Hitchcocks Forum

I'm having some problems getting my Mk1 Interceptor to run sweetly at low throttle openings. (Note, all carbs (Monobloc 389/689) as new, standard jetting, mag rebuilt, correctly timed and in fine order, valve timing spot on, A/R unit rebuilt etc) but it spits from starting from cold and the transition to the slide cutaway is very uneven. I am thinking of replacing the 25 pilots for 30s on the basis that modern fuels need larger jets by a percentage based on the ethanol content, although I use Premium grade fuel. Any thoughts? Sensible ones please!
REgards, Mark

Alright then, what plugs do you think should be in it? Regards, Mark

Having finally successfully set up a 1 1/8th" 389 for my 350 I discovered a vast range of adjustment options. I went from a 30 pilot to a 25 and reduced the cutaway on the slide to a 3 from a 3 1/2 to get rid of the flat spot. Having a source of drills in 0.1mm increments I would solder up the jets and drill them out. So in short yes do try the bigger idle circuit jet. The effects will be quite obvious and instant. I found it was a matter of small gains and a-b tests in order to decide what was best. I think the plug is less important as whilst a hotter plug might mask excessive richness and a colder one will cope with a lean bike better ( or is it the other way around) the bike will still be running rich or lean and unless you've modified the compression or the valve timing it should run fine on the modern equivalent of the recommended plug.

Thanks Simon, you make a good point! My previous experience is that if all else is correct the original Amal settings are bang on. So, I'm puzzled by my symptoms and was wondering if fuel might be an issue that I hadn't come across on previous rebuilds of lower powered bikes, the Inter being relatively high compression at 8.5 to 1. I will try a few of the ideas here and let you know! REgards, Mark

I had a similar experience with my 860GT Ducati. The bike ran incredibly rich but would run fine on leaded Race Gas ($7.00 or something equally ridiculous a litre). I used the same method of incremental changes via solder and drilling and finally got it to run fine. There is a difference in the stoichiometry of the modern fuel to the old that makes the carburation far more sensitive. I found with the Dellorto PHF 32mm's on the Duc that there was a considerable variance between parts marked as the same and as the Ducati runs basically as two linked singles (no balance pipe between carbs) quite a lot of fettling was required. I sorted out a pair of needles that were the same and soldered and reamed the needle jet tubes to the same size. Then the main jets and idle jets were sized as well. The more powerful Ducati gave an instant sign to being too lean and I think that this is the direction to come from. A slow frustrating process but ultimately satisfying. Good luck with it.
S:)