The BSA A7-A10 Forum

Technical => A7 & A10 Engine => Topic started by: huddie on 17.08. 2008 18:44

Title: Hot start difficulties
Post by: huddie on 17.08. 2008 18:44
Hi Everybody, Afraid its me again. Having got my clutch sorted (well good enough for now), I set of to give a few miles to get it really warm to see if that threw up anything. I got back ok, nothing amiss, just a bit of smoking around the rocker box gasket at the front( i'll soon fix that). Checked the plugs, they looked pretty good so I thought i would start it while it was "HOT". She wasn't having any of that! Tried tickling the carb, Tried air slide down, air slide up, checked I had a spark, I did, She just would not start. Went out for a few hours with the Missus, came back, by then she was cold, First prod,  away she went, which is normal. Obviously I can't go out far with a bike that wont start hot, so where should I start guys.
What help can i give you, well as I said I have a spark which should be no suprise as i had the magneto profesionally rebuilt, (how can i check that the spark is strong enough to survive under compression). I can tickle the carb and it floods ok but she just will not start. I'm reluctant to put ideas in your heads but could compression be an issue? She has std pistons in bores which have no ridges at the top and no visible wear marks in the cylinders but compression cold is only around 100 psi.
Hope you can give me a check list or something to work through.  Regards Huddie
Title: Re: Hot start difficulties
Post by: a10gf on 17.08. 2008 19:05
Well, it may be very simple, the trick for getting it going when hot may be very different from when cold. First of all, after some starting atempts, are your plugs wet or dry? Smell of petrol or not at the silencers?
Title: Re: Hot start difficulties
Post by: LJ. on 17.08. 2008 20:49
What plugs are you using ???? I had found in the past that the A10 is quite fussy with what plugs you run. I get on very well with NGK BP6HS. Try using a Gunsons Colour Plug to see if there is a spark under compression.
Title: Re: Hot start difficulties
Post by: fido on 17.08. 2008 22:36
Mine has recently developed a requirement for large throttle openings when starting from hot. When cold you only need a tiny bit of throttle but  when hot it likes about half throttle. Just a thought.....
Title: Re: Hot start difficulties
Post by: huddie on 18.08. 2008 08:33
Thanks for your help so far. For starters I have just tried starting her again this morning, and she fired up first prod of the kickstart. Yesterday when it failed to start hot I took the plugs out, they were not wet. I didn't try smelling the exhaust so i will try that next time. Plugs are new Champion L82C with a gap of .45mm (book says 18 thou but I dont have an imperial feeler gauge these days). As for throttle opening, again I will try your tip next time it's hot. Waiting for it to dry up here before going out and trying again.
Regards Huddie. Oh yes almost forgot , What about the air slide, Up or Down for hot start ????
Title: Re: Hot start difficulties
Post by: LJ. on 18.08. 2008 10:41
For what it costs... and it don't cost alot, an Imperial measurement feeler gauge is important to avoid damage to the magneto. Guessing in metric is not really the way to go, correct tools for these old clunkers is a bit of a must do.

Air Slider/Choke ???? Never have to use it on any of my bikes, certainly you should not be needing it this time of year. I'm wondering if your idle mixture is too weak, if engine is hot the mixture could be vapourising.

Did you try a colour tuning  plug?
Title: Re: Hot start difficulties
Post by: huddie on 18.08. 2008 11:30
Thanks LJ   I have just cleaned up my old Gunson colour tune. As soon as I get out I will try it when the bike is hot. On the subject of a weak idle mixture, I did weaken it after my first couple of short trips cus the plugs where sooting up, however I think this was because the air slide lever was to loose and I hadn't noticed it was vibrating shut. I will put it back to the start point which i believe is one and a half turns back from fully in and try again. are my L82c's ok?. I was looking at a roy bacon book where the index indicated L10S or its modern equivalent L85.
Regards Huddie.
Title: Re: Hot start difficulties
Post by: dpaddock on 18.08. 2008 21:50
From my post to the BritBike Forum today:
 "Service Sheet 801 tells us to clean the HT pickups every 6000 miles. It's easy to do (unlike the magdyno on the B models) and failure to perform this simple service leads to all manner of starting and running problems."

Title: Re: Hot start difficulties
Post by: groily on 19.08. 2008 09:20
L82c plugs ought to be fine - they are I think the modern L10 equivalent, along with the NGK mentioned by LJ. With a recon'd mag  one would hope the HT pickups and slip ring are clean from the rebuild unless you've got a nasty oil leak which is messing things up at the rear of the cylinders. If the HT leads are new too, and the plug caps, and all are screwed up properly . . . . the prob shouldn't be ignition - and you do have a spark! Assuming the basic ignition timing is OK, it might however be worth having a look at the points gap, as that will sometimes alter with a rebuilt item as it settles down. If it's 12 thou odd (or about .3 mm), and close to the same for both cylinders, as it certainly should be with your mag, then that's OK. Spark will be weaker or disappear if gap is too far out.
Choke slide should be fully pulled up - like many of us I don't even have one fitted, as a tickle is enough even in winter.
Was the engine ticking over well when hot, I wonder?
You could check for air leaks around the carb mounting flange, which isn't all that rare. Some carbs (not sure what one you have) have an O ring in the flange that goes on the head, others don't, but all should have a reasonable gasket and make a good seal. The nuts don't want to be done up too tight either, very particularly with concentrics, as that can cause distortion and leaks - as well as in extreme cases causing the throttle slide to jam open. And thinking of air leaks, does the top screw on the carb slide properly? Pilot air should, as you say, be about a turn and a quarter/half/tad more screwed out on a Monobloc.
Whatever gremlin you've got isn't too serious, that's for sure. If the carb has the right jets and slide, no leaks, and there are good sparks and a well-adjusted mag, it'll just be a question of working through the possibilities until hey presto . . .

And one more thing - tappet clearances? If they're closing up when the engine gets hot . . . . that could certainly cause you some grief. You mention compression being a bit less than you'd expect despite decent pistons and bores . . . perhaps a valve isn't seating as well as it should?
Title: Re: Hot start difficulties
Post by: huddie on 19.08. 2008 13:15
Thanks dpaddock & groilly,  Plenty to work through there, Hopefully i'll get it hot starting soon and if i'm lucky identify the actual cause along the way. I'll leave another post when i get it sorted. Regards Huddie
Title: Re: Hot start difficulties
Post by: A10Boy on 22.08. 2008 11:28
Sound like the Mag to me, the windings start to break down when hot and causes this problem. Its a waste of time testing the plugs out of the engine as they will spark in fresh air but not under compression.

A Mag refurb costs about £150 this chap is the best.

Service Exchange & repairs
Paul Lidford @
APL Magnetos
1 Maple Close
01747 852136
Title: Re: Hot start difficulties
Post by: groily on 22.08. 2008 18:28
hope not for Huddie's sake - just had the mag done he says!
Title: Re: Hot start difficulties
Post by: huddie on 27.08. 2008 13:02
Thanks for all the input you guys. The problem seems to be fixed now. What did i do, well several things, I re-adjusted the tappets, they were a bit tight, i reset the idle air screw to one and a half turns out, and i don't use the airslide to start it but i do give it plenty of throttle.
Thanks Huddie