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Re: Bonnie Carb Jetting help needed [Re: BrettF] #723660
01/29/18 4:02 pm
01/29/18 4:02 pm
Joined: Mar 2006
Posts: 1,304
Magnolia, TX
htown Offline
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Generally most of street riding is on the throttle range from the slide to the needle jet. Not a whole lot at wide open on the main jet. I may have missed it but what mufflers and air cleaner are you using. Also, what slide cutaway. You may need to go richer on the slide. What position is your needle clip. Richest setting is the bottom groove. If you are in the bottom groove and still want to go richer you can use these discs under the clip.
https://www.eBay.com/itm/MIKUNI-JET-NEEDLE-WASHERS-10-PK-826-03002/202121454094?epid=171329418&hash=item2f0f60a20e:g:Qs8AAOSw5VtaFNB7:sc:USPSFirstClass!77355!US!-1&vxp=mtr


1978 Bonneville T140E
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1972 Norton Commando
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2004 XL 1200R Sportster

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Re: Bonnie Carb Jetting help needed [Re: BrettF] #723718
01/30/18 8:55 am
01/30/18 8:55 am
Joined: Jan 2014
Posts: 398
Geneva, Switzerland
BrettF Offline OP
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Hi Htown, all was standard, air filters and exhausts, Needle is as high as possible (bottom groove) and the slide is 3 but I don't want to change that till I solve the "lean all over" problem. It should not run lean at WOT with a 270 main jet. (and when running without a mainjet I get 8 stroking on demand above 3/4 throttle so the fuel delivery is there if needed)

Dave, I've been thinking of your calculation - my own experiments with vacuum gauges give around 15 to 20 KPa vacuum at idle (5 to 6 inch HG I believe). I have never tested vacuum at WOT but a quick search of google indicates a vacuum of around 4 KPa would be normal for a road going vehicle. This seems a bit higher than the 4% you calculated. Maybe due to the vacuum created by the descending piston sucking in air against intake tube and air filter restrictions etc on top of the straight Bernoulli velocity created vacuum?


3D TV: A format that lost a format war without even having an opponent.
Bikes:
'69 T120 on average (1967 rolling frame and 1971 Bonnie engine)
'56 1/2 T110 on average (58 rolling frame - with 55 iron head engine)
Re: Bonnie Carb Jetting help needed [Re: BrettF] #723741
01/30/18 2:28 pm
01/30/18 2:28 pm
Joined: Nov 2012
Posts: 625
Isle of Wight, UK
K
koan58 Offline
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Hi Brett,
I would expect that readings by different vacuum gauges are only roughly comparable, in the same way as with compression gauges.
Also, it is important to be consistent and clear on whether a value under discussion is a pressure (a quantity above the zero of empty space), or a vacuum/depression which is a measure of how much below atmospheric it is. Easy to muddle these 2 ways of expression.

The units variously used are also easy to muddle, for quick and rough comparison,
1 atmosphere = 30" Hg = 100 KPa

As a rough guide, a healthy manifold vacuum at tickover is generally expected to be 17-21" Hg
However this intended for manifolds generally feeding more than 1 cylinder, so a 1 cylinder manifold tends to present vacuums a little lower.
This is why I suggested a PRESSURE of 1/3 to 1/2 atmosphere (= VACUUM of 1/2 to 2/3 atm = 15-20" Hg = 50-67 KPa)

Thus your vacuum readings of 15-20 KPa do seem very low - are you sure it wasn't 15-20" Hg?.

For the life of me I couldn't find a quote for WOT vacuum, that's why I went to the fag-packet hassle to get a handle on it!
But your figure of 4 KPa seems to lend support to the fag-packet surely? 4 KPa is 4% of 100 KPa (1 atm).

Re: Bonnie Carb Jetting help needed [Re: koan58] #723743
01/30/18 3:11 pm
01/30/18 3:11 pm
Joined: Oct 2012
Posts: 3,729
Running from demons in WNY
Hillbilly bike Offline
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Originally Posted by koan58
Hi Brett,

Thus your vacuum readings of 15-20 KPa do seem very low - are you sure it wasn't 15-20" Hg?.

For the life of me I couldn't find a quote for WOT vacuum, that's why I went to the fag-packet hassle to get a handle on it!
But your figure of 4 KPa seems to lend support to the fag-packet surely? 4 KPa is 4% of 100 KPa (1 atm).


