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Unread 12-03-2018, 04:21 PM   #1
Not Yet Wild
 
Join Date: Nov 2018
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Default Keihin B70F YH19 idle screw location

I have looked at pics of the idle screw adjustment online but on my carb there is only what seems to be a brass plug in the spot, I have removed this plug and there is no tapered point on it and no spring. This is on a truckster OMC engine.
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Unread 12-03-2018, 04:28 PM   #2
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Default Re: Keihin B70F YH19 idle screw location

On further investigation I am guessing that this is the idle jet location and there is no adjustment on it.
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Unread 12-04-2018, 09:30 PM   #3
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Location: Central North Carolina
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Default Re: Keihin B70F YH19 idle screw location

I don't think you can adjust the idle mixture.

Here is the Keihin carburetor exploded view and parts list. If the fuel input tube (where the fuel line attaches) has a tiny filter inside it, remove and discard this filter. Replacements for it aren't available, and a good filter on the tank outlet to the fuel pump is more than sufficient

The carburetor rebuild parts that I needed came from my local forklift dealer. They ordered and received them for me. After rebuilding the carburetor, including accelerator pump, gaskets, float needle valve, and a new float, my 87 Truckster with 22 hp OMC engine now runs great. I also installed an electric fuel pump and removed the Cushman bypass filter and fuel return line to the tank. My fuel line now runs direct from the fuel pump to the carburetor and I made a cover plate and gasket to cover the original fuel pump hole. The electric fuel pump is self regulating for fuel pressure, so the return line and bypass filter aren't any longer needed.

I need to use the choke when first starting in the morning, then it almost always starts on the first revolution for the rest of the day.

A parts manual and parts supplement manual are available for download on www.sillylittlecars.com. They cover many of the 80's Trucksters. You won't find Cushman sheet metal, but the steering and drive train parts are still available. The supplement has full information on the OMC engines and electrical schematics. The OMC 18 and 22 hp engines, and the single cylinder OMC scooter engine are almost all the same as far as parts go. The 18 hp cylinders have the spark plug located below center, and the 22 hp cylinders have it located above center. The added hp comes from increased compression. I believe that the only other part that is different is the spark advance spring. Otherwise all three engine models use the same parts.

Charley
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Unread 12-04-2018, 11:12 PM   #4
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Default Re: Keihin B70F YH19 idle screw location

Thanks very much Charley, this engine has been transplanted into a Cushman Trackster many years ago, it also has no bypass for the fuel inlet from pump, It is flooding all the time . The needle valve seems to seal good when blowing air in but the fuel pressure seems to overcome it. I should buy a new needle seat for it I guess, I have followed your other posts on cleaning the carb but still can't figure out how the acc pump gets fuel up to the carb, the passage up through the float bowl seems to be blocked by a tan coloured plug made of plastic.
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Unread 12-05-2018, 09:10 PM   #5
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Default Re: Keihin B70F YH19 idle screw location

If you are using the original fuel pump, you have to have the bypass filter and return line back to the tank, because the pump will over pressure the float bowl needle valve without it. Since those original fuel pumps are almost un-obtanium now, if you do find one, it will be crazy expensive and the rubber parts will likely not last long since they are old and haven't been manufactured in almost 20 years.

Solve all of this and put in an electric fuel pump. The one that I have was bought at O' Riley Auto Parts. It self regulates the fuel pressure and will not over pressure the float bowl needle valve. It has 2 wires that need to be hooked up. The red connects to the wire that runs from the key switch to the coil. The black wire connects to the frame. Mount the fuel pump low and near the tank outlet, with a good fuel filter between it and the fuel tank. Then connect a hose from the pump outlet direct to the carburetor. Wait a couple of seconds for the fuel pump to build pressure after you turn on the key switch, then turn the switch further to engage the starter. That's about all you need to know about electric fuel pumps.

I don't remember exactly where the passage from the accelerator pump to the carburetor throat is, but I do remember that I needed a very fine strand of copper wire to clean it out and it took 3 tries to get it clear, even after soaking the carburetor parts in carburetor cleaner for about an hour. The accelerator pump is in the bump on the bottom and just to the right of the air intake when looking into the air intake end of the carburetor. A rubber diaphram with a metal center is the pump and it needed replacing too. There isn't a tan plug in the passage as best as I can remember.
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I have a spare Keihin carb, but it's in a box in the garage. If you are unsuccessful, I'll find it and take it apart to refresh my memory, but I can remember that I just kept putting a little gas in the float bowl and manually operating the pump, then taking it apart and cleaning it again, and repeated this until I managed to see fuel spurt into the carb throat when I manually operated the pump. Then I finished assembling the carb and installed it on the motor. No more problems since then.

Air flow around the engine is very important to keep it from overheating. The air control dampers are controlled by 2 bellows assemblies that expand when heated, if they are working correctly, to switch the air flow from the flywheel fan to cool the cylinders. Unfortunately, when these bellows fail, they default to the no air flow position and the cylinders overheat. Since I live in NC and rarely see temps much below freezing, I couldn't find replacements for these bellows assemblies, so I just removed the bellows assemblies and mechanically locked the damper doors fully open. I don't get much cab heat anymore, but the engine doesn't run hot either. If you plan on using your Trackster in the Cold North and Snow you will likely need to get this temperature control system working somehow.

Charley
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Unread 12-05-2018, 11:38 PM   #6
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Default Re: Keihin B70F YH19 idle screw location

Thanks Charley
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Unread 12-06-2018, 11:07 PM   #7
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Location: Central North Carolina
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Default Re: Keihin B70F YH19 idle screw location

Keeping all of the fins on the cylinders and crank case clean is important for keeping the motor from overheating too.

OMC motor parts are available from www.directparts.com and from www.denniscarpentercushman.com. You can even buy a complete new short block (crank case, cylinders and everything inside) but it isn't cheap at about $1,500 with exchange. Cylinders with valves and springs will run you about $300 each with no exchange.

If I was doing a motor exchange I wouldn't be using an OMC engine. I would be looking at a more modern overhead valve designed engine with parts still readily available at more reasonable prices. The 2 cylinder 22 hp complete overhead valve Predator engine now being offered by Harbor Freight comes to mind. I was even considering a Honda Gold Wing 1000 cc the last time I was having trouble with my OMC engine (broken exhaust valve), but decided that if I could get new exhaust valves for both cylinders that I would just repair it again and not have to do a major conversion to get a different designed engine to fit. Dennis Carpenter Cushman had the valves in stock for $39 each, so I just bought them. I would have loved the increased horsepower though. My Truckster is WAY under powered with only the 22 hp available from the OMC engine.

Charley
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