Weird Lean Spot at Part Throttle

[Chris Hebden]

Its hard to describe but cruising on the motorway at 70mph with very light throttle the AFR's are 14.7 but if the motorway inclines a little and i compensate on the throttle i.e. press it a little more the AFR's really drop off into the 16's it'll stay like this untill i either lift off or press the pedal further so that the turbo starts to kick in, only then will the fuelling sort itself out!

My set up is 1.2mm main, 1.1 AC, 1.0 1st stage, 1.2 2nd stage. What could be causing the lean off at very slight throttle?

I have had problems with a sticking ACC arm but this is sorted, i set this up using the 5mm drill bit but is was too lean so i have adjusted the nut back out until the ARF's seemed ok on accelerating. Cheers Chris

[Andrew Cooke]

how much boost is it making when it leans out? Can you feel it leaning out?

[Chris Hebden]

It isnt making boost Andrew, i try and check the gauge and its readin -1.0(ish), rising. I can feel it leaning off as the car feels like its hesitating

[Andrew Cooke]

have you pulled out your emulsion tube to check that it's clear?

[Chris Hebden]

The emulsion tube, is that the one that feeds fuel by the ACC arm?

[Andrew Cooke]

no, it's what the air corrector screws into.

http://blog.flat4ever.com/loicnews/1...DIS+Turbo.html

not exactly a GTT carb, but close enough

[Chris Hebden]

The centre tube shown on the right? Can this be pulled out?

[Chris Hebden]

no, it's what the air corrector screws into.

http://blog.flat4ever.com/loicnews/1...DIS+Turbo.html

not exactly a GTT carb, but close enough

What about page 32 of of the doc within the link above "fuel load enrichener system" could something be going on here?

[Andrew Cooke]

full load? aren't you on part throttle?

and yes, it pulls out.

[michael tierney]

as soon as ur vacuum goes the load enricher comes into play..this is the lower diaphram which just acts on a ball valve,check this circuit especially the tiny jet in the carb base.if u pull the emulsion tube out with an m4 screw check that its clear and orientated correctly when u put it back.

[Chris Hebden]

full load? aren't you on part throttle?

and yes, it pulls out.

Sorry i meant within the first two paragraphs under the heading "fuel load enrichener system" about part loadings.

I'll give it ago pulling that part to pieces, i havent done this before is it straight forward how it comes apart when looking at the carb?

[Chris Hebden]

as soon as ur vacuum goes the load enricher comes into play..this is the lower diaphram which just acts on a ball valve,check this circuit especially the tiny jet in the carb base.if u pull the emulsion tube out with an m4 screw check that its clear and orientated correctly when u put it back.

Is there a correct orientation that it should be in the first place? Do i put the M4 bolt where the AC jet goes and then pull?

[michael tierney]

yes! try to use a nut and spacer as a pullers.if u look into the well u will see how the orientation should be.if ur afr is 14.7,that might be a bit lean,carbs need to be 1 or 2%rich!

[Chris Hebden]

yes! try to use a nut and spacer as a pullers.if u look into the well u will see how the orientation should be.

Lovely, thank you! I'll see how i get on tomorrow. If this is blocked, how will this cause the engine to lean off and not richen? The way i understand it this tube adds air into the mixture causing it to lean, if this is blocked wouldnt it mean no air could get through therefore richening the mixture, not leaning it? Have i got the wrong end of the stick? I am struggling to understand these carbs!!!!!!!!!!!!!

[michael tierney]

ur also running a longer-bigger cam so ur idle jet should be aruond a 48!if its standard this would explain the weakness at part opening,.i'd go a 125 main if that did'nt cure it!

[Chris Hebden]

ur also running a longer-bigger cam so ur idle jet should be aruond a 48!if its standard this would explain the weakness at part opening,.

Its not standard i know that, i dont have a .45mm drill bit to check but the .40 i have fits very snuggly so i presumed its that size, is this too small? When does this jet finish its usability?

[michael tierney]

its good on light throttle up to about 2500rpm and ur drawing alot more air with the cam and when u rev higher the cam is more efficient and drawing even more air so thats why i'd go for a 125 main as well!

[Chris Hebden]

Sorry (dumb sh*t tonight) just to clarify, you suggest i should try a larger idle jet around the .48 mark and try a 1.25mm main? Thanks for the help!

[michael tierney]

yes thats right,ur cam is requiring the extra fuel!

[Chris Hebden]

yes thats right,ur cam is requiring the extra fuel!

Really appreciated! I'll get a playing tomorrow!

[Spooky]

Chris, have you got the BBT GT cam ? Its also known as the BB5500327

[Chris Hebden]

Looks familiar Spooksta! All i know is whats on my profile chap, i couldnt find any disdiguishing markings on the cam when i had it out last! Why????????

[Spooky]

Just asking buddy, I've got one in my garage that should be in my car

[Chris Hebden]

I've not had much experience of other cams other than the standard ones but i like this one, it pulls well (with a T25) all the way from 2800rpm! I've got a RR printout somewhere of the car making 140something (at 16psi) with really bad charge temps and bad fuelling so should be better the next time around!

