Off idle is a bit abrupt... SU or linkage ?

[DetomasoLee]

Hi all,

My new to me '72 US 240z is now registered as of last week and so I've been able to put some miles on it.

I cleaned the carbs (2x Hit SU) and set them all up weeks ago and it idles well and pulls well. Timing is correct so I'm satisfied all is good.

But when driving and foot off the fuel, or slightly on then when accelerating from a stop or even a little bit between gears it picks up very abruptly and it's difficult to make it not do that.

Where should I start looking ? I suppose it sounds a bit like dashpot damping or lack of at least initially, though there is oil in there. How much should be in there ? Mixture screws ? I have them set to initial + a bit and ideal seems to be at best possion with respect to mixture. Also idling around 900. and steady, chokes are (now) coming off fine (were sticking).

thanks

Lee

 

[jonbills]

Is the problem abrupt pickup when you start to open the throttle?
I wouldn't think that's the damper.
I'd be looking at the throttle balance.

 

[jonbills]

or the throttle linkage binding

 

[DetomasoLee]

Thanks, yeah I'm thinking linkgage, I've balanced them with a balancing tool so they should be the same or certainly very close.

 

[jonbills]

Another thing... I think wear on the throttle spindle hole is quite common, and that could lead to a lot more air with a little movement of the throttle.
I think @Farmer42 and @toopy may have relevant experience.

 

[Rob Gaskin]

Perhaps you could adjust the threaded linkage rods to alter angles so that a small movement of the pedal doesn't result in a big movement in the throttle.

I had a problem with my modified 240Z - when driving over bumpy roads my foot moved resulting in 'blips' of the throttle, which then made things worse.

My cable operated 260Z with Webers was a lot better.

 

[DetomasoLee]

Thanks, I think this is a pretty low miles car and the engine bay and engine is very clean with mostly original parts - this leads me to think that there is very little wear. My feeling is that it is in the linkage and do I'll start looking there too and yes it does feel like the pedal is too sensitive and perhaps even a return spring on the linkage on the bulkhead would give a bit more resistance and alter the feel of the throttle.

 

[toopy]

Take off the whole linkage from bulkhead to the top of the manifold, clean, adjust and lightly grease moving parts. Made a big difference to mine a few years back and you can see then if anything is worn to the point of being scrap. I have actually now bought a cable kit, but haven't fitted it yet, as although i like the linkage look, most are really past there best now!

 

[Farmer42]

Thanks, I think this is a pretty low miles car and the engine bay and engine is very clean with mostly original parts - this leads me to think that there is very little wear. My feeling is that it is in the linkage and do I'll start looking there too and yes it does feel like the pedal is too sensitive and perhaps even a return spring on the linkage on the bulkhead would give a bit more resistance and alter the feel of the throttle.

You should have return springs for each carb that attaches to the heat shield. If you don't, I would suggest that is your problem as the whole throttle assembly will move too fast and not return. If you have non-stock exhaust headers there is probably no heat shield and you will need to fabricate something that will replicate the attachment point for the spring. I'm surprised if it returns to a stable idle speed without them. The other thing to check as Jon said earlier is the throttle shaft bushes. There should be very little movement up & down and side to side otherwise that will let in air past the butterfly valve and affect the throttle speed. You would however, get quite a bit of pinking from that as the mixture will weaken significantly.

WRT the rest of the throttle linkage across the bulkhead etc. there are adjustments that can be made and you can insert bushes to take up the slack. Cables are ok but I prefer the OEM stock look of the linkages.

 

[DetomasoLee]

Thanks for the tips, the linkage is very original (still clean and zinc plated etc) but there is some slack before movement is taken up. I was assuming that is the way it should be to compensate for engine movment a little and I'll need to check on the return springs, but I do think they are all there, but yeah it does sort of feel like the throttle opens up to quickly/abruptly.

 

[jonbills]

Thanks for the tips, the linkage is very original (still clean and zinc plated etc) but there is some slack before movement is taken up. I was assuming that is the way it should be to compensate for engine movment a little and I'll need to check on the return springs, but I do think they are all there, but yeah it does sort of feel like the throttle opens up to quickly/abruptly.

can you post a video perhaps?

