Fuel starvation after prolonged high RPM

dcioccarelli

dcioccarelli

Dominic Cioccarelli
Hi all,

I was driving to work today and (as the autobahn was nearly empty) I accelerated to about 160 km/h (6000 RPM in a 4 speed). The car is a 1300 with a single 36 DCNF. Accelerating is fine, but after running at 6000 RPM for longer than about 30 seconds the car seems to lose power. Slowing down will resolve the issue. It certainly feels like fuel starvation.

I'm running a Facet posi-flow fuel pump, which I'd have expected to be sufficient for a single carburettor. The only other thing would be the jetting or the float level I suppose. I did play a lot with the idle jets but didn't give much consideration to the primary jets when I was setting the carburettor up initially. Currently I'm suspecting that the float level might be too low. Any other suggestions? Also, does anyone know the recommended float level for a 36 DCNF?

Cheers,
Dom.
 
if the specs for the pump rate it at less than about 1.5 litre / min , then the pump just won't be delivering enough fuel... if the specs for the pump are higher than this, check all filters.

SteveC
 
Or the gas tank is not venting and you build up negative pressure which won't allow gas to flow. Try driving with the gas cap off and see if the problem continues.
 
Stopped using ANY Facet fuel pumps after two of the recently purchased died. Cut one apart to discover they are now "cost reduction" in ways that will make them less reliable-durable. This has become a serious problem for piston engine aircraft as Facet made a 24 volt version for small piston engine aircraft and they are failing (DIY small aircraft forums and else where).

After a significant amount of looking, came up with the Walbro FRB-13 marine-industrial fuel pump series as a FAR better alternative. These cost more than the common Facet fuel pumps, are superior in many ways.



Bernice
 
I second Bernice's position on Facet pumps. They were once regarded as high quality, well performing, and reliable. That no longer seems to be true. However this also applies to many of the other brands of pumps currently on the market. It is getting difficult to find a good fuel pump without having to spend a huge price, and even then the price won't necessarily reflect the quality or performance of the pump.

For Weber carbs (which like a very low pressure but may still need a decent flow rate) a adjustable bypass/return type fuel pressure regulator is a excellent idea.

Several other good thoughts on your situation also discussed already.
 
I have not had any issue using the stock Fiat mechanical pump with my carbs. As far as your question about float level, see the excerpt below:

DCNF Float Level.jpg
 
dllubin said:
the stock Fiat mechanical pump
Click to expand...
That's a good alternative for Webers, especially considering the cost of a good electrical pump. Sad really, I'm not sure why electric fuel pumps have become so problematic. Possibly something related to "modern fuel chemistry" is a factor? But there is more to it than that...see Bernice's comments when opening one up.
 
If the pump flow is adequate, have a look at the needle valve that the float operates. I had a similar problem with twin DCOEs run at full throttle, cured by fitting a slightly larger needle valve.
 
GregS said:
cured by fitting a slightly larger needle valve
Click to expand...
It's been ages since I had any Webers. But there used to be a popular upgrade for the needle valves called something like "goose valve". They were a floating ball instead of a needle. Supposedly flowed more, sealed better, not as affected by vibration or dirt, and nothing to wear out. No idea if that (or anything like it) is still available.
 
Dr.Jeff said:
It's been ages since I had any Webers. But there used to be a popular upgrade for the needle valves called something like "goose valve". They were a floating ball instead of a needle. Supposedly flowed more, sealed better, not as affected by vibration or dirt, and nothing to wear out. No idea if that (or anything like it) is still available.
Click to expand...
I think the name is "grose jet". I used them when I had DCNF's.
 
Rupunzell said:
Stopped using ANY Facet fuel pumps after two of the recently purchased died. Cut one apart to discover they are now "cost reduction" in ways that will make them less reliable-durable. This has become a serious problem for piston engine aircraft as Facet made a 24 volt version for small piston engine aircraft and they are failing (DIY small aircraft forums and else where).

After a significant amount of looking, came up with the Walbro FRB-13 marine-industrial fuel pump series as a FAR better alternative. These cost more than the common Facet fuel pumps, are superior in many ways.



Bernice
Click to expand...
Bernice -

do you run a separate fuel regulator with your Walbro?
 
Yes, set to 1.5 psi. single stock carb on the 74'. This Weber carb has a fuel return line that goes back to the tank.
Pressure regulator outlet feeds the carb, carb fuel return then goes to the tank return line.

