Faultering under accelleration

[mrmorrisminor]

Hi folks this has driven me mad today so I thought I'd ask for some ideas......

It is a bit of a long story .... basically the car is a standard 1098 which I bought about a year ago and began to experience problems under touque, going up hills etc. The engine was also starting on 3 cylinders and using oil like it was going out of fashion. Compression testing revealed very low compression so I have totally rebuilt the engine. Rebore +20, new pistons crank, mains, ends duplex chain, oil pump, head and block skim etc etc, it was knackered!!

Anyway, just put it all back together and the engine sounds beautiful, ticks over like a sewing machine at very low revs, no smoke.... wonderful I thought!

Out on the road, very similar probs as before, when accellerating it feels as if it is down to 3 cylinders at times, very jerky, when you let off the throttle and go again gently you can improve the situation. Gentle throttle does not seem as bad. It is fine ticking over and on flat/ down hill. On the drive, giving her a gentle rev, she hesitates and faulters slightly before the revs build (I'm trying to go gentle as I'm running in!) bringing out the choke does not seem to affect things either

So far I have ....

Changed plugs

Changed HT, rotor arm and dizz cap and condenser

Checked the small braded wire in the dizzy

Swapped the coil

Checked the timing and valve clearences and of course re touqued head after first heat cycle

Swapped the dash pot for the one on my other Mog that is working fine

Checked the statick timing using the strobe and starting handle (half way between long and next tooth on comb as on my other mog)

The carb its self I serviced when I first bought the car with new jet, needle and float valve

Plenty of petrol is passing through the pump and I've checked the filter in the bottom.

I have been at this all day and I'm running out of ideas. My next trick is to take the whole carb off my other car and see if that helps.
I currently have the dizzy out on the bench and I've taken the plate off where the points are situated to reveal the weights inside but I have to confess I do not really know what I'm looking at in there! I have compared it with a scrap dizzy I have (the housing is brocken) and there seems to be a difference in the number stamped on the arm inside. Mine has 16 deg on it and my scrap one has 15 deg... I'm assunming this is the amount of advance??? Not sure whether I'm barking up the wrong tree but I'm running out of things to try.... the problem seems to be getting worse....

Any suggestions welcome......

[bmcecosse]

Oh dear! You do seem to have tried pretty much everything...although you don't actually say about new points - and have you tried another coil (although as regulars will know - it's never the coil) I would be trying the complete carb swap too - although I doubt it will make any difference since you have already swapped the dashpot. However - you do say you did some work on it - so just maybe.... So - back to fundamentals - can you be sure the camshaft is timed correctly - dot to dot ?? Don't worry about the dizzy numbers - yes that's the mechanical advance in dizzy degrees (so X 2 for engine degrees!) as long as the balance weights are loose/free to swing - but I do suggest you try advancing the ignition timing and see if that helps ? Advance it to get the best/smoothest idling - and Is the vacuum advance working in the dizzy ok ? Suck the pipe and watch to see the baseplate move and relax.

[mrmorrisminor]

Thanks, yes forgot to mention the new points.

I was quite careful when fitting the new timing cogs and I'm pretty confident that it is correct. I even experiemented with a tooth in each direction to satisfy my mind that my dot to dot was correct.

With the timing I did have one partial success today, I slackened the dizzy and adjusted it by ear for idle speed and exhust note. The car perfomed much better but pinked like mad! Which is what pointed me to timing/dizzy.

When revving without load after the innitial hesitation she has a slight miss beat every so often, but back on tick over it sounds better than my other Mog!

I have tried to repeat adjusting the timing by ear but the problem persists this time.????

I was worried that maybe the head had begun to blow or a vlave not seated properly or something daft but when I've had that in the past tick over is poor and higher revs smooth it out!

I will try again!

[bmcecosse]

Sorry - yes I see now you have already changed the coil.. So - it does rather point to timing - and very possibly the vacuum advance ? If it's not working then the static advance must be put too far forward to get good idling - and in turn leads to the pinking which must NOT be allowed to continue.... So - check the vacuum carefully - is it working - is it connected correctly to the inlet manifold - any chance pipes are blocked, that sort of thing ?

[MarkyB]

The fact that you can "nurse" it to run a bit better makes me wonder about the oil in the carb dashpot and the spring.
You didn't use ATF in there did you?

[chickenjohn]

another thing to check as well as the excellent suggestions above -is for leaks on the inlet manifold. You did change all the manifold gaskets and carb gaskets for new ones??

Other than that, is the distributor worn?? Is there a lot of play in the distributor shaft?? In which case the timing may be excessively wandering. Also, strip the carb down and clean it, especially the float chamber and the piston etc, can you blow air between the jet and float chamber.

Make sure the points gap is set correctly as this could be the cause of the symptoms you give.

[mike.perry]

After reassembling the carb did you check that the piston was lifting and dropping on to the bridge with an audible clunk?

[Andy]

Hi

This sounds similar to a problem that I had when I fitted a new engine. My car would tick over really well and would rev okay when the throttle was 'blipped'. But as soon as I took it out on the road and accelerated it would start misfiring badly!

The problem on my car turned out to be how I'd fitted the distributor. I'd managed to fit it so that the rotor arm was just in range of the contacts in the distributor cap. The engine would run in this condition but when out on the road, the change in vacuum at low engine RPM when accelerating would mean that the rotor arm moved out of range of the distributor cap contacts and the misfiring would start.

