Spitfire Mark 3 - misfire

[Julian Wilson]

Hi, new to Forum. Had Spitfire a long time but has been out for limited summer drives since rebuild in 1998. It is Mark 3 but has Mark 2 1147 cc engine - only one I could get many years ago after blowing original on Westway in London. 

The problem has been how badly it warms up before driving; as long as it was left totally alone while warming up ( like a actor before a big stage role) for 10 mins or more and throttle not pressed - and never driven till warm - it drove like a dream ( through the Northumberland countryside). But any slight change to the "pre-stage process" then sooty spark plugs and no joy. Even got to point of warming it up with one set of plugs; then changing to clean set and would drive great.

So it now has new thermostat - no change (it runs with standard 4 blade fan). washed out the water system ..... no difference of course. I didnt recore the radiator when I did re-build which was probably an error. Could that make a difference ?????

Gave it to trusted garage; always wanted simplicity of electronic ignition (so electronic distributor replacement (Accuspark I think it is called) - so changed that; refurbished the carbs (new jets etc) and set ; timing set for new electronic ignition (garage says); compression test done - no problem with compression - in fact compression is good; 

It has not had heat shield for carbs for years so bought one of those the other day .......will that make a difference ?????

Garage says now starts fine but once at temp sounds like a bag of spanners and plugs still soot up. It was not tested on open road by garage and the sooting of plugs was just revving it in garage setting. 

Appreciate this could be anything but any direction would be welcomed 

Best Julian 

[NonMember]

Almost certainly a fuel / mixture problem and ignition changes or new thermostats won't help.

What's your choke protocol? The handbooks always said NOT to warm it up by idling but to just get on and drive. You should need a fair bit of choke when it's cold and progressively push it in as you drive. If it will start without choke then the mixture is pig rich and it's no surprise you get sooty plugs.

What exactly was included in your carb refurb? Did you check for wear on the throttle spindles?

[Pete Lewis]

agree something is making it run very rich 

operate  the choke when it is pushed home get a finger under the jet  see if it pops up to sit against its adjusting nut  ....if so the linkage and jet are sticking down in the choked position

air filters  What type   always make sure the carb vent holes in the filter face are not blocked by an upside down air filter   a common error

Pete

 

[Badwolf]

I had a problem with the choke on one carb. Turned out to be a bent linkage arm. Reshaped it, ran fine. So simple in the end. Even the old school garage had given up!!

[DanMi]

what needles are in the carbs, are they carbs from the mk3 and if so have you put mk2 AN needle in. MK2 and I believe MK3 didn't have heat shield so that is not an issue

[Pete Lewis]

add to that has some past work ended up with the yellow bed springs on  the  air piston 

a sure way to make it rich 

Pete

[MSZ]

I always start my spit and let it get to temperature before heading off. The choke I gradually push home after 15-30 seconds.......of course every car is different with its own temperament LOL.

Mk3 spits always run rich at idle, even when using a colourtune kit, you have to run it at different RPM's to adjust jets. but you're saying that after warm up the plugs are sooted up? Mine always are after warm up before I head off, only after I have a run do they go a slight charcoal/brown and all that soot is burnt off....................................what if its the same problem I experienced lately see my post 'Spitfire mk3 - spitting'.

 

Mike

[Pete Lewis]

there is no acceptable reason for any car to run rich at idle  never designed to and unless you are masking some other defect then 

thats not correct 

if you start on choke  then it can take many miles to de soot the plugs  and get them clean 

dont let cold start soot from using choke to decide any course of action 

you may need to reprofile the fast idle cam and adjust its screw to get a 1500/1800 fast idle on full choke a file on the cam lead allows better adjustment and an increase in idle as too slow will choke itself 

Pete

[Jeffds1360]

Julian, send me a PM., I'm just along the road and can pop along for a natter. Two heads etc etc.

[DanMi]

check that the engine is genuine mk2 fc> 50000 and collets on the valve tops. If it is a herald engine then the twin carbs are likely to be set completely wrong. Has it ever run properly since you fitted the 1147cc, a MK2 engine is a great little engine.

[Jeffds1360]

I hope Julian does not mind me mentioning that he is away for a few weeks and we are hoping to meet up early November. 

Personally I think his choke use is causing the problem and with my 1970s experience with spitfires and my 2020 experience with the mk3 in my herald, I'm sure we can find the best setting and cure his richness