pressurized to fuel pump conversion trivial question

[ovnic1234]

Hello,
I have two 50's era motors, one a 52' Bigtwin and a 55-56 7.5hp Aquasonic. The 52' I just picked up and someone had already converted it with the fuel pump mounted to the bypass cover, which looks like it came off a later model Johnson (green). The 7.5 I converted by removing the check valves and blocking off one of the channels so that the pulse and vacuum of one cylinder didn't cancel the other.
Is this just a matter of preference? I can't visualize why one would be a better choice. Does heat play a factor in shortening the lifespan of the fuel pump being mounted on the bypass cover so close to where all the action is (combustion)?

Note: Great cowling on the 25hp, very practical design team Evinrude. 10+ screws plus some spark plug hood which of course is missing. I imagine there are a lot of them in watery graves. Next stop ebay.

 

[kbait]

Re: pressurized to fuel pump conversion trivial question

I think it's just a matter of preference, and what type of fuel pump you can find cheap. For the parts, try tim's outboard 218-682-2331. Very reasonable and lightning quick shipping!

 

[rjezuit]

Re: pressurized to fuel pump conversion trivial question

Most of the later factory installed pumps are directly on the bypass covers. So I doubt there is a heat issue. Rick

 

[jay_merrill]

Re: pressurized to fuel pump conversion trivial question

Doing the bypass cover mod is easier. I have a pair of '58 Bigtwins that have been converted. One has a bypass cover off of a late 60s Johnson 33hp (same green paint that you spoke of) and the other has the middle boss drilled through for pulse/vacuum and the outer bosses drilled/tapped for mounting screws.

There is a peculiarity with the original bypass covers, in that the bosses that were never used (except on the "offbrand" models, like Gale) are cast at a slightly higher position on the bypass cover, than was done on later models. The reason why this is an issue, is because the fuel pumps sold today are taller than the older ones. That results in the top of them hitting the throttle advance rod (located under the armature plate) at some power settings.

On my '58 Bigtwin that has the original by pass cover, I had to mount the pump upside down, in order to clear the rod. Lots of people have told me that the pump won't work properly that way, but I never had a bit of problem with it.

One thing to remember, if you convert an OMC motor by mounting a fuel pump on a bypass cover, is that you will then have an open pressure nipple, at the intake manifold cover. You must remove it and plug the hole with a screw and crush washer. The screw must be very short, or it will hit the leaf plate, directly behind the cover. Failure to plug the nipple/hole, will result in fuel mixture being sprayed all over the engine, cowl, etc.



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[F_R]

Re: pressurized to fuel pump conversion trivial question

Or just leave a piece of the air hose attached and plug it with whatever you have handy. Just be sure to clamp it so it doesn't blow out.

Jay, do you know if the 18-7351 fuel pump will clear the rod when mounted on the "high" cover?

 

[ovnic1234]

Re: pressurized to fuel pump conversion trivial question

Thank you for your replies, especially you Jay. I like these 50's era motors and imagine I will be doing a conversion again. I like the bypass cover because it eliminates one hose and it's a shorter run from the fuel line connection to the pump and from pump to carb.
The nipple is still there on the 25hp looks like the guy filled it with epoxy, out it comes and replaced with a plug.

thank you again, this is a lovely and useful thing "iboats forum"

 

[restornator]

Re: pressurized to fuel pump conversion trivial question

actually used plugs from a 40hp Mercury in place of the nipples. perfect fit. i just finished a conversion on a fd-12 and sure enough i forgot to plug that darn hole. it wasn't idling as well as it did with the pressure tank, well stupid me i hope that is what's causing this

 

[jay_merrill]

Re: pressurized to fuel pump conversion trivial question

Or just leave a piece of the air hose attached and plug it with whatever you have handy. Just be sure to clamp it so it doesn't blow out.

Jay, do you know if the 18-7351 fuel pump will clear the rod when mounted on the "high" cover?

The plugged hose works too. I actually left one of my Bigtwins that way for awhile. I used a short bolt to do it, because the threads help it stay in place, by digging into the inside of the hose. Between that and a zip tie on the outside, it stayed in place well.

On the fuel pump, I'm not exactly sure of the correct answer to your question, by the part number. If the pump you are talking about is the one that looks pretty much like the ones in my photos, except smaller, I think it will fit. I have one kicking around my shop somewhere - I'll see if I can find it later and mount it just to give it a try.

Another option that I have thought of, but have never tried, is a pair of the smaller, square fuel pumps running in tandem. You would have to have "Y fittings" on either side of them, but I think the pair would deliver enough fuel on a Bigtwin for WOT operation. You would also have some redundancy, in the event of a failure on one of them. In order to make that really effective though, I would put a pair of small in-line fuel filters on the output side, prior to the Y that feeds the carb. By doing this, you should eliminate carb fouling problems from a deteriorating diaphram in whichever pump failed.


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