The engine may idle fine or it may cough and stall or cough

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panhead
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The engine may idle fine or it may cough and stall or cough

#1

Post by panhead » Fri May 25, 2007 7:52 pm

On behalf of pepsi:

I've had this bike for about 14 years and this problem has been a reoccurring issue for the last 8-10 years. The bike runs great down the road with good gas mileage. The problem is at idle. The engine may idle fine or it may cough and stall or cough and pick back up. It's fine on acceleration with no spitting when you crack the throttle. It may not do this for days or weeks and the come back with avengence. It done it with a fresh engine or a when it's had lots of miles on a rebuild. The carb is rebuilt with new shaft bushings and throttle shaft. The idle mixture screw can be set rich or lean and it will still do it. The intake has no leaks. I went as far as to go to electronic ignition to see if it was a stray spark because that's what is actually sounds like with no difference in performance. The spark plugs are light tan on the rear and slightly darker on the front one. The engine is fresh now and it still is coughing at times. It may idle for as long as you can stand it and run fine or it will just spit and die. You never know. I'm sure it would run fine with a different carb but I like the Linkert. Any help or suggestions would be appreciated.
Thanks, Pepsi



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#2

Post by Sadist » Fri May 25, 2007 8:30 pm

It's gotta be in the carburetor... I'm not intimate with linkerts (have not had to mess with mine much) but i'm very familliar with carburetors in general. The idle circuit and anything but idle circuts are totally independant of each other. A vacuum is created when the throttle plate is closed (idle) and fuel is pulled through a small orifice, more vacuum equals more fuel, (the amount of fuel / vacuum present is adjusted by the mixture screw). When you crack the throttle the throttle plates open, and the idle vacuum signal drops to nil, and fuel is drawn through the main jets.

look into a problem somewhere in the idle circuit. You'll want to even get into all the passages with a wire (like a guitar string) my guess is you'll find a partial obstruction, like a piece of cast flashing, sand nugget or gasket material that has worked its way into one of the tiny fuel / vacuum passages... my guess is something has been rattling around in there for quite some time causing the intermittant problem.

Good luck.

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#3

Post by john HD » Sat May 26, 2007 1:50 pm

i think cotten spoke about this in a recent post somewhere. something about glass beads or other debris in the idle slot.

perhaps he will chime in and comment.

john

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#4

Post by Cotten » Sat May 26, 2007 1:57 pm

Pepsi!

Your symptoms are classic signs of a vacuum leak.
How did make certain that the manifold was sound?
Did you test the pan cover screw over each intake port?

And Sadist!

The Linkert idle circuit is in no way independent of the highspeed circuit! When speeds are reached where the venturi pulls from the nozzle, the idle circuit reverses itself to meter air to the bottom of the nozzle well for highspeed operation. Thus the lowspeed setting affects the high.

If Pepsi's symptoms are not a vacuum leak, the most common Linkert tuning difficulties are shrunken venturies, borewear from the throttledisc, and floats that don't float.


....Cotten

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#5

Post by panhead » Sat May 26, 2007 5:35 pm

On behalf of pepsi:

I checked for vacuum leaks with WD-40 and carb cleaner with no leaking. These are O ring heads not plumbers fittings. I will have to pull the carb off and check the Idle circuit again. I have probably rebuilt this carb five or six times in 14 years. I have put over 85000 miles on since I built the bike the first time. I don't like to run wires into passages but I may have to use a piece of copper wire and give it a try. The float is an armstrong type that usually will hold back the fuel with the engine off and the petcock on. I don't do that on purpose but I have forgotten a few times and there has been no flooding. Damn soup cans. I'll be gone for a few weeks but when I get back I'll tear into the Linkert and see what happens. By the way back in the day I was a master motorcycle and auto mechanic. Back when fuel injection was for German imports. I like points and carburetors! Thanks again. I'll let you know.

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#6

Post by Cotten » Sun May 27, 2007 12:47 am

Pepsi!

Your leak test method is by no means conclusive.

Please pressuretest as per http://virtualindian.org/11techleaktest.html
(and don't ignore the pancover screws over the ports. Douche everywhere that may hide a casting porosity or hairline fissure. And give it a few minutes for even the tiniest leaks to make little blisters of foam. No leak is acceptable.)
Bubbles will tell you exactly where and how much of a problem may exist.

Until this variable has been completely eliminated, you risk chasing your tail endlessly.

Please do not run wire or drill bits through any of the metering orifices!
If a solvent does not open all passages with air pressure, then great care must be used to explore for blockages. The Idle bleed slot can be cleared with a sharpened feelergauge of the proper width. But above all, please do not probe the lowspeed needle seat.

Armstrongs were great floats. Until P4gas came along.
If it can still be removed from the bowl, its probably fine.
Stay away from Shell and ethanol blends with the Federally-mandated IVD additive..

...Cotten

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#7

Post by Guest » Sun May 27, 2007 4:27 am

i'm sure your spark plug wires aren't cracked or old. Air-borne pollution can cause dry-rot and allow juice to bleed-off. Grab the insulated wires the next time it's running and see if you flinch.

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#8

Post by fourthgear » Tue May 29, 2007 1:59 pm

That's going to be some flinch .

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#9

Post by Sadist » Tue May 29, 2007 9:08 pm

woah... thow all the basic carbuetor operation out of the window that was beat into my head when i was a kid when delving into a linkert.. neat. I've been around too many holleys for too long, i reckon.

Thanks Cotton...

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#10

Post by Cotten » Wed May 30, 2007 1:49 pm

Sadist!

It gets worse!

At lowspeeds (before the venturi reverses the idle circuit), air comes into a mixing chamber at the front end of the idle bleed slot, leans an emulsion already formed by air entering around the needle, and then exits back into the bore at the rear of the idle bleed slot.

(This is why tuning gets difficult if the groove worn into the bore by the throttledisc extends to the idle bleeds.)

The reversal of this convoluted and voluminous idle circuit is probably not abrupt.
So adjustments of the highspeed needle must be done at a steady state in order to feel effects, which may take a great lag in time before they manifest themselves. This means you need a long stretch of uninterrupted highway where you can leave throttle set at top speed as you tickle the carb.

Everybody please be carefull.

.....Cotten

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