Author Topic: ONLY RUN ONE CYLINDER  (Read 582 times)

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Offline Davlea

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ONLY RUN ONE CYLINDER
« on: September 04, 2021, 05:42:00 PM »
I have a RD250f with a weak spark and will only run on one cylinder have fitted new coil, cdi unit plugs. Definately not a carb issue have been cleaned and new Mikuni parts fitted. I didn’t think it would run good on the one cylinder with a stator fault but I could be wrong. Have checked all connections and earths.  Any suggestions would be greatly appreciated, before I crack up.


Offline 5port

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Re: ONLY RUN ONE CYLINDER
« Reply #1 on: September 04, 2021, 06:30:01 PM »
Because of the commonality of the ignition CDI system it is unlikely that ignition faults will only effect one cylinder.  As both plugs fire with the same discharge (it is effectively one system that has both plugs in it) you can simply swop the HT wires to the other plug and see if it runs on the other cylinder or the same cylinder or, no cylinder?  If you swop HT leads and it runs on the same cylinder, then the bad cylinder probably has a fault, like low compression, a bad air leak or carburation fault.
Just about the only ignition faults I can imagine with the RD CDI that would only effect one cylinder are, a bad plug or HT lead or a failed/missing trigger magnet.
Try the HT lead swop.

5port
 
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Offline Davlea

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Re: ONLY RUN ONE CYLINDER
« Reply #2 on: September 05, 2021, 01:11:03 PM »
Thanks I will try that, do you think it unlikely to be the stator

Offline 5port

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Re: ONLY RUN ONE CYLINDER
« Reply #3 on: September 05, 2021, 03:57:49 PM »
Thanks I will try that, do you think it unlikely to be the stator

You can test the CDI functions in the Stator if you refer to the RD250/400E (2R8/9) Supplementary manual in the Tech downloads.
I would be surprised if the CDI system itself was only firing one of the cylinders because there is only one CDI charging coil (with HIGH and LOW speed windings) and one CDI pulser coil, they fire the whole ignition (both plugs) every time one of the two trigger magnets pass the single pulser coil that is on the outside of the Rotor. The HT coil has only a single feed that passes the firing pulse every 180 degrees of crank rotation and both plugs fire every time with the same HT pulse, there is no Left or Right.  The Troubleshooting guide does not mention single cylinder running but, the faulty plug is mentioned and Coil, which would include HT lead.  The rotor is not mentioned but there is reference to replacing the complete "Magneto Assembly".  :o
There is complete guidance to testing the whole system in that FREE manual in the tech.  ;)

Tell us how it goes with the plug leads swopped over.

5port



 
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Offline Davlea

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Re: ONLY RUN ONE CYLINDER
« Reply #4 on: September 05, 2021, 09:02:38 PM »
Tried changing plug leads around the same cylinder as before still not firing

Offline Gr8uncleal

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Re: ONLY RUN ONE CYLINDER
« Reply #5 on: September 05, 2021, 09:26:13 PM »
Where did you get your spark plugs from - reputable source or the internet?

A former resident of this parish spent a lot of time and effort chasing a problem on his KH250, but it turned out to be just a crap new plug that he'd fitted (probably a counterfeit one). 
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Offline Davlea

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Re: ONLY RUN ONE CYLINDER
« Reply #6 on: September 05, 2021, 09:43:33 PM »
Tried various new plugs, swapped plugs around.

Offline RDDave

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Re: ONLY RUN ONE CYLINDER
« Reply #7 on: September 05, 2021, 10:40:22 PM »
I would check the voltage at the coil plug orange and black wires, my 250 had a weak spark on both cylinders, i rigged a 19mm socket up on a battery drill, took both plugs out and spun the engine with the ignition ON at 1000rpm, it had 0.1 A/C volts at the coil plug (orange wire positive, black negative on the tester). I changed the CDI unit and bike ran spot on, checked the orange and black at the coil plug again and had 2.8 A/C volts, make sure you set multimeter on A/C and not D/C.

Offline betty foRD

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Re: ONLY RUN ONE CYLINDER
« Reply #8 on: September 05, 2021, 11:38:11 PM »
Might be the magnet on the rotor for the bad cylinder. True, both fire together, but the wasted spark system still has dedicated trigger for each cylinder.
Rev 'em till they rattle

Offline 5port

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Re: ONLY RUN ONE CYLINDER
« Reply #9 on: September 06, 2021, 07:24:00 AM »
Might be the magnet on the rotor for the bad cylinder. True, both fire together, but the wasted spark system still has dedicated trigger for each cylinder.

Yes, by conventional timing proceedure of doing the left cylinder while doing the set-up.  You can time the right cylinder BTDC to the "F" mark if you wanted to, it's just the same with this CDI set-up.  ??? 
 However, if a single trigger magnet is missing or not functioning, you would only get a get a single firing per rev, which would only fire the one cylinder.

Cheers 

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Offline 5port

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Re: ONLY RUN ONE CYLINDER
« Reply #10 on: September 06, 2021, 07:39:13 AM »
Tried changing plug leads around the same cylinder as before still not firing

So, with plug leads swopped over the same single cylinder fires.   If everything else tested correct, I would suspect a bad trigger magnet on the Rotor.  However, you don't say if you have done any of the Yamaha manual diagnostics.  It could still be a weak spark for other reasons that is just good enough to fire one of the cylinders, but not the other one. 
Have a quick look at the trigger magnets and check they are not damaged.  Overall, you need to do the Diagnostic tests. 

Cheers

5port
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Offline bettatek

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Re: ONLY RUN ONE CYLINDER
« Reply #11 on: January 15, 2022, 03:22:01 AM »
Have you tried swapping the reed valves between cylinders?

Offline 5port

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Re: ONLY RUN ONE CYLINDER
« Reply #12 on: January 15, 2022, 03:48:59 PM »
Have you tried swapping the reed valves between cylinders?

Well, the Reeds do need to be there and in reasonable condition.  ::)
The overhaul manual has the details but basically, they should be; undamaged, clean, free to move and close within 0.5mm against the rubber at the edges.

Cheers

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Online chrisg.007

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Re: ONLY RUN ONE CYLINDER
« Reply #13 on: January 17, 2022, 10:50:26 AM »
As said, my money would be on carburation. How did you check the timing?  Has it ever run properly or have you had the engine apart?
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