Racing with 40 DCOE carbs

D

DaleS

uh, maybe
For those of you out there that have experience racing with a pair of 40-DCOE carbs, my car is a reasonably well prepared '76 1300 that seems to be running out of gas near the end of a very long (0.5 mi) straight. I'm wondering specifically what type of fuel pump you have used, if you used fuel systems with return lines or any other specifics/tricks that have proven to work.

The problem running out of gas gets worse as the car runs better. I increased main jets and that seems to cause the car to pull harder until it starts to crap-out - but that doesn't happen until I'm well down the straight.

I've looked at fuel pressure near the carbs and it seems okay but I've gotten differing views on that too from people who are supposed to know (somewhere between 3-6 psi)


I'm currently running:
  • well-vented fuel cell in the front
  • a small Carter pump (that Pierce Manifolds claims is "perfect" for the carbs) located in the frunk (I do not have a relay on this pump - tho it might be a good idea to add one, it doesn't draw much amperage)
  • a pair of small in-line fuel filters; one fore and one aft of the pump
  • a single supply line to the carbs with no regulator
 
I had a similar problem

I was running dual 40 DCOE's as well on my 128.

I had the existing fuel pump setup so it would deliver 3.5psi. Well, that is correct per carb, not for both. So, in an Autocross format, it would work fine, get on a hill climb with long sustained RPM's, and it would sputter out of gas.

You are going to need to run at least 7psi for both carbs.

Hope this helps. On Courtney's 128 site, he still has the jetting information I used on these as well.

Eric
 
Thanks for that

Thanks; that makes sense. I'm not familiar with Courtney's 128 site - can you help me find it?

Thanks again.
Dale
 
In past I run with Facet solid state competition no return way not problem with 45 DCOE 145 /155 jets ...some time before start I have switch I out for few second and than on to end of race ( have little more pressure for carbs ....) than I put some return pipe but with only 1mm -I blocked pipe in .
 
That would be a Carter rotary electric fuel pump?

Is there a problem with the fuel pick-up?

What size fuel lines are used? AN-6 on the feed, AN-5 return.

Ditch the small in-line filters, use one BIG fuel filter, spin on oil filter style or similar. This acts as a reservoir and also helps stabilize feed fuel pressure.

Fuel pressure for DCOEs should be no more than 3.5psi Max. At 6 psi, the floats will NOT be happy, fuel volume is what matters, not just psi.
If needed, a larger needle/seat can be used to increase feed volume into the fuel bowl.

Use a return/bypass fuel regulator, this helps reduce back pressure stress on the fuel pump and stabilize the fuel pressure at the carbs.

Bernice

DaleS said:
For those of you out there that have experience racing with a pair of 40-DCOE carbs, my car is a reasonably well prepared '76 1300 that seems to be running out of gas near the end of a very long (0.5 mi) straight. I'm wondering specifically what type of fuel pump you have used, if you used fuel systems with return lines or any other specifics/tricks that have proven to work.

The problem running out of gas gets worse as the car runs better. I increased main jets and that seems to cause the car to pull harder until it starts to crap-out - but that doesn't happen until I'm well down the straight.

I've looked at fuel pressure near the carbs and it seems okay but I've gotten differing views on that too from people who are supposed to know (somewhere between 3-6 psi)


I'm currently running:
  • well-vented fuel cell in the front
  • a small Carter pump (that Pierce Manifolds claims is "perfect" for the carbs) located in the frunk (I do not have a relay on this pump - tho it might be a good idea to add one, it doesn't draw much amperage)
  • a pair of small in-line fuel filters; one fore and one aft of the pump
  • a single supply line to the carbs with no regulator
Click to expand...
 
A few thoughts.....

Dale, here is a photo of the setup I put in the Dallara racer
dscn0682w.jpg

I joined both bungs at the bottom of the tank to the pump inlet with 1/2 inch hose, then plumbed 3/8 inch line to the rear. I did my pressure measurements at the union where the lines separate to the two carbs.
dscn0539h.jpg

Pressure was set to 3 1/2 psi.

So:
-- If you filter before the fuel pump, it needs to be LARGE. Electric pumps are very sensitive to inlet restriction. I would recommend not filtering before the pump at all.
-- What size fuel line are you running to the rear? Suggest as large as possible.
-- Tap the fuel pressure right at the union and use a pressure regulator to maintain 3 1/2 psi.

Good luck. Post up how it turns out.

