Piston Weights

[scottmastrocinque]

Out of curiosity, how many Moto Guzzi motorcycles have you owned?

What are you making your opinion based upon?

 

[scottmastrocinque]

I think a bent crankshaft would result in misaligned pistons and scored cylinder bores.

Jason

Not necessarily.

Precisely where would you attribute the imbalance then?

Here is the G5 crankshaft and carriers.

He claims bearing surfaces are fine and no runout is noted.

I’m curious where you think his vibration is coming from?

 

[JasonC]

Not necessarily.

Precisely where would you attribute the imbalance then?

Here is the G5 crankshaft and carriers.

He claims bearing surfaces are fine and no runout is noted.

I’m curious where you think his vibration is coming from?

Loose motor mounting fasteners can cause "engine vibration" in motorcycles that have their engine rigidly attached to the frame. Hence my previous post about loose motor mounts being a source of vibration. Another possible source of vibration is the drive shaft and/or its interface joint with the transmission output shaft.

It seems unlikely, to me anyway, that the crankshaft would be bent if the run-out is within specification and there is no cylinder bore scoring/scuffing, which would suggest a misaligned rod bearing journal.

Jason

 

[scottmastrocinque]

The vibration is present even when the bike is on the center stand and I rev it up in neutral. The vibration depends solely on engine rpms, not road speed.

Loose motor mounting fasteners can cause "engine vibration" in motorcycles that have their engine rigidly attached to the frame.

Unfortunately, the OP statement above negates your idea.

Also, it is virtually impossible for a Moto Guzzi block to have a “loose motor mounting”.

Not once have I ever encountered one on any Moto Guzzi motorcycle, ever.

The vibration pattern of a loose motor mount would also be erratic and not harmonically linear and tied to engine speed as the OP indicates.

 

[PerryB]

This is my first and only Guzzi. In the early 1990's my brother owned a 850 LeMans III that I rode a lot. It was very smooth under all conditions, with the seat the only painful spot. The vibration that I get from this bike is not normal. I also owned a 1972 Ducati 750GT. While I don't have much experience with v-twins past these three bikes, I am confident that the vibration this bike produces is abnormal and a symptom of a something seriously wrong in the engine.

When I originally rebuilt the engine, I fitted 2nd oversize pistons. These pistons bare had a 5 gram difference in weight. The machine shop that I took all the bits to assured me that they could make the pistons closer in weight. I believe that they took most of the weight off of the pin and the small end of the connecting rod. They then did the calculations needed to get a 52% figure for the bob-weight and dynamically balanced the crank with the flywheel and ring gear. I reassembled the engine and it has vibrated ever since.

I have often thought that the weird exhaust I have is the source of the problem, but I removed it and ran the bike and it still vibrated in the same way at the same intensity at the same rpms.

I even went to the trouble of disassembling the engine and taking all the parts to a different machine shop and had them check the balance job of the first shop. No revelations from this second redo.

I've put about 14,000 miles on the bike since the initial rebuild and the only problem I've have with the engine is that the alternator rotor shorted out early in the life of the bike.

The vibration is bad enough that my hands go numb if I ride for more than about two hours and it really prevents me from riding the bike nearly as much as I would like.

 

[scottmastrocinque]

I honestly don’t know what to say.

 

[PerryB]

Exactly. I've been stymied by this for the last 14 years.

As I do the next rebuild, I will document all what I find as I disassemble the motor. I will also show all the work that I do to balance everything for this next go-round.

Thanks for the input from everyone.

 

[scottmastrocinque]

Good Luck!

 

[JasonC]

The vibration is bad enough that my hands go numb if I ride for more than about two hours and it really prevents me from riding the bike nearly as much as I would like.

Perhaps the numbness can be attributed to handlebar grip pressure against the palms, rather than vibration.

Jason

 

[jr1967]

Did your balance shops take the mentioned 27 grams for oil into account?
Other than that I see that you did a kill switch test to check vibration, but you mention 3000 rpm.... Guzzi's smooth out above 4000 rpm and shouldn't really be driven below that. If you are totally aware, forget that I mentioned it. But some people try to ride a Guzzi like a Harley.

 

[PerryB]

Yes, the shop considered the oil volume in the rod journal, behind the plug.

The engine sways the whole bike side to side at idle, then, at very low rpms (below 2300) the engine is smooth-ish. As the revs climb there is a strong increase in vibration between 2500 and 3100 rpms. From there the engine smooths out some, with the smoothest section between 3800 and 4300. After that vibration increases in frequency and intensity until redline. The worst section is between 2500 and 3100 rpms, but the vibration is always there, just at different frequencies and intensities.

I know that all this is subjective and my descriptions rely on my limited ability to articulate what's happening. I wish that I had some objective, quantitative way to measure the vibration and compare it to other Guzzis.

 

[scottmastrocinque]

The more you elaborate, the more inclined I am to think that you are just describing rocking couple and the operating characteristics of the Carcano 90 degree twin engine design.

Especially your description of the motorcycle at idle. Sounds perfectly normal.

This is all academic anyway because without being able to actually start, listen to, and touch your engine while it is running, nobody could adequately declare a definitive opinion based upon facts.

Where are you geographically because you need to see, touch and feel, other 90 degree twin Moto Guzzi engines. Somebody has to be near you.

 

[PerryB]

Believe me, the thought that this is just normal vibration and I am over stating the severity has occurred to me frequently. Unfortunately, I live in a remote rural area and there are no other Moto Guzzis near me. I have not ridden any Guzzis other than my own since my brother sold the above mentioned LeMans 111 in the 1990's, so a direct comparison is impossible.

As I think about it, my brother also owed a 1981 Mike Hailwood Replica Ducati and a 1987 Ducati Paso 750. Those other two v-twins didn't vibrate at all, especially compared to what my Guzzi does.

I really think that this a problem that must have a cause and, hopefully, a solution.

As I do this rebuild with the new pistons, I will document all that I do to try to diagnosis and eliminate this problem.

I'm sure that I will ask for advice from this forum in the future.

Thank you.

 

[jr1967]

I trust that your engine is properly balanced after all that you have done. What I found is that sometimes, because of throttle cable routing, the throttle slides don't open equally through their range. If you only balanced the carbs at idle, then this could be the problem. If you did it properly ignore the rest that I wrote down. (Check this with vacuum gauges or as a poor man's tool, with the throttle set screw at a small opening, check with various size drill bitts under both throttle slides if they are at the same height.) Oh and check the slides for the same number, like 60/3 or something like that. Also could it be that the choke of one or both carbs is not sealing properly anymore? The small rubber discs can be reversed if needed. Also check your carb intake rubbers for leaks. Best of luck, I hope you'll find the problem!