Sears SS/Twin Onan carburetor - MyTractorForum.com - The Friendliest Tractor Forum and Best Place for Tractor Information
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post #1 of 22 Old 10-04-2019, 06:04 PM Thread Starter
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Sears SS/Twin Onan carburetor

Having trouble running at high throttle. Wanted to clean carburetor jets. What do i need to do? Do you use a small jet brush that I've seen advertized. ,,,,,,,,,,,, Thanks

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post #2 of 22 Old 10-04-2019, 08:41 PM
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Re: Sears SS/Twin Onan carburetor

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Having trouble running at high throttle. Wanted to clean carburetor jets. What do i need to do? Do you use a small jet brush that I've seen advertized. ,,,,,,,,,,,, Thanks
What is the model number of your Onan engine and do you see a carb ID type? (they used several different model carbs)

You can find free Onan engine downloads once you ID the engine model. Most of the old ones are B43 or B48.

AND to completely remove the carb you usually have to remove the intake manifold. (the service manuals show the different types of carb's and IPL's) (and a leaking intake manifold or the manifold gaskets will also appear to be the carb)

I've clean a few by carefully removing just the top of the carb while it's still bolted to the intake, being really careful and using a thin blade I can usually save the top gasket for re-use, then take low pressure air of about 20 lbs and I do not force direct inject the air into the holes. A good carb cleaner with a nozzle tube would be good.

Have you checked you ignition system yet so as to make sure it's the carb? A bad set of points or a condenser can make it appear to be carb. I would also install a new set of spark plugs of the correct type.


(some of them carb kits are expensive and the carbs are priced higher than gold.

Last edited by Forest Gump; 10-04-2019 at 08:47 PM.
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post #3 of 22 Old 10-05-2019, 06:08 PM Thread Starter
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Re: Sears SS/Twin Onan carburetor or ignition

I completly went through the fuel system with carb clean,new float and rebuild of fuel pump. The carb bowl is full when i take the top off.,,,, I am ready to move on to the ignition system. I have checked the points, spark plug wires and put new plugs in.,,,,,,,,,,,,,,,,,This is my engine (onan BF - MS/2379-B) ,,,,, could i be getting a bad coil or condenser. The engine runs rough on higher trottle and governor is constantly changing to keep it running. Might not be geeting good spark all the time?

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post #4 of 22 Old 10-06-2019, 04:15 PM
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Re: Sears SS/Twin Onan carburetor

Check the gap on the points, if they look old, might be time for new. (and replace the condenser when installing new points)Usually changing the gap on old points is kinda like peeing into the wind.

If you have a ohmmeter you can do a ohms test on the coil. (keep a heads up, you cannot stick just any Onan coil on that rig. They have changed design and ohms reading over the years)

Does your carb jet act ok at low idle.
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post #5 of 22 Old 10-06-2019, 11:37 PM Thread Starter
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Re: Sears SS/Twin Onan carburetor

points look good and checked gap ,,,,,,,,,, have new coil and plug wires ordered ,,,,,,,,,, hope that will do the trick. Bought coil on e-bay.Advertized to work on my onan part # 166-0535 ,,,,,, cross my fingers

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post #6 of 22 Old 10-08-2019, 10:28 AM
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Re: Sears SS/Twin Onan carburetor

Most issues on onans stem from poorly cleaned/tuned carbs. Speaking from experience... If it's hunting at high rpm, try adjusting the high speed screw in the carb. It is the adjustment screw towards the front of the carb (when standing on the muffler side) and should have an O-ring on the screw. I usually start at 1 3/4 turns off seat to start, and go from there. Some carbs I have worked on need over 2 turns, some need only one.

The carb for that tractor should be A DD11, and you should have a brass float in it. If it has a fiber or foam float, spend the $30 and buy a brass one. It will save a lot of issues. Also, make sure the intake manifold is sealed- it is made from two pieces of aluminum, and sometimes the sealent between the top and bottom half dries out and the intake starts sucking air, and causes it to run lean.

If re-adjusting the high speed screw, setting the float correctly, and checking for leaks doesn't fix the issue, then you can also change the hole the carb control rod goes into on the governor arm. It has three settings based on how "sensitive" the governor needs to be- sometimes as the motors wear out, you need to move which hole the rod is in. It is trial and error on that....

I have many of these tractors, and I love the onans. they can be finicky, but are well worth it when they run correctly. I have a couple that I have to tinker on constantly, but I can always fix them.

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post #7 of 22 Old 10-08-2019, 02:43 PM
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Re: Sears SS/Twin Onan carburetor

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Most issues on onans stem from poorly cleaned/tuned carbs. Speaking from experience... If it's hunting at high rpm, try adjusting the high speed screw in the carb. It is the adjustment screw towards the front of the carb (when standing on the muffler side) and should have an O-ring on the screw. I usually start at 1 3/4 turns off seat to start, and go from there. Some carbs I have worked on need over 2 turns, some need only one.

The carb for that tractor should be A DD11, and you should have a brass float in it. If it has a fiber or foam float, spend the $30 and buy a brass one. It will save a lot of issues. Also, make sure the intake manifold is sealed- it is made from two pieces of aluminum, and sometimes the sealent between the top and bottom half dries out and the intake starts sucking air, and causes it to run lean.

