Zenith 28IF problem solving

[mojobaby]

I've decided to start a new thread as I don't want to use Petak's post to solve my concerns and problems with my carburetor. He has enough problems of his own to solve

Because of the difficulties I've had over the past couple of weeks with irregular running and idling I decided to open the carb to check for cleanliness and any obvious problem.

The first thing that I noticed was that the surface of the top gasket was wet with fuel, so this could have been a place where air was being sucked in.


Here is a photo of the two halves when I first opened up. The bowl looked fairly clean, but the throat of the carb was dirty which I think is caused from oil fumes being sucked from the rocker cover

 

[mojobaby]

The first thing that I checked was the needle valve. The spec is 8,9 +/-0,50mm. There's been a bit of confusion as to how this measurement should be taken, (by me).

The Heynes manual says, "with the needle valve ball in the extended position.
The technical revue that I've been following, says that the ball should not be depressed.

 

[mojobaby]

So I took a photo of the measurement with the needle completely extended and you can see that it is far from the 8,9mm that is required

So I had a closer look at the needle itself. My wife has a macro lens on her camera which really helped a lot.



And then I discovered that the ball itself can be completely depressed



So I now understand that the technical revue and the Heynes manual are saying that the measurement of the needle valve has to be taken with the BALL extended and not depressed.
I'm sure that a special tool is required to take this measurement. For a part that is now 37 years old the tolerance of +/- 0,05mm seems a bit too fine for me. We're talking about 5/100's of a millimeter!! Plus or minus.
Screwing the needle valve inwards will raise the fuel level and screwing it outwards will cause the plastic float to close the valve earlier and thus lower the fuel level.
I don't have this problem so I didn't touch the levels, I just took the measurement as best I could and it seemed close enough.

Is it Pepper or Cornish who always says, "if it ain't broke, don't fix it"?

 

[Andy McGhee]

I think the manuals cause a bit of confusion - the RTA diagram seems to defy gravity!
It's worth mentioning that GTL and F6 washers are a different height (whether after decades of maintenance the originals are still in place is another story!).

 

[petak]

I was just thinking ask malcolm (nicely) to open separate topic about zenith 28if problems and how to sorth them out.
With adjustment tips, and official numbers for adjustment.
It would be place where colegues could read about it.

There are many good tip writen in my topic that could help others who would never read my topic because they so not know we were talked about zenith 28if problems.

Malcolm would it be possible to do it?

 

[mojobaby]

A very good idea, Petak, so many of our cars have Zenith 28IF carbs, but there are also Zenith 32 carbs and also Solex that I know of. I think the 28IF is the most commonly used judging by the posts on the forum.

 

[mojobaby]

My carb is the version V05 078A.
Today I removed and cleaned the jets. I couldn't remove the air jet so I left it in place and just made sure the passageway was clear using carb cleaner.
The air jet is the long thin one that sticks up on the left hand side.

I then took out the idle jet from the centre hole. It has a number 35 embossed on the top, which is according to the spec.



I then took out the main jet for cleaning. In case someone needs to know, it can be found under the carb once you have removed the drain plug. Very easy to do with the carb still on the manifold.
It has a 91A stamp on it which is also according to spec for my carb.



I read a post that Angel put up in 2010. He said that if the main jet is worn, you can reduce the flow of petrol to the carb by threading a thin piece of electrical copper wire through the top of the jet and then twisting it tightly to keep it in place.
A little like this photo. Has anyone ever tried this?

 

[harbourseal]

Mojobaby? If memory serves me right, isn't there supposed to be a copper washer in between the needle valve and the top of the float chamber?
I'll check my spare Zenith 28IF tomorrow.

 

[mojobaby]

Yes there is, it's a strange looking washer. I never removed the needle valve but you can see the washer in this photo.
The needle valve has 1.25 embossed on the side, which is correct for this carb

 

[Cornish4]

Hi guys. I would have thought that a plain copper washer as harbourseal suggests would be fitted under the needle valve originally. The washer in your photo mojobaby looks like what is known as a Dowty washer, made by the Dowty company. It's made of aluminium and rubber.

 

[mojobaby]

Yes Harbourseal, in the file that Petak sent me, its listed as a copper washer.
Thanks Cornish, nice to know what it is. In the carb rebuild kit that I received from Franzose, there's a similar aluminium coloured washer, although it doesn't seem to be as thick as the one in the carb.
It's obvious that the carb has been worked on in the past and someone has put it in to raise the level of the needle valve. Perhaps the needle valve was an aftermarket part. If it was too low it would have caused excess fuel in the carb bowl and perhaps an overflow.

 

[mojobaby]

There's also a very fine passageway between the idle jet (where the screwdriver tip is) and the hole where the thin copper wire is sticking out. It's only accessible to clean with carb cleaner spray once the idle jet is removed.
This in turn leads all the way down to where the mixture screw is positioned.

 

[JonathanT]

Reading with interest

 

[mojobaby]

Before I continue, I will post Petaks picture which shows all the pieces of the 28 IF


and the parts that I will be using to set the initial gap of the throttle butterfly No 31.

 

[mojobaby]

The gap is 0,6 to 0,8mm according to my Revue Technique. The supplement at the back of the Heynes manual says 1mm (extreme cold)

Loosen screw No 44 which holds the rod No47 in place. Close the choke plate No3 completely

The closest that I had to that measurement was electrical copper wire or 0,75mm so I put it between the butterfly No31 and the wall of the carb.


then keep your finger firmly on throttle lever No13 which will ensure that the butterfly doesn't open again. (and release my copper wire.)

then move lever arm No12 until it touches No13 throttle lever. Notice that when you move No12 it also activates the rod No17 to the correct position. You can now tighten the screw No44.

Remember that the choke plate must still be fully closed! So now when you start your car in the morning and pull the choke out to close the choke plate, the throttle plate will always have the initial opening of 0,75mm.

 

[harbourseal]

what you are describing Mojobaby is setting the passive throttle opening. Nice with all the pictures. Thank you so far.

 

[mojobaby]

Thanks Harbourseal, that little gap has made a huge difference in starting when the engine is cold. So much easier now.

Before I started this I never really knew what that front rod was for.

 

[Melissa]

Thank you for the excellent description & photos..I think this is my next task!

 

[mojobaby]

And now to set the choke gap.......

Release the bolt that holds the choke cable in place


Make sure that the choke control inside the car is closed (pushed in).

Then holding the choke cable with a pair of plyers so that it doesn't move, close the choke flap completely by hand



The lever plate No46 should now be in this position.......



Then, still holding the choke cable with the plyers, move the lever plate all the way to the left



The choke plate will now be fully open



On this carburetor there is a spring loaded locking pin/end stop No 48 which keeps the choke plate in the fully opened position. The carb on my 845 engine doesn't have this pin.



So now you should re-tighten the bolt that holds the choke cable
And then check the operation of the choke control inside the cabin.
Make sure that when you pull the choke out, the choke flap is closed
And when you push the control in, the choke flap is completely open (in the vertical position).

Thanks Andrew4Renaults for this information
As to the choke butterfly valve, the valve should be vertical (i.e., not causing any obstruction at all) when the choke is pushed fully in (that's the choke in the "off" position)

only now do I realize that I have been driving with my choke in the half closed position for many years. That is probably the reason why my plugs are black and also the reason why I failed my MOT. I don't think it had anything to do with the mixture screw!!

 

[Jurjenz]

Thanks for this extensive walk through! Interesting to read and very helpful.

Are you also planning to do a similar topic about an Zenith 32IF7?