Wide open throttle manifold pressure (vacuum) for a multi cylinder decent breathing auto engine is generally around 4 hg.....On a highly tuned engine it may be 1 hg...OHV pushrod auto engines tuned to about 1.2 HP per cubic inch like a Triumph will pull maybe 12 hg at idle.. When you open the throttle the vacuum will drop close to zero and then slowly climb up as the engine nears maximum power and the intake system becomes more of a restriction..I've tried to get tradings on a Triumph twin carb engine but the vacuum gauge needle pulsating rapidly...Need more dampening..

Last edited by Hillbilly bike; 01/30/18 3:13 pm.

I ride junk
Re: Bonnie Carb Jetting help needed [Re: BrettF] #723757
01/30/18 6:01 pm
01/30/18 6:01 pm
Joined: Nov 2012
Posts: 625
Isle of Wight, UK
K
koan58 Offline
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Isle of Wight, UK
On a lighter but still relevant note, I found this little snippet on Wiki:

"Prior to the introduction of Federal Motor Vehicle Safety Standards in the USA by the National Traffic and Motor Vehicle Safety Act of 1966, it was common to use manifold vacuum to drive windscreen wipers with a pneumatic motor. This system was cheap & simple but resulted in the comical yet unsafe effect of wipers which operate at full speed while the engine idles, operate around half speed while cruising, and stop altogether when the driver depresses the pedal fully."

and:

"Manifold vacuum is an effect of a piston's movement on the induction stroke and the choked flow through a throttle in the intake manifold of an engine. It is a measure of the amount of restriction of airflow through the engine, and hence of the unused power capacity in the engine.
Manifold vacuum should not be confused with venturi vacuum, which is an effect exploited in carburetors to establish a pressure difference roughly proportional to mass airflow and to maintain a somewhat constant air/fuel ratio."

HB thanks for your figures. 12" Hg is a bit lower than my 15" Hg lower estimate, but this is still 40 KPa, not Brett's 15-20 KPa.
Also involved in this (as illustrated by your hope for more damping) is that a gauge can only do its best to give an average of a fluctuating vacuum. In a car manifold, with more frequent draws, a steadier and higher reading is to be expected. The much less frequent intakes in a Bonnie say (1 draw every 2 revs), will reduce the average vacuum considerably, though the vacuum during the intake stroke alone (which is what matters to the pilot system) will be much higher than the average.
The 4" Hg manifold vacuum at full throttle for for a multi cylinder decent breathing auto engine of course also involves several times more induction strokes per RPM through the manifold than the 1 cylinder per carb situation (far more even flow than pulsed).

When a flow system (pipe) is designed to operate in a certain range of pressure differential, say 12+ inches Hg, then that system is exposed to much lower pressure differential (perhaps 4" Hg or less), and that differential is not focussed at the position of the system ports, then what force is available to result in anything like its usual flowrate?

Re: Bonnie Carb Jetting help needed [Re: BrettF] #723758
01/30/18 6:18 pm
01/30/18 6:18 pm
Joined: Jan 2014
Posts: 398
Geneva, Switzerland
BrettF Offline OP
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Geneva, Switzerland
Interesting stuff - Might be my pressure gauges - they were high-quality ones and interestingly they read almost the same (20 KPa) on my Suzuki GS500 which is essentially the same as the triumph, vertical twin with two carbs. Damping screws are good and allow effective damping (but this does slow response when revving etc).

In response to:

Quote
When a flow system (pipe) is designed to operate in a certain range of pressure differential, say 12+ inches Hg, then that system is exposed to much lower pressure differential (perhaps 4" Hg or less), and that differential is not focussed at the position of the system ports, then what force is available to result in anything like its usual flowrate?


I understand your point in the above statement but would this not also, by extension, reduce the main jet/needle jet flowrate to close to zero also? At WOT it must be sucking quite hard to get 200cc per minute through the little main jet hole.
If we assume a bonnie is "hot" and has only 1 inch of mercury (say 4 KPa) that gives 40 cm of water head equivalent pushing gas into the carb. At 4 "HG the head is around 1.5 meters - quite a bit of pressure! Could this be enough to make the orifices in main and pilot jets the limiting factor to the flow? Just throwing ideas about. Wish I knew more about this stuff.



3D TV: A format that lost a format war without even having an opponent.
Bikes:
'69 T120 on average (1967 rolling frame and 1971 Bonnie engine)
'56 1/2 T110 on average (58 rolling frame - with 55 iron head engine)
Re: Bonnie Carb Jetting help needed [Re: BrettF] #723761
01/30/18 6:58 pm
01/30/18 6:58 pm
Joined: Dec 2006
Posts: 3,895
Massachusetts, U.S.A.
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btour Offline
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Massachusetts, U.S.A.
Brett,

Your needle jets are cross drilled and no swarf? If so, you might want to mess with the size of the holes.

