O/T Weber on an A-series

[stephen grant]

Apologies, because this is obviously not a Caterham problem, but I know there are lots of Mini and ex-Mini owners here, so I'm hoping for some help.

 

1275 engine, completely bog standard, out of a Midget. Running well on twin SUs. I bought a brand new manifold and single 45 DCOE and asked for it to be appropriately jetted. Apart from switching manifold and carb, the only other modification I made was to blank off the vacuum advance on my Aldon electronic dizzy.

 

Symptom; Car idles just fine, but as soon as I apply any throttle, the engine misfires horribly and dies within a couple of seconds. I have checked the following;

 

1. The face of the manifold appears to be flat against a steel rule

2. I have nipped up the manifold-to-carb rubber bobbins, I have slackened them off, I have replaced the Misab O-ring gaskets in case i'd overtightened.

3. I have detached throttle and choke cables and operated the carb by hand, so it's not a badly attached cable.

4. I've taken it all to pieces, reseated the manifold on the head, and put it all back together again.

5. I had a chat to Aldon about the lack of vacuum advance and they suggested it was unlikely to be an issue.

 

Of course it's possible that the carb is incorrectly jetted (I can't get an answer out of them as to what jets/chokes they put in there), although I am guessing it would have to be horribly incorrect to be this bad. Even a tiny throttle input kills it.

 

It could be an air leak, it could be incorrectly jetted. I'm guessing it's unlikely to be the lack of vacuum advance. I am not running a fuel pressure regulator (the SUs didn't seem to need one); I have an electric pump, could this be overwhelming the carb with fuel? As I say, the SUs seemed fine like this....

 

Any help/suggestions would be very welcome

 

Thanks,

 

stephen

 

 

Edited by - stephen grant on 19 Feb 2009 11:54:49

[guilleracing]

Hi,

 

In my humble opinion,

 

The symptoms you describe can only be caused my the fueling going lean. I would check that the pump jets are actually working, an then start looking for a jetting chart for your spec of engine.

 

Look at the plugs if you can get the engine to run for any length of time.

 

A series engines do not tolerate big chokes very well. I would start small to make sure you get a good signal to pull the fuel into the inlet manifold.

 

There must be dozens of mini tuning firms still in the UK. Check with someone who has mini based tuning experience on the correct jetting.

 

You should be able to eliminate anything else by putting the original carbs on for back to back testing of the ignition etc..

 

Greg.

[Roger King]

It's impossible to guess jetting without some experience of the exact engine spec. Even then you need a rolling road and a good operator to get it bang on.

 

There are numerous possibilities for the problem (and it may be something else that I haven't thought of).

 

Firstly, it is unlikely to be an air leak because the idle would almost certainly be horrible. Basically the less throttle you give an engine the bigger the effect of an air leak.

 

You need to be sure that the idle screws are set to give a good idle mixture. Even if this is good I would recommend unwinding the screws about 1/2 a turn each. This may make the idle worse, but if the main problem then improves a little it is a clue that the idle jets are too small.

 

So it might be simply that the idle jets are too small or large. The name of these jets is confusing because they actually fuel the engine until quite large throttle openings are used. It's easy to whip one out and see what size it is, although this assumes that they are the size they say and haven't been drilled out. Take the round jet cover off and the idles are the smaller of the round brass screws you see. If the test with the idle screws improved things a bit it may be simply a question of putting in bigger jets. If the test with the idle screws made things worse you may need smaller jets.

Note that small changes in idle jet size can make a big difference to the way the engine runs.

 

It is also possible that the progression holes are unsuitable for the engine. Unfortunately, this isn't really a DIY job without a great deal of knowledge.

 

The main jets themselves may also be wrong. These are the large brass screws, but this assembly also incorporates the emulsion tubes and the air corrector jets. Starting at the bottom you have main jet, E.T. and air jet.

 

You also need to check the ignition timing. I have little or no experience of Mini engines so I can't advise on a figure, but without a vac advance you should set the figure at full advance (whatever figure someone who really knows the engine recommends). In practice this normally means setting the timing at around 5000rpm. A vac advance will normally not work with a Weber due to sever pulsing in the tube so you are correct to remove it.

 

Are the float levels correct? Is there enough fuel pressure and flow? Is there a blockage, in the fuel line or internal to the carb?

