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Another turbo slingshot


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Finally I had a chance to go for a test drive after fixing some gasoline leak from choke circuit pipe...

 

 

0.5bar wastegate spring installed.

AFR numbers are still a little bit off:
~2k idle 14-14.5 afr
~3-4k cruising very low throttle 50-80kmh 14-16 afr
When I go WOT from 4k, I get 10 afr between 4.5k and 5.5k and after that it's pretty nicely 11.5 - 12.5 afr. Something to do with boost transition? 

Carbs are now:
Mixture screw 2.5 turns out (stock size)
Needle on 3th from top (stock needle)
Mainjets 135

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  • 8 months later...

Little update for this year.

Took her out from the shed for a quick spin.

image.thumb.jpeg.56b57556f6fd8e93e76c2e2dc9866dbf.jpeg

 

During winter time I changed the pitot to different style. Previously I used pitot which was delivered by proboost. Problem with this kind of pitot is that, there is a known risk of forming void behind the pitot which will make air to bypass pitot tube when going above 0.8 bar. With this setup, I got good AFR as in the post above.

Below is pic of old pitot:

image.thumb.png.68aecacdbff4c98e212eb5f74226855c.png

 

I was advised to make a new pitot from a tube with an inner diameter of 4mm, which is inside a 2" charge pipe (charge pipe was not changed).

Below is pic of new pitot:

image.thumb.png.2db6917da79f568bfd0ea2ba498c44c6.png

 

Now I'm having problems with getting AFR right with this solution.

125 mainjets: 14-15 AFR on the whole rev range.
130 mainjets: 14 AFR on low throttle (<4k), 12.5 on boost transition (4-6k), 13 on low boost (6-8k), 14 on the higher boost (8k-10k)
135 mainjets: 14 AFR on low throttle (<4k), 10.5 on boost transition (4-6k), 12 on low boost (6-8k), 14 on the higher boost (8k-10k)

What would cause this strange behavior? Mainjet seems only to affect transition AFR, not the mixture on top.

 

Heres how my pitot pressure is shared. Is has been working well with old pitot, but not sure if its causing some problems now?

image.thumb.png.7cb0fb116351886bb70d36607a1069ec.png

 

Hoping to get some suggestions....

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Whole range AFR can't be fixed by changing the MJ alone! Both the pilots and the needles will need attention IMO. re the plumbing, I'd have 1 tee off the main line then split that with another tee to feed each pair of carbs - seems would be more equalised to me?

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Thanks for the reply. Will try with this plumbing modification.

How would you change pilots and needles?

Mixtures are 2.5 turns out and idle AFR is 13.5 at 2k rpm. Needles are 3rd from top.

Just tried without intercooler and AFR stays at 13 on WOT with 135 mainjets.

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My carb setup did work nicely (in the end:-) I had 2 pitot tubes, one for each set of carbs. No tee's in there or other stuff connected to it, just straight from pitot to carbs as short as possible, and equal length and definately no kinks!  I made my pitot tube straight and made an angled hole in the uppipe as to avoid the bend in the pitot tube. The entry should be in the middle of the airstream, I did angle cut it.  I used ID 6mm tubes into a 48ID uppipe.  Dont know how much of a difference that makes compared to your design but I noticed a difference between one and two pitot tubes.

At WOT and boost, slide is/should be fully up so needle height, idle screws and pilot jets dont influence AFR (much)  My setup was ok with all boost settings at 125mains

Check out a post that I wrote a looong time ago about this topic, maybe it helps https://oldskoolsuzuki.info/archives/396

Sometimes the FPU is set too low causing high boost lean AFR. If I remember correctly I  did set it to 3psi. 

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The pitot tube makes all the difference in the world so focus on that. 

I did a set of BST34mm last week and I ended up with stock settings on everything. (Buy genuine jets)

 

Adjust the pitot tube until you are happy.

(It's nice to have an installation that are adjustable) Don't have the tube close to a bend. 

