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Ignition/fueling problem between 2000-3500rpm s3 turbo

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Hi, this is my first post so please excuse any mistakes!

I just wondered if anyone could help me at all, I have an s3 turbo which we have recently rebuilt the engine in, all is good apart from a fault that we are struggling to place which is what I would describe as a spark breakdown between 2000 and 3500rpm, power delivery is very jumpy and feels misfirey - on all four cylinders so engine can't be the problem. Other than this the car is very smooth, below 2000rpm is smooth and likewise when exceeding 3500rpm. Considering dizzy/ignition problems and carb imbalance, have already checked timing which appears correct to manual and other esprit owners words (total advance is under 30 degrees), it's not a tooth out on the cams so thought I'd ask if any other owners/mechs have experienced something similar.

Thanks!

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Did you rebuild the carbs?

I ask , because it's important that carb float levels are adjusted correct. If not you can experience stuttering at revs where main jets are coming in .

 

Geert

Edited by gvy

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Did you check fuel pressure?

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Hi Lars,

 

Since Maddy told us that the car is OK below 2000rpm and also above 3500rpm, I do not suspect fuel pressure to drop or be low, just in between 2000 and 3500rpm.

But worth checking of course

 

Geert

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I had exactly the same thing on my identical car. It turned out to be the slow running jets were obstructed by varnish deposited by modern petrol. I had tried all the ignition stuff, the float levels, had taken the blanking plugs out of the carbs where the progression holes are and cleaned them. In the end, I had a good read of the Weber/Dell'Orto bible...Des Hammills wundatome...

 

http://www.biblio.com/book/how-build-powertune-weber-dellorto-dcoe/d/507949829?aid=frg&utm_source=google&utm_medium=product&utm_campaign=feed-details&gclid=CJGV8a6TnMQCFQ_KtAodvg4AlQ

 

Then it became clear that the slow running jets also deal with the progression of the mixture until the main jets come in. Not only that, but the choice of jets on the S3 Turbo was at the very bottom end of diameters advised for the size and power of the engine....and this got me thinking.

How can you measure the inside diameter of a vey small hole??? I ended up rummaging through my stock of used guitar strings..until I found one that read the correct diameter on the micrometer and the digital vernier caliper. Then I used a section of this....tried to insert it into the slow running jets.

It wouldn't go!! I tried to clean the jets in an ultrasonic cleaner...and using every solvent I had to hand..with no success.

So I spent an evening sitting in front of the telly and twiddling the guitar string...carefully...in the slow running jets until it would just go through. Bits of powdery varnish fell out. I could have bought new ones....but the Scots blood will out....(!) Finally I had got all four to the correct diameter.....reassembled the carbs.....and the misfire had completely disappeared!! Before it had farted and hesitated around the normal motorway cruising speed of 70 to 80 mph...the rev band of 2500 to 3500 as mentioned....after she was all clean power, no hesitation and a pleasure to drive. Some day I might even treat her to new jets!!!

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Hi,

Thank you for your replies, interesting points which I think may need more investigation. Just as a little back story on the engine, we rebuilt the engine after 2 pistons cracked - this was very soon after we had bought the car so suspect it had been over boosted, it was clearly unwell! So stripped it down, my other half has been an engine builder for years on lotus' of all kinds, Ferraris and Lamborghini so has experienced many a problem, just very few problems with carburettor'd turbos!! Anyway, having spent enough for the poor thing to start feeling loved again and replaced the pistons with omega forged ones, liners machined to fit said pistons and re honed, new stainless valves, guides, seat etc., new buckets and shims, we have also changed it to run the s4 water cooled turbo charger to combat the problematic remote waste gate, 2.5" stainless steel exhaust system all the way back and modified back box to match (giugiaro barrel back box but reduced baffles to match the gas flow), full set of silicone hoses - the list goes on!

