Dunstall Vs Irving

TBolt

Joined
Jul 19, 2013
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71
In his book, "Norton Tuning", Paul Dunstall's method of tuning twin carbs on a twin cylinder machine was to unplug one plug lead and tune the opposite cylinder when it's firing by itself, and then swap over and do the same on the other side.
Yet in Phil Irving's book, "Tuning for Speed", he states; "it is quite useless to attempt to adjust the idling setting on one cylinder only, with the other cut out by shorting the plug. The amount of throttle required to pull the engine round with only one lung operating is so much greater than with both in action that the scheme gets you nowhere."

I've never had any luck with the Dunstall method. Like Irving says, you tune them both individually but when you put them together they go completely out of whack.
How did Dunstall make this method work?
 
You slacken each throttle stop screw by the same amount until you get the idle speed that you want. It isn't the most elegant method of balancing carbs at idle but it does work.

Ian
 
Nortoniggy said:
You slacken each throttle stop screw by the same amount until you get the idle speed that you want. It isn't the most elegant method of balancing carbs at idle but it does work.
Click to expand...

It works to a degree. Both cylinders will be "pushing" the same, but may not be burning the same. An idle plug chop with brand new plugs will tell the tale.
 
Running an engine with a plug lead lifted off the spark plug is one fast way to ruin a coil.The coil voltage normally only gets high enough to fire the plug,then it can't get any higher.With the plug disconnected,the coil voltage rises much higher and its looking for an escape path.It can rise high enough to find an escape path through the coil insulation,and burn the internals of the coil.

There's nothing wrong with shorting out a plug lead,because that keeps the secondary voltage low.If I short out one plug on a running engine,I expect the engine to fire 5 times on the other cylinder before it stops.It should do the same when I short out the other plug.If the mixture screws are already adjusted for fastest running,all is well and the idle stops are then equal.

It's an eye-opener to short each plug at about 3500 rpm,and note the drop in rpm.This will get the throttle cable adjustment equal,if the rpm drop is equal.You'll be very surprised at how much error there is if you've already tried some other way to get them equal.
 
I've tried the Dunstall method and tend to agree with Irving.
I always end up tuning air screw and idle stop positions by ear.
For synching the slides to the throttle cables, the best method seems to be using two equal dia. rods or two equal dia. drill bits under the slides then adjusting the cables until the ends of both rods drop slightly at the same instant when the throttle is twisted just past slack take up point.

Glen
 
If you want exact synchronicity of two throttle stops and two throttle cables, use a vacuum gauge.

If all you want is a reliable idle, well maybe it doesn't have to be quite that exact.
 
worntorn said:
For synching the slides to the throttle cables, the best method seems to be using two equal dia. rods or two equal dia. drill bits under the slides then adjusting the cables until the ends of both rods drop slightly at the same instant when the throttle is twisted just past slack take up point.
Click to expand...
Try shorting out plugs at 3500 rpm.Like i said,you'll be surprised how inaccurate all other methods are for synching throttle cables.
 
X-file said:
worntorn said:
For synching the slides to the throttle cables, the best method seems to be using two equal dia. rods or two equal dia. drill bits under the slides then adjusting the cables until the ends of both rods drop slightly at the same instant when the throttle is twisted just past slack take up point.
Click to expand...
Try shorting out plugs at 3500 rpm.Like i said,you'll be surprised how inaccurate all other methods are for synching throttle cables.
Click to expand...

Thanks, will give it a try

Glen
 
Watch that if you have electronic ignition and 'remove a plug lead', this can burn out the ignition black box.
Some manuals also WARN ABOUT THE SPARK FROM LECCY IGNITIONSs, which can be a heart stopper - literally !!

The 'proper' way to do it with this method is to, with the engine STOPPED, remove one plug lead and fit it to a spare spark plug.
Lay the plug and lead on the cylinder head, so it can spark in air, and start the engine = running on one cylinder.
After setting each cylinder individually like this, you'll have to lower the idle speed for the pair of them working together.
This works extremely well, in the absence of vacuum gauges or any other similar help, it gets both cylinders synchronised at idle .

A magazine survey, many years back now, concluded that ~70% of twin carb bikes surveyed were out of synch, throttle wise.
Don't know how or where this survey was done, but it was to do with bikes coming into dealerships.
Lot to be said for single carb engines.... ?
 
Rohan said:
The 'proper' way to do it with this method is to, with the engine STOPPED, remove one plug lead and fit it to a spare spark plug.
Lay the plug and lead on the cylinder head, so it can spark in air, and start the engine = running on one cylinder.
After setting each cylinder individually like this, you'll have to lower the idle speed for the pair of them working together.
This works extremely well, in the absence of vacuum gauges or any other similar help, it gets both cylinders synchronised at idle .
Click to expand...
There is another way to short out plugs,and you can do it to either cylinder with the engine running.
Connect a short piece of wire between the cap on the lead and the plug.After I wind some of the wire around the top of the plug,I leave about 1" hanging out the side of the plug.A piece of fence wire or tie-wire would do.If I ground a screwdriver blade to the head,I can short out the wire at the plug with the tip of the screwdriver.

