Help Please Uncle Bob!

[exupturbo]

I bought another set of totaly stock carbs, fitted them to the bike without any of the turbo stuff connected and it was fine.
When I connected all the turbo stuff back up it ran exactly the same as my old setup even though I now have a pitot tube to pressurise the float bowls, as soon as anything is connected to the float bowl vents it seems to run really rich up to 4000 rpm, I've lowered the fuel level by 3mm but this has not helped.

Any suggestions before I take a hammer to the damn thing.

Thanks again for any help

Mark

 

[Uncle Bob]

that doesn't make much sense, the bike should run exactly the same off boost.

When you say "without any of the turbo stuff".....what are you using? Stock air box? pods?

Just like any bike, if you go changing the intake system drastically, it'll definitely effect the jetting. Are you using aftermarket needles?

 

[exupturbo]

that doesn't make much sense, the bike should run exactly the same off boost.

When you say "without any of the turbo stuff".....what are you using?  Stock air box?  pods?  

Just like any bike, if you go changing the intake system drastically, it'll definitely effect the jetting.  Are you using aftermarket needles?

When I say without turbo stuff I mean just the carbs with no filters or airbox, it runs fine like this with either dynojetted carbs or stock ones. As soon as I plumb in the float bowl lines it overfuels and splutters like crazy at low revs. Fuel pressure is set 2.5psi above boost.

Mark

 

[Uncle Bob]

When I say without turbo stuff I mean just the carbs with no filters or airbox, it runs fine like this with either dynojetted carbs or stock ones. As soon as I plumb in the float bowl lines it overfuels and splutters like crazy at low revs.  Fuel pressure is set 2.5psi above boost.

Mark

well yah, you take the air box off any bike and it will require completely different jetting. Thats your problem right there. Don't jet it with the turbo air box off.

Throw the stock needles in and see what happens. At lower RPM's there is no boost and very little flow, there can't be any real pressurizing of the system going on, unless you're using a turbo charger off a lawn mower. Are you using stock idle jets? if not, I'd put those back in too, and turn the mixture screws out to 3 turns.

 

[exupturbo]

I've tried with stock needles, dynojet needles, infinate numbers of turns on the idle mixture screws, all sizes of air corrector jets,stage 3 bungs/no stage 3 bungs, stock main jets all the way to 180 main jets, stock slides and modified slides. It seems that as soon as the carbs see even a minute pressure difference in the float chamber low speed fueling goes bad. If I connect just plain pipe to the float bowl vents and leave them at atmospheric pressure without the plenum fitted it runs fine, when they are connected to the pitot tube (bearing in mind there is no plenum fitted and hardly enough airflow to feel on your face coming from the the pitot tube in the boost pipe) it goes bad. Fit the plenum and it is just the same. Perhaps the semi downdraft type carbs fitted to the Exup are over sensitive to pressure in the bowls?

Thanks
Mark

 

[Uncle Bob]

but then why did it do this before the pitot tube? Makes no sense, there's no pressure at all when you just reference the air box instead of the pitot tube.

Something isn't adding up, but I don't know what it is. There should be no difference between hooking up the float bowl vent tubes and not hooking them up off boost. Atmospheric is atmospheric.

How is your fuel pressure regulator hooked up?

 

[exupturbo]

FPR is referenced from the plenum, the original boost signal to the float bowls was also referenced from the plenum and I had the same problem. The pitot tube is mounted in the boost pipe before the plenum and the tiny amount of air flowing through it without the plenum fitted is enough to upset the carbs

Megasquirt is sounding tempting.

Mark

 

[Uncle Bob]

when you say plenum, I assume you mean the air box, correct?

Got any pictures of the pitot and air box and vent bowl lines? I'm out of ideas otherwise. what you're discribing is impossible, from my undertanding.

 

[exupturbo]

I'll try and take some pics ASAP.

By plenum I mean the part that the carbs breath from and the turbo blows in to. The flow from the pitot is hardly enough to feel against your face with the plenum removed.
I fitted a piece of clear plastic tube to the float bowl drain and taped it to the carb, with plain pipe fitted to the bowl vents the fuel level hardly budges when I blip the throttle, with the pipe fitted to the pitot the fuel level rises by at least 1/4 inch when I blip the throttle, this test is not all that accurate because the end of the clear plastic pipe was not sealed but it does prove the point that even with a miniscule pressure change the fuel height goes mental.