If re-adjusting the high speed screw, setting the float correctly, and checking for leaks doesn't fix the issue, then you can also change the hole the carb control rod goes into on the governor arm. It has three settings based on how "sensitive" the governor needs to be- sometimes as the motors wear out, you need to move which hole the rod is in. It is trial and error on that....

I have many of these tractors, and I love the onans. they can be finicky, but are well worth it when they run correctly. I have a couple that I have to tinker on constantly, but I can always fix them.

Very good info here, especially about adjusting the cab jets. On some of the onan carbs I use a offset screwdriver attachment (about 80-90 degrees offset) so as I can really fine tune the carb without my hands/fingers getting hot or in harms way/. Like the guy says some carbs are critical on the high speed jet at 1 turn or even at 2 1/2 turns. Adjust when the engine is hot and engine may run just little bit rough when first started up cold. On some Onan carbs with only a idle JET screw adjustment this jet will also make the high end run little rough if not fine tuned correctly.

Milwaukee, Harbor Freight and few other have them. You can use a 1/4 inch nut driver taped to the driver end and select the size bit needed from an assortment of power screwdriver bits. Here is what I'm describing as a offset type.
https://www.amazon.com/Offset-Screwd.../dp/B01N489TL4
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post #8 of 22 Old 10-08-2019, 09:07 PM Thread Starter
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Re: Sears SS/Twin Onan carburetor

i spent a lot of time on the carb - new high speed adjust needle, new float. Adjusted float and cleaned carb several times. ,,,,, I am putting new coil and spark plug wires in soon to see if that helps.,,,,,,,,,, we will see what happens and go from there.

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post #9 of 22 Old 10-09-2019, 11:15 AM
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Re: Sears SS/Twin Onan carburetor

Let us know how the new coil works out?
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post #10 of 22 Old 10-09-2019, 11:35 AM
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Re: Sears SS/Twin Onan carburetor

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Originally Posted by bws1148 View Post
i spent a lot of time on the carb - new high speed adjust needle, new float. Adjusted float and cleaned carb several times. ,,,,, I am putting new coil and spark plug wires in soon to see if that helps.,,,,,,,,,, we will see what happens and go from there.
That most likely won't help. I have never replaced the coil, rarely replace the plug wires except when very badly cracked... Plugs will help, but most likely will not fix hunting. Its all carb and governor.
Check the timing too, but then again, makes it start better, but won't help hunting.

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1967 Sub 12 Diesel
1967 Super 12 Special
1967 Super 12
1966/68 Sub "Miss Emma Lee" Diesel
1968 Super 12 Hydrotrac
1970 Sub 12
1970 Sub 12
1971 SS14 Hydro Diesel
1973 SS16 Diesel
1973 SS16 (red)
1973 SS16
1974 SS16
1976 SS18
1976 Roper RT16T with Loader
1978 Roper RT16 (pinky)
1978 GT18
1991 Craftsman with loader

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post #11 of 22 Old 10-09-2019, 01:42 PM
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Re: Sears SS/Twin Onan carburetor

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Originally Posted by GT_80 View Post
That most likely won't help. I have never replaced the coil, rarely replace the plug wires except when very badly cracked... Plugs will help, but most likely will not fix hunting. Its all carb and governor.
Check the timing too, but then again, makes it start better, but won't help hunting.
Right: hunting usually not ignition. Try slowly adding little bit of choke, changing points setting just little bit, timing.

But a spare coil is always a good thing when running a onan. I took time out and mounted the coils of the up on the firewall of my old onan engine tractors and they seem to last lots longer than when mounted on the engine. The condenser is also mounted on the firewall by the coil.

I use the solid core ignition wires also not the carbon type.
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post #12 of 22 Old 10-09-2019, 06:11 PM Thread Starter
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Re: Sears SS/Twin Onan carburetor

I will try some of those things and see if they help. Purres like a kitten at idle. Thanks for your help ,,,,,,, u guys have been working on these a lot longer than i have.

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post #13 of 22 Old 10-09-2019, 07:59 PM
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Re: Sears SS/Twin Onan carburetor

You can use later model GM car coils on two cylinder engines like Onans also..

This video explains how...



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post #14 of 22 Old 10-11-2019, 12:00 PM Thread Starter
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Re: Sears SS/Twin Onan carburetor

Thanks guys ,,,,,,,,,,,,, got her going and you where right " It's all carb and governor" It was the governor. I had the engine shroud off to clean the engine that was a mess. Probabley wasn't cleaned since the 70's. When i put it back on i got governor linkage too tight. That governor is more finicky than most the women i know.

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post #15 of 22 Old 10-11-2019, 12:30 PM
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Re: Sears SS/Twin Onan carburetor

Did you get to test the new coil you ordered?

Reason I ask is I bought a NEW coil awhile back that was suppose to work on points type Onans and it would not fire any at all. It was the kit type coil, this one.
https://www.ebay.com/itm/New-Replace...72.m2749.l2649

You can bench test a coil easily just using a battery and rig spark plugs and wires together and just momentarily touching the neg post of the coil to ground to simulate points closing and opening and see if both plugs fire at same time. Set the plugs gap at about .045 to see good fire.

I immediately returned the coil for a refund and went elsewhere for a replacement.
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