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BSA B25 Starfire carb questions #564255
09/21/14 6:41 am
09/21/14 6:41 am
Joined: Jul 2001
Posts: 2,264
Gothenburg, Sweden
Morgan aka Admin Offline OP

Ride safe today!
Morgan aka Admin  Offline OP

Ride safe today!
Joined: Jul 2001
Posts: 2,264
Gothenburg, Sweden
we purchased a BSA B25 Starfire and it worked well but now I have some issues to start it so i need to work with it.
1968 and 1969 parts list say different specs for the 928 carb.
My bike has a date marked in the engine dated 24-8-67 so its a 1968 model which also the engine/frame no says B25S xxxx and then there is Y stamped in which indicates it was sold later with an extended warranty.
The records I have is that it was registered in April 69 and it also has 1969 steel petrol tank.
The carb is a 1968 marked R 928-1 and it should then have specs as stated in the 1968 parts list however it has the 1969 specs internals. With main 180, needle jet .106.
With a 928-1 carbI was expecting to find 220 main and .107. needle jet inside. See below tables what BSA parts list says.

BSA B25 Starfire parts list for 1968
Carburettor: R 928-1
Main jet 220
Pilot Jet 25
Throttle slide 3
Needle jet .107
Needle Position: 2
BSA parts number 19-2401

BSA B25 Starfire parts list for 1969
Carburettor: R 928-5
Main jet 180
Pilot Jet 25
Throttle slide 3
Needle jet .106
Needle Position: 2
BSA parts number 19-2416

The carb is old and shows some corrosion so I will probably replace it.
So if we have any B25 and or carb people willing to recommend what specs I shall go for out there than I would appreciate it. Both years appear to have air filters so I also wonder why the heck thy would use 220/.107 vs 180/.106 on those years?

Here is the bike.


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Re: BSA B25 Starfire carb questions [Re: Morgan aka Admin] #564260
09/21/14 7:15 am
09/21/14 7:15 am
Joined: Dec 2004
Posts: 9,375
Scotland
kommando Offline
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kommando  Offline
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Posts: 9,375
Scotland
928-1 signifies it as being the first 928 carb AMAL fixed the jetting for as they stamped them in sequence, as an early AMAL it would likely be setup for 2 strokes not 4 strokes, if you assume 928-5 was the correction to 4 stokes then go for -5 but make sure all the parts are 4 stroke parts. Considering today's fuel is not like 1960's fuel it can only be considered a starting point anyway and you should tune to today's fuel anyway. John Healy has posted in the past on what parts are needed to convert an early 2 stoke AMAL to a 4 stroke.

Re: BSA B25 Starfire carb questions [Re: Morgan aka Admin] #564284
09/21/14 10:01 am
09/21/14 10:01 am
Joined: Jul 2001
Posts: 2,264
Gothenburg, Sweden
Morgan aka Admin Offline OP

Ride safe today!
Morgan aka Admin  Offline OP

Ride safe today!
Joined: Jul 2001
Posts: 2,264
Gothenburg, Sweden
Kommando: I assume both were for 4-strokes as BSA presented them in their B25 parts list but the different suffix -1 or -5 just indicates the differenties.

The only thing I did was to clean and replace the .106 needle jet (correct 4-stroke part no) and pilot screw o-ring.
I'm still open for info on proper settings.


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Re: BSA B25 Starfire carb questions [Re: Morgan aka Admin] #564289
09/21/14 10:23 am
09/21/14 10:23 am
Joined: Dec 2004
Posts: 9,375
Scotland
kommando Offline
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kommando  Offline
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Posts: 9,375
Scotland
JH's posts will give the full info but the first Concentric carbs had 2 stroke parts fitted, this was not the jets but items like the jet holders etc, the Triumph 500 twin was badly effected. Assume nothing.

Re: BSA B25 Starfire carb questions [Re: Morgan aka Admin] #564294
09/21/14 10:41 am
09/21/14 10:41 am
Joined: Sep 2002
Posts: 7,813
Seattle
Alex Offline

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Alex  Offline

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Posts: 7,813
Seattle
Morgan, go with 180 and .106 as a starting point. This jetting seems to work well on 250 BSA's. That said, check the jetting with a plug chop, or whatever other means you may feel more comfortable with.


