It sounds like the vacum from the turbo is holding the vacum controlled jet needle in the carb for too long after you let go of the throttle.. A CV carb is probably not the best choice..

With a CV carb you only control the air inlet by hand, the gas inlet is controlled by the vacum.. With a non CV carb you control both air and gas by the throttle..

I thought it was a cv by looking at the pictures, and yes supercharger ofcourse...   Should've read the thread through before posting, my apologiez.

Anyway super cool project, Im looking forward to see how it turns out..!

savagebob wrote on 03/03/14 at 02:25:11:



You asked about spring pressure in your email, and that could be an issue with this setup.  It is not something I've had to deal with before, so cannot say for sure but it is worth exploring.  If there is a local shop with heavier springs available you could try, it would be worth it to see what happens.

savagebob:

I can't see for sure how you have the carb and supercharger set up.  Are you pulling air through the carb and then it goes into the supercharger...or do you have the supercharger pushing air into the carb?

If you have the air from the supercharger going into the carb....I am not sure how you will ever get it to run correctly.  The carb operates by having a vacuum in the the venturi - but if you are forcing air into the carb you may be eliminating the vacuum and pressurizing the venturi...and this will be most severe when the engine is at speed and you close the throttle resulting in "overboost".

curmudgeon wrote on 03/03/14 at 10:35:21:


That is my point however....supercharger to carb....and the carb will not function as it will not always have a vacuum in the venturi.  Without the vacuum it cannot pull fuel out of the float bowl.

Fuel injection (mechanical or electronic) works between the supercharger and engine.....carbs need to be operating at atmospheric pressure.  Think about any V-8 engine with a blower and carbs....the carbs are sitting on top of the supercharger and the air is being pulled though the carbs where it gets the fule added, and then pushed into the engine.

OK.....we got the carb on the "correct" side.

I don't know anything about the UFO.  I had a chance to use one - but first I tried the VM36 without one.....and it runs so nice I hate to change it.

Without the UFO, I was surprised to learn that when the throttle is closed and a high vacuum is created by the closing of the slide - the mixture goes very lean.  This is caused by the slide dropping and essentially closing off the main jet/needle jet and only allowing fuel to flow through the pilot jet.....while a bunch of air is pulled under the slide.  Before I had the O2 sensor I had thought the high vacuum was pulling too much fuel.....and I was proven wrong.  Our Savage backfires because the mixture is so lean when we close the throttle that the spark plug cannot ignite the lean mixture - but the hot exhaust pipe can once enough fuel builds up in the header (or you open the throttle enough that the combustion process in the cylinder resumes).

The superchager may be causing this lean condition to occur whenever the throttle is closed and the inlet vacuum remains high while the engine is still slowing down.

There must be some way to make it work.  Plenty of cars are running carbs and superchargers.....although they are running butterfly type carbs instead of slides.

I don't know what the UFO does when the slide is closed......I know the pilot jets are made much smaller because the UFO allows fuel to be pulled out the Needle Jet at low throttle settings.

I know a little bit about carbs and jetting.....I know nothing about adapting superchargers and turbos.  I might suggest that adding an Oxygen sensor would let you know exactly what is going on with the fuel mixture and take away some of the guessing.

I have actually got an O2 connected, it's it a mount bolted up the tailpipe but isn't getting a reading. Either it's not getting enough exhaust back there for a decent reading - or more likely I haven't run it long enough to get it hot enough to work.

My mate has a wide-band 02 I might borrow soon.

I have looked at the slide whilst running, and I don't really think it's being sucked open. The VM36 has a fairly hefty spring.. I'd be really surprised if that is what is happening. Personally I think Dave is on the money, the SC can suck with so much vacuum even at idle that it's leaning out and running high.

The idle screw doesn't lower the idle at all either.. it's like, the SC just drawing air under slide through extreme vacuum. Either that, or there is a leak somewhere. Everything is pretty tight though!