Run-on problem - Twin SU's

After reading a thread here about using lower octane fuel I have dropped from 94 to 91 then 89 then 87. Progressively I have experienced pinging (remedied by reducing the timing advance). However now I am having excessive run-on which I did not experience with the higher octane fuel. We have twin SU's with floats set initially to 1/8" now to 5/64" to lower the float in the chamber to get it to shut off at a lower point. We have no anti-run-on valve and no canisters.

I can go back to Sunoco 94 (at 1.26/litre!) or try to solve the run-on problem (preferably by means other than stalling the engine out).

But what to try first?

1/ Further tinkering with the timing? It is now at about 25 degrees Advanced, down from 32 (We have a mild street cam with a 2 degree offset key in there)

2/ Lean it out?

3/ Change needles?

4/ Change the float gap?

5/ Install an anti-run on valve?

6/ Go back to 94 Octane?

I just don't know where to start messing with this multivariable problem and would appreciate some input from those more familiar with the problem. Thanks in advance.

Peter, If you change timing you should always recheck your mix. I think that addressing the mix might help you, but retarded timing will also compound the problem.

The only other thing I would attempt is the anti-runon system. I retained mine for my conversion and it really does help.

Needles and floats will lead you down the path to confusion and mysery.

My $.02 (USD) Hope you figure this one out.

Peter,

The run on problem is not being caused by the SU's but fitting a fuel shut off valve will help since , given there is no fuel to burn , nothing can ignite.

In most cases it is caused by carbon deposits within the combustion chamber becoming hot enough to ignite the mixture independently of the spark plug.

One prime hot spot is the "heart" shaped protrusion between the valves and removing this point can go along way to eliminating the problem but you will need to do some major work on the engine to get this accomplished.

Personally I would go back to the higher octane rated fuel despite the extra costs as a temporary solution.

In addition I would ask around on the anti run on assembly as a possible long term fix.

Incidentally fuel here is AUS $1.34/ litre yesterday

Pete.

I wouldn't retard the timing more than a couple of degrees from the 32. Twenty-five is way too much. You may be increasing your combustion temperatures. A slightly richer needle may help but the easiest thing is to go back to the 94 octane. Too low an octane fuel also can carbon up your cylinders and reduce your gas mileage. If it runs best on 94, use it.

Pete. This engine has only 1000km on it. I am not inclined to do a tear down at this point to modify it.

The needles are spring loaded bias type, K&N filters (the 45 degree slant type) and I have a big bore Stainless Steel Exhaust.

This weekend I am going on a Spring Fling and I think I will just top up with 94, put the timing back to 32 and see if the run on problem clears up. If so, I 'll leave it dialed in that way and pay the extra few bucks on the fuel.

Thanks for the input.

I know I said needles would be confusing but after pondering the problem...

Which needles are you using? If you're still on AAA then you're probably too lean at WOT and high speed cruise which could be the source of your pinging. AAM would probably work better given your set up.

Spend $45 now on some needles instead of hundreds on gas in the long term.

Here is the AAA vs AAM, look at the top end.

Thanks Eric.
What is the scale on that x-axis? 02 - 16 is surely not revs in some scale of thousands.

The x-axis represents points along the needle, start from the base at the left and move toward the tip at the right. Since the piston in the carb moves up as air flow into the engine increases you can relate this to rpms. The y-axis represents the diameter of the needle in thousands of inch, smaller being richer, at each measured point.

The graph came from here: http://www.mintylamb.co.uk/suneedle/