MFI to Weber conversion problems

[IanK]

I have a 1972 911T. Its an awesome car.

The car came with a 2.2 engine with weber IDA carbs.

The engine needed a full rebuild and so I sourced a 2.4 engine that needed a rebuild and used that engine instead. I put the webers on the 2.4, which I was advised had previously been set up with webers.

The result was not good. The car constantly backfires, drops 1-2 cylinders and generally runs poorly. My mechanic replaced all of the leads, the points, I had the webers fully rebuilt, the fuel pressure from the new fuel pump is correct. So, we drilled out the idle jets from .55 to .7 and (almost) all of the problems disappeared. Now the car runs quite well but it blows black smoke, smells of gas and has an enormous appetite for gas. I also feel that there may be some flat spots.

We are stuck for solutions and I wondered if anyone would have some advice for me. I wonder if the cams for the MFI engine will cause issues on a weber engine?

Any thoughts or advice would be greatly appreciated.

Thanks in advance.

Ian

[chris_seven]

Ian,

0.7 idle jets is wild and will affect mixture to at least 3500rpm.

There are clearly some issues with your carbs and you need to check the venture, main jet, air correctors and emulsion tubes to see what is fitted and if they need changing to suit a 2.4.

You haven't said which 2.4 you bought T,E or S but there is no reason they shouldn't work on carbs and all of these engine have the same distributor advance curve.

I would think about how the carbs were rebuild and that none of the internal drillings are silted up.

[IanK]

Hi Chris, thanks for your response.

The car is a T.

Yes, I cant help but feel that by going from 55 to 70 jets is putting a band aid on an unknown problem. No one really knows why the car runs better on the 70 as opposed to 55.

[1QuickS]

As Chris said, blocked fuel galleries supplying fuel to idle jets. My web site has a procedure for checking this.

[IanK]

What an incredible website Paul. I only wish I had known about you before I had my carbs rebuilt...

[1QuickS]

Thanks!

Give me a ring if you want to chat; I speak "Weber" fluently, not effluently.

Paul