Zenith 28if throttle shaft looseness

[AdamWilkes]

I have a nice Zenith 28if v05083A fitted to my F6, which works well, but there is quite a bit of play in the throttle butterfly shaft. How easy is it to refurb/bush the butterfly shaft to make a better fit (and seal) ? I wonder if there is a service kit for it?

 

[mojobaby]

Hi Adam
Personally, I've never had the need to remove the shaft, but it looks simple enough. Once the throttle plate has been removed, it just slides out.
The rebuild kits that are available only have gaskets and a couple of copper rings and no bushes.

JonathanT removed the shaft as he had a leak. Possibly he'll be able to tell you what he replaced to stop the play and the leak.

 

[JonathanT]

Hi Adam

I did have a leak but it seems to have gone -at least for the time being. But I am suspect.

I cant see how you can maintain an even idle though with a wobbly shaft? - its a classic fault with old carbs and when they are properly and professionally refurbished bushes are fitted even if they didn't have them originally. Years ago I effected my own repair to a Zenith side draught carb fitted to my Austin 7. I made a white metal bearing (Solder) and bedded it in. Worked fine

 

[AdamWilkes]

Well, I took the butterfly off the shaft (the first butterfly screws that I have ever managed to undo without ruining them!), and found lots of wear but also a rubber seal designed to take up the air-seal slack when the shaft/housing increases. I suspect my mixture weakness problem is in fact an air leak from the manifold gasket, as another carb fitted had the same issue, and a tighter shaft! I've had the manifold machined flat (was surprisingly distorted, with inlet ducts all slightly further from the head than the exhausts), and fitted a new gasket which I am slightly suspicious of - it has a metal face and metal hole-linings (I like the hole-linings idea), and a 'normal' fibrous face. I placed the metal face against the head (with a smear of liquid gasket - wrong way around?), but I swear that there is a little 'chuff' from somewhere (so could also be an air leak to mess up the carb mixture & contribute to the light-acceleration judder some others have mentioned), so I am going to fit a fibrous gasket instead & see.
In my explorations of the carb, I also managed to snap off one of the two pressed-in slow-running tubes (doh!). I have no spares, so I soldered-up the remaining stub. I guess this helps weaken the idle mixture?
Are there any spares anywhere for these Zenith 28if carbs?
Seems that every time I fix something, I break something else!

 

[mojobaby]

Hi Adam, bad luck with the damage to the carb, but these things do happen occasionally, unfortunately. Are those the tubes inside the venturi?

It would probably be cheaper to buy a second hand carburetor somewhere
I priced a new mixture screw at 40 Euro, the only one I could find, and declined.

I don't think anyone carries spares.

although I don't know for sure, I would have put the gasket with the flat side to the head.

 

[AdamWilkes]

Thanks for your kindness mojobaby, I am good at breaking (and then usually fixing) things!
The slow-running tubes (? correct name) are those (3-ish cm long) that push-fit into the top half of the carb, sitting down into the fuel bowl area when in action; they don't go into the venturi.
Some Zentih 28if carbs seem to have one tube, this one had two - there are always two holes for them in the top half of the carb though. I've not yet found out quite how the tubes work or whether there are always supposed to be two, & I'm not even certain that they are called slow-running tubes!

 

[mojobaby]

Not quite sure which tubes they are. Are they brass tubes? Do you see them in either of the 2 photos?

 

[AdamWilkes]

hi again - yes, what I call the 'slow-running tube' is the long thin upright three-lobed-end brass tube in the first photo (see also the matching empty hole symmetrically opposite in the ally top casing). Mine had two of these (one now snapped).

 

[mojobaby]

It's listed as an air jet, and as you can see from the next photo, it leads to the bottom of the bowl. Or to the bottom of the fuel in the bowl.

If I remember correctly, it the tube leads into the venturi. It might be associated with the idle jet which is alongside.

I have no idea what will happen to the efficiency of your carburetor if one broke off. Hopefully when you test your engine it will be OK

 

[AdamWilkes]

I've now found another air jet/slow-running tube and fitted it - the tickover is much improved, and no longer cycles up & down as it did! Why some have two, and most have one of these tubes is a mystery to me.

I have bought an Oilite bush (7mm inside, 10mm outside, 8mm long) with the hope of improving the throttle shaft loosness, and hopefully not butchering the carb totally.... I'll need a 10mm hole where the 7mm throttle shaft sits at present - snug enough to gently press in the Oilite bush. I am hoping to drill in from the outside (there is a blanking cup in there I think, where it was drilled through during manufacture). Gulp.