Solex overflow carbs

0120raptor · 03-09-2018, 10:53 AM

I have a race 911 2.0 liter with the solex overflow carbs. On the rollers it runs really rich low and mid range (lambda 0.6) then runs really lean at the top end. It`s running twin electric pumps feeding from the float bowl to carbs. Mains are 160 ,Airs 140.These are the jets the engine came with which would explain the low down richness but not the top end leanness. The 6 banjos were already drilled out to 1.3mm but only one hole was drilled the other had been soldered up. We drilled the second hole also to 1.3mm which richened the bottom end even more but still lean at the top.
Should these be drilled even bigger.?
My second thought is that the mains are SO big that the engine is using up all the fuel in the carb well that by the time it reaches high revs the wells are almost empty because the pumps can`t keep up?

1QuickS · 08:21 AM

Sounds like fuel starvation to me.

Which venturis are you using? 36mm? Main air corrections sound small, I think 170s would be right. The 140 air corrections will advance main circuit initiation as well as richen top end. It is classic to enlarge hole in banjo bolts to 1.30mm but only one. These are required to assure equal filling of the float bowls since three bowls are fed in sequence from the supply fuel line. If these holes in the bolts are too many or too large then you risk over-filling the first carb & starving the last one.

If this is a race application then I recommend ditching the Pierburgs and re-plumb the fuel supply to have an electric fuel pump feeding a pressure regulator and then a flow control valve. Drop fuel pressure to something like 2.5 psi & adjust flow until you have 0.1 times peak HP flow in GPH. So a 200 HP engine would need 20 GPH which includes some margin. Then feed both fuel supply lines from the line from the flow control valve. Remove the floats from the float bowls and collect the fuel in the float bowls and use a scavenge fuel pump to return the over-flow fuel back to tank.

Of course you can try to make the Pierburgs work but it sounds like fuel starvation to me; possibly the first carbs get the fuel due to too many holes in the banjo bolts and the last carbs go lean as a result. There is info regarding checking fuel output pressure & volume at 3000 RPM for each pump. Quite tedious. Adjusting output flow & pressure requires shimming Pierburgs where they attach to cam cover. Also, Pierburgs are subject to wear between the pivot shaft & the internal rocker arms and replacement parts are NLA. You can buy new Pierburgs but much better to install an updated system & ditch the fuel floats & Pierburgs altogether. If you must keep the Pierburgs for sanctioning originality then use them to pass over-flow fuel back to the fuel tank.

0120raptor · 03-10-2018, 02:42 PM

Thanks for the reply. Will try reversing the fuelling tomorrow.

0120raptor · 03-12-2018, 09:54 AM

Still lean at top end with reversed fuelling. All the fuel enrichment jets are soldered up . Is that right?

1QuickS · 03-12-2018, 12:20 PM

I don't understand "reversed" fueling statement. Banjo bolts should have only one hole & that is 1.30mm. Did you install fuel pressure regulator & flow control valve & then supply both sets of banjo fittings from there? Did you remove floats from float bowls & send fuel back to tank? Did you check fuel flow rate?

High Speed Enrichment occurs at high RPM and yes, the nozzles should not be soldered closed. However, the 140 air correction jets would compensate for that.

0120raptor · 03-12-2018, 12:53 PM

By reversed fuelling I mean plumbing it as you suggested ie no floats etc.That proved difficult in the timescale I had as 1 the car is testing in one days time and 2 the petrol tank didn`t have a fitting for the return fuel.
However after unsoldering the high speed enrichment jets it made a HUGE difference.The lambda readings are now much better. Good enough to race this weekend. We`ll fine tune it afterwards and probably will reverse the fuelling. Thank you for your help.