Stromberg Snap Throttle Response

[david.g.chapman]

Just a quick question.
I am tuning my engine after putting it back together, and fitting new needles.
I have set the idle speed and the engine idles reasonable well, and when the throttle is snapped open the engine responds quickly without misfiring but falls back to below idling speed before climbing back to a normal idle.
If the throttle is more gently applied this does not happen.
Is this a symptom of a lean idle mixture?

(I have not done the piston lifting checks yet).

Dave Chapman.

[Billmack]

I'd say a small amount of extra fuel comes out when you snap the throttle closed and richens the idle. I would not worry about it much. Check float levels maybe.

[david.g.chapman]

Thanks for that Billmack.

I have a Gunsons Colourtune so will try it out to see what the idle mixture is. The trouble with my twin cam is that the plugs always blacken up when the engine idles unless you go for a run first, so looking at the plugs before each adjustment means another run. I have non adjustable Federal carbs which doesn't help, so you have to remove the top of the carb to raise or lower the needle.
I will also check the pistons for free movement and try lifting them as well.

Dave.

[SENC]

So it sounds like the damper is slowing the piston lift appropriately to enrich the mixture on rapid acceleration, but the opposite (rapid piston drop on quick letoff the throttle) isn't happening so it is leaning out to much until it returns to its normal state of depression? Perhaps try a stronger spring on top of the piston, or make sure something isn't preventing the piston to fall freely and smoothly. Also check the temp compensator, bypass valve, and idle trim screw - could these be affecting the internal pressure balance? If a new engine, you may need to open the idle trim screw a bit if it is fully seated. Alternatively, that screw is meant to be progressively tightened as the engine breaks in, until it is just seated.

[david.g.chapman]

That's deep thinking SENC. I did not know too much about the idler screw function but your explanation sounds good.
The piston springs are original so they could be tired. I will change them if all else fails.
I have just checked the pistons and they fall freely, the temperature compensators are closed at 7 degrees C ambient and spring open freely when poked. They are not the cause unless they are being held open - seems unlikely.
I will start the engine Tuesday and do the colourtune checks and balance the carbs. If I find anything that solves it I will post the result.
Dave.

[USA64]

Thinking of your plugs blackening at idle I wonder if you don't need a hotter plug. Assuming you have removed the cross-over your mixture will be cooler at idle than intended.

[SENC]

The idle trim screw purpose was only top of mind because I'm restarting a carb rebuild so have recently been rereading various notes and articles and manuals. I may be wrong, but it almost sounds like it is starving itself at the drop after a heavy throttling. If no leaks or other pressure issues found, check other things that could cause a restriction in available fuel, like the float level, fuel valve free movement, fuel line/pump.

[david.g.chapman]

I run NGK BPR6 plugs, I do not fancy going to 5's at the moment.
I have ordered new piston springs from Burlen. It's worth trying as they are only ?5 each. You have all kindly given me a list to work through, and I should know by the end of the week what the cause is.
Cheers,
Dave.

[david.g.chapman]

SENC was right in the end, although it took me a while to identify the fault.

I had "checked" my fuel pump my looking at the flow from the open fuel pipe when the starter was turned. I had a spurt of fuel on each stroke of the pump so assumed all was well.
After trying new carb return springs with no improvement I had to raise the needles to richen the mixture considerably to get the car to idle after a throttle blip and not stall. After more head scratching I reasoned that everything else had been tried so maybe the fuel pump was getting weak and not giving me enough pressure.
I tried my electric fuel pump, lowered the needles to where they should be and hey presto, all is well again!

So just seeing a spurt of fuel is not enough to test a pump - substitute to be sure if you don't have pressure testing equipment. There must have been something like a leaky valve so the pump pressure must have fell below about 0.5 psi, failing to open the float chamber valves perhaps. The fuel level in the carbs must have been dipping after an acceleration surge to give fuel starvation on the return to idle.

You live and learn.

Dave Chapman.

[SENC]

SENC was right in the end

There is, indeed, a first for everything!