Engine fade

[Worty]

This has probably been covered before, but I'm having issues with the top end speed/power.  Bike starts well and ticks over properly, pulls through all the gears and accelerates no problem, even up hills.  My problems start when I get to about 65mph up a steady incline when the bike loses interest and starts to slow.  Knocking the throttle off a bit helps a little, but it only gets going again when not under top end load.  I do have this weird 'tinkling' from the timing side, and she feels lumpy at low speeds.  When I open her up a bit, she sounds, and goes, lovely.

FYI, maggie is fresh from Andrew and no issues with pick-ups, HT leads, etc.  Plugs are not new, but don't seem to have had many problems.  Fuel from the tank is 500ml/min (slower on the reserve side but still 300ml/min).  Tappet clearances were checked not long ago.

The only thing I can thing of is maybe the carb needs a clean or something.  Bike does run a tad rich anyway, but the carb was new 8 years ago and hasn't been apart since.  Give the state of modern fuels, and the fact it can stand for some time over 8 winters and stays in hospital, could I be looking at a build up of varnish, etc.  If I do need a carby strip and changing needle settings, advice would be appreciated (with pictures if poss).  Also, would I need a gasket set??

Cheers in advance.

[Greybeard]

Could the tinkling be pinking, (pre inginition)? Have you checked the points gap followed by ignition timing? If you drop a gear when she is struggling do things improve?

My bike shows sooty plugs but she starts and runs fine so I don't mind. I'd rather have a rich mixture than lean.

Where are you? Is there someone nearby that has experience of these bikes who could take it out and give their opinion?

[Worty]

Cheers GB.  It's not pinking, I had that a lot before and makes my blood run cold when I hear it.  It's more a power loss that rights itself a bit when not under load.  Must admit, there are some strange sounds from the r/h side at times - makes me nervous of impending failure.

Points and timing are good, but don't like to drop to third at 65mph!

I live near Wakefield, by the way.

[RDfella]

I'm inclined to agree it may be 'pinking'. At what throttle opening does this occur - 3/4 - full? Have you tried raising the needle and / or a bigger main jet?

[Triton Thrasher]

Pinking isn’t pre-ignition.

You have to be certain the mixture is rich enough in the throttle range you are using.  Is it full throttle or 2/3 throttle?

[KiwiGF]

This has probably been covered before, but I'm having issues with the top end speed/power.  Bike starts well and ticks over properly, pulls through all the gears and accelerates no problem, even up hills.  My problems start when I get to about 65mph up a steady incline when the bike loses interest and starts to slow.  Knocking the throttle off a bit helps a little, but it only gets going again when not under top end load.  I do have this weird 'tinkling' from the timing side, and she feels lumpy at low speeds.  When I open her up a bit, she sounds, and goes, lovely.

FYI, maggie is fresh from Andrew and no issues with pick-ups, HT leads, etc.  Plugs are not new, but don't seem to have had many problems.  Fuel from the tank is 500ml/min (slower on the reserve side but still 300ml/min).  Tappet clearances were checked not long ago.

The only thing I can thing of is maybe the carb needs a clean or something.  Bike does run a tad rich anyway, but the carb was new 8 years ago and hasn't been apart since.  Give the state of modern fuels, and the fact it can stand for some time over 8 winters and stays in hospital, could I be looking at a build up of varnish, etc.  If I do need a carby strip and changing needle settings, advice would be appreciated (with pictures if poss).  Also, would I need a gasket set??

Cheers in advance.

Long shot but quick to check is that the vent to the float bowl is clear (its in the tickler), also that the tank vent is clear. This is based on the theory that your problem occurs during sustained max fuel consumption.

[Swarfcut]

    Reckon this is a fuel/air problem at wide throttle. Is the air filter choked?

     Here are a few other things to consider.
 
     BSA Service Sheet 708 details carburation, and once the throttle is opened past the pilot by pass up to  3/4 Throttle it's the needle controlling the mixture. From 3/4 Throttle it's the main jet.  Examining the plugs after wide throttle operation should give a pointer on how the mixture is at these higher speeds.

     The mixture is weak if at a particular throttle setting the motor runs better with a little bit of choke. The reason for the weakness then has to be found, easy fixes are fuel flow, varnished and occluded jet, or too small a main jet.

    Sheet 708 makes interesting reading, particularly the sequence of tuning. So next time it loses puff, close that choke and see if it makes any difference.

 The pilot jet is almost like a carb on its own and although this can be set up to give the perfect idling mixture, this is no guarantee that the mixture remains at optimum over wider throttle settings. That is down to the combination of throttle slide, main jet, needle jet and needle. Easy to sort if you have a full rack of AMAL Spares, not so easy for the rest of us.

 Swarfy.

[Worty]

Brilliant replies and suggestions - thank you.

Pinking - all I can say is that I've had severe cases of this in the past and it doesn't sound the same.  The pinking I've had before sounds like the head is full of broken glass and small washers.  Knocking it down a cog helped when it slowed severely.  At the moment, the bike runs fine (with the 'tinkling' on the r/h side - but not all the time).  Problem occurs at 2/3 to 3/4 throttle, pointing to the suggestions about needles and jets.  Having said that, the bike has run perfectly in the past right through the rev range.

Cap vent is not blocked and there's and excellent flow of fuel to the carb - 500ml/min on main tap.  Air filter is relatively new, although seems to get dirty quickly.

How do I 'raise the needle' (never done carb stuff before).  The carb was brand new and has covered 10k miles.

[Worty]

Plug Chop.

Que?

[berger]

worty  plug chop , cutting off the ignition at certain throttle openings and inspecting the plugs because they tell a story of how the mixture is. a nice back road with little traffic is ideal for this and you only need the plug spanner.

[Worty]

Cheers Bergs, wasn't familiar with the term.

So, I get it up to where it starts to fade out, hit the engine cutout and roll to a stop, whip out plugs and see what colour they are.  FYI, does run a bit rich (sort of dark tan colour) over normal speeds.  I'm guessing that if it looks a bit whitish when I reach fade out speed, I have a fuel/air mixture issue??

[Rex]

My first thought when reading your original post is that it's running slightly too rich at WFO and you should try a smaller main jet, but if you say the plugs are a dark tan then that sounds about right.
That said, I'd still spring for a couple of main jets slightly smaller but keep an eye on the plug colour.
Incidentally plug chops worked great in the days of four star petrol, but are rather meaningless now, at least, according to pro tuners anyway.

[Jules]

You mentioned the plugs were a few yrs old Worty, high loading on the plug is the conditions you are having issues at, maybe worth a quick plug change too, relatively inexpensive and quick to do......regarding the plug chop, be aware that cutting the spark when under high load/high fuel conditions will put unburnt fuel into the silencers, this can cause a (sometimes big!) backfire in the silencers on start up 

[Worty]

Cheers Jules, have reset timing from 5/16 to 11/32 and get no spit back on kickover.  I'm going to test the bike tomorrow to see if things improve.  If it runs past 2/3 throttle freely, I'll do a plug chop to see if I should put the needle back to P3 from P4.  Air screw is 1.5 turns out and plugs are brand new.

Stunningly, when I removed this, the FOURTH, ATD, one of the 'prongs' had broken off and was sitting in the timing cover.  What the hell is going on with these ATDs - the FOURTH one!!!!!!!