15 replies to this topic

[patslfc]

Just fitted one and plugs are black Need to change it and help

[finch661]

you can adjust it by the big nut the jet goes into (under the carb). move down to richen and up to weaken :) get either an exhaust analyser or colourtune (glass spark plug) to set the correct idle mixture. If you find that the car isnt running correctly, or is using too much petrol, consider a rolling road

have a look here if you are unsure, http://www.theminifo...0 scroll to the 'Waxstat' part, you can see the bit adjusting nut

Edited by finch661, 16 March 2012 - 12:05 PM.

[R1minimagic]

There is also a piston lifting pin on the carb which can assist setting the mixture, it should be set so the exhaust is smooth sounding with no popping

Now check the mixture strength by lifting the carburetter piston (by means of the lift pin situated on the side of the carburetter body) by approximately _^1/32 in., when if
(1) The engine speed increases and continues to run faster, the mixture is too rich.
(2) The engine speed immediately decreases, the mixture is too weak.
(3) The engine speed momentarily increases very slightly, the mixture is correct.
When the mixture is correct the exhaust note should be regular and even. If it is irregular, with a splashy type of misfire and colourless exhaust, the mixture is too weak. If there is a regular or rhythmical type of misfire, together with a blackish exhaust, then the mixture is too rich.

Edited by R1minimagic, 16 March 2012 - 12:18 PM.

[dklawson]

Setting the mixture is basically as described above but it is not a strictly trial and error process. The Color Tune or gas analyzer will be very useful. However, you can get an HS series carb pretty close without them.

Rather than repeat the entire step by step process here I encourage you to Google on the topic of "tuning SU carburetors". There are LOTS of web sites that describe the process (as do most manuals). In this case, I assume you only have one carb so the part of the practice involving balancing will not apply. The lift pin and monitoring the idle RPM as described in texts will get you where you need to be.

[goat64]

Only 10 years late to this post...

continuing saga of my mini 850 (1976)....

 

I've failed to get the mixture correct using piston lift method (single HS4).

Whenever I push up under the pin, revs rise and stay up, suggesting rich mix.

Plugs confirm mix too rich: sooty within few miles of cleaning. Not wet, not oily, just sooty and smell of petrol (all 4  the same).

I screw the adjusting nut upwards towards carb body, to weaken mixture and check again

Here's the thing: It NEVER reaches "too weak" point. I can screw the adjuster right up against the lock nut under the carb body and it STILL tells me "TOO RICH" when I lift the piston using the pin. Surely in that posiion it should shut off fuel completely.

When I enrich the mix (unscrew the adjuster downwards away from carb body), revs drop off and tries to stall at just 5 flats (5/6ths of a full turn). This is also a bit odd, as default starting posiion is usually advised as 2 full turns (12 "flats") open.

Another clue: It NEVER needs any choke, even on cold, damp mornings. It fires up brskly enough 1st time, bu if you touch the choke, it rapidly dies.

 

I have tried / checked these things so far:

• ignition: bright, regular spark at all 4 plugs, from new coil, breaerless "points", new HT leads and plugs. timing was strobe set within the last year and the dizzy is soildly clamped, so no reason to slip out of timing. .(by the way, new HT leads and breakerless points made a BIG difference to prev running probs)
• fresh fuel after new fuel tank and flush through.
• good consistent level of clean fuel in float chamber
• cleaned carb piston / needle, damper (gently but thoroughly - not convinced any gunge)
• tried new needle, but made no difference so put old one back in.
• renewed damper pot with clean engine oil (Haynes modofications chapter suggests this is fine, as do pots on here and other fora
• checked choke cable and throttle cable are not guilty.

It runs and drives OK, if a bit smokey and lumpy, starts OK (cold and hot), so I've ignored it so far. But i can't help feeling it is not right, and possibly getting worse, whatever I try.

 

I've trawled this and other groups, and the wonderful SUCarb site, but nothing quite matches the symptoms.

 

What shall I try next? I have some spanners and a screwdriver, but nothing much fancier than that!

 

Thanks in advance

 

Goat

 

850cc, 1976, no mods apart from breakerless points.

[timmy850]

I would check and possibly replace the jet, as it may be worn oversize and cause rich running

If the fuel bowl level is too high it can also cause issues

[ACDodd]

The carb is worn out replace it Or rebuild it fully.

Ac

[Spider]

Check the fuel pressure, the needle & seat (in the float bowl) and the float level.

[johnR]

When you say you tried a new needle do you mean you tried a different profile one? Although at idle I believe most are the same there are leaner ones available

[Chris1275gt]

I had numerous problems with my Hs4 I purchased a refurbished one off eBay from mini1275sub bought a colour tune plug and never had a problem since. The old one was sold on eBay as in need of a refurbishment. As a novice I had no idea what I was listening for and could never get it correct from reading it from a book but with the plug it was easy and the engine noises began to make sense.

[coopertaz]

check corrrect needle if right, is it bent? if all ok rebuild carb with full kit,

[andyt]

Go to one of AcDoddS tuning days, the only way to get your fuelling correct, been there done that👍

[Mr Piggy]

Is the needle pushed too far into the piston? The shoulder should be just level with the bottom surface.
Alan.

[goat64]

Latest chapter in the ongoing saga....

Have tried the fixes suggested:

float chamber and valve cleaned / levels checked,

needle cleaned and seating in piston checked (it was a bit too low),

carb cleaned out thoroughy , new seal in fuel line between float chamber and jet

inlet manifold checked for leaks: none apparent, but renewed gaskets anyway

new plugs and leads

 

All of thi has resulted in a car that starts and runs reliably (Phew!).

BUT:  NEVER needs any choke, even on a cold winters day, exhaust still black and smokey, pugs very black and sooty (dry), does no more than 25 mpg (850).

i.e.: It's still running rich,

All this with the needle seat screwed right up as far as it will go against bootom of carb body, that is to say as rare a mix as I can get it.

 

A wise man on another forum (there are many) once advised check for inlet manifold leak, which I have done, but appears not to be one.

However, i have noticed a strange thing which may give somone a clue: When I pull off the crankase breather tube and put my thumb over the pipe into the carb, it runs VERY differently from when it isconnected up properly. I can reproduce this behaviour by putting my hand (or several layers of cling film) over the oil filler opening (cap off) when it's running.

 

Could it be that a leak of air around oil filler cap into rocker cover area then via crancase breather tube to carb (on engine side of butterfly, I think) might be responsible for inability to tweak mixture rare enough???

 

I can't seem to get ananswer to the simple quetion: "how airtight does an oil filler cap need to be?"  (in the context of a crancase breather tube to inlet manifold).

 

I may try and seal the filler cap with layers of cling film under the cap (temporary fix) and re-attempt mixture setting, but loathed to fiddle with a car that runs OK at last, albeit way too rich.

 

Any thoughts on air filler cap clue?

[Rorf]

I believe an HS4 is too big for your 850 engine, go down a size to an HS2 which is what it originally came with