Moto Guzzi T3 1000 gearing and vibration

I have a 1980 mk2 Le Mans which shows rpm 4000 at indicated 80mph. This engine is pretty smooth.
Just purchased a 1977 t3 which is fitted with a 1000cc engine, I am told it’s a 1000cc Convert engine. The gearbox is a the standard 850t3 box.

(1) At an indicated 80mph it shows rpm 4500. Why would it seem to geared lower that the 850 Le Mans.
(2) The vibration on this engine is much more than the 850 and seems to start just over 60mph. I have no experience of a standard 850 t3 to compare.
(3) Is the crankshaft the same in both 850 and 1000 engines. If so, perhaps fitting the 850 cylinders and pistons would reduce the vibration?

Would appreciate any help / advice as I don’t think I would want to tour on this 1000 t3 if it vibrates the way it does from 70mph upwards. The 850 Le Mans mk 2 is smoother and far more relaxing to travel on though for me it needs higher bars.

Finally why do people respray petrol tanks and side panel but don’t lacquer them. I spilled a little petrol on the tank pf my 1000 t3 and …well it’s not good. I had this issue with an Yamaha RD350lc and also a 1972 Benelli tornado 650……Why do people do this….:frowning:

Possibly the speedo is under-reading? Just something to check

Vibration - carbs balanced properly? I did have to replace one on a Spada that I could never balance properly and always vibrated, because of a machining fault in one of the carbs. New carbs magically cured it. Until then I had bar end weights on because the vibration buzz through the 'bars was so bad. :frowning:

There are various gearing combinations so you may have a non-standard setup in one or other bike. I think my 1000SP shows 5000rpm at 80mph and is dead smooth. Both needles pointing up is where a Tonti bike is best!
Agree with Mike that carb balance makes a big difference but I would also suggest a thorough check of all the basics - valve clearances, timing etc.

Thank for the advice.

I adjusted the slides so they both close together, well pretty close and closer than it was before. :slight_smile:.
It’s running smoother……really nice over 3000 to say 3800 them 4000 vibration starts but it is not as bad as it was. Not quite as smooth as the mk2 Le Mans but it’s now not much of an issue, vibration increases from 4000 up but again it’s much better than it was.

I must say I am surprised by the reduction in vibration after a small carburettor slide adjustment.

Just noticed there is a little oil comming out of the little slot behind the g/box. I’m not going to bother with it unless it gets worse :slight_smile:

Really need to do it with vacuum gauges rather than ‘visually’.


I put a loose feeler gauge under the carb slide both sides so when you slowly open the throttle you get a very good visual check that they are(or are not) opening together.

Good thought, I remember reading somewhere using a 1mm twist drill bit to set both slides the same height, as a starting point. Then keeping a count of how many turns or fractions of a turn and which way, of the throttle stop screws, both together.

On a big twin carb synch is the key to a sweet, smooth responsive motor.
To get the best results you need to do the valve clearances and plugs before the carb synchronisation and use vaccum gauges. It makes a really big difference to get the valve clearances the same left and right.

Also if you arel running twin throttle cables all the way to the handlebars they need to be lightly cable tied together wherever you can or the carb synch will change as the cables flex and move around.

I’m a big fan of twin engines for motor bikes. I have owned a fair old number over the years. It is vital to get your valve clearances right before you start to synch your carbs.
One of the best and cheapest carb balancers can be made with a length of clear tubing fixed to a wood support in a u shape, filled with some ATF fluid. Plenty on You Tube to show how. You need to ensure that the fluid levels are level at idle, when the slack in throttle cables means they are out of play. Once you have got this setting then the tricky bit is to partially open the throttle so the cables are in play and adjust one of them to bring the fluid levels level.
Comparing the results against Carbtune etc this is more accurate.
It takes some practice to get this right. Admittedly, vacuum guages can’t allow ATF to suck into the engine if you take your eyes off what the levels are doing and adjust the cables incorrectly. However the manometer style relies on gravity which is pretty reliable and the same column of fluid being pulled at either end by the carb depression. Long ago Carbtune relied on mercury columns and was really accurate, however 'elf and safety means that the mercury had to go.
Usual caveats apply, seek professional help if you are unfamiliar with the above.

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