1958 Johnson 18 hp surges and dies at wot

[Eoughphily]

Hi all, I am new to boats and outboards. I purchased a non running 1958 Johnson 18 HP for cheap, and spent the last five months bringing it back to life. I replaced the ignition, broken shifter, gear case seals, carb kit. I tore the whole thing apart, stripped, primered, and painted it (this took most of my time). I converted it to single line gas tank utilizing the mikuni fuel pump commonly used in a lot of these conversions (waiting for new USCG fuel lines on order). I used the method of removing check valves and plugging one of the holes in the intake manifold (was easiest for me at the time as I had the carb off and vacuum plugs on hand). I am experiencing two issues, possibly related or not, that's why I'm here asking questions. I've looked all over on here and on other sites dedicated to small engines and can't quite find my answer, or a point in the right direction. The first day I got the motor running in a test barrel, I took it out on the lake for some fine tuning. What's happening, is when I get up to high speeds/rpm/wot, without moving the throttle, the HP will increase giving a surge of power and shortly after will start to bog down and die, unless I start pumping the primer bulb. So obviously it is running out of gas getting extra air hence the surge, like the mikuni pump isn't keeping up. Any ideas on this? The other issue is after running it on the lake for 2 1/2 hours (only burned 1/8 of a 6 gallon tank...not bad) I noticed when I got back home the pulse line had some fuel in it, much darker fuel then fresh from the tank. It is a new pump so the diaphragm shouldn't be leaking. I'm wondering if fuel is flooding the intake manifold picking up carbon from there and leaking down into the pulse line giving it that dark look. Maybe float not closing the needle all the way (primer bulb gets hard when priming before starting). Anyways, just thought I would bounce this off you guys while it picks away at my brain...lol

 

[Eoughphily]

No Title

 

[nwcove]

if i were you, i would put the check valves back in, plug the pressure nipple, and plumb the pump so it gets its pulse from the bypass cover.. the shorter the pulse line the better . the mikuni pump should have no issue feeding that motor at wot. motor looks great! nice work!

 

[Eoughphily]

Does anybody else have any thoughts on this?

 

[Willyclay]

Been a lot of years since I worked on one of those beauties but I agree with what Admiral nwcove posted previously. He probably has several in his shop as we speak. The bleeder valves need to be functioning and the transfer cover port should provide a better source of pulse AND suction for your fuel pump. The link below will get you to a Johnson factory service manual that covers your model and there is lots of good info about the bleeders and fuel pumps on pages 175-179. Good luck!

http://boatinfo.no/lib/johnson/manuals/1922-1964johnson.html#/0

 

[Eoughphily]

I've considered tapping into the bypass cover. Haven't gotten to it yet.

 

[geoffwga1]

Ditto what nwcove says re the vacuum and pulse.If you want to keep using you're present setup you need to make sure the line from the nipple to the pump is as short as possible and locate the mikuni pump accordingly.The last one i did was a 57 5.5 Fisherman and I located the pump on the front left leg of the recoil starter..The first place i located it was on the other side of the motor and it was too long and did exactly what yours is doing. As said,the bypass cover is the best bet.

 

[Eoughphily]

My pump is located right next to the starter with about a 4" line, I could certainly make it shorter.

 

[Eoughphily]

So I went ahead and tapped into the bypass cover. The pulse line is as short as it can possibly be, but its still doing the same thing. The carb bowl is not staying full at wot. I can pump the primer about every minute and motor runs fine but if I dont it dies. The mikuni pump was brand new, is it possible I got a bad pump? Anybody have any other thoughts on this?