She wont idle under 1200 rpm

[PaulR]

Greetings all

This has got me compleley stumped.

CT 24290 is running strombergs with K & N filters. Since refitting the carbs after clearing evereything out to spay the engine bay I have not been able to get a satisfactory idle. After having set an aceptable 800/850 rpm with the idle screw and balacing the carbs everything seems fine. As soon as the revs are raised either uusing the pedal or moving the linkage in the engine bay the idle rests at 1200+ rpm, with the stops on the spindle not touching the idle screws, when driving a blip on the gas sometimes takes the revs down 800. With the engine turned off the stops touch the idle screws every time and if the engine is turned off from the 1200 idle I can see the stops moving onto the screws i.e. engine off: stops on screws engine on: stops dont meet screws.

I've lubed all the linkage, topped up the dash pots and all the springs are doing their jobs. Now I'm out of ideas.

HELPPPPPPPPPPPP

[stuart]

Check your diaphragms for any splits/damage and make sure that the springs on each end of the main throttle bar where it exits the front of the front carb and the rear of the rear carb are actually doing there job. Also check that the starter bar springs are also doing the same.

Check also that the plug under the rear carb to blank the advance mechanism port is correctly seated and there is no hole in the advance vacuum tube from the front carb.

Stuart.

[PaulR]

Check your diaphragms for any splits/damage and make sure that the springs on each end of the main throttle bar where it exits the front of the front carb and the rear of the rear carb are actually doing there job. Also check that the starter bar springs are also doing the same.

Check also that the plug under the rear carb to blank the advance mechanism port is correctly seated and there is no hole in the advance vacuum tube from the front carb.

Stuart.

 

[PaulR]

Thanks Stuart

I will check all tonight and report back

Paul

[KnightsT]

Had a similar, but not identical problem (4A with HS6's - so not sure the linkage is the same), but it turned out the main link rod (the longer horizontal one) was too long and the adjustment nut was snagging on what is called the 'bellcrank' in the Moss catalogue - the triangular shaped metal bracket that acts as the contact between the long horizontal link rod and the shorter vertical one - and preventing the throttle from quite going back to its original position. Because the dia of the throttle lever arm ends is by design smaller than the slot in the throttle spindle it looks like the throttle discs are back to idle as the levers are against the idle adjustment stops, but in fact the throttles are slightly open, hence 1200 rpm, not 800. Hard to describe - but I hope you get what I mean.....

 

It took me ages to spot where it was snagging and almost as long to figure out what was happenning as a consequence, why, and then fix it, ...... but great when you do and it works!

 

If you have had need to loosen/adjust either rod then you might want to check something similar is not occurring.

 

Hope this is of some help!

Mick

[PaulR]

UPDATE

Having taken the linkage out of the equation I changed the dash pot oil for 20/50 engine oil. This seems to have done the trick. However it begs the question, What is the reccomendeed oil for the dashpots

Paul

[PaulR]

Update 2

Had a pleasant drive to the new forest this w/e and guess what. Saturday eve revs went up to 1500 at idle. I am now contemplating washing up liquid in the dashpots as it the most viscous thing I can think of. Any views on washing up liquid or suggestions for oil thicker than 20w/50?

[Steve Brabazon]

Hi Paul,

 

The oil in the dashpots should not effect the idle. It's is used to richen the mixture on acceleration. The thicker the oil the more petrol the engine gets on sudden acceleration, as the oil resists the upward movement of the piston.

 

Steve

Click on thumbnail

[stuart]

Update 2

Had a pleasant drive to the new forest this w/e and guess what. Saturday eve revs went up to 1500 at idle. I am now contemplating washing up liquid in the dashpots as it the most viscous thing I can think of. Any views on washing up liquid or suggestions for oil thicker than 20w/50?

 

You have a fundamental problem there and no dont use washing up liquid and you shouldnt need anything thicker than 20/50. Do you have the correct springs and dampers in the dashpots I take it. Have you checked that you dont have any air leaks around the carbs, some carb cleaner sprayed around the inlet tracts or round the carb bodies may help to pinpoint the leak as the revs will rise when it is drawn in any area leaking.

Stuart.

[Badfrog]

Hi,

 

Three major causes:

 

1) punctured nitrile membrane

2) air leak, notably around inlet manifold gaskets (I.Man. may even be broken), throttle axis or at dizzy depression inlet tube and/or advance cup.

3) poorly adjusted or loose throttle disc in the carb venturi (moves a bit and sits sideways)

 

I was going to advertise SUs, but repetitive humor won't help here.

 

Good luck,

[stuart]

 

 

I was going to advertise SUs, but repetitive humor won't help here.

 

Good luck,

 

I was going to do the same

Stuart.

[LGFromage]

Just a thought. Suggest that you check the advance/retard diaphragm on the distributor. If you suck on the pipe and there is no resistance then the diaphragm has broken. This allows fresh air into the inlet manifold thereby allowing the engine to rev. If there is resistance then the problem lies elsewhere, as a strong suck should actuate the advance/retard mechanism. This can be inspected if the distributor cap is removed.

