Topic: To dwell on dwell, OR how to divide duration.

[fang]

I have a nice happy dwell meter, and I like to use it to set my points. My dwell meter only has settings for either 6 or 8 cylinder engines. Lets assume I am using this on my trusty mid-1970's SOHC CB750, which we all know is a four-cylinder engine with two sets of points and a single-lobed points cam.

Normally, I will statically set the dwell with a feeler gauge, double check, and then use the dwell meter to dynamically ensure that both sets of points the exact same while the engine is running. It does this just fine, yielding splendid results.

However, I would like to correctly measure and record my points dwell angle for future reference/ other applications. I could just look at the eight-cylinder engine reading and record that number, but like I said, I would like to record the correct angle. This has been bugging me forever. Every time I think I have it figured out I start thinking myself backwards into a box. I am pretty sure that as far as the dwell meter cares, one points lobe = one cylinder. 

The real questions here are:
(1) Am I correct in thinking that, as far as the dwell meter cares, the number of lobes on the points cam is equal to the number of cylinders of the engine?
(2) What then is the math to obtain the correct angular dwell readings for my CB750 (a "single cylinder engine") when measuring with my happy old 6 or 8 cylinder dwell meter?

I believe that when I am checking one set of points on my CB750, I might as well pretend that it is a single-cylinder motor. If this is correct, then to obtain the corrected dwell angle I could take the 8-cylinder reading and multiply this times eight (or the six cylinder number and multiply it by six). For example, if the dwell meter indicates 22 degrees of dwell in the eight-cylinder mode, then I would multiply this by eight disclosing a corrected 176 degrees of dwell. (22*8=176)

Does this sound correct?

peace and grease,
fang

[keiths]

Here is what TwoTired says.
http://forums.sohc4.net/index.php?topic=41860.msg431980#msg431980

[fang]

Right.  That is what EVERYONE says.

I am trying to find out if there is a simple formula for calculating a number for our CB750s from the readings provided from a dwell meter designed for six or eight-cylinder engines.

The interwebz are strangely silent regarding the subject.

-fang

[Whaleman]

http://www.motorcycleproject.com/motorcycle/text/sohcign.html

This article talks about using a dwell meter. My bike runs smooth as silk and I just set the gap with a feeler gauge. Dan

[fang]

Thank you for all the nice links.  Please re-read my original post.  I promise I have no problems whatsoever setting the points perfectly.

I am only interested in figuring out how to convert what my 8-cylinder dwell meters says into the correct, actual dwell angle on a CB750.

-fang

[gane]

Fang, It's been so long I can't remember if my findings were calculated or derived... But, according to my 30 year old notes, on by Dad's (mine now) dwell meter on the 8 cyl.setting 23 degrees is fine for my K6 and  '78 kz650. luck G

[gears351]

Fang - your bike on an 8cyl dwell meter should read 23-24.5 degrees on an 8cyl scale - I just (12 minutes ago) finished doing this same procedure with my Actron dwell meter on a 76 CB750. Both the 1-4 points and the 2-3 points were spot on at 23 degrees.

Sorry I don't have a formula for you. But that's straight out of the Honda service manual.
Hope it helps.

[fang]

Thanks guys.

Now lets find the formula!  

[fang]

Sweet!  Now we're getting somewhere!  Keep it coming.  =)

[mystic_1]

TT said that stock dwell was 195 degrees.  People report that 23-24.5 degrees on a 8 cylinder dwell meter is correct.

195 / 8 = 24.35.

Interesting.

mystic_1

[gears351]

that formula is a miracle Mystic - have fun storming the castle!

[fang]

Fire ze missiles!!!!

[w1sa]

This is the way I've always reasoned it..........I may well be wrong!

A dwell meter uses the cylinder# setting to determine which degree of crankshaft rotation to use in its calculation. It also assumes a single distributor of spark. So it calculates based on the number of spark events  over two crankshaft revolutions   i.e. crankshaft spark event/intervals for  8 Cyl = 90*,  6 Cyl = 120*,  4 Cyl = 180*,  2 Cyl = 360*, etc.
 
It uses the basic formula,   % time closed X spark interval crankshaft degrees.

With a 4 Cyl, dual points, dual coil ignition, 8 Cyl dwell selection will give a reading based on 90* spark interval. As an example, using a dwell of 195* for a Honda, the dwell meter would calculate as  
195/360 X 90* (8Cyl spark interval)  =  48.75*dwell reading
 
Or with 6 Cyl selected
195/360 X 120* (6Cyl spark interval)  = 65* dwell........etc.

A dwell meter with 2 Cyl selection would read the equivalent 195* dwell.
 
If you were expressing in Camshaft* equivalents, the readings should be halved.     

