Hawkstone engine seizing

[kayjay283]

Hi All,

I am having some trouble with my 1961 Hawkstone Scrambler and wondered if anyone out there can help me shed some light on the problem!!!

My bike has the Vale-Onslow iron barrel type engine fitted and is constantly overheating and seizing up, even when just running on tick over - it started to seize up on the start line on Sunday!!! Below is a list of things we have changed/modified/tried:

New crank seals fitted
electronic ignition sent away and tested (all OK)
3 different carbs have been tried (2 concentrics and one monobloc)
raised cable needle to its highest setting
increased main jet size right up to 300
increased oil/petrol mix gradually from 25:1 to 16:1
replaced inlet gaskets (no leaks has been pressure tested)
someone suggested to me that it could be the exhaust but it is the same exhaust I have had on it ever since I've owned it!

ANY SUGGESTIONS???

Many Thanks

[dave higgins]

Is this a barrel that has recently been fitted?, if so, one possibility is that your piston to bore tolerances are incorrect, to tight to start with.
Insufficient piston ring gap will also cause nipping up, partial main bearing failure can do the same, need more info to say really.
Over advanced ignition timing will cause overheating, this usually ends with a hole in the crown...., a good indicator of how serious the problem is, how long do you have to wait, or how much does the temp drop before the motor will turn again?
If its nipping up, it will leave marks, & you will be able to see them. dave.

[Andy Briggs]

Hello; I wonder if the fuel/oil ratio is the culprit? I run my Scottish (iron barrel, etc) at 50:1 with no troubles, but the fact that you've reduced the ratio from 25:1 to 16:1 means that you've actually made the petrol/air ratio LEANER and this might be your problem. Just a thought.

Also worth checking: piston-bore clearance OK? (ie not too little OR too much); Con-rod not bent? etc, etc.

Cheers, Andy Briggs.

[scott151]

Hi there,

The vale onslow barrell had a special piston to go with it, have you got the right piston? I know Adrian 335 had to cut and weld a standard dimension piston to suit his VO barrell, I think the standard villiers piston is taller from the gudgeon to the piston crown. I'm sure Adrian will confirm this for you.

How does the bike run in general? Does it run well or run generally poor?

I run my hawkstones on castrol R at 20 to 1

Also what type of air filter are you running?

Scott

[johnrunnacles]

Hello Kayleigh.
I've been running-in a newly-linered Vale-Onslow converted Hawkstone amid a host of other problems (now hopefully sorted, if Sunday is anything to go by!).
I agree with what Dave and Scottie have had to say. The piston and rings must suit the bore, and the timing must be spot on. My Hawkstone runs on a 20:1 mix of Avgas to Millers MLR 40, and it seems to have run-in OK after a couple of meetings. I must admit, the exhaust I have fitted is a bit out of the ordinary - I can't recall what your's is like from memory - but it shouldn't be the cause of your problem?
As Scottie has said, Adrian is another convert to the Vale Onslow set-up. He may well be able to add to this discussion (?)
I'm sure we would be pleased to have a look at your bike at or before the next meeting, if that could be arranged? No promises, mind! These two-strokes are devilish devices, each with a mind of its own!
John R #461

[kayjay283]

Bike had a rebore & new piston. I can't recall what the tolerances were but were from Greeves specialist. The bike had a rebore because it seized up at the end of last season so I suspect that was the beginning of the current problems. This leads me to believe that the new piston/rings/rebore are probably not the cause of the problem.
Ignition has been sent away for testing & has been set up by Greeves specialist. Engine heats up & cools down relatively quickly, I would say about 10 minutes. Once cool the engine will start up and run fine.

