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Hitachi Flat Tops

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Is vapor lock a possibility on the 73 Hitachi flat tops because I am having some engine problems that can't really be pinpointed to a certain thing? There's usually a decrease of engine power after its been driven for around 10minutes. The loss is sporadic and sometimes only happens once but sometimes it will cause me to pull off the road to allow it to clear itself up.

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If you're talking about having vapor lock this time of year, I would be suspicious.


1. Do you have the heat shield between the exhaust manifold and intake manifold?

2. Do you have both the electrical and mechanical fuel pumps?

3. Have you verified fuel flow?

4. Have you verified a hot spark?

I suspect you are getting excess heat from somewhere that is affecting your carbs/ignition or you have crap in your gas tank/fuel lines that is restricting flow.

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No its not happening this time of year because its not driven in the winter, it happens from spring to early fall. It started out periodically the second summer it was on the road and as the summer progressed to fall it happened more and more until I had to take it off the road early. There is a heat shield and both pumps do work and most of the time there is fuel flow and good spark I just can't say for sure that they are when it happens unfortunately I can't check it when it happens because I'm usually away from home and it never happens at an idle.

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Hi Samshot240, The short answer is yes those carbs are prone to vapor-lock. When I had my first '73 in '74 it had the problem so bad that the dealer installed a "fix" kit consisting of an electric fuel pump and some insulation on all the engine compartment fuel lines (it's that silvery stuff). To make a long story shorter...it didn't work and by really complaining the dealer installed "round-top" carbs from an earlier Z (71-72). That worked great and I was happy. Fast forward to now...I have a '73 that the previous owner installed '71 carbs on a long time ago. I couldn't get them to balance very well so I sent the carbs to Z Therapy in Oregon. For $600 I got completely rebuilt carbs that balanced right out of the package and have worked flawlessly ever since...no vapor- lock, no backfires, no farting, etc. One of our members (Bruce Palmer of Z Therapy) is the Hitachi (S.U.) roundtop guru here. I'm hoping he will chime in here and share with you his knowledge. Try the search feature and you will find more than you ever wanted to know about this subject. FWIW the flat-top carbs are known as "boat anchors" around here. Change over to "round-top" carbs, tune them properly, and your vapor-lock problems will disappear.

Cheers, Mike

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If your avatar is of your Z she looks great. welcome to the club. I agree about replacing the flat tops with SUs, either 4 screw or 3 screws. Keep your intake manifold. In fact keep the flat tops as well. In the future anyone with a completely stock '73 Z that wishes to show the car in that category will need flat top carbs to be period correct. Since most people just throw them away , they will be difficult to find in the future. One thing you can check on your present set up is the fuel rail return line. Where the return line of the fuel rail attaches to the rubber hose , then to the metal return line. The end of the rail line has only a vary small hole . About the size of a paper clip wire. If this becomes blocked , flake of rust or something, it will not allow the fuel to flow back to the tank. With the gas setting in the fuel rail over the manifolds the gas will start to boil causing ''vapor lock''. With the excess fuel flowing back to the tank , this helps to keep everything cool. This is the reason Nissan started using electric pumps, because the mechanical pump gets hot as well as all the metal fuel rail and by pushing cool gas through it was an attempt to cool things down and prevent the problem. I have round tops on my '73 and have for 8 years this Feb. . These are so much better carbs , you won't be sorry if you make the change. You will need the linkage that fits between the two carbs because your linkage is too short , the rest is a direct fit. the Air cleaner from the flat tops will not fit so find a early one. They are tuned and make more low end power than the aftermarket units. Gary:rambo:

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