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1974.5 - 260Z (Won't Idle)

3060 Views 2 Replies 3 Participants Last post by  z74z90
I have reviewed many of the discussions that were archived, have tried several but still nothing works.

I have a 1974.5 260z that won't idle properly or at all. When the engine is cold it runs rough, when it is warmed up it does not exist and the car stalls.

The engine has had the pollution stuff off for several years and ran great until I got into a fuel problem last year. I have completely gone over the fuel delivery and I am getting 3 to 4 psi static off the fuel pump, so lots of fuel to the carbs. The bad fuel was removed, the tank relined due to rusting , the system (lines) blown out.

The carbs, unforunatley are the flat top - Tuna cans (HBW46W-4), though they did run great until this fuel problem.

As a result of the fuel problem the rear carb started to overflow fuel, so I assessed the float level and it was to high, I adjusted it and cleaned out the float bowl. It now reads on the bulls eye. I also removed the carb pistons and cleaned them, as well I looked at the needles and they are straight. Some wear but still they look pretty good, the shoulder of the needles are flush with the main bottom of the pistons not the slot of the pistion. Both were the same when assessed.

So the floats are bull eyes level and the needles are straight, I tried the lift and drop, and the piston clunk so the needle seems to seat okay and they is a good pressure to lift the installed pistons with my finger, they go up maybe a 1/4 inch when the engine is not running. The float adjustment stopped the carb from leaking.

As part of other issues I have come across, I had a head gasket problem which resulted in a warped head that was shaved, as well during this maintenance I replaced the intake valve guides which were brass and starting to sink. The valves were cold adjusted and in tolerance but now with the rough idling I can hear some small rattling of the valves.

The distributor was old and it to was replaced with new remanufactored one, I performed a "retarded" timing test and it was within spec at 600 rpm.

I have done all the checks except for engine compression prior to adjusting and trying to balance the carbs. I have the Haynes manual and FSM and have followed the procedure excately except that my adjustment system does not seem to work as it should, the Idle speed adjusting screw does not affect the engine rpm (can not get to 750 rpm) , the Idle Mixtrue adjusting screw (no idle limit switch) as well ,seems to have no effect on the engine rpm, though the balance adjusting screw does affect the front and rear carbs.

The carbs are running very rich so at this time I now have the Idle Mixture Adjustment screw turn about half way out. Though turning it when the engine is running does nothing, just like the slow idle speed adjust screw does nothing.

Any rpm below 1000 the engine runs rough and it spurts, I think the system is too rich and it not getting enough air. But, I do not know how to get more air with the four adjusting screws (Idle mixture, slow idle , fast idle and blance screws), nothing seems to work. If I keep the intake manifold vaccum hole open the engine idles better, but once I put the vaccum hose from the air cleaner back on and put the air cleaner back on even without the filter, the car wont idle.

By the way the engine runs great at high speed no problems with the carbs, except again they still run rich and eat up a lot of gas.

Another thing I have not done is assessed the power valves maybe they are the problem.

I have looked at the choke mechanism and throttle linkage and everything is okay.

I have sprayed the intake/exhasut gasket, and around all the parts of the carbs with carb cleaner to check for vaccum leaks but none found.

I do not have access to the Ztherapy video.

So, what am I missing. Any input will be accepted and I will be gratiful.

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Welcome to flattop ****....

Throw the flattops in the trash and get some early roundtops.

Some posters on this forum claim to have success with these carbs, but they number only one or two (you know who you are)

If you insist, though, try total disassambly of the the system and clean everything, especially fuel/air passages.

Clean the muck out of the idle air tubing that connects to the balance tube.

Clean the balance tube.

If the system then idles lean, pull the carbs back apart and drill out the idle jets one size (to be honest, I quit before I ever got this far)

Check for bent choke plate linkage. The choke plates should open completely, not just halfway. Bend the tangs to correct this. If the engine EVER backfires, recheck the choke plates to make sure they are still functioning correctly.

Disable the EGR system so it no longer cooks the carbs.

After you have totally exhausted your enthusiasm for getting the flattops to work properly, reconsider the round top idea. You'll be glad you did.
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Well, obviously you got some service out of the flattops. However, I also have a 1974.5 260Z (original owner) that did not fair so well. See above post by Z Doctor. Yes, some claim the flattops are a superior carb, but in the real world I did not find the right Nissan mechanic to make that dream come true. Vapor lock and associated evils caused me to sputter to a stop on many occasions. I got to meet friendly Nissan people in towns all over the midwest while getting to ride in really neat tow trucks. In frustration I gave up and went the roundtop route. I now know a little more about SU carbs and can make my own adjustments. We have gone full circle it seems as many Nissan dealerships in our area will no longer work on these "old" cars. This is no help for your problem, just as a fellow 260 owner who has been there..."I feel your pain"!
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