"http://www.britbike.com/forums/ubbthreads.php/topics/560566/re-needle-jet-cross-hole#Post560566


Bob, Lifetime bike: '71 T120R, bought in '71 at Ken Heanes, England.
Re: Bonnie Carb Jetting help needed [Re: BrettF] #723762
01/30/18 6:59 pm
01/30/18 6:59 pm
Joined: Dec 2006
Posts: 3,895
Massachusetts, U.S.A.
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btour Offline
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Massachusetts, U.S.A.
Your slides may be labeled 3 but what are they really?


Bob, Lifetime bike: '71 T120R, bought in '71 at Ken Heanes, England.
Re: Bonnie Carb Jetting help needed [Re: BrettF] #723765
01/30/18 7:40 pm
01/30/18 7:40 pm
Joined: Nov 2012
Posts: 625
Isle of Wight, UK
K
koan58 Offline
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Hi Brett,
What's going on at very low throttle isn't the same as what's going on at say medium throttle upwards.
What I've been trying to explain is that the pilot system is sized to operate at very low throttle openings. In this situation, the only thing operating on the carb is the large manifold vacuum, which only acts on the engine side of the slide wall that closes the aperture, with a leak of this vacuum under that wall that affects the secondary hole at small openings.
This is a unique mechanism for the function of the pilot system, manifold vacuum is not employed by the main carb system.
By the time the slide is open enough to allow significant air flow (~1/8 open) the manifold vacuum is rapidly reducing, and the venturi effect is increasingly taking control.
By half throttle, the manifold vacuum has diminished enormously and has little involvement in the functioning of the carburettor. Because the air flow has become significant, the pressure reduction by venturi effect is what now runs the carb function.
Imagine the descending piston acting as a vacuum pump. If you close the inlet, max vacuum is obtained. This is what happens on the engine side of the slide when it is sealing the orifice. This would not be conducive to idling if there wasn't a bleed to allow the engine to breathe, hence the hole under the edge of the slide, which cleverly changes its function as the throttle lifts slightly. The important thing to remember is that these functions occur at small throttle openings and consequently high vacuum in that specific place.
Once the slide is open beyond ~1/4, the manifold vacuum has reduced enormously, the venturi effect takes over from then. Though the pressure reduction is much smaller than the manifold vacuum, this main part of the carb is designed to work in those conditions, employing much larger orifices, and high air speeds, producing vacuum focussed in the centre of the carb.
These different carb functions are largely independent of each other.

Re: Bonnie Carb Jetting help needed [Re: BrettF] #723775
01/30/18 9:00 pm
01/30/18 9:00 pm
Joined: Jan 2014
Posts: 398
Geneva, Switzerland
BrettF Offline OP
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Geneva, Switzerland
I always thought that the 17 to 21 "Hg was more for a car engine, so I looked at the only resource I have for carbs - The Haynes Motorcycle Carb manual (1981) says

" A common reading is something
in the region of 20 - 25 cm Hg (7.9 - 10.0 in Hg). If one gauge
or setting shows abnormally low vacuum on one cylinder, say
15 cm Hg (6.0 in Hg) or less, an air leak should be suspected."


3D TV: A format that lost a format war without even having an opponent.
Bikes:
'69 T120 on average (1967 rolling frame and 1971 Bonnie engine)
'56 1/2 T110 on average (58 rolling frame - with 55 iron head engine)
Re: Bonnie Carb Jetting help needed [Re: BrettF] #723780
01/30/18 10:01 pm
01/30/18 10:01 pm
Joined: Nov 2012
Posts: 625
Isle of Wight, UK
K
koan58 Offline
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Brett, I refer you back to an earlier post:
12" Hg is a bit lower than my 15" Hg lower estimate, but this is still 40 KPa, not Brett's 15-20 KPa.
Also involved in this (as illustrated by your hope for more damping) is that a gauge can only do its best to give an average of a fluctuating vacuum. In a car manifold, with more frequent draws, a steadier and higher reading is to be expected. The much less frequent intakes in a Bonnie say (1 draw every 2 revs), will reduce the average vacuum considerably, though the vacuum during the intake stroke alone (which is what matters to the pilot system) will be much higher than the average.
The 4" Hg manifold vacuum at full throttle for for a multi cylinder decent breathing auto engine of course also involves several times more induction strokes per RPM through the manifold than the 1 cylinder per carb situation (far more even flow than pulsed).

