450 Desmo not running cleanly

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Duccout
Posts: 1285
Joined: Wed Feb 05, 2020 1:20 pm
Location: Essex UK

Re: 450 Desmo not running cleanly

Postby Duccout » Fri Feb 21, 2020 10:20 am

Whenever I've had something like this it has been an ignition issue, or a blocked carb main or needle jet. Years ago I went to Silverstone in my old Triumph car (those were the days!) in the pouring rain. Nearing the circuit, the car suddenly lost power and would only run on tickover, so I had to nurse it to a cafe, where there was a phone. The slightest amount of throttle made the engine cut out. I called the breakdown services, suggesting that water had got in somewhere, but the brilliant tech, working in the pouring rain, traced the problem to a broken condenser wire.

Jordan
Posts: 1380
Joined: Fri Dec 24, 2010 11:29 am

Re: 450 Desmo not running cleanly

Postby Jordan » Fri Feb 21, 2020 10:49 am

Nothing to do with this problem, but just another example of unexpected causes of poor running -

I had a big water tank delivered, the truck driver ringing ahead to say he was running late because of intermittent power loss problems.
He finally was able to see the cause in his mirror. There was some plastic wrapping that had become dislodged from the cargo, and was covering the intake of the air filter every now and again. Until he noticed it happening, whenever he stopped to check, the plastic would fall away so not to be obviously the cause.

graeme
Posts: 938
Joined: Sun Mar 25, 2012 12:00 am
Location: Tasmania Australia

Re: 450 Desmo not running cleanly

Postby graeme » Fri Feb 21, 2020 11:30 am

Try new fresh 95 without any added lube or lead replacement
I suggest not using 91 save that for your lawn mower.
I’ve had issues in the past with upper cylinder lube causing poor running.
Try the air cleaner off test, if that makes no difference try adding a very small amount of choke.
But only a very small amount. Might give you a direction to chase, rich or lean.
Fuel goes off very quickly so keep it fresh.
I don’t think it’s your air cleaner being too restrictive, I have the same on a 32 phf
Worth trying before you start mucking around with timing etc

Good luck

Graeme

(Where are you in oz ? )

Dan C
Posts: 75
Joined: Wed Feb 12, 2020 12:52 am
Location: Armidale, NSW, Australia

Re: 450 Desmo not running cleanly

Postby Dan C » Fri Feb 21, 2020 9:15 pm

Thanks Duccout, Jordan and Graeme, really appreciate your thoughts.

Just wondering if Duccout's brilliant tech is still alive and would like a trip to Aus? He'd probably have me going in 5 minutes....
I'm tending to discount some random cause, like the example of plastic covering the air cleaner, because the problem is so predictable and reproducible, but it is a good suggestion because it could be something that is not obvious when the bike is static but occurs when the throttle reaches a certain opening. By that I mean something that isn't directly an an ignition or jetting/fuel flow issue. Trouble is I can't think what....

I will give those things a try Graeme. Today's agenda is to connect the ignition directly to the battery, bypassing the wiring loom. Sashce say a bad contact in the power supply side or a bad ground can cause misfires. However the symptoms they describe sound more like an intermittent misfire rather than the constant misfire that I'm experiencing. But you never know!
Graeme I'm near Armidale NSW. I've ticked the location box on my profile but just wondering if its not showing?

Cheers
Dan

Duccout
Posts: 1285
Joined: Wed Feb 05, 2020 1:20 pm
Location: Essex UK

Re: 450 Desmo not running cleanly

Postby Duccout » Fri Feb 21, 2020 11:07 pm

Just a thought - is the HT lead new?

Jordan
Posts: 1380
Joined: Fri Dec 24, 2010 11:29 am

Re: 450 Desmo not running cleanly

Postby Jordan » Sat Feb 22, 2020 3:16 am


Jordan
Posts: 1380
Joined: Fri Dec 24, 2010 11:29 am

Re: 450 Desmo not running cleanly

Postby Jordan » Sat Feb 22, 2020 4:05 am

Here's a wild idea.

https://www.banggood.com/PWK-28mm30mm32 ... rehouse=CN

Cheap carburettor to try?
It might help diagnose if it's a carby fault you have.
With coupon, discounted price is under 10 bucks, delivered.

Duccout
Posts: 1285
Joined: Wed Feb 05, 2020 1:20 pm
Location: Essex UK

Re: 450 Desmo not running cleanly

Postby Duccout » Sat Feb 22, 2020 8:00 am

Let's get analytical: you have changed the ignition system, so it is not that (unless you didn't change the HT lead or suppressor cap) the carb is not blocked in any way; so we are looking for something else - massive air leak, for instance. Is the manifold cracked, allowing air in? When the bike cuts out, is it due to oil contamination - worn guides or broken rings? Have the guide seals gone hard or cracked? Are the valves sealing properly? What about doing a compression test?

ducwiz
Posts: 604
Joined: Tue May 22, 2012 12:52 pm
Location: near Frankfurt, Germany

Re: 450 Desmo not running cleanly

Postby ducwiz » Sat Feb 22, 2020 9:19 am

And why not try the 265T atomizer? Seems this is the most likely size.

Hans

Jordan
Posts: 1380
Joined: Fri Dec 24, 2010 11:29 am

Re: 450 Desmo not running cleanly

Postby Jordan » Sat Feb 22, 2020 10:09 pm

Adequate valve clearances?


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