steve2286w
Well-Known Member
Perhaps try it with one plug lead disconnected at a time and check for any difference
ExMil109
Active Member
I also wondered if it was an optical illusion, but the other symptoms suggest not.
Like others have posted, this sounds like an ignition timing problem. If you have a good strong spark from all your new ignition components, plus incorrect timing, it's possible that the wrong cylinder is firing during the start-up. If the dizzy rotor is halfway between HT posts in the cap when the coil fires, the spark could jump to either of the HT leads and cause the wrong cylinder to fire. The same thing can happen if the dizzy cap insulation has broken down and there's tracking between the HT posts. I'm not sure if that could actually drive the engine backwards, but it might fight against the cylinders that are firing correctly.
My suggestion would be to aim for a good basic static timing: no 1 cylinder on TDC, rotor pointing at no 1 HT lead, ensure the cap is good and all the HT leads are in the correct order. If that works, then set the timing by ear or by strobe, a bit at a time. If that doesn't work, or if you need crazy amounts of advance or retard to make it run (I think the spec is 16 deg BTDC @ 2000 RPM with no vac advance), I'd be suspicious of the electronic dizzy. Good luck!
thspeller
Active Member
Thanks ExMil109 - from all the above, I am leaning firmly on your suggestion that one of the plugs is firing at the wrong time (makes a lot of sense) - question is why and why only on starting. I don’t believe the timing is out so I feel there must be a fault in the dizzy sending a spark down the wrong lead - I like your suggestion of the coil firing when the rotor is between HT posts in the cap but..... if the engine ticks over beautifully at idle, could this diagnosis still explain why it only happens on start up?
I’m beginning to be tempted to throw £80 at a new powerspark dizzy (and assume this one is faulty) - does that sound rash?
Tom
I’m beginning to be tempted to throw £80 at a new powerspark dizzy (and assume this one is faulty) - does that sound rash?
Tom
Colthebrummie
Well-Known Member
Replacing the old dizzy for an electronic one is a good decision in the long run, it eliminates any problems with points and condenser. The thing that puzzles me is this grinding noise you refer to, could the starter pinion not be making proper contact with the flywheel starter ring?
Col
Col
thspeller
Active Member
I think we’re safe in assuming that one of the cylinders plugs is firing out of turn. From all the evidence, that pre-ignition could be caused by (1) dodgy distributor (whilst new, I may just have a dud), (2) a head gasket cross igniting or (3) some kind of arc-ing in the distributor. Option 2 seems unlikely since the problem wouldn’t be limited to starting and compression readings are even. So I think I need to just buy a new electronic dizzy and hope that works - would that sounds sensible to you guys? The only other thing I’ve heard is the coils discharging at slow turnover to prevent over heating as a “safety” feature. Tom
Colthebrummie
Well-Known Member
In your original post, you said the engine was running smoothly after resetting the timing to tdc. So do you think the distributer has developed a fault since then? It could be a faulty starter, I know you said a new one was fitted but the word new means nothing these days. I haven't heard about coils discharging at a slower rate to prevent over heating.
Col
Col
thspeller
Active Member
I can’t remember when this first started but I’d changed a load of components around the time so he’s to pinpoint culprit. I’ve played around with timing and that has never fully cured it, wherever it’s set. If it’s not pre-ignition of the wrong cylinder, it’s that one cylinder is firing just before it reaches TDC on cranking and the piston is being forced back down the way it came (I.e. the wrong way). Trouble is that I’ve retarded the timing miles and it still happens.
Colthebrummie
Well-Known Member
I've noticed that you started posting about this originally last year about April. Have you since checked the valves?
Col
Col
ExMil109
Active Member
I'm not familiar with the powerspark dizzy, but that would be my first suspect. I presume it has the same mechanical advance mechanisms as a "normal" dizzy i.e. a set of weights on pivots that advance the timing as the speed increases. If this mechanism is faulty (weak spring, sticking weights) then the timing could be way off when stationary and come right once the dizzy is rotating.
Another suspect is the vac advance (if there is one?). Engine off is no vacuum, so no vac advance. Once the engine is running, the vac advance can change the timing by several degrees at idle, which might also compensate for a bad static setting.
If you still have the old dizzy, I'd be tempted to drop it back in and see if that helps (assuming the points & condenser are ok). I'm not sure how far you can check the powerspark - it may be possible to see the mechanical bits below the electronic "points" sensor, or it may be a sealed unit?
Col's point about a stuck valve is also a good one - would this show up on a cold compression test?
Another suspect is the vac advance (if there is one?). Engine off is no vacuum, so no vac advance. Once the engine is running, the vac advance can change the timing by several degrees at idle, which might also compensate for a bad static setting.
If you still have the old dizzy, I'd be tempted to drop it back in and see if that helps (assuming the points & condenser are ok). I'm not sure how far you can check the powerspark - it may be possible to see the mechanical bits below the electronic "points" sensor, or it may be a sealed unit?
Col's point about a stuck valve is also a good one - would this show up on a cold compression test?
steve2286w
Well-Known Member
did you miss this one, simple and could tell a lot
thspeller
Active Member
So - the new module arrived from powerspark and there was no improvement. Having browsed a ton of forums searching for starter kickback and electronic ignition, I found a bit of a theme. It appears that electronic ignitions need a really solid voltage to work otherwise they start playing up. If the voltage falls too far, the magnetic field in the coil can collapse and discharge a spark randomly. That would also explain why the issue is purely on starting - when the battery is under real load. Whilst the voltage of the battery appears fine, it doesn’t factor in ageing wiring, I’m using a high performance 3ohm coil and I’ve heard voltage alone is rarely an indicator of battery health. So I’ve shelled out for a powerful new battery (069) and am hoping - having replaced 10 other components - a new battery might sort it. It arrives tomorrow so will report back. The current battery is at least 3 years old (and was marginally underpowered from the factory spec) so am hoping the cause might be that simple. Will update you. Will be good to get a solution to this on a landy forum as, if it is the battery, all the answers are on motorbike forums. Tom
Colthebrummie
Well-Known Member
I haven't heard that before, I don't know how the coil can spark randomly cos it can only spark when the ignition module, or points initiates. Powerspark do recommend that you use one of their coils, but they would, wouldn't they. I have used the starting handle on mine when the battery has been pretty flat and that has not caused any issues apart from a bad back. Have you tried starting it at night as previously mentioned to see if there is any high voltage leakage to earth?
