Noise issues on 350 Scrambler engine

[Rocla]

Hello,

I've achieved the restoration of my 350 Ducati (Mototrans) Scrambler 1974* and it runs very well. It is reliable (1000 km without any problem) with very good acceleration. But the engine is very noisy, not because of the exhaust, but because of the cylinder head. It doesn't not seem to be due to the valves, the slack is OK. So, where does this noise come from? Is it from the bevel gear? Is there any solution? Is it sufficient to replace the bevel mecanism by a new one, or is it as much noisy with a brand new engine?

Many thanks for your advice...



* pictures of my Scrambler's restoration: http://strv.pagesperso-orange.fr/motode ... Ducati.htm

[StewartD]

Ducati manual fig 22.jpg

Ducati manual page 87.jpg

Hi Rocla,

The Bevel gears are a source of noise, even when fitted perfectly. Before checking, I would ask someone who has been around Ducati singles for a while, whether they think the noise is excessive. To check them you can leave the head on and just take off the top cover.

The second attached diagram has note "The two lapped surfaces must be on the same plane". These ground areas on the bevel gears will be visible if you wipe off excessive oil. Rotate the engine until the ground surfaces are adjacent to each other. To check if they are on the same plane (or co-planar) place the end of a small steel ruler or the end of a vernier caliper against them, at the meshing point. Next shine a light behind the ruler and see if a sliver of light can be seen. If light can be seen at one gear's surface, then that gear is not close enough to the common intersection point (C.I.P.), of the drive and cam shafts, or the other gear is too close. Take a note of the situation.

Next check the meshing. Make sure the cam is in a position where both valves have clearance to their rockers. Wipe away excessive oil from the face of the meshing gear teeth. Hold the camshaft gear with one hand and try and rotate the drive shaft gear with the other hand. You may be able to see a slight movement of the oil at the teeth that are in contact, but if you cannot feel movement, I think this is acceptable*, even though the Clymer manual says "there must be no backlash". If you can feel any movement then the meshing is too sloppy. Both bevel gears or only one are too far from the C.I.P.

If you see no problems with these two tests, your noise is either coming from another problem, or it is normal for a Ducati single.

If you detect either coplanar or meshing backlash problem then the head will have to come off to either reshim the bevel gears or replace them if they are stuffed. Note that the shims are item 89 in the first attached drawing. They are not shown in the second drawing. For both bevel gears the shims are between the bevel gear and the inner ring of the bearing. The shims are 15.5 inside diameter and 22.0 outside diameter, available in 0.05, 0.10, 0.20, 0.50 and 1.00mm thickness. The first attached drawing has the circlip, item 98, shown incorrectly above the bearing, it should be below it.

If you had noted a coplanar problem but no meshing backlash problem, then you can put an extra shim under the gear that showed the sliver of light and remove a shim of equal thickness from behind the other gear.

If you had noted a coplanar problem and a meshing backlash problem, then you can put an extra shim under the gear that showed the sliver of light only.

If you only detected a meshing backlash problem, then you should add an equal thickness shim under each bevel gear.

To check results of your work, you must use a tool to keep the lower end of the drive shaft concentric to the tube. See the diagram; of course the tube, (they call it "tubing protection"), must be bolted in place.

This is a time consuming job. You need a bit of patience.

*Clymer manual says "there must be no backlash", i.e. no backlash when motor is being worked on and presumably cold. When the motor warms up, a small backlash will exist due to the aluminium head casting having a higher coefficient of thermal expansion than the steel bevel gears. If you are keen enough to go for zero backlash cold, then you must not overdo it inadvertently. If you set the gears too close, they will wear unnecessarily. To avoid this you should check that they are freely engaging. To do this it is best to remove the rockers and check full rotation, The lower end of shaft tool must be in place and the tool must be of high standard to allow free rotation of shaft.

Cheers,

Stewart D

[Jordan]

Check also the bush at the top of the bevel tube is not loose in the head. Gasket fit?

Jordan

[Nick]

Ear plugs.

[Rocla]

Many thanks to Steward and the others, it is very specific and relevant. I will check that. Anyway, according to your says, it seems that even brand new engines are noisy. Is it right? So, once the mecanism is perfectly set, it could be relevant to add a strong rubber layer in each housing of the head cylinder. Did somebody try that?

[StewartD]

Ducati manual fig 90.jpg

Hi Rocla,

I rode my 450 Desmo (1974) as day to day transport for 10 years and I never for a moment thought the noise was a problem. It's part of the deal, I loved that machine, it took me everywhere and on the few occasions it stopped, due to the simple design, I was able to fix it roadside.

The bevel gears are nearly as efficient as a chain drive in terms of frictional losses but noisier. The design of the Ducati bevel gear cam drive makes head removal a snap compared to removing the head of a machine with chain cam drive however. At some stage Dr Taglioni may have considered chain drive, but I don't think it would have been a serious starter. I don't think the chain technology was good enough then. Even Honda used a train of spur gears to drive the double overhead cams of their 1964 CR77 305cc Factory racer. In the 1950 - 1965 era, race machines that had overhead cams nearly all drove them with either bevel gears, (Norton, Velocette, Jawa, NSU, B.M.W., Moto Guzzi, C.Z.,), or straight cut spur gear trains, (MV Agusta, Gilera, Mondial, Moto Guzzi V8, Benelli & Honda). The only successful race machines of this era to use chain drive for the camshaft are the Matchless G50, A.J.S. 7R and the Moto Morini 250cc of 1958.

