Fuel evaporation?

malcolmpaul

malcolmpaul

Well-Known Forum User
My 240Z after several miles starts to misfire badly but improves if I give it half choke. This also happens after a short journey if the engine is stopped for a few minutes and then re-started. I at first suspected ignition problems and have replaced literally everything on the ignition system with no improvement.
In cold weather it runs fine so I now suspect fuel evaporation. The engine is standard apart from a 6 branch manifold so I knocked up a couple of heat shields to isolate the carbs from the manifold but again with no improvement.
Has anyone any ideas or suffered the same problem? Any advice would be much appreciated.
 
Sounds like the mixture going lean when hot. Check for air leaks around the inlet area. Have you checked the colour of your plugs after a run?
 
Having a similar problem with my '74 260Z. Misfire and massive power loss, particularly at constant speed running. I have recently moved to unleaded fuel and use a valve-protect addative. Does anyone know whether I should retard the ignition a little to compensate? In looking at the plugs they seem to be overheating. The car also pops on deacceleration. Is it best maybe to stick with 4-star?
 
Do you still use the standard 2 rail system i.e. unused fuel returns to the tank? This should keep the fuel 'moving' and cool. If so this is good together with an electric pump to circulate the fuel before start-up.

I don't know if this helps but I still have the standard fan which keeps the air moving under the bonnet at all times when the engine is running. I know I lose a few bhp but it's very reassuring to know that the fan is 'on'.

I have had problems with a blocked standard fuel filter which didn't look blocked but very fine rust particles restricted flow. Check it out, but it doesn't sound like that is your problem. My car was suffering fuel starvation after a couple of minutes on the track.
 
malcolmpaul@fre said:
My 240Z after several miles starts to misfire badly but improves if I give it half choke. This also happens after a short journey if the engine is stopped for a few minutes and then re-started. I at first suspected ignition problems and have replaced literally everything on the ignition system with no improvement.
In cold weather it runs fine so I now suspect fuel evaporation. The engine is standard apart from a 6 branch manifold so I knocked up a couple of heat shields to isolate the carbs from the manifold but again with no improvement.
Has anyone any ideas or suffered the same problem? Any advice would be much appreciated.
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What carbs do you run & how long has it been misfireing,has it gradually got worse? If it has standard carbs maybe the needle could do with lifting a bit, just unscrew the grub screw a touch & lift needle slightly. As for fuel grade I use 2star in mine with skimmed head & 40DCOE's with ign timing at 14 deg & it runs spot on up to 7000rpm.
 
Got mine sorted. First, take a look at those dizzy springs (almost always the lighter one has gone! Dunno where it goes though! Then take a look at the fuel filter. The tank tends to give off a fine particle dust which, although it looks reasonable, severely chokes fuel delivery. Then check all ful line connections for air leakage. If this fails, turn the carb grub scews (SU type) 3/4 turn clockwise and tweek from there.
 
Thanks for the advice fellas, I think i've sorted it, found an air leak on no.1 carb and after curing that did a run of about 70 miles with no probs. Off to France to do the Laon Historique on 25th May so keeping my fingers crossed!
 
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