 

The carb should have around 8mm of up and down movement on it's mounts at the ram pipe end without forcing it. This is essential to damp out vibration from the engine. In extreme cases, a rigidly mounted carb will froth it's fuel and the thing won't run properly.

 

 

[Klunk]

Hi Stephen ,

 

I have a friend who used to build race engines for minis so he would probably have an opinion or two.

 

Can you give me the choke size, jets etc so I can go to him with the full detail?

 

Regards,

 

Giles

[bstark]

Stephen,

 

Apologies if this isn't very helpful, but I have only heard mixed to poor reports of webers on A-series. If the engine is bog standard why change from the SU's (which seem to work very well)?

 

SWMBO's Midget is also bog standard and goes very well on SU's and electronic ignition...

 

Bob Stark

Supersprinter

[Manxseven]

Stephen,

 

Having run a Mini on a single 45 and then twin 45's DCOE, if you are still running a standard cylinder head, I would be sticking with the twin SU's.

 

Get a modified head and then the 45 will be a real benefit.

 

Get one from Bryan Slark - here .... go on it's only money 😬

 

Hope that helps,

 

Simon

 

---

Caterham BEC here

 

Edited by - Manxseven on 19 Feb 2009 21:06:28

[Nick Pargiter]

With a standard engine you probably won't gain much by fitting the DCOE other than a lot of noise

As a guide though check that you have the following:-

Choke 35

Main Jet 150

Air Corrector 180

Em Tube F2

Pump Jet 40

Idle Jet 50F2

Aux Vent 4.5

 

These were taken from 'The Bible', or 'Tuning BL's A-Series Engine' by David Vizard 😬

 

Nick

[Anton]

Many, many years ago I had a Morris 1300 GT with the engine bored and stroked out to 1430ccs, 10.75:1 compression ratio, high lift cams and electronic ignition. Initially the builders tried out Webers, but found bigger SUs far better suited to the characteristics of the engine over the whole rev range except 7500 plus.

 

If my memory serves me correctly I had about 120BHP - not bad for an A Series in the early seventies

 

LEO 3eggs

Another Slightly Vider SeVen

 

[oilyhands]

In the past I have used just about every combination of 40 and 45 on short, swan-ncked and long manifolds and a set or two of split webers on a variety of capacities between 970 and 1550. I'd agree that unless you have a heavily modifed head and a lot of cam (544, 649 or better) then a properly modified set of SU's are better suited to your engine.

 

That said the calibration offered looks similar to what I have here for a regular 1275, except I have a 55F8 idle jet.

 

Oily

[stephen grant]

 

Thanks all - much appreciated comments, especially since I really know nothing about Webers at all (or the A-series).

 

I do indeed plan to put a modified head on, hence I thought I'd move to a Weber. I thought moving to a single carb would make balancing it easier (and remove the need for linkages, etc.) and I don't know anyone who's found a manifold to get a single HiF44 under the bonnet of an eleven. Hence I ended up with a single Weber. My plan was to get it running, and then change the head and rockers and rejet it appropriately.

 

However - I do hear the prevailing sentiment here, which is that I might have been better sticking to my SUs. Having paid for the Weber though, i'd like to at least get it running so I can see for myself. I've still got my 1.25" twin SUs sitting in the garage, so if the Weber really is a dumb move, I can always move back. I'm loathe to abandon the Weber without getting it running properly though.

 

 

I took all the various jets out last night and found the following;

 

idle jets 50f2

emulsion tube f2

main jets 170

air corrector 180

aux venturi 4.5? (looks more like 15 with small1 but prolly 4.5)

choke 38mm

 

Choke seems rather big for a standard 1275 ?

 

 

thanks,

 

stephen

 

 

 

[Dave B]

Stephen

I have just found an old book I'd forgotten I had " How to modify your Mini" by David Vizard. This was the bible back in 60's 70's and goes into great detail on various carb installations including weber 45. If you want it, yours for postage.

Cheers

dave B

[stephen grant]

Hi Dave - that's very kind. I sent you a blatmail, but it seems to have come back to me undelivered. I'll try again.

 

Regards,

 

stephen

 

 

[JampJ]

Flog the Weber set up and fit a HIF44 SU on a Titan Motorsport manifold. Much easier to play with and pretty much the same performance, with probably better consumption.

The Weber should be fine, but you need to get it to an experienced tuner to get the jetting correct. Not something you can do yourself without a lot of expence. Is it a DCOE 45 152?

Cheers John

 

JFDI

(Just F*****g Do It)