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MG Midget and Sprite Technical - Help needed with DCOE 45 please

I have a Mk1 midget with a 1275 engine - moderately tuned - big valve ported and polished head, 649 cam with 1 - 1:5 rockers, extractor manifold, virtually straight through exhaust and a 45 DCOE on a swan neck manifold (Oselli?) Crappy oiled gauze filter (no room for a K&N?) which may not filter well but doesn't appear to constrict its breathing.

It runs superbly at all speeds providing I don't use more than half throttle although it does seem a little hesitant to rev from 4.5 k on.

If I use full throttle it spits back and really doesn't want to know.

I am guessing that it is too lean at full throttle???

I have done some research and the following people recommend the following as a starting point.

45 DCOE ACTUAL S/PRO MOSS VIZARD STA/TON

Aux Venturi 5 3.5 4.5 4.5 3.5
Choke 34 36 36 38 36
Main jet 160 165 150 165-170 165
Air correction 170 175 180 180 175
Emulsion tube F16 F2 F2 F2 F2
Pump jet 50 50 50 45 50
Idle jet 45F9 55F9 50F8 50F2 55F9

Universally they all recommend a F2 Emulsion tube - I have a F16 which as far as I know is the starting point for a 1500 BL??

The engine is in good mechanical condition and there are no air leaks around the manifold area, none of the jets are blocked and the carb is in good condition.

Any suggestions most gratefully received.

If the chart above doesn't display properly I can email the Excel chart.

I know I should stick it on a rolling road but as far as I know the nearest roller with a technician who can really 'do' Webers is 240 miles away!!

I don't mind spending a few quid on bits to get it right by trial and error.

Thanks in advance.
David Banks

OK that didn't display at all well!!!

I will try again - see attached image.


David Banks

Hello David you are talking my favourite subject

Here is my rule of thumb for the weber carb that I have published on the MGCC Midget register website.

http://www.mgcars.org.uk/mgcc/midgetreg/Weber.shtml

From that you will notice that I think an F16 is OK so that is NOT your problem

Also you are using smaller venturis than our tuners listed above although 34mm should be ample for your engine. However with smaller venturis a jet becomes more efficient so generally the jet size will reduce as the venturi size reduces so yours is not a straight comparison with any of those listed above.

All that said if anything your mixture will be too rich yet I think you have identified that in reality the engine is weak at anything over half throttle or 4.5KRPM. so what does this mean?

Well the points you make 1/2 throttle and 4.5KRPM are just about the changeover point from primary to main jets so it sounds to me as though you have a problem with your main jets. Generally I would expect this to be caused by incorrect instalation of the venturis.

So who built the carb? Give us also some history of the car and problem.

If the venturis are not installed correctly the main jets become blocked and the correct installation is secured by the screws below the barrel, I have seen these fallen out or not installed.

It should be possible to rev the nuts of the engine and see fuel sprayed into the carb throats from the centre of the flow tubes down each barrel this ought to answer the question as to whether or not they are installed correctly.

Please feedback and I will answer shortly.

Bob
Robert (Bob) Midget Turbo

Thanks for that Bob.

I haven't had the car long and the carb was set up by the PO - I have just started stripping it bit by bit and correcting problems as I go. The Venturis appear to be installed correctly and the retaining screws were all correctly aligned and wired shut.

It has had this issue since I got it - it hasn't got better or worse.

Not sure if I can shed any more light on it. I can't start the engine now so i cant see if both mains are spraying juice in.

Thanks in advance.

David Banks

Thoug i've never had to do anny thing to the inner workings of my 45 (thankfully) i have had some experiance with diferent filters and manifolds

if you 45 is on a swanneck there should be more than ample room for a k&n with sub stacks or smal rampipes

if ther is no place then go for a good pair of rampipes with filter socks
filters slightly less but does not have fitting issues

if you realy want to spend monney
http://med-engineering.co.uk/product_details.php?p_id=193&vlang_id=237
is the way to go
Onno Könemann

Try Mike Stewart Performance Engineering up at Errol (near Dundee) 01821 642576. . He really knows his stuff.
He looks after a lot of Scottish competition cars, plenty of them A series engined.
I was put on to him a couple of years back. Hes very good. No financial connection etc
Neil
na munn

I'm always doubtful if the carb is too blame for this kind of problem.

