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TR4 SU Choke linkages


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Hi,

I've always had bad cold starting problems.

Problem is I've got loads of loose connections in the choke linkages. So when I pull the choke there's a lot of travel before the jets start to drop. The rear jet only drops a fraction compared to the front. Is the interconnection rod in the right place? Should it be in the upper to add more leavarge. I've tried to find some pictures of the linkage connections just in case there's something amiss. Plus it requires a considerable pull on the cable. I've got a twisted wire inner cable. Jets move freely in the assembly and are both centred. Car behaves perfectly when warm starting.

Would love the engine to start on a rich mixture for a change when cold.

Thanks

Darran

 

IMG_6926.JPG

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On my 3A I had some choke problems. Now I have only the front carb operated by the cable and gave up on the connecting rod. The position of the fast idle cam is difficult on my carbs. They were rebuilt and I think that the cam is in the wrong position. I also used a new cable lubricated with special grease and moved it to a more direct route, which would be different for a 4/4a. My bellcrank lever was also very worn but the replacement from Rimmers was made back to front and had to be altered by my local welder. After a long layoff I prime the carbs by hand and push the choke manually. After the first start it is all working for the next week.

Replacing the bush on the pedal support made a huge difference to the tick over but it is an awkward job with limited access. The old one was a ST original and probably 50 years old.

Good luck Richard & B

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Thanks. I ended up stripping the carbs yesterday and then found I need new jet washers and probably fuel line too as they're a bit hard. Ordered some new parts and put it all back together for the time being with no leaks. Took up all the play with the cable and back off the fast idle cam too. Anyway first time ever that I've been able to pull the choke cable out fully ( still a struggle) and hey presto started first time. Now to get the jets back to normal without having to push them back up with my hand. Hopefully new rubber will address that.

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Following this with interest as my car has the same problems. Renewed the linkages, cable, replaced the cork rings (nr 13) by a Vinted (or something called like that - a rubber-ish O ring made to withstand petrol) but still it needs a hefty pull. And then you never know if the jets will be pushed up again, always have to check manually. A horrible choke mechanism.

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On 10/27/2020 at 12:56 PM, Geko said:

that's quite a bit of wear isn't it ?

Screenshot 2020-10-27 at 07.52.29.png

That's what I thought. There's a considerable amount of slack in the system. Is it supposed to be sloppy on the linkages or nice and tight? I've nothing to compare it too so just took up all the slack in the cable. Still a good effort to pull the jets down though. Thoughts anyone?

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5 hours ago, Geko said:

Thanks Stef - that's a very good explanation - plus this article too - https://thosbryant.wordpress.com/2014/03/30/su-carburetor-rebuild-h-6/

I've still got some free movement of the jets for the fast idle - will have a bit more of a play with the linkages to understand fully how they all work together - plus ways of making it easier pulling the choke in the first place. 

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On 10/26/2020 at 6:27 PM, badhuis said:

Following this with interest as my car has the same problems. Renewed the linkages, cable, replaced the cork rings (nr 13) by a Vinted (or something called like that - a rubber-ish O ring made to withstand petrol) but still it needs a hefty pull. And then you never know if the jets will be pushed up again, always have to check manually. A horrible choke mechanism.

Viton, I assume.  Viton doesn't work well at all.  You need Teflon o-rings, as described here:

https://thosbryant.wordpress.com/2014/03/01/su-carburetor-rebuild-h-4/

The o-ring discussion begins a bit less than half-way through the post, so scroll down until you find a photo showing some small white Teflon o-rings.

As someone else has noted, the "slop" in the jets is supposed to be there, and I assure you, that slop is not the cause of hard-to-pull jets.

That home-made choke linkage rod, shown in Darranh's original query could be a part of that problem, but I rather doubt it.  If he replaces his jet seals with the Teflon o-rings, and checks his choke cable for proper function, I suspect his problem should go away, as there doesn't appear to be anything visibly wrong in his photo.  Although, I personally would recommend installing the original adjustable linkage rod, I don't think it likely to make pulling the choke any easier.

Tom Bryant

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57 minutes ago, Tom Bryant said:

(snip)

I personally would recommend installing the original adjustable linkage rod.

I just had another look at Darranhs's original photo.  It appears to me that the choke cable is misconnected and misaligned.  The original-type adjustable linkage rod is supposed to attach to the bottom hole in the jet lever arm.  And the choke cable is supposed to attach to the stud that holds the linkage rod in place (Items 65, 66, 67, and 68 in the above parts diagram).  The homemade linkage rod has been attached above its intended location, which by itself will reduce the amount of travel of the jets.  So, my recommendation is to buy the correct parts, install them properly, and see how it works, then get back to us.

Tom Bryant

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Thanks Tom, that's my intention for the Christmas holidays to do a full strip down and rebuild. Will add the adjustable linkage. I did try the homemade rod by previous owner(s) in a different position but no success as no adjustment... hopefully if I order the parts now then they will be here in 6-8 weeks. Joys of living far away from the main suppliers. Probably go for Burlen rather than Moss or Rimmer or eBay?

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One more comment...

