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MG Midget and Sprite Technical - 45 DCOE tuning

Can anybody help with a tuning problem I have with Weber 45 DCOE on modified 1275 midget engine?
Engine set-up is Kent megadyne 266 cam, stage 3 head high lift rockers sport coil and Newtronic ignition. Weber 152 was originally set up according to specs in Dave Vizard's 'tuning BL's A Series engine' i.e. 38mm choke, 50f2 idle 165 main jet, 180 air corrector, 45 pump jet 4.5 aux venturi and 12mm float level.
The idle was found to be way too lean and is now replaced with a 55f9. This produces a smooth idle and good progression untill 2500 rpm and above where a pronounced irregular misfire occurs and power falls off. This is especially noticable when driving the car. Pulling the spark plugs reveals a white colouration to electrode; therefore weak mixture in the main jet circuit. I have gradually increased main jet size and decreased air corrector size and am presently running 200 main and 160 air with a 50 pump jet with no effect on enrichening the mixture. I suspect the choke is to large. Any other ideas welcome.

Philip
P.N. Baker

Hi Phl I have been using 45 DCOEs for the past 30 odd years so I like to think I know a bit about them, unlike those who simply repeat urban myths. So anyway your car.

I refer you to this information on the MG Car Club Midget Regster website.
http://www.mgcars.org.uk/mgcc/midgetreg/Weber.shtml

You will see that the first part of setting up a weber is to choose the correct size of chokes. You should try to use only the biggest size suitable.

From the chart you wll see a 1380 engine with a 286/296 cam capable of power up to 8000 RPM is possible. However your engine with only the mild 266 cam doesn't need such big chokes. A 1275/1300 with a rev range of no more than 7K would be quite happy with 32/34mm chokes. With smaller chokes you need to realise suction on the fuel jets is a lot higher and thus jet sizes reduce accordingly. As I wrote in the article I have found that when using the rules of thumb for jet sizing (ratio'd to the choke sizes)I always end up with the primary 1 or 2 sizes too small.

As you rightly identify the mains do the work above 2.5K or whatever so make sure the mains are clear and the spray tubes in the throats are correctly orientated. I think that with a 165 main jet the car should be fine and defiately with a 200 which leads me to think that you have not got the spray tubes in correctly (you should locate the sprays with the screws underneath that locate into a divit in the sprays) and thus the jets are not supplying fuel to the engine.

Does the engine pick up at all?
Robert (Bob) Midget Turbo

If you have a 200 main jet on a 38mm main venturi and 4.5 Aux vent and the car is lean something is wrong.

I suggest you've either got a fuel starvation problem or as Bob suggests you haven't got the Aux vent fitted properly. Maybe you even have them fitted upside down.

Once you've sorted the problem I think you will find the engine will run nicer with 34mm chokes and the 165 main jets.

Bob appears to be older than I am!
Daniel Thirteen-Twelve

Having met you Daniel I think of that there is no doubt!
Robert (Bob) Midget Turbo

Bob -

I don't understand when you say "As I wrote in the article ... using the rules of thumb for jet sizing ... I always end up with the primary 1 or 2 sizes too small." I can't see where it's mentioned in the article and when you use the word 'primary', is this the main or the idle jets?

Your principle of 'choking down' seems highly logical and presumably not more advocated because of the ingrained ethos of Webers as race carbs. I have a 40DCOE with 34 venturis which I'd like to try - would you have a recommendation for the same spec as for the 45? (The engine is actually 1301cc (+30) with a Kent 286SP cam.)

Tom
Tom Coulthard

Bob & Daniel

Thanks for your responses. I have dismantled my carb a number of times to check passages and jets are clear, that all works correctly and then reassembled carefully. The aux venturi is assembled the correct way round and the car accelerates up to 4K although very roughly. fuel pump is new and delivers a healthy gulp of petrol. Float level at 12mm is correct for this model carb. Althogh symptoms suggest fuel starvation, fuel delivery and level are OK. I am convinced problem lies in primary choke/main jet circuit. I think next step is to order 34mm choke tubes.

I'll let you know how I get on.