A smattering:
'53 Gold Flash
'67 Royal Star
'71 Rickman Metisse
'40 Silver Star
'37 Rudge Special
sixtyseventy Lightboltrocket road racer...and many more.
Re: BSA B25 Starfire carb questions [Re: Morgan aka Admin] #564338
09/21/14 2:30 pm
09/21/14 2:30 pm
Joined: Jul 2001
Posts: 2,264
Gothenburg, Sweden
Morgan aka Admin Offline OP

Ride safe today!
Morgan aka Admin  Offline OP

Ride safe today!
Joined: Jul 2001
Posts: 2,264
Gothenburg, Sweden
Alex it looks like i have a mismatch of parts in the carb or at least early/late stuff in it.

Here was in it
Carburettor: R 928-1
Main jet 180
Pilot Jet 25 it is a screw in jet not the bush type
Throttle slide 3
Needle jet .106 two stroke needle jet with no bleed hole in the side
Needle Position: 1 - two stroke needle with 3 marking rings

So here is what I should have I think
Carburettor: R 928-1
Main jet 180
Pilot Jet 25 it is a screw in jet not the bush type
Throttle slide 3
Needle jet .106 four stroke needle jet with bleed hole in the side
Needle Position: 2 - two stroke needle with 2 marking rings which is longer that the 3 ring type

So far I have changed the needle jet .106 four stroke with bleed hole
have no correct needle so it might be a mismatch here.

The bike is starting now but 8-strokes a lot in pilot and going to cut away but then it four strokes fine.


Alex what is a plug chop?


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Morgan Johansson
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Re: BSA B25 Starfire carb questions [Re: Morgan aka Admin] #564341
09/21/14 2:44 pm
09/21/14 2:44 pm
Joined: Dec 2004
Posts: 9,375
Scotland
kommando Offline
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kommando  Offline
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Posts: 9,375
Scotland
Quote:
Alex what is a plug chop?


Mark the throttle at 1/4,1/2, 3/4 and full. Take the bike for a ride and ride up a hill at a set throttle opening for at least 100 yds. Then as you are still running at the set throttle opening turn off the ignition and pull the clutch. When you are stopped by the roadside take the plug out and using a magnifying glass inspect the bottom of the ceramic portion of the firing end of the plug and look at the colour, this will tell you if the mixture is rich,lean or ok at the set throttle opening you chose. Look for a coffee colour for ok, white for lean and black for rich. With today's fuels it does not always work.

Re: BSA B25 Starfire carb questions [Re: kommando] #564458
09/22/14 11:01 am
09/22/14 11:01 am
Joined: Sep 2002
Posts: 7,813
Seattle
Alex Offline

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Alex  Offline

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Joined: Sep 2002
Posts: 7,813
Seattle
Originally Posted By: kommando
Quote:
Alex what is a plug chop?


Mark the throttle at 1/4,1/2, 3/4 and full. Take the bike for a ride and ride up a hill at a set throttle opening for at least 100 yds. Then as you are still running at the set throttle opening turn off the ignition and pull the clutch. When you are stopped by the roadside take the plug out and using a magnifying glass inspect the bottom of the ceramic portion of the firing end of the plug and look at the colour, this will tell you if the mixture is rich,lean or ok at the set throttle opening you chose. Look for a coffee colour for ok, white for lean and black for rich. With today's fuels it does not always work.


Thanks for that commando. With modern fuels it very much does still work but not in the classic sense shown in many older auto maintenance books like clymer or haynes. The brown color of old is gone with the absence of lead and the addition of detergents in fuels. What you are now looking for is a clean insulator and a 1.5 mm band at the base of the insulator. That would be considered ideal but often you can just use it as a coarse indicator: if the insulator is black, you are waaaaaay too rich. On a B7 ES it's hard to see the dark band when your mixture is correct, you can either use a lighted magnifier or cut the threaded portion of the plug off.

