gp150 misfire help needed please now updated

[grandpa]

Hi Chaps,
I have been restoring a second Vijay 150.After obtaining MOT/ registration I took it on the road for a few short runs to check the gears selected ok and check for other problems it went ok with no misfire.
The engine had worn mag bearing and worn gear selector bush so an overhaul was done with new bearings ,seals and rings etc.After the overhaul it misfired under load and after changing all the ign parts stator ht coil and carb etc I discovered I had fitted standard rings/head gasket instead of 57.8mm.To cut a long story short( The piston nibbled the standard head gasket and some bits ruined the big end) I had to fit new 200 crank correct rings with gap of 15 thou.My inner tube leak test showed no leaks from gaskets.Its a points stator it starts first kick, the plug is chocolate brown.But it still misfires under load.Today i have fitted the stator flywheel HT coil plug lead and plug from no1 scoot and it still misfires.
When I had barrel off I noticed crankcase transfer port had been welded but there was still a crack on gasket surface see arrow on pic so I had it repaired but the engineering shop appeared to have ground off excess weld resulting in a slight gap of about 15 thou when a piece of glass was placed across crankcase.We used some emery paper on the glass to make gasket surface perfectly flat.this has the effect of lowering barrel by about 15 thou

Would lowering the barrel by such a small amount cause a misfire.Squish was 1.8 with .5mm head gasket
Would honing the barrel with emery to remove scratches cause loss of compression it seems ok but how can I check compression remember it starts first kick unless its been left a week. It revs up ok in garage.
would an exhaust cause this fault remember it was ok before rebuild.I have been assisted by my neighbour as this was my first engine rebuild (Jaguar trained 50 years in the motor trade own repair garage)I know I made a mistake with wrong size rings and gasket.............Run out of ideas ...any help appreciated
sorry its a bit long winded
Thanks Grandpa

[Adam_Winstone]

Did you put a new plug in it after the rebuild? If so, is it the right heat value and did you close the gap up a bit to give it an easy life?

[IaninDorset]

Could you not use a thicker gasket to compensate for the lost height?

I don't know much of how the lambretta fuel timing cycle works and how essential having it all "in sync" to such a tiny tolerance is unusual on two strokes I would have thought, the only thing is, the weakness is perhaps only patched rather than given actual strength and could it be wilting outwards when a load is placed upon it thus changing the profile of the gasket face allowing air/fuel mix to be disturbed?

What would the thrust pressure of the crank itself exert onto the webbed area? Could that be enough on a suitably weakened section to do this?

[sunrisemac]

If you have lost 0.015" off barrel then the compression ratio will have increased, so an idea would be to try a smaller plug gap and change nothing else to see if that makes any difference.

[grandpa]

Hi Chaps,
With regard to plug I have tried 3 all b7es tried different gaps initially also tried a different stator at 17,19 21 degrees with strobe light.Admittently not tried different gaps since new crank put in the gap is 15thou as per Stickys book for Indian points.One can get tunnel vision and start going over same things. By taking the stator ,flywheel ,ht coil, ht lead and plug off my running scoot I was hoping to eliminate ignition.I am in process of refitting the ignition components back onto scoot no1. As regards Ians point about pressure on repaired area under load it was ok before I started the rebuild the engineering shop did not touch the original repair inside only the gasket face but obviously had to heat up the area so may have weakened it .Something to look at if the barrel comes off. The plug is brown which I think is a sign of correct mixture.
Adam would lowering the barrel by say 15 thou have any effect... note that I had to use .5mm head gasket to get 1.8 squish?
How can I check compression?.....would low compression cause misfire?
I will be refitting original ignition components next week as changing all did not change condition.
Thank you all for you help and I welcome any other suggestions
Grandpa

[sunrisemac]

Have you checked everything that needs earthing has a good connection?

[Adam_Winstone]

I wouldn't think that this minor change would have anything to do with a misfire. Your squish is less than it was and this is actually better than previous so that should be all good. In trying to get the most out of motors before I have raised compression ratios too high and run into all sorts of pinking, overheating and difficult kick starting but this has never caused a misfire. Such a small change will IMO have nothing to do with the misfire.

My reason for suggesting plug gap was because I wasn't sure if this was an older flywheel that might have dropped Its magnetism ( I note that you've since stated Indian points ignition). Too wide a plug gap (even the correct gap) can cause running issues with older points flywheels and a frequent bodge cure is to close the plug gap to make the job easier for the ignition.

Unfortunately, there are lots and lots of things that can cause a misfire but I doubt that a tiny increase in compression would be a factor.

Good luck resolving this.

Adam

[rogscoot]

Main jet in carb partially blocked?

