Alans835 Posted June 16, 2022 Report Share Posted June 16, 2022 I have a mk4 spitfire, starts and runs smoothly on the drive but under acceleration it bogs down and eventually cuts out. The car is fitted with accuspark electronic ignition and has a good spark. Compression test carried out and all are around 150psi. Timing has been set to 10degrees before tdc. The twin su’s have been stripped and rebuilt using a rebuild kit when running I have checked the butterfly shafts to ensure they are not passing. Jets have been set to 10 flats down from the bridge and both carbs balanced. New spark plugs fitted. There is a new inline fuel filter and I have removed and cleaned the mesh inside the fuel pump. Floats have been set and looks like a good fuel level in the chambers. But I still get bogging down when trying to drive. I’m running out of things to check. Any help would be appreciated. Frantically trying to get it sorted for a show on Sunday! 🙈ðŸ˜ðŸ˜‚ Link to comment Share on other sites More sharing options...
johny Posted June 16, 2022 Report Share Posted June 16, 2022 Do the two carb air pistons rise and fall ok when you look into the carb mouths with the air filter housing removed and rev the engine? Link to comment Share on other sites More sharing options...
Alans835 Posted June 16, 2022 Author Report Share Posted June 16, 2022 Hi Johny yes both move freely. I have used 3in1 oil in the dash pots. Might try engine oil instead as I have read that the wrong oil can be an issue but not 100% on this. Link to comment Share on other sites More sharing options...
johny Posted June 16, 2022 Report Share Posted June 16, 2022 yes engine oil better and will give a richer mixture on acceleration... Link to comment Share on other sites More sharing options...
Alans835 Posted June 16, 2022 Author Report Share Posted June 16, 2022 Will give that a try tomorrow. Thankyou fingers crossed. Link to comment Share on other sites More sharing options...
Josef Posted June 16, 2022 Report Share Posted June 16, 2022 You say 10 flats down, that sounds like the ‘get the car started’ position? Have you checked / adjusted the jets? Bogging down sounds a bit like it could be way too rich or way too weak mixture. Link to comment Share on other sites More sharing options...
Badwolf Posted June 16, 2022 Report Share Posted June 16, 2022 For goodness sake, don't let PeteL even think that you have something like 3in1 in the dashpots. Good old fashioned clean engine oil. Just wait for the lecture... it's coming!!! Link to comment Share on other sites More sharing options...
Pete Lewis Posted June 16, 2022 Report Share Posted June 16, 2022 too late  spotted the 3 in one  have to ask why you go through all the rework and setting up then ignore the manual when it comes to dashpot oils the basic is a straight sae20 but 20/50 is perfect i have no idea what viscosity 3 in one is or what happens to it when its hot ,, a bit cats pee comes to mind the dashpot is your accelerator pump to richen when opening the power if it rises too quick you just make a giant flat spot does that help  without a lecture  Ha ! Pete  1 Link to comment Share on other sites More sharing options...
NonMember Posted June 16, 2022 Report Share Posted June 16, 2022 7 minutes ago, Pete Lewis said: the dashpot is your accelerator pump All very true but... 2 hours ago, Alans835 said: under acceleration it bogs down and eventually cuts out Note the word "eventually". Too thin dashpot oil means you don't get the transient enrichment you need for the first second or two after opening the throttle. If that were the only problem then the engine would pick up again after the initial hesitation. So, Alan, how long do you mean by "eventually"? Link to comment Share on other sites More sharing options...
Alans835 Posted June 16, 2022 Author Report Share Posted June 16, 2022 So the 3 in 1 was something I had heard, however I can’t remember where or when I heard it haha will change it tomorrow! 🙈😂 when I say eventually probably a few seconds but depends on what I do with the throttle i can sometimes catch it before it dies and keep it running but only sometimes eventually it will stall. And at that point it’s a pain to start again. The mixture is rich according to the plugs as they were black and sooted up tonight. Gave them a clean and managed to fire it up again to make it the 600yards home before it died again. At that point I said screw it time for dram!  Link to comment Share on other sites More sharing options...
