PK backfire

[Vespista_ZD]

Hello...i was wondering if someone can give me some advice...my PK is equipped with 133DR,Keihin PE24 (main jet 95, idle 48), original PK 12V ignition and flywheel, giannelli right hand exhaust and casings are just ported to match the cylinder and intake manifold(nothing too much)...i noticed in last few weeks that my PK backfires in low rpm(when i add a little bit of gas just so it revs up a bit for few times it backfiers strongly) i changed the other ignition also original from PK, bought new spark plug and it backfires again...at high revs it runs ok but sometimes at this low rpms it backfires and when i check the spark plug it is completly black...so i tried with idle jet 45 and did a little ride around town it still backfired but now the spark was a bit on the lighter side(leaner mixture)...now i ordered a brand new BGM pro CDI unit and will see how it runs with new CDI unit...but i just cant figure out what is going on???

[pfupfu]

Hi,

 

normally when I hear from backfire I think of defect pickups (that little black thing that generates the signal for your CDI). You can easily check for a broken pickup with a timing light just by activating it and looking on the motor.

 

the second thing could be extreme rich air/fuel. I am not firm with your carb, slow jet 42-45 should be good depending on the needle, main jet 95 sounds reasonable

 

best regards 

[Vespista_ZD]

Pickups are in great condition on both of my ignitions...i measured the resistance on them the way one of more experienced member of my club told me and they seem to be like new...PE is older version of PWK carbs i think...so i tried with smaller idle jets but turned out the mixture was leaner and the backfire was still present...so should i try and put a thicker needle or what?....now i am running on a pretty slim needle.

[pfupfu]

Do you know kyajet? http://www.kyajet.de/

 

Should help you with jetting.

 

The resistance of the pickup does not tell you the Full truth, because there more elements in it you can not measure with a simple ohm-meter. But as said, pickup can be checked quite easily with a timing light.

 

You can also disconnect the cable tree for testing purposes. Sometimes the green wire goes bad over the years and can make some weird shortcuts 

[Vespista_ZD]

Thank you...i will sure try it!

[pfupfu]

Another remark:

 

Normally while checking the ignigition timing it should look like shown in the video (beginning 12:00) so the rotor should appear standing still or just lightly moving with the standard ignition. A broken pickup results in a more unpredictable and unstable appearance. 

 

 

[ronin]

Are you shure that it´s backfire and not afterfire?

[Vespista_ZD]

It fires when i release the throttle...

[Vespista_ZD]

Loud bang and lot of smoke...

[ronin]

Back fire is when it bangs into your inlet and carb. If the gas bangs into your exhaust it´s after fire and your explaination sounds like after fire.

With a lot of smoke it looks like you have a very rich ratio when you close the throttle. If you have a leakage at your exhause system (flanges?) and you close the throttle oxygen comes into your exhaust gas and the hot exhaust will fire up the mixture of unburned gas and oxygen. That may also happen when there´s a leakage at the cylinder head.

[Vespista_ZD]

Ok...thanks for the advice and explanation...i will sure check it!

[ronin]

You‘re welcome