Mark

 

[Uncle Bob]

if the pitot tube was over pressurizing, the fuel level in the bowls would DROP not rise.

You sure that FPR is set up right? Have you watched the fuel pressure on a gauge while blipping the throttle? It shouldn't move at all.

 

[exupturbo]

if the pitot tube was over pressurizing, the fuel level in the bowls would DROP not rise.  

You sure that FPR is set up right?  Have you watched the fuel pressure on a gauge while blipping the throttle?  It shouldn't move at all.

The fact that the end of the clear plastic tube was not sealed would proberly explain the wierd fuel height thing, but the fact still remains as soon as I plumb in the pitot tube fueling goes rich, fuel pressure has been checked and set at 2.5 psi above boost and does not change whilst bliping the throttle.

Mark

 

[Uncle Bob]

ah, I got yah, wouldn't work with it capped anyway, have to have a place for the fuel to go.

since you had the same problem without the pitot tube, your air box is pressurizing. What turbo are you running again?

 

[exupturbo]

IHI RHB 52 with R-20 exhaust housing and bigger compresor wheel.

Compressor map.

Mark

 

[Uncle Bob]

well, that looks fine. I must say I'm stumped. I can't say I ever tried experimenting with the float bowl level while blipping the throttle, so I don't know if mine does that anyway. Some pressure, I wouldn't think should make much difference, but as you suggested a down-draft carb setup might be more of an issue since there is much less pressure needed to get the fuel to the outlet in the carb. This MIGHT be the real problem.

But I also know that down draft carbs have been used on many turbo'd bikes, so it still seems strange to me that its making such a big effect on your bike.

I can think of a couple rather complex fixes for this, if there really is no other way, and that would be to have a pressure redirection switch, that would redirect the bowl vents to atmosphere normally and then switch back to the pitot tube at some boost number, say 2 psi. Although its a pain that you might have to do this, its not overly hard to set something like this up. All you need is a pressure switch and a air divertor (you could either use one of many used in EVAP systems on cars, or even use a dual tank switch.....lots of options). These are just some ideas that I'm pulling off the top of my head.

I'd try lowering your floats even more first.....can't hurt to try, no? The lower the better, it sounds.

 

[Rhythm]

Dayyumm, talk about tech support.  You Da man Uncle B

 

[exupturbo]

Dayyumm, talk about tech support.  You Da man Uncle B

I'll second that  

Pic of plenum/intercooler in place

Pic of underside of plenum.

Pitot take off point

Close up of pitot tube

temporary plumbing of pitot to  float bowl vents

Any ideas?

thanks again for your patience

Mark

 

[Uncle Bob]

well, everything looks good there. You have a fan for that intercooler? Seems like you'd get a lot of hot air from the radiator.....but thats not related to what we're talking about

You mentioned that you've tried all sorts of combo's of jetting, but have you tried the large range of jetting with the pitot?

What I'm thinking is, maybe you are still using a fairly rich setup from before the pitot tube. Just for humor, I would put the stock needles and idle jets in, put a very small main jet in, say a set of 100's and try that. Between the stock needles and the smaller mains, it very well could reduce the large amount of raw gas getting dumped into the system at small positive pressure increases in the float bowl.

I wish you were in my area....I'd love to come over and check out whats going on......

But if none of that works....could always try the valve idea....

 

[exupturbo]

The area under the cooler is normally sealed from any hot air rising from the engine and is fed by the two chrome pipes that originally fed cold air to the air box.
The main jet is not meant to have much effect at lower rpm and below 4000rpm there is no boost showing on the gauge, stock carbs with stock jets,needles etc behave the same as dynojetted ones----badly

Thanks again

Mark

 

[Uncle Bob]

Main jets do effect the whole range, although its most notable at the higher RPM. If things are far enough out of whack, they will definitely effect the lower throttle openings.

Humor me....give it a try. All you'll lose is time.

 

[exupturbo]

Main jets do effect the whole range, although its most notable at the higher RPM.  If things are far enough out of whack, they will definitely effect the lower throttle openings.

Humor me....give it a try.  All you'll lose is time.

OK, i don't know if I have any main jets smaller than 120 though but I will give it a try. Back to work now for another 5 1/2 days so no progress reports 'till Saturday evening UK time.

Cheers

Mark