Replacement carbs?

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FM350
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Replacement carbs?

Postby FM350 » Sat Apr 16, 2011 4:27 pm

Read the various comments on the subject of carbs with interest, and have never been able to grasp exactly why some people insist on OE type carbs, which in most cases dont work that well, and often wear out very quickly (Amal and Bing springs to mind here).

An efficient carb is something that seems to me to be comparable to modern tyres or suspension units, in that its something thats easily available to anyone and often costs less than the rather poor OE type parts. Here in the UK OKO carbs are available from Ebay for around $85 Aus, and jetted correctly seem to work very well on most machines.

Obviously if you have a completely original old TS trials bike and intend to exhibit it at shows rather than actually use it, then fitting a modern carb would not be appropriate, but I am not sure that compromising performance by sticking to OE type carbs for actual competition is that great an idea?

Maybe those who feel that older bikes should be restricted to using carburetion that doesnt work that well, and is more costly than easily available modern alternatives, would like to explain the thinking behind this?



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Re: Replacement carbs?

Postby David Lahey » Sat Apr 16, 2011 6:24 pm

My understanding is that the fuss is not whether a twinshock class bike has an OE carby or not, but if it has a round slide carby or a flat/square/crescent slide type carby that is upsetting some people.

There are two aspects to fitting a modern carby, as you have indicated - the appearance and the performance.

Appearance - the modern OKO round slide carby has a clear float bowl which may look a bit out of place on an authentic/otherwise-original-looking bike, and some people have strong feelings about what an era-class-based competition bike should look like (beauty is in the eye of the beholder)

Performance - OKO sell both a round slide carby and a more modern design carby that are both suitable for fitting to twinshock bikes. Both are economical to buy but the modern design is perceived to have superior performance by some people. Flat slide carbies made by Keihin and Dell Orto are also perceived by some riders to have superior performance to new round slide carbies. I suspect that 99% of the difference in performance between modern flat slide carbies and modern round slide carbies are more to do with how well the carby in question has been set up to suit the bike it is on, rather than the shape of the slide per se.

My reasoning for this belief is that:
1 Some modern trials bikes have round slide carbies and some have flat slide carbies, yet there is very little difference in the price of the item, so the bike manufacturers themselves are probably not convinced about which carby performs best.
2 I have ridden many bikes both twinshock era and modern with 1970s OEM, modern round slide and modern flat slide types of carby and have not been able to detect any difference that could be securely attributed to the type of carburettor fitted.


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FM350
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Re: Replacement carbs?

Postby FM350 » Sat Apr 16, 2011 8:38 pm

So if there is no performance advantage in fitting a modern carb, then would it be correct that the only issue here is related to not retaining an authentic appearance?



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Re: Replacement carbs?

Postby David Lahey » Sat Apr 16, 2011 8:59 pm

It depends on who you are asking. The appearance bit means nothing to me personally and same for the type and age of the carby.


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Re: Replacement carbs?

Postby FM350 » Sat Apr 16, 2011 10:12 pm

Just trying to figure out why exactly there is so much concern about what carb is fitted to an older bike?

Here in the UK we have £10k+ modern Brit twin-shocks being ridden in the "P65" class, and if the Scottish "P65" 2 day next month is anything to go by, pretty much the only modification thats strictly no go is running a modern carb! No real problem for the top guys though, as they simply fit Jap internal parts to highly modified S25 or Amal carb bodies!

Appearance wise the modern Brit twin-shocks dont look like anything around in 1965, so maintaining an original look certainly doesnt have much to do with the ban imposed on modern carbs in Scotland, and Jap internal parts fitted to Brit carb bodies work as well as the Jap carbs, with the only difference being the cost of this is 4 or 5 times that of a modern carb.



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Re: Replacement carbs?

Postby Jon V8 » Tue Apr 19, 2011 6:01 am

I think in many cases it is down to form over function,people believing their bike cant run properly unless it sports a flat slide Mk2 etc.
Yesterday I rode a classic trial on my HT5,running a mk1 Concentric - it runs perfectly a real pleasure to use.Looking around at other bikes there were allsorts of carbs,old and new.I notice that the Dellorto's with clear float bowls seem popular on Cubs and Bantams now.But then those are the two bikes that seem to gain most from a carb change.A couple of other HT5's were running the Villiers carb,but not doing anything different to mine.
Apart from the fact that in most cases you cant see it,I think the ignition system is what makes most difference,a reliable spark under all conditions can be hard to gain,wheras most carbs can be made to do a reasonable job unless totally worn out as already stated.



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Re: Replacement carbs?

Postby Stu » Tue Apr 19, 2011 12:10 pm

So David, can I use an OKO carby in Twinshock or Post Classic comps if the original carby is worn out? I have a new one ready to go on the TY if possible.


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Re: Replacement carbs?

Postby FM350 » Tue Apr 19, 2011 5:13 pm

Jon V8 wrote:I think in many cases it is down to form over function,people believing their bike cant run properly unless it sports a flat slide Mk2 etc.
Yesterday I rode a classic trial on my HT5,running a mk1 Concentric - it runs perfectly a real pleasure to use.Looking around at other bikes there were allsorts of carbs,old and new.I notice that the Dellorto's with clear float bowls seem popular on Cubs and Bantams now.But then those are the two bikes that seem to gain most from a carb change.A couple of other HT5's were running the Villiers carb,but not doing anything different to mine.
Apart from the fact that in most cases you cant see it,I think the ignition system is what makes most difference,a reliable spark under all conditions can be hard to gain,wheras most carbs can be made to do a reasonable job unless totally worn out as already stated.


Have you tried a modern carb on your HT? They do make a great deal of difference, and this is the main reason top runners in UK "P65" events have Jap internals fitted to Brit carb bodies.



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Re: Replacement carbs?

Postby David Lahey » Tue Apr 19, 2011 7:03 pm

Stu wrote:So David, can I use an OKO carby in Twinshock or Post Classic comps if the original carby is worn out? I have a new one ready to go on the TY if possible.

Stu it depends on the way that the rules in the MOMS for Twinshocks Class are interpreted.
Earlier this year I made a few suggestions through my club for clarifications of a few issues relating to Twinshock Class that could be included in the 2012 MOMS, and one suggestion to remove carburettors from the list of regulated parts for Twinshock class bikes. If that happens, then there will be no problem using an OKO carby next year.
With the 2010 MOMS, it is possible that if there was an eligibility scrutineer at an MA trial, having an OKO carby on a bike in Twinshock class might be a stumbling block.
Have you tried the OKO out yet? I'd love to know what you think of it on the TY250.


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Re: Replacement carbs?

Postby Stu » Tue Apr 19, 2011 7:30 pm

thats next on the list to do David!


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