I see you have a 5.0 Aux vent and a 34mm main venturi and as Bob says 34mm is small though this won't cause the problem your car's engine has.

Are you sure the cam is a 649? Isn't this an out and out race cam with 300 degrees duration?

I'll disagree with Bob that 4,500 is just about the point of changeover from primary to main jet circuits. I'd expect the changeover from primary to main to be at 3500 if not 3,000 rpm.

Have you got a photo of the installation?

Can you do a plug cut at the point the engine stumbles and see if what colour they are?

I'd fit F2s on the basis that they are known to work ok and are a known quantity once you look at the other stuff.

Daniel Thirteen-Twelve

Hi all - it IS a 649 cam - about 300 degrees duration and I kind of think of it as top end of fast road or mild race cam.

I can take a photo tomorrow if that will help although the carb is on the workbench at the moment.

I did do a plug cut and they were all digestive biscuit brown - not white and not oily.

One other thing that occurred to me was that the engine still uses vacuum advance although I cant see an obvious link between that and the problem - it does run superbly up to about 1/2 throttle which is more than enough for 99% of my road driving!!

David Banks

I've conflicting data on the 649 cam with a Kent catalogue stating it has duration of 287/281 while DV states 300/300.

How have you got the vacuum advance connected to the Weber? Assumedly you have it drawing on one inlet runner? I'd plug the manifold and distributor vacuum advance for a starter

You don't say what distributor you have but it should be designed for use with the 649 cam - Aldon red or something??

When you say it spits back when you use full throttle is then when you floor it from any revs or floor it from 4K?
Daniel Thirteen-Twelve

The vacuum pipe is connected to one of the inlet manifold branches.

The dizzy is a 45D with Lumenition electronic ignition, silicone leads and new plugs (BPR6HS) etc.

My normal driving pattern is to use up to 1/2 throttle - on the occasions I have 'nailed it' whether its low revs or mid revs - it spits back and doesn't accelerate - back off to half throttle and its away - although its worse when nailed from mid revs than low revs.

I am still considering the crappy air filter - from what I remember the distance from the carb throat to the cover of the air filter should be at least the same as the diam of the carb throat - makes sense. The one I am using is only 40mm and the throat is 46mm - would that be enough to strangle it when nailed??

Is it worth posting a couple of pics of it in its disassembled state or shall I put it back together and take some pics - what would be most helpful???

Thanks in advance.
David Banks

David
I had a problem just like this once.
A mate had fitted a brand new Weber on his MGB-- It would idle fine ,run well at part throttle but would just run off onto two cylinders when you nailed it.
It ended up being one of the secondry venturies fitted incorrectly--- It had the locating screw screwed into the fuel supply passage of the venturi and this left the blank side lined up to the passage in the carb body-------No fuel
Turned it half a turn to where it should have been and away she went.
I'm not saying this is what is wrong with yours but it might be if it's running off onto two cylinders----worth a look, Willy

That air cleaner sounds like it needs to live somewhere else---although I don't think that is causing your problem, it would make it run rich if anything not make it spit back
WilliamRevit

The 45 D dizzy is historical right. Fuel has changed though. So you HAVE TO CHANGE THE ADVANCE CURVE. I had similar problems. It vanished, thanks to FRM, with a 123 dizzy curve 5. (See one eyed frogeye). See if the 45D dizzy fires at 32 degrees BTDC at 4500 rpm with and without vacuum attached, while not driving so part throttle and much vacuum. And how new is your coil?

Flip
Flip Brühl 948 frog 59

Plenty of people running standard dissies with modern fuels and webers without these problems Flip so I am puzzled by that. Was your Problems EXACTLY as these and was your original dissy in good repair?


