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MG Midget and Sprite Technical - Hesitant under acceleration

Hi Guys

Just took my 1500 midget round the block (1st and 2nd gears only) and its seems hesitant when I go above about 2k.

Now its been SORN of the road for the last 6 months and the following has been done since I last drove her.

New K&N, MGOC advised new needles were not needed.
RC40 backbox, checked and is not blowing ata all.
New fuel pipes that run around the K&N.

Now she'll be taxed on the 01/04 so will put some fresh fuel in as its on miniumum and go for a blast to try and blow the cobwebs away.

Now, does anyone have any suggestions as to what may be the issue as last time I used her she was fine, initially I though about the needles but would this be an issue at such slow speeds?

Help appreaciated!
Thanks
Arron
A Burgess

All depends what you mean by "hesitant". If it splutters and coughs it's just as likely to be ignition related as the carbs. Check the points, HT leads etc. It could also be fuel starvation if your tank is almost empty. Fill her up to eliminate that.

Will it rev higher when stationary?

ant
Ant Allen

Cheers Ant, by hesitant the best I can describe its a bit of kangerooing or as if you keep dabbing the throttal.

And yes, seems to rev perfectly when stationary, just when under load.

Thanks
A Burgess

Sounds like ignition problems to me, but I look forward to knowing what fixes it.
Ant Allen

Arron,

The question is realy what to check first, i.e. what's easiest. You have made a change to the carb set up, by way of reducing the upstream air depresion, so I would start there. When it starts spluttering, pull the choke out slightly (aim to drop the jet by about 2-3mm - I am assuming SUs). If you pull slowly, it may clear up a bit, then go back to spluttering as it will have gone too rich. If this makes a difference I would revert back to the old filters and see if this makes the difference, this is the acid test.

I fitted K & Ns to my V8 and did have to change the needles, a good resource for needle comparison is http://www.mintylamb.co.uk/suneedle/. But you could also consult this forum with a dedicated thraed on 1500's with K & Ns.

I would also check the dynamic advance, make sure it moves the timing as engine speed increases and make sure the timing is correct. If you don't have a strobe, check the static advance. At the same time I would check the dwell and points gap, do this before the timing check, as if you need to change the point's gap, it will need to be re-timed after anyway.

If you haven't replaced them for a while I would replace condenser and the points, as they are in-expensive and need replacing from time to time anyway.

Another good check for the ignition power is to set up an increased gap for the spark to jump from the HT lead to earth. The gap should be about 8mm, which equates approximately to 12kV. This check fault finds the coil, distributor, rotor and HT leads in one hit. DO NOT DO THIS IS HAVE ELECTRONIC IGNITION.

Let us know how you get on.
J E G Eastwood 1

Thanks for the advice, will head back out to the garage and tinker, first will try some new fuel I think and then revert back to standard air filter.

Thanks
Arron
A Burgess

Simple first test - does the rev counter needle leap about when the engine is hesitating? I don't mean does it change, but does it flicker vigorously? If it does it is likely to be an ignition problem, if it doesn't then look for a fuelling problem.

Guy
Guy Weller

SU Dashpots need topping up with oil?
Adrian Jones

Air filter on the right way up? (so breather holes a not obscured)

A
Anthony Cutler

How long has it sat un-started, and how old is the fuel?

Id pull the plugs and giving them a bit of freshing up, esp. if sat for a long period of time....even new ones can get a bit scaley if sit in engine for a long time...

also, try letting it run for a while at idle...20-30 min. it may just have some small amount of varnish build up on the valves and they may be sticking just a bit.

Like the Above said... check the easiest 1st. and go from there

Prop
Prop***The End in 2012

what about the fuel filter...give that a looksy


Try some spray on brake cleaner with the tiny straw thur the carb hose that goes from the float to the jet...Gunk really like to collect in those hoses...go in thur the float side and disconnect from the jet 1st.

prop
Prop***The End in 2012

Your engine will run very weak if you still have the standard needles with K&N filters, whether you have any other problems or not.ABT are usual standard ones I seem to recall. Try AAM needles with the jets dropped to 70ish thou in both carbs.

Peter
peter burgess

Hi Peter, just checked out the Moss catalog and the std jets are either ABT (early) or ADT (late), they recomend AAQ for K&N's and a tuned exhaust, they recomend AAM for K&N, exhaust and a modified head.

AAQ or have you experience of the AAM and my set up.

They don't have a recomendation for just the K&N's.

Prop, will have a go at your suggestions as well.

Thanks for the help
Arron
A Burgess

Arron, if Peter tells you to try AAMs you would do well to follow the advice

On his rolling road he will have tried out almost every combination you could imagine
Bill sdgpm

Cheers Bill, will try and source some AAM's today.
A Burgess

Ok, new AAM needles fitted and a fresh tank of fuel, car runs smoothly upto 3k revs but after that in all gears starts to splutter, (revs fine at standstill), would I be right in thinking that its running to lean and I should richen it up?

