Hesitation on overrun

[David Morris]

Hi there,

We took 'Black Shadow', our Javelin DCB 246, to the 'Wheel Nuts' rally at Stroud on Sunday, which was the first outdoor event on the calender for this year. A lovely day, with blue skies and about 300 classic cars, including the Jowetts of Maurice Benning and Jem Wilding. However, while DCB 246 ran well, there is a slight problem and I would appreciate any ideas?

She starts and pulls well and loves to climb hills but its a slightly different matter when going down hills! On the overrun, using the engine as a slight brake and doing around 55 to 60, you can feel she is not very happy. It feels as though at least one or two cylinders are mis-firing. Putting your foot down when this is happening sort of overcomes the problem, but not when there is a corner coming up!

Before I start changing plugs, adjusting/checking the timing and cleaning carb jets etc. it would be great if someone who has experienced this themselves could suggest a more direct solution than my rather 'hit or miss' approach?

Many thanks,

David

[robert lintott]

On the overun intake manifold vacuum is at its highest level . Rough running could occur if the small passage which bypasses the closed butterfly is not sufficiently open or is obstructed so that ignition is affected . With two carbs this is when a minor difference in idling settings could cause rough running ? . Maybe these passages contain the mixture screws ? The other possibility is that one butterfly is slightly more open . With the high vacuum just a minor difference could affect the mixture .None of this would show up with open throttles
You could try raising the idle speed a bit ,as you well know a low idle speed sounds good but try pulling away! good luck Bob

PS interesting chat at the show , it seems as if jowett at least intended the engine and body numbers to be the same Bob

[Nick Webster]

Worth checking that the pintle valve is working Ok and is not blocked.