Hunting at idle - starting from scratch

[fool73]

Since my recent engine rebuild, one of the things on my snagging list is that the revs vary on idle within a few seconds it varies by about 100 or so revs. I believe the fuel mix is not too far out from the colour of the plugs. I have tried spraying wd40 in various places around the carbs and pipes and it doesn't seem to reveal any air leaks as far as I can tell. When the engine was first rebuilt I did balance the carbs using a borrowed syncrometer. However, in the absence of inspiration for anything else to try, I thought I might as well go back to square one. Has anyone tried this type of balancing tool

http://www.ebay.co.uk/itm/CARB-BALANCER-CARBURETTOR-BALANCE-BALANCING-Tool-With-Air-Flow-FOR-STROMBERG-/251053689768?pt=UK_Diagnostic_Tools_Equipment&hash=item3a73f74ba8

Looks a bit mickey mouse, but it's half the price of a syncrometer like I borrowed before
e.g like this

http://www.ebay.co.uk/itm/Carburettor-Carb-Syncrometer-Syncro-SX-Balancer-Weber-Dellorto-40-45-DCOE-/261495399589?pt=UK_CarsParts_Vehicles_CarParts_SM&hash=item3ce25724a5

The carbs had new diaphrams, needles, floats, valves etc not all that long ago and I am not inclined/skilled enough to work on carbs beyond that. What I really need is a known good set of carbs to try a substitution test (i.e to be sure whether or not it is the carbs causing it), but I that's not the sort of thing anyone has knocking about unused.
Any other suggestions for things I could try or symptoms to look for or tests to do to help narrow down the likely cause ?

[ozzymac2011]

HI i,ve got a newish set of strombergs on my car about 3 weeks,until i changed them for a weber,your welcome to borrow them,but you,ll have to collect,i,m on holiday ,ring if you want any info. (0779 2317656) regards Mac.

[Seaking]

I had Hunting on mine yes it has a holley carb but still turned out to be a air leak.i had to fit a clip on the rocker cover breather and I also discovered my brake servo pipe on inlet manifold had a crack so I fitted new pipes with a banjo type adaptor now has stopped hunting

Cheers Glenn

[jakesmig]

Agree with Glenn, most likely a small air leak somewhere.

I used a CARB BALANCER and a COLOURTUNE on a mates midget recently with good success. trick was in setting up the linkages properly, everything fell into place after that.

John

[wilf]

Air leak somewhere, biggest culprit is usually the crankcase breather system.

Does tickover vary (drop) when you remove the oil filler capo? It should.

[fool73]

Thanks for the replies.
Mac. Thanks for the offer. Shame you are not nearer otherwise I would take you up on that. I am in hertfordshire.

Glenn, what do you mean by putting a clip on the rocker cover breather ? Would the servo leak show up if you sprayed it (e.g wd40/carb cleaner etc).

John. What kind of carb balancer did you use ? Not one of these gunson ones by any chance ?

Wilf. Tickover drops if I remove filler cap. And I think it increases if I squeeze the rocker cover breather if I remember correctly.

Re. air leaks in general, should these be easy to spot by spraying something on the leak and if so, would wd40 do it ?

[Seaking]

Here you go 767.jpg770.jpg769.jpg

[Stagsongas]

'73

Over 95% of hunting at idle issues are caused by weak mixtures, and an air leak can cause that for sure.

BUT! There are other causes. A fault in the engine timing Advance/Retard mechanism or the vacumn supply to the Distributor can result in the same effect. To prove or eliminate this theory is easy. Idle the engine, disconnect and blank the vacumn hose with your finger. If it doesn't hunt, your hunt is over

Regards

Steve

[Ian Durrant]

I am very confused by this thread. I have always understood the term hunting to be the rythmical rev pattern caused by an over rich mixture. Like when you leave the choke out too long and you get the brmm brmm brmm. Whereas an overlean mixture is heard as a splashy slightly irregular misfire. ( I am talking carb systems here, I know there are circumstances where a closed loop injection system can have faults that make the engine hunt for enirely different reasons).

Have I had this wrong for the past 30 years? Certainly if I have the terminology wrong, I have been able to interpret things anyway.

[jakesmig]

Thanks for the replies.
....................

John. What kind of carb balancer did you use ? Not one of these gunson ones by any chance ?

