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

[r_towle]

how are the carbs configured?
Is this a new issue? been fine before at this RPM?

Rich

[David Stowers]

Hi Rich
What do you mean by configured? Theyre twin 40 DRLAs. It has happened once before in a similar situation, top gear foot hard down for a prolonged period then a splutter. That time I was low on fuel so went straight to fill up thinking it was that.when running through the gears it's fine through the rev range but then you rarely have it to the floor for as long

[gunny]

I sugggest getting a wideband air/fuel gage to monitor your ratio. You may be running very leaner as RPMs go up and just need to increase fuel from carbs at higher RPMs.

[David Stowers]

Just looked up the wideband gauges, what's involved in installation? Presumably it won't rectify the issue just confirm it? What would be next if what I suspect is true? I'm thinking perhaps just be kinder to the car and ease off when I get to 100mph!

[gunny]

The Wideband O2 sensor needs to be added to your exhaust at a point after all cylinders exhausts are combined. Mine is just before the muffler but after the resonator. You have to run the wire set to the gage in the console, I run mine thru the same hole as the speedometer cable to get it in the tunnel. I also added a Vacuum meter to monitor load, but your issue is at full throttle so load is high and vacuum is very low.
If you see that the mixture gets to lean at high RPMs then a carb adjustment would be required, maybe new Jets.

[Krieger]

Does your distributor have the rev limiting rotor?

[David Stowers]

Does your distributor have the rev limiting rotor?

No, it wasn't revving to the max

[David Stowers]

What model of fuel pump are others running? It seems to increase the flow, the pressure goes up too high so then I'd need to but a pressure regulator aswell

[ThePaintedMan]

Facet pumps, while cheap, are typically viewed as inadequate/unreliable with carbs for these cars. A Carter or similar style rotary pump is preferred as they provide a constant volume of fuel at an adequate pressure. CB Performance over here in the U.S. sells one such pump: http://www.cbperformance.com/ProductDetail...roductCode=3193

This pump is self-regulated at 3 1/2 PSI, which is perfect for DRLAs or most Webers and eliminates the need for a pressure regulator. I would start there and see if that clears up your issue. From what you're describing, it does sound somewhat like fuel starvation.

That being said, go through the normal (read: FREE) checks first. Check/set your valve clearance, verify timing and check to ensure that the carbs are synched at both idle and 3,000 RPMs. Then, when you have the time and money, replace the Facet pump to eliminate variables.

[David Stowers]

Facet pumps, while cheap, are typically viewed as inadequate/unreliable with carbs for these cars. A Carter or similar style rotary pump is preferred as they provide a constant volume of fuel at an adequate pressure. CB Performance over here in the U.S. sells one such pump: http://www.cbperformance.com/ProductDetail...roductCode=3193

This pump is self-regulated at 3 1/2 PSI, which is perfect for DRLAs or most Webers and eliminates the need for a pressure regulator. I would start there and see if that clears up your issue. From what you're describing, it does sound somewhat like fuel starvation.

That being said, go through the normal (read: FREE) checks first. Check/set your valve clearance, verify timing and check to ensure that the carbs are synched at both idle and 3,000 RPMs. Then, when you have the time and money, replace the Facet pump to eliminate variables.

Sounds like a sensible and affordable plan, Thanks

[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

[McMark]

If you set float height, or if they do, make sure they/you don't compress the ball-spring in the needle valve. If they/you do then your float settings are too low. Set the upper/raised height without compressing the ball.

I'd trust Jake's numbers, but the ball is marked Sf in this pic.
(IMG:http://www.914world.com/bbs2/uploads_offsite/img.photobucket.com-419-1392137068.1.jpg)

[Jake Raby]

Compressing the ball changes it all.. I blow on the fuel inlet to know if the ball is open, or not during this setting.

[yeahmag]

I think I set mine to 5-6mm closed per the CB Performance book, but I've written down Jake's settings and will try them sometime as I'm running a non-standard air correction jet to correct my high speed enrichment.

[David Stowers]

Wow! I'm trying to keep up but fear I'm out of my depth. I'm gonna print all this out and take it with me to the shop later this week.
Thanks everyone for the input so far.

[David Stowers]

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 was the only graph I got, torque and power. What's AFR?

[Tom]

Air to Fuel ratio.
Tom