David can we just confirm what your problem is. If you have a weber fitted and you plant the throttle and the engine mis fires, backing off and slowly applying the throttle and engine speed increasing would be a problem with the throttle pump.

I assumed yours was not this and was in fact that when you try and get above say 60MPH or accelerate using high throttle settings that the engine refuses and holds back, maybe spitting and stuff.

This will NOT be caused by the air filter or even the dissy unless the dissy is faulty. Even a poorly specified distributor will not cause this to occur except if it is faulty or if you have NOT SET the IGNITION timing correctly. (Even with timing not set right it should still go pretty well). Did you set your maximum mechanical advance at about 32 degrees BTDC.

Also as Will rightly points out an air cleaner would need to be almost block to cause an engine not to rev well enough to reach higher speeds, a too small a filter will not cause these symptoms.

Finally all of the webers I have used I always used the vacuum plumbed into the manifold on one barrel or the other with no problems at all, you must remember with full throttle the vac advance should be negligable and is therefore simply a fuel saving device and is not an issue here. If you are concerned simply disconnect it and plug the hole.
Robert (Bob) Midget Turbo

If the plugs are NGK, which seems to be the case from the number then they are too hot a grade for the engine spec. You'll need to drop to a colder plug such as NGK 7 or maybe 8. This is something to bear in mind once you've got the engine running properly and I don't think is the current problem but one you've yet to get to...

You do have ram pipes on the Weber don't you?

when you talk about nailing at low revs or mid revs what revs are they approx to 500rpm? Also is nailing it pressing hard on the throttle or flooring it? Not least have you stayed with it to see if you drive through out and finally, what gear would you have been in.

I still recommend fitting F2 emulsion tubes.

Flip might be onto something about the coil. If it's past its past it will be higher up the rev range that you'll have problems.
Daniel Thirteen-Twelve

Why do you think the plugs are too hot Daniel? I believ that you should only go to a cooler plug if there is good reason, such as maybe pushing the CR boundry just a little or running at full throttle for long periods. Do you think there are other reasons?
Robert (Bob) Midget Turbo

Bob,

The plug needs to shed heat to prevent it being a cause of detonation, pre-ignition or being damaged. The amount of heat the plug needs to shed or lose will vary according to how hot it is getting in first place.

For an A series the standard plug is rated for standard power. As the power increases the plug needs to shed proportionally more heat but reaches a point where it cannot shed heat as fast as it needs to and so a cooler plug is required.

NGK 5 and 6 are for standard and mildly tuned engines. For this spec of engine which when running correctly ought, I guess to have peak power at 6K or higher and run to 7K the NGK6 will run too hot. I'd guess that running it to full power even for a few seconds (just to see what it'll do) will be enough to cause damage. The ultimate damage is to the piston crown.

Hence my recommendation to switch to NGK7 or 8.

What grade do you run in your turbo A-series? I have seen a turbo A-series running on NGK 9s.

Daniel Thirteen-Twelve

Standard Midget runs NGK5s, Standard Metro Turbo runs NGK 6s.
Daniel Thirteen-Twelve

Hmmm I use 6s in my turbo as I have never had detonation so unless I do it will remain on 6s
I have used 7s and 8s in race engines as they tend to create far more heat than any road car I would imagine. Unless of course the road car is used for long periods of time at speeds above 100MPH :-)then I would perhaps think twice!
Certainly if an engine is suffering detonation due to its CR or high performance use then a cooler plug is definately essential. :)

Never had to use a 9..............
Robert (Bob) Midget Turbo

BPR6HS? Don't you mean ES? HS are 12m length and are the type I use for the VW and the Morris Minor sidevalve.
rob multi-sheds thomas

Bob, you're just not trying hard enough!

I suppose it's a comment on my driving that I've run NGK8s for years on my road car! Since fitting the aluminium Pierce head I considered dropping down to NGK7s but since it doesn't foul the 8s I've left them in.

