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MG Midget and Sprite Technical - help ID-ing Twin40's and a crankcase pressure ???

Hi all,

Having a few issues with the new twin 40 setup on my 1500 midget, to help resolve this I am trying to ID the exact model of 40 DCOE's that I have, The stamp is 40DCOE126 with the part no. as 9H. Any ideas where I can track this down, I know it's an early style one but not sure just how early!

Secondly, on first start session of a brand new rebuilt engine, I noticed a reasonable amount of oil vapour coming out of the crankcase breather, I know some oil vapour should be visible from this point if all is ok, but how much is too much? I have yet to run the engine for more than a minute or two whilst trying to get a decent carburation setup, but some oil has been spat out as well as the vapour. I was originally using an alloy rocker cover but changed it to the original which has a small baffle in the top of it, am I being over paranoid about this or rightly concerned?

As always thanks for any and all comments :)

Dave



D Prince

Hmm not sure of that Dave and I wondered if hey were series 12s?

As the numbers only indicated the "extras" fitted to the carbs (IMO) why is it necessary to identify the exact series?

Do you think the vapour coming out of the crankcase is just assembly lubricants and dampness being burnt off the new engine?
Robert (Bob) Midget Turbo

Hi Bob,

The only reason to check the specific series of carb is to check base settings against the Haynes Weber manual.

At present I am having real trouble sychronising the 2 carbs, front carb (cylinders 1 & 2) is spitting the odd flame from ram pipes, yet check plugs and cylinders 1 & 2 are fine 3 & 4 are black. When I pump the throttle linkage and look down the ram pipes you can see the fuel pumping into the venturi of front carb but less so in carb 2. Also after running, if you open the throttle with engine off, carb 1 releases much more vapourized fuel mist than carb 2, 2-3 times so. These results suggest there is fuel reaching both carbs , though probably more to carb 1 than 2, yet spark plugs contradict this!!!!!

I have checked all parts of the ignition system, the timing and the compression, so I am sure it is down to the carb setup. The jetting is luckily as per Telegizers, and others, recommended base settings for my setup, and both carbs have been rebuilt with full service kits. I am not trying to get perfect settings, I am booking in with a rolling road for this, but simply running happily on all 4 cylinders at idle and through most of the rev range!!

Phew!!! glad I got all that off my chest ;)
Any help welcome

D
D Prince

Dave,

Maybe call the UK Weber people and give them the tag details. I have had 6 Dellortos from Alfa Romeos and giving the tag detail to Contact Developments, the Dellorto Consessioners at the time, they knew what it was originally fitted to and the specs.

On the subject of a new engine build and blow by, I have done a couple and the blow by in the first hundred miles was seen to be quite high. In my experience the rings had settled in by 100 miles but prior to that the oil being blown out into a catch tank was high. From my limited experience the initial 20 miles was chucking oil out, the next was 1/2 the first 20, etc, etc, to the point that by 100 miles of use oil caught in the catch tank had become negligable and reached a normal level which was maintained into high mileages.
David Billington

It suggests that either the throttle pump strokes are not the same or the spill back jets are not the same size. I would end to go and check the throttle pumps and compare one against he other. What size chokes do you have fitted?
Robert (Bob) Midget Turbo

Chokes are 30's, I have to admit you lost me at throttle pumps, these are my first webers! I would say SU's were so much easier but i'd be lying, Lol!

I assume you mean what pumps the fuel into the aux venturi when you work the throttle (sounds so obvious when put like that!) how is this adjustable?

D
D Prince

Hi Dave, I used to have twin 40's on my 1500 and found them to be excellent top end power producers with bags of torque and brilliant throttle response but a little problematic at small throttle opening cruise. Originally they had 32mm venturis and it was a little better when I fitted 28mm venturis but it was never perfect at small throttle openings cruising at 30 to 40mph. I would get occasional pops, spits & backfires at the trumpets. Nothing too terrible but a little irritating nonetheless.
The pump jets are often not precisely the same in terms of the amount they deliver. They only provide a boost of overrich fuel for when you are snapping the throttle open so dont need to be that exacting. Having said that, if they are noticeably different by eye I'd say that they need further investigation. Listen to Bob(R) and Dave as they are known old hands with sidedraft Webers :o) One thing I will say though, is that the twin 40DCOE setup on a 1500 is very much different in behaviour to the single 45DCOE on an A series due to the higher gas velocity so anyone used to setting up an A series may not necessarily be the right person for getting your setup right. Try finding a RR used to setting up twin 40's or 45's.
As for the breather producing vapour, mine has always produced a fair amount of oil vapour and a constant fine mist of oil. It will no doubt get better as the rings bed in but I believe these engines are rather susceptible to blowby due to the piston & ring design. They are often a little smokey too due to the lack of seal on the valve guides.
A catch tank will work wonders I'm sure. You can redirect the caught oil back into the sump for less aggro and then just either redirect the fumes to atmosphere via a filter or feed it into the air filter - depends on whether you want to find maximum power from the octane of fueling or whether you want it to comply with MOT standards ;o)
Most of these little niggles are some of the reasons why I went ahead with the EFi design on my car. I saw it as a way of combining the more precise low speed fueling with top speed gas flow. It took a while to complete and perfect but is very reliable now.
BTW what is the spec of your engine? Head spec, cam timing, CR, exhaust etc...
Bob T

