[[email protected]]

On my 70 cb450 the left cylinder was not firing. I pulled the the plug and held it against the head, started the engine, no spark, but when i hold the plug about 1/4 inch from the head there is plenty of spark. Both plugs are set to spec. I have never seen this before. Any ideas of whats going on?

 

[66Sprint]

Replace the plug... the porcelain is cracked internally (been dropped)....

 

[[email protected]]

Been there done that, changed both with new ones.

 

[66Sprint]

And the problem persists?

Were parts from the same supplier?.... Auto parts guy could have dropped the whole box of plugs......

Must replace with KNOWN good plug (like the one that was previously running the other side)

 

[[email protected]]

Funny you would say that. I did swap the plugs and still the same thing on the left side.

 

[66Sprint]

TRY this....install known plug but simply hold cap over, but not all the way down (wear insulating gloves and hold cap that 1/4" up)....Does cylinder now fire?

OR, swap caps and see if side also swaps....

You are looking for any alternative path to ground that excludes the spark-gap of the plug within the cylinder......
Is plug wire nicked and touching head anywhere?

 

[Rob Axel]

Clean ur coil grounds

 

[66Sprint]

Clean ur coil grounds

WHAT???? .....The coil grounds are the points.....Or the spark plugs...

The coil bracket IS the condenser ground, but a points ignition will fire without the condenser in circuit...... I would suggest he temporarily disconnect the condensers and see if he then gets spark....

 

[Bill H]

go to the local parts store and pick up a plug tester. About $10.00. It will light up and tell you if you have power to the plug. If it doesnt light up you have another problem instead of the plug.

Very fast and easy test.

Bill

 

[66Sprint]

Bill....He's got power (can get spark, just not at plug spark gap.....)

 

[Rob Axel]

funny you say that..(points are the grounds for the coils). I have my coil on my Triumph hanging from a ziptie(not grounded).. and one time my 450 wasn't firing proper. Noticed the contacts where the coils mounted to the frame were rusty/corroded.. cleaned and havnt had a issue since....
.. WEIRD!!!..

 

[Bill H]

Thanks 66sprint I read the post wrong. I have not had that problem and will file the info away.
Bill

 

[66Sprint]

funny you say that..(points are the grounds for the coils). I have my coil on my Triumph hanging from a ziptie(not grounded).. and one time my 450 wasn't firing proper. Noticed the contacts where the coils mounted to the frame were rusty/corroded.. cleaned and havnt had a issue since....
.. WEIRD!!!..

NOT WEIRD....The bracket IS the ground for the condensers, which help the points maintain their correct gap and increase their lifespan.....Plus likely help increase spark intensity once they discharge and draw higher current through the coil primary......

 

[WintrSol]

The plug caps may need replacing; could be carbon tracking on the inside of the caps bypassing the plug when the plug is held against the head.

 

[[email protected]]

Well I changed the plug gaps on both plugs from 035 to 028. Starts and runs fine. However now I'm back to the original issue, it idles at 3K. I can get it down to 1200 but when I give it some throttle it comes back down to 3K. Both throttle plates move at the same time. I checked the springs and movement of the spark advance unit and it SEEMS good and smooth.

 

[Rob Axel]

Thanks sprint ... makes sense.. Triumph has a E.I...no condensers

 

[66Sprint]

Well I changed the plug gaps on both plugs from 035 to 028. Starts and runs fine. However now I'm back to the original issue, it idles at 3K. I can get it down to 1200 but when I give it some throttle it comes back down to 3K. Both throttle plates move at the same time. I checked the springs and movement of the spark advance unit and it SEEMS good and smooth.

Did you do the "straw" test, and are the slide pistons in the correct carbs?.....gaskets beneath them (dome gaskets) dry and sealing?... Excessive wear on the chromed center-tubes of the slides or the brass they fit into in the domes?.... Domes reversed?

 

[[email protected]]

I don't know about the straw test, sounds interesting. I did the carbs one at a time, put in new gaskets and made sure the slide pistons are free. I've heard the rubber carb to engine boots can develop leaks and that boiling them in water can bring them back.

 

[66Sprint]

Boiling might soften them so the carb will insert more easily, but it won't repair cracks or leaks......

 

[Rob Axel]

"straw test"... please elaborate...