Rough idle/flat spot on take off

[Dave_EH]

Hi,

Yesterday I had the time to do some engine tuning work (my car only sees the road once or twice a month), and ended up with success.

It is a 179 (192), bosch HEI ignition (all new components). Plugs are NGK BPR5ES11, mild cam, YT head.

I changed to 61 main jets on the Holley 350, and am running about 12 degrees static advance and it was really singing along.

This morning I started it again and it let out a real crackle in the exhaust on start up.

It had a rough idle (usually idles steady at 15In/Hg manifold vacuum) and drops down to about 10In/Hg every few seconds.

There is a flat spot off the mark and a slight hesitation on throttle application.  It does not rev out as cleanly and I can't get a crisp rev on a double declutch.

I checked timing and mixture screws and all is ok.  I thought it may be running slightly lean, so richened the mixture up an 1/8th of a turn.  On inspection the plugs indicate the mixture to be a little rich, but nothing too bad.

The only thing I can think is that the metering block has got some gunk in it, although I am running a fuel filter before and after the fuel pump.

My other guess is a vacuum leak somewhere, but I can't find one at the moment.

Any help appreciated.

Cheers
Dave

[oldjohnno]

Start with the basics I guess.. Does the problem get better or worse as the engine gets hotter? A too-lean mixture will be worse when cold and improve as it warms up, while too-rich won't be so bad when cold but will get worse with more heat. Sometimes if the engine coughs back it can rupture the diaphragm on a Holley's power valve, making it run rich though I think the later carbs have a ball-check to stop this. Flat spots on take off often indicate lean-ness, I'm assuming you've checked that the accelerator pump squirts with the slightest movement of the butterflys, as well as all the vacuum hoses and the pcv valve. If you think you may have a leaky gasket you can try spraying WD40 around the suspect joint to see if that smooths the idle momentarily... All pretty basic stuff I know and it all assumes that the problem is fuel related.. I once spent a lot of time trying to track down a similar misfire on a Holley equipped Chev, only to find later it was a corroded coil output terminal.... 

[Dave_EH]

It turned out to be the powervalve afterall.  I missed a tiny hole in the diaphram...

I travelled from Brisbane to Warick and back to watch the Historic race series today and it didn't miss a beat.

Cheers
Dave