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Paul Coleman

S1 backfiring

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As part of my restoration I've attempted to ressurect the carbs (DHLA 45s) but when I run it and try to set the mixture I've got a lot of back firing which I don't seem to be able to get rid of.

 

The car has never run during my 8 year ownership and wasn't running when I bought it. I got the carbs handed to me in a box off a shelf where they'd sat for years. The owner said they were the original carbs and I have no reason to think otherwise.

 

I stripped them down and cleaned them and blew my air line through every passage way and hole I could find. I bought the Dellorto rebuild kits and set the accelerator pump to be 8mL for 20 pumps of the throttle.

 

When I run the car at tickover I can't seem to adjust them so that they don't back fire. I've been using a colour tune and just when I think I've got one cylinder running correctly it will pop and back fire  and I can see a flash.

 

The timing is set to 8 degrees and I've double checked it.

 

The other strange thing is that I have one of these manometers... http://www.carbtune.co.uk/carbdtls.html but only one cylinder seems to register and even then it's very low?

 

Very confused... anybody got any ideas?

 

Thanks, Paul.

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Upgrade today to remove Google ads and support TLF.

Would suggest a compression test...might be leaking valves?

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I would agree with John especially after sitting all that time. I had a 907 engine when it had been sitting for 5/6 months the cam's would sometimes knock as if one bucket was stuck and she would missfire due to sticky valves.

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Yes the leads are on the correct way and a compression test gave 160~162psi on all cylinders.

 

Latest thought is the distributor is worn as the timing seems to flick now and again. I'm waiting to borrow another distributor to check...

 

Paul.

Edited by Paul Coleman

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Any more updates on this issue? My S1 was running ok when I bought it, got me home from 160 miles away without missing a beat. Since then I've replaced the fuel tanks, lines and pump and fitted a carb service kit - but the engine pops and splutters, but doesn't die. I used a carbtune II and the carb nearest the gearbox was perfect, but the carb nearest firewall showed no readings whatsoever! I've made every adjustment there is, but still no readings. I did wonder about a compression test, but cannot find my gauge at the moment, but you found good compression in yours so perhaps it is (fingers crossed) something less serious. I'm thinking air leak between the carb and inlet manifold, although I'm stripping the carb again just to be sure I didn't miss something the first time around. Failing that, then might have to purchase a rebuilt carb and swap it out and see if that cures it.

Oh, and I've got new leads, plugs, distributor cap, rotor arm, coil and the timing with the strobe is spot and doesn't waver.

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No I haven't made any progress on this whatsoever. I've reached the end of my knowledge, ability and patience with it and it now sits with a dust sheet over it.

 

I stripped, cleaned and rebuilt the carbs about 4 or 5 times (lost count in the end) and it made no difference whatsoever. I have a 4 channel carbtune and I get nothing on any of the cylinders when the car is ticking over. I have checked the plug leads, timing etc over and over again. I've sent pictures of the cam pulley wheels to Steve at SJ and he can see nothing wrong with it. I've taken the distributor apart and cleaned it numerous times and it all looks fine.

 

Nobody I've spoken to has ever seen an S1 backfire so badly that it blows the O rings out of the carb mounts. And some of the explosions are taking place in the back box of the exhaust - it got so hot when I ran it one time that the paint on the rear valance blistered.

 

I have no idea what's causing it and don't know what else to try. When you're at that point the best thing is to walk away.

 

I am reluctant to put it into a Lotus dealer as it could end up an open cheque book which isn't going to happen but it needs somebody with more ability than me!

 

Paul.

Edited by Paul Coleman

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If your compression tests come out ok are you overfuelling? Sticking valves may still be a possibility as well.

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Sounds like you are 180 degrees out on your timing.

 

Buddsy

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Paul - I'm gutted for you and know how you feel. Sounds like we've had very similar experience and I too was getting fed up with attempt after attempt, rebuilding the carbs yet again etc. so much so, I had started looking at replacement engines. However, your post gave me new faith in this not being a timing issue or a poor compression issue. So I went with my last and final theory, the spacers! I took off the offending carb (with no vacuum according to my Carbtune II) and inspected the spacers. I noticed the rubber seal inserts were extremely flat and the surface of the spacers, the manifold and the carb were quite dirty too. So I cleaned all of the surfaces. I then checked SJ's for replacement seals and although they have got some, these seemed to be just ordinary O rings, so I decided to see if I could improve the old ones. I filled the slots where the seals sit with Loctite SI 5980 quick gasket and pushed the seals on so that sit a little more proud than before and left the Loctite to set. I've just finished fitting the spacer and carb to the car and what a difference! I've not tested the vacuum yet with the monometer yet, but it now idles smoothly and only coughed (a little) when revving the engine. Seeing how I've not tuned or balanced the carbs yet, I'm confident I've now solved the main issue. You cannot tune or balance the carbs until you've got them both working properly and that isn't going to happen until you have good air flow, which means making sure you've not got any vacuum leaks - especially from between the manifold and carb.

How she runs now, before tune and balancing. https://www.facebook.com/bill.mant/videos/10207112852487978/

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If you are rebuilding a carb im sure Paul would have changed the large O rings!! They do have to seal.

 

Buddsy 

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I have so far got through two complete sets of the large O rings, all new of course. Most of them have ended up 1.5 times bigger than they started out :huh:

 

I threw the towel in today and booked my car into SJ Sportscars next Tuesday for Steve to have a look at it. I'm out of ideas, talent and patience!!

 

Paul.

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I hope Steve sorts it OK Paul.

 

Please let us know what he says and finds.

 

Fingers crossed its not too costly for you.

 

Alec

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Yes, please let us know what Steve comes back with. Going by the way you've gone through those O rings I'd still bet those are the root of your problem - I'm glad I stuck with the originals and just improved them, I've normally got a bad habit of buying new just for the sake of it. Anyway, lucky you for being near enough to put it into SJ's, I wish I could drop mine into Steve occasionally.

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Okay, will do. He's actually about 5 hours from me so not that close really and from what I gather it won't actually be Steve who works on the car.

 

I didn't know what else to do with it and I was concerned that I might start to damage something (if I haven't already!).

 

Paul.

Edited by Paul Coleman

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I've never been to SJ, only seen them on Wheeler Dealers, but I've bought so many Esprit parts from them over the past 15 years that I'm sure they know my address off by heart! Lol. Spoken to Steve a few times on the phone, he's always been happy to give advice and certainly knows these cars like the back of his hand. If they cannot sort it, nobody can.

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I decided to have one last crack at this and managed to get my brother-in-law to help as he's pretty good with cars. He reckons the distributor is causing the problem and we did actually manage to get it running without blowing the O rings out - which in my book is progress. So I'm going to buy a new distributor and just rule that out before I have it shipped all the way down to SJ.

 

Paul.

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Sorry to hear about your problems - I have not disimilar issues with my car. I purchased all my stuff through SJs but then had a word with Pete Musgrove at PNM Engineering. He can rebuild your distributor and currently has mine. I would have a word with him as he can test first to determine whether or not the fault is here or somewhere else.  

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