Twin-choke Webbers

[Smegnoguk]

I have a Caterham Classic built by me in 1999 with two twin-choke Webbers.

When I first set the car up it was perfect. Even drove all the way from

London to N Devon and back without a hitch. Mysteriously, in 2000, it

developed a fault whereby one carb would cut out when warm making the car

undriveable. After two attempts the Caterham Factory identified a sticking

float in one carb and said they had fixed the problem. However, on the way

home I joined a 30 minute traffic jam and it was all I could do to keep the

car running as it got hotter and hotter. I eventually limped home and found

that the plugs where heavily contaminated with soot. Since then I have

attempted to re-tune the engine many times without success. I get the

engine hot, tune the idle screws according to the manual but it still runs

badly and the plugs are constantly sooty. Does anyone have instructions for

a fool-proof carb tune? I love my car and this is REALLY beginning to spoil

my fun! ☹️

 

 

[Super_Rich_Bernie]

My 1600 Vx car idles OK in most traffic, but the one time I got caught badly on the North Circular it strarted to get a bit objectionable - cleared OK when I got out of the traffic. I imagine this is the same thing - plugs sooting up.

 

It certainly sounds like the tickover on your car is far too rich. You could try quarter of a turn in on each idle mixture screw, and see if it improves things - if it does try another eighth etc. I know this isn't the proper way of setting things up - but if it makes a diffference you know you are on the right track. Make sure you don't wind the screws into their bases hard, or the carb body will be damaged.

 

The other option could be the plugs themselves - they will foul in traffic if the heatrange is wrong. I think the correct ones are FR8LDC Bosch - if the plugs are branded GM, like the spare set I have, the FR8LDC code is still stamped into the metal in small letters.

 

The previous owner of my car had it rejetted - and it runs very cleanly - with larger main jets and bigger chokes. The chokes which had been in the carbs were with the car - the finish on them is apalling with a nasty ridge in the casting - I looked at the new ones to see if they were the same - they are much better quality. Weber's quality control may be variable.

 

The weird jetting on these cars is presumably a function of the hideous Z shpaed inlet manifold needed to clear the diagonal chassis tube.

 

I live in Wimbledon and have used Southern Carbs nera South Wimbledon tube in the past. Believe Jam Mad had a less than satisfactory experience with them, however they did a good job setting up the pair of twin choke carbs on a Lancia Fulvia for me. May be worth a try if all else fails.

 

Jonathan

 

 

Jonathan

 

Edited by - JonathanG on 23 Aug 2002 10:36:05

[Casbar]

Take it to a rolling road, spend a £100 and get it tuned and jetted. 😬

[oilyhands]

Sounds more like the carb is brimming, if the float valve isnt sealing properly or is being overcome by the fuel pressure then fuel will brim in the chamber and spill out through the aux vents / main circuit which will kill the engine at low RPM/part throttle. This is a very common problem, you can spot it by peering down the mouths of the carbs at idle and seeing if you can see fuel spilling/dripping from the aux vents.

 

Do you have a fuel pressure regulator?..

 

Oily

[Smegnoguk]

Thanks for the help. In my motorcycle days (going back a bit to when a trip to the coast was an adventure what with clutch slip, exhaust pipes falling off, oil leakage on the the rear tyre, rampaging mods etc) I used to tune my Gold Star so that the plug was chocolate brown. Do you thing the same approach would be appropriate today JonathanG?

 

 

 

Edited by - Smegnoguk on 29 Aug 2002 12:32:53

[Mr Locust]

Make sure that the thackeray washers and gasket plates are tip top and not too tight. These provide isolation from engine vibration and gave me a hell of a game on dellortos fitted to a triumph engine. If in doubt 'rock' the carbs (filter end) up and down to ensure they are not tightened up hard.

 

Also avoid bright, plated thackeray washers. In my experience, they are not properly de-embrittled and are prone to breaking relatively quickly. Use black finish ones. Also avoid rubber 'bobbin' packers as these just seem to go hard quickly and lose their resiliance.

 

While I'm waffling - if the seal plates are o-ring carrier types, check that an o-ring hasn't been sucked in or spat out. I have always found bonded aluminium types more reliable. I seem to remember someone on blatchat recommending you spray the joints with wd40 or similar to see if there are air leaks.

 

Remember that these are consumable items so it may be worth a precautionary change?

 

When the engine idles - see if the carbs look like they start shaking excessively i.e. resonance - this will foam the petrol up and it will never idle satisfactorily.

 

Hope this makes sense because it's a real pain - I've been there - Hope this helps - Bemused of Stafford

[Smegnoguk]

Rah Mr Locust! Didn’t know I had any Thatcher Ray washers or Basket Plates! No wonder my washers are embittered! From now on I’m giving Bobbin Packers a wide berth however resilient they are and I promise I haven’t spat out any o-rings – honest! Thanks. I’m glad that’s sorted!

 

 

[Smegnoguk]

Aah! So this is double posting!

 

Edited by - smegnoguk on 28 Aug 2002 09:02:15

[Mr Locust]

Sorry I took the time to offer a few suggestions for your problem 🙆🏻

[ashaughnessy]

I thought Mr Locust''s advice was quite interesting. The bits he refers to hold the carb on to the manifold and need to have some compliance to avoid frothing the petrol in the carbs. It''s interesting to me because I''m having similar trouble on my car with my newly installed engine and this is one of the things I need to check.