I too am a proud owner of said manual, dated 1980.
So Haynes 8-10" Hg (26-33 KPa) recommendation still doesn't compare well with your measurement. That Jap 4-stroke twins expect to idle closer to 1500 rpm than the 700-900 of a Brit twin suggests more gas flow at idle, in turn suggesting lower manifold vacuum. But I wouldn't make to much of a Haynes fact anyway. The general function of the carb as I describe holds true.

I fully understand your hope that the change in pilot jet will reward you with 5% mixture enrichment throughout the range, I cannot make you happy by agreeing with that idea, irrespective of these debates of technicalities. I fear we're going round in circles now, time for other contributions perchance? Dave

Re: Bonnie Carb Jetting help needed [Re: BrettF] #723819
01/31/18 7:25 am
01/31/18 7:25 am
Joined: Jan 2014
Posts: 398
Geneva, Switzerland
BrettF Offline OP
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Thanks for all your insight Dave, rest assured I'm not "hard-headidly" trying to force the 5% on this discussion, I just think I see a thread of logic in Mr Healy's original point which I was interested in unpacking. I have learned a LOT from this thread but I agree - I think we have gone as far as we can on this issue.

Cheers


3D TV: A format that lost a format war without even having an opponent.
Bikes:
'69 T120 on average (1967 rolling frame and 1971 Bonnie engine)
'56 1/2 T110 on average (58 rolling frame - with 55 iron head engine)
Re: Bonnie Carb Jetting help needed [Re: BrettF] #724456
02/06/18 4:38 am
02/06/18 4:38 am
Joined: Jan 2014
Posts: 398
Geneva, Switzerland
BrettF Offline OP
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Dave, I reached out to Burlen to get some background on the above pilot supply debate - and their response supports your point of view.

FWIW and FYI I inhaled some petrol fumes again and directly measured the fuel level in the new bowls. It is, in fact, better than I could achieve with the old bowls and significantly higher than the fuel level achieved in the original bowls as supplied from the factory. The tube measuring method really is for the birds - did not have the same error between direct/tube as when previously measured...


3D TV: A format that lost a format war without even having an opponent.
Bikes:
'69 T120 on average (1967 rolling frame and 1971 Bonnie engine)
'56 1/2 T110 on average (58 rolling frame - with 55 iron head engine)
Re: Bonnie Carb Jetting help needed [Re: BrettF] #724494
02/06/18 2:32 pm
02/06/18 2:32 pm
Joined: Nov 2012
Posts: 625
Isle of Wight, UK
K
koan58 Offline
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Hi Brett,
Thanks for your update.

This bowl business is curious. I read your article re bowl evolution, so the difference of the earlier bowls is understandable.
The differences between the modern bowls is the odd thing. Are they visibly different? Are the floats the same ones that you had, or have they exchanged them for another pair of stayups?

And just out of interest, are the needle seat drillings both better centred?

Let us know what the fuel level is that you eventually proceed with, and how you get on...

Re: Bonnie Carb Jetting help needed [Re: BrettF] #728290
03/11/18 3:42 pm
03/11/18 3:42 pm
Joined: Jan 2014
Posts: 398
Geneva, Switzerland
BrettF Offline OP
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Geneva, Switzerland
Hi Dave, To respond to your comments, the new bowl material did look different to the previous material - in the image below one can see the new bowl, it has a different sheen to the body. The old bowl had the same look as the body.

Holes were all spot on this time- floats I believe were the same ones.

[Linked Image]

Will summarise current situation in this ongoing saga in the next post (summary - "it's broke and I still can't fix it...")



3D TV: A format that lost a format war without even having an opponent.
Bikes:
'69 T120 on average (1967 rolling frame and 1971 Bonnie engine)
'56 1/2 T110 on average (58 rolling frame - with 55 iron head engine)
Re: Bonnie Carb Jetting help needed [Re: BrettF] #728291
03/11/18 3:45 pm
03/11/18 3:45 pm
Joined: Aug 2001
Posts: 7,455
scotland
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Who the heck is Tuff Nell!


Amateur Loctite enthusiast.
Re: Bonnie Carb Jetting help needed [Re: BrettF] #728296
03/11/18 4:50 pm
03/11/18 4:50 pm
Joined: Feb 2008
Posts: 4,359
New Jersey USA
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You should be grateful that you have not met up with her in an alley on a dark night!

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