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+--- Thread: Constant misfire ?????? (/showthread.php?tid=9472)
RE: Constant misfire ?????? - Bluebird7 - 14-08-2024
Hi Tony, Dizzy appears ok, good spark etc,
so guess looking a carb next ?????
RE: Constant misfire ?????? - Ruairidh Dunford - 14-08-2024
I find D21 plugs are much better suited to Austin Sevens, than D16.
Sooty suggests too rich and would tie up with miss-firing when warm.
What SU needle do currently have fitted?
RE: Constant misfire ?????? - Bluebird7 - 14-08-2024
(14-08-2024, 06:40 PM)Ruairidh Dunford Wrote: I find D21 plugs are much better suited to Austin Sevens, than D16.
Sooty suggests too rich and would tie up with miss-firing when warm.
What SU needle do currently have fitted?
Hi, Ruairidh, good to chat yesterday !!!
I'll check out carb in the next couple of days and get back to you.
RE: Constant misfire ?????? - Ruairidh Dunford - 15-08-2024
I recently had a misfire (more of a spit) issue at around 3000rpm that appeared to give symptoms of a weak mixture.
After many frustrating adjustments, and with little else to actually try, a much weaker needle was installed and has allowed clean and sweet running ever since.
RE: Constant misfire ?????? - NBS - 16-08-2024
Is there oil in the damper pot?
RE: Constant misfire ?????? - Bluebird7 - 16-08-2024
(16-08-2024, 07:59 AM)NBS Wrote: Is there oil in the damper pot?
Yes.
RE: Constant misfire ?????? - `Gordonhbm - 16-08-2024
It is worth checking that the piston travels freely in the dashpot. I would talk to Burlen on how to carry out a piston drop test for your specific model and what the drop time should be. Also make sure that the holding down lugs on the dashpot are not damaged in any way as this can resulting the dashpot not sitting down correctly and interfering with correct piston movement.
RE: Constant misfire ?????? - Bluebird7 - 16-08-2024
(16-08-2024, 12:49 PM)`Gordonhbm Wrote: It is worth checking that the piston travels freely in the dashpot. I would talk to Burlen on how to carry out a piston drop test for your specific model and what the drop time should be. Also make sure that the holding down lugs on the dashpot are not damaged in any way as this can resulting the dashpot not sitting down correctly and interfering with correct piston movement.
Not sure SU were ever fitted as standard to Austin 7's, so it's unlikely Burlen could answer the question, but as so many SU's have been retrofitted, what's the best needle for my 1.1/8th dia model possibly ex 803cc Morris ????
RE: Constant misfire ?????? - Malcolm Parker - 16-08-2024
EB needle is mini 850
AN is Morris 1000
Both the above should be okay
I am using an EK which works well.
On the trials Chummy I believe Steve used a GG (for extra Horse power
)You can compare different needles on the mintylamb website
On my set up whether there is any oil in the dashpot doesn't seem to matter.
RE: Constant misfire ?????? - `Gordonhbm - 16-08-2024
Dashpot drop test figures are dependant the SU model not the car so there is no reason that Burlen can't give you the details for the particular SU carb number that you are using.
The last car that I had an SU on developed the sort of mid range hesitation that has been mentioned on this thread. Cleaning the dashpot interior with methylated spirit showed several bright spots where there had been contact with the piston. I removed these with wet and dry 2000, lubricated with thin oil, applied very gently to just the bright spots until there was no evidence of contact the you spun the piston (with piston rod lubricated with WD40) with the dashpot inverted and allowed it to fall under gravity. It is easy to hear if there is any piston to dashpot contact. This resolved the problem.
The carb had originally been set up by my local rolling road by an SU expert. His method was firstly to ensure that the SU piston would rise to nearly the top under full power adjusting piston weight, (or spring strength if carb had one). He then started with a needle that was weak over the full range and then proceeded to delicately file a flat progressively along the taper starting at the top of the needle and checking the mixture was right at that point and then increasing engine load to raise the needle about an 1/8th off an inch and repeating the filing till the mixture was correct for the new needle position. In this way he tuned in the needle along its whole length with a variable filed flat along the length. That the needle had a flat is not important as it is the relevant cross sectional area difference between needle and jet that controls the flow of fuel. Apart from when the piston/damper pick up occurred I never needed to adjust the carb again whilst I owned the car