Looking at the photo at the top of this thread, I notice that the pins at the bottom of each jet are installed backwards.  The cotter pin should be placed on the back side of the jet, as shown in the parts diagram, out of the way, so that there's less chance of injury when adjusting carburetors.  Trivial, yes, but it's good practice.

Tom

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  • 1 month later...

Tom Bryant's comments and WordPress tutorials are a treasure of info on SU carbs. Thanks, Tom.

My SUs have been sorted out fine now that I understand what Tom has said in his links.

I wanted to express a few opinions for Darran H as I believe he was going to work on his SU carbs during this holiday season.

So, I am not as experienced as Tom, but maybe I can encourage Darran in his garage work this week or next by reviewing things that Tom said and maybe a thought of mine.

First, here is Darran's pic.

Screenshot-2020-12-10-205644.jpg

The first thing I noticed, and what Tom shows well in his picture that follows is that Darran's fast idle cam is hopelessly positioned above.

Here is Tom's correct cam linkage.

sketch-1607652679489.jpg

See the difference. I even read somewhere that a slight choke pull will produce an elevated idle just before jets start to lower. This would be helpful after starting on a cold morning with full choke, but needing 500 more rpm for a few minutes with no choke to get car ready to go. You can sit there for 5 minutes with foot on pedal or use the cam while you are inside your warm kitchen waiting.

Next, I noticed that Darran has his jet adjusted all the way up, flush with carb inner platform.

sketch-1607652796050.jpg

It seems to me that you haven't lowered the jet the 2 1/2 turns for mixture.  My jet top sits about 1/8 inch or more down. Plus, with led lamp, I can see the fuel level about 1/8 inch below that.

Tom points out in his tutorials on WordPress that the linkage between jets should be adjustable. The rod you have installed is wrong for good adjustment.

sketch-1607652762670.jpg

With an adjustable rod, you can make the rear choke position its jet properly.  Without the adjustability, you are stuck with unequal jets.

Moss sells this and it looks just like mine, and mine is original.

sketch-1607652850547.jpg

Darran, I wish you success. I am sure you will find help here if you need it. 

Opie

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7 hours ago, Opie said:

Tom Bryant's comments and WordPress tutorials are a treasure of info on SU carbs. Thanks, Tom.

My SUs have been sorted out fine now that I understand what Tom has said in his links.

I wanted to express a few opinions for Darran H as I believe he was going to work on his SU carbs during this holiday season.

So, I am not as experienced as Tom, but maybe I can encourage Darran in his garage work this week or next by reviewing things that Tom said and maybe a thought of mine.

First, here is Darran's pic.

Screenshot-2020-12-10-205644.jpg

The first thing I noticed, and what Tom shows well in his picture that follows is that Darran's fast idle cam is hopelessly positioned above.

<snip>

Opie makes some good points here.  And I'm somewhat abashed that I didn't notice the mis-positioned fast idle cam shown in the above photo.  It also appears that the lower end of the linkage rod is not even connected to the jet lever arm, as it's supposed to be.

What I'd now like to point out, however, is that the slotted cheese head screw that's holding the fast idle cam to the carburetor throttle body is incorrect.  That screw is supposed to have a hex head on it, not a cheese head.  And it's supposed to have a shoulder on it to prevent over-tightening.  And it's supposed to have a spring washer on it, designed to maintain a bit of friction-producing tension on the fast idle cam to help keep it in place.  The flat washer(s) shown in the photo above are simply not up to the task.  And I'll bet that cheese head screw has no shoulder on it.  Take a look at the parts blow-up posted above (on October 24, 2020) and notice Find Numbers 30 and 31.

That hex-head shoulder screw is also shown in the photo that Opie copied from my blog and posted as his second photo above (the one with a red arrow pointing to the fast idle cam, and having a pair of green-handled nut drivers at the lower right of the photo).

Tom Bryant

Edited by Tom Bryant
Corrected typos.
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Many Thanks for the extra info and insights - very useful. Re the fast idle cam - that was intentional as I was fitting a new serrated washer at the time as the old one had fallen off, you can see it danglining at the other end!

I still need to get the proper choke linkage but I can now confirm that with some minor adjustments the engine fires first time and the choke does actually work. Still requires some effort to pull out but much better now. When I do get a new choke linkage that will solve the uneven balance between the two carbs and hopefully will ensure that the jets move back to their normal position - which doesn't happen at the moment. 

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Darran,

I think Tom recommends teflon instead of cork on the SU jets, but the other non-cork jet o-rings that I have purchased and installed have leaked and dragged.  I went back to original cork seals, soaked in motor oil overnight. Oldie but goodie design. That may help your jet to slide back up into jet bearing. 

Opie

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Try the Teflon... you'll never go back.  They're available here:

https://www.theoringstore.com/store/index.php?main_page=product_info&cPath=367_93_96&products_id=4168

Or, if you're just looking for a small quantity, I can sell you a kit, sufficient for two carburetors, with 8 Teflon jet seal o-rings and 2 Buna-N jet bearing o-rings.  And yes, I can mail them to New Zealand, too.

The Buna-N o-rings may be purchased here, or at most any hardware store:

https://www.theoringstore.com/store/index.php?main_page=product_info&cPath=367_23_25&products_id=1179

Tom

Edited by Tom Bryant
Added link for Buna-N o-rings.
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