Engine tickover speed and adjustment

[ChrisE]

Engine tickover speed.

Two of us have similar engines. Mine a 1929 and Alistair has a 1927. Mine seems to tickover just fine, Alistair’s is racing away, and never stable. It also overheats very quickly when stationary which is a function of the engine speed.

We’ve had the carb to bits and messed about but can’t really seem to get it lower or stable. Any ideas? Chris Spencer has a carb for us so that’s our next trial but I’m not convinced it’s that simple as I found an old carb and put that on which doesn’t seem to help.

What RPM would you expect? I checked mine, which idles just fine, with a simple smartphone strobe app (see https://photos.app.goo.gl/pYex8Q8JepCc4Nmf6) and it is 450RPM which seems believable. Alistair’s is running at well over 2,000!

Obviously the fast tickover is making it difficult to drive. When going downhill the brakes are even more of a problem than they would otherwise be, the car is trying to run away and gear changes are more difficult than they should be because with clutch in, moved to neutral, but the engine isn’t falling back to make it easy to select a gear!

Chris

[k. rogers]

Without being too obvious, but I'm assuming the throttle stop has been adjusted and the linkage isn't somehow pushing it open when connected? Sometimes, it's possible the assembly doesn't return properly due to the linkage snagging on something. I wonder if the throttle butterfly is seating correctly.

[ChrisE]

Hi Ken

This is with the throttle stop wound all the way off

With the carb off the engine and put to my lips, blowing, it’s not 100% airtight but it’s pretty good. How airtight would you expect?

[k. rogers]

The throttle butterfly would have to be noticeably open for it to be revving as high as it is. How worn is the throttle pivot, can you move it up and down considerably?

[David Kemp]

My two bobs worth, is check for air leaks by squirting oil around all manifold joints if smoke starts poring out the exhaust you have a flange keak. This is not to be unexpected with a car almost 10p years young.
If that fails please check your ignition as many a carb has been blaimed for ghe distributir faults.
Does the dizzy shaft have play? Are the bob weights returning completely ?

[Jeremy]

I would look at the carb, and specifically the jets. This is exactly how my Bradford reacted when it had a the wrong jets - since it has been very stable at tick over and the overheating has gone away.

[Chris Spencer]

20240730_172949.jpg

These are enroute to Chris & Alistair and should hopefully provide a suitable replacement or enough good parts to make a decent carb out of the 2

[ChrisE]

What I don’t really understand and need better knowledge of, is how ‘airtight’ should the carb be when the throttle is closed and the tickover screw wound right off? Is it supposed to be absolutely “submarine-door-type-airtight”? If so does any wear, any air that comes through enough to start causing problems and if so how much wear can be tolerated before it’s time to take serious steps?

I can see that the likely culprits for leaks are the butterfly valve (wear) and the throttle spindle (wear). I can’t see any spare spindles and discs (valves) on sale anywhere that fit our carbs. It can’t be a complicated bit of manufacturing, does anyone sell them?

Chris

[Forumadmin]

I look through them at a strong light. Really they should be air tight. Fitting new butterflies is often counterproductive because of worn bores.

[Jeremy]

Firstly they can't be air tight or the engine won't run at all - the butterflies have to let air through to mix with the fuel etc or no bang - as simple as that.

But worn bores, discs and spindles will introduce too much air and could upset the mixture - so you need these to be in good condition. as you say parts are difficult but a good engineering place or classic garage with knowledgeable staff can sort out the spindles as these tend to be the weak area - they certainly are on SU carbs. Brass bushes can be made and fitted with new spindle bars etc.

[Chris Spencer]

I don't think that you need to look at remanufacture of early carb parts - I'm almost certain that there is a good quantity of new old stock early carb spares in a substantial hoard of parts that I recently recovered on behalf of JCS - I need to dig through and find the tray that they are in

[Forumadmin]

The close contact of the butterfly with the inner wall of the venturi means that there is friction and wear here. It is impossible to drive without opening the throttle, so it is inevitable that the butterfly will wear, but if there is a gap, the air being sucked in cannot be managed and the engine may modulate.

The position of the slow running jet metering hole relative to the butterfly closed position is critical. Even if, in a worn carb the butterfly still closes well (let me say light tight with no throttle and stop backed off), the idle may not be even as the flow across the slow running metering hole may not be stable or be too little for the adjustment to compensate.

https://japan.webike.net/moto_news/webi ... n-the-sun/

[Keith Clements]

My further point on reconditioned carbs is that often the new butterfly does not mate accurately with the worn body, meaning there is either (or both) wrong positioning of the closed butterfly or too much airflow to get a slow idle. Manufacturing carburettor bodies and butterflies is an extremely accurate process and would be difficult to get done economically.

Reconditioning also requires accurate machining, especially when inserting sleeves for new throttle spindles. I have seen reconditioned carbs that do not pass the 'light test' and so did not idle properly.

[Chris Spencer]

20240805_093343.jpg

I've also managed to secure a full new old stock OE Zenith horizontal carburettor rebuild kit that should assist in the resolving the carb issues - This I will deliver along with the carbs + other parts next weekend at the Reunion in Bradford