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I had my bike out with a group of vintage riders with old bikes about three weeks ago. Good long ride through the foothills of the Canadian Rockies. I think a little over 200 km, the longest ride I have had on it by far. I had some trouble keeping up with traffic for the first stretch, which was a major highway. Flat out, on a relatively flat stretch, the most I could do was about 120 km/hr (about 75 mph).

Not trying to set a speed record with this old bike, especially since it is so easy to go fast on a newer one and a lot safer, but should I expect better performance? I do have new Amals and the right jets for the altitude. Engine completely redone last winter. It also has EI.

I see on the internet that it is supposed to go 110 mph from factory, which is partly why I ask. Do I have a problem with the bike? If so, what is it most likely to be?
 

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Does she idle OK? Fuel/air mixture good and valve lash in spec?
 

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Starts first kick, idles nicely after 30 seconds, maybe a little fast. Pulls really well up to about 65, then kind of powers out at 75 mph. Hey, it seems great. But maybe another way to ask the question is what should I check this winter after maybe 800-900 miles on a rebuilt bike? Check idle, fuel/air and valve lash. I will do those first.

I did not do the work on the engine, though I am confident the person who did was an expert. Should I change the plugs too and is it possible the timing is a little out? Just trying to get a list of the most likely suspects. It is really buzzy at 75 mph if that is a clue that means anything. Maybe that is just the normal limit for a bike like this one?
 

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Hi Jiminy,
I too have a 70 T120R with exactly the same problem as yours, fast idle and lack of top end power. Turns out that this model was the first to have a thicker O ring between the carbs and manifold. There is also a O ring recess on each carb. Mine was rebuilt with thin O rings that do not seal properly and is only evident at highway speeds. Embarrassing when attempting an overtake to run out of puff at 70 mph. I have now fitted the correct O rings (E9711) and problem solved. Pulls strongly past 100 mph. This was Triumph's attempt at rubber mounting the 2 concentrics and there is a danger of warping the carb body when tightning the mounting nuts which have a rubber and normal washer attached. When fitted at correct torque, you should be able to fit a 30 thou feeler guage between the carb body and manifold just inside of the mounting studs.
I added a dab of paint on the nut flats so I can see if these have moved after a ride. Have a look at the replacement parts catalogue (No 8) and all will be revealed.
Hope this sorts your problem.
 

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Hi JiminyCricket, That is not right. Was this fairly level road without out strong head wind.

I don't know if they can really do 110mph, but they pull pretty strongly to 90 or so anyway.

Depending on your balance factor & dynamic factor, vibration getting much worse at higher rpm is normal.

What air filters & exhaust are you running? What was the altitude? Have you verified choke is fully open & the slides actually get full up at full throttle? Have you verified fuel flow through taps & inspected banjo screens? As was said any air leaks at intake manifolds are a disaster.

Timing is very important. Mixture is very important. I would mark grip to see how much throttle you were using. Study tuning guide for feeling how power increases slightly when backing off throttle slightly. Takes some practice to get the feel of it. If too rich will 8 stroke at this speed/conditions you were in. An ah ah ah ah miss fire that is unmistakable.

Here's link to tuning guide. Print as needed, mark grip, find road to simulate the poor power conditions.


I've had my '73 Tiger 750 to 80 often passing cars. It will pull very strongly to 90. I took it to indicated 112 once. I'd never do it again. Vibration was horrendous & I thought it was going to blow up. I did dumb things when I was young. Bike was still just barely in warranty. My bike ran very strong at 8000' coming west over pass out of Silver Lake Hwy 88 California. I took it to 75 effortlessly just to see what it could do. It could have done more. Afraid of ticket as speed limit was 55. 650s do the same good power up this pass.
Don
 

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Common reasons:
Choke slides not lifting fully out of the Venturi;

Fuel starvation- should flow something like 300 ml per minute out of the float bowl drain. Maybe more. I’ll try to look that up.

Retarded ignition timing or points badly adjusted.

Someone smart Alec changing sprocket sizes, making it too high geared.

It should do over 100 mph with you lying on the tank, even if you have to take it to peak rpm in 3rd gear before changing up.
 

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i could rarely read my gauges at speed on my 1970 T120R, but think it should go better. baring tuning issues a gear change may help + remember besides the brakes the VIBS were the biggest downsides compared to todays rides, although my traded 2006 porksters brakes were prolly WORSE as its suspension was, BUT being 20 something + not ever riding something better i cared NOT. the most BEAUTIFUL bike Triumph ever produced IMO
 

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Both fuel taps must be open and flowing freely.

Ignition timing may be slightly off.

Could be other things including valve adjustment.

