Spitfire bogs down under acceleration.

[Alans835]

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! 🙈😭😂

[johny]

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?

[Alans835]

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. 

[johny]

yes engine oil better and will give a richer mixture on acceleration...

[Alans835]

Will give that a try tomorrow. Thankyou fingers crossed. 

[Josef]

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. 

[Badwolf]

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!!!

[Pete Lewis]

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

 

[NonMember]

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"?

[Alans835]

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!  

[Peter Truman]

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!"

[Wagger]

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.

[AlanT]

I had this problem after tuning. Was a couple of flats too lean. 

[Nigel Clark]

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

[dannyb]

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

[Pete Lewis]

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

 

[Alans835]

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. 

[Alans835]

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. 

[Pete Lewis]

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

[Alans835]

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 🙈😂

[DanMi]

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

[NonMember]

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.

[Pete Lewis]

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

 

[Alans835]

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.

[Badwolf]

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.