I have a problem

bobsp16

bobsp16

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Its not like me to ask for some advice but i need some now,,,ive been having a big misfire on my new race car and cannot seem to find out why.its been set up on the roller which it gave out good bhp but now it will not rev over 4500rpm all it does is splutter,
I dont think its carberation and inlet i think its eletrical i have change everything twice now ie,plugs,leads,dizzy,coil even wired it straight to a battery cutting out my wireing loom and it still missing.
It seems to go off a plug which it suts up just as if the spark stop around this rpm and its not fussy which one it is,1,2,3,4 it changes every time.....
I have even had the head off and checked for valvas not seating properly and crackes and a compression tester all very good

ANY IDEA'S WOULD BE WELCOME
 
could it be running really rich? i didnt quite get what you said about the spark plugs, but is one definatly not firing at high-rpm? if your not sure then maybe changing the fuel filter would be good, if K10s have them.
 
rlees85 said:
could it be running really rich? i didnt quite get what you said about the spark plugs, but is one definatly not firing at high-rpm? if your not sure then maybe changing the fuel filter would be good, if K10s have them.
Click to expand...

it was setup on the rolling road to about 3%co all the way though the rev range.
and it has a new fuel filter
yes it just like you have a rev limiter fitted,
 
If you really have changed everything on the ignition side of things then It doesnt sound like the ignition is the problem. It cant be.

Has anything changed between when it was run on the Dyno and when the problem started? Has the ignition timing been re-set?

Other than that my best guess would be something in the carburetor that has vibrated loose after its run on the rolling road. An air jet perhaps?
 
'Sounds like the float chamber in the carb might be emptying quicker than it can be filled up again at high revs???

Float height adjustment?

fuel pump?

Just an idea......
 
I would try a totally different type of spark plug. And gap them at 0.8mm. You can tell when driving if its a lean misfire or rich. But I would probably stick a wideband on the exhaust to see for sure.
 
Inspection would reveal if they are hot enough or not. I have had some pretty strange plug breakdowns in the past, In any case. faults like this can be surprisingly tricky if you dont have the means to test them.
 
thanks for all your idea's
nothing has been changed since the roller,the carb was also fully reconditioned but i will check the float level in the carb.
As for spark plugs i use what the book says champion RN9YCC4 would there be a better or hotter plug avaliable.
 
Get a spark strength tester. This will hightlight any igntion problem.

Ed
 
Champions are graded numerically with hotter being higher, therfor an rc10ycc4 would be a grade hotter copper plug. like Ed said though just pop a plug out and take a look at the colour.
 
AMY said:
Champions are graded numerically with hotter being higher, therfor an rc10ycc4 would be a grade hotter copper plug. like Ed said though just pop a plug out and take a look at the colour.
Click to expand...

i no me plug colours three are always a nice brown colour but one is sooty black and it changes between the four every time..it can't be a plug breaking down its something else????
 
This might sound stupid, but is there any chance that your petrol is contaminated with diesel or oil?
 
AMY said:
This might sound stupid, but is there any chance that your petrol is contaminated with diesel or oil?
Click to expand...

its all new tank,pump,pipes the filter is a clear one and there doesn't seem to be any think floating about in there or water,,,thanks
 
Sure is. I can supply you one for £10inc postage if you wish.
 
I can supply as required! Its basically a calibrated gauge that you use to measure the distance the spark jumps. the further it goes the better the spark. It can show up problems immediatly that would other wise be missed.
 
bobsp16 said:
i no me plug colours three are always a nice brown colour but one is sooty black and it changes between the four every time..it can't be a plug breaking down its something else????
Click to expand...

Heat grade of a plug cannot be judged by the Hayne's method - that just shows hugely rich/lean/oily. You need to look in a lot more closely really.
 

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