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vapor lock


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i was driving my 1973 240z up a winding medium grade highway, on a hot day (about 85 to 90 degrees ferenhight)in S.California; then it lost power and the rpms were becomig irregular, going up and down! i put it in 1st gear but that didn't seem to help! i got worse when i pushed on the gas. it also didn't help when i put on the choke i all the gages read mostly normal 'cept for the engine temp. was about 75% but it seemed normal since it was a hot day. later when it cooled down i reparked it and it seemed fine? in case you need to know when i purchased this car the original '73 carbs. were swiched with '71 carbs. and the smog junk was removed, mostly, and everything else is stock(it was left in a garage for 20 years with 90,000 miles, by the origonal owner) i got it from the second owner who only had it for 4 months after replacing the muffler(afer it fell off) and swiching the carbs,and cutting off the smog. has no rust, good paint job and almost perfect interior, 'cept for the cracks in the dash! i'm getting carried away! any way....... i think the problem could be vapor lock, but i'm not positive!

how would i fix this problem?

P.S. most of the fuel line isilation is still there, 'cept for about 1 and a half inches directly in front of the cam.

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  • 4 months later...

The vapor lock is caused by the exhaust manifold heating up the fuel in the carb. bottoms.You need a shield between them.I made one from aluminum.On the intake there are 4 threaded holes just waiting for a shield.The foil type insulation wrap was a datsun attempt to cure vapor lock.The problem is that it acts as insulation until it finally get hot.Then it acts as a blanket that holds the heat IN.They also tried adding electric fuel pumps as a recall item to cure this.The shield is the fix. Daniel

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It sounds like you'er being robbed of fuel. Another possible problem might be your fuel filter. They can plug up enough to allow enough fuel to idle but RRRun. Incipient plugging can be intermittent. Good Luck and Merry Christmas.

Doug Quammen

'73 240

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Don't confuse the silver thingon factory exhaust manifold as a heat shield.It's job is a heat collector to provide warm air to your carbs.Thats why that hose hooks to your air cleaner bottom.You probably would want a shield attached to the carb intake,its cooler than the Exhaustand the air gap between exhaust and shield helps also.Bad coil,maybe .but my experence has been a intermit bad coil ,stops working till cool. Have Fun!!

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