This makes it dead easy to short plugs and compare cylinders with the engine running.
 
While this may be true, if you are going to be adjusting one carb at a time, its much more convenient to have that spare splug sparking away on its own,
rather than doing the screwdriver trick on one side of the bike while adjusting a carb on the other side....

Plus, if you can start your bike with only one plug working/active, you KNOW that the basic carb settings are good...
 
It's all much easier with vacuum guages........loosen off throttle cables, set carb idle speed screws for same vacuum, then adjust mixture screws for fastest idle. Go back and reset idle speed screws for your choice of idle revs AND same vacuum.

Adjust throttle cable adjusters for same vacuum at about 2000 revs.

All done on an engine that is up to operating temperature, after valve and ignition adjustment.
 
Just to throw another spanner in the works, some manuals advise NOT to adjust air for necessarily the fastest idle but rather for the smoothest, strongest sounding idle. I have encountered this with some engines, where a high rpm uneven idle can be found and also a bit lower rock steady idle at a slightly different air setting.
This is very common with two strokes but also occurs with four strokes.
With my two stroke trials bikes , a strong idle is needed for actual operation rather than only with engine unloaded as with street bikes. So for those I play with the idle until the bike will sit with the front wheel against a tree or fence post while the rear wheel sits and slowly spins on gravel at idle in first gear, rider weight removed, feet flat on the ground. The ideal air screw position for this is never where the Max no load idle rpm is.
I have often thought it would be good to tune a road bike idle in a similar fashion, but of course the gearing and bike weight generally do not allow for spinning the rear wheel at idle. One needs an idle dyno of some sort, or perhaps a certain slope could be used.

Glen
 
You blokes are much more technical than I am. The first thing I usually do is wind the throttle wide open with the motor stopped and make sure the slides are both at the same place at the tops of the bores. Then I back off the stops which provide the speed adjustment when the slides are closed. I wind the idle mixture screws out a couple of turns . I then start the motor and gently close the throttles, until the motor starts to idle (preferably rich ). I screw in each idle mixture screw until I get the spit back through the carb, then back off half a turn, and the motor should run smoothly while I am holding the throttle slightly open. If the carbs have throttle stops, I then screw each of them up until the motor speeds up, then let go of the throttle and adjust them to get smooth running.
If the throttle cables are unequal in length, one slide will come off the stop before the other as you open the throttle. If one carb is lean enough to cause a spit back, and the other one is correct, the idle will be uneven, as it will be if the stops are not set correctly.
 
Here is a link to the North Texas NOC site for carb adjustment. Mostly same as method of Chasbmw except "ears" are used rather than vacuum gauges. I like vacuum gauges....they don't lie.

I like to print out stuff such as this and have it before me when I'm working on the old Atlas.

http://www.ntnoa.org/Carbs.htm

Slick
 
I understand that Dunstall did some good stuff using the ex Doug Hele Domiracers etc etc. Also he must have had some good people working for him. However Phil Irving is a genuine hero of motorcycle (and Formula 1) engineering. At the end of the day you are comparing an entrepreneur/real estate developer to a highly experienced, highly qualified Engineer. I know who's opinion I would have faith in.
cheers
wakeup
 
Yes I'm more inclined to go with Phil Irving, the man was superhuman.
Just look at a Vincent.........and then Jack Brabham's F1........case closed.
I like Dunstall though, he got some good stuff done.
 
wakeup said:
I understand that Dunstall did some good stuff using the ex Doug Hele Domiracers etc etc. Also he must have had some good people working for him. However Phil Irving is a genuine hero of motorcycle (and Formula 1) engineering. At the end of the day you are comparing an entrepreneur/real estate developer to a highly experienced, highly qualified Engineer. I know who's opinion I would have faith in.
cheers
wakeup
Click to expand...
+1 on that statement wakeup. Very well said!
 
Phil Irving was from a different generation to Paul Dunstall, as probably I am to most of you guys. I had a conversation with Phil when I was in my 20s, and he would have been in his 80s. He was the old school professional engineer - the sort of guy who was active in the Society of Automotive Engineers. When I was working in Defence at Maribyrnong, he was around the corner in Repco Brabham, and one of the guys who worked with us came from there. What you are talking about is intense, do everything yourself innovative engineering. We don't do much of that these days. Dunstall was a bit later and I don't believe he was quite the same type of guy. I didn't think much of his hot-up bits, however they seemed to work. I think a later 850 would have run rings around his 800 (?), however at least he made an attempt. You couldn't buy much trick stuff back then.
I seem to remember there were some strange bits came from Repco back then and ended up in my friend's Vincent - might have been titanium valves.
Did Unity Equipe have something to do with Dunstall ?
 
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