It took me a while to work out what the cause was and I only realised when I noticed that the burn mark on the rotor arm was right on the edge of the contact area!

Make sure that the dizzy cap contacts line up with the middle of the rotor arm contact area. Mark the outside of the dizzy cap with tip-ex where the No.1 cylinder contact is, fit the cap and transfer the mark to the dizzy body. You can then remove the cap and adjust the dizzy body until the middle of the rotor arm lines up with the mark on the body.

Hope this helps

Andy

[bmcecosse]

Can't happen with the 45 D, but I guess may be possible with the 25 D if the vernier is not centralised.

[MarkyB]

It can definately happen on a 25 D, it's happened to me.
Took ages to work out what the problem was.

[dalebrignall]

have you checked the connections to the coil are tight this happened to me they could nove a little and it made the car run with the symptoms you describe .

[mrmorrisminor]

Hi,
thanks for all the suggestions above, it is good to bounce ideas round, the folks at home try to nod in the right places but really they have no idea nor are that bothered!!!!

With regard to the carb, all went up with new gaskets and the oil in the dash pot is the same as my other minors that are working A OK.

The problem was the same with my spare coil. I will inspect coil wiring, especially spade connectors.

The timing I have checked using a strobe attached to HT lead 1 whilst turning the engine very slowly on the starting handle. It flashes when the timing mark on the pulley is half way between the longest tooth on the comb and the next one, which is where my other minors are set. I believe this is called setting the static timing?? Would the vaccume advance mess this up if the vernier adjustment was set in the wrong place?

The vaccume advance does look a little sus.... still running on the original steel pipe.

I will return to my mother's garage on Fri eve for another session.... priorities will be-

1) re-assemble dizzy, checking weights and swap vaccume unit and check hose

2) full carb swap with my traveller


I will update you all on progress!

[minor65]

Sounds like the points to me !!

[mrmorrisminor]

Well Well Well........

Another day of tinkering

Re assembled the dizzy, greasing weights, checked vaccum advance- all in good oreder, maintains vaccum and pipes clear. Put new points on (again) for good measure. Car still no different!

Swapped the carb from Traveller which is running A ok, car still the same.

The symptoms were still the same. Perfect tick over, faultering under accelleration and loosing a cylinder. I was beginning to get the HUMP!

I was listening to her ticking over when every so often there would be a momentary mis as if one spark had failed and you could hear a click from inside the dizzy cap-which was replaced last week and all new leads!

In despiration I tool the cap off, and had another look. . . .

The braded earth lead (replaced last year) that goes from the terminal to the points is slightly longer than the original and the loop of cable slightly sticks up. I re routed the wire, tucking it behind the condenser cable which I avoided innitally as I wanted to keep all the wires separated and not risk chafing the cloth brade on the advance mechnism' which was what happened to the original......

Hey presto problem solved!!!! The spark for cylinder 1 was earthing to the cloth brade of the cable when under the load of accelleration! Hence loosing a cylinder. Not sure why it wasn't doing it at tick over???

All that fuss for something SOOOO simple!

Thank you for allowing me to pick your brains and I hope that someone can learn something from my cautioinary tail

[MarkyB]

Nice fix!

Always a good idea to use all your senses when diagnosing engine problems.

I think that there is more resistance against the plug sparking when it is under pressure.
It is possible for a plug to spark out of the engine, but fail in use, if it's very marginal.

[bmcecosse]

Well done - good detective work.

[chrisryder]

The spark for cylinder 1 was earthing to the cloth brade of the cable when under the load of accelleration! Hence loosing a cylinder. Not sure why it wasn't doing it at tick over???

is it possible that the acceleration was, as it's meant to, rotating the baseplate in the distributor via the advance and retard unit. and that the movement of the baseplate was moving the wire just enough to cause the short circuit? it would explain why it was ok on tick-over... possibly!

[mrmorrisminor]

Yes, that's very possible and the direction of travel of the base plate (clockwise) would shorten the distance the wire has to travel and would therfore increase the size of the 'loop' possibly putting it more in the way of the rotor arm?.... Good thought

[xpress]

I we just reading this, is this set with a timing light? Also, if the rotor arm is burnt on the right side, is the timing too far advanced?

Hi

This sounds similar to a problem that I had when I fitted a new engine. My car would tick over really well and would rev okay when the throttle was 'blipped'. But as soon as I took it out on the road and accelerated it would start misfiring badly!

The problem on my car turned out to be how I'd fitted the distributor. I'd managed to fit it so that the rotor arm was just in range of the contacts in the distributor cap. The engine would run in this condition but when out on the road, the change in vacuum at low engine RPM when accelerating would mean that the rotor arm moved out of range of the distributor cap contacts and the misfiring would start.

It took me a while to work out what the cause was and I only realised when I noticed that the burn mark on the rotor arm was right on the edge of the contact area!

Make sure that the dizzy cap contacts line up with the middle of the rotor arm contact area. Mark the outside of the dizzy cap with tip-ex where the No.1 cylinder contact is, fit the cap and transfer the mark to the dizzy body. You can then remove the cap and adjust the dizzy body until the middle of the rotor arm lines up with the mark on the body.

Hope this helps

Andy

[mrmorrisminor]

I have to confess I have never really looked at where the mark is on the rotor arm. I have set it up with the strobe timing light using the timing mark on the front pulley and the teeth under the timing chest.

Since re-routing the earthing strap all has been well..... I completed a 75 mile round trip at the weekend without fault