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I have been working on a '78 Onan B48M opposed twin cylinder engine. The engine runs fine when I pour gas down the carburetor. However, when I feed gas through the fuel line, it leaks out the bottom of the fuel pump, which is connected directly on the carburetor. I suspect that the gaskets in the fuel pump will need to be replaced. I think the carburetor is a Walbro, but I am not sure.

Is it possible to remove the fuel pump and bypass it? I plan on having a gravity fed fuel tank so I didn't think it would be needed in this instance. Sure, I could get replacement gaskets for it and it will probably fix it but they are quite expensive for what they are. And if I were to bypass the fuel pump on the carburetor, would is still need an external fuel pump? I do have some Briggs and Stratton pulse fuel pumps that would work and even an electric fuel pump.

I am not that familiar with Onan engines. When it comes to opposed twins, I've only ever worked on Briggs and Strattons and Kohler's since they were a lot more common.

I have seen this many times on older Onan engines. With time the seals in the pump tend to dry out. Don’t know which carb and fuel pump you now have, because you did not give us your spec number. Most of the older Onan engines came with a Marvel Schebler carb. I have seen this engine with a Marvel Schebler carb with mounted pump with both three and four hoses. Have also seen some engines which were retro fitted with a different carb and electric pump. Carb kits are not that expensive and if you have an electric pump, they are reasonable also. If you have a carb mounted pump, we will need to know which style, because if it is part of the carb, with no visible fuel outlet, you will need to rebuild or replace the pump, even if you try to convert to a gravity system. Please post all engine numbers so we can pinpoint exactly which carb and pump you have. Pics of the carb and pump would also be helpful.

Arwing .......... You said that the pump was built on the carb...... Is it like the Briggs opposed twin carb with the pump made into it ????

I'm not asking if the carbs are the same, just curious as to how the pump is made onto the carb....... If it is built in on the carb then like Rivets said it has to be rebuilt...... They usually last for years and years with non etho gas and also running the carb dry for winter storage.......

Yes plus 1 on posting some pics ...............

Plus Tard Mon Ami ~!~!

Boudreaux In Eunice La. said:

Arwing .......... You said that the pump was built on the carb...... Is it like the Briggs opposed twin carb with the pump made into it ????

I'm not asking if the carbs are the same, just curious as to how the pump is made onto the carb....... If it is built in on the carb then like Rivets said it has to be rebuilt...... They usually last for years and years with non etho gas and also running the carb dry for winter storage.......

Yes plus 1 on posting some pics ...............

Plus Tard Mon Ami ~!~!

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If it is the carb style that I worked on in the past with the Case 446 mowers, it looks similar to the briggs the way it is part of the front of the carb housing.

ILENGINE said:

If it is the carb style that I worked on in the past with the Case 446 mowers, it looks similar to the briggs the way it is part of the front of the carb housing.

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That's what I was thinking..... The Briggs opposed has the pump on the front of the carb...... I relly don't think that the pump can be deleted and an external pump put in it's place....

But I haven't worked on one of those in quite a while...... It might can be done, BUT the gaskets on his carb shouldn't be too pricey.......

I did a little more searching and I would like you to take a look through this old manual and tell is whether the fuel pump, shown on pages 2-16 & 2-17 looks like yours. If it does then you have a three hose pump. Inlet, outlet and vacuum. These can be easily rebuilt. Take a look and get back to us with an answer and your engines spec number. http://ccmanuals.info/pdf/Onan B48G Service Manual.pdf

SRTsFZ6 said:

https://www.onanparts.com/index.php?main_page=index&cPath=1_12&sort=20a&page=5

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I'm having a lil trouble on my end with the forum.....

Anyway I don't think he has that Nikki carb... I could be wrong though......

Plus Tard Mon Ami ~!~!

Boudreaux In Eunice La. said:

I'm having a lil trouble on my end with the forum.....

Anyway I don't think he has that Nikki carb... I could be wrong though......

Plus Tard Mon Ami ~!~!

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I don't know either, but once that's determined (by pic's, #'s, etc) that place carries OEM parts (no Chinese knock offs)
so that should help repair that carb properly W/O modding anything..

**It'd be ALOT easier if the Op can shoot and post some close up pic's of the carb here. (as already asked)..

Thanks for all of the replies. I have some pictures showing the carburetor and the engine.

Just to clarify, the model is an B48M-GA018/3417A. Serial #: M783372445

It appears the be a Marvel Schebler Carburetor based on where the idle jet and high rpm jet screws are located. Are these carburetors any good?

As you can see in the pictures, the fuel pump is located directly on the carburetor and has a vacuum line going to it. There are several gaskets and diaphragms and has a spring inside. It is attached with two flathead bolts.

In truth, I do not know the exact origins of the engine. I do not know the exact year or what machine it came off of. I thought it was around the late 70s or early 80s.

I am missing some exterior parts to this engine such as the air cleaner cover, the screen on the flywheel, the intake tube, and the flywheel cover that attaches on the cooling shroud. I've tried looking on ebay without any luck. If anyone has those parts, PLEASE let me know and I'll make you an offer.

Looking at your pic's and description, there's several carb kits for that carberator (fuel pump as well-rectangular diaphram/gaskets) on Amazon.

I tried posting a link here, but it won't load.

Type in: iFJF K7523 Carburetor Repair Kit for Onan BG B43M B48M Marvel Schebler Model DD11 DD13 DD15 and they'll come up..

Here’s a parts manual for your engine. http://www.electricmotorsvc.com/pdf/onan-parts.pdf Marvel Schebler makes a very good carb.

Here's another link from a company that's not on Ebay and Amazon ..........

www.okp.com/onan-parts-carburetor-repair-kit-fit-142-0570-bf-bg-b43m-b48m-model-dd.html

Boudreaux In Eunice La. said:

Here's another link from a company that's not on Ebay and Amazon ..........

www.okp.com/onan-parts-carburetor-repair-kit-fit-142-0570-bf-bg-b43m-b48m-model-dd.html

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That's about the same price (and parts included) as what I found.

I'd certainly go that route for the repair vs modifying, etc.

Thanks for everyone's help! I installed the fuel pump fixed with a carburetor gasket kit. Now it isn't leaking fuel and is pumping fuel like it should.

I did get the engine running but it is running way too rich. So I adjusted the fuel mixture screws on the carburetor, but it seems like it will only run when the choke is halfway open. It smokes like mad when the choke is closed, and sputters and pops when the choke is all the way open.
I've tried adjusting both the screws in many different ways but the engine doesn't seem to want to run smooth at all. I did replace the gaskets that are on the intake manifold that mate with the block and the carburetor. I thought there might have been a vacuum leak or something.

I looked down the mouth of the carburetor when its running and there is fuel splashing out of a port. I would have thought this was some sort of accelerator pump or something, but I'm not sure. I think this is normal but adjusting the valves will either make it run way too rich or way too lean. For some reason I can't get that perfect fuel mixture. It runs the best at part throttle. When I push the throttle from part throttle to full throttle, it puffs a ton of black smoke out the exhaust and can't run right. It sometimes surges as if it isn't getting enough fuel which I can't believe.

I'm stumped on this one. Normally tuning a carburetor is relatively simple but this one is certainly putting up a fight. I cleaned it one more time and reinstalled it and it does the same thing. Any suggestions?

Here is a service manual for your engines. Review the fuel section on cleaning and adjusting the carb. I’m thinking you may not have set the float level right. They can be tricky as well as adjusting the mixture needles.
https://manuals.casecoltingersoll.c...NGINES SERVICE MANUAL 9-51393_watermarked.pdf