R25/3 Surging at Idle, Until it Dies?

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swagsherman
Posts: 3
Joined: Sat Jul 25, 2020 9:09 am

R25/3 Surging at Idle, Until it Dies?

Post by swagsherman »

I have a 1955 R25/3, starts right up, nice and smooth idle for about 1 minute, during 2nd-ish minute of idling, starts surging, smoothing back out, surging again, until it cannot surge past stalling - then it stalls. I'm running a reproduction bing carb from ulismotorradladen. If I pick the idle up, run it a little high, eventually the pattern repeats itself (even at the higher idle), until it dies. Eventually, needs to rest before it will start again.
No leaks from the carb, float needle in position 2, air mixture screw at about 1 1/2 turns.

This bike is new to me. I owned a 1954 R25/3 25+ yrs ago and had to sell; replaced with the '55 just this past spring. Any thoughts? Appreciate any and all....

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schrader7032
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Re: R25/3 Surging at Idle, Until it Dies?

Post by schrader7032 »

Welcome to the forum! What was wrong with the original carb, do you know? I wonder about the reproduction carb. Is there a carb number that is similar to Bing? What about the other parts of the carb...the idle jet, the main jet, etc.

I'm having some issues with my R25/2 not nearly as persistent as you. I do get some surging after I've ridden the bike for a while. At one point, I thought my air mixture screw was the wrong kind...there's a short on and a long one...the long one has a hole drilled in the side to allow air to get in. But that didn't help. Plus, whenever I screw the mixture screw in or out, it makes no difference on the way the bike runs. The latest suggestion for me is to get the bike warmed up again and spray something like carb cleaner around the carb and its connections. Somehow air is getting into the carb at low throttle settings but not through the air mixture screw. You say there are no leaks, but I'd do something like that just to be sure.
Kurt in S.A.
'78 R100/7 '69 R69S '52 R25/2

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San Arthur
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Joined: Sun Apr 22, 2007 2:08 pm

Re: R25/3 Surging at Idle, Until it Dies?

Post by San Arthur »

After a few minutes it stalls or can you ride it at higher RPMs or more throttle?
Coil failing after warming up?
Have you check the obvious, fuel lines, spark plug and sp cable, jets, key vibrating out of contact...
San Arthur in San Antonio, Texas
'58 R26 '76 R90S '94 R100GS/PD

swagsherman
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Joined: Sat Jul 25, 2020 9:09 am

Re: R25/3 Surging at Idle, Until it Dies?

Post by swagsherman »

schrader7032 wrote:
Thu Sep 03, 2020 7:11 pm
Welcome to the forum! What was wrong with the original carb, do you know? I wonder about the reproduction carb. Is there a carb number that is similar to Bing? What about the other parts of the carb...the idle jet, the main jet, etc.

[...] The latest suggestion for me is to get the bike warmed up again and spray something like carb cleaner around the carb and its connections. Somehow air is getting into the carb at low throttle settings but not through the air mixture screw. You say there are no leaks, but I'd do something like that just to be sure.
Thank you for the welcome! When I got the bike, it had a Mikuni carb... The reproduction Bing carb is made in Germany and it claims to be to exact specs as the Bing 124/41. I haven't check the jets themselves. Thanks for the suggestion re the air getting into the system; I'll try that next - makes sense.

swagsherman
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Joined: Sat Jul 25, 2020 9:09 am

Re: R25/3 Surging at Idle, Until it Dies?

Post by swagsherman »

San Arthur wrote:
Thu Sep 03, 2020 7:15 pm
After a few minutes it stalls or can you ride it at higher RPMs or more throttle?
Coil failing after warming up?
Have you check the obvious, fuel lines, spark plug and sp cable, jets, key vibrating out of contact...
It doesn't seem to die when riding - higher rpms keep it going. Within 1 minute or so of stopping, same surging pattern then dies. I have checked fuel lines, flow from petcock, have not checked jets since carb was a repro with no time on it but didn't look to see what was in there.

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jwonder
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Re: R25/3 Surging at Idle, Until it Dies?

Post by jwonder »

Sounds like fuel is not getting TO the carb.

I would look at the petcock flow and the screen in there. Take the fuel line off the carb and put the end in a clear glass jar. Turn the petcock on and make sure the flow is consistent and fast.

I have seen the following items do this:
  • Gas cap not venting properly.
  • Dirty fuel filter
  • Someone put an EXTRA filter in the fuel line. Don't do it!
  • Rubber in the petcock has swelled. It happens.
Check those items.

James
James Wonder
1967 R69S, 1966 R50/2, 1965 R27, 1963 R27, 1952 R51/3 and 2018 R1200GS

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