I've now done probably 300 - 400 miles in my box since its engine re-build, literally just come back from touring mid wales, now got a list of jobs to do as long as my arm......

One problem that cropped up is a constant backfire on the over run. lift of the throttle and I get lots of burbling and the odd quiet pop, I think the timing is just about right, it doesn't backfire on the handle, but the rear main just starts to rumble pulling hard, but not a lot. I've also tried retarding more to no effect.

Any ideas?

I have a later sidedraft zenith fitted (rebuilt), a new manifold gasket, but a home made carb gasket. My gut feeling is that I have an air leak somewhere. Also on my list are a set of plugs, a condensor and a swap of points, I also had an intermittent missfire when pulling hard, but only now and again.

Oh!knew it would come to this..

Hedd

2 things...

I know the rear main is rumbling, BUT just try advancing a bit more. Popping on overrun is often due to retarded ignition. I can make mine pop or backfire loudly on purpose by retarding it with my manual control.

Air leaks do cause it as well, so will to weak a mixture. I have experienced all of these probs recently! The new SU conversion has sprung a leak, due to faulty assembly on my part . I am in the midst of refitting it with more care this time.

The only point I wil make ref air leaks, is that my sidedraught Zenith had an air leak that popped slightly, BUT the most noticable effect was dreadfull kangerooing down the road at low speeds which was cured by lapping the surfaces and refitting with a homemade gasket which worked fine until the SU conversion.

Steve .

have tried advancing more, the only diference I got was more rumbling from the rear main!.

.

Hedd,

Out of interest, at what speed (or revs) do you experience the roughness?

Mike

The poping only occurs when I lift off the throttle, but worse at highish revs when I lift off in top gear.

Hedd,

I wonder if you have managed to adjust your idle mixture OK.

In my very limited experience with the Zenith side draught carbie, I am aware that the idle can be a little bit difficult to get spot on.

You should be able to screw the idle mixture jet in until the idling begins to "hunt" and then just slacken off for slightly leaner smooth idle.

If you set the idle too lean, or if you have air leaks then you will encourage "popping back" on the "over-run."

Does the car seem to perform OK apart from this?


Regards

Barry R.

perhaps, I am a total novice in engine tuning!.

The car preforms ok, it needs choke to start, and a little until warmed up or it will not idle. Once warm it runs ok, like I said it has an occaisional missfire, but as the plugs are off a second hand engine, the condensor the same, well you probably get it, all of considerable age and unknown use, the odd miss isn't a big suprise. I'm currently procuring new ones, though this doesnt effect the backfiring problem. The only other problem I had with its running was that it got hot going up out of Dollgellau on the way there, and going up the 'pass' on the way back. probably 2 mile or so of 2nd and bottom gear, needing me to **** around with the choke to keep it going. Once back in top and not pulling so hard it returned to its current state of normalness. I put that down to the petrol evaporating in the carb as I have no fuel pump not much 'head' of fuel and a fairly warm day.

As I said the carb is a side draft job, procured through the northern division of the seven workshop, so I understand it to have been re-spindled and should have the correct jets.

Hedd,

If the engine runs better with the choke out then it may well be that this is correcting very weak mixture.

The most likely cause ( in my book and I agree with Steve Martin) is the joint between the carb and the manifold.

Take the carb off and check the face of the joint on a flat plate when you may well see that it is hollow.

When you close the throttle with the engine on overrun there is a lot of 'suck ! ' from the engine which will draw air in from any weakness in the joint causing 'popping' and back firing.

Hi Hedd

I must agree with Malcolm, take the carb off and check the face of the joint, it is very rare to find a flat one!!!

Hedd, here is a good mistake I have made. When replacing the exhaust manifold studs, if the centre one is a bit long it can pass throught into the water jacket. At this point it can partially or completely obstruct a little water passage which runs up between the centre two exhaust ports. Have a look at a block and you'll see the one.
I did this unwittingly to my '33 Tourer. Result; ok on the flat or mild incline, but on a hill of any significance sufficient heat would transfer to the inlet / carb flange to cause the flange to distort, just a little. Then, sudden loss of power, pull the choke out and off you go. I didn't appreciate the cause till I had the head off some years later and saw the obstruction. Cured temporarily by some none- setting petrol proof gasket cement on the carb joint.
Might be worth checking? Not hard to take the centre stud out, measure the depth of the hole and make sure all is well.
Just a thought,
Regards, Stuart

Stuart

Interesting what you have to say? My centre stud used to poke into the water jacket, and I thought nothing of it. I now have a 1/4 BSF CSK screw instead of a centre stud to allow the SU conversion flange to fit in place, and I have Loctited the screw to try and stop the water running out.

What you say about the drillng is interesting because last year during engine strip number xyz blah blah blah.... I lapped the top of the block and the head, and they were distorted in this very area? Could it be that the previous stud had blocked the drilling and caused localised overheating and thus distortion?

At the time I didnt know about any probs in this area so I havnt seen the drilling myself as yet, but I may inspect my spare block?

Steve.

Just an update,

Removed my carb, the flange is bent like a bannana, and the throttle spindle is like a dick in a bucket.
This carb was purchased some 4000miles ago (5years) as 'rebuilt'! from an Austin specialist........

Will be sent of to Burlen's shortly.

Hedd,

Can i reccomend the services of Steve Hodgson who rebuilt my Zenith 'carb. Very proffessional job and very cheap. Does a postal service, will post contact details when I get home this afternoon.

Cheers

Ruairidh

Just to let you know, havnt got round to sending carb away, however I file'd the carb flange flatter, and ground it on a plate of glass and refitted. That has eliminated all but the very occasional pop .Coupled with the new NGK plugs and condensor the car is running better than ever. Either that or because there is no oil to drag the back axle ive got more power.....thats another story.

So those who pointed to the carb flange, many thanks

Another one to be aware of is the exhaust manifold face which mates with the block. In recent years all that I have looked at have serious corrosion in the middle and required attacking with a big square file to get them anywhere near flat.