ignition prob. 911 SC ROW with webers

[jon sigurdsen]

Hi

I have a 911SC 1981 ROW(204hp) with rebuilt weber carbs.
The engine backfires alot at constant speeds, and I suspect the dissy is way off.
If I understad the diagrams posted here Best advance curve? (page 2)

I might have the worst dissy to combine with carbs , as the advance is all in by 3000 (1500rpm at the dissy)

Am I right?

thanks alot

Jon Sigurdsen

[Flat6pac]

You need vacuum for the distributor to advance all the way. Another possibility with the SC distributor is the inside of the unit isnt free to respond to the vacuum because its frozen and that keeps it from advancing.

[Bob Kontak]

Jon,

See this link. Pretty sure later ROW and US SC distributors are the same. In short - wimpy even when working correctly. I AM NOT SURE IF EARLIER SC DISTRIBUTORS ARE MORE RADICAL THAN LATER ONES.

I get full mechanical advance at 3000rpm?

I would think if you put a timing light on the wheel and you are able to estimate 20 to 25 degrees advance (mine does not have total advance marks) your advance systems are working properly.

Here is the thread where Vereeken was struggling with timing on his ROW SC. Long story short, his ROW and my US SC's were both the same - wimpy and ran out of advance fast.

Timing. What was Ferry thinking?

Depending on what gas you are using you may be able to cheat just a wee bit - I am talking very low in the degree change - like two. However, I don't understand this enough to feel safe tweaking my engine let alone yours.

[jon sigurdsen]

Hi.

Thanks for answers!
I'm pretty sure the distributor is ok, but the question is:
Is the curve way to steep for use with carbs...
My 911L 1968, was all in by 6000rpm and the SC is all in by 3000rpm.
Is this what causes the backfiring from 2000-4000RPM?

Jon

[timmy2]

Not a carb expert but I believe there is a transition from idle to main jets in the rpm range you are having backfire issues. Around 3000 rpm?
Or you may want your distributor re-curved.
Stock camshafts?

[jon sigurdsen]

Hi

If the Webers are not jetted correctly you will have a flat spot around 3000RPM.
Yes I have stock cams.
My car tends to backfire at constant RPMs, therefore I thought the dizzy curve was wrong..