classic motorcycle forum

Motorcycle Discussions => Japanese Bikes => Topic started by: christ044 on March 03, 2008, 01:50:09 AM

Title: CB72 Ignition
Post by: christ044 on March 03, 2008, 01:50:09 AM

Hi there, iv been rebuilding a 1965 Honda CB72 for my dad, iv got all back together but im having big problems with the ignition / points.

It has the type 1 system (two sets of points) but they seam to be open all the time and close to fire the plug? Is this correct?

I also cant get the plug to fire at the right time, it should be about 5 deg before tdc, it is firing about 45 deg before tdc! But no amount of adjustment on the back plate moves this?

Title: Re: CB72 Ignition
Post by: L.A.B. on March 03, 2008, 04:06:23 AM

Quote

It has the type 1 system (two sets of points) but they seam to be open all the time and close to fire the plug? Is this correct?

No, something is definitely wrong there? The spark always occurs as the points open.

I'm just wondering, I think the CB72 has a chain primary drive?
So it may be one of those rare bikes where the crankshaft rotates in the opposite direction to normal, so rotates 'backwards'?

Could you be turning the crank the wrong way?

Title: Re: CB72 Ignition
Post by: christ044 on March 03, 2008, 04:47:24 AM

No the crank turns in the normal direction, with both the starter motor and the kick start.

Im totally lost by it to be honest, there is just no other way for it to go back together?

Title: Re: CB72 Ignition
Post by: L.A.B. on March 03, 2008, 05:15:00 AM

You could try asking on the forum here?: http://www.honda305.com/

Title: Re: CB72 Ignition
Post by: christ044 on March 03, 2008, 05:25:10 AM

I have mate, thin i need to either contact honda themselves or find a cb72 expert

Title: Re: CB72 Ignition
Post by: neale on March 26, 2009, 07:56:10 PM

Hello pal, Just a quick few lines, it sounds as if you have the valve timing incorrectly set up , as a very quick and very rough check , remove the valve covers,turn over the engine and watch when the valves begin to open,inlet should start to open just before TDC and the exhaust at about BDC.But,before doing that recheck your points ,set them to 12-15 thou gap at max lift,make sure that the back plate is the correct way up, go about this one at a time.
Best of luck !! Good bikes ,I am after one to restore.
Cheers Neale

Title: Re: CB72 Ignition
Post by: chris p on March 26, 2009, 10:07:38 PM

i agree with neale sounds like the valve timings wrong, as the points run off the end of the cam.

Title: Re: CB72 Ignition
Post by: huron on April 02, 2009, 10:35:41 AM

Hello,
As LAB said the spark occurs when the points open.If you put a piece of thin paper in the closed points,when can you pull it carefully out in relation to the timing marks on the Nr1,left cyl,is this point around the mark?If all otherwise is ok?Could the timing plate be not on right and turned 180 degrees
wrong?I had this wrong on my cb250k mot and it would not time right!
I can well understand what you mean!
Huron

Title: Re: CB72 Ignition
Post by: BarryBiker on May 03, 2009, 06:04:53 AM

If you took the cylinder head apart you probably put the point cam shaft in 180 degrees off.

Quote

Quote

It has the type 1 system (two sets of points) but they seam to be open all the time and close to fire the plug? Is this correct?

No, something is definitely wrong there? The spark always occurs as the points open.

I'm just wondering, I think the CB72 has a chain primary drive?
So it may be one of those rare bikes where the crankshaft rotates in the opposite direction to normal, so rotates 'backwards'?

Could you be turning the crank the wrong way?