Club 80-90 Forums

Just how easy is it to tweak the carbs etc to stop my engine running rich?

Thanks,
Sam

Rolling road for a proper job

thanks Kev,

simply cant afford to take it to a garage at the mo though. Is there a 2nd best DIY fix thats not too difficult?

what carbs are they?

it's the standard twin solex s

put it this way, 80 quid on a rolling road will ensure your motor is right and safe.

fiddle with it and run it lean and you will be buying a new engine.

save up, its worth it.

ok, I hear ya! Guess I'll have to take the hit in the pocket in the short term though from running rich!

Once it's set up properly should it stay right for a while?

I'm no expert on your particular setup but what I do know is that on fixed jet carburettors the 'running mixture' is controlled by changing jet sizes to richen or weaken mixture under real time conditions (thats normally a rolling road under load and at revs)
At tickover the carb resorts back to using the idle jet (CO screw) and or the air by-pass valve (screw) so when you are adjusting the mixture (CO) you are effectively only doing this for idle.

Martin

So to continue from Martins post...
it begs the question how do you know it's running rich?

to test this theory you must take the engine up to working temp, run it above 3000 rpm for a mile or two switch it off and check plug colour

Carburation is a interesting subject, with the ideal air fuel ratio being in the region of 14.7:1 (where there is enough oxygen prestent to burn most of the fuel) this is called Lambda 1.0 or stoichiometric 1.0. Trouble is, this ratio is an ideal with the engine at running temperature with a set load and engine speed. However, we know that engines run under all sorts of temperatures, loads and speeds and so your typical carburettor is fitted with several devices that are designed to change the ratio of petrol to air.
These can be:
Two or three staged jetting (delivering progressively richer mixture depending on load)
Accelerator pumps (adding an extra jet of fuel under initial excelleration which richens mixture) more fuel to air.
Cold start enrichment devices (these typically add more fuel whilst the engine is cold)
Bi-metalic choke mechanisms (that restrict air flow through the venturi which richens fuel)
Also devices that weaken mixture (for fast speed cruising)

All this devices in addition to the normal 'JETTING' of the carb, infinately varying the fuel/air ratio depending on conditions.

Incidently....ECU (electronic control unit) controlled fuel injection was introduced in direct response to ever stringent emmision regulations, especially in the US and Japaneese markets. Computer controlled fueling and ignition systems have the ability to deliver even more accurate air/fuel ratios and precise ignition points to attain the best burn/power and economy.

Martin

I'm not 100% certain but just an educated guess from a few symptoms. Worse (even for an Aircooled) MPG, 'fumey' smell at times, old plugs were black.

oh, and when in 1st or 2nd crawling in traffic i can tae my foot fully off the clutch and it wont stall. It just keeps slowly trundling on. Could this be a symptom too?!

Most rich running problems on the Solex carb (and many others including weber etc.) can be controlled by the idle mixture adjustment screw. The reason for this is because the idle circuit is still in operation up to about 2500rpm, at which point the main jets take over. If these are the carbs designed for your engine then you shouldn't need to re-jet them as you should already have the correct jets in place.
If you suspect the mixture is wrong or a long way out then a simple tweak can be performed quite easily as follows:-
Start engine and run up to normal operating temperature to ensure chokes are not closed or partly closed.
Turn mixture screw out slowly. If running improves ie. less misfiring, smoother and faster idle, continue to turn out until revs just begin to drop. If running gets worse, turn in until running is smooth and idle rpm is at maxium. Then turn out a little (about quarter of a turn) and the idle rpm should drop a small amount.
You can then be fairly sure the mixture is about right.
It would still be advisable to get the mixture checked professionally but this can be done at a local garage using a gas analyser. Rolling roads need only be used if re-jetting a carb and the whole range of revs have to be checked but if these are the carbs for your engine then that isn't necessary.

Still moving when foot is off clutch is not a bad thing providing your idle rpm isn't too high and pulling you along at motorway speeds! . Should be around 850rpm at normal operating temperature.

Thanks for the extra input Martin and Mark. I may try tweaking the screw ever so slightly. I guess, if i did accidentally go slightly lean, i'd hear the engine pinking?

Only under load ie. uphill, possibly in a high gear.
If adjusted as I mention, turning the adjustment screw out a little to reduce the rpm a touch will make the mixture rich rather than lean. Not too much to be runing rich but enough to prevent lean running and pinking which can lead to overheating.