Onan twin carb connection

i have an onan twin it runs good but eats gasoline im unsure of the size of delivery pipe going to the carb im thinking 1/2 is to small? the opening is about 1 1/2 wide
anyone else has done a setup on an onan 4.0 bfa the intake has very strong vacuum when i put my hand over it


I would match to the size of the carburetor’s mouth if you’re talking about the gas delivery pipe size.


The onan twin cylinder 18 hp motors had all liked to drink with the best of them. They are just a very poor motor for fuel efficiency. I have 4 of them in different case ingersoll 448 garden tractors. 3 of my motors where junk with I got them because the fuel pumps went bad and people ran them too lean causing piston scoring and blow by into the crank case to the point they blew the oil dipstick right out.
I replaced one with a 10hp diesel I had from another failed project and haven’t bothered fixing the other good motor I have. But my advice is let them drink because fuel is cheaper than a new motor.


Hello Ryan,
This is the opposed cylinders flathead (valves-in-block) engine? Correct?
What is your engine displacement size, please?
What is the power loaded RPM you are running it at, please?

The only OTC with OHV Onan made was a very rare diesel verssion of the CK…

Its going to require surgery to the tin work but you can fit a set of BG heads to the BF engine. Mill them flat so the head has no pocket over the combustion chamber ( About 0.060 milled off ) Use the same plug as the BGE ( not sure what that is off the top of my head ).
A CCKA or B or really any of the high compression heads for the 3600 rpm CK engines are also a fit for the BF BUT also will require some tin work surgery.

This little boost in compression will help with wood gas, you should bump the timing too.

My personal feeling is the BGE head is better because they are much cheaper and easier to find and its a newer design with a more central located plug and the camel hump for better gas mixing. Beleive it or not inspite of the different engines and decades they were made these all bolt on any 9 bolt cylinder head.

Stock BGE head
Milled BGE head
The high compression CCKA head


nice machine work reminds me of extra credit in vocational school for hopping up a briggs push mower lol
this generator didnt get ran hard i cleaned the valves set lash and put gaskets on the intake i kind of live off grid and fuel cost is something that is difficult my line went down in iceamegeddon they want to much to fix it so i need this to just run basic things like no more than a washing machine and well pump ive never managed to load it more than a little off [email protected] its runs the 3/4 hp shallow well pump without much of a grunt im semi new to wood gas/ simple fire i could distill things to a liquid fuel even turpentine since i have many pines that have fallen this is to get me though until i can build a water wheel and experiment with wood as fuel many ideas to explore i have watched the mr tesolonian build basic stuff right now i have to magyver something together with a old well pressure tank an old style galvanised pressure tank a few pieaces of pipe and misc around here im welding with an oxy torch the big tank i have is i think big enough to cool the gas down the old pressure tank i welded a pipe and cap to load it in the idea is to make charcoal then burn the charcoal and cool it down with my other parts i was thinking if i angled the 1/2 pipe and resticted the intake enough to get it to draw the gas i thought reduceing the intake down to far would not let it run and cause a strong vacuum on the 1/2 line in some ways that might be good it might be possible to go to 3/4 or 1 with what i have laying around but i wanted to ask i will get pictures of what im working with

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I did it by hand in a vice by draw filing and finished on a glass plate with a dial indicator to check for depth of cut and squareness.

I recommend you have a machinist mill to be sure two are identical…
I only needed one to “soup” up my LK. ( an engine that is basically one half of your BA but cast in iron )

Then get your best tin snips out and make some head gaskets out of this.


Theses engines tend to have issues with carbon build it. It has historically been caused by the quality of the fuel and the deposits it leaves, oil consumption, and light load. Woodgas is not going to leave the kind of hard carbon deposits in the engine that gasoline would ( also NG and propane ). You can tighten up the clearances on the BF and not worry about developing a carbon problem.

You do not have a filtration system with the BFA so you should change your oil more often. AT least until you are sure you have good clean gas… I might even be inclined to come up with an adapter to let you run the carb for start up and a gasoline wash down. You can run an oily mix of gas and 2 cycle oil for shut down to make sure the bore does not rust up in case your gas is wet.

Please look for the model an spec number on the unit and post it. I will find you the electrical prints for it and a service manual.


This is a 4 stroke Johnson "Chore horse " Canadian slant on the Maytag engine that many folks without hydro used when they wanted to wash cloths ( made them right up to 1970 ). When I was a kid we still had an Inglis washer with a Chore Horse doing the wash at the cottage…

The engines outlasted the wash machines.

If your smart you can convert an older wringer washer to run directly on woodgas.

Same could be said of the pump problem.
You can use a gas engine to pump a well, or a wind turbine, even solar if you move to a water tower instead of a pressure tank system.

Hot water is made easy if you cut a small fire box opening and convert a gas hot water tank to wood burning.

