376/097 on 1958 SR

[Slymo]

I believe the carb on my 58SR is the original and in pretty astounding nick. I replaced the slide with a new 3.5 cutaway hard anodised one and the bike starts, idles and runs very well. The Porklips site has the settings for the carb in this application using needle position 4 and indeed using position 3 I had pinking in the mid throttle range. The only fly in the ointment is that the plugs tend to the slightly Rick look and tooling along at low speeds in town the motor tends to burble a bit feeling slightly rich. The Porklips site has the needle jet as a .11 which is clearly bollocks as no such jet exists and I believe the 016 needle jet is correct. However whether it's a bit worn or just a wrongun  I'm tempted to try a .017 needle jet and try position 3 instead.love to hear your thoughts.

[morris]

Pilot jet/pilot air settings have some influence at almost closed throttle openings so would have a look at them first.

[Triton Thrasher]

If needle position 3 caused pinking, don’t try position 3 again: it’s not safe.

Are you trying to say 106 and 107 needle jet sizes?

If your 106 seems rich, it may be worn.  Try a new one.

Another possible cause of richness at a small throttle opening is a too-small throttle slide cutaway.  Sometimes you can tune it out by weakening the pilot screw.

[JulianS]

The original monoblocs are assembled with no fibre washer between float seat and carb body - the "new" monoblocs have a fibre washer at this joint and the body is machined to accept the washer.

An old carb assemble with the fibre washer (which is provided in the carb washer set) will have a higher fuel level than designed and may run rich as a result.

Worth checking. If it has the washer take it out it is not supposed to be there.

[Joolstacho]

I thought SRs all had 389 s... shows how much I know!

[Slymo]

Yes I've adjusted the air screw and it works as one would hope going from a lean unreliable idle to a sooty one. I've set it on the marginally lean side of optimal at the moment. Like I say this is a bike that goes very well on the whole but I am on the endless search for perfection as are we all I suspect. Cutaway is definitely a possible area of exploration as is a worn needle jet. I'm loath to mess with the cutaway because the slides are not cheap and I can reverse any mods I make to the needle jet. I'll try the micrometer on the needle and some go no go tests on the jet to see if it is bigger than it should be and report on the results.

[Slymo]

Had the carb apart and everything measures to spec. What was gratifying was that the hard anodised slide after about 7 or 8000 miles is absolutely unmarked. No witness marks of any kind and it still fits in the body like a new carb.  Why Amal didn't fit them from new is hard to imagine. Still doesn't sort my richness issue. I'll try another decent ride and check the plugs again.

[Joolstacho]

All very well having a nice 'hard' slide, but the mazak wears in no time!

[Black Sheep]

Book settings were fine 60 odd years ago but these days they are really just a guide. Running rich? First thing to do is drop the needle. Conversely, running weak and/or pinking, raise it.
You run on the needle jet most of the time, so that's the first place to look.
Cutaway affects picking up from low revs. Same logic applies. The Pitmans books always have a chapter on tuning your carb, well worth a re-read.
I too thought that all SRs had 389 carbs, though they run well enough an a 376.

[Slymo]

No fortunately like white metal bearing shells on a hardened crank journal the hard anodised slide reduces wear on both the slide and the body. Wear is worst when friction occurs between two parts of the same metal.
Just thought for the very first time to check the BSA Parts manual. Whilst it doesn't show my 376/97 it does show the 376/89 with a 1065 needle jet and a 3 setting. Also a 4 cutaway. More experimentation to follow.

[Slymo]

Going to try a 4 cutaway slide to see what effect it has on my plug colour. Rather than investing in a new rather expensive hard anodised slide straight off I’ve sleeved the old buggered 31/2 slide that came with the carb and will raise the cutaway 1/32”and try it out. If it’s good then I’ll go the whole hog and buy a new one. I won’t mod the 31/2 anodised slide as it’s still mint and grinding through the surface may have unintended consequences.

[mikeb]

symo - here's a few references to the 376/97, and there-abouts

appear to say slide 3 1/2, needle 4 - or 3 depending on which one you look at.
i've thought the thing to do is set it at standard-ish and work thru the monobloc tuning procedure.

??

[Slymo]

It seems bizarre how many variations of carb and setup there are but I suppose that petrol varied all around the world then as it does now and it depends what market the machine was headed to. I've made the cutaway just shy of .250" and left the needle jet as a .106 with the needle on position 4. The weather in Auckland was atrocious today so I didn't get a road test or try it warm but just starting it up and blipping the throttle it does seem to answer the throttle better than before. If this does the trick then I'll leave well alone but if not I'll try the .1065 needle jet with the needle on position 3 next. The fun never ends! 😁

[Slymo]

Ok got the bike good and hot today and whilst open road was excellent there is an inclination to pink at takeoff when properly warm. I’ll go back to the 3 1/2 slide but might try the 1065 jet on needle clip three. Question is is that a richer or leaner setting than 106 on position 4? Presumably it alters the timing of the needle jet but makes in richer overall?

[RDfella]

If it's running OK apart from pinking, try knocking back the ignition timing. Is your plug the correct heat range?