 

Tim

[roy53]

Hi i have a car that on heavy acceleration at say 2500 revs [su h6s] faulters as in wont pull but wait a while and it goes like a train, could this be the oil in the dash pots being to thin or non existant?ROY

[67_gt6]

Hi i have a car that on heavy acceleration at say 2500 revs [su h6s] faulters as in wont pull but wait a while and it goes like a train, could this be the oil in the dash pots being to thin or non existant?ROY

 

 

perhaps, or wrong needles

 

andy

[roy53]

perhaps, or wrong needles

 

andy

 

 

 

I am sure the needles are fine as they were when removed from the car,now re fitted this problem has arisen ,the only difference is that the filters have been removed and trumpets fitted50mm long.my suspicion is the oil as i only had thick engine oil so removed it as the resistance seemed to much but now wonder if the lack of oil is the problem i was working on the basis that the carbs were either fully open or closed in competion driving situations so pistons need to slide up quick

[stuart]

I am sure the needles are fine as they were when removed from the car,now re fitted this problem has arisen ,the only difference is that the filters have been removed and trumpets fitted50mm long.my suspicion is the oil as i only had thick engine oil so removed it as the resistance seemed to much but now wonder if the lack of oil is the problem i was working on the basis that the carbs were either fully open or closed in competion driving situations so pistons need to slide up quick

 

You do need oil in the dashpots otherwise the piston will lift too quick and weaken the mixture thus giving the faltering. For SUs I use either engine oil in the hot weather or thin it a bit with 3in1 for colder conditions, bit of trial and error to suit your driving style

Stuart

[88V8]

I am sure the needles are fine as they were when removed from the car,now re fitted this problem has arisen ,the only difference is that the filters have been removed and trumpets fitted50mm long.my suspicion is the oil as i only had thick engine oil so removed it as the resistance seemed to much but now wonder if the lack of oil is the problem i was working on the basis that the carbs were either fully open or closed in competion driving situations so pistons need to slide up quick

 

With the dashpots fully open or closed, the oil doesn't matter, but during acceleration they're in a transitional state and the oil counters the Bernoulli effect to ensure the mixture doesn't lean out. You may even get max acceleration with EP90 in the dashpots during summer. Too rich and juicy for the road, but worth trying for the track.

 

Ivor

[PaulR]

Furter Update

Possible cause

Thanks for all the advice.

I can confirm that sucking the advance/retard pipe from bottom of the carb moves the plate in the distributor, so all ok

Dashpots have correct springs and no splits in diaphragms, so all ok

No leaks around manifold gasket and new gasket fitted

Pistons rise smoothly and drop with a ping onto the bridge

HOWEVER spraying carb cleaner onto the inner exit point of the butterfly spindles of each carb drops the revs right down. This says to me that air is being sucked in through these points, is this possible? If this is the case is there a seal inside the carb body that may need replacing?

[Kiwifrog]

Moss Part number AUD3080 bush throttle shaft stromberg carbu 2 per carb required may be worth replacing the throttle shafts themselves 512285 as they are more than likly worn. Strombergs are fairly easy to rebuild you can buy carb kits with all the service parts for one carb from Moss

 

Cheers

 

Alan

[88V8]

....spraying carb cleaner onto the inner exit point of the butterfly spindles of each carb drops the revs right down.

But.... if carb cleaner is sucked into the engine it acts like a slug of fuel and should speed it up....

 

What CO reading do you have at idle?

 

Ivor

[PaulR]

I am a mere mortal and have no way of measuring my co2

Any other suggestions?

[stuart]

Have you checked that the timing is correct and the dizzy is firmly clamped. As the idle dies with carb cleaner sprayed on the ends of the shafts it sounds as if this is helping them to return so maybe end bush breaking up could be the cause or one of the springs on the ends of the throttle shaft has fractured. I gave up in the end with the Strombergs on my 4a after having a lot of problems with them and went to twin 40 Dellortos and have never regretted the change (They came cheap otherwise I would have gone to SUs but still very pleased with the result )

Stuart.

[TR4Geoff]

Moss Part number AUD3080 bush throttle shaft stromberg carbu 2 per carb required may be worth replacing the throttle shafts themselves 512285 as they are more than likly worn. Strombergs are fairly easy to rebuild you can buy carb kits with all the service parts for one carb from Moss

 

Cheers

 

Alan

 

 

If you are thinking of buying Stromberg parts, would suggest you try Southern Carburetters in Wimbledon www.southerncarbs.co.uk For obvious reasons they know a hell of a lot about carbs and carry a great stock of Stromberg spares. Their spares are also cheaper than Moss, to whom (I believe) they supply Stromberg bits!

 

Regards

 

Geoff

 

PS. Not connected with SC other than as a satisfied customer!

[stuart]

If you are thinking of buying Stromberg parts, would suggest you try Southern Carburetters in Wimbledon www.southerncarbs.co.uk For obvious reasons they know a hell of a lot about carbs and carry a great stock of Stromberg spares. Their spares are also cheaper than Moss, to whom (I believe) they supply Stromberg bits!

 

Regards

 

Geoff

 

PS. Not connected with SC other than as a satisfied customer!

 

Seconded! In fact you could do worse than just sling the carbs over to them and have them rebuilt as they probably havent had one in quite some years

Stuart

[Badfrog]

HOWEVER spraying carb cleaner onto the inner exit point of the butterfly spindles of each carb drops the revs right down. This says to me that air is being sucked in through these points, is this possible? If this is the case is there a seal inside the carb body that may need replacing?

 

 

Hi,

 

Air leak around spindle is the major cause of bad function. There is no seal to change, so you have two possibilities:

 

1) the spindles run in the original aluminium carb body. The ali has been ovalized along the years. Reconstruction kits include copper bushings that you have to get machined into the ali. Not for the amateur, unless you're good at machining. Have them rebuilt or get good ones.

 

2) Your carbs already have inserted bushings, then it's the spindle that is eroded. Just change it, easy job. Watch out for correct butterfly sitting.

 

Cheers,

 

Badfrog, the oilman....