I've done long stretches at 100mph+ though as the motorways are busier even on a Saturday it's harder to do so.

The hardest work-out the engine usually gets is on the rollers and I did a particularly long final pull on the last session which the oil temp stood up to ( a concern of the operators).

There is a view that while you can hear pinking it is impossible to hear high speed detonation...

ES is probably correct as I couldn't find a reference for HS - not in NGKs anyway.
Daniel Thirteen-Twelve

Thanks Bob - by planting it I mean suddenly giving it full throttle - it does the same thing from anything over about 1500 rpm - I haven't tried it at lower revs than that as it is quite lumpy to start with. It makes no difference what gear I am in although it feels more laboured when in top.

If I keep to 50 - 60% throttle it will rev quite happily all the way up to its (self inflicted) rev limit of 6k although it progressively takes longer to get their as the revs increase - feels like it needs more fuel - suddenly giving it full throttle is when is spits back and coughs and actually slows down.

The maximum mechanical advance is set to about 32 degrees BTDC

Daniel - AFAIK the plugs are NGK BPR6HS but I am away from the car (by 300 miles) until Weds afternoon - I will check when I get back and let you know.

It doesn't have ram pipes - just this crappy air filter. I have ordered two ram pipes and a decent air filter as whatever the real issue is the current air filter is junk. (Don't worry - I will only change one thing at a time!)

The coil was fitted when the electronic ignition was fitted - about 12 months ago - I know that's no guarantee that it is OK

I haven't 'stayed with it' as my fear was that if its that lean and at the top end of the rev range I might do some serious damage. I have a real dislike of melted pistons!!

I will gladly fit F2 tubes but what else in the carb setup would need changing??

Thanks for all your help and advice so far....

One other thing I have discovered whilst investigating this issue is that the manifold is actually for a DCOE 40 - not a 45. The internal airway has been suitably flowed and matched to the carb ports but the studs have been butchered as they are a different spacing - they are splayed out from each other to fit the carb. I am intending to re-drill the manifold and helicoil it with the correct spacing. The misab plates are in good order but the thackeray washers are missing. I tested it for air leaks at tickover and found none but maybe it is leaking air when its hot and under load like when full throttle is applied ?????
David Banks

Your problems sound a lot like a DCOE with no ram pipes which is why I asked my question. Fit the rams and things will improve dramatically.

Don't expect an engine with the cam you have to take anything other than a kiss of the throttle once it's in any gear and rolling at 1,500rpm. It takes all the right parts to change that situation.

The engine sounds like it needs more air and not more fuel. More air comes with large main venturi but they won't easily work unless you really have that engine set up properly.

I think you'd be better off with decent rams and an average air filter - what rams are you buying?

Daniel Thirteen-Twelve

This I will find very interesting.

As Daniel is probably aware I am a total ram pipe skeptic and I believe they add next to nothing so if you can test them for us to see if they have any affect on this situation I for one will be very happy. In fact if they cure th problem I will be a born again ram enthusiast. :-)
Seriously report back on any progress as you say 1 thing at a time.
Robert (Bob) Midget Turbo

David, Bob and Daniel....

I've just picked up on this thread and whilst there are a lot of valid comments regarding air filters, coils etc, I think the problems are compound.

David mentions that the stud spacing on the manifold is different for the 40 and 45 DCOE.... NOT as far as I remember.

The Thackeray washers are missing.... I think the installation may be part of the problem here. As we know, the DCOE on most engines need to be mounted flexibly, so I would put new studs into the manifold with a little Loctite 270, fit the Misab plates then the carb, bolt it up with new rubber mounting washers and new nyloc nuts allowing a little float on the carb.

The debate regarding ram pipes is interesting, I have experienced, like Daniel, that an engine with a race cam will run better with rather than without them. BUT, here is what I think may be happening... The ram pipe on a Weber (not a Dellorto) holds the choke assembly etc into place and tight, if it's not there, vibration will cause a disrupted air and fuel flow at higher revs.