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9 hours ago, peter1127 said:

My carb setup did work nicely (in the end:-) I had 2 pitot tubes, one for each set of carbs. No tee's in there or other stuff connected to it, just straight from pitot to carbs as short as possible, and equal length and definately no kinks!  I made my pitot tube straight and made an angled hole in the uppipe as to avoid the bend in the pitot tube. The entry should be in the middle of the airstream, I did angle cut it.  I used ID 6mm tubes into a 48ID uppipe.  Dont know how much of a difference that makes compared to your design but I noticed a difference between one and two pitot tubes.

At WOT and boost, slide is/should be fully up so needle height, idle screws and pilot jets dont influence AFR (much)  My setup was ok with all boost settings at 125mains

Check out a post that I wrote a looong time ago about this topic, maybe it helps https://oldskoolsuzuki.info/archives/396

Sometimes the FPU is set too low causing high boost lean AFR. If I remember correctly I  did set it to 3psi. 

Where did you take pressure for fuel regulator if you only had 2 pitots?

 

 

With current setup I ended up with 142.5 main jets and now WOT AFR is 12.2.

When I open the throttle all the way open at around 5k rpm, AFR drops to 10 (min. on the gauge) and bike is coughing for a second. I was told this happens because sliders are too lightweight and I should glue 8mm nuts on them. Is there any truth on this statement (mk1 bandit bst36)?

Pitot is plumbed now like this. All tubes and fittings are 8mm ID.

image.thumb.jpeg.107f2e02bbb425c5b2ec2f2cfca87b58.jpeg

 

Yes, the "pressure reg" is a line to Malpass with a 90 degree fitting due to space constraints.

image.thumb.jpeg.a720029ee0c245c88e2bc0abd1f3782d.jpeg

image.thumb.jpeg.4629cb28b6a4ff06f8cd331c1dbc8300.jpeg

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I had FPU pressure take-off from plenum when I first build this bike (a couple of years back) and it didn't work. I think there is too much pulsing, because it dropped fuel pressure and caused high idling. I fixed it by taking pressure from pitot.

@Fredrik_Steenhow did you make adjustable pitot?

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2 hours ago, Baron said:

I had FPU pressure take-off from plenum when I first build this bike (a couple of years back) and it didn't work. I think there is too much pulsing, because it dropped fuel pressure and caused high idling. I fixed it by taking pressure from pitot.

@Fredrik_Steenhow did you make adjustable pitot?

I didn't, but Truls did. You can see the images in his build thread on this page:)

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9 hours ago, peter1127 said:

Just take the pressure to the fpu from the plenum. Doesn’t need dynamic boost. 

Wouldn't that give a lean miss? The pitot should always be a better choice as it acts faster to pressure change - that's its whole reason! I've no axe to grind here - carbs are hard enough without having to contend with pressurised air AND fuel . . . . . . . I prefer EFI!

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2 hours ago, Baron said:

I had FPU pressure take-off from plenum when I first build this bike (a couple of years back) and it didn't work. I think there is too much pulsing, because it dropped fuel pressure and caused high idling. I fixed it by taking pressure from pitot.

@Fredrik_Steenhow did you make adjustable pitot?

Weird I didnt have that. Anyway you could make a separate pitot for fpu. Best that nothing interferes with the carbs dynamic pressure.

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1 minute ago, Gixer1460 said:

Wouldn't that give a lean miss? The pitot should always be a better choice as it acts faster to pressure change - that's its whole reason! I've no axe to grind here - carbs are hard enough without having to contend with pressurised air AND fuel . . . . . . . I prefer EFI!

As long as pressure is high enough to fill the bowl its ok. tis not like efi where fuel pressure directly influences amount of fuel injected. In baron's case maybe the spot on the plenum was unlucky so he has pulsing problems. I did it before, worked fine.  Converted to efi as well and like it a lot, although I have to say it also has its challenges.

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