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Dual Carb Manifold Pressure #269139
08/07/09 8:43 pm
08/07/09 8:43 pm
Joined: Jan 2009
Posts: 327
Montana, USA
D
Dr. Z Offline OP
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Dr. Z  Offline OP
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D

Joined: Jan 2009
Posts: 327
Montana, USA
I have new AMAL 626 carbs on a 68 T100R, and I have been trying to balance by adjusting the cable length to each carb. This is done with a homemade manometer with an old petcock restrictor and ATF as the fluid. Both ends of the tube are connected to the balance tube connections on the intake manifold and holding the throttle slightly open off the stops it is possible to equalize the pressure by adjusting the cable lengths.

I have done this at about 3000 rpm, but when the engine speed changes the relative levels in the tube changes. For example, going up to about 4000 rpm there is a relative height difference of about 50 cm. This probably corresponds to a pressure difference of about 1-2 cm of mercury.

Is this difference expected for AMAL carbs, or can this be the result of the valve design or something else? Does the old drill bit method work just as well?


Craig Zaspel

T100R
BSA A10
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Re: Dual Carb Manifold Pressure [Re: Dr. Z] #269143
08/07/09 8:49 pm
08/07/09 8:49 pm
Joined: Aug 2001
Posts: 1,839
British Columbia
hh Offline
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hh  Offline
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Joined: Aug 2001
Posts: 1,839
British Columbia
I would suspect the relative condition of each cylinder being different since the carbs are new and presumably near identical in flow. Have you done a compression test?


"Beer is proof that God loves us and wants us to be happy" - Benjamin Franklin
Re: Dual Carb Manifold Pressure [Re: hh] #269147
08/07/09 9:03 pm
08/07/09 9:03 pm
Joined: Oct 2003
Posts: 11,267
North Georgia, USA
RF Whatley Offline
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RF Whatley  Offline
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Joined: Oct 2003
Posts: 11,267
North Georgia, USA
Craig -
Your method will work, but there's a little more to it. The intake vacuum is pulsing wildly, so you should have a tiny jet in the hose as a restrictor. I'd also think you need a heavier fluid or real vacuum gauges.

On a twin this type setup is hardly called for. Adjusting the cables and idle setup is good enough. Now a triple or 4-cyl is another issue.


Don't hide 'em, Ride 'em !!

RF Whatley
Cornelia, GA
Re: Dual Carb Manifold Pressure [Re: Dr. Z] #269148
08/07/09 9:07 pm
08/07/09 9:07 pm
Joined: Nov 2008
Posts: 2,849
Bishop, Calif.
D
desco Offline
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desco  Offline
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D

Joined: Nov 2008
Posts: 2,849
Bishop, Calif.
The drill bits are quick, easy and accurate, providing you've done all the other steps before hand. It's been working for me for about ten years.


1968 T120R
1972 T120RV
Any advice given is without a warranty expressed or implied.
Re: Dual Carb Manifold Pressure [Re: desco] #269154
08/07/09 9:41 pm
08/07/09 9:41 pm
Joined: Aug 2001
Posts: 7,604
scotland
triton thrasher Offline
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triton thrasher  Offline
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Joined: Aug 2001
Posts: 7,604
scotland
If the levels are equal at idle and at some small throttle opening, that's all you want


Amateur Loctite enthusiast.
Re: Dual Carb Manifold Pressure [Re: triton thrasher] #269157
08/07/09 10:15 pm
08/07/09 10:15 pm
Joined: Mar 2005
Posts: 10,222
Boston, Massachusetts
J
John Healy Offline

BritBike Forum member
John Healy  Offline

BritBike Forum member
J

Joined: Mar 2005
Posts: 10,222
Boston, Massachusetts
Assuming you have AMAL Concentrics:
1. Unscrew the both throttle stops so the slides touch the bottom of the bodies.

2. Adjust the cable adjuster so the slides lift off the bottom of the body at the same time. You can use your fingers, drill rod or sound.

It is interesting to note they will also hit the bottom at the same time when you drop them. If you get click, click you got it wrong. When dropping them you should get just a single click the slides hit bottom in unison. With practice this can be done without taking off the air cleaners and done by ear.

