Su Carby Issue.....

[Smithius]

I am currently running a 2 inch su of a rover that i hae fully rebuilt to go on my supercharged 4k.

 

The setup as it currently sits is..

 

2 inch SU carby

 

SC12 toyota supercharger

 

locked electronic dizzy

 

basically stock internals apart from full rebuild with a cam and double valve springs

 

 

 

Now the problem that i have is the piston in the SU will not open up enough to allow more fuel to pass into the carby. I have replaced the needle in the carby with one which i bought from a carby specialist who has tuned a couple of similar setups on corolla's and datsun's. Some were turbo and some supercharged but all ran an su of some descript and he ran this needle.

 

The needle i have gives a good idle but taper's down quite quickly to give a rich mixture with more throttle.

 

The problem is that i can get the car to idle properly and rev up slowly to around 6000 rpm so far, but if i give it full throttle, the piston does not open fully and the car only revs to around 3500rpm before leaning out and detonation kicks in.

 

So this is one for the su experts out there becuase this one has me stumped.

 

 

The motor previously ran twin 1 1/2 su and they ran perfectly and opened properly but this one has me stumped.

 

 

 

Cheers

Matthew

 

As the motor was previously

 

As it stands now

Edited March 9, 2011 by Smithius

[altezzaclub]

the piston in the SU will not open up enough

 

Have you video'd this with a webcam to know that it is correct??

 

Leaning out is having the piston open too far or too fast, as having the piston too low makes it rich.

 

I suggest a heavier oil in the dashpot. If you're using ATF go to engine oil and see.

 

Its easy to film it-

 

http://img9.imageshack.us/img9/8783/5qb.mp4

 

from here-

 

http://www.rollaclub.com/board/topic/42407-the-girls-ke70/page__st__30

 

Once you really see what is really happening we can talk about dashpot springs, sanding needles and shimming dashpot shock absorbers...

[kickn5k]

Just use the "penrite dashpot oil" it's not expensive and was the best I had used before I went sc myself.

Or you could do it right and put efi on it. Even a cheap d5 microtech fuel only is better than a carb. Lol

 

How much boost you running? Ignition timing? Have you put a sniffer up it? Nice looking setup though hope you get it sorted.

Stu.

Edited March 9, 2011 by kickn5k

[altezzaclub]

I'm sure you can get the carb running as well as EFI. I think the supercharger sucks harder and faster than stock when the throttle is opened, so it may be slamming the piston up hard and needs really thick oil. I've read of people using everything up to SAE 75.

 

I use thinner than Penrite (Johnsons Baby oil) and it runs fine, the needle can handle the faster throttle opening & leaning on medium throttle speed opening (going up a hill) and still enrichens it when I boot it quickly. This took O2 sensors and an FMD to sort out...

[snot35]

The oil's major effect is to limit how quickly it opens. If the piston has only gone so far it's because the suction the motor is asserting on the piston has reached its limit. The only thing that resists this is the weight of the spring in the dash pot, and probably the weight of the piston itself. If it's a problem use a lighter spring, but generally you'd tune the steps on the needle to provide the correct mixture for a given engine air requirement (load/revs etc).

 

Changing the oil dictates the resistance of the piston to movement and has the effect of enriching the mixture for acceleration only.

 

Also check that the butterfly is indeed opening all the way to ensure the full vacuum signal from the motor is reaching the piston. I'd probably also check for air leaks, and make sure that the piston does not bind at any stage in its full range of movement. You should also be able to see down the throat and see that the needle never touches the side of the jet either.

 

Good luck!

Edited March 10, 2011 by snot35

[Smithius]

alltezzaclub, i was under the impression that the further the piston opens, the richer the mixturew becomes?

 

I have a jaycar oxygen sensor display hooked up to an o2 sensor out of a falcon (bosch) and i can get good idle mixtures (what do you recomend?) but as soon as i give the car a big bust of throttle it shows lean and i get detonation.

 

Now all of this is happening all while free reving. I have not driven the car yet because i do not have the throttle cable hooked up yet.

 

I have the dizzy locked and set on 10deg btdc at the moment.

 

I am having no luck with this at the moment and i am very close to ripping the su off it and putting a side draft webber on so i can tune it properly.

 

I am pissed off because there is no reason why it shouldnt work as it has been done before.

 

 

Cheers

Matthew

[Smithius]

kikin5k, from my calculations it should be running around 6 to 7 psi of boost with the pulley that i made.

 

I have a boost gauge but it wont show any boost while free reving for some reason?

 

will yours produce boost while free reving?

[altezzaclub]

Well Smithius, the theory is that the lower the piston is held for a certain air flow, the richer it is.

 

So if you have a constant demand of airflow, pushing the piston down increases the air velocity across the bridge and hence the depression in the venturi that sucks the fuel out of the jet. Pushing he piston up leans it out if the throttle is constant, which is what happens when you're setting the mixture and use those little finger pistons to lift the slide at idle. If it increases idle speed its rich, if it dies its lean, so lifting the main piston up leans it.

 

I run an idle of 14.7 to lean, so it idles around 17 grumpily, pops up to 14, down to 17 again, on each carb and they are extremely similar. That gives me a resonable 12 to 14 on a slight throttle when driving, and it goes rich to "R" when I boot it. On over-run its down at 18 to 20.

