Tag Archives: outboard

LPG outboard hack 2

A short video showing how we got my Honda BF5 engine running on LPG by removing the carburettor completely and controlling speed using a needle valve.

I’m starting to wonder how many parts I could take off and still have a workable engine!

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2015-06-10 · 15:36

LPG outboard hack

I’ve been having some problems with the Honda BF5 outboard engine. Last week it failed to start at all  When Dad and I opened up the float chamber under the carburettor, it was full of white gunk.


I’m not sure what it is. It looks a lot like salt, but it is squishy and doesn’t taste like salt. We cleaned it out, cleaned the main jet with a needle, and eventually did manage to get the engine to run, but it wouldn’t idle. There’s a second path through the carburettor that’s probably the slow-running jet, and we suspected that was clogged. I took the engine to Hendy Marine at Swanwick and their engineer cleaned the carburettor in an ultrasound bath and fiddled with the engine until it ran OK. But it still wasn’t behaving properly.

This sort of thing worries me because it shows the carburettor is very sensitive to problems, and isn’t easy to fix. I don’t like things that I can’t fix myself. I’m sure I could come to be much better with engines given time, but for now it’s a problem.

Last year I thought about buying a LEHR propane outboard instead. Purportedly reliable and clean, it would also mean that I didn’t have to carry petrol on board. I personally know one person whose boat was gutted by a petrol fire, and I’m really not keen on having it around. There’s also the appeal of having a single fuel for both cooking and propulsion. LPG (propane or butane) is a lot less dangerous than petrol. On Yachting Monthly’s crash test boat they had trouble getting it to explode even when trying their best!

Then I heard that four-stroke petrol outboards could be converted to run on propane. I found suppliers of kits (mostly for generators) and there are even some DIY conversions on YouTube.

Talking to my engineer father about it, it all sounded pretty simple. And the more I thought about it, the fewer parts I thought I’d need to just get the engine running. Finally, I went outside, bodged a connection between the gas bottle and the carburettor, and got the engine to start without much trouble. I immediately did it again on video to show you all.

All that I’ve done here is rely on the gas bottle’s cooker regulator to provide a constant flow of gas into the outlet of the carburretor float chamber, so it’s squirting through the jets into the Venturi. It so happened that the gas pipe fitted over the casting. No extra hardware was required!

Of course there’s a big difference between the engine running and it being reliable and efficient. But I think all I need to achieve that is to add a regulator that is driven by the low pressure in the Venturi. One of these might do it. And I should make a proper fitting that screws into the jet inlet.

If that works I will have made a completely reversible and easy conversion. Watch this space!

Edit: I did some more research and found that a component often used to control the flow of gas into the engine is a “demand regulator” or “demand valve”. This is similar to the valve used by SCUBA divers to suck air from their tanks. This makes a lot of sense since we want the engine to suck gas from the tank in proportion to the amount of air it is drawing in, rather than squirt in a fixed flow of gas. The IMPCOGarretson KN” seems to be commonly used, and there even seem to be clones. This video shows a demand valve mounted on the side of a generator running on natural gas. This does seem to be a large component, possibly sensitive to movement.

The LEHR outboard carburettor seems to have a disc-like valve on the side as well, but it’s much smaller than the Garretson.

LEHR outboard engine carburettor

LEHR outboard engine carburettor

Edit: I checked with Calor Gas, the suppliers of the gas bottle that came with Tammy Norie and discovered that it contains 25 year-old butane not propane as I originally thought. As far as I can tell that doesn’t change things very much.


Filed under engine, Equipment, Repairs and Modifications

Engine starter fixed

When Tammy Norie’s original engine broke, I bought a second hand Honda BF5 from Seamark Nunn in Felixstowe. Unfortunately, its recoil starter broke almost immediately, stripping the teeth from the starter sprocket.


I bought a new sprocket, but that jammed during my messing around in strong winds off The Naze and I had to dismantle it and use the emergency starting cord.


I’ve been using the starting cord ever since, and the engine has been fine in all other ways. But I don’t like running it with the cover off. There’s a chance of a rope or clothing getting snagged in the flywheel.

Seamark Nunn offered to take a look at the engine under guarantee, and I finally had a chance to take the engine to them yesterday (2014-09-10). Josh, their engineer, immediately helped me diagnose the problem by pulling out a similar engine and finding a diagram from the service manual.


I thought perhaps something was bent out of shape, because my sprocket teeth didn’t engage very deeply with those on the flywheel, but Josh pointed out that my fixed cap (part 9 on the diagram) and split pin (part 7) weren’t locked together in the same way as on the other engine, and that this would mean that the sprocket wouldn’t drop out of the way of the flywheel when the engine started. That would account for the teeth getting stripped: when the engine fired up it would push very hard on the sprocket.

I looked at the other engine and noticed that I had a piece missing.


There’s a large plastic washer (part 11) that spaces out the sprocket (part 4) from the recoil assembly (part 3). I’m certain that my engine never had one of these, and I did notice some home-made plastic spacers that I found suspicious. I reckon that the original owner lost his washer and then attempted a bodge. The result is that the split pin is too low and falls out of position beneath the cap. You can see it escaping in this video.

Josh found a spare washer and a spare sprocket. We put everything together and things worked much better. I might also bend the split pin slightly to make double sure that it can’t escape.

I recommend Seamark Nunn, who have been friendly, helpful, efficient, and professional.

I also had a great chat with Josh, who is restoring a Cornish gaff ketch and has plans for a six-year circumnavigation. If he starts a blog himself I’ll link it from here. She looks lovely.

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Filed under engine, Repairs and Modifications