3. Screw in the throttle stop adjusters until they JUST touch the slide. Use your sight or finger to feel the point where the slide just begins to move or: This also can be done with out taking the air cleaner off. Just raise and lower the slide with the throttle as you slowly turn in the throttle stop screw until you feel the slide hit the screw. Stop. Do this to both carbs.

4. From this point screw the throttle stop screws in 3/4 of a turn (this can vary from bike to bike and you should note the rpm at 3/4 and make note so that you can repeat the next time If you are anal you could get the bike to the point where you like the idle and back out the screw noting at how many, or fractions of, turns before the slides hit the bottom of the body. This way you can get repeatability).

5 Clean the pilot jet and turn the pilot air screw out 1 1/2 turns.

6. At this point the bike should idle evenly at between 500 and 1,000 rpm (probably closer to 500).

7. Making 1/8 turn adjustments to each throttle stop screw at a time, set the idle.

I like this site as it shows you what you can expect when using a vacuum gage:
What to expect when using a vacuum gage!

HTH


Re: Dual Carb Manifold Pressure [Re: John Healy] #269174
08/08/09 12:58 am
08/08/09 12:58 am
Joined: Apr 2006
Posts: 1,708
Virginia, USA
SBoyd Offline
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SBoyd  Offline
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Joined: Apr 2006
Posts: 1,708
Virginia, USA
"" 5 Clean the pilot jet and turn the pilot air screw out 1 1/2 turns.""

What method do you use at this point to clean it?


Thank you

>>sb


Stop the insanity.
Re: Dual Carb Manifold Pressure [Re: SBoyd] #269214
08/08/09 8:03 am
08/08/09 8:03 am
Joined: Jul 2009
Posts: 3,549
Vic. Australia
P
Pete R - R.I.P. Offline
In Remembrance
Pete R - R.I.P.  Offline
In Remembrance
P

Joined: Jul 2009
Posts: 3,549
Vic. Australia
Your carb-balance device is super-accurate, congratulations! 15.5" of ATF @ 0.875 specific density=1" of mercury.The variation is approx. 0.04 atmospheres;still significant.There are 2 causes to investigate:1 is that you are opening more throttle area on one carb.A twin-cable twist-grip can pull more cable on one side(different radii).Check cables for kinks etc.It is unlikely that this is caused by dimensional difference between carbs.Check that one carb top has not been installed backwards(crazy,but I have seen it).You could swap the carbs,with cables,as a check of option 1.
Cause 2 is that you are getting different pressure signals back through the intake valve into the port.The list just got bigger.Valve seat leakage,tappet-setting,cam timing:I install follower-blocks using a straight-edge and measuring with a vernier-caliper back to the cylinder bores,when fitting.Within 0.002",I still get timing variation about 2 degrees(sometimes improves by re-checking with one follower rotated 180 degrees:- follower ground a little off centre).I'd expect a difference of over 5 degrees in intake closing to get this variation though.
Cylinder/piston condition is unlikely to have much effect.Exhaust system is.Any variation in ports,exhaust pipe length(not the diameter so much),and muffler dimensions,can have large effects during overlap and it will vary at different RPM.
Sorry I can't narrow this down for you,but you have a few things to check.I'd like to hear what you find.

Re: Dual Carb Manifold Pressure [Re: Pete R - R.I.P.] #269265
08/08/09 3:52 pm
08/08/09 3:52 pm
Joined: Mar 2005
Posts: 10,222
Boston, Massachusetts
J
John Healy Offline

BritBike Forum member
John Healy  Offline

BritBike Forum member
J

Joined: Mar 2005
Posts: 10,222
Boston, Massachusetts
SB asked "What method do you use at this point to clean it?"
My little #78 drill mounted in a small length of 1/8" modelers brass tubing.

Pete R said: "Cylinder/piston condition is unlikely to have much effect.:

With the pistons and rings the source of the vacuum we are measuring, as they fall in the cylinder bore, why wouldn't the condition of the rings and cylinder bores have an effect upon the manifold vacuum read from each. In as much as rings and cylinder condition will have an effect upon the amount of compression each has it makes sense that variations would also effect their ability to pull vacuum. I certainly wouldn't expect a piston with one stuck ring to pull as much vacuum as a cylinder where the rings had perfect seal.

