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What Should A Properly Set Up 998 Behave Like?

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#1 bluequinn

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Posted 28 November 2023 - 05:53 PM

This is a question rather than a problem.

 

My 1991 Mini (bog standard 998cc) is almost certainly running a little rich. It runs great, but there's a fair bit of fuel smell and when I cold start it I only need the choke to start it, then I can immediately blip the throttle and I push the choke cable back in again. (Kind of like my old Vespa actually)

Reading this forum (and comments about clothes pegs on the choke knob) leads me to believe it's not supposed to be this easy.

 

Adjusting the carb mix will be no problem, but this is the first Mini I've owned, so my question is how will I know when I've got it set up right? What do they normally behave like on a cold start?



#2 greenwheels

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Posted 28 November 2023 - 06:25 PM

First thing I've always done to a newly acquired Mini is to give the carb a new jet and needle spec'ed as the manual. Set it up as the Haynes manual, the jet two turns down from flush if memory serves me right for an HS4. Clean out the float chamber also and if the float valve looks worn renew it. Make sure the choke cable etc. is working OK. New set of plugs also. Timing set accurately and the dizzy advance weights all OK.

Then after a good run(s) tune the carb to give plug colour as per the pictures in the Haynes manual. should be a nippy little fun motor.

I once miked a well used needle and was amazed how much it was worn.



#3 cal844

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Posted 28 November 2023 - 08:06 PM

If its running rich (sooty plugs off choke) I'd say you need the engine tuned and optimised by someone like AC Dodd (or someone with his training)

Well worth the money

#4 timmy850

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Posted 28 November 2023 - 08:14 PM

With mine I normally use the choke till I’m off the driveway and driving, then I can push it back in. If I come to a stop before it’s come up to temp I may need to pull the choke out again to keep it idling

Adjusting an SU carb to run correctly is much more detailed than simply adjusting the mixture nut on the bottom. If you have the idle mixture set too rich to make it drive better, then it’s likely the needle is wrong or the carb is worn

The best way to set up the carb is using an air fuel ratio gauge and an O2 sensor in the exhaust. Using the gauge you can set the idle mixture correctly, and then from there measure if the mixture is correct across the rpm and load range

#5 Bobbins

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Posted 28 November 2023 - 08:28 PM

The first part of the choke cable movement is just “fast idle” which often needs using until there’s enough heat in the engine, it doesn’t richen the mixture. If you can run the engine from pretty well full cold without using the fast idle the mixture is definitely rich!

#6 bluequinn

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Posted 28 November 2023 - 08:49 PM

I can't take it to AC Dodd. Well, I could, but that's in the wrong country for me, sadly. I don't know any local French specialists.

And while I think a rolling road setup would be worth it if I did find a good person, I'd probably save that excitement for after I'd fitted a stage 1 kit.

 

The car is super low mileage (33,000 miles give or take) so needle wear isn't all that likely.

I had forgotten about the timing though. The previous owner may have mentioned an electronic ignition conversion. I guess it's time to take a look and see if he was right.

I really should check the plugs too. I've not bothered and it is so easy to do!



#7 timmy850

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Posted 28 November 2023 - 09:19 PM

If it starts easily and runs from cold the ignition system must be in decent condition (perhaps not optimal but good enough). Of course checking the leads, plugs and gaps, coil type will be of benefit but that can’t be a big problem in your case

#8 mbolt998

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Posted 28 November 2023 - 10:25 PM

Yes you should normally need a bit of choke for the first maybe half mile of driving or so. I would start by just leaning it off (turn the big nut underneath the carb, I think you probably want anticlock as viewed from above-- i.e. screwing it in-- to lean it but I may be wrong). If you just set it to whatever runs best, then turn the idle down a bit to something sensible (as it will probably have gone up), and then check again (just set the mixture at which it idles best) and that will put you in the right ballpark.

 

I also have a "Gunson's Colourtune" now which is pretty cool but I always set it in the past just sort of by ear like that.

 

You can also tell if it's too lean if pulling the choke a bit makes it run better when it is warmed up (but I think yours is too rich currently). Also check there's oil in the dashpot. There's a proper oil you can buy but engine oil is also fine.

 

This is my experience. Others on here are much more knowledgeable. Just advising on what's good enough and works for me.



#9 Ethel

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Posted 28 November 2023 - 10:53 PM

As above, the clothes peg is really just enough to hold it on fast idle.  So it may just need the idle adjusting. Plug colour after a run that's long enough to get it up to running temperature is a better indicator. You can use  the piston lifting pin, if it has one, or a bent coat hanger wire substitute down the filter elbow if it hasn't. Of course being right at idle doesn't guarantee it'll be right under load, or vice versa.



#10 coopertaz

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Posted 29 November 2023 - 11:07 AM

a 1991 car in uk will most likely be fitted with hif carb (large flat base) not hs (jet visible below cars and float bowl on side) so no lifting pin or adjusting nut. on these carbs the mixture screw is in a recess on the carb bodies lower left side and screwed out (anti clockwise when viewed from end) to lean mixture till engine speed just starts to fall. obviously when engine up to temp.



#11 Ethel

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Posted 29 November 2023 - 12:31 PM

If it's a HIF then there's at least one choke jet O ring worth checking for failure.



#12 bpirie1000

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Posted 29 November 2023 - 02:48 PM

May I also suggest checking that the choke cable actually goes to the off position. If the cable has been replaced(face it, very likely) the could have been done and be constantly on hence lack of need for,choke to,be on for long....

Worth checking...

#13 ac427

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Posted 29 November 2023 - 06:52 PM

I can't take it to AC Dodd. Well, I could, but that's in the wrong country for me, sadly. I don't know any local French specialists.

And while I think a rolling road setup would be worth it if I did find a good person, I'd probably save that excitement for after I'd fitted a stage 1 kit.

 

The car is super low mileage (33,000 miles give or take) so needle wear isn't all that likely.

I had forgotten about the timing though. The previous owner may have mentioned an electronic ignition conversion. I guess it's time to take a look and see if he was right.

I really should check the plugs too. I've not bothered and it is so easy to do!

 

AC Dodd does offer remote diagnosis too.



#14 bluequinn

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Posted 04 December 2023 - 03:28 PM

Oh my! That’s a service I may well consider.

#15 ac427

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Posted 07 December 2023 - 06:54 PM

Oh my! That’s a service I may well consider.

 

AC released a video on the subject a few of days back.

 


Edited by ac427, 07 December 2023 - 06:55 PM.






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