Peter Truman Posted June 16, 2022 Report Share Posted June 16, 2022 I do use engine oil, but as I service the car with Penrite 20/60 that's the weight I use, in the past I've used 3 in 1 & hadn't really noticed any great difference, but it empties faster and becomes expensive with 3 cars, I brought the special SU Dashpot oil and it felt quite a light oil (perception only) felt more like a 3 in 1 than engine oil rubbing thro the fingers, any way it's too hard to obtain and expensive so nowadays engine oil it is! No mention of auto trans oil, tried that too, seemed ok but just messy! So engine oil it is!! why 20/60 well check your oil after a run on a 40+C day and normal engine oil runs off the dip stick like as Pete says "cat's p=e!" Link to comment Share on other sites More sharing options...
Wagger Posted June 17, 2022 Report Share Posted June 17, 2022 Check your fuel pressure if you can, and/or the fuel flow when turning over with the solenoid. Disconnect the fuel line at the carb(s) and watch it pump into a bowl. Link to comment Share on other sites More sharing options...
AlanT Posted June 17, 2022 Report Share Posted June 17, 2022 I had this problem after tuning. Was a couple of flats too lean. Link to comment Share on other sites More sharing options...
Nigel Clark Posted June 17, 2022 Report Share Posted June 17, 2022 A starting position of 10 flats down for the jets sounds a little weak. I always thought 12 flats down to start then warm the engine up and tune to the perfect setting. Nigel 1 Link to comment Share on other sites More sharing options...
dannyb Posted June 17, 2022 Report Share Posted June 17, 2022 My Spit was tuned on a rolling road and the timing is set for 18deg btdc. But the dizzy was also modified to give mor advance. I think 10deg is a starting point but to get the best is to advance the timing enough without pinking. Also I would check fuel pump is putting out the right pressure. Danny Link to comment Share on other sites More sharing options...
Pete Lewis Posted June 17, 2022 Report Share Posted June 17, 2022 many SU suffer from the jet not returning after using the choke so after a start up its open bonnet and give the jet base a quick prod if it jumps upwards to meet the adjusting nut the linkages are all fouling up very often needs some pry and bend to realign smooth operation some have shortend the jet tube by 3mm once the jet is working properly after choke just set both idle screws to 1.5 turns in from closed this gives a mechancally set balanced idle setting it may need the spindle couplings adjusting 3 turns down is the general base setting of the jet nut once she is idling just turn the adjuster nut to see how it responds turn each one in turn to find the best /fastest idle so any prap prap goes to va room now its getting better you can use the lifting pins  these are very light and touchy feely things and you must have air filters fitted you lift the pin till if just touches the air piston  raise the piston a few mm do not just yank it up you are listening for a hint of change in the idle for a few seconds so if it feels like you increase by 50 rpm its rich if nothing happens its about right if it does a quick falter of -50 rpm its weak hope that helps is this car all standard or has it got some engine modifications ??? Pete  Link to comment Share on other sites More sharing options...
Alans835 Posted June 17, 2022 Author Report Share Posted June 17, 2022 Hi folks, changed the dash pot oil for 20/50. Started the car still stalling out. Double checked my carbs are balanced. Will give another go with the mixture settings as I’m am sitting around 10 flats down from the bridge. Have only been tweaking it a flat either way. Plugs are very peelie wally not much colour to them. Hopefully it’s just a weak mixture. Unfortunately I have nothing at present to check the fuel pressure, I have new pump coming Monday just to rule that out but I’m not convinced that’s where the problem is. The engine is standard from what I know, the previous owner didn’t know either of any modifications choke is returning ok I have had a little bother with it but it is ok now. I have the timing at 10 deg btdc will maybe try advancing it slightly see if there is any improvement. When the car does stall out I normally whip the plugs out for a look and a clean regardless of whether the plugs have been sooted up, sure enough the car will start right up after this. Thankyou all for the help it really is appreciated. Link to comment Share on other sites More sharing options...