Of course these mechanical differences are most problematic when the carburetor is running on manifold vacuum (idle and just off idle where most of these bikes are ridden) and not venturi vacuum where a lot of the variations you listed above have an effect.

Of course anything that effects the velocity of air through a venturi will effect the signal on the fuel supply and performance as the negative signal increases with the speed of the air flowing through a venturi. So with so many possible variables, using manifold vacuum to set the physical lifting of the slides becomes an art.

So besides doing all of the normal things like adjusting valves, setting timing, etc. it would make a lot of sense to check compression. Any variations in compression related to ring or valve sealing will certainly show up on variations in manifold vacuum.

I would encourage any mechanic working on these old bike to have a set of vacuum gages at hand. Though you will need to do a lot of reading and practice before you fully understand what they are telling you. They can be really handy when diagnosing problems.

I would not encourage the average Brit-Bike rider to run out and buy a set, unless they need them to complete their collection of tools. WIthout study, practice and constant use they will not tell you anything.

As Pete mentioned above most people pay little attention to the route their throttle cables take. If you want them to pull evenly you must make sure they only have two bends. If you introduce a third, or more, bends you have made a spring and movement is not predictable. And remember when there is difference that you cannot get out of the cable movement between closed and wide open it is better to let the error be at wide open throttle. As a percentage of error, differences at low throttle openings are more important to balance out.

It's like a lot of things we do. Some like Dick Harris, who at 70 plus puts more miles on these bikes in a year than most brit bike owners do in their lifetime, who likes to keep it simple. He builds his motors in his basement shop sans all of the specialized modern equipment uses pump gas, and good old mineral oil. I have ridden his bikes and they perform as well, or better than any others I have ridden. AND THE BRAKES WORK!

Then there are others who have built all kinds of specialized equipment to check clutch spring pressure before installing them, pressure vessels to check the oil feed seal to see if it will hold before installing them, and have all sorts of other specialized equipment and worry all this to death. To which I admit I must plead partly guilty.

To answer Dr Z question: "Does the old drill bit method work just as well?" For most people, this method or some variation, will work very well indeed.
Just MHO...
John


Re: Dual Carb Manifold Pressure [Re: John Healy] #269281
08/08/09 4:37 pm
08/08/09 4:37 pm
Joined: Jan 2004
Posts: 3,849
ca, us
D
DMadigan Offline
BritBike Forum member
DMadigan  Offline
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D

Joined: Jan 2004
Posts: 3,849
ca, us
A drawback of the drill bit method is it only measures the slide height, not the airflow. Fit of the slide in the carb and mismatches will affect the flow. Your manometer would be less pulsating if you put a reservoir (say, a 4-6 oz. model airplane fuel bottle) in line with the vacuum line. That will serve as a damper and your manometer will read the average vacuum instead of reacting to every intake pulse.
If you want to balance the carbs, make a U tube from glass or plastic pipe and attach the top of each leg to each intake. Again, put a reservoir in each leg with a restrictor hole between the reservoir and intake to dampen the pulses. Now the height of each side gives you the relative reading compared to the opposite side and the RPM does not matter.

Re: Dual Carb Manifold Pressure [Re: DMadigan] #269297
08/08/09 7:12 pm
08/08/09 7:12 pm
Joined: Jan 2009
Posts: 327
Montana, USA
D
Dr. Z Offline OP
BritBike Forum member
Dr. Z  Offline OP
BritBike Forum member
D

Joined: Jan 2009
Posts: 327
Montana, USA
Thanks for all the replies. This morning I balanced the carbs according to John's steps leaving out pilot jet cleaning, and there is no noticable difference in performance (it ran very well after using the manometer and it runs very well after using the drill bit method). So at this time I will forget about the small difference in manifold pressure and ride while the weather is still good.


Craig Zaspel

T100R
BSA A10

Moderated by  John Healy 


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