New chrome airfilter causing fuel mixture problems on 948??

[mrmorrisminor]

Hi,
We have recently fitted a chrome/gause type air filter to my brother's 948 minor brought from a well known supplier (in an attampt to 'pimp' his ride!!).

The car was running fine before, having only covered 10,000 since a major rebuild, including new needle and jet in the carb. After fitting, the car was 'missy' giving the signs of a weak mixture. This seemed to make sense as the new filter would probably be able to pull in more air thus affecting the mixture.

Several tuning attempts have not been totally successful. We can get her to tick over beautifully but performance is poor, often coughing and missing especially around town (pulling the choke helps so I'm assuming weakl mixture still). The best we have found is a compromise of a slightly uneven tickover but ok round town performance, up to about 50-55. Beyond this the car will (suprisingly) still accelerate quite hapily but is very uneven when kept on a light constant throttle to maintain cruising speed.

We have spoken to the supplier as I have heard that there are a variety of needles etc about but they said they were unable to advise and have not had these problems before......

So.....anyone got any ideas.....?? my vote is back with ye olde paper filter but this is not popular with the teenage driver!!



HELP!!!<br><br>

[Onne]

Best thing to do is get to a proper rolling road, and have them find a suitable needle.

Bit more weekend work for the aforementioned teenager :P

[eastona]

You probably need a new needle, and Onne's correct, a rolling road's one of the best ways.

An alternative would be to buy a best guess richer needle or reprofile your existing one using a file and or wet&dry/drill to get it right. I guess it's the mid to high throttle openings that are a problem, as you're getting a rich tickover when performance is good, but having to lean it off to get it to tickover smoothly, so taking some off the middle to thin end of the needle may help.

You're unlikely to get it spot on and would need several "seat of the pants" tests to get right but may be good enough given enough time/experiments. You can't put it back on though! so only take off a little at a time.

Don't run it lean though, you can cause damage, (apologies if you knew that already )

I got twin carbs on my MG set up like that but it took ages (mainly because I was too tight to go to a rolling road )

Not sure those wire mesh filters are that good (someone correct me if I'm wrong), but they look shiney!

Andrew

Nice car/grille BTW!

[bmcecosse]

Do you have a picture of the filter ? If it's the 'pancake' type - then they are not very good - the flat surface being too close to the carb mouth. You really want a Cone type filter - K & N or clone is best. What needle is in the carb right now - and what oil is in the carb damper ?

[minor_hickup]

These things are terrible, they flow badly, but better than the original air filter housing with the tube so upset the mixture and make a lot of noise. You will need a different needle but may as well get the right one for a better flowing filter such as a K&N with a stub stack.

[mrmorrisminor]

Thanks for the ideas so far......

I believe the needle currently in the carb is 'BX1' ( this was what I was sold when servicing the carb as it is a 'late' 948.)

The oil currently in the damper is an SU type oil that I bought specifically for going in the dash pot

And unfortunatly it sounds like we have one of the pancake filters!!

I was interested to hear about altering the profile of the needle as this was going to be my next course. I have read that it is somewhat of an artform so I was thinking of getting a replacement needle to experiment on just incase I made a mess of it (car is in daily use so need to keep it going)

I was a little confused as to why it would still accellerate at quite a pace (for a 948) over 55 if it was running that lean at higer RPM??

Anyhow thanks again

[linearaudio]

There is a book in the "speedpro" series called performance tuning SU carburettors. A lot of the stuff is madly radical, but there is a section describing how to determine where the weak spot is on the needle, and how to remove metal from the right part by the right sort of amount. It's all to do with taking the dashpot plunger out and putting a calibrated cocktail stick in its place, then watching its response as you rev the engine (slowly). Haven't had need to try it yet, but sounds feasible. Got my copy of the book out of the library-free!!

[eastona]

If you're going to get a spare one, why not get one that's roughly right out of the bag? I just tweaked mine because I was tight to buy two new needles!

Does someone have WINSU or a similar setup and can suggest a needle? Or as minor_hickup suggests, what's the recommended one for a K&N setup?

They're right, the pancake is too close to the inlet and the sharp edge on the inlet isn't good. Added to that, it's doubtful if the scouring pad type filtration actually filters very well!

Andrew

[bmcecosse]

Do you not have the old needle from the carb to 'play' with ?
But first -I suggest you try using 20W50 engine oil in the carb - and report back how that works ? It will delay piston lift - and may just give you the result you need! Normally I use 3 in 1 - but this could just be the answer to your problem.

[minor_hickup]

Well I got mine free and used it because the silencer pipe fell off my air filter. However I suffered the same problems as you and the engine hunted at fast cruise which was utterly infuriating on a long run. However putting back the old filter case less the silencer was a revalation, the car pulled much better, was quieter but still had a nice induction roar when pulling hard!

Seriously if you're going to mess about with needle profiles you may as well do it with a series of worth while power/economy upgrades unless you want to experiment for a bit of fun. Why put a K&N filter on a 1 1/4" SU when you could be putting one on a 1 1/2" SU and a decent manifold?

[bmcecosse]

The needle that used to be suggested to 'cure all ills' was I think the E3 - but hickup is right - you would be better looking to a larger carb and an MG Metro inlet manifold.

[mike.perry]

If you say that the problem started when you changed air filters then the obvious step is to refit the original filter and reset the carb mixture. If the problem was caused by the new filter then you should be able to get the engine running correctly again.
Once the engine is set up again you can be certain that the problem is with the air filter and not some coincidental cause
If your car has an H2 carb then the standard needle is BX1, for an HS2 the standard needle is an M. In some stages the BX1 is richer, in other stages the M is richer. The richer needle options to go for are the S and AH2 respectively. I would not recommend filing needles down as they are precision parts being produced to a tolerance of .001 in and once filed you have lost all references to the correct dimensions.
If you obtain a book of needle charts you will be able to study the profiles and make an informed guess as to the correct needle.

[eastona]

once filed you have lost all references to the correct dimensions.

Quite true, there's no going back. I did mine with a micrometer and very fine emery. It takes ages, and should be taken very slowly.

I agree that the difference between lean and rich is only a few thou, and you shouldn't just go at it like a man posessed.

I had that speedpro book too, it's really good. Didn't try the stick method, but always fancied a go.

Andrew

[bmcecosse]

As long as it's a single carb - you can simply file a 'flat' on the needle - it doesn't need to be round! Just always put the needle in the carb with the flat on the engine side. But yes - it does lose the reference on that needle - but if it's an old one anyway, there's nothing to lose!

[mrmorrisminor]

Thank you again folks!

Will try suggestions, my vote is to re set the thing up with the paper filter and start from there.......watch this space!!

With regards to purchasing alternative needles, where is best to try? The supplier we use only seems to be interested in 'standard' types.

[bmcecosse]

Try the 20W50 oil in the damper before doing anything else! Burlen will sell you needles - but to be honest - it's a waste of time/money fiddling with that carb. Go for a larger carb and manifold off a Mini!

[mike.perry]

Burlen Fuel Systems, Salisbury, tel 01722 412500, www.burlen.co.uk
I would comment then that filing needles should be left to the experts and that doesn't include me.

[Longdog]

Hi Burlen services in Salisbury should be able to help.
The have a web site and remanufacture all things S.U., hope this helps.