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Sporadic miss cold and hot

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Datchew
Posts: 20
Joined: Sun Dec 21, 2008 10:38 pm

Sporadic miss cold and hot

Post by Datchew »

1964 r50/2

Magneto ignition with fresh coil
Rocky point ignition switch
12v system with "lathed" 1980's airhead rotor and stator with under-tank regulator.
Harms points plate.
Everything but crank and heads done by your truly since it was built from a frame and 15 boxes of parts in 2005 era.
That gives the background. I know the bike well. Have had it titled in 5 states now. Runs well.

One gremlin I can't solve is a sporadic miss.
When it's freshly started and idling low and cold, about every 10 seconds and sometimes not at all it'll miss a fire. It slows down for a sec and returns to normal.

Then when I'm riding, and also sporadically, if I'm too low rpm in a gear or heading up a hill in too low, it'll stumble similarly. Just 1 miss. Like red October "give me a ping Vasili, one ping only please."

The 2nd scenario makes me think carbs.
The idling scenario makes me think spark or timing.

My advance puts out a single image under a timing light. Have that merit badge.
Timing works to spec from OT to S.
Valves are right, carbs are right.

My next thought is to put my spare set of carbs on with a fresh rebuild of all the carb guts, new plugs and wires, points, condenser, coil, etc. And just start elimination of options but before I create so many backup spares, I'd love to hear if anyone has any experience with something like this.

Thank you

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Micha
Posts: 693
Joined: Fri Oct 12, 2007 2:24 am
Location: Israel

Re: Sporadic miss cold and hot

Post by Micha »

Hello,
I don't know specifically about the problem you describe, because as mentioned it can have many reasons, but checking the carbs for tightness is simple:
During idle rpm, brush engine oil on the carb's body, on it's connection to the engine head (flange) and to the air intake (at the pipe connection).
If you hear a change in the sound of the engine you will know where to look for a leak.
Last edited by Micha on Tue Oct 04, 2022 3:32 am, edited 1 time in total.
Michael Steinmann
R51/3 1952
Engine Nr. 529466

Daves79x
Posts: 630
Joined: Sun Aug 09, 2015 8:13 pm
Location: Knox, PA. USA

Re: Sporadic miss cold and hot

Post by Daves79x »

Look for a bad spark plug cap.

Dave
Dave

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VintageJim
Posts: 54
Joined: Thu Oct 07, 2021 2:07 pm
Location: Arizona

Re: Sporadic miss cold and hot

Post by VintageJim »

If you haven't tried a new set of spark plugs I would try that. I had a Honda 305 years ago with a similar problem and rebuilt the carbs, replaced everything in the electrical system (except the plugs because they were new) and spent over a year with no results. Then as part of my yearly maintenance I changed the plugs and the problem was gone and never returned. Vech has a post here about spark plug problems after flooding and it is a great one to read.
Jim

cwf
Posts: 96
Joined: Sat May 15, 2021 5:52 am
Location: London, UK

Re: Sporadic miss cold and hot

Post by cwf »

How about the wiring harness, any plugs/sockets and key/lights switch? I had a similar misfire that was an intermittent short to ground/earth.

Charlie.
75/7+ offroad sidecar; 50/2; R 35; XR125V; XR200A; Solex; 1939 Hillman Minx DHC.

Datchew
Posts: 20
Joined: Sun Dec 21, 2008 10:38 pm

Re: Sporadic miss cold and hot

Post by Datchew »

Thx guys
I feared it would be a brute force "check everything" scenario but was hoping for a telltale sign I wasn't aware of.

I'll start with cheap and easy and go down the line.
Systematic isolation of variables.

Experience says it'll be something dumb.
I'll report back when discovered.

cwf
Posts: 96
Joined: Sat May 15, 2021 5:52 am
Location: London, UK

Re: Sporadic miss cold and hot

Post by cwf »

Or condenser, if it has one.
75/7+ offroad sidecar; 50/2; R 35; XR125V; XR200A; Solex; 1939 Hillman Minx DHC.

FL54
Posts: 188
Joined: Sun Aug 12, 2018 5:09 pm
Location: Central Oregon

Re: Sporadic miss cold and hot

Post by FL54 »

Pure speculation on what the issue is but I would suggest one solution at a time and road test. Using the shotgun approach may solve the problem but you won’t know how it was resolved.
Central Oregon

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