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Play in Choke connecting plate 1 and connecting rod


Johnny280

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Hello, 

I was chasing down (again) a problem with a stuck open nozzle when easing out the choke on my SU carbs when I noticed some weird play between the Connecting plate A and the attached 'unknown name plate' on the choke shaft. Both carb have it - more pronounced on the front one. Also the connection between the connecting rod and the fast idle lever (the connection with the small spring) looks very loose. I added three videos to show the current situation. Any help and feedback is appreciated.

If someone is wondering about the throttle return spring custom setup => i had to circumvent some fitting issues with the heatshield and Zstory headers. Also the dome screws are not tight on purpose.

 

 choke_plaes.jpg



1. Connecting Plate A
This play results in the nozzle to not fully stay up when the choke is off. I wonder if this play is normal or if the linkage is worn out.  I tried to decipher the parts manual (Carburetor L24 (To Aug.-'71)) and don't see any part missing (the 'unknown plate' seems to be 41 - PLATE "D" CONNECTING). The nozzle is able to roughly move 0.5mm. I can only assume that this is not ideal for fine tuning those carbs. If the play is normal would I need to adjust Connecting plate A to mitigate the up and down movement of the nozzle? Or do i need to somehow sleeve the pin on Connecting Plate A?
There is also some play where the connection plate A is screwed into nozzle head (longitudinal).
 
I added two videos, one showing the play on the rear carb and the movement of the nozzle on the front one (was unable to film both videos on one carb. But they behave very similar).

 

 

 



2. Connecting rod to fast idle lever
My second concern is the slopiness between the connecting rod and the fast idle lever (the connection with the small spring). I added a video showing it on the rear carb.

 

 

 

Edited by Johnny280
changed parts manual reference to earlier SU types
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Yeah, most of the linkages on the SU's are a sloppy affair. I wouldn't worry at all about the slop in the fast idle actuator, and I also wouldn't worry at all about the fact that the linkage that connects to the nozzle wiggles around a bunch. Except...

When the choke is off, that linkage that connects to the nozzle should always be pulling the nozzle up tight against the underside of the carb. In other words, even though the linkage is sloppy, there should still be some spring pressure pulling that nozzle all the way up "with a snap" when you push the choke off.

So, I got two quick thoughts... First thought is that you simply have the choke cables mis-adjusted and pulled to tight. If that's the case, even with the lever pushed all the way to "OFF", the nozzle will still be held down a little bit by the cable. To check that, what happens if you loosen the clamp screw onto the cable and let the lever down further? Does the nozzle come all the way back up then?

If that isn't it, my second thought is that maybe one of the return springs on the choke linkage mess is not installed correctly. Take some good pics of the springs on that choke actuation linkage so we can have a look?

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Hi,

good to hear that sloppy is normal.

First I will check the choke cables. Last time I worked in that area I was adding a floor mount for the choke lever and swapped the old cables with new piano wire (very smooth action now). I know that i brought the 'clamp lever' up to the point that i could feel the spring resistance starting to have immediate action when moving the choke lever. Should I leave some margin there? 

If that doesn't solve the problem I assume that the up and down play in Connecting plate A to the 'unknown plate' needs to be adjusted by bending Connection plate A in such a fashion that 1. the nozzle is not canted, to prevent any binding (current situation) and 2. in case of choke off keeping the nozzle completely up (in that position no up-and-down play should happen).

Currently i would need to swim to the car. I will try to get fresh pictures of the return spring assemly as soon as possible. 

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I'm not sure this is what you're talking about but thought I'd share my experience on one I bought.

Make sure the J hose that connects the nozzles to the float bowls have the soft rubber hoses and that's not holding them in a bind.

I bought one that had regular fuel hose instead and the nozzles weren't coming all the way up. Garage smelt like gas all the time and the carbs were adjusted witout knowing that so there was some back it up and start over issues to get things right.

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When I connect choke cables I do not lift the cable coonection at all other than to make sure they are even to each other. The choke handle moves more but you know they are all the way released when the handle is down

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The flat bar (ish) choke linkage that attaches to the nozzle can cause the nozzle to bind and not return to the top.  The ends of the linkage must be parallel to their mating surfaces. A slight twist to the linkage piece, with pliers, is usually all that's required to align it. Unfortunately, the carbs have to come off to check the alignment accurately.

