450 Desmo Starting issues

[Stratos23]

HEllo,

I feel I've seen a few posts on this topic before, but I'm nearly at my wits end on this one. Story goes like this:

I've been bringing an NZ new 450 Desmo (Silver Shotgun) back to life. It was apparently restored about 20 years ago and then left sitting. I've discovered a bit of dodgy work though, so most things have needed attention. Anyway, I know they're hard to start, so I've paid particular attention to anything that will improve this issue - for example:
- Completely re-wired
- New Electrex (UK) 12v 120W alternator/flywheel
- New Electrex CDI ignition
- New 12V Battery
- Rebuilt head (new valves, seals) and shimmed to spec

It starts first pop on the race roller-starter (and runs beautifully) - but to get it to start with a kick seems impossible. It just kicks back viciously if the choke is out. It's got incredible compression. And I've studied the endless threads about technique etc.

The timing is spot on. These units are 25º retarded until the revs get to 4000rpm, which means that with max advance set to 28º BTDC, the spark should be discharging at 3º BTDC when it's kicked over. And this all theory appears to hold true when I run it up on the timing strobe. I've even experimented by advancing and retarding either side of the marks, but nothing seems to have any effect. It's a 'wasted spark' system with two magnets on the rotor, so there's no chance of getting the timing 180º out either (meaning the spark is firing on both compression and exhaust strokes).

Has anyone else had such issues? I know that big singles sometimes throw up issues with CDI, since the speed of the crank varies much more during one rotation when the engine is kicked over manually - and therefore the CDI electronics struggle to average what rpm it's actually doing. This is less of an issue with smaller singles, because the crank speed is more constant (I've even heard of some CDI manufacturers limiting their models to 350cc and under). Electrex on the other hand insist that this shouldn't be an issue with their unit.

Short of towing the rolling road around on a trailer with me, I'm a bit stuck - and about to reluctantly try going back to points as another experiment..?

Any suggestions out there?

[Nick]

Try the points, and if it solves the problem please let us know.

[Madman]

I thought that as he has a CDI then he has no points. Am I wrong?

[Jordan]

I had the same model Ducati, that was OK at starting once I learned the technique.
A friend was to borrow it, and although his own bike was a kickstart only 850 Norton twin, he could not get my 450 started at all, and gave up.
It sounds like your bike is particularly bad at starting though.
Do you have the original VHB carburettor? These are pretty good for starting I think.
Are you using fresh fuel? I'm told that aromatics that are there to help starting tend to evaporate out of old fuel.
Trying your points is a good idea, to get some data.

[graeme]

I don't know how the Electrix World ignition is set up for timing but if your ignition is set at 3 degrees advance at kick over it Will bite you unless you kick it very forcefully.
Sachse on my 450 Desmo is set at 1/2 degree retarded so it doesn't bite.
It starts easily.
450's don't have huge compression but the starter gearing makes it feel like they do.
Try setting the ignition at 0 or 1/2 a degree retarded to get rid of the kick back until you get used to the starting technique?

Graeme

[Stratos23]

Thanks for the suggestions thus far...

Nick, yes, I'll try the points and let you know how I get on.

Madman, yes I still have the points from the original 6V set-up.

Jordan, I could get it to start - SOMETIMES before I rebuilt the head (with CDI fitted). But it still kicked back ferociously every second kick. But your point about technique is very much appreciated. I know one guy who struggles to start his 450 and another who gets it every time. Once they kick you in the shins a few times, you tend to get a bit tentative - which is probably the worst thing. And yes, I'm using the original VHB with std jets - and fresh fuel.

Thanks Graeme for the suggestion about retarding the ignition. I had exactly the same thought, so tried that but it didn't seem to make any difference - which is why I'm so stumped on this.

Someone last night suggested dialling in the cam to spec (using offset woodruff keys) - but I can't see why it shouldn't start being a completely std engine (with CDI being the exception).

[Nick]

You almost certainly know all this, but here's my technique for the 350:

Fuel and choke ON, ignition OFF, kick through once to prime.

Put transmission in gear, roll bike backwards until compression (this locates the piston just after compression in normal direction of running).

Ignition ON, choke OFF, kick. Fires first kick every time. Points ignition.

(Whether the 450's piston should be located just after compression prior to kicking, or a bit further past compression you'll have to determine with experimentation. And as the others have noted, kick-back is often a sign of excessive timing advance.)

When Yamaha designed their now-famous SR400/500 single they put starting marks at the cam sprocket and a small window that made it easy for the rider to position the piston at the ideal location prior to kick starting. (Easy starting is no doubt one reason that the SR400 has been a top seller in Japan for so many years.)