My carburettor has developed a problem that manifests itself in two ways (could be two problems, I suppose )

Firstly, when pulling in top at about 30-35 it sometimes stutters a bit, as I would expect it to if it was a bit blocked up somewhere.

Secondly, when sitting at a juntion at tickover, as I put my foot on the gas to pull away, the engine often hesitates (especially if I'm too unsubtle with my right foot), it makes "that hissing sound" and sometimes then stalls.

After I first got the first problem (as it were), I cleaned the two jets in the bottom of the float chamber, and the problem persisted. So I then took the whole things off the manifold and cleaned everything I could get at (including the little jet underneath, behind a blanking screw). That is when the second problem started! I think the lesson there is to leave it alone if it goes at all!

Having re-installed all the jets properly, and knowing that there are no blockages and no leaks, I can only see two possible adjustments, the screw on the top and the screw underneath (tickover). How far out should the top screw be unscrewed? If I undo it much more that half a turn I get the dreaded hissing noise and the engine stalls (unless I adjust the tickover too high). Whatever I do, there is still a hissing/hesitancy when I open the throttle.

Last thing (!) the carb sometimes "spits", especially when cold - is that anything to worry about?

Thanks for your help! (I hope)

I had/have the same or similar problem with mine.
It would seem that a certain amount of hesitation under progresion is normal, and is a characteristic of the Zenith. However mine was quite bad, as indeed yours sounds?

There are several points...

The "hidden" jet you mention underneath is easy to clean BUT the drillings for it in the body are not easy to clean, and I suspect this causes some of my hesitation. Ultrasonic bath cleaning may be the answer here?

The flanges can be distorted, mine was very badly warped to start with which caused a hesitation coming off idle because of an air leak. This can easily be lapped or draw filed flat. The throttle spindle should also be rebushed if required. All of the above are to stop air leaks.

Once you are certain that it all drillings and jets are clean and your flanges are airtight I would check your fuel supply and float height. Make sure that you have clean filters and the pump is supplying fuel and that the needle valve is operating correctly. You can then experiment with washers under the needle valve as we have discussed recently on the forum. Go for a good burn up, say 20 miles or so and then remove plugs and look at them. you can then "experiment" with washers....

Add some shim washers to weaken, and remove or substitute thinner to richen mixture. This realy does work quite well and after a few runs and tweaks I got my plugs a realy nice sandy colour. unfortunately I still have some hesitation when coming off idle and as mentioned I suspect that the progression drilling is still blocked.

Incidentally I have the "mixture" screw turned in almost till it bottoms and this seems to work best for my set up.

One point worth mentioning which others will also advise you , is once you have it reasonably sorted DONT be tempted to keep playing. The Zenith seems to be a temperamental beast and once you are "there" dont risk upsetting it again!!!!


Regards

Steve

Hello Urgent,

I HAVE TRIED TO MAKE SOME SUGGESTIONS ABOUT YOUR CURRENT WOES. HAVE TRIED TO ANSWER IN CAPITALS AGAINST YOU QUESTIONS IN LOWER CASE

Firstly, when pulling in top at about 30-35 it sometimes stutters a bit, as I would expect it to if it was a bit blocked up somewhere.

IT MAY NOT NECESSARILY BE THE CARBURETTOR WHICH IS THE CAUSE OF YOUR “STUTTER.” COULD BE CROOK SPARK PLUG, PLUG LEAD, CONTACT BREAKER POINTS DIRTY OR OUT OF ADJUSTMENT, DISTRIBUTOR CAP TRACKING ETC., ETC., etc.

Secondly, when sitting at a juntion at tickover, as I put my foot on the gas to pull away, the engine often hesitates (especially if I'm too unsubtle with my right foot), it makes "that hissing sound" and sometimes then stalls.

THE PROGRESSION JET (THE ONE "UNDERNEATH” AND COVERED BY A CAP) IS PROVIDED TO GIVE YOU A LITTLE BIT OF EXTRA MIXTURE, BETWEEN THE TIME THE IDLE JET IS DEPRIVED OF HIGH VACUUM AS THE THROTTLE DISC IS OPENED, AND THE TIME THE MAIN/COMPENSATING JETS COME INTO THEIR OWN. THE JET, AS STEVE SAYS, IS RELATIVELY EASY TO CLEAN, BUT THE DRILLINGS IN THE BODY OF THE CARBURETTER CAN BE DIFFICULT. YOU WILL NEED A SOURCE OF COMPRESSED AIR AS A MINIMUM. I CAN SEND YOU A SECTIONAL DRAWING WHICH SHOULD GIVE YOU A GENERAL IDEA OF WHERE THE PROGRESSION JET IS FED FROM FUEL WISE.

