BSA Owners' Club Forum

The BSA Workshop => Twins => Topic started by: HUGH on 26 May, 2017, 17:19:24

Title: Loss of Power
Post by: HUGH on 26 May, 2017, 17:19:24
Realise BSA forum but wonder if anyone can help with my 1971 T120R OiF Bonneville since its a very similar bike to the later A65L OiF which I had in 73 and was a brilliant bike in my opinion. Also my 1970 A65 pre OiF doesn't suffer the problem outlined but is very similar to the T120 in terms of maintenance (new coils, new carbs etc. regular service and so on) and goes well under equivalent conditions to those described. T120 has electronic ignition which is about 15 years old. New twin AMALS, new 6 volt coils and spark plugs. Valve clearances fine and cylinder head correct torque to new gasket so should not be sucking in air through gasket joint weakening mixture. Bike starts easily and ticks over fine. Near top of hills it loses power where is a long climb getting steeper and the hill is slightly higher than usually causing the engine to work hard. It accelerates well at bottom and initial climb. My normal test to see if my fettling to solve problem has done any good here is Caerphilly mountain (for those who know it)-  it is relatively steep at top and about 889 feet high so not exceptional by any means for some sort of air starvation. Air filters cleaned. Bike accelerates well as said on lower slopes but at top bike just loses power and have to change down to 3rd even 2nd. It might be on one cylinder by this time but can't really tell in the general slowing down and can't really hear with helmet and slightly deafness. When pull into the car park after a short flat section ticks over fine and restarts easily on kicking over and comes back home (down the mountain) with no problems despite a number of short climbs. Hope someone can help as you can imagine its very frustrating and trust you will all excuse a Triumph post on a BSA forum - excuse is do have an A65. Enjoy a safe weekend and hope weather is fine. Cheers, Hugh
Title: Re: Loss of Power
Post by: DEREKANDERSON8 on 26 May, 2017, 17:54:58
could it be a fuelling issue ie not getting enough when it is working hard? just a thought
Title: Re: Loss of Power
Post by: EDDIE SIMPSON on 27 May, 2017, 13:38:18
is the gearing standard?
are both exhaust pipes as hot as each other and one cylinder not dropping out?
is the carb needle in the right possition as per manual?
and a thousand other possibilites.
Title: Re: Loss of Power
Post by: HUGH on 27 May, 2017, 14:49:48
Eddie as you say that's the problem. Its absolutely standard bike (apart from electronic ignition - or how I wish it were points). it is fitted with 190  jets. What's so frustrating is had its MOT yesterday + went straight through. Came back up hill and down dale without any issues what so ever. Anyway we'll see what happens as we continue the hunt. It is an intellectual challenge but one I could do with out. As a complete novice in engineering terms I am learning more all the time which is one reason I developed an interest in classic British bikes. What is a relief (touching wood) is the A65L is excellent. Cheers Hugh
Title: Re: Loss of Power
Post by: Editor on 27 May, 2017, 14:54:41
Hugh, whereabouts on the throttle are you when this happens? If its up to half way and you open it up further and this persists it could be your main jets are too small. Try fitting a pair of next size up, or maybe get the size after that aswell while your at it and see if it makes a difference. They're cheap enough.
Caerphilly mountain is a good testing ground by the way, as is the Bwlch up from Nant-yMoel.
Let us know how you get on.
Title: Re: Loss of Power
Post by: HUGH on 28 May, 2017, 10:37:24
Hi Chris Know the Bwlch quite well - bike was slightly worse here. Have uprated jets from 180m to 190 after this display but have some 200s in garage which will now add to the list of new bits.   Cheers Hugh