ATTTN :BEN WIGHT

[SHEEL]

having teething problems with my blow thru set up using a 32/36 dgv.
runs rich on idle and acceleration but leans out on cruise when the accelerater pump cuts out, although it feels like its leaning out i cant be sure if it is actually flooding.
have tried playing with different jets, needle and seat, float level ect to no avail. do you have any suggestions that may help. post here or email Sheel2@Optusnet.com.au
cheers
SHEEL out
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[Ben Wight]

First off, I would like to say that you need to know what your mixtures are, not only to get good drivability, but to protect your engine, as they are too easy to blow.

There are two ways to do this, the best is to use a wide band oxy sensor, usually whilst on a dyno, and find out what is going on and fix any problems. The other way is to use a cheap narrow band oxy sensor and find out what is going on. I honestly dont know how you will ever tune the thing without knowing your mixtures.

I have a narrow band sensor and mixture meter permantly in the car, even though I am running efi.

Ben

[martin]

i thought that running a mixture meter on a car that uses super or leaded fuel kills the sensor real quick??

is that right? thats why i never ran one

[SHEEL]

i've had the car hookeed up to a gas analyser machine but i can't get a reading coz the mixtures fluctuate too much, like i said before its rich on idle the machine stops at 10% co and its richer than that. but when i hold the revs up it stalls from leaning out or flooding before the machine can give a reading. i dont think the problem lies in the tuning of the carb. it runs fine when u disconnect the piping and run it with no boost, as soon as u put boost into it the show is over it runs shit as. basically what i was asking was have u done anything to prepare a weber for boost other than solid float, rejetting, higher fuel press ect. i thought maybe i've missed somethin along the way....
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[GeminiCoupe]

Checked the malpassi rising fuel regulator (or whatever the fuck it is). If its running too rich at idle, then its got too much fuel pressure, then as the boost rises its supplying way too much fuel and flooding it?

just a thought

[GasGem]

sounds like you need a rising rate fule pump and regulator so that as the boost rises so does the fuel presure so that the bowl will not run out of fuel when the boost comes on.

try this link for some idea's

http://datsun1200.com/modules/nsections/index.php?op=viewarticle&artid=35

cheers
Eggy

[Ben Wight]

I see, you never mentioned it was boost related before. It does sound like your boost pressure is coming close to or overtaking your fuel pressure.

What fuel pump are you running? You do realise that you have to keep fuel pressure above boost pressure for the carb to function? For this you need a method of increasing fuel pressure as boost rises, a boost referenced fuel pressure regulator works best, providing you pump has the pressure output required.

You could also try using a boost switch to trigger a solenoid in the fuel return line, or switching pump voltages, etc.

Let me know more about your current fuel setup before I can recommend any changes.

Ben

[FLYINLoW]

hey ben u seem to have a bit of knowledge on the carburettor side of things have u heard of SK carbys

they have SK racing printed on them and the throats are close to 48mm - 50 mm they are twins.

[SHEEL]

running an AC DELCO low pressure high volume pump, using a Malpassi rising rate fuel reg set to 4 PSI on idle and it goes up to about 12 psi under boost conditions, i know this is a bit high but like i said before. the car runs fine( not too rich) without the piping on(N/A),with the same fuel pressure. it is wierd how on boost it runs really rich? the only thing i can think it is doing is pressurising the float through the vent at the top of the carby and thus pushingmore fuel through the jets? i dunno. i was gonna ditch the 32/36 and try to source at DMTT which would be better just thought i'd ask u before i gave up on the 32/36
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[Ben Wight]

Keep in mind that if you disconnect your inlet plumbing, your fuel pressure regulator will not alter fuel pressure as you will not produce any boost.

How do you know what fuel pressures you have, do you have a gauge hooked up? 12psi of fuel pressure will only be high if you have around 4psi of boost. If running 7psi of boost, it should be fine.

The fuel pressure wont affect the mixtures much, it cant push fuel through the jets, it simply effects the fuel level in the carb, which can alter the mixtures, but only slightly. If you have a brass float, I wouldn't worry about the float as I have run over 20psi consistently with a 32/36 and brass float.

I need a bit more info such as boost, and fuel pressure at that boost. It may be a case of turning your fuel pressure down further. Or maybe its running lean, you wont be able to tell without an oxy sensor.

How have you jetted the carb, its quite simple to let the primarys stay fairly stock, whilst the secondary is rich to give enough fuel for boost.

I dont know what a DMTT is, but the 32/36 should work well for you.

Ben

[SHEEL]

fuel press is at 5 psi on idle and 12 psi under boost. full boost is about 5psi. i know this is a little high but thats the best i could do with a variety of pump configurations.
jets are 135 primary and 150 secondary using a .175 needle and seat and 160 primary and 165 secondary air jets.
like i said i've tried less and more fuel press and diffent jet combos from 120 primaries right up to 160 secs. jetting and press aren't the problem i think.
could it be anything else apart from these things?
ps there are no vacuum/boost leaks either
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[SHEEL]

also some education for u...
a 34DMTT is a downdruaght weber very similar to a 34 ADM. it origionally comes out on a turbo citreon and are of course DESIGNED for a blow through configuration. you can get rebuild kits and what not for them easily BUT the carbies themselves are rare as rockin horse shit as u could imagine but i know where to find one...
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[Ben Wight]

I actually have heard of them, long time ago though, my first thought is that they are bike carbs, but they would be too big.

I dont know anything about them.

[Ben Wight]

I didn't know Citreon ever sold a turbo car, let alone a blow through one.

I would be interested to see the carb, you know someone who has one?

[Ben Wight]

So it feels like it leans out at part throttle, with and without being on boost. Or runs too rich, you cant tell.

What have you done with your dizzy, any boost retard system? That might give a similar feel.

Does it seem to cut out, or die slowly? If its the needle and seat restricting fuel flow, you will feel the car die suddenly.

If its lean, you will feel a severe lack of power, if it leans out whilst under boost, it will detonate and might blow, so be careful. If you have the jets, keep your primary fuel jet at 135, but try a 180 in the secondarys. If it runs worse, then its definitely too much fuel.

It might run better, which means too lean.

As another check, you could turn your fuel pressure down to 3psi at idle to see if its something to do with too much fuel pressure under boost.

One thing I just thought of, it could be that due to your positive displacement blower, you are making boost, even at part throttle, which means you are still on the primary fuel circuit, which is probably just too lean for the engine on boost, as the engine sort of thinks its at say 140% of throttle, but the carb thinks its at 40% of throttle, if you get what I mean.

At the end of the day, its either too rich, or too lean. Or its not a fuel issue. You should be able to find out by altering fuel pressure, and by going relatively extreme with the jetting. If you cant get your fuel pressure any lower than 5psi, then your return line is too small. You can tee into your pollution line that runs under the car to the tank, and use that as another return line, running in parallel. You could also try a base fuel pressure of 7-8 psi, as see if that makes it better with bigger jetting.

BTW, have you considered installing an oxy sensor yet?

Ben