Author Topic: Odd running on a closed throttle  (Read 951 times)

Offline BSA500

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Odd running on a closed throttle
« on: 28.04. 2021 13:16 »
Not sure this is a carb issue but I will start with that. When riding bike runs really well when engine is under load no issues starts well,ticks over etc etc. But it has always(and I mean for years and years) pops,hesitates runs not smoothly, difficult to explain, with the throttle closed or backed off. The list of checks etc etc...
1. No visible air leaks around the carb or exhausts
2. Mag always starts well and timed with the timing buzzer
3. No head leaks
4. Last couple of years the carb resleeved/engine rebored/head new valves etc

As I said I have owned this bike many years (30+) and it has always done this. Am I expecting to much and is this a case of they all do that sir

Offline Swarfcut

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Re: Odd running on a closed throttle
« Reply #1 on: 28.04. 2021 14:44 »
 Popping back in the Silencer on the over run was a sign of a weak mixture. Try raising the  throttle needle as a first try.

 Swarfy.

Offline BSA500

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Re: Odd running on a closed throttle
« Reply #2 on: 28.04. 2021 15:07 »
Cool thanks I shall try that

Online chaterlea25

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Re: Odd running on a closed throttle
« Reply #3 on: 28.04. 2021 23:09 »
Hi,
What carb is fitted?
Recently while trying to sort out an A10 that was "acting the maggot" I found that the jet block had come loose in the carb body, (monobloc)
A worn needle jet / needle or a "T" needle jet (2 stroke) will also cause problems

John


1961 Super Rocket
1963 RGS (ongoing)

Online Triton Thrasher

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Re: Odd running on a closed throttle
« Reply #4 on: 29.04. 2021 06:42 »
Richness at small throttle openings, such as is caused by a worn needle jet, can cause symptoms that vaguely match your vague description.

Offline BSA500

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Re: Odd running on a closed throttle
« Reply #5 on: 29.04. 2021 10:10 »
Richness at small throttle openings, such as is caused by a worn needle jet, can cause symptoms that vaguely match your vague description.
Trust me I wish I could be less vague *smile*
In reply to others its a 1 inch 376 monobloc, resleeved a couple of years ago(no play in slide) 25 pilot,1065 jet(new couple years ago) position 3 on the needle clip. MAIN i THINK IS A 240. Its a alloy head with 8:1 compression recent new valves/guides/rebore big ends etc etc

Offline BSA500

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Re: Odd running on a closed throttle
« Reply #6 on: 29.04. 2021 10:11 »
Oh its a 'c' needle

Online berger

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Re: Odd running on a closed throttle
« Reply #7 on: 29.04. 2021 10:45 »
do you mean it runs odd when just on tick over as in touching nothing stood back listening to it's odd running or the very first bit of throttle opening. also if it is when touching nothing is it on side stand or centre stand or slung up against a wall, as I used to do and I bet most others have too ;)

Offline BSA500

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Re: Odd running on a closed throttle
« Reply #8 on: 29.04. 2021 11:13 »
No when riding and you are slowing/coasting with throttle closed/part open. Just popping/not running clean etcetc

Offline Angus

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Re: Odd running on a closed throttle
« Reply #9 on: 29.04. 2021 13:04 »
My A7 (to SS spec with open goldie silencers) has always done this (pop and bangs from the exhaust), back when I was 17 I loved it. But two rebuilds (last one by Roger at Cake street classics) and two carbs, plus lots of other changes over the years it still does it. She runs slightly rich. ps I still like it doing it as it brings back good memories.
1961 A7 since 1976
1960 A10 Gold Flash Super Profile Bike
1958 Matchless G80 Project
1952 Norton Model 7 Plunger
1950 T100

Offline BSA500

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Re: Odd running on a closed throttle
« Reply #10 on: 11.05. 2021 13:35 »
Small update put some Denso plugs in and running a lot more even but plug look a little sooty/black

Offline BSA500

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Re: Odd running on a closed throttle
« Reply #11 on: 15.06. 2021 12:46 »
Update-pulled the carb and found the needle was bent at the tip(prob my fault) had a spare, also removed the washer that they supply with new gasket sets from under the petrol feed. Now she stumbles from idle and low speed running is lumpy, never used to do that. So I will put them back on(bent needle as well) and see what happens. Raised the needle and no improvement

Online Triton Thrasher

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Re: Odd running on a closed throttle
« Reply #12 on: 15.06. 2021 18:21 »
Usually better to make one change at a time, when looking for the cause of a problem.

Offline BSA500

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Re: Odd running on a closed throttle
« Reply #13 on: 15.06. 2021 21:19 »
Well back to how it was and one plug seems to have failed. Strange thing it was nearly white??
Tried with spares and seems to not like slow running and spitting, didn't do that before FFS

Offline Swarfcut

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Re: Odd running on a closed throttle
« Reply #14 on: 16.06. 2021 08:07 »
 BSA Service Sheet 216 lists Shooting Star 1" Monobloc as Main Jet 270, with Throttle Valve 376/3 1/2 and Needle Jet 0.1065. Position 3 is suggested for needle clip.

 Another basic check is the fuel level in the float chamber, plenty of discussion on the pitfalls  and  what to look for already posted in the carb. section of the Forum. This section contains some of the finer detail of carb assembly gained from experience so worth a look. Also the float valve position in relation to the carb body is dependent on the thickness  and position of sealing washers. Goes without saying check the pilot jet while you have it in bits and also  the mounting flange face for bow and poor sealing that cause puzzling air leaks.

 

 Swarfy.