After I first got the first problem (as it were), I cleaned the two jets in the bottom of the float chamber, and the problem persisted. So I then took the whole things off the manifold and cleaned everything I could get at (including the little jet underneath, behind a blanking screw). That is when the second problem started! I think the lesson there is to leave it alone if it goes at all!

A VERY WISE APPRECIATION OF THE ZENITH 26VA.

Having re-installed all the jets properly, and knowing that there are no blockages and no leaks, I can only see two possible adjustments, the screw on the top and the screw underneath (tickover). How far out should the top screw be unscrewed? If I undo it much more that half a turn I get the dreaded hissing noise and the engine stalls (unless I adjust the tickover too high). Whatever I do, there is still a hissing/hesitancy when I open the throttle.

THE "TOP" SCREW IS THE IDLE MIXTURE CONTROL. THE MORE YOU TURN THE "TOP" SCREW ANTICLOCKWISE, THE WEAKER THE IDLE MIXTURE BECOMES. I WOULD START OFF BY GENTLY SCREWING THE TOP SCREW IN AS FAR AS IT WILL GO TURNING CLOCKWISE. THEN CAREFULLY UNSCREWING THE MIXTURE SCREW ANTICLOCKWISE BY, SAY 1/8 TO ¼ OF A TURN. IN THEORY, YOU SHOULD BE ABLE TO MAKE THE ADJUSTMENT BY SCREWING IN TILL IT “HUNTS” (TOO RICH) AND THEN UNSCREWING TILL IT STARTS TO RUN ROUGH BECAUSE IT IS TOO WEAK, THEN GO BACK TO THE BEST MID POINT BETWEEN THESE TWO EXTREMITIES, WHEN IT SHOULD BE RUNNING LIKE A CLOCK.

THE PROBLEM IS THAT UNLESS EVERYTHING ELSE IS SPOT ON, AND YOUR THROTTLE SPINDLE IS ABSOLUTELY AIRTIGHT, AND YOU HAVE THE ACTUAL ENGINE RPM AT EXACTLY THE RIGHT IDLING SPEED, THE IDEAL POINT WILL TEND TO BE MASKED.

STEVE’S ADVICE ABOUT THROTTLE SHAFT BUSHINGS IS VERY IMPORTANT FOR THE ACHIEVEMENT OF GOOD IDLE.

IF THIS PROBLEM JUST DEVELOPED OUT OF THE BLUE (THAT IS YOU HAVE HAD THE CAR RUNNING OK PREVIOUSLY) I WOULD TEND TO DOUBT THAT IT WILL PROVE TO BE A MATTER OF FLOAT LEVEL


Last thing (!) the carb sometimes "spits", especially when cold - is that anything to worry about?

JUST BAD MANNERS, AND TYPICAL OF AN ELDERLY AUSTIN. WOULD WORRY ABOUT THAT AFTER YOU GET THE IDLE RIGHT.

JUST ANOTHER POINT, IF YOU HAVE CLEANED OUT THE CARBURETTER, I PRESUME THAT YOU HAVE HAD THE UNIT OFF THE MANIFOLD. I SUPPOSE THAT YOU ARE CERTAIN THAT YOU HAVE STILL GOT A WELL SEALED JOINT BETWEEN THE CARBURETTER AND THE MANIFOLD!

IF YOU DIDN’T TAKE THE CARBURETTER OFF THE MANIFOLD TO CLEAN IT OUT (AND I MEAN A GOOD BLOW JOB WITH A COMPRESSED AIR LINE) THEN YOU PROBABLY HAVEN’T REALLY CLEANED THE CARBURETTER SUFFICIENTLY YET. YOU HAVE TO GET ALL THE INTERNAL DRILLING CLEARED OF ANY DETRITUS AS WELL AS THE JETS THEMSELVES.

Regards

Barry R.

Steve and Barry,

Thanks for the very detailed responses!

I did have the whole carb off the mainfold. The seal between the two came off intact and I put it back (and have tightened everything well home). The mounting bolts have spring washers, so I would hope they don't work loose.

I did use compressed air (of the homo sapiens variety) to clean out the progression jet passages underneath, after I spent some time with a small screwdriver coaxing the jet out. I put my fingers over the various holes under the throttle spindle and either side of the jet, and gave a good blow down the jet hole, no obvious blockages.

