Heat seize and snapped stinger

[Simon ELC]

Had a classic four point heat seize on the way up to Lincoln the other week.

About 40 miles after filling up on a motor that had done 4000 trouble free miles including Davos and C2C.

It happened just after overtaking a lorry(wagon) doing about 70 decelerating to about 60. Loss of power, clutch in, hard shoulder, locked solid.

Mate towed me along the hard shoulder using bungee straps for 4 miles until we reached a service station.

Strip down required. Pipe removed first. But as I remove the end can bracket nut the can comes away in my hand, stinger snapped where it joins the main body.

Top end stripped and freed and rebuilt, but with what looked like superficial damage. Broken pipe held together with spare carb rubber and Jubilee clips.

Carb up jetted half a clip and off we go. Do the last 60-70 miles over rich and get there OK.

Engine spec:
TS1 230 (Darrell Taylor Touring Tune(more like a fast road tune))
Ron Moss Head
VHSH with power jet
Augusto 7000
AF NK Road pipe

On the Saturday at Lincoln I had a chat with the dyno operator and he agreed to do several runs at all throttle positions with the Lamda sensor on. No lean spots to worry about.

Rode home on Sunday with no problems (apart from rain and the voices in my head).

Stripped the motor down and realised the amount of damage caused. Piston scored deep, bottom ring seized, four areas of nikasil missing from barrel. And it still made nearly 24 BHP on the dyno.

There were no obvious signs of a top end air leak or lean jetting and no pinking. And I'd done the same overtake manoeuvre 100 times.

So, if you are still reading, could the broken pipe have caused the seize?

I don't think so. Do I have to put it down to one of those things?

[Scooterslag]

Strange, maybe a shit tank of fuel? did you fill up just before your nip up?

[Simon ELC]

Strange, maybe a shit tank of fuel? did you fill up just before your nip up?

No 40 miles down the road.

[MickYork]

When you say "no sign of air leak" how would you know ? Did you air test before strip down ?

saying that I would have thought an air leak would more likely hole your piston.

Looks like rolling off the throttle may have caused a fuel shortage and as you say a classic heat seize. might be worth checking the timing and see what it had retarded to as you were doing 70. Unless you were doing 7000 revs the Augusto won't have fully retarded so depending on gearing you may have been running slightly advanced.

Just some thoughts, it's always a problem pinning down the cause with so many variables.

[Adam_Winstone]

I ran into fuel starvation recently, which resulted in me nipping up when shutting down from a starvation/weak high speed distance run, when running an unmodified VHSH 30. This immediately reminded me of the issues that I'd previously had with VHSA/VHSB unmodified body, which I'd overlooked with the new VHSH. I then 'blueprinted' the carb, a service that MB used to offer and is covered on a number of websites...

["A word of warning! We have found that all flat slide Dellortos have a fault from the factory and require what we call a blue print modification. None converted carbs will get fuel starvation below 3/4 of a tank, (mainly on fast high revving engines)." http://www.lambrettaspares.com/info/set ... -+151.html]

... and I've not suffered fuel starvation since. Lloydy's Lambretta site used to take you through the mod's in photo 'step by step' but I can't upload that today (still uploaded?). I'd check your carb (easy to do if you can find photos on the web) and consider getting it modified or doing it yourself if you have a pillar drill.

Adam

Edit: Fuel would be the second thing I'd consider.

[Simon ELC]

Thanks for the replies fellas.

I've rebuilt the motor with a spare TS1 I had on the shelf which I going to try with Franspeed Supertourer.

I'll check the timing tomorrow once its up and running.

Adam, the blueprint mods you mentioned MB do are listed on the Scooterhelp site. I checked my carb over and found that the carb's little filter was still in place and had twisted and collapsed slightly. I have always discarded these on VHSB carbs in the past and so have now done so on this one. The float valve fitted is a 300 which I think should be big enough. I just need to drill out the restriction now.

[Simon ELC]

After rebuilding the motor I had some intermittent spark problems. Swapped plug, cap, lead, CDI, but still had an on off spark. Put a multimeter across the stator and all checked out ok. Thought I'd fixed the problem until I tried to strobe the timing.

It would barely retard more than 2 degrees and wouldn't rev out and then it cut out again. Turned the flywheel by hand to feel it binding. Took the flywheel off to find the stator had cracked



I think this may have been the cause of the seize as it was not retarding properly and running advanced.

The stator is less than a thousand miles old and is the third one I have had fail. All for different reasons. The last one all the rivets came loose. I won't be buying any more of this rubbish.

[Captain Pugwash]

Check the stator against the maghousing, the centre part of the mag housing is often a closer fit to the stator than some realise, yours looks like it's bottomed out then as you've tightened the stator bolts it's stressed the alloy of the back plate to the point that it's snapped.
It sort of depends on the BGM's dimensions.

That's my best guess.

[Simon ELC]

Thanks Captain. That's something I'll have to check.