110 is ambitious for a stock classic Bonneville (not impossible, but the bike must be PERFECT)
 

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Hi Jim
It is a good idea to check the ignition timing and the valve clearances.
Firstly, are you running the standard exhaust and air filter, this will have an effect on running.
Straight through exhausts on my T140 will have the bike hitting a wall at 70mph just like yours, unless I retune the carbs.
I expect that you would be over half throttle at 75 mph, this is where the mixture control moves from the slide to the needle and needle jet (ultimately controlled by the main jet).

As a quick and dirty test without the need to buy parts, I would by pass conventional carb tuning and jump in on the needle and raise it by one notch, then go out and see if it makes a difference. If it improves then tune the main jet first then the needle jet. If it is worse then lower the needle down one notch from standard, if performance improves then tune the main jet first then the needle.

This is not a methodical method, just jumping in at a likely point, neither is it a final fix, but might help narrow down the cause of the problem.

Hi JiminyCricket,I would mark grip to see how much throttle you were using.
Don
Don has already given you the best advice that you will ever see on carb tuning.

Regards
Peg.
 
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Make sure fuel flow is sufficient and carbs are opening completely. Does your air filter flow enough ?
No flow, no go.
 

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I had a bonneville 650 in Sicily in 1971 and the road to work was level concrete. Laying on the tank, the speedometer read 118 at least 3 different times.
 

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the site software won't let me edit. i see that you have jetted for your elevation.

what jets are you running?

have you advanced the timing for your elevation as well?
 

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yes. i can edit posts above and below, but not that particular one.

...

and of course, now i can, and this one too. i am not a fan of this software.
 

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Check the engine sprocket size as well as all suggestions. It should be 19 tooth but someone might have used a smaller one. A 650 standard T120 will easily get to 100 mph indicated speed sitting upright on a level road. If i use the standard silencers, it will bog down at around 85mph. I have used many other silencers to get a lot more speed at higher rpm.
 

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Discussion Starter #18
We got a little snow here the past couple of days. It will likely clear at least for a few days between now and the end of October, so I will try some of these things this weekend and hopefully get a couple chances to test it.

The elevation is high. 3800-4600 feet above sea level. I am pretty sure I do not have a leak between the carbs and the manifold. I put those on with the correct o-rings and nuts with rubber washers, though maybe not as I cannot put a feeler gauge in there (maybe they are too tight or the o-rings too thin). I do not have the choke installed (I need a slight modification to the cable and have not got round to that). The slides travel the entire way. The exhaust is standard, though not original. Air filters are the paper ones, not oiled, but good quality and still clean.

I will check the sprocket, but I think the most likely candidates are carburation and maybe timing. I did not have both fuel taps open, just the right, so that is easiest and the first thing to try. I will try some of the carb tuning tricks suggested next, starting with Peg's suggestion. I better make sure I actually installed the high altitude needles I remember ordering separately after I bought the carburetors. If that does not work, I will check the timing. If that does not work, I think I will pull the carbs off and get new o-rings. At the same time, I will clean the carburetors. While I do not think I have a fuel flow problem (I put new clear lines and taps on this last winter), it may be that there was some crud or scale in the tank that has come loose over the summer, though I think that kind of issue would show up at idle.

I will report back, but it may be a week or two. Thanks everyone for the replies.
 

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it does sound like fuel flow to me. thats a five minute test.

take out the drain plugs, put a jar under each, turn on both taps. you should be able to get 300ml per minute out of the drain on each carb to keep up with full throttle.
 

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Hi,
Both fuel taps must be open
Why on a 650?

@Truckedup has previously posted the amount of fuel required to produce 60 bhp. A single standard tap is marginal supplying this quantity of fuel. However, a 650 twin's peak power is less than 50 bhp.

it does sound like fuel flow to me.
take out the drain plugs, put a jar under each, turn on both taps.
Before this test, ensure a 3/16" OD rod, drill-bit, whatever will pass through each tap in the open position. That's the ID of original tap openings, pattern taps have been known to be smaller.

When checking the taps, that you remove them from the tank (so you don't poke the rod, drill-bit, whatever through the tap's filter) will ensure there isn't any fuel-flow restriction due to blocked filters.

I put those on with the correct o-rings and nuts with rubber washers, though maybe not as I cannot put a feeler gauge in there
As @[email protected] posted earlier in this thread, and I've posted previously, you have to be careful you're supplied the correct 70-9711 O-rings ...

Regrettably, for some years, the Co-op equated 70-9711 to Amal 622/101; they aren't the same O-ring. More regrettably, people that set up as 'dealers' in spares for old Triumphs (particularly on eBay) do not have to pass any sort of competence test. If you "cannot put a feeler gauge in there", I wouldn't take bets you haven't been supplied the thin 622/101 rather than the thick 70-9711. :(

Btw, the whole point of the 'shouldered' mounting studs and collection of mounting nuts and washers is you should be able to tighten the nuts up to the 'shoulders' and the thick 70-9711 O-ring seals the gap between the carb. and the manifold. Otherwise there isn't any point having shouldered studs?

Hth.

Regards,
 
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