High speed misfire

[dickie]

Ok. I've just done the same test using an old fluke 23 which is not true rms, but form factor corrected (a sort of average that cheaper meters use). I got pretty much the same result.

Started at 115v but by the time I'd got it up to around 6000rpm (ear rev counter), it had dropped slightly to about 108v.

Seems like that's the problem. I've ordered a new lt coil.

[Fast n Furious]

Ok Dickie..... Defo sounds like the LT coil. Unless the magnets are very weak which I think can be ruled out.
For peace of mind, try the same test on a mates steed. See what you get.

[dickie]

Ok. I've done the following:

Replaced stator.
Replaced plug
Replaced plug cap
Replaced ht lead
Replaced cdi
Removed green wire
Flushed and refilled tank
Flow test = 600ml per minute
Run without filter

Always does the same thing.......
Takes off great and if I hold onto 3rd long enough it will splutter and eventually fail. If I change up to 4th it will happen earlier. If I ride conservatively it will last longer; maybe all day but I haven't tried very far. The first faulter makes me nurse it then I usually fail and have to pull over. Then it will start easily but ONLY on the choke.

I'm at my wits end.

This was set up on Darrel Taylor's dyno. Since I got it home the only change has been a new exhaust stub (as id borrowed one from him so had to return it).

[bolzenanker]

I feel your pain having had a similar problem recently. Mine may be totally unrelated to yours but I thought I'd share anyway.

Accelerate hard on my Avanti 238 and it would go fine, 1st, 2nd, 3rd and then get to 4th and it would start to misfire and slow down. Was convinced it was fuel starvation related but did all the swapping of electrical bits anyway just to check. The problem remained. So started on fuel system, changed the cap on the tank, changed the fuel tap, checked the internal filter and all jets on the carb, change the carb for a new one, problem still remained.

Then I swapped the carb from my Series 2 with similar set up onto this engine and the problem was gone. Stripped both carbs to see what the difference was and found the float valve in the carb that worked was a 350 (PHBH) and the carb that didn't work was a 300. Such a small difference but this has solved my problem, put a 350 float valve in the new carb and all is good.

It may help, it may not.

Warren

[dickie]

I feel your pain having had a similar problem recently. Mine may be totally unrelated to yours but I thought I'd share anyway.

Accelerate hard on my Avanti 238 and it would go fine, 1st, 2nd, 3rd and then get to 4th and it would start to misfire and slow down. Was convinced it was fuel starvation related but did all the swapping of electrical bits anyway just to check. The problem remained. So started on fuel system, changed the cap on the tank, changed the fuel tap, checked the internal filter and all jets on the carb, change the carb for a new one, problem still remained.

Then I swapped the carb from my Series 2 with similar set up onto this engine and the problem was gone. Stripped both carbs to see what the difference was and found the float valve in the carb that worked was a 350 (PHBH) and the carb that didn't work was a 300. Such a small difference but this has solved my problem, put a 350 float valve in the new carb and all is good.

It may help, it may not.

Warren

Thanks, unfortunately, I've already swapped the valve from 300 to 400. I should have put it in the list above but ive got to the point where so much has happened that I'm beginning to lose track (as well as the will to live)

[foremanbob]

May be of interest...

My scoot would start coughing, backfiring etc when i was pushing on hard... back off and all was OK...

Issue was fuel tap... causing a bit of fuel starvation... only way I was able to discover that was when, despite having a full tank of fuel, i switched to reserve and the symptoms disappeared...

Changed the fuel tap and now all good...

[dickie]

May be of interest...

My scoot would start coughing, backfiring etc when i was pushing on hard... back off and all was OK...

Issue was fuel tap... causing a bit of fuel starvation... only way I was able to discover that was when, despite having a full tank of fuel, i switched to reserve and the symptoms disappeared...

Changed the fuel tap and now all good...

Both you and bolzenanker are describing the same symptoms that i have. And it feels fuely (if you know what i mean). However, I've got a 400 float valve and 600ml per minute of flow from the tank and tap with the fuel cap in place.

Ive never heard of anyone adjusting the float height on a lambretta and this puzzles me; it's critical on proper bikes. It doesn't matter how big the valve is or how good the flow can be, if the valve is open too little of the time then not enough fuel will get through. So I'm going to bend the tang and see what happens.

[Adam_Winstone]

^... lots of people adjust float height on Lambrettas to address fuel starvation or flooding. I'm surprised that you've not found threads on it.

Adam

[dickie]

I reckon float height is the problem. The valve barely opens due to hitting the thingummybob (baffle?). Can I just leave it out?

[dickie]

^... lots of people adjust float height on Lambrettas to address fuel starvation or flooding. I'm surprised that you've not found threads on it.

