Super easy conversion for vertical Briggs on zero turn

Propane, Butane, LPG, GPL, C3H8, C4H10
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evranch
Posts: 21
Joined: Thu Dec 13, 2018 11:11 am
Location: Saskatchewan, Canada

Super easy conversion for vertical Briggs on zero turn

Post by evranch » Thu Jun 27, 2019 10:31 pm

A rainy afternoon and I'm moving along with converting more of my equipment on the farm to propane. Next on the list was this pesky JD Z225 mower that has been a hard starter ever since I bought it and has a bad habit of draining the fuel into the crankcase. There are lots of expensive conversion kits out there for these but I got it done for under $50CAD using some modified commodity parts.

Images:
https://photos.app.goo.gl/V2ZvDK9x9GqpCRXh7

It's a "18HP" vertical 500cc Briggs that has been JD-ified by making some parts nonstandard. There is an identical motor in regular Briggs trim on the brushcutter beside it in the first photo, which might end up on propane if I can figure out where to mount the tank.

I have a Honda generator running this carburetor: https://www.amazon.ca/gp/product/B07PLMJ1Z6 which appeared to be very similar in form factor. It turns out to have the same bolt pattern and a similar throat size. It bolted up with the simple use of a pair of 3.5"x1/4" bolts and the gaskets supplied, and it even fits the original air horn and molded in air box! The only issue was that the throttle butterfly turned the opposite way in the generator, so I had to cut off the top stub and epoxy on my own small aluminum arm to mimic the original carb's control arm. Then I just strapped the negative pressure regulator to the side of the intake manifold. See images for details.

I would like to advance the timing a bit but I think I'll have to make a bracket to move the magneto, or pop off the flywheel and rotate it? Too much work for today. So it's running stock timing for now.

A little tweaking of the idle stop and mixture and this thing runs great out of the box and chews up tall wet grass without stumbling. The only issue appears to be under prolonged load it starts to freeze the BBQ regulator, causing occasional spontaneous stalling? It will restart immediately with no issues. The regulator shouldn't be freezing though, as I'm drawing vapour and the tank is not frosty... Maybe a high flow protection trip in the regulator in case the BBQ hose breaks? Not sure where to get a higher flow version of these guys.

Thought I would post this as these vertical Briggs motors are very common, as are these generator carbs, but I've never seen the two put together before.

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BigBlockMopar
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Re: Super easy conversion for vertical Briggs on zero turn

Post by BigBlockMopar » Fri Jun 28, 2019 1:07 pm

Sounds like you had a fun, rainy day.
I don’t have any small engines to tinker with but if I had I’m sure they wouldn't stay ‘stock’ very long... :)
Last edited by BigBlockMopar on Fri Jun 28, 2019 5:14 pm, edited 1 time in total.
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GLHS60
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Joined: Wed Jan 10, 2007 10:32 pm

Re: Super easy conversion for vertical Briggs on zero turn

Post by GLHS60 » Fri Jun 28, 2019 1:30 pm

Subscribed and following along!

I have a riding mower with a 15.5 OHV Briggs that looks like your Engine.

Had to install a manual valve to keep fuel out of the crankcase.

Adjusted the valves yesterday to cure a slow crank / hard start issue.

I'm a big propane fan and am intrigued by your conversion.

Thanks
Randy

evranch
Posts: 21
Joined: Thu Dec 13, 2018 11:11 am
Location: Saskatchewan, Canada

Re: Super easy conversion for vertical Briggs on zero turn

Post by evranch » Fri Jun 28, 2019 11:37 pm

I'm glad my project is of interest to someone!

Sounds like my engine all right. The first time it filled up the crankcase I blamed crud on the needle/seat. The second time I installed a valve as well... what a pain. I also added a boat style squeeze bulb primer to get fuel flowing to the carb, otherwise it required lots of cranking or a shot of ether to get going.

One thing to watch about these carbs is they are a little... cheesy I guess compared to the full size systems. They were designed primarily for generators, which don't idle. At greater throttle openings they run great, but at idle they rely on the vacuum drawn against the air cleaner to suck the fuel in. So if you are initially setting it up with the air cleaner off, it requires a much higher idle setting and enrichment to idle. Once the air cleaner was installed, I had to back the idle screw out many turns as well as lean it out to get a proper idle.

I think the BBQ regulator, which was fine for the generator, is badly undersized. A standard BBQ regulator is 80kbtu which translates to 31HP according to online converters. However... I realized that is 31HP as in 23.4kW of heat. Assuming around 20% thermal efficiency, that's only capable of supporting an engine with ~6HP of output. No wonder it stalls when the cutting gets heavy! It looks like there are economical single stage regulators available that will handle 150kbtu, which would double my peak horsepower and get it into the actual range of the engine.

evranch
Posts: 21
Joined: Thu Dec 13, 2018 11:11 am
Location: Saskatchewan, Canada

Re: Super easy conversion for vertical Briggs on zero turn

Post by evranch » Sat Jun 29, 2019 1:07 pm

It did turn out to be the excess flow protection in the black QCC fitting on the cheap regulator. I swapped the black one for a green 400kbtu QCC and was able to cut the entire yard with no issues. The regulator frosts up a bit after an hour but doesn't get excessively cold or change its behaviour. The tank is mounted on one of those black plastic "tank feet" that I bolted down to the back deck of the mower. It fits snugly and doesn't shift around at all.

I cut some heavy weeds and really bogged down the motor. This carburetor clearly has enough CFM to support the motor at WOT - it ran cleanly at this load with a deep growl rather than coughing smoke and stumbling as it previously did on gasoline. Fuel consumption seems good so far.

There are still some idle issues, once warmed up the motor will idle quite a bit higher than it does when cold. But it doesn't really matter as the mower is governed and is only idled to cool down the hydros after mowing. The idle speed setting is very sensitive at the bottom end, only a quarter turn from running quite fast to stalling. I think as the RPM falls below a critical point, the vacuum behind the air cleaner likewise falls and the motor leans out and stumbles down. If I look inside the carb there are those 3 tiny fuel ports beside the throttle plate, and I think they are coming into play in this throttle range, but they may not be properly sized for the CFM of this engine at idle. The idle enrichment screw doesn't do a whole lot, since this is a dual-fuel carb I wonder if it only adjusts gasoline flow? There is no documentation on these carbs whatsoever, unfortunately.

I'll play with it some more and see if I can figure out the idle. I definitely have an interest in doing so since I have an old 200cc ATV that should be capable of running on one of these carbs, but it will need to be able to idle nicely to be usable.

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