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MG MGB Technical - Running lean?? Help.

Hi all.

This week I changed my old pancake air filters for the bigger K&N filters. Car is a '73 BGT with 272 Piper cams.

These filters provide considerable more air to the engine and air IS Power.

MBG's haven't electronic injection, so I know I have to provide more fuel coming in. I've got the AAA needles and rotated the carb screw a further half turn.

Car pulls very well in 1st and 2nd gear through the whole rev band. But beyond 4000rpm in 3rd and 4th it just runs out of fuel momentarily. Very noticeable.
Could it be that it's still lean, or is the carbs' fuel reservoirs that are empty and therefore it's not replacing fuel at the comsumption rate?

Because during short and quick accelerations it's ok (1st and 2nd gears).

Thanks a lot.

Valter.
Valter

Do a quick check Valter and see if pulling the choke partly out solves this problem. If it does then it will certainly be a needle problem.
Iain MacKintosh

Valter

When I installed the full size K&N filters on my 1973 B, twin HIF SU's, I went from the original ABD needles to AAA. These are quite a bit richer than standard and are often installed on cars used for racing. Since you only have the roblem is the higher gears, could it be a fuel pump problem - i.e. having trouble supplying enough fuel at max horsepower conditions?

Good luck.

Barry
73B
B.J. Quartermaine

Barry, that's why I suggested using the choke. If the choke corrects this then its needles but if not it's fuel delivery. I run two 72Bs and one needs AAAs with a fully standard set up the other has K&Ns and also runs on AAAs. I fear that with the Piper cam that Valter might need even richer than that but would hope that the cam manufacturer would make a recommendation. Lets see what he comes back with before we jump to conclusions.
Iain MacKintosh

Ok.

I rotated the air/fuel mixture screw on the carbs a further half turn. What I can say is that it improved a lot! It's not perfect yet, but it's the way to go.
I did a stint of 8 min at full throttle in 4th gear and had no serious fuel starvation problems.
Now, I'll rotate them again a another half turn. The question is: when will I know it's time to stop? When will it be perfect?

Thanks!

Valter.
Valter

Oh well at least we know now that its probably the needle profile that is incorrect for your set up. However I doubt that your low speed performance and idle in particular will now be acceptable. Will the cam manufacturer provide you with further information or perhaps Peter Burgess or someone with a similar engine set up on this board could help. Have a look through the archives as well. If all else fails then have a look at this
http://www.teglerizer.com/mgstuff/SUcarbs.htm
Iain MacKintosh

This thread was discussed between 11/08/2005 and 12/08/2005

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