• oakley
    18 December 2007 at 08:53 #47078

    I have asked this before but, not yet being familiar with how to use this Forum, it was in the wrong place. Therfore, my question again in a new thread. The problem is that the downdraft SU carburetters (see image) on my 2L engine “spit back” quite a bit of petrol (in small clouds of very tiny droplets) through the air inlets when the engine is running, especially when the car accelerates. This creates a highly explosive atmosphere under the bonnet and a nasty petrol smell. As I am currently rebuilding the engine I would like to solve that problem – only I haven’t a clue what causes it. The car ran very well in spite of this and the valve overlap was 15 degrees. Ideas anyone?

    Attached files

    Colin M34
    26 December 2007 at 13:13 #47094

    Hi there. Looks like a very tidy job you have done with your conversion. Mine was one of the first. It’s wonderful and transforms the 2 Litre – I have been running on it for about 12 years. I would be happy to discuss your problems off-line and you can find my phone number in the register of club members, M 34.
    Colin Mallett

    27 December 2007 at 07:40 #47097

    I will contact you after new year Colin – many thanks!

    Roger Seabrook
    21 January 2008 at 18:17 #47125

    Have you cured the problem yet? It sounds like valves sticking or not seating properly.
    Did you make your own inlet manifolds? If so, can you please advise where I can get some pipe, and get it bent to 60 degrees?

    Best wishes,

    22 January 2008 at 08:53 #47127

    Unfortunately I have had no more reactions with advise how to cure the problem. However, I have discovered that the engine has rather unusual camshafts with very flat cams which means that the valves stay open longer. This could very well be the reason for the petrol “spit-back” problem. I am now replacing them with new camshafts supplied by the club – they are quite diffrent in shape with much more rounded lobes. I expect that this will take care of the problem.
    As for downdraft carburetter inlet manifolds – they were done by Maniflow in Salisbury – tel. 01722 335378. They know exactly how to make them and often work with Burlen (carburetter manufacturers).

    John Hugh
    23 January 2008 at 10:25 #47132

    Oakley, can you please keep us notified how you get on? I have the same symptoms with my 2L, though it has just a single SU in the normal place. The cams I’ve inherited have the flat-topped profile you describe and are timed symmetrically with about 40 degrees of overlap which seems OTT to me though it is, I understand, a recommended change to the very conservative Lagonda setup.


    26 January 2008 at 17:44 #47147

    As I said above, if the lobes are very flat the valves stay open longer. Especially with much valve overlap, this causes petrol to be pushed back through the inlet valves and then the air inlets of the carburetter; no matter if it is sidedraft or downdraft, although it would be worse in the latter case as the distance between valves and carburetters is shorter.
    The solution is to buy camshafts from the club which have rounded lobes (see picture in my posting below). I could also recommend to reduce the overlap to about 24 degrees or so – more overlap is only useful at very high revving; at lower speeds it actually works counterproductive.

    Tim Wadsworth
    26 January 2008 at 22:32 #47148

    I would be very surprised if, on it’s own, the change of cam profile solved this problem. The square top cams go back to pre-war days and were standard for 2 litres for many years. I must have run them in my car for at least 30 years without a spit back problem. About 8 years ago I redesigned the cam profile from basic principles and came up with a more rounded top profile and a slightly concave flank. This was adopted by LMB (with my agreement) and subsequently the club produced new shafts with a similar shape. I run with 45 degrees of overlap. (IVO 20 BTDC EVC 25 ATDC) and the car is totally tractable, ticks over sweetly at 600 rpm. It is true that this amount of overlap does not enhance the performance below 2500 rpm, but after that she pulls strongly all the way up to 4500. I do think you need to rev the 2 litre engine to get the best out of it, strange for such a long stroke engine.

    27 January 2008 at 09:44 #47149

    Well, the engine is now being rebuilt and we’ll see if these new camshafts will solve the problem. Everyone who is working on it says it will. I too believe it is the logical answer. In the 1930’s, flat-lobed camshafts would not have been a problem with side draft carburetters and the standard valve overlap for the 2 Litre engines being only 9 degrees. However, I’ll keep the Forum posted. Meanwhile, here is a picture of the two types of camshafts – with flat and rounded lobes.
    Personally, apart from the spit-back problem, I find much valve overlap and high engine revving on a long stroke Lagonda 2L when it is only used on the road quite unnecessary and it is not good for the engine. Also it makes the car less original and increases petrol consumption. I seldom rev my engine as high as 3500 and the car very nicely keeps up with modern traffic. At 3100 revs in forth gear (wonderful relaxed cruising) it does just over 70 MPH – quite fast enough for me! (And indeed the law).
    If one races the car it’s a different matter and these modifications make sense. Besides, everybody has his own idea how to use or alter his car – I respect that.

    Attached files

    Colin M34
    27 January 2008 at 22:33 #47153

    Thanks for the interesting post from Oakley. I completely agree with you observations?.my car sparkles with the downdraft head!

    Thanks Phil Ridout! To those who do not know the late Phil Ridout, he was a long term 2 Litre owner and telecommunications researcher. I used to work with. His retirement project was to design the downdraft 2 Litre head and we spent many happy hours over beer discussing the project. Wessex Workshops took his design into production and their product is a great credit to them. I have had one on my car since 1994. Phil passed away a few years ago aged about 83.

    I agree with Oakley that 3100 revs in fourth gear gives wonderful relaxed cruising and you can belt along with all the law abiding drivers in the middle lane on the motorway. This is quite fast enough for me as well! In fact I get scared going much faster because fools cut you up and I feel very vulnerable with excellent but still 1920?s brakes. Much more fun is that the car is GREAT at 2800 rpm because you can put your foot down and roar past lorries with plenty in reserve in top ? up to 3500 rpm – and then relax back to 2800 rpm when the road is clear.