[Chris Hebden]

I've made it bloody worse!!!!!!!!!!

I took the carb to pieces a moment ago, took the idle jet out and gave that a good clean (inc where it screws into) took the AC jet out and checked the inner dimension (1.1mm) cleaned it. Took the part of the carb out that the AC jet screws into, it was clear, i made sure with carb cleaner. Took the main jet out cleaned that and where it screws into the carb and checked the diam. 1.2mm.

The only thing i changed was i drilled the idle jet from a .40mm to a .45mm. I re-assembled, got it idling around 14.5-14.8 AFR and took it out on the road!

The revs build up to exaclty 3000rpm with the boost gauge reading +1.2psi this was when the AFR's started to drop last time and they still do, i then accelerate a bit more and the car almost stalls at 3200rpm with the AFR's in the 17's, boost pressure un known but in the +'s! I thought this was a little worse than it was before i started playing so i tried it like i was driving normally and pressed the throttle a little more to try and accelerate through 3000rpm, at exactly 3000rpm and a positive boost pressure of 8-10psi the car jerks forward and the AFR's dive off so i tried to accelerate through but it wouldnt rev out so backed off, drove home keeping the AFR's up and the revs low and here i am!

Whats going on!!!!?????!?

[Spooky]

Chris, sounds like what my car was doing.

That idle jet will need a good blow out more often than not especially as you've modified it, to help keep it clear.

When you accelerate and then back off, does the car want to stall ? It probably feels like massive hesitation yea ?

[Chris Hebden]

Chris, sounds like what my car was doing.

That idle jet will need a good blow out more often than not especially as you've modified it, to help keep it clear.

When you accelerate and then back off, does the car want to stall ? It probably feels like massive hesitation yea ?

I shouldnt be allowed near these cars!!! I didnt have the reference to the FPR attached to the top of the carb thats why the "masive hesitation" was occuring. You got it in one Sukh, if this is what your car is doing i would be looking around the FPR!!!

SO this part is now sorted BUT i still have a slight hesitation before boost with the afr's down in the 15's its not as bad with the slightly larger idle so once its cooled i might see if the 1.25mm main changes it!

[Chris Hebden]

Chris, sounds like what my car was doing.

That idle jet will need a good blow out more often than not especially as you've modified it, to help keep it clear.

When you accelerate and then back off, does the car want to stall ? It probably feels like massive hesitation yea ?

As soon as i backed off mate the car settled and drove normally untill i went to positive pressure

[Mart]

In my experience, drilling out the idle jet isn't a wise move, and even with the hottest of cams, it isn't necessary.

[Chris Hebden]

In my experience, drilling out the idle jet isn't a wise move, and even with the hottest of cams, it isn't necessary.

How come Mart? It had .45 stamped on the side but would only accept a .45mm drill bit unti it was drilled out

[Mart]

What's the tolerance of that drill bit? The idle jets are usually .4mm or .42mm bore. No need to increase them further.

Have you checked that you haven't an air-leak somewhere?

[Chris Hebden]

Unknown tolerance mate. I thought the idle was .45! I havent checked for air leaks yet.....................

[Chris Hebden]

Why would a lean spot be caused by an air leak? Is there anywhere particular i should be looking!?

[Spooky]

If you're going lean you are getting more air into the air/fuel mix from somewhere...could be carb, inlet manifold etc.

Chris, my fpr is ref'd from the compressor housing, all is good

I've got a .45 idle jet with no brass torpedo, idles/drives fine.

And I'd look at opening out the 2nd stage more as that will affect on boost afr which is where you seem to be having an issue yea ?

[Chris Hebden]

If you're going lean you are getting more air into the air/fuel mix from somewhere...could be carb, inlet manifold etc.

Chris, my fpr is ref'd from the compressor housing, all is good

I've got a .45 idle jet with no brass torpedo, idles/drives fine.

And I'd look at opening out the 2nd stage more as that will affect on boost afr which is where you seem to be having an issue yea ?

All is well then Sukh! Thanks for settling my mind with the idle jet, the car idled alot better with it !

On boost the AFR's are great with WOT being 12.1! Its hard to describe, sometimes its just before boost starts to increase i.e. -1.3psi and sometimes its fine upto +2.3psi and this is when the AFR starts to drop! What doesnt change is that it seems to only be when the throttle is in a certain position!

"If you're going lean you are getting more air into the air/fuel mix from somewhere...could be carb, inlet manifold etc." of course!!

[Spooky]

Your cars ****ed then

[Chris Hebden]

Your cars ****ed then

Would increasing the first stage help with this? Or should i be looking at increasing the main jet first and see what happens?

[Matt Cole]

Pop the 125 main in, or even a 130 for test. See what that does. There is also a preloaded spring that can be adjusted for the first stage enrichment circuit. The threshhold can be moved when adjusting.

[Chris Hebden]

There is also a preloaded spring that can be adjusted for the first stage enrichment circuit. The threshhold can be moved when adjusting.

Could you go into this a bit more please? I'm going to try a 1.25 today and see how it goes