Motoscrubs.com

Hi,
I have recently purchased a restored 1973 450 Scrambler. The bike runs well and starts reasonably easily but has an issue once the throttle is more than 1/3 rd open. This is particularly noticeable up a steep hill and the engine runs rough until the throttle is backed off. I suspect this is due to an over weak mixture rather than over rich, but I am not entirely sure.

The carb is a Dellorto VHB30AS which looks to have been rebuilt. However, I believe the correct carb for this model should be a VHB29AD with a V17 needle and 130 main jet. I am assuming that the extra 1mm of choke diameter shouldn't make too much difference provided that the needle and jet are sized accordingly. As yet, I have not stripped the carb down to check what jet and needle it has.

However, I would be most grateful for any help solving the rough running and any thoughts on which needle/jet combination I should use.

Cheers. RogerO

As it is not the standard VHB, could it be from a Moto Guzzi?
These have an accelerator pump, which might affect your tuning.

Thanks Jordan,

According to 'Ducatipaddy' it is the VHC carb that has an accelerator pump. If anyone has a VHB30A carb fitted to a 450 Scrambler, could they tell me what main jet and needle they are running?

Cheers,

Roger

Hi Roger

a 450 scrambler should have a V7 needle and a 265T needle jet .

time to get the carb off and have a look wat is inside . a 30 mm carb should have the same needle and needle jet as the 29mm . only a little bigger on the main jet .

Eldert

Thanks Eldert,

As you say, it is time to look inside the carb. Thanks for the information.

Cheers. Roger

RogerO wrote:According to 'Ducatipaddy' it is the VHC carb that has an accelerator pump.

This is a square slide pumper carb: http://guzzino.com/devhbcdi27.html

The supplement to the Ducati single workshop manual has this information:
http://www.ducatimeccanica.com/67-70_si ... ual/4.html

While you have the carb off look at the rubber manifold gasket. They completely suck, especially with the bigger hole in them for the larger carbs. No point in trying to tune your carb if the manifold is leaking. I'm not saying it is or isn't your problem now but check it out anyway.
I got a sheet of phenolic so I can make some but I haven't had time yet.

Aaron