Cheers

Philip

P.N. Baker

Philip.

First of all, I would move from the 38mm chokes down to 34mm and then alter the jetting and emulsion tubes to suit.

Unless you are a very experienced tuner of these carbs with a massive jet and tube selection, I would take it to your local rolling road and get them to set it up (providing they are very familiar with the Weber DCOE carb).

First of all though, make sure you have your ignition system set up correctly.

Mark.
M T Boldry

Can I suggest you make a visit to the "sidedraft central" group on Yahoo. They are a fantastic DCOE and DHLA resource and have solved problems on my twin DHLA lotus engine easily..
Cheers
Tim
T Dafforn

Sorry Tom I mislead you slightly, Normally using the rule of thumb that I learnt many years ago I end up in reality increasing the primary(idle) jets slightly and also the same for the main jets. So I got confused with other articles I have written on setting up the Weber and thought I had stated ths on the MGCC Midget register website. The advice I always give is if buying jets buy the size calculated along with the next size up so you save on postage. :-)

Tom a common misconcepton is that a 45 is too big for a 1275 A series. This of course is nonsense in fact many Mini modifiers use twin 45s :-)

The size of the carb is determined by the choke size, therefore a 40DCOE with 34mm Chokes is the same as a 45DCOE with 34mm Chokes. So from your point of View Tom a 40DCOE with 34mm chokes would be ideal.

Mark a rolling road is a great tool, but what did you used to do 20/30 years ago? We used to set things up by feel. If done correctly it is a good enough technique for a road car. The only time my MGA has been on a rolling road was when I made up the numbers for Dennis at Aldon's. They confirmed it was fuelled correctly. :-) However if someone is not happy with their ability to set up their own car then a rollng road will help enormously.

Back to poor old Philip as agreed with by Daniel the setting you have in your weber are more than capable in fuelling the engine. Something else is wrong, and you need to find it. What carburettors were on the car before you fitted the weber?

Robert (Bob) Midget Turbo

Robert.

20 years ago I HAD my own Rolling Road.... A Clayton 200bhp water brake linked up to a Sun 1212 Diagnostic Tuner.... so I used that... lol

However, I see where you are coming from and yes, I can, given time and a good selection of jets and tubes, set up a single or bank of Webers (or Dellortos) by 'feel' and 'dead cuts'.

Mark.
M T Boldry

Bob,

The car was running acceptably on twin SU HS2's up to approx 5500 rpm when power would tail off, hence the switch to Weber.
One other peculiar fault with this carb which I have not mentioned in previous posts is that the cold start circuit when actuated has the effect of WEAKENING the mixture. It is as if operating the cold start mechanism opens a huge vacuum leak in the inlet tract and kills the engine. Again, I have checked the jets for blockages, they are reciving petrol but this fault persists. Don't know if this has any relavance to the lean condition in mian jet circuit.
I have that sinking feeling in the pit of my stomach that the expected precision Italian carburettor has turned out to be a Spanish Friday afternoon job!
As for the rolling road, I am aware this is the the optimum tuning method. Just two drawbacks, I want to satisfy myself that the carb is at least functioning correctly before shelling out on a rolling road session (clearly I am not satisfied yet). Secondly,I live in Cyprus, the facilities which we take for granted in the UK are not neccesarily available here. I very much doubt that I will find here a rolling road tuner with a full set of Weber chokes and tubes. As Bob says a road car can be set up by feel; that will have to be good enough for me.

Philip
P.N. Baker

No such thing as a Friday afternoon job with the best carburettor in the world Philip, something is very wrong.
Where did you get the 45 from?
Have you a picture of the carb installed?

I suspect this is a very easy soluton if someone was stood next to the car.
Robert (Bob) Midget Turbo

Philip,

I don't wish to doubt your abilities, but DO you understand how the DCOE works?

Before you can even attempt to set up one of these, you MUST have at least an understanding of them.

You haven't mentioned anything about emulsion tubes yet, so give us the FULL spec of the carb and engine and lets start from scratch....

Mark.
M T Boldry

This thread was discussed between 27/09/2009 and 04/10/2009

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