Onan surging/backfiring. Brand new fuel pump/filter/carb

Anything is possible, If you suspect it, fix it. Process of elimination

jaredmahone wrote:
The fuel line connected to the fuel pump between the carb used to have a proprietary hose clamp but I had to cut it off when I replaced the fuel pump and put an after market clamp that uses screw and it won’t fit down around the fuel pump like the old one. Is it possible I’m getting an air leak there? Could that really be it?

The section between the fuel pump and the carb is under pressure, not suction. So if the hose clamp and fuel line was not sealing you would be leaking gasoline instead of sucking air.

The opposite can happen between the fuel pump and the gas tank. That is under suction. So a leak will suck air.

I dont think your issue is gas/carb related. Which sucks because its an easy fix. To rule out your fuel pump, fuel filter, fuel line, it helps to get a water bottle can cut off the bottom. Then drill a hole in the cap slightly smaller than your fuel line. Use that setup to gravity feed gas into your carb. Its a quick way to help troubleshoot a genny or ATV or lawn mower. The one pictured here usually has a much longer fuel line with an inline filter. I borrowed the filter for a motorcycle and keep forgetting to order more.

In terms of your prime button mine sounds the same. It gets quiet once the carb is filled. Does make bubbling sounds. Look at your carb and on the bottom is usually a drain screw. You can pop that open to drain the carb when not in use. Its also a good way to determine if your fuel pump is priming and working decent. Lets say the genny is running rough and dies on you. One trick is to get a small container or a rag and pop open the carb drain. You should see a good 3-4 tablespoons of gasonline come out of the float bowl. If very little gas comes out, that means its a good chance the genny died because of lack of gas. Could be clogged filters, bad pump, out of gas in the tank. Its a quick way to start diving into the problem.

If your genny has breakers on it try turning them off. See how it runs when its not under any load.

Thank you so much for the response. I was told there were a couple of Onan gurus on this site. It's hard to believe it would be my fuel pump/filter. I walked into cummins in Fort Myers, FL a week ago and purchased both new to match my model number Onan. Replaced them that day.

Carb bowl is filling up. I popped open the carb drain and out came the 3-4 tablespoons of gas. I made sure to fill the gas tank to full right before we parked a couple days ago knowing I was going to change the carb. I don't think it's a clogged filter. The only filter between the gas tank and the carb is the brand new one attached to the back of the brand new fuel pump.

I always start the gen with the breakers off so there is no load.

I tried priming and starting it again and I didn't need to spray starter fluid in the carb to get it to turn over this time, which was exciting. But it immediately started surging and backfiring when it turned over so I cut it off. I don't want it backfiring and dirtying up my brand new carb.

I'll try the gravity feed tomorrow. Any suggestions if it's surging and sputtering even with a gravity feed? Thank you for the help.

Have you checked the air filter?

jaredmahone wrote:

I'll try the gravity feed tomorrow. Any suggestions if it's surging and sputtering even with a gravity feed? Thank you for the help.

To answer several questions there are much smarter guys here. They often link to a different website. Might be a cummins site. Im sure that link is coming soon.

Can you clarify how old the different gasoline is inside your tank? Might be a mix of old and new.

The part that bothers me is im pretty sure you have 100% fresh gas feeding the genny when you bypassed the tank and used your own fuel jug.

Top of my head things to check:
1. Double check the spark plugs. I had a motorcycle running rough. Decide to check spark gap. Looked a little tight. I stick the feeler gauge in there and the entire electrode could move up and down!!! Never seen anything like that. Visual inspection did not confirm the failure. I had to physically move the electrode. Its possible 1 or 2 of your plugs needs replacing, even if the other guy inspected them visually. Did you see him gap the plug? Or just check to see how black it looked?

2. Make sure the plug boots are tight.

3. Start researching how to ohm test parts of the ignition. Maybe this is an intermittent spark issue.

4. Compression test or even better a leak down test. I believe this is a used unit that has always run a bit rough?

5. This should be done when you check the plugs but also make sure both plugs are getting spark. Touch the bottom of the plug to bare metal when you turn over the motor.

did your new carb come with automatic choke or is the original still on? I believe you can move it manually to see if its working correctly

As someone else mention, check the air filter. Ours did that once and that is what it was.

The easiest way to diagnose your problem is to pull the inlet hose to your pump and attach another hose and feed it into a gallon can of gas. If it runs well that means your fuel pump is good and your problem is the line that goes into your main gas tank. From what you describe it sounds to me that is your problem. I would check the hose clamp on the top of your tank it could be loose and allowing air into the line. You may have to drop your tank to get to it. How old is the MH? If older than 15 years I would just replace the line, they can deteriorate inside, they are cheap to replace. Gas will also get to your carb bowl with a loose hose clamp but not enough to keep it running. The test I described will tell you. Let us know how you make out.

are you sure the spark is good? pull the gas line off and prime it should be pumping a decent amount of a fuel

i have a 4k 2000 era onan, it starts perfect, but starts cutting out, i have determined the coil goes weak and stops firing. i just got the ignition coil, my generator has to come out and be taken apart to replace. it was 12f here today, its going to be a while

if you pull the air filter and spray carb cleaner and it doesn't pick up, start looking at spark closer.

Any chance your fuel tank had some water in it? If that water made it to the fuel bowl that would kill the engine. Then if the fuel bowl was not cleaned out the water would taint what ever tests were done with auxiliary fuel supplies. This just happened to my riding mower.