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I’m having a strange issue with my 450 Desmo. I’m using Electrex ignition from Lacey Ducati and running a VHB29 Dellorto.

The bike will start first or second kick and runs and revs but very quickly dies within 5 seconds or so. I can then start it again with the same symptoms. It feels like fuel starvation but the float bowl is full. Could this be an issue with the Electrex set up? I’m stumped.

Ian

How soon after stopping are you checking float chamber? I had a slightly similar issue on a recently acquired bike. Mine ran out of go at about 35mph, so not as bad as yours. After much messing about it turned out to be a partially blocked fuel hose. So it would start and run, but if the fuel requirement was greater than the dodgy hose could replenish the float bowl emptied.
Ian P

Checked the float bowl immediately after stopping. In any case, it should still run for more than 5-10 seconds on a full float bowl so, although it feels like fuel starvation, I’m not convinced it is.

Hi Ian,

Have you checked for a blocked pilot jet? Or could the pilot adjuster be set too weak?


Good luck,

Colin

Aye Ian,

My '74 450 MKIII runs a AMAL MKII 32mm carb and Electrex Hall ignition. However, It reads as though you have a fuelling problem rather than an ignition problem.

I would be inclined to take the carb off the bike, dismantle the carb and go through every orifice of the instrument with either an airline and or an aerosol brake cleaner, ensuring that all of the jets, drillings, including that bleed drilling between the slide and the inlet valve are clear and flowing freely. Check that the float needle is sealing against it's seat, you say:

It feels like fuel starvation but the float bowl is full.

How full is full?

You could be drowning the spark! So, check, clean and re-gap the spark plug. Check the spark being produced, if its blue and gives a crack, as my Electrex Hall ignition set up does, I would have thought that was not the source of your problem.

Clean/replace any filters between the carb and the tank, including any filter on the fuel tap. If you aren't using a Viton fuel hose or a co-extruded fuel hose with a Viton inner, then I would replace all of your fuel hoses, before connecting up your instrument to the inlet and the fuel tank. I use additional filters on both fuel lines, as the filters on my fuel taps only cover the "Main" feed and not the "Reserve" feed.

My apologies if this reads as though I am trying to teach you to suck eggs, But!

I hope that you can find the problem pronto. Good health, Bill

For diagnosis, put your spare carb on and see if it stops cutting out.

Or open the choke lever very slightly and see if it keeps running.
It’s an easy way to see if it is starving for fuel.

Graeme

Thanks all, I think I’ll try putting on a PHBH 30AS Dellorto I’ve been meaning to use. If the problem persists at least I’ll know it isn’t fuel.

Ian

Electrex stuff is not totally foolproof so don’t eliminate it yet.
I only have their reg-rec but the first one failed completely after a very short time and the replacement no longer controls the charge warning lamp although it does still manage the battery voltage.
Ian P

Your carb is almost certainly picking up a small piece of something in a jet or passageway almost as soon as the engine starts due to engine vacuum and as soon as the engine stops its falling away again only to be picked up when you start again........scrupulous carb clean job I think ?????? cheers, George