Fuel starvation when maxing out? Options

[David Stowers]

My 914 is running a 1.8 (ish!) 40 Dellortos and recently recorded 111BHP and 121lbft of torque after a carb set up so yesterday I went to have a little thrash about.
A couple of times when the traffic cleared I really kept my foot flat down and pushed to over 100MPH on a slight uphill. It was still pulling strongly for a few seconds then seemed to lose power and splutter slightly. I backed off and it returned to normal. It seemed like the carbs emptied as if the fuel pump couldn't keep up. I had the heater on and the rich oily aroma changed to a hot sharp smell like it suddenly ran lean. It's running the small square facet pump mounted up front near the master cylinder.
Any ideas please
Thanks
David

[Jake Raby]

Float height is the first thing I'd go to.. If its set too low the float bowls never fill completely and therefore they run out of fuel soon, no matter what pump you have. The pump can aggravate the issue, though.

Find a dyno, you'll learn more in an hour than you will in a year of driving and tuning.

The Facet pumps are horrible and honestly 3.5 PSI is too much for these carbs. They love 2.5 PSI dynamic and anything above 3 static can bypass the needle valves. Having more pressure might help to mask a float height issue a bit, until it runs out of fuel.

I like 10 mm closed and 28mm on the drop with those carbs. Your needle valves may be too small, as well.

Don't forget, going too rich will create just as big of a power loss in the top end as going lean. Nothing kills top end power more than being overly rich. Thats why you need to find a dyno (with AFR) and quantify the condition before applying corrective action.

This post has been edited by Jake Raby: Feb 11 2014, 01:06 AM

[David Stowers]

Float height is the first thing I'd go to.. If its set too low the float bowls never fill completely and therefore they run out of fuel soon, no matter what pump you have. The pump can aggravate the issue, though.

Find a dyno, you'll learn more in an hour than you will in a year of driving and tuning.

The Facet pumps are horrible and honestly 3.5 PSI is too much for these carbs. They love 2.5 PSI dynamic and anything above 3 static can bypass the needle valves. Having more pressure might help to mask a float height issue a bit, until it runs out of fuel.

I like 10 mm closed and 28mm on the drop with those carbs. Your needle valves may be too small, as well.

Don't forget, going too rich will create just as big of a power loss in the top end as going lean. Nothing kills top end power more than being overly rich. Thats why you need to find a dyno (with AFR) and quantify the condition before applying corrective action.

Thanks Jake, it was on a dyno a couple of weeks ago for a basic carb set up so I'll take it back with these ideas and see what they say.

[Jake Raby]

Thanks Jake, it was on a dyno a couple of weeks ago for a basic carb set up so I'll take it back with these ideas and see what they say.

Have any graphs? Were they plotting AFR?

This post has been edited by Jake Raby: Feb 11 2014, 09:39 AM