annoying hesitation

[ajm]

hi everybody,

I have a slight problem it isn't major ,just rather annoying.I have a stag it's just done 87000 miles and does not seem to burn a lot of oil and has no undue rattles,so I assume the motor is in reasonable nick (ahem!)

the problem I have is when the motor is up to normal operating temperature (needle halfway on the gauge) there is occasionallyan annoying hesitation,it occurs when I putthe throttledown quick during an overtake or pulling away from a junction, (if I need to get out quickly) .if I ease up on the throttle it becomes very smooth again.When the engine is cold and warming up there are no such problems.

I have replaced carb diaphragms,sparkplugs,plug leads,coil,distributor cap, checked mixture,timing(electronic) and timing advance.and checked the dash pots.

any ideas? could it be the ignition system? or is this a characteristic to live with,due to the modern unleaded fuels?(possibly pre-ignition?)I use castrol valve master plus which "seems" to help but maybe I'm kidding myself!!!!!!!!

what is the best fuel to use?

thanks

allan.

[Mpower738]

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Hi Allan.

Fixed choke carbs have an accelarator pump usually in the form of a diaphragm which gives a squirt of fuel when you put your foot down quickly to avoid momentary weak mixture, if this were to occur it would create a flat spot.

Stromberg and SUcarbs do the same thing but on a slightly different principle, when you put your foot down the piston does not have time to rise instantaneously due to the damper, and as the manifold absolute pressure has dropped, the amount of fuel sucked through the needle/jet increases, thus momentarily richening the mixture.

I would definitely aim investigation at the carbs, they can be very frustrating things to get right. Make sure your mixture settings are spot on and you have no air leaks downstream of the butterflies. Check the o-ring at the base of the dashpot where the needle is seated and make sure the oil level is correct.

I personally wouldn't suspect the ignition system, not unless there is something seriously amiss with the timing. Check the advance diaphragm aswell, if you have a vacuum pump you can make the baseplate move with the cap off to make sure this isn't perforated, which would also weaken the mixture. I have heard people say it is possible to move the baseplate by sucking on the pipe, I've never managed it, maybe mine is stuck !!!!

Doug.

[WildStag]

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I think it is worth checking the vacuum advance mechanisim - take the distributor cap off, disconnect the connecting tube from the carburettor and and give it a good suck. You should see movement in the carburettor. If there is none or just a twitch, then suspect tht the diaphram in the vacuum unit has perished.

Tony Hart showed me this (with the explanantion)on my own Stag - I have exactly the symptoms you describe but as I have no interest in traffic light starts, I have still to get it fixed. Perhaps someone can comment about how easy it is to DIY.

[Kevin Garrod]

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Wildstag, when you talk about sucking on the tube don't you mean see if there is movement at the distributor rather than the carb? suction from the carb should provide movement in the dizzy base plate to advance the ignition, or have I got the wrong end of the stick altogether?

[WildStag]

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Kevin Garrod wrote:

Wildstag, when you talk about sucking on the tube don't you mean see if there is movement at the distributor rather than the carb? suction from the carb should provide movement in the dizzy base plate to advance the ignition, or have I got the wrong end of the stick altogether?

Just testing to make sure y'all awake now - almost 12 hours before someone spotted it! :shock:

You are so right -it is movement of the distributor base plate that you are looking for and not a mouthful of Shell's finest. My excuse can only be that I was overexcited by sight and sound of Denis Waterman's Stag on Monday night soI shall retire back to the depths of the forest (garage?). . .

[ajm]

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thanks for your input chaps

I will let you know how I get on

allan