On the TR6 which has twin SUs, the Walbro pump is at the bottom of the tank inside the trunk. Pump feeds a 3/8" line to the engine compartment where a Bosch EFI fuel filter is located for ease of servicing and access. That Bosch filter is big enough to behave some what like a fuel reservoir. Outlet of the filter feeds the fuel rail above the two SU carbs. End of the fuel rail has the pressure regulator which holds the pressure inside the fuel rail mostly constant 1.5 psi (same scheme as most EFI fuel rails). Outlet from the pressure regulator goes to a 5/16" line back to the fuel tank.

Another fuel pump story. Neighbor has a TR3 with the twin motor SU fuel pump set at 3-4 psi. One would believe a fuel pump made by SU would work no problem with SU carbs.. not so. Turns out, 3-4 psi was high enough to over pressure the needle-seat valve causing fuel to run out the over flow and flood the carb resulting in an absurd fuel mixture. The cure was to make a pressure feed fuel rail with a regulator. No fuel return on this as there was no easy way to add a fuel return line. Once this was installed and set up (1.5 psi), the fuel problems went completely away and the engine runs FAR better than previously.

We have been using FORD inertia fuel off switched mounted vertical as would be in the stock position. No problems with false tripping so far. These are pretty common and not to $ and seems to work ok. FORD inertia switches uses a magnetic ball seat. If the rate of change is high enough, the ball escapes the magnetic pull breaking the circuit.


Bernice


Agnelli said:
Bernice -

do you run a separate fuel regulator with your Walbro?
Click to expand...
 
Rupunzell said:
Yes, set to 1.5 psi. single stock carb on the 74'. This Weber carb has a fuel return line that goes back to the tank.
Pressure regulator outlet feeds the carb, carb fuel return then goes to the tank return line.

On the TR6 which has twin SUs, the Walbro pump is at the bottom of the tank inside the trunk. Pump feeds a 3/8" line to the engine compartment where a Bosch EFI fuel filter is located for ease of servicing and access. That Bosch filter is big enough to behave some what like a fuel reservoir. Outlet of the filter feeds the fuel rail above the two SU carbs. End of the fuel rail has the pressure regulator which holds the pressure inside the fuel rail mostly constant 1.5 psi (same scheme as most EFI fuel rails). Outlet from the pressure regulator goes to a 5/16" line back to the fuel tank.

Another fuel pump story. Neighbor has a TR3 with the twin motor SU fuel pump set at 3-4 psi. One would believe a fuel pump made by SU would work no problem with SU carbs.. not so. Turns out, 3-4 psi was high enough to over pressure the needle-seat valve causing fuel to run out the over flow and flood the carb resulting in an absurd fuel mixture. The cure was to make a pressure feed fuel rail with a regulator. No fuel return on this as there was no easy way to add a fuel return line. Once this was installed and set up (1.5 psi), the fuel problems went completely away and the engine runs FAR better than previously.

We have been using FORD inertia fuel off switched mounted vertical as would be in the stock position. No problems with false tripping so far. These are pretty common and not to $ and seems to work ok. FORD inertia switches uses a magnetic ball seat. If the rate of change is high enough, the ball escapes the magnetic pull breaking the circuit.


Bernice
Click to expand...
Thanks Bernice. What model regulator with the Walbro please?
 
Been SO long, don't remember any more. It is one of those genetic "billet" aluminum "hot-rodder" jobs found on eBay and else where.
These are not really designed to regulate down to 1.5 psi, they are designed for 6-7 psi for Holley carbs. The fix is to take it apart, remove the spring and replace it with a much lower tension spring.

Think there are fuel pressure regulators that go down to 1.5 psi easy today. Check this.


Bernice

Agnelli said:
Thanks Bernice. What model regulator with the Walbro please?
Click to expand...
 
Fuel supply issues are fairly easy to test on electric pumps. To check the flow capability of the system, pull the feed line from the carb and place it in a bucket. Run the pump for 60 seconds and then measure the amount of fuel in the bucket. This will tell you the capacity of the feed, which includes the pickup, pump, lines and filters, all the way up to the carb. If the volume is sufficient then its likely a float or feeder issue.

If the volume is insufficient then remove the various potential restrictions; lines, filters, etc... to eliminate them. Then isolate the pump and test it.

I ran facet pumps for ever and never had a problem but I wouldn't be surprised to find out that their quality has degraded, likely with a relocation of manufacturing to a lower quality, off-shore source.
 
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