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Carb needles on modified GT6


Richard Heath
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Hi. I have a Mk II GT6 that has been off the road for about 10 years, and am looking to get it back there for its 50th birthday (September). My issue at the moment is that the engine is running very rough (misfires and very little power). I'm gradually working my way through potential causes (timing, valve clearances, etc) but my suspicion is that the problem is with the carbs, and one thing I want to check is the needles. The issue is complicated by the fact that the previous owner had some modifications done, the relevant ones here being that the engine is stroked out to 2.5 litres and has a Moss fast road cam. There was also fuel injection fitted, but I had that changed back to carbs (Stromberg 150 CD). My questions are (1) does anyone know what the correct needles would be for this setup, and (2) if the current needles were incorrect, would that alone be sufficient to cause the problems I am experiencing?

Thanks

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I thought the 2.5 normally has 1.75 carbs so you're probably going to need the help of an expert on here to set up the smaller items well. However I would have thought that you could get it running reasonably as it is if the carbs are in good condition (diaphragm, piston free, float level etc)

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if they are 150CD whats the manifold??  apart from the cam what air filters are fitted , std or some free flow like KN 

a browse through the triumph tune manual suggests  a 2500  + 175 CD with modified head as a fixed 1AS needle .

where as 2000 +150CD   and KN as 7B    no other are shown 

i fear you have a bit of a hybrid there 

there  are far more needle options for SU HS6 

why did you opt for the CD150  not the CD175 ???

Pete

 

 

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Hi

Thanks for all the replies. Firstly, in answer to the questions...

  • The manifold is a Stanpart V3186. Is that the original?
  • K&N air filters
  • I don't remember choosing the carbs - they were fitted by the Triumph specialist when removing the PI. I suppose it's possible they had forgotten about the other mods.

Anyway, I have now had a bit more time to investigate and am wondering whether the needle question might have been a bit of a red herring (or at least not the fundamental issue). Pulling the plugs, I noticed that most were black and sooty, but number 1 was wet. Using a Colourtune, I can see that cylinder 1 is firing very intermittently, and there also seems to be a (not quite so bad) misfire on 3. I had thought the black deposits were just because the engine has only been running for short periods, but now I'm wondering if a chronically rich mixture might also cause the misfire?

My thought at the moment is to adjust the carbs to weaken the mixture and see what the effect is; if that doesn't sort, check compression for more sinister problems. Does that sound sensible, and/or any other obvious things to test?

Thanks

 

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Theres a post about spark plugs and problems we se that never worried us in previous use , the ceramic insulator is not now glazed and it absorbs products of carb combustion that short the plug  out, the sooty over rich will exasperate  this , so maybe replace the plugs as well as lean off the mixture, finding a needle to run with KN is a challenge as these make a weak mixture due to less depression in the carb throat,  

Dont use plugs with an R in the suffix  they dont suit our low HT

All needles start with the same idle diameter, its the taper when it rises thats  going to wreck the running 

You can add  thicker diaphragm top plates to richen mid range running, or find a stronger spring. (make own weight is possible)

But 150 cd on a 2.5  with KN is going to be a challenge to get it somewhere near right

Pete

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Hi Richard

Could be a difficult nut to crack with that combination.

I run 175CD2s on a Mk2 GT6, which were already fitted when I bought the car in 1977.

I have checked my inlet manifold and the Stanpart casting numbers are 308671 and V3153. This has had work on it to suit the larger carbs and the balance the branches internally , but is otherwise original.

It may be that a carb swop to CD175s or HS6s will be your best long term option, but first of all I would check the basics:- valve clearances, static timing, float levels and a base setting on the needles (ie three turns down from jet level with the bridge).  A new set of plugs might not be a bad idea as well.

Let us know how you get on.

Ian

 

Edited by Ian Foster
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If I remember a 2500 Australian Saloon used 150CDS carbs. I have run a 2.5 Litre engine on 150CDS it had loads low end torque, but lacked some top end. This all makes sense due to the restricted gas flow. The engine was in TR6 125BHP spec. So 150CDS carbs will work on the 2.5 Litre engine. The needles were the standard 2 Litre Mk2 i.e. 6 AC on CD150S carbs. I have used 150CDSE on the same 2.5 Litre but found the mixture was weak and the range of needles are very limited.

However, I understand this engine has been breathed on so I agree there is a question mark on what needles should be used.

Dave

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Hi

Thanks everyone for your input. I think the conclusion is that the misfire is unlikely to be down to the carb specs so I need to get to the root of that first (but carb info very useful for later reference). It's likely to be a few weeks before I get the chance, but I'll post an update when I get anywhere.

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  • 3 weeks later...

Some answers! I checked cylinder compression and all registered between 150 and 160 (cold), which I believe suggests pretty healthy. I then disassembled the carbs and found the problem: the fuel inlet valve on the rear carb was seized shut, so there was zero fuel going in; I must at some point have tried adjusting them to get the car running - as I said, it has been off the road 10years - and ended up with the front carb feeding the rear cylinders, and completely drowning the front ones.

So, a massive improvement and the car should now be at least driveable (once I have attended to brakes etc!). Thanks for all the input. The advice seems to be that getting the car running well on 150s might be challenging, so I’ll consider my options in the longer term. To return to the original topic of my post, the needles are B5DG. Any views on whether those are likely to be the most appropriate?

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Hello Richard,

B5DG needles are usually fitted to the 150CD3 Stromberg's and were later style carbs IIRC.

Additionally, a lot of "B" needles were used for emission carbs.

I would have expected your needle to be along the lines of a 6AC (as you have 150CD carbs) which is a standard needle; as such you will probably require a richer needles with your current set-up.

These are purely my interpretations from the info you have supplied above. I guess it would be too much to hope that the carb ID brass tags are still in situ ?? !!

A chat with Burlen would be useful and of course a RR session in due course.

The downside with Stromberg carbs is the needle range is quite limited unlike SU's.

Regards.

Richard.

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Okay Richard, you have the correct standard needle for the correct carb fitting; as such using a richer needle may benefit the current set-up you have - that certainly warrants a call to Burlen.

I would say your B5DG is too tame at the moment.

Best wishes.

Richard. 

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