Another carb problem over fueling/no fueling

[SamBam]

Hey all,

After revision of my carb its running too rich, and I cant get it adjusted right.
I can adjust the mixing screw but its ineffective.
The AFR stays around 10.0 - 10.5 while cruising.
On boost is okay, around the 11.5
Idle around 11.5

I thought the mainjet was to large or the a/c to small. so i changed them.
There is now a smaller main jet (first 120 now 100) and larger air corrector jet.
He has now gone from 10.0-10.5 to mid 11 So that's a lot better.

This went well a few days until Saturday.
If I let go of the throttle and pressed the clutch, the AFR shoots to 17 + and the engine stals. When I gear down, the AFR goes to 20+ It backfires like hell and black clouds of smoke come out.

After some adjustments on stationairy and mixture no problem with backfiring or lean afr. but still too rich!!!

I know a 100 mainjet is way too small. So i dont think the problem is on the carb.

Does anyone knows where to look?

Carb specs:
- air corrector 1.2
- 2nd stage 1.2
- 1st stage 1.0
- main jet 1.0

[andybond]

Sounds like an issue with the idle mixture screw to me.

It hasnt jumped out or anything has it ?

Try putting it all the way in, and then turning it 3 full twists out.

That cured my car doing exactly the same thing

[SamBam]

I will try that. tnkx.

But is it okay to run with a 100 mainjet?

[SamBam]

idle is fine now.
While cruising its still too rich!
I dont get it because of the 100 mainjet???
Hope someone can help me out.

[SamBam]

Alright i still dont get it!!
I just went for a decent test run.
Idle is fine (high 14)
Cruising is fine (around 12)
On boost way to lean (14-15)!!!!

If i use a larger mainjet cruising will be way too rich but on boost it would be fine.

I know there are many carb guru's here
Someone please help for a solution!!!

[tubbyG]

Leave the main jet standard just now, just Drill out the second stage to supply more fuel when on boost.

[Logg]

I'd check that the fuel pressure regulator reference pipe has no leaks as this can cause running lean on boost

Also check your voltage while on boost. A friend had a running lean issue on boost and it was found that while on boost one of the wires from the alternator was coming loose and voltage was dropping so the fuel pump wasn't seeing 14 volts.

[Sparkie]

do you happen to have the enrichment vent attached to vacuum?
you know.. the pipe that is supposed to lead no where.... if you attach a vacuum pipe to it by ermmm accident, on idle it will be too rich, due to the vac sucking the diaphram open, but on boost, the diaphram gets pushed shut by the boost, so no fuel passes through when its supposed to causing it to go lean.

[SamBam]

2nd stage is already drilled out to 1.2
There are no leaks
Nothing is connected to that hose

Ive read something about the ac jet. It ads more air on high revs.
So I just changed the ac jet from1.2 to 0.9 jet
Mainjet is still 1.0 if i change it to standard (1.2) the afr is to rich. And im where i begun.

Idle is 14.7 Perfect!
Cruising is around 11.0
On boost the afr goes to 13.0

Should i be happy with those values?

[BluntyR5GTT]

try taking the 2nd stage out to 1.3 mate as your full boost afr is to lean you want arround 12.2 max really

[SamBam]

I could drill out 2nd stage. But isnt 1.2 big enough?
When i change the main to 1.2 it will be fine on boost.

The mistery here is: Why is the cruising afr to rich with a standard 1.2 mainjet?

[fishead]

my moneys on a fault with the throttle housing part of the carb or enrichment circuit.
put the 120 main back in

[SamBam]

Ive tried the 1.2 mainjet again and now it feels hesistant and its to rich while cruising (low 10's)
So im back where ive started out now!!

[Nayls]

you do have a needle jet fitted dont you?

[Jimmy_GTT]

I also suspect the enrichment system. Not the turbo enrichment, but the normal full load enrichment. I think there isn't enough vacuum on the back of the membrane so the ball valve is open all the time.
It could explain all the behaviour.
On full load it should be open anyway. So it is lean because of the 1.0 main.
Cruisin is very rich. It should be around 15. The membrane should not open the enrichment.

So you have to check the enrichment circuit. The little housing. If it is not blocked somewhere. If the seals are good. If the correct spring is behind the membrane. If the membrane is OK. And if the ball valve is closing correctly.

The easiest could be to ask someone who's carb is OK to lend his the enrichment part for a try.

[SamBam]

I have notices the revision set has two different membranes for the enrichment part af the carb. one has a slightly shorter pin then the other. maybe this will sort it.
I will try on friday or saturday.

tnkx for now

[Jimmy_GTT]

Yes, that could be the cause.
Use the one with the short pin!

[Woznaldo]

I have notices the revision set has two different membranes for the enrichment part af the carb. one has a slightly shorter pin then the other. maybe this will sort it.
I will try on friday or saturday.

tnkx for now

Could you take a picture of the two different membranes as I have a similar problem and want to confirm before sourcing replacement parts.

Thanks SamBam

[SamBam]

Ive tried a original carb and its still to rich.
So the problem isnt with the carb/jets/gaskets.

Im going to get a new fuel pressure regulator.
Hopefully that sorts it.

to be continued.

@Wozalno, As u can see the problem isnt with the gasket. Ive compared the gaskets (the red one) there is a difference of 1mm. The gasket with the shorter pin didnt make a difference.

[Woznaldo]

Thanks for the info.

[Thundercat]

Just another idea that might be worth trying change your main jet back to 1.2mm and also change your 1st stage jet to a 0.9mm. Main jet should richen off throttle and 1st stage will lean the on throttle part before boost comes into play.

[SamBam]

Well im back again.

Ive tried the new fuel pressure regulator.
Unfortunately no difference.

With original carb (accept mainjet 100) afr on cruising is 13.0.
On boost is to lean of course.

Then ive tried the old carb.
Mainjet 100 and 2nd stage larger.
afr while cruising is 10.5

Im all out of solutions now.

[Woznaldo]

I would strip your carb right down, give it a damn good cleaning and overhaul it with a rebuild kit from the club shop. Put all jets back to standard, apart from the 2nd stage which can stay at 1.2mm.

[SamBam]

I did that already.
That s where the problems all started.

[car.crash]

im having the same issues, i read through all this thread to be lead to another dead end .

[Fordy]

seems a bit strange how your still rich on a 100main

[s3irios]

New poster! (less than 10 posts)

Got same issue except the o boost being ok with mine.

https://www.rtoc.org/boards/showthread.php?t=28580

Sorry for bumping old thread but it's too identical thought someone found a solution till now!