Hi folks
I bought a '73 Triumph Stag last October and have been using it for 3 or 4 days a week. It's a great car, and while not a "fully restored garage queen" has had all the usual Stag problems sorted out. It's very much a daily driver if I need it.
However, I had it serviced a while back and asked for the carbs to be tuned/balanced. This car has a pair of Zenith Stromberg 175 CD-2's. Since then, it's been difficult to restart when hot. The car smells gassy at those times too. Only way to get it going is to hold the throttle wide open when cranking.
I don't trust the crowd that serviced it, so rather than bring the car back to them I took it to a pro - while he knows carbs, he was unable to get the CO2 etc down, they were very high. In fact, adjustments at the car were not making much difference.
Any ideas ? Carb-rebuild time ? The latter doesn't look too difficult, there are rebuild kits available.
Thanks
Frank
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If I can remember that far back I seem to recall split diaphragms in the carbs. were a cause of over rich mixture and dodgy starting. The diaphragms are simple to replace, if they are still available.
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That's easy to check ! And I just did - diaphragms look okay. There's a large piston with a needle extending out the bottom - the needle has quite a lot of sideways movement in it, which I wonder is correct or not. It's seated, but "wobbly". The damper for that piston mechanism is full of oil too, which is as it should be.
BTW - thankfully all the parts are available, these things can be rebuilt from scratch
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The "wobbly" needle is quite correct, it is either to help it align with the jet, or to keep it "biased" to one place in the jet, I can't remember which.
You need to have a good look at the diaphragms, and carefully stretch them about abit, splits are not that easy to see in them.
If the Stag is starting ok cold, but not hot, it could be vapour lock in the fuel lines, or the float chamber level is wrong. You said it has just been serviced, maybe there is a heat shield out of place, or a fuel pipe has been unclipped and is picking up heat from somewhere.
PS my wife always fancied a Stag, Robert Redford, and an Afghan hound. She ended up with a Spitfire mk 2, me, and a border collie!
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Apart from what mjm mentioned.It has been known for the carbs to be set up too weak.Ok on cold starts as you use the choke..But when hot the mixture was too weak and they struggled to fire..a little tweek to make slightly richer and no problems when..They also didnt as I recall like timing being slightly off.Just a thought
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Steve
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Thanks guys, I'll check the floats.
I have a good workshop manual for the car (a legit reprint of the old British Leyland manual) which goes into detail about overhauling the carbs. Doesn't say much about troubleshooting, just instructions on strip-down and rebuild.
Looks like I need gaskets and bits before I can look at the floats, so I'll get those and just do a proper job of it.
I'll report back when done just for closure. If all else fails, exchange carbs are readily available.
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Meant to say: timing is fine, and the carbs are reportedly too rich, they just don't want to be tuned any less. It's the smell of petrol around thing after it's hot that maybe makes the "float" idea make sense.
Ta
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Check the floats aren't punctured, the levels are adjusted correctly, and that the needle valves are OK. They might be worth replacing anyway as you have the carbs dismantled.
I recall a large O -ring sealing the carb assembly onto the inlet manifold, held by a single bolt down the middle. This was difficult to make airtight, and my bolt pulled out having damaged its threads.
Also mine had a cold/hot air intake thermostat on top of the inlet duct. Could that have stuck in the cold-running position?
As said, the jet needle is meant to be moveable, but the jet itself still needs to be very accurately centralised. Try removing the dashpot damper and lifting the piston about 1/4". It should fall back onto the bridge with a satisfying clonk, unimpeaded by binding needle or perished diaphram rubber.
One sure indicator of a bad diaphram (apart from holes) is when it seems too big for the recess ring in the carb body - ie it has swollen. I found they don't like switching from leaded to LRP to different additives.
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As an aside about un-adjustable rich running at idle on this family of carb -
There is commonly a plastic cover on the side of one of them (about the size of a finger) held on with a couple of screws, under it is a bi-metallic strip and plug which is meant to alter mixture a bit during warm up.
