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Living with an old carb.

Submitted by robert_tuck on Mon, 03/08/2020
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Heavy Twins

The monoblock on my 99 is orriginal  but has very little body wear and a new chromed brass slide.What wear it has is less slop than the new carb I bought some years ago for the Atlas,so you may understand my reluctance to buy new!. I continue to have a regular minor spitback just off idle that bugs me.I have tried smaller cutaway slides ,richer pilot screw settings etc. I'm going to try a bigger pilot jet,--not because I want more fuel at tickover but because I think that the bypass flow is unregulated by the tickover settings and may flow a bit more fuel at the transition from pilot to cutaway. What do you think?. I'm also going to fit a better air filter , so its possible that alone may richen things a tad and do the trick.

Submitted by lionel_yexley on Mon, 03/08/2020

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I'm debating whether to do a full overhaul and parts replacement of my 1959 376 but when I priced up just the guts of it and reached about £180 (using a Surrey brass slide) I conclude that it's false economy.  A complete new one costs £202 and the overhaul of the old one didn't include the body and top cable cap bits.  If I buy new and just do a a cleanup and minor overhaul of my old one I can sell it and put the cost against a new one!

Submitted by robert_tuck on Mon, 03/08/2020

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Hi Ashley, I have already worked out that the old type brass float is a better bet, I will try a brass needle too.

Submitted by jonathan_newton on Tue, 04/08/2020

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... stepped or chamfered needle jet? Seem to remember it helped with Commando issues on lean Tickover.

No experience, just a possible solution

 

Cheers

Jon

Submitted by robert_tuck on Wed, 05/08/2020

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Hi Jon,The problem occurs before the needle jet is in operation.Although I can see where you are going.  Usually a slightly richer pilot setting or a smaller slide cutaway is suggested . I have tried those and they just soot up the plugs without solving the issue.. There is a transition between the two thats covered by the by-pass drilling  . The function of which is more complicated than one would think.My understanding is that it supplies both air and fuel  in diferent ratio's at different  depressions but is not controlled by the pilot screw . There may be some change   with an increase in pilot jet size though.Too big a pilot change means having the pilot screw too far out for security though.Hopefully a  boffin will chime in! Mr Berisford?. He helped me before with mods to a carb for a Rudge that needed special attention that Amal did back in the 1930's but did not publish. I blame it on the fuel which now has all sorts of components it never had in the past !.(don't mention the eth word).

Submitted by john_holmes on Wed, 05/08/2020

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Wonder if the issue is electrical as you would expect a rich pilot setting to cover the issue, certainly worked for me in the past when using a lean cutaway and checking if it was lean or rich by changing the pilot mixture setting.

Submitted by robert_tuck on Wed, 05/08/2020

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Hi John, I doubt electrical as the bike idles nice and slow and will go like stink if asked. Usually if the points need a clean there is an accasional miss which always gets worse till fixed.Plugs are clean ,they usually foul up if an electrical fault.Going to try a proper air filter ,that will be a bit more restrictive .

Submitted by george_farenden on Wed, 05/08/2020

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If the aircleaner you want to fit is more restrictive at idle/just off idle, it will probably act as if the choke slides are on at anything above that!

Super duper aircleaners usually cause a restriction at max rpm full throttle airflow of around 2-3%, more normal ones 4-5%, and very restrictive ones up to 7-8% and on all of them there is virtually no restriction upto around 2500rpm and 50-60% throttle. A typical full throttle airflow is around 70 times greater than the same engine when idling.

Regards, George 

Submitted by michael_sullivan on Thu, 20/08/2020

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You can always trade her in..........

Oops..Sorry  I mis-read....

I thought that the original posting said "living with an old crab" 

Submitted by robert_tuck on Thu, 20/08/2020

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Going to try a slightly bigger pilot jet  as the feed to the bypass is unregulated (I think) .Might also raise the fuel level a couple of mm.

Submitted by jonathan_newton on Thu, 20/08/2020

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Robert, I have a mono lock on my V...cet.  As per usual starting was an issue. I was suspicious of fuel level and picked up a float extension with  glass window to observe.  What became clear was the amount of frothing in the bowl which sunk the float early   It did not reflect the “static” level I had observed previously.  I know your working with a twin but the vibration may give you a false reading...

just a thought....

 

it’s still an odd starter but they are quite different to Nortons...

cheers

jon

Submitted by robert_tuck on Fri, 21/08/2020

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Hi Jon,   After 50 years I still remember  trying to bump start  the Rap  with wet plugs.and the side saddle half throttle take off when it fired!.I recently fitted a rubber insulated remote float to my 250 and it has solved a few longstanding issues.  No room for that on a 99 DL.

 



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