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GT6 engine advice please?


mr2toGT6

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I've been rebuilding the top end on my Mk3 swing spring for a while now.... It's all running and working after some set backs but I don't seem to have any power above 3000rpm

I've had to retard the ignition to aprox 8 degrees BDC to overcome the high compresion (14 - 14.5 bar) I've got after skimming the head. and this allows it to idle well and drive freely to 3000rpm but then I lose all power.

I've a few things that might be causing issues
one carb loses oil from the dashpot
I've swapped the timing chain
The carbs pistons don't lif at the same rate


My main question is what could cause my carb pistons to lift at different rates after balancing idle speed on carbs and topping up dash pots. I'm sure the fact that the rear carb lifts slower could be causing my issues

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mr2,

assuming you have su carbs,check that the damper springs are both the same rate,also that the oil damper rod and valve are ok and not bent.
are the vent holes on the front mount face of the carbs clear and not blocked by an incorrectly fitted or wrong gasket?
do a drop test to make sure the needles are not binding in the jet by lifting the piston and letting it fall, it should fall freely and 'click'down on the jet bridge,(mostly applies to the fixed needle type as bias types are less prone to sticking)
next ,take off the piston housing and make sure the piston runs smoothly up and down with no sign of any binding or scuffing.if there is you may be able to smooth off any high spots with some fine wet and dry paper but only remove the absolute minimum.

think i've covered most points. :)

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mr2toGT6 wrote:

My main question is what could cause my carb pistons to lift at different rates after balancing idle speed on carbs and topping up dash pots. I'm sure the fact that the rear carb lifts slower could be causing my issues


Tried swapping the springs? Are you sure the pistons weigh the same?  :-/

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if the diaphragms are distorted wash in petrol resumes their shape.
make sure the lugs that position the piston and the lugs on the perifery are located correctly in the body cutouts,
the 2 holes in the piston base should be facing engine side.

on some CD150 the float can be fitted upside down as they have ambidexrous needle valve arms
(thats two)  and you can get a low fuel height , the bottom of the float should be at an angle to the fuel
if its parallel to the fuel its wrong,  (looks better, but wrong) if you had to guess you would fit it to look flat,
wrong.  Pete
as for srings there are 2,         0.7 and 0.9mm thick wire

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  • 5 weeks later...

So after much tinkering I've made some progress. It didn't help that my Carbs had two different springs in so I've changed these and now running at around 6 degrees BTD timming to avoid pinking.

I've now the following issues:
1 - Piston number one is fouling its spark plug with oil. I'm hoping this is just because it's spent so much time idling will I've been tinkering and it's oil from the dashpot maybe....
2 - No power above 3000rpm
3 - Position of needle within carb piston is very different between carbs. one is proud by 1mm and the other recessed by 3mm
4 - Mixture adjustment on rear carb is seized and I can't work out how to get it apart. I've taken the grub screw out which should realse the needle but it will not budge, does the adjustment mech come out, if so how?
5 - The grub screw on the front carb has sheard off, I need to get this out to replace the o-ring seal.

Anybody got any good ideas, or know where I can lay my hands on replacement pistons?

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First off you need everything un seized. carb cleaner, WD, Plusgas, Whatever. A possible cause of the height of the pistons is that the needle has not been centralised with the jet and is sticking not allowing the piston to seat properly. You need to unscrew the jet until everything is loose, make sure the piston is all the way down and then raise and lower the piston as you tighten the jet back up centralising everything. UNLESS THIS IS RIGHT IT WILL ALWAYS RUN RICH.

BLUE  

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93 wrote:
First off you need everything un seized. carb cleaner, WD, Plusgas, Whatever. A possible cause of the height of the pistons is that the needle has not been centralised with the jet and is sticking not allowing the piston to seat properly. You need to unscrew the jet until everything is loose, make sure the piston is all the way down and then raise and lower the piston as you tighten the jet back up centralising everything. UNLESS THIS IS RIGHT IT WILL ALWAYS RUN RICH.

BLUE  


Thanks Blue

It's the needles which aren't sitting in the same position within the pistons, the pistons are fine. These are the CD3 carbs so the needle is not centralised by design.

Do you know how the needle and adjusting mechanism is removed?

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It's the needles which aren't sitting in the same position within the pistons, the pistons are fine. These are the CD3 carbs so the needle is not centralised by design.

Do you know how the needle and adjusting mechanism is removed?

Wouldn't no off the top of my head. Try and speak with Andrew Turner, Carb Specialist, he will Know.

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