Forum Discussion

Geocritter's avatar
Geocritter
Explorer
Feb 13, 2015

Need help with carburetor adjustment on my Onan generator

I’ve been working on The Behemoths generator lately. My rig has a 7kw Onan gasoline powered generator. When I first tinkered with it I learned that it needed a new circuit board, which I was able to purchase from a Dinosaur Electronics distributer. I also replaced the oil, oil filter, and spark plugs. However, it didn’t run long before the spark plugs fouled out. The fouled plugs and the exhaust odor pointed to carburetor problems and a possible bad valve (one of the two original plugs I’d replaced was heavily fouled). I serviced and thoroughly cleaned the filthy carburetor inside and out and also gave the engine a compression test. It passed the compression test with flying colors so I replaced the spark plug wires, since that was the most logical thing that would have caused a weak spark in only the one cylinder.

I fired the generator up today and it ran much better, but here’s the problem. The carburetor I have has environmental issues and is no longer being made, nor are service parts available for it. What I’m looking for is the initial starting point adjustments for the main jet adjustment screw and the idle jet adjustment screw. As it was, before I took the carburetor apart, I screwed both adjustment screws all the ways in, counting the number of revolutions on each screw before taking it all apart. When I put the carburetor back together I set the screws to those same adjustments. The generator initially ran too rich but after turning the main jet screw back in I had it running better but still not to my standards. I’d like to know what the factory recommended initial starting point jet screw adjustments are. It’s an Onan #7NHMFA261060 generator equipped with a Nikki #146-0580 carburetor. If anyone has a shop manual that covers that carburetor it might have those initial adjustment settings.

BTW you’ll never guess, I think this is funny and ironic, the recommended EPA blessed carburetor, which costs $380 is #146-0666 (the devil made them do it! LOL).

Steve
Class A
  • romore's avatar
    romore
    Explorer II
    Feb 13, 2015
    Start at 1 turn out. Once it starts and warms up turn the main in very slowly with the engine under load until the it starts to die then back out 1/4 plus a tich which is a bit more than a hair. Holding the throttle against the stop screw, no load this time, do the same with the idle mix screw. When you release the throttle shaft, it will snap open and the engine should pick up cleanly.
  • Geocritter's avatar
    Geocritter
    Explorer
    Feb 13, 2015
    Romore, thank you! That's exactly the information I'm looking for. I have one final question. Do I start the idle mix screw at 1 turn out?

    Thank you again for your clearly detailed answer!

    Steve
  • dougrainer's avatar
    dougrainer
    Nomad
    Feb 13, 2015
    1. The CALIFORNIA air control board is what determined Tamper proof Onan carbs, NOT the EPA. California 25 years ago had strict requirements on small engines and since Onan could not just build APU's just for California, they made ALL their carbs California compliant.
    2. When you rebuilt the carb, you DID remove the needle jet? That is the brass looking thin cylinder that has about 16 or 20 microscopic holes in that jet. If ANY of those holes get gummed up, the Carb and Genset will not run right. The ONLY way to clean that is to remove and clean out with Carb cleaner and compressed air. Doug
  • midnightsadie's avatar
    midnightsadie
    Explorer II
    Feb 13, 2015
    do what doug says I,ve fixed meny a carb for people just by claning the jets , then add a can of sea foam to the gas. you did clean the old gas out?
  • Geocritter's avatar
    Geocritter
    Explorer
    Feb 13, 2015
    Guy's I appreciate your concern about the micro-holes in the jets and yes I thoroughly cleaned them all out with spray carb cleaner and compressed air and then visually inspected them with a bright light and a magnifying glass after I cleaned them. A few needed actual picking but for that I used a sharp wooden toothpick. Even though I have fresh gas in the tank I also ran the fuel line into a catch basin and flushed it all out via the units electric fuel pump. This ain't my first rodeo by a long shot.

    BTW thanks for the California heads-up I'm not surprised the issue had California's interference written all over it.
  • wildmanbaker's avatar
    wildmanbaker
    Explorer
    Feb 13, 2015
    Geo, when you get it straighten out, could you get back here and enlighten us as to what actually worked? There is very little information about the carbs to be found.
    Thanks
  • Carm's avatar
    Carm
    Explorer
    Feb 13, 2015
    Geocritter: Re - California interference: A couple of years ago my 7kw Onan started to stall under load. I removed the carb from the generator and stripped it down. I found a bit of crud partially blocking the needle valve so I blew everything out with compressed air. Since the gen set was 15 years old I figured it might be time for a carb overhaul.

    I headed off to my local Onan dealer and asked for a carb rebuild kit and was told that:

    1. Due to California emissions rules it's against the law to touch the carb - if there's a problem you just throw it away and we'd be happy to sell you a new one for $800. (Must be a special Canadian price)

    2. The carb is factory sealed anyway so you can't take it apart.

    I already had the carburetor in pieces. I'd apparently missed the part about it being 'factory sealed'.


    I declined the offer to sell me a new carburetor. I re-installed the old one and the generator has been running fine every since.
  • Geocritter's avatar
    Geocritter
    Explorer
    Feb 14, 2015
    I'm going to be busy with other issues all day tomorrow and maybe Sunday but I may get a break on Sunday afternoon or Monday to finish work on the generator. I'll be sure to post the results of my work so that others here can benefit too.

    $800 for a new carburetor in Canada! All over a California based environmental issue. Don't say it Steve, bite your tongue, just bite your tongue!

    Steve
  • Geocritter's avatar
    Geocritter
    Explorer
    Feb 19, 2015
    As promised here are the results of my tinkering with my Onan generator.

    To obtain preliminary carburetor settings I emailed Onan customer service and was referred to one of their techs. The fellow I spoke to recommended that I preliminarily set both idle and main jet adjustment screws to 1-3/4 turns and adjust from there. I followed his directions and wouldn’t you know it, by the time I was done both jets were adjusted very near to the 1 turn setting recommended by romore earlier in this thread (BTW his “1/4 plus a tich” adjustment worked great). My generator now runs fine.

    Thanks to everyone who posted.

    Steve

    PS I found that it’s much easier to keep track of adjustments if I put a dab of White Out on the adjustment screws. I commonly use the fast drying White Out for marking up mechanical parts. It’s handy because it not only dries fast but the little bottle has a built in paint brush.

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