Here's a helpful link:
http://www.dragstuff.com/techarticles/reading-spark-plugs.html

All this said about plug chops, use every means at your disposal: If you are already 8-stroking in a given throttle range, you know you're rich without doing a plug chop. If you're lacking power but the bike surges when you roll off the throttle, you know it's running lean in that range. I find plug chops useful for fine tuning and sorting out tuning issues when simpler ones like the above aren't pointing to an obvious cause. Usually for me that's in the idle/cutaway region...sometimes it's tough to diagnose and cutting a slide is permanent so I like to make sure.

One more thing, Morgan, how is the fit of the slide in the carburetor? If it's really loose (they should have ~.004" of clearance) it will really confound the idle circuit, often leading to the symptoms you're seeing.

If you do decide to get a new carb, order one with the 1969/70 specs. We are about to put one of those on Skip's TR25.

Cheers!

BTW, just went to Fred's on Saturday, he says Hi.


A smattering:
'53 Gold Flash
'67 Royal Star
'71 Rickman Metisse
'40 Silver Star
'37 Rudge Special
sixtyseventy Lightboltrocket road racer...and many more.
Re: BSA B25 Starfire carb questions [Re: Morgan aka Admin] #564483
09/22/14 3:34 pm
09/22/14 3:34 pm
Joined: Jul 2001
Posts: 2,264
Gothenburg, Sweden
Morgan aka Admin Offline OP

Ride safe today!
Morgan aka Admin  Offline OP

Ride safe today!
Joined: Jul 2001
Posts: 2,264
Gothenburg, Sweden
I just ordered a AMAL Premium R928 with stay up float, the old had a mismatch of 2stroke parts in it and Phil at AMAL confirmed they used to work poorly with the original setting, still it worked well when I bought it. never mind I'll await the new carb from UK next on friday or next week.
OK I now about Plug chop just didn't know the EnglishUS term for it. I'll go for the 1969 version
So here is what I should have I think
Carburettor: R 928-5
Main jet 180
Pilot Jet 25
Throttle slide 3
Needle jet .106 four stroke needle jet with bleed hole in the side
Needle Position: 2 - four stroke needle with 2 marking groves

I'll read the article in the link later this week. let me know how Skips bike takes the new carb and if you have same settings.
cheers


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Morgan Johansson
BritBike.com.
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Re: BSA B25 Starfire carb questions [Re: Morgan aka Admin] #665835
08/30/16 8:07 pm
08/30/16 8:07 pm
Joined: May 2014
Posts: 4
NOVA SCOTIA
S
Scotian Starfire Offline
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Scotian Starfire  Offline
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S
Joined: May 2014
Posts: 4
NOVA SCOTIA
How did the new carburetor work out?

I have a 1970 B25 Starfire that has been running rough for about a decade, and stumbled onto this thread. It has an electronic timing/ignition system, and has always required spark plugs to be rotated through. It will run for about a week and then need either a new or a cleaned-up previously used spark plug before it will start again. The plugs looked too oily/sooty when taken out, leading me to believe it was running rich.

I always had the pilot air screw almost all the way out (only about one thread holding it in). It appears to me that the pilot circuit pressed-in mixing valve was installed incorrectly (you can see that it was pressed in off center and see the non-Concentric mark of the press). I ended up prying off that pressed in piece since it needed to be re-seated and it turned out to just be a brass cap. I hammered it back in and it worked the same as before.

Furthermore, my workshop manual calls for a R928/4 carburetor but the one on my bike is a RB928/15. I'll have to take it apart to see what identifying markings I can find on the jets, needle, and throttle valve.

So, interested to know what your findings were after you changed out for that new carb.

Re: BSA B25 Starfire carb questions [Re: Morgan aka Admin] #665914
08/31/16 12:26 pm
08/31/16 12:26 pm
Joined: Jul 2001
Posts: 2,264
Gothenburg, Sweden
Morgan aka Admin Offline OP

Ride safe today!
Morgan aka Admin  Offline OP

Ride safe today!
Joined: Jul 2001
Posts: 2,264
Gothenburg, Sweden
Scotian

The new carb worked just fine i was inded satified. however the bike is now sold it had wrong color according to my daughter.. wink


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Morgan Johansson
BritBike.com.
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