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1976 Spitfire 1500 Rough running when cold ??


alan.gilbert_6384
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Hi All, Hope you're all well.  So here is my problem, spitfire 1500 1976, when its warm, its great runs really well, but from cold its rough, won't accelerate properly etc.  As it warms up it stays the same rough running until a few minutes after maximum temperature is reached ie the thermostat opens, then a few minutes after further running all is well.  It starts OK on choke with small amount of accelerator pedal etc.  The timing is 10 BTD, dwell is good (yep still on points but will replace later with a spark rite unit).  The carbs are balanced and the mixture set as per book and colour tune checked (plugs a little black after a good run, not too much, I think this maybe old piston rings leaking a little oil, but no smoke out of the exhaust).  Is it me just not remembering how things were in the past ??

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Could be something like the carb jet is not returning to its seat after the choke has been out. Is the inlet manifold coolant tube in good condition?  Have you check the valve clearances at cold?  
Have you oiled the dizzy including the felt pad under the arm.  Check the position of the carb linkage arms have clearance at the end to the hex nuts as the small springs sometimes are not man enough to overcome the friction when they bind.    

Maybe need to describe rough running a bit more.  Normally engines develop misfires and other symptoms at operating temperature and under more load. 

start car on drive from cold and gradually release choke to all the way in and look for anything not being quite where it should be. Particularly, push each red jet on the bottom of each carb to see if it’s not quite all the way home. 

Does the idle ignition timing look very different between rough running and when it is warm?

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How quickly do you put the choke in? Have you tried different approaches? Several of my cars have had a tendency to want different choke for driving from idle, until they're warm. Mind you, I'd expect that behaviour to clear before "maximum temperature".

Also, do you have a compression tester? I've come across engines which misfire during warm-up but not when fully warm, due to valve clearances, which often show up as poor readings on a compression test.

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Hi thanks for the suggestions I will check them all out.

already checked the carbs and all levers all Woking ok and returning ok, the coolant tube is good and no losses anywhere, dissy is new and oiled, jets are non wax and operating correctly will check the small springs etc.  Timing remains the same warm or cold.  Just ordered a compression tester gauges and will check these when it arrives.  The head was fully refurbished approx 5000 miles ago.  I will check all the valves again, thanks all, will let you know how it goes.  I usually start to gradually push the choke in as it get warm, will try other things next.

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Most SU do  need a very good fast idle or the richened fuel  mixture overcomes the running  so with choke out it needs over 1500  + rpm 

or it will choke itself 

then sooted plugs can be unretrievable   do not have plugs with an R in on std coil ignition

 

worth a browse     

 

 

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  • 3 weeks later...

HI All,

Update;

I have now done the compression test cold and hot, good news is that all cylinders are the same 118 psi cold, 120 hot.  I thought there might be some variation but not as it seems. :)

Does anyone know the correct set up for the coke system (I have looked in the Hayes and the workshop manual, but unless I am going mad, a possibility, all they mention is tuning and not basic set up to start with of the coke)?

Hopefully you can see the attached video.  It seems to have 3 phases of operation.  The first is large amount of movement which does not seem to do much at all.  The second then lowers the jets.  The third rotates the butterfly's, very marginal as can be seen in the video.

Does anyone have any ideas if this this is good ??????  Thanks for all your suggestions and help so far.

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well the choke cable is stiff in its outer as when you push the control in the  outer rises out of its abutment 

i would remove the inner and grease it up before refitting 

the first part of pull should rotate the cam and the contact screw that  should have a small gap (choke off)  will start to open the throttles 

not the first time the cam needs a attack with a file so a theres a small gap  and   the cam  actually raises the idle  ie reprofile the cam .

the 2nds part should start to drop the jet    and fully puled should fully drop the jet

plus the more the choke is pulled the cam should apply more increase to the idle upto 1500/or more 

Pete

 

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On 10/09/2021 at 19:31, Pete Lewis said:

i cant see the choke cam wears but on many the lift in idle   is just rubbish 

on many i file a flat to increase the amount you screw the tappet screw out , always keep a small gap 1-2mm but then the cam does some work and now opens the taps 

pete

That's exactly what I did on my 1500 Spit to get more lift. Made all the difference. 

Danny

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  • 2 weeks later...

Hi All,

 

Update on the rough running when cold.  Having looked at all the above and tested everything etc, I decided I push the boat out and buy some new plugs.  I did not do what I did before and buy some fancy plugs (see pictures, I could never workout how to set the blooming gaps anyway, these blighters did about 700 miles :().  Did a bit of research ref iridium, platinum, copper etc  As it turns out the fancy (iridium, platinum) is only good for plug coil types (these are the individual coils attached to each plug on modern engines) due to the much higher power throughput, so not only was copper the cheapest but the best for single coil distributor type ignition types.  Bit more looking around and it was concluded that a hot type pug was needed.  So invested 10 quid in some NGK BP5ES and 10 quid in Champion N12Y (the champion pugs are slightly hotter than the NGKs).  Fitted the NGK's so far all is good and running very well now.  Just goes to show the work shop manual is usually right and modern stuff is not always better than the 70's engineering.  I can only think that the fancy ones or one of them, only produced a decent spark when very hot, difficult to get to when its not firing.  So it relied on the residual heat from the the rest of the engine to get it hot and then started to work.

Interestingly the bloody lawn mower decided it was going to play up as well.  It would start and run very well for about 5 minutes, then stop, and not restart.  If left for a while it would start and run again but for a short period.  After looking at the manual fault finding guide it was decided this was fuel related and the bowl not filling up quick enough, sounds plausible ?  Stripped it all down, found no problems, blew it all out with the compressor, all the way from the tank and through the carburettor.  Put it all back together, no flipping change same as before.  So as I was ordering plugs for the Spit, decided to splash out 4 quid on a new plug for it as well, with the full expectation that this won't fix it, but hey oh.  Fitted the new plug before I did anything else (change one thing, at one time, a hard lesson learnt many years ago) and gave it a try, it works perfectly now.  The reason I wrote this last bit is that it seems to have the opposite route cause to the spitfire's ie the plug works when cold but not when hot, go figure, as our cousins across the water say.

Hope this helps.

Cheers to Pete for the plug stuff above.

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