Interesting point today, having driven it a little very tentatively the problem seems to have reduced, all carb settings we did check prior to fitting, the CO levels have checked out actually dead on what was expected BUT the point about the fuel leaving deposits is very interesting as this could well be our problem! Having driven it a bit more today the problem has reduced without me touching or altering anything - possibly down to fuel washing old deposits? Looking forward to having the carbs apart tomorrow and will let you know how we get on.

Thanks again, any thoughts are much appreciated,

Maddy.

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Welcome to TLF Maddy. :welcome:

 

Best of luck with the problem.

 

Duck over to the Introductions area and say hi in there as well. :)

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Can we have a few photos please - of ALL your cars as it looks a very interesting collection you have there  :thumbsup:

 

Whereabouts on the UK are you?

 

Hope you are bringing one or more to Brooklands on 10th May.

 

Chris

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Its a good question.   I had to disconnect my rev limiter to cure a similar misfire (you can just pull the plug).   Also a failing rotor arm caused a misfire on mine.   If you have Lucas Constant Energy fitted, check you have the correct col and all the correct 'blue' cap and rotor arm fitted.

 

I would also check you have the correct idle jets fitted.  Also, if you have improved the volumetric efficiency you may struggle  as these are fixed jet carbs that can't adapt to new demand.  Finally - mine won't run at the 'stock' CO2 levels.  I need to be closer to 3.5% for good low speed drivability    

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Hi Maddy this is a really intersting topic I was just about to post somethign very similar as I have an s3 turbo with what soulds like exactly the same issue. Can you let me know if you cure it with the suggestions above (I think I might opt for new jets rather can use the strings from my daughters Guitar for cleaning!) 

 

Cheers Christian

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Hi again,

Sorry for lateness of reply, update for you all!

From the last time that I posted, I've gone away and double checked pretty much every suggestion, carburettor balance is perfect, purchased a new fuel pressure regulator (correct lotus one), have double checked main jets, idle jets and emulsifier tubes are all exactly as specified and are clean and correct, also checked float heights which are also correct...

So has the misfire gone? Unfortunately the answer is no! But on my particular car (whether this is of any use to anyone else I don't know), we've calculated that we now have a larger volume of air being produced by the s4 turbocharger and I believe the slow running jets and crossover emulsifier tube are too small, as on full throttle I have no misfire, no hesitation and the engine sounds perfect. I have played around a lot with it today and I have found that at slow running if I crack the chokes open my horrible lowdown hesitation seems to vanish. So I've currently ordered a selection of slow running jets and a collection of emulsifier tubes, which will be here next week. As having checked virtually everything I think it is just down to the correct jetting for the car as currently the jets are totally factory spec. That's where I've got to so far, any thoughts?

Also it is living up to it's name and like all good lotus's one problem is never enough; so of course since last time it decided to destroy it's alternator for no rhyme nor reason, so replaced with updated alternator so rock on the next fault!

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I think they all have a slight "thing" around 2500 rpm to 3500rpm dependant upon how hard the engine is working when the boost starts to come in. Up to and past this cross over is fine but Im sure that cross over point is always going to be felt?

 

Buddsy

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Hi all,

Think we have pretty much got it! We have changed the slow running jets out from 40's that were in the original carbs to the 50's that were fitted into the dellorto 45's, these have a longer tail and obviously a slightly bigger hole (not easily noticeable by the naked eye!) so provide it with a little more fuel compared with the air ratio. Am stunned by the difference this has made! It is monumentally better, idled perfectly, accelerated smoothly and feels so much better! Such an improvement!

Thanks for all your suggestions and I hope this can help someone else too.

Would be interested to hear if anyone else finds this useful or any other problems you have had!

Maddy.

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Told you it would be the jets.....(!) You cannot tell the bore by looking...and it only takes a little reduction in hole diameter to give trouble. Once I'd got my jets back to the correct book size, it's been fine ever since. I did my guitar string twiddle because a) I'm too mean to buy things and b) it was an instant way of finding out if it made any difference, without messing about waiting for new jets! Once I found it was now perfect, there didn't seem any point in fitting new jets...(!)

Glad you seem to have got to the bottom of it and can now enjoy driving again....

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