Hi, all:

This is my first post here. I am a middle-aged dad of two young sons and have owned 40-50 vintage cars, trucks and motorcycles over the years. I mostly taught myself to wrench on them, so while I am generally competent, I have some glaring gaps in my mechanical knowledge. Recently the boys and I picked up a '74 260Z 2+2 that had not been started in seven years, and we have been working on getting it back on the road. The car starts right up and runs but the twin SU carb setup was running lean and way out of balance.

I took the tops off, did some cleaning, replenished the oil in the dashpots, reset the mixture jets to the two-turns-below-deck baseline prescribed in the procedures I have been reading and put the Uni-Syn on there — only to find that the front carb is sucking in TONS more air than the back one, and the engine idles at like 2500 rpm with the choke disengaged. Fiddling with the throttle screws, I discovered that even with them backed out all the way, there is no way to drop the engine speed. 

Can anyone tell me what the heck is going on with this?

You can see for yourself at the end of the video linked below. (My kids and I are documenting or progress and/or lack thereof on a YouTube channel.) 

Thanks, all!
Michael

 

 

I worked on a friends 240Z a few years ago after he bought it and it had a similar problem.  What we discovered was that even with the idle screws completely unscrewed the idle was still too high (between 1000-1500 RPM).  The throttle plates were not being allowed to fully return to the "idle" position (ie. nearly closed) because the throttle linkage from the gas pedal was keeping them "opened" even when the gas pedal was not being pressed.  He had a '73 240Z with round top carbs.  The 1973 240Z's and the 1974 260Z's came with "flat top" carburetors and not the "round top" carburetors so someone has replaced the original "flat tops" with "round tops" from a 1970-72 240Z, similar to yours (ie. a '74 260Z with '70-72 carbs).  From what I've seen, the flat top carburetors have a thinner insulating spacer (the reddish brown plastic looking thing) between the carburetors and the intake manifold) than the round top carburetors and this was affecting the throttle linkage and keeping the throttle plates partially open even when the idle screws were completely out.    I would start by disconnecting the main throttle linkage rod that connects the main throttle bar to the carbs (the little 4-5" or so rod with the 2 plastic end pieces).  That should disconnect the carbs from the gas pedal and allow the throttle plates to fully close.  You should now be able to use the idle screws to adjust the idle speed of the carbs.  Once you get that set, you can start to figure out where the problem is in the throttle linkage that is keeping the throttle from returning back to "fully" off.  I'm guessing (because I've never worked with the flat top carbs before as I have a 240Z with round top carbs) that the throttle linkage from the flat top carb setup is slightly different in some way from the throttle linkage from the round tops and I'm guessing that you may have the the throttle linkage setup from the flat top carbs.  Let me know if this helps.

I just took a second look at your video and it looks like someone has modified the little throttle rod (with the plastic ends) and yours only has one plastic end.  That's the part you want to remove to get the carbs disconnected from the throttle linkage.  On the round top carb setups there is a main (or fast idle) screw above that linkage and yours appears to be missing that entirely.

Edited July 3, 20222 yr by 240Znomad

2 minutes ago, 240Znomad said:

I worked on a friends 240Z a few years ago after he bought it and it had a similar problem.  ... Let me know if this helps.

Damn, dude, this is exactly the quality answer I was hoping to hear. It will be a day or two before we get back to working on the car but I will 100% check this out. THANK YOU. Even if that's not the problem, it confirms my suspicion that these are in fact the earlier style carbs I have seen described in numerous other posts on the subject and helps me figure out which set of references I should be looking at. I think I actually have disconnected that throttle linkage while it was running, to no avail, but it's all a blur to me now and will def give it a shot next time I'm out there. Much obliged!

After a rebuild I installed the carbs linkage wrong and the idle shot up to around 2500rpm. These are '72 round top carbs FYI. 

These "flats" or "stops" were stacked wrong. This is the correct position. Good luck with your 260. Glad you're here, more the merrier.

 

Just now, siteunseen said:

After a rebuild I installed the carbs linkage wrong and the idle shot up to around 2500rpm. These are '72 round top carbs FYI. 

These "flats" or "stops" were stacked wrong. This is the correct position. Good luck with your 260. Glad you're here, more the merrier.

 

 

Hey, thank you! Will def check that out. And I appreciate everybody here being kind... I have done lots of shade-tree mechanic work over the years but this twin-SU setup is new to me, and I feel like a bozo. lol

I agree with 240znomad's recommendation that you look closely at the throttle linkage. When I converted from flat-top carbs to round-tops on my '73 I also replaced the intake manifold, throttle linkage and balance tube to match the earlier 1971 carbs. Even doing all that required adjustment of the throttle linkage rods.  If you disconnected the linkage and still can't get the idle speed down you may have a bent or damaged throttle plate. I experienced this on my wife's Toyota years ago when a backfire through the carb bent the throttle plate and the car would idle above 2000 rpm. The fix was to replace the throttle plate.

2 minutes ago, jpc3006 said:

I agree with 240znomad's recommendation that you look closely at the throttle linkage. ... The fix was to replace the throttle plate.

Thanks for your input, this is all very helpful!

I just modified my original post slightly but I'll mention it again hear.  It looks like someone has modified the throttle linkage, probably to make the round top carburetors work when they switched from the flat top carbs to the round top carbs.  That last piece of throttle linkage (with the single plastic connector on the end) should actually have plastic connectors on both ends.  In addtion, on the top end (where there is no plastic ball), there should be an additional adjustment screw (I think they call it a fast idle screw) that appears to be missing.  I don't use it (unless I'm tuning and want to keep the car at high idle) and normally the fast idle is completely out (unscrewed) so you shouldn't have a problem running without it.  I have attached a picture of that throttle piece from my '71 240Z (stock setup) so you can see what the original round top linkage looks like.  By disconnecting that throttle linkage you should be able to get the carbs idle set properly (using the idle screws and a flowmeter).  Then you can go back and try to figure out how to fix that modified linkage.  I'm sure there are lots of people on this site with '73 and '74 Z's running round tops (it's a very common thing to see) that should be able to help you with the proper linkage setup.

19 minutes ago, 240Znomad said:

I just modified my original post slightly but I'll mention it again hear. ...

 

DUDE THANK YOU... I keep looking at reference material and manuals and they all mention a "high idle test screw" at this position but there ain't any. All the other diagrams show some sort of vacuum biz, too, but on our engine there looks like a blockoff plate there? I am just completely unfamiliar with this engine setup before this so I don't have anything objective to compare it to. This is VERY HELPFUL!

Here's the high idle test screw.

On my car it doesn't touch anything during normal operation. It's just for adjustment.

21 minutes ago, siteunseen said:

Here's the high idle test screw.

On my car it doesn't touch anything during normal operation. It's just for adjustment.

 

Yeah it's not there at all on mine!

Your original carbs (the flat tops) had integral return springs built into them. Your current round tops do not have integral springs and require two external return springs (one for each carb) instead

In normal round top configuration, those return springs attach to the heat shield mounted below the carbs. Do you have those return springs installed?

Can you take some decent resolution pics of your carbs and post them here for scrutiny?

Awesome videos with your kids!   Gearheads in training.  👍