Slow running problems...

[Colin Bromley]

My Chummy is playing up, this time last year it became very hard to start, so I had the carb, mag and bacon slicer rebuilt. It seemed ok after this and certainly went a lot better, but I never seem to have been able to set the slow running up properly, it refuses to tick over, with the hand throttle set it will run fine, but when one reduces the revs, aiming for something close to tick over it dies.
I assume that I have got something wrong in the carb setup, I have looked in Woodrow & the green book, but nether offers any advice on how one should set the slow running up. Could someone offer advice? It has a bronze up draft Zenith...

[Mike Costigan]

Air leak, float level, dirt in the carb? Is the slow-running tube split?

[Colin Bromley]

I have sprayed all the joints with brake cleaner with the engine running, and there is no increase in the rpm, so I assume no air leaks, earlier this evening I took the carb to bits, it was all clean inside and there was no obvious damage to anyhting...

[Mike Costigan]

Dave Mann will probably be along soon with chapter and verse, but I would start with the slow-running tube: slacken the nut on the top of the tube assembly (under the adjusting knob), then slacken-off the retaining grub-screw and withdraw the complete assembly. Separate the two halves and inspect the top half for cracks where it sits over the bottom half. If all is well, assemble the two halves so that they are just hand-tight together, then slacken off half a turn and re-assemble the carb ensuring the top nut is still loose. Start the engine and adjust the top half to get the best tick-over, tighten the top nut and the grub screw. If that doesn't solve the problem, then I would suspect the float height, but I'm not the one to advise on that!


  Zenith FZ.jpg (Size: 56.24 KB / Downloads: 210)

[Ruairidh Dunford]

I have increased the compensating jet size on a number of cars recently to obtain decent tick over, I suspect modern fuel may be the issue.

[Dave Mann]

With the 22FZ & 22 FZB carburetors I have found that a lot of the slow running screws are splayed out and split where they touch the slow running jet by being screwed down tight, replacements are available from Willie Mackenzie.      slow running jet assembly. I've yet to find a carburetor mounting flange and the manifold mating flange that are flat being bowed like this fuel pump flange.     . Then with these attended to I check the float chamber fuel level with a test rig like this     . It is easy on the mazak 22FZB to see the fuel level in the main jet cover by taking the top off the carburetor not so easy on the one piece bronze 22FZ, it should be 1mm mm below the top of the main jet cover as so 
  Carb Comp 1.jpg (Size: 32.79 KB / Downloads: 137) and is adjusted by sliding the collar on the needle valve up to lower the fuel level and down to raise it. Due to the design a it is not a 1:1 ratio and the collar only needs moving a small amount. The collar is usually locked with solder so needs heating to release it. I should have added that to do this correctly the needle valve must seat and shut the fuel off as any leakage will confound the issue, I use T cut to re seat them never grinding paste,