 

Just toss some engine oil in the dashpot and see if it helps it. Then try diff oil. I think the supercharger is sucking much harder than the cylinders ever would, you are pushing a shitload more air into the motor. If that improves the acceleration mix to rich or at least reasonable, but its lean when you hold the revs, you can increase the spring tension, add a weight to the piston, or sand the needle thinner at those stations. All those three things will make it richer at constant throttle.

 

I've sanded the needles to get my mixtures.

 

That picture of your carbs was an inspiration to me when I was planning my setup- beautifully done! Mine is functional but agricultural! :laff:

 

 

 

I wouldn't be surprised if it doesn't show boost on free revving, the throttle opening to do that is extremely small. The motor isn't doing any work at all so the revs wind up with the slightest amount of fuel. When you drive it I think the fuel demand will be much higher.

Edited March 14, 2011 by altezzaclub

[kickn5k]

kikin5k, from my calculations it should be running around 6 to 7 psi of boost with the pulley that i made.

 

I have a boost gauge but it wont show any boost while free reving for some reason?

 

will yours produce boost while free reving?

 

Cool that should be fine(6-7psi) and you have barely any ignition timing in it. I run about 25+btdc(only cause it's past the end of the marks). My sc12 is the same I may get about 1-1.5psi at max on the free rev. The sc14 will boost on a free rev and give instant boost on throttle crack. My car runs 5psi in 1st, 6psi 2nd, 7-8psi 3rd and 9psi in 4th and 5th. In my experience there is little to no peak power difference between the two blowers it's all in the way they deliver boost.

altezzaclub is right you need to slow down the piston. Being that the su carbs only run about 2psi of fuel pressure if the piston moves too fast there is not enough fuel behind it to keep up. Yes su's will work almost as good as a fuel only ecu, but it will unlike efi test your patience with setting it up. Best thing about a draw through carb is no need for an intercooler.

 

Stu.

[Smithius]

cool, thanks guys for your answers.

 

I will try putting some thicker oil in the carby.

 

 

I'll try to get a throttle cable on it so i can take it for a spin

[SLO-030]

Any luck? Keen to see it setup in the 38.

[Smithius]

I tried some thicker oil in the carby this morning, but it still runs lean on heavy throttle and starts to ping.

 

I'm trying to source a dcoe weber so i can actually tune the stupid thing properly.

 

There is no logical reason why it runs lean while free reving because the needle i have is a very rich needle. it is very hard to get the mixtures right at idle to the point that its fouling plugs really fast. Its almost like it doent have enough spark but the afr meter shows that the mixture go's lean as soon as you give it a big hit of throttle.

 

So if you know of a dcoe weber sitting around the place I'm after one. Must be a weber so i can get the jets easy. Dellortos are to hard to source parts for.

 

 

Cheers

Matthew

[kickn5k]

Man it's so close your only going to leave the frying pan and into the fire.

Been thinking what main jet size do you have, are you running an electronic fuel pump and fuel pressure reg if so what pressure are you running?

Main jets come in three sizes .090" , .100" , .110". Might have a small one in it stranger things have happened.

 

Stu.

[altezzaclub]

So, you could watch the needle go up?? What if you stuck a finger in there and stopped it going up?

 

Maybe that jet can't flow enough fuel at all...

 

Do you have a needle you can sacrifice to The Great God of Experimentation and sand it thinner where it leans out?

 

I had the original AAP needles,which were too lean all over, and I bought a pair of ABH, which were too rich. So I sanded the AAP needles thinner in the low to mid-range to give a lean idle, a nice mid and a 14.7 top end.

 

Can you fire it up without a needle and get it to rev straight away??

 

------------------------------------------------------------------------------

 

Does retarding the ignition change anything?? As soon as you boot it the cylinders fill up and burn faster, so an advance that will be OK on empty revs will pink on full throttle.

 

What is the advance curve like?? I'm 80% through sorting the electronic dizzy I bought recently, and it was a dog stock! No advance low down then it all rushed in at 3000rpm. So I've had the wife in the car holding the throttle steady at every 500rpm while I check it with the timing light, then change the spring setup and do it again.

 

Currently its at-

 

1000rpm 10deg

1500 12

2000 15

2500 20

3000 25

3500 30

4000 35deg

 

How does yours compare??

[Smithius]

kikn5k, i have a massive facet electric fuel pump with a fuel pressure reg set at 3 psi i think. anything higher and the needle and seat can't hold the pressure and fuel sprays out the overflow.

 

The carby has a .100thou jet so its the correct size for the needle. everything in the carby that can be replaced has been, including the float.

 

 

altezzaclub, i can watch the needle going up but i havent tried stoping it moving. i do have anouther needle that i can modify if need be.

my dizzy has no advance mechanism in it at all. i have had the mechanical advance locked and the vacuum advance is not connected so the timing is set at a very conservative 10deg btdc. i should be able to crank nearly 20deg into it when i get it running properly.

 

what oxy sensor are you running in yours? I'm not sure if mine is displaying right.

 

at the moment I'm having trouble with fouling the spark plugs which sugest's that the mixture is too rich but it still does not want to rev for me. I'm not sure if I'm having problems with not enough spark?