Thanks for the info Bob,

Specs for the engine build are

+0.040 overbore
fast road 83 cam
Stage 3 head
Highlift roller rockers
self made 1.75" straight through single box race pipe
4-2-1 manifold
Cam timing @ 103
Electronic ignition
New cap, rotorarm and dizzy body
Sports coil
the list goes on but this is most of it!

D
D Prince

Bob,

Your experience with small throttle openings issues sounds like what a mate had on his Caterham. The carbs were basically OK setting wise but needed some small tweeks. He was too tight to go to a rolling road so set about it himself, he read a lot on the internet and found a load of bollucks but the good info is out there. He digested the info and diagnosed the issue was the transition region between the idle and main circuits and IIRC the idle and idle emulsion tube needed changing. He got it almost bang on from a small selection but didn't want to buy the last size he thought was required so drilled the emulsion tube IIRC about 0.004" larger and sorted his issues.

I'll most likely speak to him this evening so will see if he has any useful links bookmarked.
David Billington

OMG too many Daves...

Dave P, that spec is pretty similar to mine apart from the roller rockers and the fact that your head is probably a little more developed. Mine is ported quite nicely with bronze guides but with std valve sizes. Same cam though which is a really nice one in this engine. With carbs it does almost nothing until 2500rpm then takes off like a scalded cat to 6000rpm. Looking at the spec and quoting purely from my own experience, maybe something to consider for the future is a programmable ignition setup such as a megajolt with EDIS wasted spark. That mod had made my car so much more smooth and driveable on the road at low speed, certainly better than any dissy could ever hope to do. Major transformation - drove like a different car.

Dave B, yes I did get the carbs set on a RR and they did run really well at all speeds but it was at slow cruise it played up with an odd pop every now and then. Just lightly feathering the throttle at tiny openings where I had a large overlap on the cam combined with the butterfly edges just hovering over the progression holes. I was told by someone far more knowledgable with Webers that the way to cure this was to sharpen the edge of the butterfly plates to a knife-edge so that they were angled away from the progression holes allowing the fuel to drop in more freely. I never tested the theory as I had already started the EFi project but it sounded feasible. Besides, when you have large overlap and slow speed combined, nothing will ever get the fueling as precise as electronic injection ;o)
Bob T

BTW Dave P, my engine looks nowhere near as pretty as yours - mine is more a Frankenstein's lab engine LOL
Bob T

Well a day spent fiddling all to no avail!!!

I started to doubt myself in the morning so did another compression test across all 4 cylinders, 1 & 2 consistantly 135-140, 3 about 125 and 4 down at 110! Tried putting oil in top of 4 to check rings, no change on compression test.

Head off to investigate...headgasket intact but god how close are each pair of pistons to each other!!! Checked the valves by filling with a solvent and checking for leakage on a bench, no worries there. Head back on compression now 135-140 across 1,3 &4 but best I can get with all tappets set is 120-125 on 2.

After this time spent on the head I decided the compression was close enough to bolt the carbs back on and see what if any effect this had had. None is the answer, 1&2 fire fine 3&4 lumpy, so on to the carbs I guess. Fuel supply checked, definately getting to carbs at a decent rate. Check float heights, jets and emulsion tubes which are all blown through with an airline, all are the same between carbs.

Decide to try and start the engine with only 2 leads attached, start with sparkplugs 1&2 attached 3&4 earthed, car starts on the turn and idles away, no revs required. Reverse the sparkplugs and try again, wont start, will with full choke and throttle!

What am I missing here guys? The jets etc are the same between carbs yet each carb differs so widely from the other in fueling to the cylinders.

As picture often speak louder than words, I have attached a pic of the tops of the pistons when the head was off 1 on the left 4 onthe right. This is after a few starts and a couple of 3-4 minute warm ups.

As always advice welcome :)

D



D Prince

Both Davids and Bob.
I think many people are totally unaware that 95% of the running with weber carbs is done on the pilot jets. These therefore are extremely important and are very rarely selected correctly

In Bobs case whilst I understand what he is suggesting sounds to me like a simple case of lean mixture on the pilot jets and a easy upsize of the jets ought to have cured it.

All the webers I have set up over many years have always needed a slightly larger pilot jet than the ones that were fitted. (generally larger than the rule of thumb sizes we all use for jetting a weber.)

As Bob says EFI should always be better at metering fuel than a carburettor but a weber can compete with EFI on outright power without a problem. However the "quality" EFI will be overall better throughout the rev range. Just need to ask Bob T if he megajolt is actually a quality piece of kit? I must amit I have not tried one but have heard differing reports.