Adam

I did a quick search and the last member to do this was....me! Age is doing its thing i guess. Anyway, it appears to me that the float height is correct, so that it closes correctly, but can't fully open due to hitting the thingummybob.

[Warkton Tornado No.1]

I reckon float height is the problem. The valve barely opens due to hitting the thingummybob (baffle?). Can I just leave it out?

I still need to work out how to use dropbox properly too.

I think your "thingummybob" (baffle) is upside down. The purpose it serves is to create a 'well' of fuel (admittedly an upside down well) which is why it has those little holes to ensure no air entrapment.

The other comments that I would make is that I'm not convinced that a fuel valve that is massively oversize (400) is actually a 'help' as I think that there needs to be 'finer' control. Definitely the number needs to be greater than the largest main jet you are likely to use, but consistency of the float level might be better with lots of small sips that fewer gulps, if you get my drift?

Sorry to harp on about racing, but I think the highest I heard of for (probably) the most powerful Group Four engine was a 350 in a Ø39 mm VHBC running flat out on the long straight @ Cadwell Park, so a 400 on a road machine sounds like overkill to me.

Whilst I'm throwing my hat in the ring, I hope you can get this problem sorted, but if it's not the carburation, "misfires" tend to be electrical in my experience & I think that you could do worse than swap out, temporarily, the complete ignition for any other known system from the aspect of elimination.

[dickie]

I think your "thingummybob" (baffle) is upside down. The purpose it serves is to create a 'well' of fuel (admittedly an upside down well) which is why it has those little holes to ensure no air entrapment.

Thanksfor that. I thought that was its purpose but wasn't sure. I trird it upside down and it mafe no difference. I can puah it on as far as i like as there is no hard stop; mayne mine is missing? Also ive been advised elsewhere to leave it off altogether.

The other comments that I would make is that I'm not convinced that a fuel valve that is massively oversize (400) is actually a 'help' as I think that there needs to be 'finer' control. Definitely the number needs to be greater than the largest main jet you are likely to use, but consistency of the float level might be better with lots of small sips that fewer gulps, if you get my drift?

yes, i see your point but i was beimg a little desperate. I'll put my 300 back in after I've proven this latest theory beyond doubt.

Sorry to harp on about racing, but I think the highest I heard of for (probably) the most powerful Group Four engine was a 350 in a Ø39 mm VHBC running flat out on the long straight @ Cadwell Park, so a 400 on a road machine sounds like overkill to me.

If that's your source of experience feel free. Its generally a good source in my experience

Whilst I'm throwing my hat in the ring, I hope you can get this problem sorted, but if it's not the carburation, "misfires" tend to be electrical in my experience & I think that you could do worse than swap out, temporarily, the complete ignition for any other known system from the aspect of elimination.

I already have done albeit a piece at a time. "Misfire" is a bad description to be honest, but i couldn't ithink of a better one.

Thanks for your advice

[Warkton Tornado No.1]

Warkton Tornado No.1 wrote:
Whilst I'm throwing my hat in the ring, I hope you can get this problem sorted, but if it's not the carburation, "misfires" tend to be electrical in my experience & I think that you could do worse than swap out, temporarily, the complete ignition for any other known system from the aspect of elimination.

I already have done albeit a piece at a time. "Misfire" is a bad description to be honest, but i couldn't ithink of a better one.

Thanks for your advice

You're welcome, but I would replace the whole of the ignition (including the flywheel) with anything that functioned, just to prove the particular area of the fault.

It wouldn't be the first time that any of us have overlooked the obvious, & as the carburettor has , no doubt, been on & off more times than a whore's drawers, is the needle retaining plate (another "thingummybob")above the needle circlip as it should be?

[dickie]

Sorted, thank God. Or more accurately lcgb forum members and Chris Sturgess.

Goes like stink. Great feeling but a little unnerving with a low back tyre.

I don't know how fast it goes mister but it used to do 67mph gps and it's much quicker now. But the main thing is how quickly it gets there. Cars were holding me up for a change.

Thanks gents.

[MickYork]

and the problem was .......................?

[dickie]

and the problem was .......................?

Oh sorry.

The "anti surge washer" was upside down. That caused it to stop the float from dropping fully which in turn stopped the float valve from fully opening.

So unsurprisingly it all makes sense in retrospect: Ignition is independent of road speed but not engine speed; fuel demand is proportional to road speed and/or acceleration. But aren't we all clever in retrospect?

[dickie]

The electrics is your first point of call but.......(there's always a but) I wouldn't rule out a fuel issue.

My mates TV regularly back fires when he goes on reserve......and, I ran out of petrol at the weekend which preceded a couple of backfires then the engine died.

Maybe check you flow rate and give the carb a once over.

Ooh look. Clever lad.