irregular idle

[gerard-K]

I just finished the restoration of my 1957 BSA A10 Goldflash. Engine , fork, wheels and mudguards were all badly damaged in an accident 1.5 years ago.The K2f Magneto was rewound,  new intake valve guide and new exhaust pipes.

The engine has been completely stripped, and I followed the Haynes manual to the letter. It now starts first kick, and it idles fine but only for a  minute or so.

Then the right one cylinder seems to fail and idling becomes very irregular, with the one side kicking in now and then. Increasing the revs helps to prevent an unwanted stop at traffic lights but I need to keep the revs far too high.  At normal riding speed the bike runs without problems. , the 376 amal monobloc has a 240 main, 106 needle and 25 pilot jet. Needle is in second groove from the top.  Everything cleaned and all the carb passage ways are open. I double checked the magneto timing  11/32 before TDC on both cylinders. CB point gap is 0.012''

New NGK plugs, new HT leads and pickups, the mag slipring is clean.  I tried a thick uneven gasket correction between carb and cylinder head but no change. Intake and exhaust valve tappet gaps are as in the Haynes guide .  After a 30 miles ride the sparkplugs on both sides are light coffeebrown and look OK to me.

It is always the right  pot that cuts out at low revs. I never had this irregular idling before the accident

There must be an obvious reason, but so far I am puzzled. Any ideas, please. Thanks

Gerard

[Greybeard]

Try swapping the plugs to see if it's a duff plug. Also try swapping the HT leads.
I had something similar and changed one of the plugs but I now think it was due to too many short, low speed runs. After a good burn-up the bike ran much sweeter.

[Topdad]

Welcome Gerard, 11/32 I've always timed mine at 5/16 btdc with the auto unit wedged fully advanced . points as per yours at 12thou . The plug lead /plug change may sort but also not letting her idle for to long 'til you've got a few miles on her and have blown the cobwebs out off her . You may have a slight bias to the left pot what colour are plugs when its idled for abit before the miss fire comes in ,maybe worth checking .By the way the Haynes manual is ok for basic points and help but don't treat as the bible , they do fudge a few bits in various chapters to say the least . bobH  ps how about abit of back ground re you and your history with the bike ,poss some pictures as well best wishes .

[Triton Thrasher]

You get these symptoms with a rich idling mixture.  Did you do the screw adjustment with engine at working temperature?

[gerard-K]

You get these symptoms with a rich idling mixture.  Did you do the screw adjustment with engine at working temperature?

Yes, you mean the pilot air screw.  its at 1.5 turns out, and there is a very slight improvement at 0.5 turn richer.  I wondered if the irregularity can be explained by a sticking inlet/exhaust valve when the engine gets warm.  Can you run the engine with open rocker covers to check on the valve action??

Try swapping the plugs to see if it's a duff plug. Also try swapping the HT leads.
I had something similar and changed one of the plugs but I now think it was due to too many short, low speed runs. After a good burn-up the bike ran much sweeter.

You may be right but the spark plugs are new and there is no no carbon fouling.  Thanks both.[

quote author=Topdad link=topic=8815.msg62957#msg62957 date=1411469359]
Welcome Gerard, 11/32 I've always timed mine at 5/16 btdc with the auto unit wedged fully advanced . points as per yours at 12thou . The plug lead /plug change may sort but also not letting her idle for to long 'til you've got a few miles on her and have blown the cobwebs out off her . You may have a slight bias to the left pot what colour are plugs when its idled for abit before the miss fire comes in ,maybe worth checking .By the way the Haynes manual is ok for basic points and help but don't treat as the bible , they do fudge a few bits in various chapters to say the least . bobH  ps how about abit of back ground re you and your history with the bike ,poss some pictures as well best wishes .
[/quote]
I take your comment on the Haynes, but it served me very well.  The plugs look both OK, light brown, no carbon, not even  on the ring.
 My  history with the bike is 10 years now, a pleasure to ride and no real problems, always reliable. But in Feb 2013 a trucker decided to overtake a horse rider on the other side in a Surrey country road bend. I ended up in hospital with broken bones, wrist, knee, collar, fingers, and the bike looked even worse. The restoration took 1.5 years weekends mostly but was part of the healing process.
 

