Main Jet

[Goo'Roo]

Hello, I'm just curious about your main jet in turbocharged blowthrough sacs engine.

I know that main jets are effected mainly by the pilot tube.
I have 36mm mk1 gsf1200 carbs

I used to have a 127,5 with a 11,5/11,8 afr at wot before.

Now I've changed the plenum completely with a much larger volume different duct and pilot pipe, moved from a gsf1200 intake cam to a gsx1100f one, changed the turbo from a Td04-16G to a TD05-20G.

Now I'm running a 135 but it's still lean, reading more than 15, even spikes at 17. 

I'm going to change them to 150 and try but I'm starting to think that the pilot pipe design I've done is wrong.
 

Edited December 8, 2018 by Goo'Roo

[Gixer1460]

Sounds like it! Blow through carbs shouldn't need much more on the main jet compared to NA as the pitot tube should ensure equal pressurisation.

But as you have changed so many things at the same, time it will be difficult to pin it down to one thing!

[Goo'Roo]

When you change cams and pods filters on that Na engines you have to go to 145/155 main jets, so I think that I could still be in that range, because the volume of the plenum is pretty big.
Anyway I'll go on trying with the main jet and see what's going on.

For sure if I have to go over 155 I'll consider to redesign the pilot pipe. But I have a strange dual chamber plenum design and it's not that easy

 

Edited December 7, 2018 by Goo'Roo

[Gixer1460]

Carbs function is all about airflow - cams don't generally help that, just change how much can be ingested - slower airspeed = bigger jets req. Most plenum volumes don't come close to std. airboxes volumes so should improve gas speed. Switch to EFI - and all the problems evaporate! 

[Goo'Roo]

I can't switch to efi at the moment, even if I had everything ready.
I have to understand properly how to set carbs because I have to do 3 turbo carbed bike for different customers.
My plenum is pretty huge. The total volume is more than 3 liters

Anyway going to 150 solved the problem, now I have 12.2 afr at wot with 10psi of boost

[no class]

Why would you make such a large volume plenum ? 

[Arttu]

53 minutes ago, no class said:

Why would you make such a large volume plenum ? 

Why not?

[no class]

Ok....thanks.

[Goo'Roo]

Bike runs really smooth with that plenum and I see 10psi of boost at 4k rpm with a td05-20g with 7cm exhaust, that I think is pretty good.

Bike start to take off at 5-5,5k 

It works pretty good.

The only thing I cannot figure out is how to solve is an overly rich condition (10,5/10,7 afr) when at low revs (2,5-3k rpm)  I give full throttle. The turbo start to push but the engine is still out of cams, the afr goes down.

I lowered the needle, but it just smooths this weird transition.

I'd like to try harder springs on the slide

This is the bike

[clivegto]

Air mixture screws on carbs maybe.

[Goo'Roo]

1 minute ago, clivegto said:

Air mixture screws on carbs maybe.

I'm going to play with float level, air screw and springs load today

[Arttu]

13 hours ago, no class said:

Ok....thanks.

I mean, from technical standpoint there shouldn't be any disadvantages from large plenum. As far as I know increased volum should only improve performance if anything. As long as we are talking about any remotely sensible size.

Ok, to be fair there are a couple real life problems with making a big plenum. The first and obvious one is availability of space. Usually these projects are continuos fight to get everything fit in. So even if you have space to make the plenum larger that space might be valuable for placing other components. Second problem is mechanical rigidity under boost pressure. Larger the volume gets also surface areas get bigger and that increases forces that pressure applies on them. And that might become a real problem surprisingly quickly.

[Gixer1460]

I can't knock your fabrication skills but for commercial fabrication reasons wouldn't a regular round tube be a better choice for the 'snorkel' ? Arttu makes a good point re : volume vs surface area - i've seen some smaller, more regular shaped plenums get blown out of shape so badly the guys gave up and switched to SS material - 2 bar boost can be brutal LOL!

[clivegto]

On 12/8/2018 at 7:58 AM, Goo'Roo said:

I'm going to play with float level, air screw and springs load today

How did you get on with this, any Intresting results to report.

[Goo'Roo]

On 12/9/2018 at 10:51 AM, clivegto said:

How did you get on with this, any Intresting results to report.