The plug can corrode in the hole, and you'll get rich tick-over.
(either that, or it's been "interfered with".)
If you take it to bits to clean it up, on re-assembly the basic adjustment is plugged when cold, and out when hot, but I'm sure there must be a more precise way of setting it which someone alse here will know..
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If the carbs need serious work, I'd give serious consideration to changing them for SUs.
BTW: is it on the original engine or a Rover V8 convertion?
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To AnotherJohnH: thanks ! Found a great article here on the temperature compensator thing:
www.vtr.org/maintain/temp-compensators.shtml
To Cyd{P}: they're the original carbs - don't see any big reason to change them, so I'll just go the overhaul route. One of them has "varnish" on the outside so looks like it is or has flooded itself at some stage, and the spilled petrol is baked on. Parts are readily available and if I really mess up the overhaul, new carbs are just as easy to find.
My main worry is that after I fit the shiny, overhauled carbs, the settings may be so far out that I can't get the car to start.
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Thanks for posting the link for adjusting them!
The carb with the "varnish" on the outside may well have a sticking needle valve - I have distant memories of running away from my 1854 Dolomite after pulling in at the side of the road when it started running very rough and smelling of petrol. On opening the bonnet and there was white "smoke" where the petrol was dribbling onto the hot bits.
Incredibly enough it didn't catch fire, but I never really trusted it again, even with the new needle valves.
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As a rough guide, centralise the needles in the jets first then get the throttle and choke linkages on both carbs working together but make sure that the choke is off. Then remove the dashpot dampers and screw the jets up until you feel them touch the bottom of the piston/needle assembly(a small screwdriver down the dashpot to contact the piston will let you feel this). Then unscrew the jet(lower it) by one and a half turns. This will give a setting good enough to start with. Don't forget to re-fit the dashpot dampers. Open the throttles with the idle speed screws and start it, being ready to turn it off if the revs rise too high. If they do, close the idle speed screws a bit and try again.(You could see how many turns from closed they are before you dismantle and start there.) When it does start, let it warm up a bit no matter how rough it ticks over before adjusting anything. With a similar setup,(SU s, but similar principles) I used to get the engine at a low idle speed, adjust mixture(s), lower idle speed again, adjust mixtures etc.
Hope this is some help.
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As a general rule the hot idle compensators or "coffin valves" are more trouble than they are worth on an old carb. I generally close them up tight when tuning them.Check the O ring under the carb mounting block. You may have a problem with incorrect mixture either too rich or lean. Inexperienced techs may try and set them up at idle to the 'book', from experience again I set them at around 2%CO at around 2000 rpm to give a good balance of fuel at light throttle and cruise but usually this gives a rich mixture at idle if the carbs are worn. Setting the carbs to the 'correct' CO% at idle will result in weak mixtures, lack of power and high engine temps at cruise.
I can highly recommend the Carb Exchange at Leighton Buzzard for a rebuild, 01525 371369.
See you at Prescott??
Andrew
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Simplicate and add lightness!!
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>>Inexperienced techs maytry and set them up at idle to the 'book', from experience again I set them at around 2%CO at around 2000 rpm to give a good balance of fuel at light throttle and cruise but usually this gives a rich mixture at idle if the carbs are worn. Setting the carbs to the 'correct' CO% at idle will result in weak mixtures, lack of power and high engine temps at cruise.
Those symptoms also happen with the hot idle compensators closed all the time - you need the air bleed to get correct tickover mixture and correct running mixture with a half decent carb.
If they valves are jammed shut, or gunged up, it's got to be worth getting them open when hot and then re-tuning. The critical adjustment described in the Triumph link isn't necessary to prove the point about the state of the carb, as you're through the warm-up point of operation in a matter of minutes.
Then consider exchange if the carbs are too worn.
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