The choice of Bevel gear drive for the high performance overhead cam 250 road machine would have been a no brainer. The option of a train of spur gears would have been ruled out due to the noise of all those straight cut gears meshing. The use of Helical cut spur gears could have reduced the noise, but this would have required each gear to have thrust bearings to take the transverse loads that helical cut gears induce. Very heavy and expensive. The use of chain drive would have been ruled out due to rarity, unreliability, heat masking problem of the drive casing and difficulty of assembly.

Check the attached picture. The Ducati bevel gears are not straight cut which is the cheap way to do it. They are spiral cut, this is more expensive, but results in smoother, continuous engagement/disengagement. The major benefit of this is reduced noise. In the early 1960s when the 250/350 narrow case singles came out, an all alloy, overhead cam/spiral cut bevel gear cam drive engine was state of the art technology for a production machine. Compare this level of technology to what else was around. The engines of the 250cc BSA C11s, Royal Enfield Crusaders and Triumph Tiger Cubs were pushrod valve actuation and had cast iron barrels - grey porridge all!

To line the cam box with rubber or put rubber blocks between the cooling thins is not a good idea. The rubber lining will reduce heat transfer from inside the head to the outside cooling air. Also the rubber lining could become dislodged, or break up and block the oilways. Rubber blocks between cooling fins would be held in derision by most Ducati afficianados, - it would start to look like a 'nice' Japanese machine!

What I'm trying to say is that the Ducati of the mid 1960s is an absolutely iconic design. The performance was a revelation; the 250 won the Barcelona 24 hour race outright, the 250 won the Isle of Man production race. That they were able to continue with very similar designs until 1974, and moreover carry a lot of the design over into the bevel drive twins shows the soundness of Dr Taglioni's design.

Cheers,

Stewart D

[JimF]

There are many YouTube videos that you can watch and listen to that might give you a better frame of reference.

Also you could start and rev your bike in a video, put it up on YouTube and then post the link here to solicit some commentary on whether the noise is excessive or normal, and if abnormal what it might be.

Jim

[Bevel bob]

I rode a Mach 1 from new, I do not remember any clattering but there was always a scrapey rustling whine unlike anything else. I always admired the Velocette for its mechanical silence.Narrowcase singles were around from the late 1950's.My first sight of an Elite off the Old Kent road London around 1960 left me with a lifelong affliction!.I cycled to Elite Motors of Tooting to be able to touch and hear one in action ,the salesman started one up and allowed me to work the throttle,Bliss!, there was nothing remotely like it , a Ferrari amongst Dump trucks.When i got my current 250 (50 years later) i was delighted it had an Elite Motors badge riveted to the rear mudguard.

[Rocla]

Hello again,

"Rubber blocks between cooling fins" would be heretic! It would be sacriligious. I don't want my Ducati looking like a Japonese bike By far!
Anyway, I've noted your remarks about rubber blocks inside but I think it could be efficient. Have you noticed that there are already rubber blocks inside the head cylinder, in front of the rockers path external extremities. This kind of rubber is strong enough, and have a udge silencer power.

But it is not obvious to stick such lay inside the bevel housing (the aluminium part with the 4 screws) but I don't think that this part (the housing with the 4 screws) is usefull for head transfer because there is no fin on it...

Of course, I agree, noise is a part of the deal. And such a deal! This bike is one of the most mythic and beautiful. But I am sure the mine one is especially noisy. I'll check the bevel gear. If I compare with Youtube Videos as JimF suggested, in most of the cases, my 350 is noisier. Some videos show very "silent" engines, in comparizon with the mine one. On my bike, the clinking noise covers the silencer's sound! Maybe the gears have too big slag, or are not at the perfectly right position...

[ducwiz]

Hi,

other noise sources I experienced while working on a lot of w/c engines between 1980 and 2000:

- excessive backlash at the coupling between lower and upper bevel shafts, and/or inner dia. of coupling sleeve too large
- steel pot containing upper bevel bearing loose in it's bore in head; it has to be slightly pressed into seat by bevel shaft's protective sleeve. You can adjust this by the thickness of the gasket and/or by adding a suitable shim between bearing's outer race and the small pedestal on the protective sleeve's top
- steel pot holding lower bevel shaft bearings loose in it's seat in engine carter; cure as described above
- incorrect adjustment of the bearing pair's end play holding the lower bevel shaft
- incorrect shimming of crankshaft, too much end play; due to the primary skew gears a small lateral movement of the crankshaft is forced by each combustion cycle
- incorrect adjustment of lower and upper bevels; see http://i.minus.com/1377259954/RsUcCTfpEBllAckIwmvEXQ/d8gmyql9spk4P/mesh_job.pdf

cheers Hans