I had some other problems that stop me at the moment..

I had to modify the exit pipe in the wg and some weather issues slowed me down.

I went for a leaner needle setup, now I am at 3 and 1/2 from the top (gsf1200 MK1 carbs half step leaner from stock) and the bike runs at 14,7/15 when cruising and with a crispier response when wot from low down.

I've found that the float levels were wrong, like they have been bent by fuel pressure. I'll give another check soon, now they're back at 16mm.

 

I'm waiting for some softer wg springs, because for unknown reasons at the moment the boost pressure rises from 10 psi to 16/17. I've checked for leaking in the wg, but I haven't found any, polished the valve and greased it with a graphite grease, works perfectly but the pressure is still higher than the spring suggests.

I am without intercooler at the moment and with afr issues it gets dangerous to do some wot tests.

But I keep the post alive when I'll have some news.

Rising from 10 to 15/17 (I can't read it correctly due to a shit shaking Autometer boost gauge) it has gone leaner at the top end even on 150 mains that still suggest the wrong pilot pipe design.

But I'm waiting for the softer wg springs to start again.

 

 

 

 

[clivegto]

@Goo'Roothought my WG spring was 8psi turned out to be 15.9psi. You could cut a bit of the spring. 

[Goo'Roo]

19 minutes ago, clivegto said:

@Goo'Roothought my WG spring was 8psi turned out to be 15.9psi. You could cut a bit of the spring. 

What kind of wg do you have?

 

[Goo'Roo]

The shorter is the gsf1200 mk1, the much harder I can't really remember where it comes from, but it could be from a gsxr1100w with 40mm carbs. I'm going to use them to exaggerate the reaction and see if I can cure some rich condition when I close suddenly the throttle like after a burnout or a very hard braking condition after a wot, and see what's going to happen during the transition from n/a to boos

[clivegto]

1 hour ago, Goo'Roo said:

What kind of wg do you have?

 

Think it is a tial or a copy of a tial.

[no class]

4 hours ago, Goo'Roo said:

 

 

I'm waiting for some softer wg springs, because for unknown reasons at the moment the boost pressure rises from 10 psi to 16/17. I've checked for leaking in the wg, but I haven't found any, polished the valve and greased it with a graphite grease, works perfectly but the pressure is still higher than the spring suggests.

I am without intercooler at the moment and with afr issues it gets dangerous to do some wot tests.

But I keep the post alive when I'll have some news.

Rising from 10 to 15/17 (I can't read it correctly due to a shit shaking Autometer boost gauge) it has gone leaner at the top end even on 150 mains that still suggest the wrong pilot pipe design.

But I'm waiting for the softer wg springs to start again.

 

 

 

 

sounds like you may be experiencing boost creep......can't really tell by your pics but is the wastegate tube exiting the turbine housing at 90degrees or so ?   

[no class]

[Goo'Roo]

6 minutes ago, no class said:

sounds like you may be experiencing boost creep......can't really tell by your pics but is the wastegate tube exiting the turbine housing at 90degrees or so ?   

I've found the wg was leaking from one camera to the other because one of the 3 bolt that keep the valve attached to the piston was missing..
I'm going to test the bike in few minutes.
Turbine housing has been ported a lot to avoid boost creep and the pipe that goes to the wg is the biggest than possible.

[Goo'Roo]

Sorry..the wg is seriously leaking.. 

[Goo'Roo]

With the harder springs the afr goes from 15,0 cruising to 12,5 

Probably the higher vacuum sucks much more fuel.

Pressure now is 12psi that is much closer to the 10 psi it should be

[Goo'Roo]

I've sorted the bowls pressurization moving the port to the front of the plenum duct and the main jets now are 125 with a really nice transition and 12,2-6 afr at wot.

It remains a problem

When I shut off the throttle under boost, from mid to high revs, the afr falls to sub 10:1 choking the engine of fuel.

If you keep slowing down, without touching the throttle the overly rich condition after few seconds returns normal with a 13,5-14,4 afr. 

I've the feeling that the intake ducts remains wet after a full pressure run, and the fuel starving without air when you shut off chokes the engine.

The pilot jet is 37,5 and the screw is at 1,5turns on 36bst from a bandit.

Never had this problem before.