I also found out why I had been taught to fill the float chamber with petrol before re-fitting it....that way the float doesn't get stuck and end up pumping petrol all over the exhaust mainfold. Good job I spotted that one!

I shall, however, take it off again, and make doubly sure that the passage is clear.

As far as the mixture control is concerned, I can screw it all the way in and there is no sign of "hunting", which I guess suggests there is air getting in somewhere. The throttle spindle has been rebushed, and everything looks clean and in good nick inside the carburettor.

I went out for a drive three-up yesterday, and there is no lack of urgency when it comes to going up hills (well, the sort of hills we get in Norfolk, anyway!), but there is still an annoying "will it or won't it" when sitting at a junction.

It will be a while before I start phase two of this project, as I live 150 miles away from my car (getting fed up with London....), but I shall print out your advice and get to it next time I'm up here.

Regards,

Geoffrey

Sounds more like a weak mixture due to air leaks than anything else. As well as making sure the manifold/card joint is air tight (use a smear of Red Hermatite on both faces), also cheque the anifold/block joint. Just because the exhaust part of it doesn't blow, doesn't mean the inlet part is air tight (use a smear of exhaust paste on both sides of this joint. Finally, make sure that the spindle is forming a good seal in the casting. Only after all this has been done can the carb be set up.

STEVE

I think both of the Steves have a valid point, your symptoms seem to indicate that air is getting in somewhere. When you had the carb off of the manifold did you check that the carb face was flat? Being a relatively soft material it is not unusual for the face to get distorted where the two nuts pull the outer edges inwards toward the manifold leaving very little 'grip' in the middle. If you didn't do a detailed check, take off the carb, draw file the mating face then use a new gasket with just a touch of blue Hylomar sealer.
If nothing else this will eliminate this joint from the equation.

Good luck

Your problem may not be the carb. I have a 32 Opal with an SU fitted at the time of sale. (great addition) My car had similar problems and I found the fuel tank filter had been crusted over so the pump was being starved.
Now here is the problem. over the years the fuel had turned to varnish and cacked the filter. you can tell by opening the fuel tank drain and watching the flow. How to clean this out is an issue.
I used paint remover but maybe someone has a better idea. The painy remover sort of worked but......

Pat USA with a 32 Opal

It's been a while since I had the car out, which is why I'm only just getting round to trying out some of your suggestions!

The hesitation on acceleration has now gone. Having spent an age fiddling with the carb, I decided to turn my attention to the ignition system. I cleaned and gapped the plugs (they looked fine in terms of colour/soot), cleaned and gapped the points, cleaned the contacts in the top of the distributor cap (these were very cruddy), and low and behold, no more hesitation, in fact, pulling like a steam engine once again.

Next problem, however, is that during my attentions to the carb I've totally messed up the carb settings (although at least I now know for sure that every nook and cranny is clean). This manifests itself in "hunting" at tickover. The book says that this means the mixture is too weak, so I screw that down and it all starts to smell a bit petrolly, but it's still "hunting". The idle adjustment can be screwed all the way out and it doesn't stall, so I guess there might be issue there as well

Before the usual questions arrive(!): It ticked over perfectly before I messed around with the settings, and the throttle spindle had lovely new bushes put on it last Summer. I have also tried adjusting the ignition timing and it seems to make no difference to the hunting where it is set (within acceptable limits).

Any ideas how to stop my car being an idle hunter?

Thanks!

This appeared half way up, so people might not have seen it, so here it is again hopefully at the bottom of the list this time...

It's been a while since I had the car out, which is why I'm only just getting round to trying out some of your suggestions!

The hesitation on acceleration has now gone. Having spent an age fiddling with the carb, I decided to turn my attention to the ignition system. I cleaned and gapped the plugs (they looked fine in terms of colour/soot), cleaned and gapped the points, cleaned the contacts in the top of the distributor cap (these were very cruddy), and low and behold, no more hesitation, in fact, pulling like a steam engine once again.

Next problem, however, is that during my attentions to the carb I've totally messed up the carb settings (although at least I now know for sure that every nook and cranny is clean). This manifests itself in "hunting" at tickover. The book says that this means the mixture is too weak, so I screw that down and it all starts to smell a bit petrolly, but it's still "hunting". The idle adjustment can be screwed all the way out and it doesn't stall, so I guess there might be issue there as well

Before the usual questions arrive(!): It ticked over perfectly before I messed around with the settings, and the throttle spindle had lovely new bushes put on it last Summer. I have also tried adjusting the ignition timing and it seems to make no difference to the hunting where it is set (within acceptable limits).

Any ideas how to stop my car being an idle hunter?

Thanks!