...............

The type was this ebay item, 180748349770, costs about £20.00 delivered.

You take off the filters and press the balancer over 1 carb and note where the ball is in the attached calibrated tube, then you repeat the action on the second carb and note the ball location. Keep repeating the process, adjusting the idle settings on the carbs until both show the same level on the tube.
I had to adjust the mixtures as I went to get the best result, but the engine I was tuning was getting its initial tune after a rebuild.

I daresay most balancers use near enough the same system, It was the first time I'd used one but it really was a doddle.

I thought I had a reasonable set on the engine before I used the balancer and was surprised how far out I was, and how much smoother the engine was on completion.

John

[73stagman]

I am very confused by this thread. I have always understood the term hunting to be the rythmical rev pattern caused by an over rich mixture. Like when you leave the choke out too long and you get the brmm brmm brmm. Whereas an overlean mixture is heard as a splashy slightly irregular misfire. ( I am talking carb systems here, I know there are circumstances where a closed loop injection system can have faults that make the engine hunt for enirely different reasons).

Have I had this wrong for the past 30 years? Certainly if I have the terminology wrong, I have been able to interpret things anyway.

Ian, if you have it wrong so does Gunson... the people who make gas analysers.... I think you are right too!!

[Stagsongas]

To be fair, there are many terms that are unclear and undefined, 'hunting' is just one of them.

The all-time classic has to be the term 'Misfire'. In OBD terms this is clearly defined as a combustion stroke where the cylinder has been 'Fired', but the products of that combustion are outside of parameter.

Conversely, every day you will hear people use the term 'Misfire' for a cylinder that is not firing at all. In that case, it is not a misfire, but a failure to fire. 'Hunting' suffers from the same problem, one term being used for two quite different effects.

In any event, what was said is relevant in terms of 'hunting', however the term is being applied. It can be caused by variation or oscillation in dynamic engine ignition timing.

Regards

Steve

[fool73]

Thanks for the further input.
Steve, I will try putting a jubilee clip on as per your photo and blank the vacuum hose as per your suggestion. I guess I can also disconnect and blank the servo pipe to eliminate a crack in that ? I'm not sure what you mean by "It can be caused by variation or oscillation in dynamic engine ignition timing." Sorry if I'm being dim. Can you explain the meaning/cause/fix of that.

Ian and steve. Sorry if I have confused with the use of the term hunting. I'm not sure I am attuned to differentiate between "rythmical rev pattern" and "splashy slightly irregular misfire". What I am trying to describe is a variation in the revs of the engine, but I would say it is more random than rhythmical. My new water pump still has a slight whistle to it so the variation is most noticeable by a slight change in the pitch of this whistle, or of course by watching the rev counter.

I also have a feeling the variation is only at idle (i.e not noticed it if I increase the revs). I need to double check that, but would that be significant ?

[Stagsongas]

Fool '73,

No need to apologise, terms like 'hunting' have caused so much confusion and misunderstanding over the time cars have been around that it is fully and absolutely understandable. It's as bad as the term 'Misfire'.

Fact is that your quick definintion of 'hunting' [variation in revs of the engine] is probably the best of all. This effect can occur with both rich mixtures (e.g. Choke in use when it is not required) or weak mixtures (e.g. an air leak for instance) but BOTH conditions can result in your 'hunting' definition [variation in revs of the engine].

You'll notice that most people, myself included, will reach for and suggest the 'too weak' option as the likely cause, simply because an air leak is far more likely to be the cause. On a carburetted engine like the Stag, I have yet to see an air leak cause the mixture to go too rich, but it it still could be 'hunting' because the mixture is too rich for another reason.

None of that was my point though!

If you find neither an air leak (weak mix.) nor the other likely problem (rich mix.) then the engine timing advance/retard unit can cause 'hunting' just at effectively as either of those.

Good Luck

Steve

[73stagman]

Basically there will be other evidence that will determine if the engine is running weak or rich.

If it misses and falters at higher loads then the chances are it will be running weak

If it coughs and spits or backfires on an trailing throttle (assuming the timing is right) then it is probably too rich and burning fuel / air mixture in the exhaust system

But I do agree with Steve that it is much more common for air or vacuum leaks to occur causing the engine to run weak. (A very weak mixture will also cause more damage long term)