7 replies to this topic

[FDW]

HI everybody, my Mini Mayfair 998 came to a grinding halt last Friday evening.

 

It ended up leading us to a new condenser and I've got it running since however not very well. It's had new points and new plugs along with the condenser and is currently running smooth at idle and revs freely on the drive in neutral. The problem comes when its being driven, there is a point in the rev range where the engine gives in and has a horrendous misfire so much that I physically cannot rev it higher. It doesn't matter whether I approach this point at full throttle or half and the point on the rev range decreases the higher gear I'm in so can't go above about 35mph currently. 

 

• Compression is  ~ 150psi on all cylinders
• Have used two separate ignition coils and sets of leads to no avail
• Can't see anything wrong with dizzy cap but a new one is on the way anyway
• Tested timing at idle and full advance with strobe light and is fine (vacuum advance working too)
• Bob weights in dizzy seem to be moving freely and fine
• Have cleaned and inspected dashpot, piston and needle and refilled with my usual 3 in 1
• Spark plugs are all black and sooty suggesting suggesting rich running, yet the jet (non waxstat) is in usual position and is at a happy medium when running at idle. Hard to tell if its running rich or just soot from the misfiring 
• Fuel tank vent is clear
• Float bowl and needle valve working correctly 
• Air filter clear
• No obvious vacuum leaks on carb. 
• Valve clearances, points gap and plug gaps all set.
• Points open on all lobes of dizzy cam 
• Have been through two condensers since the break down, first was crap but I still had this problem, the second one from a different manufacturer improved the running but this problem still remains.

 

Before it broke on Friday it was running very well and had been for many hundred of miles. I've running out of things to check. I have a different air filter and corresponding needle that I may fit to see if there is a change. I'm thinking that something may still be going wrong with the timing under load, or the carb is not getting enough fuel for some reason. A sticky dashpot piston is an option but having rebuilt it last year and checked today I can't see it. I would usually think of a vacuum leak in this scenario too but that would usually affect running at idle too. The problem doesn't come on gradually, it just hits a point where it gives up. Any help anyone has would be much appreciated, I'm struggling with this one big time. It drives and starts very well with plenty of power as long as I keep it under the rough 2-2.5k rpm where problems start. 

 

Cheers

Edited by FDW, 24 March 2022 - 06:26 PM.

[sonscar]

Rotor arm could be breaking down and allowing the spark to leak through it to the dissy shaft.Try new sparkplugs.Good luck,Steve..

[Earwax]

Hi FDW

 

I had a old Landcruiser petrol with very similar problems - ended up being crud in the tank and especially the fuel pick up.-  The coarse fuel filter if running electric pump could also have picked up a flake from inside the tank? I tried changing filters very regularly (just cheap inline ones), i tried metho in tank to bind water   -both of which had some success, but in the end tank out and complete thorough clean showed just how much crud had accumulated .

[FDW]

Hi FDW

 

I had a old Landcruiser petrol with very similar problems - ended up being crud in the tank and especially the fuel pick up.-  The coarse fuel filter if running electric pump could also have picked up a flake from inside the tank? I tried changing filters very regularly (just cheap inline ones), i tried metho in tank to bind water   -both of which had some success, but in the end tank out and complete thorough clean showed just how much crud had accumulated .

Cheers, was considering taking a look at the inline filter and fuel pump, I know the inside of the tank is very clean inside but nothings out of question further down the fuel lines at this point. 

[MatthewsDad]

Hi, just about to post a new thread myself relating to a similar problem on my SPI. For me, the problem appears to have been a combination of a partly broken spade connection on the feed to the coil, and a faulty coil. Check resistance in the coil.

[cal844]

Coil failing or too thin an oil in the dashpot.

[FDW]

So today's update, spark plugs cleaned up, fitted the new air filter and needle I had been planning on for a while. It's immediately running much better, isn't coking up the spark plugs and I can accelerate through the whole rev range. It does bog down a bit at lower revs when half throttle accelerating and doesn't seem to have quite the top end it did before but much better. Swapping the 3 in 1 to 10w40 helped a bit, so will try some 20w50 in dashpot next but I'm not convinced it'll solve it fully. Could be the needle isn't 100% for purpose so maybe a proper tune would work but for the most part its driveable and much better than before. Yet I still can't quite fathom how the set up the car has had for nearly 5 years can suddenly 'go bad'. 

[cal844]

Your mini would have been set up from the factory with 20w50 engine oil in the dashpot, so worth a try.

perhaps go back to the other needle and try that with the 20w50 in the dashpot, so you don't make too many variables