    What makes the downdraft head spectacular is the effect it has on hills. The classic 2 Litre problem is that an unmodified car runs out of puff up a gradient. Not with mine; drop into third, snick into second – spin the engine to 3500 rpm – and as the car speeds up, go into third again when it will happily accelerate to 3500 rpm and maybe a touch more. While still going up the hill, having picked up speed, my car will want to go back into top and positively sparkles when it drops down to say 2500 rpm, and you say to yourself ? that was fun!

    I agree with Oakley that sustained high engine revving on the road is generally unnecessary, though having more revs in reserve makes the car more useful in modern road conditions without straining it too badly. With valve timing, I have done the usual 15 degree overlap but have not got round to changing my camshafts. The main difference is that generally I only get 19 mpg which is a bit disappointing with the 25 mpg I used to get. I might change it back and see if I get back the MPG without loss of useability.

    So why did Lagonda not put the downdraft head into production? Phil reckoned that in 1930 it was more fashionable to add a blower, and transforming the unblown car would have confused buyers. Then later it must have been very much cheaper to buy-in the Crossley engine than continue with making the 2 litre engine, and of course by 1932, 6 cylinder engines were more fashionable than 4 cylinder ones.

    The 16/80 engine loves to rev but this should be a topic for a later posting.

    Regards to all Colin Mallett M34

    28 January 2008 at 09:07 #47159

    Colin, thank you for your reaction. You say that you might “change the camshafts back” – my question is which are you using now? I presume it’s flat lobed ones; did your engine ever (or still) suffer from the petrol spit-back problem?
    As I wrote earlier, with the flat lobed shafts I got this problem of petrol spit-back, which of course resulted in much fuel consumption, and indeed a very inefficient engine.
    If 100% of the petrol is properly burnt it will enhance the performance and efficiency. I think that changing to round-lobed camshafts will make a world of difference.

    Tim Wadsworth
    28 January 2008 at 11:19 #47160

    I think we are all in agreement over this. The downdraft head is the starting point for any development of the 2 litre engine. We should remember that when Arthur Davidson designed the engine for the 14/60 it was only a “semi-sports” at best. Just 3 years later Lagonda were using the same basic engine to race at Le Mans and elsewhere, a tough call for a semi-sports ! We know they toyed with a new head casting but to produce it would have been costly, an admission that the existing head was inefficient and probably a surplus stock situation. Marketing dictated (as always) that a “blower” would be seen as more attractive and could, most importantly, justify a premium on the price. In any case continued development of a 4 cylinder, when 6 cylinders were “the thing” was hardly justified.
    We have to be very thankful to Phil and Peter for rectifying the situation with a product which, in all ways, is much superior to the original. Thanks also to Arnold for setting all this out so clearly in his new book (highly recommended)
    Incidentally, due to that one illustration of a downdraft head in the 1930 2 litre Special catalogue, the VSCC allows our cars to compete as “modified” rather than as specials.

    Julian Messent
    28 January 2008 at 13:03 #47161

    hi all,
    Something that has been missed with regards to cam profile design is this.
    The cams that Tim designed and we made were designed with concave flanks, this allows the engine to perform much better than an engine fitted with flat sided flanks (ala the club profile) due to the fact that it “tricks” the engine at low revs into thinking that it has mild cams with less overlap fitted when in fact they have quite heavy overlap and high lift.
    please remember that timing figures and lift are not everything, Flank shape is VERY important if not more important and Tim worked very hard to develop this profile for just this reason.
    And because of this unusual profile they are rather expensive to manufacture, but so much better than having a slow 2 litre Lagonda!


    28 January 2008 at 17:22 #47164

    All very well, but none of you have offered a real explanation for the petrol spit-back problem. And if I myself, after much thought, come up with the very plausible answer that it might be the combination of downdraft carburetter/valve overlap/camshaft profile, all I get from one of you is: “I would be surprised if, on its own, changing the cam profile would solve this problem”. I already knew that and it is not what I said.
    The fact that we all agree on the merits of the downdraft head is not the issue. What we are discussing and concerns us now is applying this cylinder head the correct way, with the right camshafts, valve overlap etc. and getting the most out of it without creating a modern mongrel. I have tried to help those who have the same problems (H45 John). And to be honest, with all respect for Arnold, he doesn’t “set it out clearly” at all; actually he hardly mentions it in his new book. He only claims that none were produced at the time and that the picture is a mock-up. However, I myself know the location of an original 1930 Lagonda downdraft cylinder head.
    Anyway, Colin – I’d love to have your answers to my qustions above.

    Colin M34
    29 January 2008 at 09:16 #47165

    Hi folks,

    Some more thoughts….take a look at this graph I did in 2003. The green line is Tim’s profile as supplied some time ago – is this still correct Tim?

    The black line is a proile given to me by ‘John’ , another 2L Lagonda owner curently not registered on this site – (I will call him up and ask him to join)

    The green line is my profile. Ignore the higher lift – I should have normalised it. I believe my camshafts are original 1927 speed model ones. Anybody like to comment on this graph?

    I think I will save up for a pair of the ‘Julian’ cam shafts.

    By the way, can others tell us hat MPG they get?


    ps I cant remember any more about this – think I was resetting my camshaft for 15 degrees BTDC so don’t read too much into any of the figures.

    Attached files

Viewing 15 posts - 1 through 15 (of 17 total)

You must be logged in to reply to this topic.


Request to join the Lagonda Forum

To avoid rogue requests we are currently manually approving all forum applications. Please fill out your details below and we will forward a link and password to complete your application.