Lean out on boost
Turbo Mini Forums

I have posted on this before and still have a real problem.

Accelerating hard, good AFRr,s, but after about 4-5 seconds as boost starts to build, the AFR,s go off scale, lean.

I have changed pump, filter and FPR as well as all associated lines, re-set float level and fuel pressure.
Fuel flow at the carb, with feed line disconnected, is massive. (I have even removed the tank and enlarged the outlet)I have checked the gaskets carb to plenum, but they were not changed and the problem has been with me for some time.There was a lesser problem with high speed lean out with the T3, but its impossible now with the GT17.

The really weird thing is that it is (was ?) intermittent.It first showed after filling up and ran like it for about 150 miles, then went back to normal operation.Then later, on the same journey, it went "off" again, only to return about 100 miles later. There is no correlation between tank empty or full as far as this problem is concerned.

One other thing. If I lift off as soon as the AFR,s go lean and then open up, it will pull for a second or so on the right mixture and then go lean again.In the initial stages of the problem doing this restored performance to normal for many miles, then the intervals became shorter and shorter, to the present position.

It seems that the fuel pressure is not being maintained above boost pressure consistently but would two FPR,s exhibit the same tendency -the second one was brand new.

I cant, short of running fuel lines through the bulkhead to the cabin, check that the fuel pressure is rising in step with boost.I,m reluctant to do this.

On top of all this, I leave for Rotterdam on Thursday afternoon with the car.

Any more helpful ideas, please ?

Tried the t-piece trick?

If something is worth doing, it's worth doing half of.

No, John, I had forgotten about that - thank you

The FPR will have no affect on AFRs, so it is not surprising that it is the same with two different ones on.

Then increase in fuel pressure in the float bowl is done by a hole from the carb flange (where the plenum mounts) and the float bowl.

This is what the plenum restrictor does. It basically makes sure that the float bowl is pressurised at the plenum pressure, and that the pressure across the jet is around 3psi less.

So the T-piece trick may be your salvation, a blockage in the existing feed to the float bowl may be the issue.

I find the fuelling with my carb is inconsistant at best.

Fastest 998 mini in the world? 13.05 1/4 mile 106mph

Now you have confused me wil.

Surely if the FPR was not keeping the differential fuel pressure up then the mixture would lean off ?.

Just going out to try the T piece trick.

Worrying that you have inconsistent results with a carb!!.

Dave. If the FPR was not keeping differential fuel pressure your float bowl would empty out. Your AFR's wouldn't be affected until the float bowl was completely empty.

What Wil is talking about (and what I was hinting at) is that there could be an internal blockage or restriction in your carb preventing the float bowl from being pressurised ~3psi above boost pressure. If this is the case then as boost rises you will experience a lean AFR.

Edited by John on 14th Jun, 2011.

If something is worth doing, it's worth doing half of.

No. All the FPR does is ensure that the fuel pressure is 3psi above what the float bowl is. This ensures that the float bowl continues to fill up.

Turn your pump off and go for a drive. Your car will run as normal under boost, until that is, the float bowl empties.

The float bowl pressure, as I described above, is generated from a feed through the carb from the plenum.

The 'T-piece' trick just adds an additional feed signal to the float bowl from the plenum.

When I say inconsistant, I really mean consistantly inconsistant. Thiungs lit it leaning of with sudden throttle changes, an if you keep you toe down long enough it just get richer. Plus cruise is too rich, but nat at all rpms/loads.

I could start again with a new needle, but you will always end up with a compromise somewhere. So I've settled with it's idiosycratic nature.

Fastest 998 mini in the world? 13.05 1/4 mile 106mph

I had a similar problem, I tried the t-peice trick. And made no difference, however when on the rolling road, at bogg brothers it was back firing massively, and we couldn't figure out why, until we removed the t piece trick then it stopped? I still havnt sorted my problem yet as I've been off the road for a month or so.

Audi s4 b5 - 470bhp & 486ft lbs

On 30th Apr, 2012 Brett said:
yeah stick the bit in and give it a wobble

Has the carb been rebuilt at all? If the seals where worn, under some situations they could be allowing air/in out and skewing the mixture

Well I,m afraid the T piece trick bid not work for me either.

TBH I was not optimistic as the carb has been stripped twice and all passages blasted through with carb cleaner and I,m confident that the passage from plenum to float bowl is clear.

Still not clear if the FPR is malfunctioning it wont affect mixture. As the level in the float bowl goes down surely the mixture will lean off as the airflow will struggle to pull the fuel from lower down in the jet.

I have not rebuilt this carb with new seals, but have a spare one which is rebuilt, but untried.
will try this when I get back from Holland.If my throttle spindle seals were leaking all I would experience is loss of boost and a worrying smell of fuel -yes ?.

I can drive the car with part throttle and staying on low boost and its still pretty lively,and will cruise at 75 easily so I expect to get there and back,as I did to Dundee at the end of last month.

Any more ideas ?.

Dave.

I know you dont want to hear this but I'd tee in a fuel line and pressure gauge to give indication on the outside of your windscreen wiper rather than inside the car. Fit it between the FPR and carb, make sure the idle fuel pressure is 3-4 psi and go for a run. Check the fuel pressure rises 3 psi above whatever your boost level is from low to high boost. If it doesn't, there's your problem, you'll not be getting enough fuel.

You could also try clamping the part throttle lean off pipe with a brake hose clamp or small 'G' clamp and see if that makes a difference. Reinstating it if it aint fitted is another option. Worth a try surely.

On mine, the only way I could get fuelling consistency at Slarks rolling road was to remove the Avonbar Hi Boost needle and fit a BAP needle and SAE80 gear oil in the dashpot.

Is it worth trying the above if you can get hold of the parts in time ?

Dare I also suggest split signal hose somewhere ?

I'm feeling your pain mate, took ages to get mine sorted.

Good luck.

Richard.

Minivanless, but reluctantly happy living with the decision. There'll be another one day.
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