Starting when hot.

[phi010]

I have a 1927 600cc Super. I’ve had it for 10 years now and I am getting fed up with poor starting, which has been pretty much the same since I first got it. When cold it will always start, but it does always require 10 to 12 kicks.
When running it is absolutely fine.
However when hot it is a real pain. I’ve taken it to a nearby hill & tried all sorts of combinations of throttle, air-slide, stopping by stalling, choking with hand & magneto cut-out. But nothing predictably helps.
It is fitted with a reconditioned Amal (with the 4 hole round base), but I have also tried a big 1960’s Villiers carb…no difference.
The bottom end was carefully rebuilt when the big ends were done a few years ago. I also re-ringed it at the same time, I didn’t think the bore was badly worn by 4 stroke standards. The BTH mag was also rebuilt. Machined A6 plugs.
Apart from a couple of Lofty Avis articles Technicalities steers clear of starting problems, so…
CAN ANYONE HELP… PLEASE?
Paul Hirons

[richard tann]

Paul

You are not overwhelmed with views on poor starting. I have been eagerly been awaiting expert pronouncements on this matter.

I have two 2speed Scotts about the same age as yours. One starts easily, whatever the circumstances. The other doesn’t. Like you, I’ve been struggling for years with it. In desperation, I’ve even fitted a Mikuni carb. To my embarrassment, it features on the Club calendar, September 2005, complete with bath plug and chain !

I am not an expert, this is a completely uninformed theory.

Do you think that a lack of primary compression might be our problem ? It would fit in with the poor starting, but good running scenario….once it’s going, it’s OK. When mine will not start, I take the plugs out, splash a drop of petrol down the plug holes, kick it over a few times, plugs back, it goes first kick. It works every time. It makes me wonder if the correct charge is getting into the combustion chamber.

How you quantify or measure crankcase compression is beyond me. There seem to be plenty of places a leak could occur. How do you fix it ? Roger Moss has kindly offered to look at my engine when he gets time. I’ll let you know how we get on.

Kind regards

Richard Tann
tannandsons@aol.com

[Shaun Matthews]

Guys – I also have the same problem but mine ‘developed’ – used to start 3rd kick when warm every time, one day she woke up and turned into a bitch and has stayed that way ever since, carbs have been changed ( I’ve tried a Mikuni VM 30 as well as another Amal) – no improvement, plugs (A6’s as well as modern iridium jobs), door seals, transfer cover seals – you name it, I’ve tried it – One theory is a slightly weakening mag – the other is she has succum to my 15 year old daughters fog of teenage hormones and come out in sympathy – whatever when she goes she pulls ( actually pulled until I broke a crank) like a steam train,

[Roger Moss]

Clean out wells
Drain oil tank and wash out with petrol
Fill with Castor R40 Castrol R 40 is what I use
Pilgrim works better
Thicker oil film on bores stays after engine stops for better hot start sealing. Engine lasts longer and is more smooth.
And it smalls great.
If you leave the bike stored for more than a month, take off doors and mop all oil from wells before starting
If you get stuck for supplies on long journey, go into Boots and buy a big bottle of their pharmacutical grade castor oil. Elf used to make an R40 Castor racing oil called White Ideal Racing. They bought it from Boots!
Try it! If it works for you, write your experiences for us.
Kind Regards
Roger

[BRIAN MARSHALL]

Hi Paul. Do make sure that you only use petrol that is a bit better than ordinary unleaded, which seems to me to be getting worse by the month. I recommend Shell Optimax, which seems to make a surprising difference to two strokes in general. Also make sure that you drain the float chamber by running it dry if the bike isn’t going to be used for a while or you will have a float chamber full of something from which the vital “aromatics”, or lower flash point hydrocarbons have evaporated. You could also try as an experiment a tin of “Easy Start”. If the bike starts easily after a Squirt of that it suggests that the low speed suck of the engine is poor, and piston ring gaps would be my first point of inspection. Regards to Ian. Cheers Brian.

[richard tann]

Hello

Thanks for your help on this topic. With the greatest respect, the suggestions are “tweeks”, as opposed to getting to the root cause of the difficulty. My other bike starts easily, even after months of neglect. Old fuel, old magneto, a couple of kicks and it goes. Living in an urban area, I like using synthetic oil, as not many of my neighbours are enthusiasts.

For the beginner, “The Book of The Scott” is worth a look on this topic. Last night, armed with said book, enthusiasm generated from the forum, and a gallon of Homebase paraffin I set about conducting the crankcase test. I got paraffin everywhere, in my left shoe, all over the patio. I got a look of incredulity from Lynda that only a wife of 30 years standing can muster. However, it is a worthwhile exercise. In my case, a steady drip from the bottom on the flywheel, slight weep from the doors, and a gush from the joint between the exhaust pipe and the barrel confirms my suspicions. I need expert help.

For those of us so blessed, has anyone considered the effect that faulty Half Compression Valves would have on starting ?

Anyway, no more views from me as I am now on jankers, with a bucket of hot soapy water and a scrubbing brush !

Richard Tann

[Stan Thomas]

Hi Roger,

How good is your magneto – and does it produce a spark at the plugs under compression?

Have you got suppressors fitted (which should not be so with a magneto). Also, I trust you have proper copper cored H.T. leads – not modern carbon trace leads.

Other than that, Roger could be on the right track saying the compression suffers when hot – but if you have tried “hill starts” and the like, the engine speed at cranking would be high enough to overcome any dimensional alterations which might otherwise affect starting.

Are you sure your bikes not been sabatarged to stop you going out so you can spend more time on producing “Yowl”?

Stan Thomas.