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A very muched stressed topic in the ztherapy.com dvd. The flat bar isn't flat at all. LOL

You bend it until the opposite ends look the same as far as parallel goes then put it on and then you may need to twist it to get it working smoothly. There's some videos on you tube you can see what we're talking about.

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9 hours ago, Johnny280 said:

Currently i would need to swim to the car. I will try to get fresh pictures of the return spring assemly as soon as possible. 

That's not good. I heard about the flooding. Hope everything recedes and turns out OK.

So definitely the first thing would be to check the choke cables to see if they are just too tight. And yes, I would leave a little margin there. In other words... You should be able to move the choke lever a little bit without the nozzles moving at all. I don't know if you need as much safety margin as what Patcon suggested, but if you've got the travel in the choke lever, then even that much doesn't hurt.

Here's to hoping it's that simple.

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I'll check the next time I'm looking at a set of round tops. The amount of throttle increase is "adjustable"* but I thought the throttle increase is about the last thing that happens. Memory says (take that for what it's worth  LOL) the nozzle has already started to drop before the idle increase kicks in.

 

* By bending the linkage

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11 hours ago, Captain Obvious said:

 

So definitely the first thing would be to check the choke cables to see if they are just too tight. And yes, I would leave a little margin there. In other words... You should be able to move the choke lever a little bit without the nozzles moving at all. I don't know if you need as much safety margin as what Patcon suggested, but if you've got the travel in the choke lever, then even that much doesn't hurt.

Here's to hoping it's that simple.

 After a few tries at adjusting choke cables and not being satisfied, I settled on what I think is the opposite of @Captain Obvious's method. I would pull the choke knob about a 1/4" to the on position, then connect the choke cables. My logic was the choke cables having a bit extra slack would ensure the nozzles were being pushed all the way to the top by the choke cables. I lost a bit of choke because the cables wouldn't pull the nozzles all the way down but I never had starting problems in cold weather.

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Thanks a lot for everyones input so far. A lot of stuff I have to think about. I will try to analyze the clamping situation first and fiddle around with all your different methods, then probably get the carbs out to check the linkage for canting. 

@siteunseen: The carbs are suppose to be ZTherapy rebuild ones and the rubber hose seems to be of the right type - flexible silicone ones. Regarding the Choke attachment. Yes, i replicated that one after wasting half a day building my own idea (wasn't aware that the internet had a pretty good solution). Mine was working but too bulky to fit under the console. One day I will place an order with ZTherapy and on top get one of those famed DVDs.

@Captain Obvious: The flooding was pretty close to us but luckily we dodged that bullet. It was more to the south but a lot of the water was flowing down via the rhine which is only 300 meters away from our place. Waterlevels were not critical but unusually high for summer. Fortunately yesterday was only constant heavy rain (real German summer).

I read somewhere that original SUs are first opening the butterfly and then enriching the mixture. Are our Hitachis the same in that regard?

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I just took a look at a pair of (Hitachi version) SUs here, and the throttle opening cam starts to push on the throttle linkage at close to the same time the other linkage  starts to move the nozzle down. So the answer seems to be... "Pretty much at the same time."

Depending on where you have your nozzles set (how many turns down) and how you have the throttle opening linkage arm bent, you might get one a small amount ahead of the other, but not by much. I wasn't there when they designed it, but I suspect they were aiming at "the same time".

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  • 2 weeks later...

Update: 
I removed the carbs and had a good look at the linkages. There were multiple things wrong:

1. the choke cable was indeed too tight and was preventing the spring to get rid of the play and keeping the nozzle completely up.
2. a wrong bend in connecting bar A resulted in the sticky nozzle situation. After some time I was able to bend it in a fashion that a) the nozzle stays up b) the nozzle is not sticky c) both nozzles move the same amount
3. had a look at the connection rod to the butterfly and bend it to match the specs in the service manual (between 0.59-0.69mm butterfly opening on max choke)

Next step is to refit the carbs and get the choke cables on a little bit more loose then the last time. 

The original plan was to just swap the SM needles to SB needles to see if i can lean out the idle (had some filthy rich idle hot @10.5 while cruising @16+) a little bit while keeping a richer mixture at full throttle and cruise - lets see if that works out. 

Thanks again for the input and the reassurance that there was nothing wrong with the linkage.

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