Also, as I recall, the swan neck manifold was designed for use on a Mini to allow the carb to be fitted without having to move the speedo. These were NOT the ideal manifold and did restrict flow and thus performance. on the Spridget,the Weber will fit with ram pipes on a long manifold with a slight modification to the top of the inner wing.

Once you have the completed the build of the carb and installed it correctly, then I would suggest you go back to the setting up....

I have had many years of 'playing' with these carbs and like Bob and Daniel mention, In my opinion they are the best.... You will need a very good selection of tubes and jets to get it perfect and a rolling road will make things easier, however, I have managed without and achieved good results.

Good luck and keep us posted.

Mark.
M T Boldry

OK - a bit more info and some photos - firstly - this is what I referred to as a 'crappy' airfilter. Its barely 35mm deep.


David Banks

These are the trumpets I bought - chosen as they seem to most closely match the ideal trumpet as described by Vizard - full radius and big mouthed (a bit like the Missus then!!)


David Banks

and with the sock fitted - although I am not sure about this sock as it seems to be too small????


David Banks

Now then - serious stuff - a few posts back I suggested that this manifold may have been for something else other than a 45 Weber. Now I have removed it and had a good look I am even more convinced.

Firstly the studs have been removed, the holes opened up and bolts put in as the distance between the mounting holes on the manifold is greater than the distance between the mounting holes on the carb.

Due to the close proximity of the swan neck the bolt heads cant sit flat resulting in the bottom bolt being angled upwards - see photo


David Banks

also see this photo


David Banks

My guess is that although the carb mountings appeared tight the carb wasn't sealing properly as there is over 4mm greater distance between the manifold mounting holes than the carb mounting holes ie it was effectively being tightened onto a wedge. (No wonder the Thackeray washers were missing!)

In addition the manifold outlets are strange - someone has started to machine them to match the head ports then realised that there wasn't enough meat to carry that through and stopped leaving a horrible lip - see photo.


David Banks

The size they started off out does match the ports in the head - see photo


David Banks

and here you can see the elongation of the holes to try to get the manifold to match the carb


David Banks

Does anyone recognise this manifold or at least confirm that a manifold for a 45 should fit better than this!!!

Anyone got a spare manifold for a Weber 45 DCOE they would like to sell??

Thanks in advance - you have all been most helpful and kind so far....


David Banks

it does look like it was for a weber but could it be for a 40?

the rampipes and sock around it look fine

you mention the o-rings where missing as wel

i think with this filter set up a proper manifold o-rings fitted you will be sorted
Onno Könemann

David,
i use a 7" Longman inlet with my DHLA45 & it leaves just enough room for a decent sized K&N.
I just don't like the look of those sock filters, i prefer something more solid than a piece of packaging foam; i've used K&N for decades & they have never failed me.
Brad (Sprite IV 1380)

Well I have convinced myself that I need a new manifold - I could make the old one fit but it would still be a very short swan neck and not really match the inlets on the head.

Question is whats the longest manifold I can fit and still keep the trumpets and either the sox or a K&N filter without modifying the inner wing???? 6 inch??

Any reccomended suppliers????
David Banks

this any good? it's 4.75" face-to-face and I just offered it up to a DellOrto 45 OK.


David (davidDOTsmithAT stonesDOTcom)

t'other end


David (davidDOTsmithAT stonesDOTcom)

David Smith - that looks perfect and I jhave measured it and it will fit - just!

Could do drop me an email with your price and address - my email address should be 'clickable' above.

Many thanks.

David Banks

David, That's a lovely C/T period manifold, perfect for your car.

May I suggest that you have the 'O' ring groves welded up and machine the face flat again, then use the Swedish Rings (Misab plates) and correct rubber washers with new nylocs.

Start a fresh with the set up and all should be well....

Mark.
M T Boldry

This thread was discussed between 21/11/2009 and 26/11/2009

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