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GT6 stalling on deceleration to idle


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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.









Photo 1. Plugs before enriching front carb.jpg

Photo 2. No 2 plug before enriching front carb.jpg

Photo 3. Exhaust gas after enriching front carb.jpeg

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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 ??


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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? 



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11 hours ago, WWT338J said:


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


just screw the small nut to shut the blasted thing ,its an unwanted emission add on .

Edited by Pete Lewis
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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%.


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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.

Hawk AF Tester.JPG


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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 


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