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Triumph TR6 - Stromberg 175 CD mixture adjustment

I have a 71 TR6 with 175 CD carbs. My carbs have brass plugs on bottom with a screw slot. Are these meant to be used for mixture adjustment? They are not mentioned in any of my old manuals; mixture adjustment directions refer to the needle under the dampers on top, which I have not had good luck with. My car runs great with choke out and when its cold. When its warmed up and no choke, it sputters badly in 1st gear and 2nd below about 3000rpm, but runs like a champ at higher rpm. I had carbs rebuilt last year by the roadster factory thinking that would solve my problem but it didn't. Any ideas would be appreciated. Thanks.

The adjustable jets are brass and have a slot & knurled end for turning - you should be able to turn it by hand. Screw it in to lean it out, 1/4 turn at a time so as to not upset things too much.
Brent B

Thanks Brent. Actually it does not have a knurled end/edge but it would be nice if it did. So what you are saying is that this is indeed the mechanism for enrichment; my Chilton manual and other don't show this adjustment but instead refer to the needle adjustment on top for mixture; all very cofusing. I have looked at my plugs and they are pretty much white so I am running lean as I have imagined. I will concentrate on this lower adjustment and see if I can get a richer mixture. How does this coordinate with the upper needle adjustment? Thanks again Brent.

There shouldn't be adjustable jets & needles on the carbs, so I'm at a loss. Early carbs had adjustable jets & fixed needles. Later had adjustable needles & fixed jets. I want to say there was even a year with fixed needles AND jets, but I'd have to look that up.

If you have both adjustments, consider setting one fixed at some reference (needle shoulder even with air valve surface, for instance) and using the other for adjustment. Otherwise you risk getting something "out of whack" as we say around the bb-q grill.


2 fixed jet, 1 fixed needle carb
Brent B

Hi Tim

Not sure but I think your seeing the bowel drains if there underneath each carb about 3/8ths in size? Early ones were brass with a coin type slot? Some rebuilds use as well.

If your looking at the needle seats not sure if there screw out on that carb? Im not sure why your in that far on rebuilt carbs? Carb rebuilders set everything to a startable condition and leave the rest to you in most cases. Seats are not adjustable.

There is no other mix adjustment other than the top one. And you need the $25.00 thingamagigi to adjust. That being said there are lots of other things that screw up the mix.

Heres a good sight for info go down to carbs.

First off don't feel alone! I think just about everyone has a lean problem? Your carbs were redone so they should be OK if you have the tune procedure right. You won't get it from Chilton? Read the techsight backwards and forward.

Other things to look at that may cause lean are intake manifold leaks/restricted fuel supply IE: plugged filters/low compression if valve problems/even the oil filler cap on the valve cover leaking? Timing and a few I have forgotten for the moment.

Let us know

Bill Brayford

To add to Bill's list, I'd check the hoses from the carbon cannister for leaks. Plug them up to see if your idle improves. if yes, there's more air being sucked in there than intended.

Also, I have a '71, and have no knurled adjustment. It must have come along on later models. Only the top as Bill described (need the tool) and the fine trim screw on the side.

Mark Hauser

Thanks for all your suggestions. It's nice to know I'm not alone I guess. I'll keep you posted on progress.

If the assembly you describe is situated directly below the needle/damper and the small screw with slot is just below a larger "hex" used to screw the whole assembly into the carb body, then it's an adjustable jet. I don't own any carbs with bowl drains so not familiar with them.

I actually wish all 3 carbs had adjustable jets as they are easy to tune with no oily mess like the needles. They went away because of 1)cost and 2) desire to limit tinkering for pollution control. The ultimate was non-adjustable jets/needles - but you can imagine the problems those babies caused!

3 carbs, one with adjustable jet
Brent B

This may be relevant. I had a stumbling problem ( quite common after a Saturday night out) at lower RPMs that I could'nt figure out, yet all plugs were a uniform pale coffee color. Had my distributor vac retard converted to vac advance ( by Richard Good,{Goodparts}, $40) and Bingo, pulls so smooth and strong it's like a different car.It will involve,(did on my car anyway),pulling the distributor drive out and setting the gear counterclockwise 1 tooth to get back into adjustment range w/o bending the tach drive too much.Distributor is now set some 20 degs from it's old position - I don't understand this unless something internally is altered in the conversion. Took a day to get it figured but when I found the sweet spot, oh was it sweet.

Peter 74 TR6
Peter Gooch

This thread was discussed on 24/08/2003

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