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Ironhead Sportster Motorcycle Talk (1957-1985) For all those that wanna talk about Ironhead Sportster Motorcycles

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  #41  
Old 12 Hours Ago
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Chuckthebeatertruck Chuckthebeatertruck is offline
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You're correct -- the bendix from the factory did not come with an insulator.

The insulator is a -78 part number (29250-78). . .but many a person discovered adding an insulator can make a big difference on bendix in hot weather.


However, this is one of those times where you need to put on your thinking cap. With the M linkert, there was an insulating block; DC linkert; there were insulating gaskets. With the Tilly, there was a bit insulating block. With the bendix, there was nothing and with the original Keihin -- ditto. But, in 78 an insulator becomes available . . .

We don't too often hear about running issues with DC linkerts; we hear about starting issues with the Tilly, but not running issues; and no one really says anything about Keihin . . . but there's a whole lot of threads about Bendix and hot running problems.

There might be a cause and effect . . .or maybe not. That's up to you to decide. This particular post might put it into a different perspective: http://xlforum.net/forums/showpost.p...58&postcount=4


So, this might be one of the few times I'm suggesting you don't "do it like the factory" and instead "do it like the factory once did" before the Bendix came out. That said, if you're only experiencing this seasonally and you understand the parameters under which it happens . . .you may not wish to mess with an insulator. Conversely, most ironheads seems to show themselves a bit better with a longer intake tract.


Similarly, yes, your motor was "cooling down" but as it did so . . .the heat went somewhere. First law of thermodynamics -- heat always flows from hot to cold . . .and the carb is colder than the manifold and heads. So, it absorbs a lot more heat then we realize.

The vaporization is the effect -- the cause is the heat soak. So the heads may be cool when you come back out . . . but float bowl is still down.


In theory, yes, a higher flowing petcock should break the lock -- but theory and reality don't always coincide. It's one of those cases where only you can truly tell what it going on. The internet denziens, myself included, can only guess and share our personal experiences. I've personally never sprung for an "authentic" pingle so I can't comment. I've only had generic repos of a pingle or el cheapo no name single handle petcocks. I've not had issues with any of these -- only with oem or AM wing petcocks.


I leave it to others to debate over avgas. I assume not go out of my way for fuel or to store special fuel when I can adjust the bike as necessary.
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  #42  
Old 8 Hours Ago
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I have most commonly seen needles sticking in seats on Amal equipped Big Bore Triumphs. If there's and engine that vibrates worse than Ironheads, it's them.

I think you'll find the biggest restrictor in your fuel path is the need;e/seat. Compare that orifice to your petcocks orifice and you'll see what I mean.

If the flow your getting into the float bowl is sufficient to run the engine at WOR (it is) there won't be a problem at lower rpm's.

So, something else is going on. You don't really need a new petcock.

Are you riding around with a "full to the brim" fuel tank ? If so, drain about 1/4 of it out and try again.
Some gas caps have a labyrinth vent and some use a one way valve. If your cap has a one way valve it's possible to pressurize the tank.

I think it was Ed Green (?) Cycle World editor who dropped an XLR engine into a KRTT and went to Willow Springs to "see what it would do".
The XLR engine had been "breathed on" and probably made good HP. In practice the bike was a rocket ship. (And just as maneuverable).

Come the start of race 1 the bike missed and stumbled so badly he pulled in on the 1st lap. Seems they wer worried about the length of the race and fuel consumption so wanted to make sure the tank was completely full. They might have even cooled the fuel. Don't know.
But the heat from the engine warmed the tank of gas which expanded. With no where else to go it pushed the needle open on the seat and flooded the carb. Race over.

If you haven't yet removed the filter from the equation I have to wonder why not. It's very simple thing to do.
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  #43  
Old 7 Hours Ago
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I tried for about a year to get an electric fuel pump to work. Did actually race with it for a while.
The basic problem is motorcycle carburetors have a too small float bowl. That limits the size of the float. They work well on cars running big four barrel carbs with large fuel bowls.

There are pumps made for carbs. In their specs they will say 4 PSI. Don't believe it. They don't go down that low. Now you need to add a pressure regulator. I even tried running two.

What we need here is low pressure high volume.

To get the volume (I'm running Meth) you need to run an alcohol needle and seat. These are supposed to hold 4 PSI Don't believe them, they won't.

It becomes a very fine line between enough pressure to get the flow rate where it needs to be and exceeding the floats ability to close the valve. Seat pressure.

During this whole process I became fairly aware of how these things work. Flow rates, pressures, needles and seats and vibrations.


Yes, funnily enough, vibrations. When I had everything balanced correctly (an adjustable furl regulator) the fuel would just slightly dribble from the over flow at idle. Lift the revs just 200 rpm and the dribble stopped.

The guys running Mikuni's on the Big Honda Fours were having a problem with vibrations too. At the end of the straight on full noise the needles were bouncing on the seats and flooding their engines. Took a wjile to figure that out and come up with a fix.

But the long and short of it is, gravity works. I now use that exclusively.

I have never had a vapor lock on any of my sidecars. Even sitting on the dummy grid when the air temperature hit 107 F. That was on gas, not meth. The engine was HOT.
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