[Rgs-Bill]

You get these symptoms with a rich idling mixture.  Did you do the screw adjustment with engine at working temperature?

Yes, you mean the pilot air screw.  its at 1.5 turns out, and there is a very slight improvement at 0.5 turn richer.  I wondered if the irregularity can be explained by a sticking inlet/exhaust valve when the engine gets warm.  Can you run the engine with open rocker covers to check on the valve action??

Try swapping the plugs to see if it's a duff plug. Also try swapping the HT leads.
I had something similar and changed one of the plugs but I now think it was due to too many short, low speed runs. After a good burn-up the bike ran much sweeter.

You may be right but the spark plugs are new and there is no no carbon fouling.  Thanks both.[

quote author=Topdad link=topic=8815.msg62957#msg62957 date=1411469359]
Welcome Gerard, 11/32 I've always timed mine at 5/16 btdc with the auto unit wedged fully advanced . points as per yours at 12thou . The plug lead /plug change may sort but also not letting her idle for to long 'til you've got a few miles on her and have blown the cobwebs out off her . You may have a slight bias to the left pot what colour are plugs when its idled for abit before the miss fire comes in ,maybe worth checking .By the way the Haynes manual is ok for basic points and help but don't treat as the bible , they do fudge a few bits in various chapters to say the least . bobH  ps how about abit of back ground re you and your history with the bike ,poss some pictures as well best wishes .

I take your comment on the Haynes, but it served me very well.  The plugs look both OK, light brown, no carbon, not even  on the ring.
 My  history with the bike is 10 years now, a pleasure to ride and no real problems, always reliable. But in Feb 2013 a trucker decided to overtake a horse rider on the other side in a Surrey country road bend. I ended up in hospital with broken bones, wrist, knee, collar, fingers, and the bike looked even worse. The restoration took 1.5 years weekends mostly but was part of the healing process.
 
[/quote]
    You can run with open valve covers but you are going to have oil every where, to clean up.  Unless you can figure a way to get plenty of oil up to rockers, then pinch off the flexible part of the oil line going to the rockers, and then a short ride to see if valves are sticking with heat, in their guides. There has to be a better way to find out if the valves are nipping up in their guides, than possibly running the risk of scored rocker arms, from running with the oil line pinched off.  Musky why don't you address this one, you must know a way from your race years.

[muskrat]

G'day Gerard.
If a valve is going to stick in a guide it is usually in the first 5 minutes of running by the valve heating up quicker than the guide. Once hot it's usually the guide to give trouble coming lose or the gap closing up and holding a valve slightly open. Weak or broken valve springs can be another culprit.
As your problem is at idle I'd say bias as it is more noticeable then than at revs. Have you done an accurate compression test, cold and again hot? A colourtune would show an imbalance at idle.
Cheers

[KiwiGF]

I understand the carb could not have worn since you last run the bike but poor idling like you describe can be cuased by air leaking into the carb which occurs with excessive slide to carb body clearance eg a worn carb.

The effects of leaks past the slide can be reduced by sealing up the cables where they enter the carb.

Could also be a leak between the carb and head?

[Topdad]

Hi Gerard, glad to see you are getting help with the idling prob,those pics prompt a memory from me which I'll continue in the chat section, cheers bobh

[muskrat]

I recently had a RE in with a bad air leak problem. It was sucking through the inlet guide. Only noticeable at idle.
Try running the motor with the inlet tappet cover removed (warm her up before removal) and spray WD at the top of the guide that's suspect.
Cheers

[morris]

I recently had a RE in with a bad air leak problem. It was sucking through the inlet guide. Only noticeable at idle.

Just out of curiosity Musky, but didn't it smoke then? I assume that there also should be some oil sucked in with the air?

[muskrat]

Yes morris it did.
Cheers

[wilko]

Has anyone mentioned tight tappets on the missing side?

[Rgs-Bill]

Good point, too much engine heat from improper jetting, and or settings, a really hot day, and too tight of valve lash adjustment, can cause the valve overlap, and cause the missing.  Might even be shooting some smoke, or even black smoke, from the affected exhaust pipe maybe.
Does it only happen when it is totally warmed up to operating temperature, or when first started up also?