Propane fridges are expensive to run.
There are Kerosene conversion kits that do the same thing . Do the math on that and decide if kerosene is cheaper than propane ( I also tend to think its safer, but that’s a subjective thing… )


i looked into converting it to 220 but this one is the one that cant be i have hand filed heads down then used the glass method too i did a honda odessy once and went to far burning a hole in the piston hot spot formed it was a brand new wiseco too i got in a lot of trouble for taking it down to far lol i couldnt find an engine spec tag just the voltage output i have considered a repower with a diesel engine then put this beast in a cub cadet or log splitter i was planning on setting up this gen and log splitter next to each other so i could run the splitter on wood or charcoal as well its a 12 hp
on a side note the situation is i am left with tractors and other things i could sell before my father passed we started to do logging and well im here alone my mother moved back to ohio so my funding for prodjects is limited i also am born unique meaning i have id/birth certificate issues so no income from a job since my power line broke and transformer burned up they want to much ti fix yeah i know its their responsibility but im not the only one being gouged by alleged mystery/ancient billing the power company here is a pill your late on a bill a few days they come and yank the meter requiring an inspection then reconnect fee they give people issues over septic systems so this has to be labeled as a seasonal cabin…
excuse the grammar im on a phone…
if i can build something to run it i have a little 2 cycle generator i run most of the time small fridge but washing clothes and well pump won’t run on it its a 800 900 watt deal the onan as far as i know if a bfa 4.0 no oil filter i think it came out of a 80’s rv or earlier

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yes i was/am working to off grid i would love to get these old type of machinery it is just me and with id issues driving isnt possible but im in the mountains of kentucky heres some pics of what i have so far please excuse the welds im oxy/torch welding


opening for loading fuel into

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this is the tank that would have fuel loaded and fire built underneath it that would hold the charcoal or plastic pine to either be distilled or to burn/ make charcoal in

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this is the first tank after the firebox/fuel chamber
the idea is the gas would enter the lower portion that would help cool and can condense and drop out any nasty stuff
going from mr tesolonians setup
im assuming the lighter gas would go to the top of the tank from the charcoal tank it is 1 inch pipe

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this is a water cooling setup
after the first tank then the gas would flow though this and be cooled by water surrounding the pipe it goes completely though pipe is 3/4

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this is how im heating water


I am going to have to get my best glasses out to read that Onan name plate…
Just because it says its only 120 volt does not mean it is. It just happens to be more complicated to do and it was only done on items intended to export to 50hz 240 volt countries.

Wash machines.
You should watch this video.

Most of the electric bits will not work… Obviously. BUT and here is a HUGE but… Most of the time everything but the clock " you set the wash machine cycles on and it makes a lot of clicking noises ) will run on 24VDC if its designed to run on 120 AC ( read that over be sure you understand what I mean, just relays and controls ect valves .
Knowing this you can add switches and buttons to fake the signals the clock used to send ( but it never will work on DC only AC )

You really want to look around for a gas or oil fired hot water tank, they will burn significantly less fuel wood than what you are doing, trust me…

I will post more detail on Generator and look at the gen end connections when I have some time be patient…


inside the cover of the box it has a schematic but i do have the m1234 wires im not heating hot water with it just using it to run the pump and washing machine on cold water the washing machine is in a mobile home im in a 25x30 cabin the mobile home my mother lived in then moved back to ohio after my father passed the only reason to convert it to 220 would be to use a stick welder on otherwise 110 works fine at night i run a 2 cycle generator but its not big enough to run the well pump and i think a washer run time would be over 30 min i would love to have a ringer washer but i have to work with what i have it would be nice to have a partner that wanted to live a rural off grid lifestyle i only need to run the onan for a brief time but even if i run it on charcoal the power wont be wasted the way it runs now it would slurp 5 gallons in no time i heat my water by an old gas water heater
by burning waste wood/yard waste then i save the biochar/charcoal i have a tankless thats runs on propane my well pump is shallow well 110v washer is same as yours separate pump belt drive
but i dunno if a push mower engine would pull it
i thought about a push mower engine and building a wood heated clothes drier
as the one i have is propane but could be adapted to wood by an duct externally for now i hang dry
my water wheel should be able to give me the power i need but since im in a 25x30 not a lot of room i plan to build a cob home then i can integrate space to have it under one place i had the drier outside and beat myself to death getting it out of the weather into the mobile home
i was kind of left holding the bag when my mother moved to ohio its taken a while to organize the mess and build things to suit my needs…i have no income no id/DL no bc because the hospital lost it going from paper copy to digital so you can understand im working with very little…

ok so here is where im having the issue looking down the onan carb it restict’s down to about a 3/4 but the throat is about 1 1/2 or 2 inches i can mount a rubber coupler and the take a metal exhaust pipe and fold it over or restict it down then add in a gate valve for fine tuning the air to gas ratio but im unsure of the size of delivery hose to big and it wont draw to little it starves for air i was thinking 1/2 for fuel and 1/2 for air combined …

I thought I heard somewhere, it used the frequency of the pulses as the clock/timer.