Ciao,
 
Fuel pressure

Some are saying 3 or 6 and Mark is saying 3 1/2 would it safe to say 3 1/2 is good..
Thanks
 
Well, I think I still have metal splinters in my hands....

From building those hoses.

The instructions say: "Just tape off where you want to cut, then slip on the nut, insert the fitting and tighten the nut." Riiiiiight! The hose build up was one of the worst days in the ten months of work in building the racer.

Apologies to Dale for stepping on his thread.

Ciao,
 
I don't have problems on the track

I have a 1500 with all the normal high output mods
I am running 3psi with a Holly Red pump but all of the plumbing is near the fuel tank in the rear and no return line. I am running

Idle 50
Main = 135
Emulsion Tube = F15
Air Cor = 190
Pump = 30
Choke 34
Fuel line an fittings -6
Earls Fuel filter

I have plenty of fuel through the rpm range and am normally a bit on the rich side right up through 9000 plus rpm
126_2629.jpg

Check your emulsion tubes and make sure it is not something else like an ignition problem - not enough advance etc
 
Welcome Dale, don't believe I've seen you here before...

... but hey...

In reading through this thread I can't seem to find just WHERE everyone is taking their fuel pressure readings.

I have NO real idea of which I speak... but it would seem to me that if you took a reading BEFORE a TEE or a SPLIT to the two carbs it might read one thing (say 7 psi) and at the end of the line, say AT the carbs... it may read another, (like 3.5 psi).

If i'm all wet, please disregard... if you don't know, then I think I would ask the question again of Mark and the others...

Again... welcome aboard.
 
Great thought!

Yes, I have my regulator after the filter close to the engine. In my case the fuel lines will have negligible differences in pressure because they are the same inner dia, have limited bends, and are short tube length . Yes certainly a long fuel line would make a difference, lots of assumptions in calculating too but basic eqn is hl = (f*(L/D)*(Vavg^2/2), based on how many bends, restrictions, etc there are additional losses that are tacked on to calc the hlt (total head loss). The short story is if you have a long Length/Dia and lots of bends you will have a significant head loss (pressure difference). One last point volume flow rate is Q = Vavg*A so tube id really affects the amount of fluid you can flow. Side note - lots of turbo guys miss that point and focus only on max psi (boost) and miss map their applications.
 
regulator

i would try a regulator in after your pump but use the bypass type with 45 dcoe and 40 dcnf setup
works on my car very well
 
Thank you for that but...

I've been around here for a few years now; although, I do try to listen more than I talk and usually tend to be the guy standing in the back of the room. Thanks for the welcome anyway.
 
5 years experience running dual side draft Webers. 2.0 Pinto.
Fuel cell, HP rotary pump mounted next to cell, feeding large fuel filter. High quality adjustable fuel regulator, no return line, pressure set at 4.5PSI for single line to splitter to both carbs. 6500-8600 RPM range. All aeroquip.
Never any problems.
 
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Hmmmm... Why no return line?

Maybe on the Pinto ya never had one in the first place... but on the Fiat there is one in place, so why not use it? Or are you just stating that you just don't have one, not necessarily recommending not to use one?

It sure would help with any potential "vapor lock" issues we have a tendency to have.
 
Racing w/ 40DCOEs - closing the loop

Ok - everybody repeat after me: "Dale, have you checked the easy stuff first?" I don't know why but that lesson just doesn't seem to stick! :dunce:

I've only run the car once since I made my changes but it ran clean all day; tho' I'm still not sure I'm making power like I should. My biggest issue? Float levels! After removing my two small fuel filters and replacing them with a single canister filter before the pump and carefully adjusting my float levels the car ran great all day!

I don't doubt anyone's advice but I still don't have a return line or any regulator and I'm going to have to develop another problem before I add those things. I like the idea of making small changes and learning along the way. Thanks for the help!
 
Very Interesting point Tony! I am also running dual 45DCOE Weber Carbs and have my fuel pressure regulator set to 3.5psi. My fuel pressure gauge used to be in-line before the tee going to the two carbs and would read 3.5psi but when I switched to AN fittings I purchased a fuel block and so my fuel pressure gauge is now ON the tee but what are the individual carbs actually getting? I guess the only way to know would be to have an in-line gauge for each carb after the tee.
There's probably as many articles written about the correct fuel pressure for these fuel sensitive carbs as every thing else out there but I used to read that 2.5psi was the correct amount and now more and more am reading that 3.5psi is the correct amount but never in any article does it specify for single or multi-carb setups?

I'm very curious what the XWeb experts have to say.


Before Setup





Current Setup



'PeteX1/9
 
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