 

[toopy]

Thanks for the tips, the linkage is very original (still clean and zinc plated etc) but there is some slack before movement is taken up. I was assuming that is the way it should be to compensate for engine movment a little and I'll need to check on the return springs, but I do think they are all there, but yeah it does sort of feel like the throttle opens up to quickly/abruptly.

Before i took mine apart, i tried to adjust out the roughness and thought the less slack the better, but as Rob mentioned above, this can make the throttle too sensitive and indeed mine was to. Adjusting the ball socket ends especially on the bulk head end and taking time to work out what gets the smoothest action is well worth the time.

 

[Farmer42]

Just because the linkage is zinc plated or clean doesnt mean that it is original or hasn't been apart. It's quite easy to get things zinc plated and a lot of people do to 'bling up' the engine bay. If it has been apart it may be out of adjustment or worn.

The only slack needed to compensate for engine movement should be in the hook-shaped rod that goes from the bulkhead bracket to the top of the inlet manifold (see pic). any other slack should be kept to a minimum but not so tight that it makes the throttle too sensitive as Toopy mentioned. It will take some fettling to get it right but you do need return springs (see 2nd pic).

 

[DetomasoLee]

Thanks, yeah it's good to hear similar stories of throttle sensitivity so I'll have a look at that. I have been restoring for 30 odd years and I have a few different and low and high mile cars. The 240 looks to be very very original with the right amoung of cosmetic wear to indicate a low mile car, but it's been worked on so I do suspect some linkage adjustment might be the key.

 

[toopy]

Actually Farmers image reminded me, the bit circled in red, there is a doughnut shaped plastic washer/bearing thingy and the rod passes through it, when i dismantled mine this was full of dust, gunge and fine grit, even though on the outside it looked ok and moved ok!

 

[DetomasoLee]

So I've just been going through everything. Car is LHD so the linkage is a bit different than the photo above. Where the linkage comes through the bulkhead it then attaches to a lever to translate the motion 90 degrees. The lever bracket flexes a bit when blipping the pedal indicating it's harder down the linkage to the carbs. When I disconnect each significant ball joint in the linkage everything is easy to move. Zero resistance at the pedal all the way to the single down rod to the carb. When I move each throttle at each carb there is resistance from the return springs - expected of course and I have no way of knowing what is the expected resistance. My feeling is the springs are a little heavy, they look original and undisturbed for 50 years so I have every reason to think they are originals.

It all feels very normal when you have more throttle open but then you are actively pressing the pedal. The annoyance is when just feathering the throttle a little, or going from idle i.e. moving off from a stopped position. Even noticeable going to 2nd a little but less for 3rd and 4th but then of course the gearing gives less of a dramatic jerk at higher gears.

 

[DetomasoLee]

I had another tweak today. I did find that one of the pivots (upright shaft) in the linkage was a bit tight, so I cleaned and greased it. Didn't make any notable difference. Next I weakened the return springs on the carb levers as an expermient - put a small extension wire in so there were still under load, but less. No notable difference. I checked the pistons on the carbs and note that you can lift them with your finger about 1mm until the damper starts to make it harder - intuitively this doesn't quick seem right. Damper oil level seems fine, I added a little - no difference.

Then I adjusted one of the linkage rods to remove all free play from the accelerator linkage... it made a noticable difference. Pedal is still very sensitive really, particularly off idle but now I can touch the pedal and bring the revs up more smoothly (again just off idle). It is certainly a better feel but it does demonstrate how much you run on idle or thereabouts when driving round town.

I'm still not sure if this is how they all feel ? and I was suprised that removing any free play improved it so much. Throttle plates are still on the idle stop so whilst the linkage is not tuned to a very precise level the carbs still idle down to their minium setting with the screw stop. I was concerned that movements in the engine would effectively jerk the linkage and essentially cause the throttle plates to move a little, but nothing and the car drives smoothly.

So please give your thoughts on this and in particular the fact I can lift the carb pistons a tiny bit before I feel damping resistance as this doesn't sound right to me.

ta

 

[240L31]

In my opinion, you're describing what's absolutely normal with the stock rod type linkage. The only way to get a smoother low end throttle response is a cable throttle conversion (which is totally worth it).

 

[DetomasoLee]

Ahh thanks, Zeds are new to me so this is the first I've worked on and even driven. I didn't know it was a thing and there was a cable conversion. I'll take a look for it.

 

[240L31]

You have to build it, there's no kit available.