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RickR90s
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Farmington Hills, MI
Joined: 11/28/2016
Posts: 66

'74 R90s with stock Dellorto's, stock ignition system (points and condenser)

Poor or inconsistent idle when warmed up. Sometimes, it will drop and idle smooth at 800-900 rpm. Sometimes, it feels a bit rough.

stock carbs with stock jets. mixture screws out 1 1/2 turns.

Valves are set at '.004" intake, .008" exhaust

Points contacts are clean and nearly spotless (I've had a Dyna Ignition booster installed for a couple years now)
Points gap, .016"
Timing is set at the upper horizontal mark on the flywheel

Just wondering what else could effect idle?

Condenser failing? I can measure it on my Fluke meter and it charges and dis-charges normally (set to resistance)
Plug wires and caps?

Thanks,

Rick

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RickR90s

schrader7032
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VBMWMO #7032
San Antonio, TX
Joined: 10/27/2006
Posts: 6805
I guess it's either carbs or

I guess it's either carbs or ignition? That's helpful!! Not that it will help anything, but I'd run the intake clearance at 0.006"...this is what BMW settled on after many years...doesn't hurt anything really, just lets the valve sit on the seat a bit longer and cool down some more.

A new condenser might be good to try. Measure the resistance from spark plug cap to plug cap...should be around 20-22K ohms. Are the coil towers clean and free of corrosion?

Might have an air leak somewhere. Try spraying each carb with some brake cleaner and see if the idle speed changes when it sucks in some combustible fluid.

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Kurt in S.A.
'78 R100/7 '69 R69S '52 R25/2

RickR90s
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Farmington Hills, MI
Joined: 11/28/2016
Posts: 66
Idle

Thanks Schrader for the info.
I tend to think ignition since the bike wouldn't idle at all last fall, while on a trip. In that instance, I messed around with the idle screws with very little change in running. I then pulled the front cover to see the points had closed down to about .004".

Set them to .016" and idle was restored.

Rode another 1,000 miles or so since then so I'll check the points gap again and verify the condenser is good.

Opening up the intakes to .006" sounds like a plan too.

As I said, the ignition system is mostly stock with a Dyna green 2 plug coil in place of the stock Bosch 6v units. I have stock wires and end caps that I believe measure out to around 1.2k, but I'll confirm.
And I added a Dyna Ignition Booster a few years ago, which I think mainly serves to prevent increase points life.

btw, stock Dellorto's, stock jets and mixture screws 1-1/2 turns out from stop. Timing set at upper horizontal bar on flywheel.

Thanks again,

RickR90s

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RickR90s

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