[johnrunnacles]

It is sounding like your new re-bore just needs running in. Mine, as I have said, took a couple of meetings, admittedly fraught with other ignition problems, so probably about a dozen gentle laps over those two meetings to allow the engine to be revved and laboured a bit more on Sunday.
If the work was done by a specialist, the tolerances should be OK, I would have thought, although my parts were supplied by a well-respected specialist and I did find I needed to lift the barrel after a first few of those circuits and rub down an obvious high spot on the piston.
You say that the underlying problem may date back to last season. Did anything unusual happen to bring on that seizure? Can you recall making any modification or adjustment to the bike before that earlier seizure? You seemed to be going well enough before that.

[John Wakefield]

As John says looks like it needs running in. Us older riders will know all about that from the 'old days' when for a roadster you ran it in for about 500 miles at about 30 to 40 mph keeping the revs down. John says "If the work was done by a specialist, the tolerances should be OK" Trouble with todays engine reconditioners is that they are used to working with modern water cooled 4 stroke engines which run to much tighter tolerances. In the day when our old bikes were new the reborer always gave a little bit more clearance if the bike was going to be used for competition.
Best thing is to remove the barrel & examine the piston, it has almost certainly 'picked up' so as John says you need to rub the high spots down. If alloy from the piston has picked up on the bore this will need to be removed & it would be a good idea to have the barrel honed.
Once assembled find somewhere where you can steadily run the engine in for a few hours, gently at first keeping revs down stopping regularly to let it cool down.
This should do the trick but if it siezes again then its barrel off again & more filing. 20:1 petroil mixture should be fine, maybe increase it to 16:1 whilst running in.

[Rob]

Just to add to this lot.....

Hundreds of years ago, I borrowed a Hawkstone from Andrew King. This had had the Herman Meier treatment and a MX1 expansion chamber. It would go like the clappers for the first lap or two and then it would start making 'distressed' noises indicating overheating. I would whip in the clutch and it would promptly seize solid. Having waited by the side of the track until the traveling marshal came round, it would start OK and run as if nothing had happened. There was hardly a mark on the barrel but it did this all the time and I did everything listed here plus honed a bit of extra clearance into the barrel.

I think the problem was that the poor old iron barrel couldn't cope with the heat generated by the 200+ bhp of the tuning mods!

I finally cured the situation by handing the bike back to Andrew!

[dave higgins]

200 bhp rob!, even pinocio couldn`t get his nose that length...

[Rob]

Dave, It was a long time ago but I think that figure is correct but possibly a little more! Still came last every time though but at least the rest of the bike didn't wear out!

[johnrunnacles]

Hope you'll be pleased to learn that I have taken over that position, now, Rob
....................... on my super-tuned Hawkstone, too!!

[ADickerson]

"I finally cured the situation by handing the bike back to Andrew!"

Nice one Rob, an excellent solution. Enjoyed Daves response too, re bhp.
I can't speak for any one elses experiences but from my own limited past, I can report that I've only experienced piston seizures on two occasions. On each one it was after I'd dressed an over size piston to fit into a smaller bore, by hand, merely with wet & dry abrasive paper and a digital Vernier. After each seizure, the piston was dressed, again with wet & dry over the scored area, although not necessarily removing the score marks, and putting back together. Neither piston seized on any subsequent occasions despite my best efforts during competition. Compression ratio's were set at 10.5:1 with a .030" Squish clearance. Ring gaps were set to .012". Bu**ered if I would ever have had the patience to run in for 500+miles. That would be a seasons worth of competition
There, I given up all my trade secrets, such as they are, bar jet sizes. Unfortunately these relate to the Monobloc rather than concentric, so probably not much use, but if Kayleighs interested, I'll happily send them to her in an encrypted message.
Of course all of this is in relation to a rider whose weight is around 50kg's dripping wet.

[kayjay283]

Many thanks for the reply.
The bike seized at the last meeting of last year, no adjustments had been made to the bike before hand. It did then what its doing now - gets very hot and seizes. The piston rings in the bike last year were very worn so it couldn't be that the tolerances were too tight then!
Its a mystery!!

[John Wakefield]

Is it the piston seizing or the big end or a main bearing? If its the piston then there will be scuffing. Seems strange it was seizing on a well worn piston & bore.