I''ve tried WD40 in the past to see if the O ring seal is good but it didn''t tell me anything, the WD40 just ran off and it didn''t seem like you''d be able to tell if it was being sucked into the inlet or just running off the way it does.

Anthony

[Mr Locust]

Ashauughnessy - If the engine is running and the gaskets are leaking, the engine note will change momentarily as the WD40 introduced is extra fuel being sucked in through the leak. If nothing happens to the way the engine is running - there is no leak.

 

On the installation I had problems with (1500cc Triumph engine / 2x40DHLA dellortos) the carbs visibly settled into a resonant shaking at tickover (if the carbs were too tightly squeezed onto the gasket plates). Once this shaking started, the engine''s ability to idle went completely (fuel frothing?). If kept a hand on the carbs to stop the shaking - it was fine. Slackening off the nuts but still just maintaining the seal the engine idled just dandy! Presumably because the resonant frequency had changed.

[Smegnoguk]

Only kidding Mr Locust. Going to try everything you suggested next weekend (had to go visitin' kin folk this weekend). I am honestly grateful for your help.

 

 

[Nigel Riches]

Something else may be the crappy unleaded pseudo petrol has different burn characteristics to that lovely lead enriched carcinogenic real fuel of days gone by, this may be causing the fouling at idle, but a session on a rolling road should fix things O.K.

[Graham Sewell]

From what I can see, the real man's fuel may outlast the current pretender as more of the petrol outlets are stopping sales of LRP due to it only accounting for 2% of sales - they would prefer the extra unleaded/diesel pumps instead. However, sales of 4* (at £1 per litre) is doing well ❗

 

Low tech luddite - xflow and proud!

[Nigel Riches]

HOW MUCH IS THAT PETROL!!!!!!!!!!!!!!!!!, we pay the equivalent of approx 35 pence a litre, and only about 20 pence a litre for diesel, but have to pay a levy on the weight and distance travelled for diesel vehicles, still it's a lot cheaper but we still gripe about it.

[Graham Sewell]

I just figure that at £1/litre it is saving me from having an unleaded head fitted - based on 3000 miles per year and a 5p/mile premium it would take 2-3 years before the cost of the petrol outweighs the cost of the work (£300 plus fitting).

 

Not only that, but whilst the car is having a work done on the head, I may as well do the bottom end and that £300 rapidly turns to £3000!

 

Low tech luddite - xflow and proud!

[Smegnoguk]

This is all very well - hi-jacking my Topic and talking to hobbits on the other side of the world about the price of petrol! What about my twin choke Webbers! And anyway, I use unleaded!

 

P.S. How much is a leg of lamb in New Zealand Nigel?

 

 

[Graham Sewell]

OK, back to the original thread - what to do about fouling plugs on a Caterham Classic (presume xflow unless told otherwise). Firstly, it is well known that unleaded fuel is more prone to fouling the plugs so go 1 grade hotter (I use the plugs from a 1600 in my supersprint BP6ES or BP7ES (aka Champion N9Y of any variety as suggested by Len Union about 6 years ago) as the BP8ECS were useless after 100 miles).

 

Secondly, set the idle according to the manual and then wing 1/4 turn weaker (screw in).

 

Otherwise, pay the price for real fuel.

 

Incidentally, the fouling issue was raised a couple of years ago in most of the 'classic car' press as the number of MOT failures due to emissions was rising rapidly unless the plugs were changed on the way to the test.

 

 

Low tech luddite - xflow and proud!

[Smegnoguk]

Yes this sounds more promising. My engine is the 1500 Vauxhall job that, as far as I can tell, they only fitted for about 3 or 4 years. As I say, I would have expected my motorbike plug to look a nice chocolate brown after a run. Is that what I should be aiming for here do you think? I have booked it into Redline in Caterham just as a precaution (+ MOT) but I would prefer to be able to sort it myself first.

 

 

[Nigel Riches]

Smegnoguk, the leg o' lamb would be about $10nz so 3 quid to you my boy, but I'm not a big fan of lamb, get some good venison or beef though and count me in. Did you see me as any particular Hobbit? Have fun, best regards Nigel.

[Graham Sewell]

You will never get 'chocolate brown' from unleaded fuel. That colour was produced by one of the lead oxides. The best you'll get from unleaded fuel - if you pull the plug immediately after a blat, is a dull grey! The rest of the time it will be black.

 

As other evidence of this, look at the exhaust pieps, even from catalysed production cars with pukka EFi - they are black. My Seven, with genuine 4* comes out a pale chocolate colour . So with unleaded, you basically cannot rely on plug colour anymore. ☹️

 

Low tech luddite - xflow and proud!

[Smegnoguk]

Ahh! Thank you Graham. Very helpful. I will persevere with "tuning by ear".

 

P.S. If you want a leg of lamb for the weekend, I can get one very cheap!

 

 

[Nigel Riches]

That leg o' lamb might not be in to good condition by the time the post office has put it through their system, bye from your upside down Hobbit.

P.S. Edora is only an hours drive away, but not really a suitable road for a 7.

[Smegnoguk]

Really? I'm impressed. My daughter returns next weekend from six months travelling around the world. She has been to India, Thailand, Australia, New Zealand, Fiji and now the United States. She says New Zealand was the best of the lot. How about that!