WWT338J Posted June 27 Report Share Posted June 27 Dear Wise Men The engine on my late mk3 GT6 was running fine..... The dashpots kept loosing oil so I thought it would be a good idea to renew the o-rings around the needle adjusting screws inside the air valves. This was straightforward and I thought I had reset the needles to their original levels (a fraction below the bottom of the air valve). The diaphragms were both intact. The car starts easily and runs fine except it now stalls when dropping down to idle at traffic lights. I assumed I had unintentionally weakened the air/fuel mixture. The no 2 plug looks lighter compared to the no 6 plug (photo 1). My Gunson colortune supported this showing a yellow combustion in no 6 (video 1) compared to blue in no 2 (photo 2). So I have raised the needle in the front carburettor as much as I can. This has helped a bit but the mixture in no 2 (video 2) still seems lighter compared to no 6. The engine still tries to stall when dropping down to idle. You can see at the end of video 2 how light the mixture becomes on deceleration (is this normal?). Before I started this adventure the exhaust CO level would float around 5% at idle. Enriching the front carburetor has increased the idling CO to 8%+ (photo 3). So I have enriched the air-fuel mixture - possibly too much - but not solved the new stall on deceleration to idle problem. Please let me know your thoughts. David Video 1. No 6 plug.mov Video 2. No 2 plug after enriching front carb.mov Link to comment Share on other sites More sharing options...
Pete Lewis Posted June 27 Report Share Posted June 27 i would check the diaphragms for any small holes, give them a stretch if they are baggy wash in petrol to renew the shape check the lugs are located ok and air filter gaskets are not obstructing the front face ports std setting is the small delrin needle washer should be level with the base of the air piston have you checked the temperature compensators are sealed and fully closed at normal temps ?? Pete Link to comment Share on other sites More sharing options...
WWT338J Posted June 27 Author Report Share Posted June 27 Hi Pete Thanks for the quick reply. I will check the diaphragms again tomorrow but they looked ok when I changed the O rings a couple of days ago. I didn't look at the temperature compensators. If I've understood your previous posts correctly it would be simpler if the temp compensators on both carburetors were adjusted so that they are permanently closed? Is that correct? David Link to comment Share on other sites More sharing options...
johny Posted June 28 Report Share Posted June 28 I believe your carbs also have an idle speed control screw so maybe they just need adjusting as per the workshop manual? Link to comment Share on other sites More sharing options...
Pete Lewis Posted June 28 Report Share Posted June 28 (edited) 11 hours ago, WWT338J said: correct in a word ...yes if they bleed air when they should be closed you cannot set the idle mixture they should open in hot adverse under bonnet conditions not in normal running never found an old one set correctly always open ,its a simple bi metal spring and plastic plunger and in line with jonny make sure the throttle cable has a small amount of play/slack so temp doesnt affect the idle stop Pete just screw the small nut to shut the blasted thing ,its an unwanted emission add on . Edited June 28 by Pete Lewis Link to comment Share on other sites More sharing options...
WWT338J Posted July 3 Author Report Share Posted July 3 Thanks Chaps I dismantled the temperature compensators, cleaned the plungers, screwed the bimetalic strips up tight and replaced the o rings. This has solved the stalling on deceleration to idle problem. I've also been able to lean the front carburetor again so that the CO level has fallen to around 4.5%. David Link to comment Share on other sites More sharing options...
Pete Lewis Posted July 4 Report Share Posted July 4 pleased that has worked ok Pete Link to comment Share on other sites More sharing options...
SpitFire6 Posted July 5 Report Share Posted July 5 Hi David, What exhaust analyser are you using? It looks very interesting that it measures HC. Cheers, Iain, Link to comment Share on other sites More sharing options...
WWT338J Posted July 6 Author Report Share Posted July 6 One of these. Second hand from Flea bay several years ago. Will measure oxygen and therefore Lambda if I replace the fuel cell except these only last for about 6 months. David 4 gas analyser.pdf 1 Link to comment Share on other sites More sharing options...
SpitFire6 Posted July 7 Report Share Posted July 7 Hi David, Ebay is good for exhaust analysers. I bought the Hawk for £150 & after fixing some dry-joints its as good as new. I really wanted to be able to measure HC as without it, Lambda can be meaningless. Won the Camic for around the same price. I enquired what you had as the display looked like a standard DIY unit. Cheers, Iain. Link to comment Share on other sites More sharing options...
Pete Lewis Posted July 8 Report Share Posted July 8 reminds me i took the 2 Horiba units we had when we closed the petrol emissions dept but nightmare to keep calibrated brit ox would not supply me with the span gas needed and they ended up Up the Tip happy days Pete Link to comment Share on other sites More sharing options...
Waynebaby Posted August 25 Report Share Posted August 25 Reading this thread finally got me to pull my finger out and check the temperature compensators - a job I've been putting off for about eight years. In the end it took me all of 15 minutes (10 if you discount the time spent looking for a tiny spanner to fit the nut) It has to be the simplest performance improvement mod' I've ever carried out. As WWT338J found, the tendency to drop below tick-over after revving has been cured and the tick-over itself is very steady. What I didn't expect though was the general improvement in response throughout the rev range. This simple tweak has given the engine a new lease of life. The moral of this story, as always, is listen to Uncle Pete. You know it makes sense. Wayne Link to comment Share on other sites More sharing options...
Pete Lewis Posted August 25 Report Share Posted August 25 not sure about the uncle pete bit but pleased this simple fix makes a world of difference if you set the small delrin washer on the needle just about flush with the base of the air piston you have the base factory initial setting Pete Link to comment Share on other sites More sharing options...
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