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Old 11-15-2014, 03:56 PM   #1
pennbarts OP
Joined: Nov 2014
Oddometer: 1
r75 /5 cold start


First post, so appreciate the patience. I've got an 1973 r75 /5 that has trouble starting in cold weather (tried unsuccessfully at 32 and recently 45 F). Appears on various posts that this is attributable to a wide range of problems. I have attempted to measure the voltage drop at the starter (original Bosch), and the battery output reads 12.6 V. The voltage across the starter reads 10.2 V. So, what's an appropriate voltage drop to the starter? Battery voltage drops to around 9 during starting, starter drops to around 7 or so during starting attempts. Haven't tested solenoid yet.

Any thoughts? Bike starts well above 50 degrees. I have the Dyna electric III system installed.

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Old 11-15-2014, 09:44 PM   #2
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Joined: Nov 2006
Location: North Shore Mass. USA
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Battery.....all Electrical Connections.
Enricher Circut (not really a choke)
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Old 11-15-2014, 11:39 PM   #3
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Location: Silver Spring, Md
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Ignition timing is critical to starting especially in cold weather. Have you checked the timing on this Dyna set up?

Battery voltage looks good but the drop is a little high. Battery voltage should be available at the starter before cranking. Sounds like ALL the wires need their terminals cleaned.

Has it been a long time since the valves were set? Too tight valves cause multiple problems. Poor starting just one.

Old heavy 20w50 engine oil? You are allowed to use fresh 10w40.

After all of this stuff is taken care of maybe we'll have to get into the carbs. But don't start with the carbs. One thing to check now is report the carb numbers. There is a metal tag on the side of each carb. It has a number. This number should be 64/32/??. What are the last two digits of the carb numbers? BTW, both carbs. Different numbers on each side.
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Old 11-15-2014, 11:49 PM   #4
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Location: Melbourne, Oz
Oddometer: 2,400
Tune: Timing, valves, carbs.
Carbs: ensure gaskets are in good condition. Poor gaskets let in too much air at startup.
Ensure the starting jet in the bottom of the bowl is clear. Blocked jets mean not enough fuel for the enrichener.
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Old 11-16-2014, 03:34 AM   #5
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Joined: Feb 2013
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If mixture and ignition are correct it should fire near instantly at 32 F. Thinner oil will help raise the cranking voltage and cranking speed but all the connections also need to be good too. I wonder if the dyna is voltage sensitive ? Do you know if there is a spark generated during the failed starting attempts ?

What about technique. Assuming you are using full choke the enricher only functions fully with the throttle shut so that enough vacuum is generated to pull mixture through the enricher circuit. The usual technique is to hit the starter with the throttle shut and then catch it with some throttle once it fires.

On some carbs the enricher disc only has one fuel hole which makes the enricher an on off device i.e. it will only provide enrichment when full on. Half way seems to do nothing at all.

I wouldn't pull the carbs to bits if it otherwise runs OK but one easy non disruptive check is to clear the small jet at the bottom of the float bowl. The one that feeds the enricher well.
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Old 11-16-2014, 03:50 AM   #6
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Location: Bath Uk
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These comments assume that you have followed the other suggestions abot starting technique and tuning the engine and you still have a problem.

You have a pretty wimpy Bosch starter in there. Bmw in 76/77 changed the gearing of the starter motor and increased its capacity, this was because owners were having problems starting their bikes from cold, even with relatively new components.

One wear issue with Boschs is that the bushes at either end of the armature wear, and this allows the armature to cock sideways under load, the starter turns very slowly and current consumption goes right up and the battery gets drained very quickly.

Fortunately you can now get 8 tooth replacement starter motors made by Denso, Valeo or no name Valeo copies. Any ot these will kick the motor over, much quicker than the Bosch and will make cold starting much easier
R90s 1070 replica, R90/6, 1958 R50
1971 Commando Fastback
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