[Stan Thomas]

Further thoughts regarding your poor starting when hot.

If it starts O.K. cold, it probably is not “old fuel” symptoms which can affect starting, given modern fuels tend to “go off”.

How well does it start when cold???

If it does not need much flooding or choke, it could be the fuel level is too high. O.K. to start from cold, but this will give too rich a mixture when starting hot.

Check the float and needle, and try turning the carb on the stud ( I think you can do this on your bike) to “lower” the fuel level.

Just a thought.

Stan Thomas.

[Stan Thomas]

Further thoughts regarding your poor starting when hot.

If it starts O.K. cold, it probably is not “old fuel” symptoms which can affect starting, given modern fuels tend to “go off”.

How well does it start when cold???

If it does not need much flooding or choke, it could be the fuel level is too high. O.K. to start from cold, but this will give too rich a mixture when starting hot.

Check the float and needle, and try turning the carb on the stud ( I think you can do this on your bike) to “lower” the fuel level.

Just a thought.

Stan Thomas.

[John Farrar]

One of the more obscure reasons for poor starting when hot ,is the position of the carb and the length of the bellmouth. In my experience you must not use a bellmouth of any appreciable length. As an example a std Amal TT bellmouth is too long and gives rise to a rich mixture flatspot just off idle(it cannot be cured by changing the slide). I believe that the extra inlet length on the atmospheric side of the carb means that the inlet pulse wave is reflected across the primary “jet” (for want of a better word) several times , picking up extra fuel each time. This makes the mixture too rich for hot starting. If you have a std amal carb , at the std distance from the crankcase then this is not applicable.
When I corrected the bellmouth length, I was rewarded by much better hot starting.
The other point that has been mentioned is fuel height in the float chamber. This is very critical, and if the bike is leaning the wrong way when parked . fuel will bleed into the motor(even with the tap off) sufficient to give poor hot starting. As alredy ststed by others, in order to check this , it’s worth draining the carb, preferably by running it dry, before trying a hot start.
Stan’s point about the mag is well made, but I believe the comments about “modern” petrol are more to do with the fact that a poor ignition system will not ignite petrol that has lost some of its “volatiles”.
I have used modern petrol that is 18 months old in my old bikes (with good mags) that has been a nightmare with modern machines. Where I have been able to drain the float chambers on the trouble bikes, they start fine – I think that some of the volatiles “boil” off in the float chambers due to engine heat/natural evaporation, leaving a fuel which gives the ignition system a tough job.
Incidently I have a lawn mower which starts promptly with petrol that has been left in it rom the previos summer – it has a cheap and powerful CDI system which produces a mammoth spark!

[Roger Moss]

Hi John. So nice to have your input. For newcomers to Scotts,– John is well worth listening to!
All interesting points, but so often you can have an unappreciated build up of wear that can cause such problems. Generally if you do the same thing, then a mechanical system in the same state will give the same result. The question I would ask, is, was it always like this, or is this a problem that has developed? If it has developed, then how quickly has it done so. Problems due to wear develop slowly, so if the decline was fairly sudden, then it isn’t that. You must think carefully if you have changed anything that could co incide with the onset of your problems.
If the carburation was ok and you have changed nothing, then I suggest it might not be the problem. In the general cause of elimination, however, by all means dismantle it and clean it out and do not forget the float chamber. When you finally get it to fire, after it has beenn reluctant when hot, does it blow lots of smoke from the exhaust? You do not, by any chance, have a cylinder wall oiling system in working condition I hope. The valve system under the tank is prone to seep oil so that when you stop, if you have left the oil on, it gets choked with oil. I remember getting the “red staggers” pushing a bike in this condition!
If you come home and the bike is hot, try draining the wells and see how much is in there. Put the doors back and try starting it with empty wells and see if it improves it. The most likely culprit is the mag losing voltage when it gets hot. As the voltage required to fire is relative to the plug gap, reduce the plug gap to about ten thou and try again. Does it make any difference? The bit about the oil, was valid for wear related problems, and incidentally, Castor R40 is the most excellent gear box oil you can use, but do not use it as a substitute for ensuring the outrigger can not move under low gear acceleration. Correctly done, the high gear bush should last for decades, but strip a few boxes and look at the wear, and consider how much power has been dissipated in this unwelcome friction. Such a simple looking engine. how deceptive!

[phi010]

Thank you everybody for your input.
After behaving quite well this year, it disgraced itself at Banbury. I had to have a push start. After 15 miles it went down to one cylinder… and when I stopped to clean the plug it just would not go again. I’m fairly sure that that was magneto trouble in the heat. Of course, next day there was a great spark, but I’ll have to have the mag rebuilt again. My car experience tells me that when manetos are good, they are great; but when they are bad they are horrid! What appears to be a great spark outside the cylinder just won’t work under compression.
I think the problem may be also one of bore wear, maybe some castings which are slightly distorted over the years, & the cumulative effect leads to the problems.
I’ll keep you all posted, IF there is something positive to report!

[Peter Scott]

There is even a “Limerick” on this topic…

Some leads are a real voltage dropper
and high tension much prefers copper
Those silicon leads
won’t fill your Scotts needs
They get hot, and the sparks come a cropper !

Good Luck
Peter Scott

[Stan Thomas]

So we have degenerated to toilet-wall poems have we?

Stan Thomas.

[Roger Moss]

I think Stan is Jealous that he did not think of a suitable poem first.
If Stan is in the right mood, he is the most entertaining company you could wish for
but
You sometimes have to keep strong, as he has a humour that is a bit sharp for the overly proud!
I long ago found that objectivity was more use!

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