[grandpa]

Hi Chaps,
Thank you for your replies,The engine is earthed,I did try a different carb initially when it misfired a new jettex with 122 main jet and also tried different plug gaps with no improvement, but not since 2nd rebuild with new crank and correct rings.I have refitted all the parts back to respective scoots.and both start.( I was reluctant to strip parts off no1 scoot but desperation made me bite the bullet) Next week I will spend some time with a different carb and try different plug gaps.I have also bought a B8ES plug to try.With regard to magnets flywheel no2 scoot seemed better than no1 scoot.I have not taken carb off no1 scoot so might try that.Scoot no1 has rusty clubman exhaust that needs painting I could put that on no2.It did run ok with its present carb, stator ,flywheel and exhaust so it must be fixable
Any other ideas appreciated
Grandpa

[lammydave]

the only misfire I ever had was from a faulty ignition switch

my guess is that's it electrical and nothing to do with the other issues you have posted

[grandpa]

Hi Chaps ,
Just an update The green wire to ign switch is not connected but thanks for reply Lammydave. .I have tried plug gaps of 10, 15 and 20 thou as suggested by Adam W.and also tried a b8es plug.I still have misfire.There is a 400 yard incline outside my house if i go up at full throttle in second it will misfire today I noticed if i change into 3rd on full throttle the revs drop and it wasn't misfiring suggesting it was rev related.I have tried different carb but not since rebuild with new crank.I have tried 3 different stators including the one off my good no1 scoot so dont think its points bounce
Could it be exhaust ? It did not misfire before rebuild but I only took it out on short runs after registration.
Any other ideas anyone
Thankyou Grandpa

[Wack]

Just because you've fitted a new crank it doesn't mean its true so have a look when the scoot is running how the flywheel looks, then check for any movement when motor is turned off. Also check the points black wire terminal isn't touching the flywheel etc.

[sunrisemac]

Most high speed misfires I have had to sort out seem to be more commonly caused by HT components or connections

[grandpa]

Hi,
thanks for replies sunrise and wack. I would have thought it was ignition but have fitted stator flywheel ht coil ht lead and plug from good running scoot.also tried stator kindly sent to me by mick york a few months ago. Have not noticed flywheel running out of true but will look closer.One starts to get tunnel vision so all ideas welcomed.Starts first kick plug is brown, no white smoke.Possibly next week ,I need to paint exhaust from no1 scoot so might try that .Now getting desperate
Grandpa

[sunrisemac]

Have you checked the actual wires to the coil and the terminals on the end of the wires, they can appear ok but might be a bit loose and cause a bad contact under load and give a reduced spark. Sometimes wires can break inside the insulation but appear to be good. Check any terminals that were disturbed when the overhaul was done.

[grandpa]

Hi Chaps,
Just an update .Tried downpipe and clubman from scoot no1 no difference.Re Sunrisemac about checking wires from coil when I changed coil I took coil with its wire and earth braid from good no1 scoot.
Bought compression tester. Scoot no1 ( good scoot ) compression on kickover 95 pushing in second gear 100. Compression on no2 (bad) scoot is kickover105 pushing in second 110.with no carb attached.I did not open throttle when testing no1 scoot which apparently I should have.
Engine is now on bench no apparent leaks from my innertube leak test but not left overnight.
Questions hopefully someone can answer
1.what should compression be (gp150)
2.Would it run with piston in wrong way? There was no arrow but I did centre punch top of piston before dismantling.(Misfiring under load after overhaul)
Don't want to dismantle engine just yet until I have spoken to my neighbour (motor engineer) and another contact with Lambretta experience
All help appreciated
Grandpa

[Scooterlam]

Here's another thought,
I had a taffy copy pipe fitted at one time and every time it went onto reserve it would backfire, even before I could feel it .
It was great as I never got to that worrying point when the engine runs super lean.
Changed the pipe and lost the backfire with out changing anything else.

So in my own experiences in certain situations fuel starve can cause backfires.
I only mention this as you say yours happens under load which is when demand for fuel is at its highest.

Very unliklry to be your Isue but a flow test takes minutes and costs nothing.

[grandpa]

hi chaps
Thanks Scooterlam,I should have mentioned fuel flow is half pint a minute also tried different fuel tap ,carb off scoot no 1 and a new jetex.Just checked my innertube leak test .it has not gone down in the last 4 hours.This engine did not misfire prior to overhaul.Will take head and barrel off possibly tomorrow. I will fix it somehow
what should the compression reading be on gp150
would an engine run if piston in wrong way round?
Grandpa

[sunrisemac]

Hi Grandpa,
I can't quote a compression figure for gp150 but if you can get a figure with a compression gauge after a decent kick and let me know the result I will do a similar test on my gp150 on Monday and let you know the result, mine starts easily and runs with no problems.

As far as the piston being in back to front, you would probably find it detrimental to the running of the engine, as if I remember correctly, the port cutouts and gudgeon pin are not symmetrical in the piston and must line up correctly with the inlet and exhaust ports in the barrel, I think there is a mark on the piston crown that needs to be lined up with the exhaust port.

[grandpa]

Hi sunrisemac
See thread Fri Sep 19 3;23pm compression kickover 105 pushing in second 110. my good scoot 95 and pushing in second 100psi
Thanks for your ideas
Grandpa