34

The restoration is a 4AC 6eo the donor is a 5cs 6m3
If it don't bog down with the 5 carb then it's all good yes? if the bore is smaller on the 4 it will bog down no?

phantom 19 · 29 March 2018, 21:37

Quote:

Originally Posted by Lee1

I've checked the carbs, the 4 intake is a lot smaller than the 5 I've not run it with the 5 carb yet but will do as soon as I've finished the overall restoration. The carbs are exactly the same? I will pull the 5 carb off and pair it with the 4 for some pics

Are you saying the powerhead is different?

Look closely at the powerhead. They look the same at a glance but they are completely different- the 5hp is bigger bore. Only thing they share is the crank and conrod; the block, head cover and exhaust covers are different.

The 4 will run on a 5hp carb but bogs down.

The very last 5hps Yamaha sold in the UK had a new design plastic internal fuel tank, why they didn’t fit plastic fuel tanks in the 70’s when they designed the 4/5hp instead of the rusting metal ones is beyond me.

Davie · 29 March 2018, 22:00

Quote:

Originally Posted by phantom 19

Look closely at the powerhead. They look the same at a glance but they are completely different- the 5hp is bigger bore. Only thing they share is the crank and conrod; the block, head cover and exhaust covers are different.

The 4 will run on a 5hp carb but bogs down.

The very last 5hps Yamaha sold in the UK had a new design plastic internal fuel tank, why they didn’t fit plastic fuel tanks in the 70’s when they designed the 4/5hp instead of the rusting metal ones is beyond me.

As you say above but the crankshaft is also different.

What year did the 4/5 hp two stroke have a plastic internal fuel tank? I sold my last new 5hp two stoke a few weeks ago and I'm sure it had a tin tank.

phantom 19 · 29 March 2018, 22:14

Very last batch of 5hp YMUK brought in had plastic tanks on stainless steel mounting brackets.

Lee1 · 30 March 2018, 00:27

I think your right as I can't find a 5hp 6eo on searches, I guess the 4hp was the 4hp block at that that time so no upgrade will work, I did see on diagrams that the bigger later 5hp block used the old 4hp carb for the 4hp restricted version, oh well thanks for your posts! I guess I will have a totally restored 4hp for a lucky someone as a 4hp of this size and weight is no good for me as a back up if it would of been a 5hp conversion I would of suffered the size and weight, on the positive side I will have a wanted 5hp carb for sale to sweeten my labour lol I will stick to a trusted 3.3/3.5 direct drive outboards for my back up

it's all good healthy fun lol
I am tempted to test it out though as a back to standard 4hp once finished just to see if it beats the 3.3/3.5s by the extra 1/2hp!

Lee1 · 31 March 2018, 19:09

The steering bracket fitted, half a kilo of corrosion wire brushed off and in primer

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Lee1 · 01 April 2018, 20:21

Quote:

Originally Posted by Davie

As you say above but the crankshaft is also different.

What year did the 4/5 hp two stroke have a plastic internal fuel tank? I sold my last new 5hp two stoke a few weeks ago and I'm sure it had a tin tank.

So is the bore/cc on the 4hp 6eo outboard already running at its maximum fuel air compression with no way of getting more power by re-jetting the original carb?