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Stomberg carb issues

Dan Cook

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I have Stromberg carbs on my GT6 mark 3. I think I am correct is saying they are CD150SE?! They are the ones with temp compensators on. The car has run pretty lumpy on tickover since I had it. It runs fine under power and goes really well and smoothly when you have your foot down. So I think there is an air leak causing the idle to be rough. 

If anyone attended the tinker day at club HQ mine was the white Spit/GT6 conversion that Pete started the day with. Since the day I feel much more confident with the carbs and can now remove from the car and strip down in about 40mins!

Anyway I am still trying to rule out symptoms as I am going along. The air compensators are closed, I now have some proper asbestos gaskets on the carbs and have fitted the gaskets so the the air hole is unblocked on the butterfly side of the carb. I have a proper air filter box too. I did have heat shield between the carbs and the manifold which I know are not standard, I did take them off when I got the asbestos gaskets, but put them back on again as there was an issue with the  front carb (nearest radiator). The manifold is so narrow where it meets the carb that the air hole that I assume goes to the bypass valve and the temp compensator was overhanging the manifold. This is led me to believe that this is why the heat shields were put on in the first place to make a better mating surface for the carb. 

I have experimented with the fuel/air mixture and have tried using colour tune, lifting piston, inspecting plugs etc. The timing is set to 8 deg before top dead etc. I have set the fuel float level to the recommended level in Haynes manual. I have checked diaphragms for leaks, checked the floats are the right way up. I have damper oil in the pots. I have checked the carbs are balanced with a meter.


So to my question. Below is a pic of the butterfly, they don't look particularly worn to me or that the gap around them is a particular issue but I don't have any experience of what constitutes unacceptable wear? I have held them up to a strip light with piston out of the carb.

If they are worn can you just replace them I am assuming you don't necessarily need to replace the spindle. The spindle seems OK and doesn't wiggle about when opening and closing.

Any advice much appreciated.


Dan Cook


IMG_20180307_184734820 (1).jpg


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Those look OK to me, but here is a link to the Buckeye Triumph site.


A very detailed explaination of how Strombergs work, how to refurbish them and how to set them up. These are bigger Strombergs off a TR6, but essentially the same. There are 3 parts.

In the third part is this list, my first thought reading your thread was manifold gasket air leak!

Possible causes of poor idle quality:   

  • improper synchronization
  • air leaks around manifold
  • air leaks around throttle shaft
  • temperature compensator stuck open
  • damaged diaphragm
  • stuck or binding air valve
  • misadjusted float
  • incorrect timing 
  • misadjusted idle trim screw
  • Doug 
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Im please the twiddle day gave you the confidence to have a go, what you have covered is pretty much all you can do ..well done 

just what is the running/idle problem , lumpy/ mis fire/  cant adjust the idle ???

the plates both look balanced with equal daylight showing ., i hope thats with the throttle idles screws set, if its supposed to be fully closed then tehy are not

the only real wear is where the disc contacts the sides of the bore, only visible if you remove the disc. if its got a step by the spindle its worn the edges away

most wear is the spindle and lastly the spindle hole, some have a replacable bush , some not, if the hole has worn places like the carb exchange in leighton buzzard will rebore and bush for a few ££s 

get a compression test done at some time  to rule out any valve problems 


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Pete again thanks for this. 

Goodpoint about idle screw, forgot about that so I hadn't backed them off.

I have done a compression test and all cylinders well within 10 per cent of each other and were in the green section on the meter. 

There is another issue I have noticed. I did a compression test as cylinder 3 had noticeably more black carbon deposits than any other cylinders. I have checked all of the valve gaps and they are fine. I have held off doing anything as I thought it might be a head off job. I did also check spark was arriving at spark plug. Have changed plugs too and checked plug gap.

I know that this could well be idle issue. Tried to rule out carbs first though.

To answer question idle really the only issue. It doesn't mis-fire. I think there was a problem with air leaking which probably just compounded issues. But better with thick gasket between manifold and gasket.

Thanks Dan

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So is the idle too fast or too slow  or just lumpy

Have youmtried retarding a bit , hold the idle at a aprox 1500 and rotate the dizzy to get the best fastes running and back it off 

It may be too far advanced,  , ignore the timing marks,  this might a case of the damper ring has moved on the pulley

If  thats proven  johnD will be after it 


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