450 Desmo not running cleanly

[Dan C]

Thanks everyone for all your suggestions they have been really helpful.

So I was able to discount the massive air leak fairly early, ditto worn guides and seals. I don't have a proper compression tester but my rather puny right leg could tell there was plenty of compression.
Jordan, thanks for searching out the carb but I didn't feel comfortable ordering the Chinese carby, despite the amazing price, mainly because I had no idea what jetting it might need, and I would need a new inlet stub to fit either a 28mm or 30ml and I had no idea where to source that.
And Hans a 265T atomiser seemed counter intuitive because the plug was indicating running rich.
I did check valve clearances. Inlet was spot on. Exhaust a bit loose opening rocker 0.2mm and closing was interesting one fork was 0.15mm and the other 0.05mm. I figured Phil probably worked on 0.05 on purpose based on experience to allow for thermal expansion on the exhaust.

So I cleaned the carb twice yesterday and rechecked the float level. I also ran a wire directly from the battery to the ignition and bypassed the wiring loom. Made no difference. I ran the bike without an aircleaner which made no difference. But when I played with the choke I started to get some surging which seemed encouraging but it was followed by some backfiring and then the bike totally cutout...However it did indicate that most likely the problem was fuel because it was the first time I had been able to get anything to change. So thank you Graeme for that idea, it put me on the right track!

Today I dropped the needle one notch and put a 130 main jet in, new plug and 95 fuel only. I also increased the freeplay on the choke to the maximum amount of freeplay. And you know what? IT'S REVVING CLEANLY. I haven't had the opportunity to road test it but "on the bench" it revved cleanly to 6000. no way could it do that before. And the mixture screw seem to be much more responsive than previously. I tried to attach a short video but copy and paste didn't seem to work....

But once again, thank you very much to everyone that contributed suggestions. Your thoughts got me on the right track.

Cheers
Dan

[Jordan]

Happy ending.

Re: Chinese carb - it would have been for a quick test only, to see if it made a difference that could help diagnosis.
A rubber hose might have been enough to attach it.

[graeme]

Dan
For what it’s worth I’ve never been able to get a 29 square slide to work reliably for any length of time.
One day it was ok the next week it was shit.
I gave up on all 3 450’s and fitted a 32 phf to Scrambler and rt and a 36 to the ‘74 Desmo which has head and valve work.
Never looked back.
The Desmo cams aren’t ground very well and can bind if checked throughout 360 deg rotation.
Some worse than others
Possibly why your clearance s were what they were, Phil should know this.
Also never had any luck with fuel additives, just shit running and fouled plugs.
I’ve stick with BP and Caltex 95, Shell was a plug fouler.
Main problem with a 32 is fitting a decent air cleaner due to the 450 frame gussets
But I know many people in Brisbane using 29 square slide carbs that are reliable.
I have lots of 29 carb bodies and bits if you need any parts but they are old.

Regards
Graeme

[Dan C]

Hi Graeme,
Thanks for your thoughts on that.
Funnily enough I have a 350 Mk3 desmo that has been in the shed for nearly 25 years and I'm in the process of getting it roadworthy. It has a 30 PHF and fired up straight away and settled into a nice idle....Didn't even pull the carb off, just a new aircleaner. But I haven't ridden it yet still giving it a few oil changes..so it may be a different story on the road....
I used upper lube on my 750GT back in the day and never had any issue with fouled plugs but I guess things were different then. Petrol probably still had lead...Now that I have the bike reasonably sorted I'll keep on using upper lube and see what happens. The worst that can happen is a fouled plug and if it saves me having to pull it apart and redo valves and guides even once I'll be happy. And if plugs start fouling at least I'll know what to do.
Luckily the 95 I got was Caltex....I usually pull into the shell servo!
Even with the square slide and the small K&N type air filter I had to remove the trumpet to clear the gusset. Didn't really concern me because I'm not after performance, just a nice easy ride.
I'm not sure what to think about carbs. I haven't got a bucket load of money so buying a new carb isn't really an option at this point. Also its not an everyday ride, I'm averaging once a fortnight and maybe 2 or 3 rallys a year so I think I can put up with an erratic carb especially after having gone through this learning curve the past couple of days....So bottom line is I think your spares could be handy. Is there a way to PM me about what you have, price etc?

Cheers
Dan

[graeme]

PM received and reply pm sent.
Get it running nicely without upper cylinder lube then add it if you want.
Then if it doesn’t keep running well you can delete the lube.
Talk to Nulon tech regarding lead replacement if you are concerned about valve seat wear.
They told me (years ago) that their octane booster could be used as a sort of lead replacement.
I used it for a short while.
I haven’t used any addictive for many years and have never had seat problems.
Glad you’re having some luck
I have found that 450 s seem to run better with leaner jets compared to what was recommended back in the days of leaded fuel.

Graeme