250 Desmo idle issue

[Dan C]

Another one back on the road. A Bitsa. Standard 250 desmo engine (probably about 1969 according to the engine no. but was a basket case so who knows....) 450 desmo frame, replica silver shotgun fibreglass, KYB forks, Cagiva triple clamp etc etc. Dad had the engine rebuilt by Road and Race (Phil Hitchcock) in the late 1990's or early 2000's but never finished the bike. With the help of a very handy clubmate I've got it going and today was the first opportunity to give it a run on the road. By and large it went well, the main issue is an erratic return to idle.

When the throttle is shut the engine doesn't always return to idle it will hover at about 3000 for a while. Sometimes not returning at all.

Apart from that the carburation seems fine. Starts easily, runs cleanly (up to 5000 rpm anyway, running in) and plug looks nicely tan

Carb is standard 29VHB. Pilot is a 45, Main is 110, Needle Jet 265M, Needle V13 on mid notch. The slide is loose maybe 0.5mm back and forward play and I suspect that is causing the issue but was just wondering if others had had the same issue and cured it?

If all else fails a new carb with no wear should fix the issue but as the rest of the carburation is fine it would be nice to get that little glitch sorted and stay original.

Also I've seen various recommendations for Main jets from 110 to 122. Be interested to know what others run?

Cheers
Dan

[themoudie]

G'day Dan,

That's a bonnie looking bit of kit, congratulations on completeing the build.

I would tend to agree, with your diagnosis of the slide trying to tip sideways in the carb under deceleration. Try a new return spring or put a wee lead washer, 1mm or 2mm thick under it, just to increase the spring rate and see if that seats the slide positively.

When my 450 MKIII was giving all that nonsense, I fitted a stronger spring and added a lead weight (lead flashing cut to shape) to try and sort out the idle that all turned out to be harmonics and AAU bob weights! Sorted, with Electrex ignition.

Have you points or EI of some sort fitted?

As for carby jetting of a 250 Desmo, I suspect that George (NI) will be along soon and be able to give chapter and verse.

Good health, BillR

[Eldert]

Looking good Dan

did you check of the intake manifold is leaking ? lots of times people over tightening them and you land up with a air leak .

Eldert

[Jordan]

I shouldn't have thought an air leak probable with a VHB, which is designed with a "tilted" spring.
That should impart a slight thrust towards the flat face on the exit side of the carb, so no front to back play would happen while it's running.
The slide is anodized and slow to wear. But a big mileage might cause its face to be not perfectly flat. That could allow it to leak.
Suggest you first confirm no problems with the throttle cable, especially adequate play when in the fully closed throttle position.
Otherwise - bent jet needle? weak spring? idle mixture wrong?

[Duccout]

Great looking bike, well done.

[ranton_rambler]

After I changed my 175's camshaft to a more sporty profile I could never get a good idle. I put it down to a mismatched combination of valves/camshaft/carb and lived with it as it ran well otherwise.
Since I have split the cases and rebuilt it, it idles fine. Explain that!
I didn't touch the carb, although I may not have got the ignition timing exactly back where it was.
Ian

[Dave354]

Now that’s a very pretty bike! Congratulations on a fine build. I’m sure you will get the problem sorted, my thoughts were a worn air slide as it runs fine other than idle? Either way there are more knowledgeable folk on here re carbs!

Best regards
Dave

[blethermaskite]

Would love to be able to give you jetting info for the VHB on your 250 (looks super by the way ) but my 250D is on a 930 Mk1 Amal concentric I personally really dislike the VHB carb and have never got any Ducati single to run nicely on one (just my opinion others may well disagree). So I would check out all the above advice from others........have you got the thick rubber gasket between the inlet carb stub and the head nicely seated?, and is it in good condition? this in my opinion is a bad bit of design on the standard setup, I certainly have had bad sealing in this area ( If I was using this I would make a tufnol spacer instead of that rubber one).
I would be looking for an air leak in this area.
Cheers,
George

[Dan C]

Thanks all for your help and for your compliments on the bike, really appreciate that.

I should have mentioned in the original post that we have double checked, triple checked, and quadruple checked for air leaks. Doesn't mean there isn't one but we haven't been able to detect it!

Throttle cable has plenty of play, if anything maybe a smidge more than I would like, and with the engine not running it always returns properly. But an interesting point - I have not checked it with the engine running.....so that will be worth doing. So thank you for that suggestion Jordan.

I will also check the ignition again thanks Ian and Bill. Its points and it was right when checked but always possible that something has changed.

I agree with George and Graeme (who says VHB's were invented by the Devil) - I'm no fan of the VHB and have Mikuni's on the 450 and 350 because I couldn't sort VHB's, so thats a well trodden path! But seeing as how this is so nearly right I'll persevere for a bit....probably like banging my head against against a brick wall - it will feel good when I stop......


Cheers
Dan

[graeme]

Ha !!!

Dan, I have lots of VHB bits if you need parts or jets.
I should have binned all of this years ago, but didn’t.
Nice job on your 250

Graeme