Gear Change Issue

[RichardJ]

I have an odd question for you. Since acquiring my 654 I have been trying to find as much information as possible. So far I have dug up three old road tests. What is slightly worrying is that all three refer to occasional problems changing gears.
Can anybody tell me if this is a known problem on the 654 or any of the other De Tomasso era bikes ? Of course, it might just be that they were all testing the same bike.

My bike is still in 1,000 pieces, so if there is a gearbox modification to be made I think this would be the best time to do it.

Cheers.

[Cosmo]

Hello Richard,
All the multis have a "slow" gearchange due to the design of the selector drum and forks.
Having built both the Honda CB 500/550 and Benelli engines, they are remarkably similar and the Benellis are an obvious copy excepting slight differences in manufacturing tolerences and sizing. Therefore while internal parts are not all directly interchangeable, many Honda parts can be fitted with modification. My own Sei special runs Yoshimura CB 550 pistons, valves and guides and a Ravasio cam with modded rockers to clear the "wild" cam lobes.
The Benelli valves are however slimmer for gasflow and on paper should give more power, but that's not always the case if the engine explodes!

The gearbox issue particularly with worn engines is often largely down to the selector drum tensioner arm sprung scissors design - Look in the parts book to see what I'm talking about.
The Honda one comprises 2 pressed steel rivetted washer rollers, all very cheap and nasty.

The Benelli one has been "improved" to a design with a bombproof roller bearing on one side, but due to space constraints retains a rivetted washer on the other arm. Obviously, this rivetted part now suffers ALL the wear and as a result the rivetted spindle tends to go oval VERY quickly with hard use. This has the effect of leaving the drum under tensioned in rotation and you can get stuck "between gears". This causes missed gears and in the extreme, possible selector fork/gear dog damage especially if you have a heavy boot.

Theferore when re-building, pay attention to this element and replace the rivetted part regardless of any wear.

These bikes will never we as slick as a modern machine, but then again neither are the CB Hondas of the period. Don't expect Suzuki GS bulletproof gearbox performance from these engines and don't get paranoid and you won't be dissapointed.
I hope this helps more than hinders!

[RichardJ]

Thanks Cosmo. I wasn't expecting such a comprehensive answer. That is certainly very helpful. Having ridden BMWs for many years I am used to having a slow gear change, so that's not a problem. Also, I can see how a missed gear may have led to over revving which may then have been the cause of the dropped valve in my engine.
It will be some time yet before I am in a position to start on the gearbox, but I'll be sure to update this thread again to let you know what I find when I go in.

[Tornadoboy]

Hi Guys,
I've never had a problem with the gearshifts on my Benelli multies and I've had a few. The 900 could baulk occasionally if I was in too much of a hurry. If you have trouble look for bent selectors due to previous owners' quick gear changing techniques.

I hear standard Honda 500 pistons are lower compression than the Benelli piston. People have used modified GPZ305 pistons on the 600/900 but there have been problems with the thin lands between the rings breaking. You can't use Honda valves with Benelli guides as the split collett slot is lower. The CB650 engine or just the top end can be used in the Benelli frame with a bit of ingenuity making a good power to weight ratio especially in the lightweight 4 frame fitted to earlier Benellis.

Chris

[Cosmo]

Hi Guys,
I've never had a problem with the gearshifts on my Benelli multies and I've had a few. The 900 could baulk occasionally if I was in too much of a hurry. If you have trouble look for bent selectors due to previous owners' quick gear changing techniques.

I hear standard Honda 500 pistons are lower compression than the Benelli piston. People have used modified GPZ305 pistons on the 600/900 but there have been problems with the thin lands between the rings breaking. You can't use Honda valves with Benelli guides as the split collett slot is lower. The CB650 engine or just the top end can be used in the Benelli frame with a bit of ingenuity making a good power to weight ratio especially in the lightweight 4 frame fitted to earlier Benellis.

Chris

You are quite correct the C.R. of the Honda pistons is far lower, and you only have to look at the valve cut outs on the Benellis to see that. Yoshi or other speedshop pistons get somewhere near, but I have had valve bounce/bent valves with missed gears in extreme use myself using high comp pistons, so the assumption in the previous post was correct.

You are also correct about the valves and guides, the Honda guides need to be modified to heat fit to the Benelli heads so they can take the Honda valves, collets, springs and cups, the seats also needed re-cutting to suit. The Honda kit MUST go in as a whole, not piecemeal.

The Honda CB500/550 fours drop straight into the frames using the original bolts although don't look quite so good as they are more "upright" due to the barrel angle and the exhausts would need modding, but I didn't know the 650 parts went in as well.
Years ago, I have also seen a GPZ550/Z650 engine fitted in a 354 but this would have needed bespoke engine mounts and a lot more work.

All good info.......

[Tornadoboy]

Yeah, I've been thinking of doing something with the much ignored SOHC CB650's motor. They are dirt cheap and with 63bhp probably have 5 to 10bhp on what benelli claimed for the 654. In the lightweight 354/500ls/GTS frame there should be a lot of go for not much money but it would look ugly. I guy from the Dutch club installed the cb650 top end on the Benelli bottom end to retain the slope. The result is quite pleasing. There was an interesting comment about the CB650 someplace saying how odd reputation was - everyone wants to drive a CB750 with 50lb more weight and only 7 more bhp.

[Cosmo]

Reputation is indeed a strange thing......................The reason the 650 wasn't popular is because it's "only" a 650 and not sought after for the same reason a Ducati Pantah isn't.
Often seen as a little bike as it's "only" a 500, yet will run rings around anything Japanese south of a 900cc of the same vintage. Overlooked then and overlooked now, and only coveted by those in the know.
No I haven't got one..........I prefer multis myself.