Dave P the reason I mentioned the throttle pumps is that when you press the throttle it is the fuel pumped in by the throttle pumps that you can see evapourating. So in essence these should be the same and if they are not then when you open the throttle the car will stubble due to inequality of throttle pumps. I think that haynes weber manual calls them accelerator pumps so check the manual to understand their action. There is a pump and a jet associated with these things.

If one of the carbs is spitting back through the ram pipe then that is a sure sign of weak mixture. If it is in normal running then it is the primary jets that are possibly too small.

The other vitally important thing with multi carb set ups is to be absolutely sure that the fuel level is identical in each carburettor. Check the float levels carefully to be certain.

I placed his on the MGCC website which deals mainly with a single weber that twins although it stacks up the same. I have always done as is written in the feature without real problems
Robert (Bob) Midget Turbo

Sorry forgot to place the link
http://www.mgcars.org.uk/mgcc/midgetreg/Weber.shtml
Robert (Bob) Midget Turbo

Dave,

Did you get the carbs new or 2nd hand. On my mates Caterham we noticed that 2 of the idle mixtures screws on the DCOEs made no difference to the running, turned out the needle end had seized in the body and broken off. Most likely due to someone in the past closing them down to tight as they are very thin.
David Billington

Dave Prince,

You haven't checked the things that Robert (Bob) told you too and so it's not surprising the problem remains.

The pump jets are on the side of the carbs and the bleedback valves sit in the bottom of the float bowls. The first thing you need to check is whether each carb has the same sizes.

The other thing to check is that the aux venturi are all in the right way up though I'm not sure if it's possible to fit them upside down on 40s.

Daniel Thirteen-Twelve

<<Just need to ask Bob T if he megajolt is actually a quality piece of kit? I must amit I have not tried one but have heard differing reports>>
Bob, I have heard that rumour banded around too but never really understood it. Since the megasquirt and megajolt are self built units/systems, the build quality is down to only one person at the end of the day!
My own system has been installed and running for three years now without much of a hiccup. I had the engine cut out a while back when the ECU lost power but that was due to one of the tags at the fusebox snapping off.
Bob T

What I heard Bob T was that it was not the quality of the build but the specification of he "brain" was not powerful enough to keep pace with the function...
I took this with a pinch of salt as the enthusiasts explaining this had a good motive? They were all trying to push their own products. :-))
Robert (Bob) Midget Turbo

I should imagine MegaJolt/MegaSquirt have put a reasonable dent in the sales of some of the more advanced aftermarket ECUs. (whilst extending fully mappable ignition to a huge number for whom it would have been seimply out of reach before)

If they could sort the injection for the A/B series siamese ports I should imagine they'd be even more people looking at it.

A mate at work as a comeplete megajolt set ready to go onto his Mini....only it hasnt been out of the grage in 3 years. I keep pestering to let me have the whole lot cheap...but no joy yet :-)
Dean Smith ('73 RWA)

Dean, <<If they could sort the injection for the A/B series siamese ports I should imagine they'd be even more people looking at it.>> The problem isn't the ECU, it's the inherent features of the head/cam design - easy fix, fit an eight port head :o) You could of course rig up a MS kit to a correctly sized single point EFi throttle body.

Bob, as you spotted there is usually other motives in those assertions. The spec of the ECU handles the instructions and cycles of the engine perfectly well and anyone that says otherwise is taking b*lls. The MS1 chip can do EFi, ignition and lots of other things like traction control very easy to a rev limit of about 8000rpm. As the MS hardware designers suggest, for rev limits over 8000rpm on stuff like bike engined kitcars you may prefer the MS2 spec chip just to be sure.
The people who knock the MS kits with no evidence are really irritating. Megasquirt and Megajolt have opened up a whole new world of engine management by taking an "open source" attitude to its development. In such circumstances, the challenge is always "if you know better, modify it and pass on the knowledge for everyone else". So, if you find a real problem it's up to you to find a way to fix it not whinge about it! This is all good for enthusiasts and developers alike. The only ones losing out are those producing overpriced competitors...
Bob T

Bob - I know.....but for example the canems link that was posted a few days back has a several examples where its successfully been used on std 5 port heads.

e.g.
http://www.canems.co.uk/canemsjenvey.php

Whilst the canems if far from the most expensive ecu - its still more than a MS/MJ
Dean Smith ('73 RWA)

Yes thats right. I ended up buying the Weber programable kit and I am very impressed with it and I am still learning how programmable ignition can be used to easily increase BHP potential.
That said I think the benefit of EFI is not enough for me to try it on a 5 port A series head although as Dean points out a few Mini owners have apparently succeeded in getting working systems on their engines. Including one chap who lives near me who built his own system completely and has promised me the detail to put on our website. Just need nice weather to return to be able to go and have a long chat in his back yard. :-))
Robert (Bob) Midget Turbo

This thread was discussed between 22/01/2009 and 25/01/2009

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