Lotus Elan

stromberg running very rich

PostPost by: tedtaylor » Thu Aug 02, 2018 12:31 pm

that's the short of it.... running rich
I've read the shop manual and it takes you through every system to ascertain every other component is in top working order......my car runs great and doesn't miss a beat, so I refuse to look elsewhere and would rather focus on the Stromberg carb. there has got to be some tweaking or adjustment to lean it out some. I've got black soot on my tailpipe and sitting at a traffic light it smells rich....
is the cure a needle change out? I don't want to make matters worse and can live with it, but would prefer to find a simple fix to stop it from running too rich.
any 1st hand knowledge/experience and cures welcome.
thanks!

'73 Elan Sprint, Big Valve, emission cross over tubes blocked, federal plenum butterflies removed & plugged, headers, stock original air breather, tubing and AC air filter arrangement as original, all vacuum tubing correct from catch tank, charcoal canister, to porting to carbs and air breather, headlight pods vacuum work well, no leaks points, alternator upgrade, larger Triumph radiator
just detailing my particulars....
TED
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PostPost by: gus » Thu Aug 02, 2018 1:06 pm

soot on the tailpipe?

pretty much situation normal

I started writing causes of major issues till I reread your post.

Try replacing needles and seats

CD2SE there is a very small adjustment available in the needle position, or you can change to adjustable needles from a Volvo or TR6

If it is not fouling a plug, it is within normal parameters.

The stock intake is crap, it holds fuel on the crevices, hunt down a euro one.

Major issues are temp compensators stuck closed

throttle bypass valve diaphragm dead

leaky needle valve

wrong [high pressure] fuel pump
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PostPost by: 69S4 » Thu Aug 02, 2018 2:31 pm

Strombergs running rich is not that unusual an issue - it doesn't take much to get them like that. I'm not that familiar with the federal crossover assembly used in 'bypass' mode so I've no idea whether if there are any issues with it but with 'euro' parts the running rich hit list (some of which Gus has already mentioned) for me would be:

Air filter blocked?
Fuel pressure too high
Needle valve not shutting (you often get fuel coming out the breather hole and everything stinking of petrol if that's happening)
Float level wrong.
Worn jet / jet too low in carb body
Needle issues - wrong one, worn out, bias spring broken (sometimes pulls needle upwards), adjustable needles adjusted too high.
Temp Compensators not working.


Do you have any problems with hot starting? On my car the difference between it running ok and running rich can be as little as 10 thou movement on the needle.

With Strombergs any alteration in mixture control is done with the needle - through selecting the correct profile primarily but fine tuned by moving it up / down slightly. If you're changing them I'd suggest you stick to the std needle profiles as given in the workshop manual - B1Y or B2AR were the usual ones iirc. With a std profile new needle and new jet set in correct relationship with each other (easier said than done!) the mixture should be pretty much spot on throughout the rev / throttle range - until everything gets hot when it'll run rich.

The temp compensators are supposed to bleed some air in to weaken the mixture under those conditions. If they're stuck shut you'll get a rich hot idle (hot = the carb bodies heat soaked to around 50C or hotter - usually when stuck in traffic).

Just in front of the bottom of the temp compensators is an screw that bleeds air into the mixing chamber when you undo it. Opening it a small amount will weaken the mixture particularly at idle, but it's not there as a regular control - it should normally be fully closed. If opening it / them slightly improves matters then you know there's a fault somewhere else.
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PostPost by: collins_dan » Thu Aug 02, 2018 6:02 pm

Ted, Do you know what you have in there now for needles? Are they the standard fixed piston needles or the adjustable piston needles? Are any of the spark plugs more sooty than others? Front 2 or back 2? If you pull out the piston, are the needles mounted flush with piston, proud, or recessed? I'm trying to remember which causes rich, I think it is if they are proud. Pushing them in further makes it run leaner, I believe, but someone check my memory.

I used to have B1Y and it ran rich at idle and around town, and lean when I did more highway time. I went to Gower & Lee and got:
2 x B1BT metering needles
1 x B20379 adjusting tool
2 x second-hand pistons, chemically cleaned and fitted with new adjusters & fixing screws
and they have been great ever since. The other thing to check is sparkplugs. I use NGK BP6ES. I think it had 7s in before and they built up a lot of soot.
Good luck. Dan
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PostPost by: david.g.chapman » Fri Aug 03, 2018 10:24 am

It's the other way around Dan. Raising the needles into the piston makes the mixture richer (smaller needle cross section in the jet).

Sounds like this is a Federal engine with "fixed" needle carbs - like mine. If the piston is removed there is a screw in the side to release the needle, then you can re-position it up or down by about 50 thou - plenty of range.

The needle position is the single most important thing to get right. I would note where each needle is and then lower them by 5 thou to see what that does to lean out the mixture - it's quite sensitive. I would use a 5 thou feeler gauge against the needle to start with. Hold the piston up to the light and compare the width of the gauge with where the needle shoulder is, then re-position the needle downwards by the gauge width.

If the shoulder is hidden then it's more tricky. Just try and get the needles down by visually comparing the change against the gauge. Others might think of a better way but this has worked for me in the past.

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PostPost by: 69S4 » Fri Aug 03, 2018 10:53 am

I've been playing with 10 thou movements of the needle and you can really notice that. My pistons are the adjustable needle sort so it's easy to see when the shoulder is level with the bottom of the piston and use that as a starting point. Pushing the needle further up into the piston richens the mixture and dropping it down weakens it. As a guess if you have to move it more than 30 thou either way there's something else wrong.
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PostPost by: RogerFrench » Fri Aug 03, 2018 2:36 pm

I have a Stromberg-carbed Europa TC, and found that my jets were worn.
New jets and needles 2BAR in my case, though that's not standard for Federal cars, and all was fine.
As said above, it would be helpful to know what kind of Strombergs you have, whether fixed or adjustable needles and`/ or jets.
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