mystery air gap

Ken Graham · Mar 9, 2022

Hi everyone
I have been happily running my 850mk2a with Tri Spark ignition for about six years, on a run in 2021 it oddly started to backfire and I have checked all the usual simple things like (plug gaps - timing - carbs - air leaks ) today I had a look at the ignition stator and rotor and the "air-gap" is way too large ( like 5mm instead of 2mm)
is it possible and has anyone experienced the camshaft recessing itself into the crankcases ? what possible issues am I looking at.
Thank you all in advance, looks like I could be heading for splitting the cases.
Cheers
Ken.

baz · Mar 9, 2022

Ken Graham said:
Hi everyone
I have been happily running my 850mk2a with Tri Spark ignition for about six years, on a run in 2021 it oddly started to backfire and I have checked all the usual simple things like (plug gaps - timing - carbs - air leaks ) today I had a look at the ignition stator and rotor and the "air-gap" is way too large ( like 5mm instead of 2mm)
is it possible and has anyone experienced the camshaft recessing itself into the crankcases ? what possible issues am I looking at.
Thank you all in advance, looks like I could be heading for splitting the cases.
Cheers
Ken.

Long time since I had one apart but I seem to remember the conical shaped thrust washer (if fitted?) Can be fitted backwards giving this amount of play on the camshaft
A mate had a MK3 commando and I couldn't get the Boyer that he'd bought for it to spark because the rotor was too far away from the pickups

Marcelx · Mar 9, 2022

Ken Graham said:
Hi everyone
I have been happily running my 850mk2a with Tri Spark ignition for about six years, on a run in 2021 it oddly started to backfire and I have checked all the usual simple things like (plug gaps - timing - carbs - air leaks ) today I had a look at the ignition stator and rotor and the "air-gap" is way too large ( like 5mm instead of 2mm)
is it possible and has anyone experienced the camshaft recessing itself into the crankcases ? what possible issues am I looking at.
Thank you all in advance, looks like I could be heading for splitting the cases.
Cheers
Ken.

Is your Trispark the first model without the test-button? This Version is liable to fail after a few years. In my case it started also with some backfiring, if the the engine was cooled down it was OK again, but it went worse by the time. After I replaced the Trisapark with the improved version, I do not have problems anymore, ( I got a spare one from Trispark in exchange with the faulty one).

Ian James · Mar 10, 2022

baz said:
Long time since I had one apart but I seem to remember the conical shaped thrust washer (if fitted?) Can be fitted backwards giving this amount of play on the camshaft
A mate had a MK3 commando and I couldn't get the Boyer that he'd bought for it to spark because the rotor was too far away from the pickups

Ian James · Mar 10, 2022

Do you mean the chamfered spacer ? I think it was so made to alow the tacho drive gear room to reach the worm on the cam and if fitted reversed would not . Then you have to undo every thing. As for changing the position of the cam nada ?

baz · Mar 10, 2022

Ian James said:
Do you mean the chamfered spacer ? I think it was so made to alow the tacho drive gear room to reach the worm on the cam and if fitted reversed would not . Then you have to undo every thing. As for changing the position of the cam nada ?

Really can't remember what the setup of the camshaft / thrust washer is to comment
It's been a long time since I had one apart
I know my mates MK3 camshaft was too far recessed for the pickups to get a signal
We had to bodge it up with a spacer
Many many moons ago

Ken Graham · Mar 10, 2022

Ian James said:
Do you mean the chamfered spacer ? I think it was so made to alow the tacho drive gear room to reach the worm on the cam and if fitted reversed would not . Then you have to undo every thing. As for changing the position of the cam nada ?

Thanks Ian

That makes sense about the tacho drive and the tacho is still working which would infer as you say the camshaft hasn't and won't move. Guess I need to delve deeper as to why the gap is so large.

Cheers

Ken.

Ken Graham · Mar 10, 2022

baz said:
Really can't remember what the setup of the camshaft / thrust washer is to comment
It's been a long time since I had one apart
I know my mates MK3 camshaft was too far recessed for the pickups to get a signal
We had to bodge it up with a spacer
Many many moons ago

Think I could be looking at something similar Baz, I have a spare casing so could mean machining a couple of mm of the plate mounting surface.

baz · Mar 10, 2022

Ken Graham said:
Think I could be looking at something similar Baz, I have a spare casing so could mean machining a couple of mm of the plate mounting surface.

Yes but your pickup was working and now it's not
Something has changed
I know the cam followers will be pushing down on your cam but can you move the cam in and out at all?
With my mates MK3 it was a none runner
He bought the Boyer for it but couldn't get a spark
I ran a magnet past it and got a spark,then it dawned on me the magnets were to far away from the pickup coils
And this was a brand new Boyer
That's when I put a spacer behind it
Cheers

Ken Graham · Mar 10, 2022

Need to look further into the cam position, I will take the tacho drive out and see if that allows any significant end play. what your saying is what I was think in that the magnets are now too far away from the stator plate . fingers crossed.

marshg246 · Mar 10, 2022

Ken Graham said:
Hi everyone
I have been happily running my 850mk2a with Tri Spark ignition for about six years, on a run in 2021 it oddly started to backfire and I have checked all the usual simple things like (plug gaps - timing - carbs - air leaks ) today I had a look at the ignition stator and rotor and the "air-gap" is way too large ( like 5mm instead of 2mm)
is it possible and has anyone experienced the camshaft recessing itself into the crankcases ? what possible issues am I looking at.
Thank you all in advance, looks like I could be heading for splitting the cases.
Cheers
Ken.

The Tri-Spark instructions are a little confusing on this. The basic instruction it to be sure you have at least 2mm air gap but then it's says 2mm +/- 0.6mm so -1.4 to 2.6mm. In the specifications page is says 2mm +/- 0.6mm so again 1.6 to 2.6mm. If you were in that range when installed, I can't imagine how you have 5mm now unless the camshaft sprocket is digging into the case - I would have a good look at that before splitting the cases.

When I install them, before installing I screw in a bolt, turn the engine while pulling on the bolt to make sure the camshaft is all the way towards the timing side and then check that there is an air gap - I've never actually measured or worried about the max size of the air gap.

I can see a too large air gap causing missing, or maybe even exhaust bangs, but not backfires.

mystery air gap

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