Alans835 Posted June 17, 2022 Author Report Share Posted June 17, 2022 Disconnected the throttle linkage and tried each carb individually found the 1st carb to bog down and the 2nd revs no problem, have now removed the float bowl and jet and have them in my ultrasonic bath to clean them. The jet is brand new so hoping it will clear. Link to comment Share on other sites More sharing options...
Pete Lewis Posted June 17, 2022 Report Share Posted June 17, 2022 whilst hoses are being disturbed you can make the darn rubber slivers that block float needle valves and float back and forth if the fuel was low level check behind the float valve  and give the system a squirt into a jam jar you might catch the little sods you cant do any harm give the jet adjuster a good few turns there should be a noticeable difference in its better or its worse so a full turn or more from the basic is not unusual just tinkering a little is not telling you any story so be more agressive with the jet adjustment so you get a feel for how its responding Pete Link to comment Share on other sites More sharing options...
Alans835 Posted June 17, 2022 Author Report Share Posted June 17, 2022 Car was back together, running lovely on the drive reving freely after both jets were cleaned and blown through. Went a quick run made it 20seconds out the drive and it started playing up again aaaahhhhhh then died had to push the bugger home! Will have a check of the plugs next see what colour and adjust accordingly. At this point though I’m feeling a tad defeated! Lol 🙈😂 Link to comment Share on other sites More sharing options...
DanMi Posted June 17, 2022 Report Share Posted June 17, 2022 have you checked that the fuel lines are clear from the tank, as a partial blockage can cause a lack of fuel when under power or a cracking fuel hose allowing air to be sucked in, rather than pulling petrol through Link to comment Share on other sites More sharing options...
NonMember Posted June 17, 2022 Report Share Posted June 17, 2022 That does sound rather like a fuel pump or filter problem. I had a clogged gauze on a Toledo once - it ran perfectly until I came to a hill (I'm in the fens, so "hill" is a relative term) when it gradually faded to a stop. In that case it was no problem to start it again, turn round and head home, though. Link to comment Share on other sites More sharing options...
Pete Lewis Posted June 17, 2022 Report Share Posted June 17, 2022 this has the hall mark of the dreaded rubber slivers the more you mess the more you make they hide in the supply line and do check the coil polarity is correct with ign to the + and dizzy to the -neg that can realy mess with the stop start get out kick a wheel it starts  and how is you electronic wired in do you have it on 12v or ballasted ?? Pete  Link to comment Share on other sites More sharing options...
Alans835 Posted June 17, 2022 Author Report Share Posted June 17, 2022 So I have stripped the fuel pump, diaphragm is in good order. Found a lot of sediment sitting at the bottom of the pump so cleaned it all out. Rebuilt and tried to no avail. Will try blowing through the lines see if that helps. I have cleared the jets and cleaned the bowls. Looking like my show on Sunday will have to wait for another day! Thankyou again for the help folks. Link to comment Share on other sites More sharing options...
Badwolf Posted June 17, 2022 Report Share Posted June 17, 2022 Go on Alan, you can make it. I got to a show with sooty plugs, choke stuck on, kangaroo 🦘 petrol...the works but I was going!!!! Oh yes, I got back too. ...and, by the way, another time, took her out the day before..running beautifully. Morning of the show wouldn't start, backfired, ran ragged dreadful. I was really upset. Stripped down everything, put back together. Mucked about with the carbs..everything that I could think of. Lady BW was really sympathetic. Made coffee, soothing noises etc and then said "But what have you changed since yesterday". Just cleaned the plugs....???!!!! And put the leads back on in the wrong order. Leads reset, coffee drunk and roared off. So simple..eventually. 1 Link to comment Share on other sites More sharing options...
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