Idle is also very low but I can fix that.

Thanks
A Burgess

Do dashpots rise/fall smoothly?

Is hesitation also there on med throttle as well as WOT?

A
Anthony Cutler

If you unscrew the piston damper if lifts the piston up smoothly yes, and when you drop it down there is a dull metal clunk.
A Burgess

Ok; but you should try to lift the piston from the front of the carb, and check that it rises fully without impediment...

A
Anthony Cutler

Do you mean take the air filter off and lift it from inside by poking a finger through, quite new to carbs as you can probably guess?
A Burgess

I have had a problem like yours in the past on two occasions - for completely different (and unexpected) reasons.

In the first case, part of the rubber washer/seal where the float joins the main body of the carb broke off. When not under load the engine ran well but put a bit of load on it and the flow of petrol pulled the fragment of washer upwards and partially blocked the petrol supply. Took weeks to find that one.

More recently (when I still had twin SUs) the throttle link was badly adjusted (that bit between the two carbs that opens both throttles together). When I pressed the accelerator, the link was pulled slightly to one side and the throttle on one of the carbs was able come free of the link and snap shut. Result - so one SU open and one SU closed). When I released the accelerator, the throttles on both closed and the link slotted back into place so nothing seemed wrong. This was a quick fix after a long period of trying to diagnose.

The moral of this story is that poor running can result from the most minor and silly faults. Check everything!

Chris
Chris H (1970 Midget 1275)

Yes!

best to do the simple things first to eliminate them.

A
Anthony Cutler

It sounds to me like the dashpots are empty. When this happens the mixture will get very lean under load as this causes the piston in the dashpot to rise too fast. the oil dampens this effect and richens the mixture during acceleration (this is how it was designed). The other thing I would look for is manifold leaks. If your car came with vac advance and you have no vacuum line on the car and the connector on the manifold is not plugged, you will be leaning the mixture and causing it to sputter. Other cuprits could be a clogged fuel filter I suppose, but start with the dashpots. Use either 20 weight oil, or automatic transmission fluid.

Take off the aircleaners and put your finger down the throught of the carb and lift the piston. ideally it should have a fair bit of resistance on the way up, but it should fall down again easily with a dull clink. if you are lazy unscrew the black plastic cap on top of the carb and pull it off along with the rod attached to it and look down inside to see if there is any fluid in there. if not, then there is your problem. Fill that space up with either of the fluids i've listed, screw the cap back in and take it for a drive.
S.A. Jones

Arron has just fitted new needles so I hope he thought to refill the dampers afterwards...

But we often forget the basics dont we

Arron, I'm not sure if you know how to set the base level for the jet, but if you screw the jet adjuster nut up until the jet is flush with the bridge of the carb (both carbs) then undo the nut two full turns [that is twelve "flats" of the nuts passing the same spot whilst turning down], you will achieve the base setting from which unscrewing further will richen the mixture and screwing it "in" will make it leaner

for checking out the bridge "level" it's best to take the dashpot(s) off and remove the spring(s) and piston(s) so you can see the jet position and measure its height with the tip of a steel rule (or similar flat ended device)

The bridge is the place where the air is pinched as it passes across the throat of the carb and begns its trip into a tight warm space, and is where the jet releases its measured amount of fuel into the carb

By varying its height you decide whether it is running rich or lean, but you need the "twelve flat" base setting to begin assessing the mixture

My crude little picture shows the jet at the start point before lowering to base level and when it's down you will see the difference in position just from looking

the picture is deliberately crude and misses off the centring nut and the resistance spring etc.

hth


Bill sdgpm

Thanks all for your assiatance, its looking much better now after following some of the tips above.

1, The air cleaner cork gasket had folded over a little and was trapped under the dashpot itself possibly causing an air leak.

2, Vacume pipe was a little loose

3, Dash pots 'may' have had a bit too much oil in, now burnt off.

Car now running much better but seems to be drinking a fair bit of fuel, but is ok to drive which is the main thing, booked in for a tune up in a couple of weeks so fingers crossed it'll be 100% aftre that.

Thanks again
Arron
A Burgess

well if there were any air leaks it's possible that the car was tuned richer to compensate and make the engine run. lean out the carbs a little and your economy should get back up.
S.A. Jones

The Vac pipe was my main concern, when I first took ownership of the car I found it tucked away behind the dizi so I think it was tuned to compensate, I had since found it and reconnected it but not done any miles so this could be ojne of the problems.

Thanks
A Burgess

This thread was discussed between 27/03/2009 and 07/04/2009

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