> Sorry to break away form all the hot Nationals News and Protest
> Updates, but
> I looking for help on a Z. I trying to assist my son who has
> been building a
> BSP 240Z. He wants to keep the stock carbs, but we are having no luck in
> tuning them. We have rebuilt them very carefully a total of 3
> times and are
> following the books to the letter on tuning the carbs, but are
> still having a
> major problem with the engine running so rich it fouls the plugs with a
> couple of miles.
Bob, I haven't tuned stock 240Z carburetors in ages since I sold my '71 240Z
to my nephew back in '88. However, I raced and rallied the car successfully
and really didn't have much of problem tuning and then synchronizing the two
Hitachi carburetors. Here's some items to look at that may get you in the
ball park:
1. If the needle and jet in either carb has over 60K, replace them with new
units. They tend to wear and richen the air/fuel mixture over time.
2. Be sure that you're using the correct weight oil in the pots to dampen
the floating needle/piston assembly. I remember using straight 20 weight
oil. Any 10W-X oil is too light and will allow the floating needle/piston
assembly to richen the mixture too quickly.
3. If plugs 1 - 3 are fouling, the front carb is the problem. If 4 - 6 are
fouling, it's the rear carb that needs attention.
4. Disconnect the dual linkage so that you can tune each Hitachi
separately. You will need a UniSyn air flow meter to do this accurately.
Otherwise, you'll have to tune each one by ear listening to the air flow
through the mouth of each carb. The object is to equalize the flow between
the two carbs. Take your time to do this. Make sure that when you
re-connect the dual linkage that the individual idle settings are disturbed.
5. Make sure that the base of each carb is not warped and that there are no
vacuum leaks around the mating surfaces of either the intake manifold or the
phenolic resin carburetor mounting blocks. Any leaks will cause a rough
idle and usually results in overcompensating the idle rpm for both carbs.
Then you have one carb running too rich compared to the other.
6. When rebuilding the carb, make sure that the fuel level is set
correctly. Too high a level over richens the mixture. If you have a stock
fuel pump, you should be okay. If you use an aftermarket electric pump,
check its output pressure to make sure that it's within specs. Too high a
pressure can raise the fuel level.
- Greg Smith, BSP Wannabe
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