Don't shoot me, I'm still learning to play the piano

[fang]

You're a freaking genius!

With a 4 Cyl, dual points, dual coil ignition, 8 Cyl dwell selection will give a reading based on 90* spark interval. As an example, using a dwell of 195* for a Honda, the dwell meter would calculate as 
195/360 X 90* (8Cyl spark interval)  =  48.75*dwell reading

Yes... I think you are really close here.  However, since this is a 4/stroke engine (and thus a camshaft timed motor) we need to halve the figures you came up with... Or in other words, if there are 8cylinders, then eight hits in one rotation; 360/8 = 45.

So.... IF X = target dwell for the CB750, and we want to calcualte from the output of a 8-cylinder engine's dwell reading,  and Y = what to look for on the dwell meter, then the formula would be:
(X/360)*45=Y   The simplified version of that equation is x/8=y

So wow.  It really is just take the stupid 8cylinder reading and multiply it times 8.   I like how we were able to verify that.

Thanks all you all for your help.   Case closed.

To find dwell on our CB750s with a dwell meter for an 8-cylinder motor, simply take the reading and multiply it by eight.

peace and grease,
-fang

[Rampage1967]

Actually, each half of the 750 ignition system would fire once per turn of the crankshaft.
On a V8 car engine the ignition will fire 4 time per turn of the crankshaft.
This is due to the fact that ignition on the 750 is 2 separate systems firing 2 cylinders each while a V8 uses one system to fire all 8 cylinders. When you take into account that it takes 2 turns of the crankshaft to complete a complete cycle of firing all cylinders you get this result.

[Caseygroh1]

to get the dwell angle for 4 cyliner engines multiply the 8 cylinder scale by 2. mine says that right on the meter. so you want 25-25 on the 8 cylinder scale.

[w1sa]

to get the dwell angle for 4 cyliner engines multiply the 8 cylinder scale by 2. mine says that right on the meter. so you want 25-25 on the 8 cylinder scale.

No, that only applies for conventional single distributor style/single coil set-ups.

[lordmoonpie]

Clue's in the question isn't it Fang? You have a dwell meter for six or eight cylinder settings or put another way, it's meant to be used on cars, don't use it on your bike, set the timing up properly using a method meant for bikes!

Just my $0.02, dwell meters suck! 

[Deltarider]

Why do you SHOUT at us?

[lordmoonpie]

Hey Delta - it's along way from the USA to be heard in the Netherlands - he probably has to raise his voice a little 

[fang]

Why do you SHOUT at us?

Mostly because I am a half-blind jerk who was mostly drunk when posting all this crap last night.   

No offense intended.

Clue's in the question isn't it Fang? You have a dwell meter for six or eight cylinder settings or put another way, it's meant to be used on cars, don't use it on your bike, set the timing up properly using a method meant for bikes!

       I've been setting the timing on bikes for about 25 years now.  That never was the problem.  This whole time I just wanted to figure out how to covert the dwell reading on my dwell meter to the correct dwell angle.   

Oh, and the secret formula still seems to be correct this morning:  just multiply the number *8 on my car dwell meter to get the correct reading for a cb750.

peace and grease.
-fang

[76supersport750F]

Hello and I got a question.

With the changing of today fuel properties compare to 1976 and the age and condition and design and maybe other things.

Question    Does any one know if the bike response or runs better with Less Dwell example letting the plug fire longer or more dwell letting the coil saturate longer for a greater spark?

With the newer fuel do you need a touch of advance?

Or Stock dwell is right on and that's ok for today's fuel.

[kenolds]

I knew you guys were correct, but I couldn't figure out why... Then it hit me. 
A typical V-8 car's distributor sends a total of 8 sparks per distributor revolution (it turns at cam speed or 1/2 engine speed). The points cam lobe pitch would be 45 degrees (360/8=45).  So what the dwell meter is actually measuring is "time points are closed per 45 degrees of distributor rotation".  A V8 dwell meter is set up to display units of " degrees of points being closed (or is it open?) per 360 degrees of distributor rotation".  Therefore, In order for the meter to display the correct units, the scale of the display must divide the actual measured dwell by 8.  On a CB750, the points are attached to the crank, and therefore rotate at engine speed.  CB750 points only fire once per rotation.
IF the target points duration is 195 degrees for a cb750, that is 195/ distributor rotation.  In order to convert to what the V8 dwell meter would read, you simply have to divide 195 by 8. 

195/8 = 24.3.

Kenolds
 

[fang]

Disco, exactly right on.

The dwell measures the amount of time the breaker point is closed.

Otherwise very well explained.

peace and grease,
-fang

[fang]

Thanks again to all of you for contributing to the answer.  When we all work together without bickering, everyone is a winner.     Fire ze Missiles!

peace and grease,
-fang