1972 Bonneville T120rv Perfomance Issue

[oldbloke]

The carbs have been blown with compressed air only, and the vent hole in the filler cap is clear.Had a look at the carbs today as I am trying different jets and needle positions and I noticed there was fuel inside the slides is this normal, the slides have a couple of large shiny spots on them, looks like a touch of wear and tear. While the bike was on the lift I ran it in top gear, and no probs straight to 6500 rpm, wish it would do that when it is under load as it only gets to 4000 rpm / 60 mph. It appears not to be making max power.

 

[grandpaul]

...never mind...

I see we already addressed the choke slides.

Carbs may be too worn. You can go with new ones, but some supply is backed up as Burlen / Amal had issues with the foundry that was casting them, and has gone to a new foundry. I believe they are talking about a couple more months.

OR, send them to Lund's for sleeving.

But wait, see the next post!

 

[grandpaul]

AHA!!

It sounds like your Auto Advancer Unit (AAU) is sticking on the points cam, and not advancing fully.

I would pull the points cover & pillar bolts, and pull the points assembly completely out. Take the points cam between your thumb and forefinger and turn it counter-clockwise, it should turn fully 12 degrees to full advance, then spring back smartly.

I believe you will find that it will either not turn fully, not snap back, or both.

If so, remove the AAU thru bolt and install a thicker bolt in the nose of the AAU. Tap the bolt with a screwdriver handle, from all 4 side; it will simply fall off. Pull it apart, clean it, lubricate it, then see if it rotates and snaps back correctly.

With everything else you've checked, this is perhaps your last hope.

 

[oldbloke]

Hi GP, yes the carbs do look worn and I have been in contact with Burlen/Amal and have ordered new carbs, unfortunately they will not be available until the end of August at the earliest, and they have recommended carbs; 930/66 &67 (pack 126) as they say they will have the correct mixing chamber cap, not sure what that means. I am very interested in your comment about the auto advance, this makes sense to me. The bike is fitted with a Boyer Bransdon ignition so, I assume the auto advance is controlled by a unit. Is this true ?as I am clueless about these systems.

 

[grandpaul]

AH, sorry, there is no AAU with the electronic ignitions, it's all done digitally.

So, either the problem is all in carbs, or POSSIBLY you have a bad ignition "black box" that is not fully advancing the ignition as it should. I believe Boyer can test it and will replace it if defective.

 

[oldbloke]

Thanks GP, I think I will wait until the new carbs arrive and see what happens from there. The new ones are the 'Premium' with removable pilot jets.

I am not sure how old the ignition system is, the bike came with a lot of history including a total rebuild in 2003 by 'Grin Triumph' in Scotland. The ignition system is mentioned in the paper work so it could be somewhere in the region of 14 years old. Have you ever heard of one of these systems going wrong ?, I have strobed the timing and it appears to be bang on the mark.

 

[Rocky]

Some of my vintage bike buddies have fitted those new carbs and have had great results.

 

[oldbloke]

Thanks Rocky ! I will let you know how things go when they arrive (looking at 3 months)
Hope this solves the problem.

 

[Rocky]

Fingers crossed for you

 

[harper6t]

I was out on a ride today when the thought occurred to me that I hadn't seen anything in the thread about fuel lines and fuel taps being checked and that maybe its running lean but still getting some fuel through.
Have you checked the filters in the fuel taps and and all the fuel lines to make sure there is adequate flow to the carbs at higher speeds?

 

[oldbloke]

Thanks H, new taps and lines fitted early on, balance pipe between the manifolds also renewed as I thought there might be a partial obstruction due to the original pipe being slightly out of shape. Actually put a clamp on the balance pipe while the engine was running to see what it did - killed it immediately.

 

[oldbloke]

Hi Guy's, still waiting for the new carbs after all these weeks. Noticed last week one of my silencers/mufflers had a little bit of chrome missing so I have bought a pair of BSA Gold Star and fitted them to my Bonne. Hard to believe after all our discussions that the new exhausts have solved the performance issue, the bike has now got a new lease of life. Earlier in this disscusion Grandpaul said ' sounds like it's choking ', looks like you were bang on the money GP. It would appear that the motor was unable to breath properly with the old mufflers. It was just pure luck on my behalf. Has anyone else come across anything like this before, this is one thing I would never ever have looked at.

 

[harper6t]

Never had that problem with a Triumph, but Velocettes have an issue with the internal baffles coming loose and blocking the outlet, which results in the bike not revving past 3500. I had this happen on my Velo a couple of years back but it needs the exhaust to be almost completely blocked, not something I would have thought possible on a Triumph twin.

 

[grandpaul]

Even a blind squirrel finds a nut once in a while!

 

[oldbloke]

The mufflers on my Bonne were those long mega's, presumably replicas fitted by a previous owner or the firm who rebuilt the bike. Had a look inside with a torch and near the end (outlet) there is a round plate the same diameter as the pipe with two small holes in it, which are, I presume inadequate.
This is the first time I have come across this problem, but it is sorted and I am a very happy old biker.
Thanks to all you guy's for your input.

 

[Rocky]

I was wondering what was on the bike when you got it. Thanks for the info.
This is a pretty rare problem I think and it wouldn't have occurred to me that this might be the problem.
Glad you stumbled on the answer and all is well

 

[Greyfell]

The mufflers on my Bonne were those long mega's, presumably replicas fitted by a previous owner or the firm who rebuilt the bike.

Do you suppose that was why it was sold, because it no longer performed as it used to, and they, too, never thought to check if it was the new pipes?

 

[grandpaul]

That could very well be a contributing factor.

 

[oldbloke]

Not sure GF, my late brother bought it from a guy in Kirkcaldy Scotland, who apparently owned it for 20 years and says he did not use it much and it spent a lot of time lying in his shed, this could be true, because when it arrived it looked totally neglected, shabby paint and bits of rust. It came with a lot of history, including a detailed rebuild by 'Grin Triumph' Kirkcaldy in 2003. They fitted two new exhausts after the head had been modified to take 'push over pipes'. I don't know where they acquired the mufflers, but I would have thought the owner would have queried why the bike did not perform, or maybe he did and nobody could work it out and as you say, that could be why he sold it.

 

[oldbloke]

Just out of curiosity before disposing of the old silencers/mufflers I decided to have a look inside one of them. I wrongly assumed they were straight through purely because of the sound they made. Anyway, got the angle grinder out and cut one open. No wonder the bike had trouble breathing. Your probably familiar with these mega's as I think they were standard on 70's bikes. The unit is approximately 26 inches long tapering from 1 1/2 inches to 4 inches. For the first 12 inches there is a 1 1/2 inch pipe which stops about 1 inch from a round plate ( the same diameter as the as the silencer body ) with 3 holes about 5/8 inch, offset near the body of the unit, altogether there are 3 of these plates in the last 6 inches of the silencer and none of the 5/8 holes actually line up. So basically, the gases from the exhaust pipe are hitting the middle of the first plate and then has to find its way up to the three holes and then through the other two plates. I hope this makes sense, I have a couple of photos on my phone of the internals. So if anyone is replacing silencers/mufflers on older bikes please have a good look at them before you put them on your bike. I would imagine there will be differences in all silencers depending on who makes them.