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  #1  
Old 4 Weeks Ago
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Default Ironhead high speed cruise prob

Hi,

Got me a '66 Sporty with a Mikuni carb yesterday and when I cruised at steady 70 mph on a cool evening it ran nice for a while, then started bogging with a popopopopo sound and lost one cylinder. I just let the throttle go, pulled the clutch and let it idle for a couple of seconds, and the engine ran nice again and I stayed around 60 or below.

This was very similar that happened regularly to me on a Shovelhead, never figured that out. Only thing that I managed to associate with the bogging was that the oil cap popped off every time.

Didn't notice if the oil pressure rose on the Sporty.

Ideas?

Accelerating coldish, the bike lost power too. Ran better warm. Could this be just a fuel feed problem? The needle on the Mikuni had been raised some, on the 5th notch now. Could be a hint in that direction.

Thanks,
Larry

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  #2  
Old 4 Weeks Ago
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Default

Is the carb float set correctly? How about the petcock or the screen/filter on the petcock? Old fuel line may be swelled up inside, restricting the flow? Sounds like it's not getting enough fuel. Sounds like letting it idle lets it refill the float bowl.
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  #3  
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kitabel kitabel is offline
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The air vent in your gas cap is plugged.
The next time it happens, pop your gas cap and watch it go away.
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Default

Hey, I just realized the fuel lines may be from before they raised the ethanol percentage in the fuel here. So the lines may very well be swollen or otherwise gunky. The filter looks clean and neat.

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Default

Quote:
Originally Posted by kitabel View Post
The air vent in your gas cap is plugged.
The next time it happens, pop your gas cap and watch it go away.
Definitely could be that, too. I heard the cap hiss today as the sun warmed the tank. It should breathe more freely, shouldn't it?

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Default

i had the gas cap issue as well. similar carb choke under extended load. i switched to a better vented cap and everything is good again. this was after changing all the filters and lines and rebuilding and tuning the carb to no avail.
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Check your oil level. Seriously, my 80 has this same problem if I let the oil level drop too much, something about not enough oil getting to the exhaust valves during constant high speed causing them to stick, letting off the throttle allows the pump to catch up. This will lead you down the path of valve seals which is a dark alley I don’t recommend you explore.
But really, top up your oil.
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Default

If you have a clear fuel filter on the carb, you can watch and see if it is starvation. As previous posters mentioned. Loosen cap significantly and see if issue disappears, if not I suspect fuel starvation even if you can’t see it. Possibly low float, but you said steady load. On the mikuni the float level is simple to check. See mikuni reference in Sportsterpedia.
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Default

Thanks guys. All of this is educating. The cruising issue has not repeated itself, but can't get the top revs in the game. The needle has already been lifted pretty high, so I'll get a bigger main jet anyway and check the possible starvation causes like float height.

That sticky valve theory sounds interesting. I'll keep that in mind as well. The oil level was fine but that doesn't rule out any other possible oil circulation problems.

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Default

The needle has already been lifted pretty high, so I'll get a bigger main jet

Not going to do it, they control different functions. The engine will run quite well with the main jet removed until the needle is fully retracted.
When the needle taper is exhausted as a variable, the next step is the next larger needle jet. If P-4, go to P-5 etc. and re-set the needle down 1 notch.

Some really wild guesses here...
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