Col
Col
TheMegaMan
Well-Known Member
Not entirely accurate for electronic ignition. The power to the coil is controlled by an electronic module, and it's possible that if the supply to that isn't adequate to let the module function correctly, then it could plausibly fire the coil regardless of the actual engine/rotor arm position. It's possible a cylinder could fire significantly before TDC.
Hmmm. Given what's happening here, while I agree with what Col is saying that using the handle should remove the starter load from the battery and avoid the dip in voltage to the ignition module if this theory is correct, the consequences of the engine kicking back against the handle might break your wrist. I don't think I'd be brave enough to try that. But them I'm a wimp!
Colthebrummie
Well-Known Member
You do need to be brave or desperate to try starting the engine using the handle, and that's in normal circumstances.Not entirely accurate for electronic ignition. The power to the coil is controlled by an electronic module, and it's possible that if the supply to that isn't adequate to let the module function correctly, then it could plausibly fire the coil regardless of the actual engine/rotor arm position. It's possible a cylinder could fire significantly before TDC.
Hmmm. Given what's happening here, while I agree with what Col is saying that using the handle should remove the starter load from the battery and avoid the dip in voltage to the ignition module if this theory is correct, the consequences of the engine kicking back against the handle might break your wrist. I don't think I'd be brave enough to try that. But them I'm a wimp!
Col
thspeller
Active Member
Hi Everyone - new battery. Problem still there. However, I have discovered that the carb (Weber 34 ICH brand new) is producing a decent amount of vacuum at idle - enough to up ideal revs from c. 600rpm to 750rpm. Could be as much as 8-10 degrees. If I set timing to TDC as per the book (with vacuum disconnected) and then connect the vacuum, could it be that the vacuum is advancing the timing enough to generate the timing advance which is causing this infuriating kickback on starting?? For info, it’s a 8:1 5MB emission controlled with Weber 34 ICH and Powerspark electronic dizzy. What do you guys think and is idle vacuum advance normal and any cures? Thanks as always. Tom
ExMil109
Active Member
The vac advance range is 11-13 degrees, so what you're seeing is in the right ballpark.
Your symptoms would make sense if the static timimg was badly retarded, then once the engine is running, the advance (mechanical and vacuum) brings the timing back closer to where it should be. But your static timing is TDC!
So I'm still skeptical about the distributor. Even if the timing is good, the phasing (see http://www.mgb-stuff.org.uk/ignitiontext.htm#phasing) might be off if you had to make a big timing adjustment. It'd be worth lining the crank up on the TDC mark with no 1 cyl on compression and looking at the position of the rotor - if it's not pointing directly at no 1 HT lead then there's a problem.
Some electronic modules change the point at which the spark occurs relative to the rotation of the rotor, but normally only by a few degrees. So it's odd that you had to make a big adjustment to get back to TDC when you went electronic.
When you converted to powerspark, did you replace the whole distributor, or just put the electronic module in place of the points? If you still have the original dizzy, can you drop it back in? It would show if the issue is with the dizzy or somewhere else.
Your symptoms would make sense if the static timimg was badly retarded, then once the engine is running, the advance (mechanical and vacuum) brings the timing back closer to where it should be. But your static timing is TDC
!So I'm still skeptical about the distributor. Even if the timing is good, the phasing (see http://www.mgb-stuff.org.uk/ignitiontext.htm#phasing) might be off if you had to make a big timing adjustment. It'd be worth lining the crank up on the TDC mark with no 1 cyl on compression and looking at the position of the rotor - if it's not pointing directly at no 1 HT lead then there's a problem.
Some electronic modules change the point at which the spark occurs relative to the rotation of the rotor, but normally only by a few degrees. So it's odd that you had to make a big adjustment to get back to TDC when you went electronic.
When you converted to powerspark, did you replace the whole distributor, or just put the electronic module in place of the points? If you still have the original dizzy, can you drop it back in? It would show if the issue is with the dizzy or somewhere else.
thspeller
Active Member
Hi - that’s useful to know. Presume I’m right setting the timing at idle using a strobe WITHOUT the vacuum connected? I set it at TDC at c. 600rpm - does that sound roughly right to you?
I swapped the whole distributor when I went electric so I agree that might be the next thing to change out. I’ve swapped out the electronic module within the dizzy however and I sort of assumed the remainder was quite failsafe mechanical. Tempted to go back to points.
Setting the timing manually, the rotor does line up with cylinder one.
Its quite down on power now and popping on overrun so that tells me too retarded now. Urrgh.
Just so infuriating - having changed starter, battery, ignition, coil, leads, carb, pump... it’s becoming quite an expensive diagnostics.
I swapped the whole distributor when I went electric so I agree that might be the next thing to change out. I’ve swapped out the electronic module within the dizzy however and I sort of assumed the remainder was quite failsafe mechanical. Tempted to go back to points.
Setting the timing manually, the rotor does line up with cylinder one.
Its quite down on power now and popping on overrun so that tells me too retarded now. Urrgh.
Just so infuriating - having changed starter, battery, ignition, coil, leads, carb, pump... it’s becoming quite an expensive diagnostics.
