Spitting back ana popping ana banging on the overrun - standard C7 1700 Supersprint

[tompollock]

Hello Gurus,

My recent 320 mile round trip to Arch Motors was my first long journey in the 7, giving me plenty of time to appreciate the car's idiosyncrasies.  My only real complaint is the engine's part throttle histrionics.  "They all do that Sir" seems to be the general opinion on the forum but I can't help but think things can be improved somewhat.

Refurbishing the 40 DCOE's is next on the to do list, but before I do I'd love to get advice from forum members on getting the engine to run more smoothly on part throttle.  I suspect a lot of the problem is due in part to having a distributor optimised to give an ignition advance curve that produces maximum engine power output on full throttle rather than smooth running on part throttle.

My 1700 Supersprint engine is standard, specified with the optional 4 into 1 competition exhaust and Lucas High Performance ignition system, and it is in good health with all compressions around 160psi.  It has subsequently been dry-sumped and at some stage the distributor was modified by Roger King - bob weights and springs changed to alter the advance curve perhaps?  Instructions are to set the timing to 35degrees BTDC at 5000rpm). 

I have quickly set the idle mixture and balance with a manometer, with the only observation being that the two outside chokes are drawing a tiny bit more than the inner ones which I'm sure would be sorted with the rebuild.

I came across a detailed post from one member saying replacing the main jets to 130 (originally 120), air correctors to 180 (165) and idle jets to 50F9 (45F9) really helps matters.  Have any other members tried this or can offer other suggestions that have worked?

Thanks in advance,

Tom.

[orangepeel61]

My first port of call would be idle jets, they should be 50F9. Weaker jets (ie 45F9) will cause the symptom you describe.

 

 

 

[Roger King]

If it's a dizzy I modified you should set the ignition timing to 34 degrees BTDC at 4500rpm and then check that it falls in the range of 10 to 14 degrees BTDC at idle. Once this is done you need to balance the carbs and set the idle mixture for each cylinder. If the problem still occurs it is highly likely that a change from 45F9 slow run jets to 50F9 will cure the problem (as stated in #2). This problem exists in around 85% of Supersprints with standard jetting.

[tompollock]

Thanks Roger.  Yes it is a distributor you modified. Still have the original instruction sheet that came with it. I'll do a timing check first, then balance the carbs again and reset the mixture.  If no better I'll change the slow run jets to 50F9's

[Grindrod]

Hi 

maybe worth viewing the reply I received Re Throttle bodies ; Engine management system for a crossflow.

maybe worth considering.

thanks

[tompollock]

I will check that out.  Fitting the 50F9 idle jets have more or less cured the problem, even before checking that the timing is as it should be. 

I am hoping to extract more power from the engine in the spring.  Probably need to get it on a rolling road to optimise everything. We get so spoiled by modern engines with their perfect engine management systems. 

My daily is a bog standard 80000 mile 2007 Cooper S and it goes like the clappers.  Though I haven't compared them back to back, I'm sure the Mini is quicker off the line, and especially at higher speeds when brick wall aerodynamics defeat the Caterham.

 

[eric]

The charm of a crossflow  ...  I will always regret it