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Do all twins have a downdraught on the rear carb, or is it just my bike?

hadronuk

Well Known and Active Forum Website User
VOC Member
[FONT=&amp]If I place a digital level gauge against the rear 276 bellmouth on my '51 Rapide, it shows the rear carb has about 5.8 degrees of downdraught. The front 276 has about 1.2 degrees of updraught.

This is assuming the bellmouth is perfectly square to the bore of the carburettor of course.

However, when I used a sight tube to check fuel levels, it showed that moving the rear float chamber fore and aft had a big effect on the fuel level, whereas move the front float chamber only slightly changed the fuel level, confirming the above measurements are correct.

So is this some quirk of my bike or are all twins like this?

I can find no mention of it in MPH or elsewhere[/FONT]
 

sidmadrid

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VOC Member
What size wheels do you have, what tyres, what pressures? Whar springs, new, old, modified, sidecar, Petford?
Who is sitting on the machine and had they had breakfast and what had they had?
You can see where I'm coming from?
But good question.
 

hadronuk

Well Known and Active Forum Website User
VOC Member
What size wheels do you have, what tyres, what pressures? Whar springs, new, old, modified, sidecar, Petford?
Who is sitting on the machine and had they had breakfast and what had they had?
You can see where I'm coming from?
But good question.

I can see where you are coming from. I agree that the attitude of the bike does change the angle of the carbs of course, but there is a still 7 degree difference between the angle of the front and rear carbs that is absolute and not affected by the pitch of the bike. Even if I raised the front by 5.6 inches, although this would bring the rear carb level, the front carb would then have 7 degrees of updraught.
I am pretty sure the attitude of my bike does not differ greatly from standard, though I haven't measured it.
 

greg brillus

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VOC Member
Hi there Hadronuk, well i just checked the rear carb on my stock standard series B,C,D Rapide.....well thats how Harpers built it in 63' supposedly....anyway, i dont have a protractor at hand at the moment, but you are correct in that the rear carb is definately angled down. But by my digitally enhanced eyes i can't quite make out if it is angled at 5.7 degrees or 5.9 Oh well i'll have to get back to you on that one........Greg.
 

vibrac

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VOC Member
I always assumed (which means I read somewhere) that the term "downdraft" had nothing to do with the attitude in relation to the earths surface and everything to do with the angle of the caburettor and inlet tract in relation to the fed cylinders center line.(although space does dictate a relationship to some extent) sematics aside that angle I would have thought is of much greater importance than the angle to the ground (fuel level aside)
To further confuse the issue the word "downdraft" in relation to centerline of cylinder or ground I am sure refers to the angle that is pointing UP, as in "That Manx Norton has a big downdraft GP carb" this I suppose came from the car world originally where carbs were sidedraft or downdraft as in 'downdraft webbers'
thus you front carb (by your ground level explanation) is downdraft I dont know what you would call the back one I have never heard the term updraft in relation to carburation
 
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Howard

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VOC Member
Hi Rob. Is it a problem or is it just niggling you. If it's just a niggle, LEAVE IT ALONE! I've just "put right" a niggle that's now causing me a problem.

If it's a problem, you could probably skim the carb adapter faces, but I don't know what angle(s) you'd choose.

H
 

hadronuk

Well Known and Active Forum Website User
VOC Member
Hi Greg,
Thanks for that info, so it appears to be the norm, or at least not uncommon.

Hi Vibrac,
I should have perhaps used some other word rather than "downdraught". I agree it usually refers to the angle of the inlet tract relative to the cylinder, increased downdraught usually giving better gas flow of course. I have read of some old cars having a carb referred to as "updraught" because the inlet pointed vertically down.

Hi Howard,
Don't worry, I wasn't planning to "fix" it! The reason I brought it up is the effect it has on the fuel level at the jets when the position of the float chamber is changed. I have already compensated for this by moving the float chamber forwards as far as possible.

If it is universal, I thought it would be useful for twin owners to be aware of when setting up carburettors, as it has big effect on the fuel level in the rear carb.
 
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jim burgess

Well Known and Active Forum Website User
Non-VOC Member
Hi Greg,
Thanks for that info, so it appears to be the norm, or at least not uncommon.

Hi Vibrac,
I should have perhaps used some other word rather than "downdraught". I agree it usually refers to the angle of the inlet tract relative to the cylinder, increased downdraught usually giving better gas flow of course. I have read of some old cars having a carb referred to as "updraught" because the inlet pointed vertically down.

Hi Howard,
Don't worry, I wasn't planning to "fix" it! The reason I brought it up is the effect it has on the fuel level at the jets when the position of the float chamber is changed. I have already fixed this by adjusting the carburettor float chamber.

If it is universal, I thought it would be useful for twin owners to be aware of when setting up carburettors, as it has big effect on the fuel level in the rear carb.
I have often disliked the practice of fitting diferrent tempreture range plugs to a twin, it didn't make sense to me, maybe your investigation now gives reason for a long standing practice. The float bowl position allied to the angle could effect the mixture.
Jim Burgess
 

bmetcalf

Well Known and Active Forum Website User
VOC Member
updraft in relation to carburation

Here is my updraft carb ('31 Ford)

PICT2911 a.jpg
 
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