260z in Japan?

[Carl Beck]

Flat tops in OZ have ALWAYS been problematic, that is why I'd be interested in how Roger managed to fix a problem that Nissan wasn't able to do?

Hi Mike (everyone):

Nissan was able to fix the problem... more or less... after some time...

From the Datsun Fuel System Modifications Manual: (related to hard re-starting)

PERCOLATION: Percolation causes a richer than normal air-fuel mixture. It affects the carburetor float bowl and the high pressure fuel line (the outlet side of the fuel pump), and may cause hot start problems. Percolation can come about in several ways:

1. Heat Soak - back into the fuel pump and high pressure fuel lines can raise the pressure enough to unseat the float bowl needle valve. This condition is known as "after fill".

2. Float bowl evaporation may lower the float level while there is residual pressure in the fuel line. This pressure then forces an excess amount of fuel into the float bowl.

3. Fuel evaporates in the float bowl, and the resulting vapor pressure may not vent out quickly enough. The high pressure developed in the float bowl forces fuel into the intake manifold.

After explaining what was causing the problem - Nissan did supply the parts necessary and the procedures to be followed - to fix the problem.

Given that every fine detail had to be perfect, and every modification had to be made in the order Nissan specified when the cars were new - I seriously doubt that approach would be useful today, even if you could still get the Modification Parts Kits..

When all else failed.. Nissan suggested cutting a large hole in the hood - and mounting a Competition Hood Scoop over it. <a href=http://www.classiczcars.com/forums/showthread.php?t=24781 TARGET=NEW>See the Hood Scoop Thread</a>

FWIW,

Carl B.

[RIP260Z]

Hi Carl,

I am planning to stick with flat tops on my long term restro project, and I am always on the look out for tips and parts for these carbs. Do you have details on the parts (nos) and the proceedures that Nissan implemented to overcome evaporation? I know from a old post Tony D posted, that a fuel passageway was drilled out slightly larger. If so which one and was there anything else?

Thanks

Ian

[Carl Beck]

Hi Ian:

Darrel of Darrel's Datsun in Santa Clara, California was one of the few people that could actually make the 73 240-Z's and 74 260-Z's run properly with the Emissions Carb.'s. So when I was in that area around the mid-80's - I stopped by his shop to see how he was doing it.

He had Nissan's "Fuel System Modifications Manual", an 85 page manual that pretty much laid out the processes and listed the various parts for the 3 Stages and the Modifications Plus fixes.

I had never been able to find a copy of that Manual and Darrel was a very busy guy. He wouldn't sell me the manual, but I offered him $150.00 to photo copy his manual and send me the photocopy ( a couple of hours of Shop Time).

Another Z Car Friend in the area, offered to stop by Darrel's and pick up the photocopy and mail it to me... he also stopped by from time to time to encourage Darrel to get it done...

Looking through it I see that the fuel nozzles are to be replaced if they are ovaled out - but don't see any thing that recommends re-drill a fuel passage (it might be there somewhere, but I'd have to study this in far more detail than I have time to do..).

There are also Technical Service Bulletins on the subject... that may or may not have been included in the Manual. Technical Service Bulletin TS73- 38 is HLS30 Carburetor Float Modifications and adjustment Procedure (HMB46W F2 and R2). 7 pages. See TS74-053, BE74--010 or the A.C. Section of the 260Z Service Manual.

FWIW,

Carl B.

[Tony D]

Coming back just to check, and the orifice I was referring to is NOT part of the Nissan Manual! I need to make that very clear. Reading the Nissan Manual will give anyone who wishes to undertake that modification a much better understanding of what goes on in those carburettors. The orifice I was referring to is (and lacking a copy of the manual sitting here in front of me for exact reference) involved with the idle speed circuit. The fuel for idle speed is metered from the front carburettor only. As a result, you have to open it up a bit to flow enough to allow a better transition to main circuitry. This added volume of fuel acts like a jet change in a Solex. Some people do this modification, some dont. I have found that by drilling it out, tuning of the idle mixture can be made...er...'outside the CO Parameters' of the stock setup. Basically the size in the few I have done was smallish for a performance engine to fuel it properly. On a STOCK engine you may be able to get it right without drilling this passage. But on something like a 3L, some tweaking of the passages might be a good idea. I'm not really clear when I post about it, because frankly I've moved on to EFI, and suggest others do as well! Anybody who says "Carbs Are Easy" simply hasn't spent enough time trying to get them working dead-on correct! My hat would tip to Darrel had I known him. He would make four the number of shops that didn't dismiss out-of-hand a Flat-Top. They do require EXACT settings, and melticulous attention to detail when setting them up. This was their downfall---a precise metering device pushed to the field without expertise to follow it up or maintain it properly. If you look to the Honda World, you will see SU-Derivative Carburettors with FAR more emissions junk on them that our "Beloved Boat Anchors" the Flat-Top Hitachi SU...and that is into the 80's that Honda used them!!!