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Discussion Starter · #1 ·
Looking for information on upgrade for cam chain tensioner, and the modification to add a primary transmission chain tensioner.

Tom Beasley
1979 750 SEI (purchased new)
 

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modification

Hi Tom,
As a future rider of Benelli 750 sei I would like to ask you a questions.
What made you to think it is a good idea and wanting to fit a primary chain tenssioner to you bike?
Carles mentioned noises coming from the engine and he thinks it is caused by primary chain.
What are the signs of this problem in your bike Tom?
Cheers
Jerzy
 

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Discussion Starter · #3 ·
primary chain

Jerzy,

I am not suggesting that I wanted to make any modifications. I was only inquiring to see if anyone had done this. The hy-vo primary chain is known to last for a very long time. I have checked mine several times, and it does not seem very loose. It is nowhere near to the point where it would be contacting the case. It has been my experience over the 35 years and 30 plus thousand miles I have put on this machine that the noise comes from having the idle set too low or the engine is not running smoothly, needs carbs adjusted, engine timing, etc. The condition seems to be worse when the chokes are on and before the engine is not thoroughly warmed up. I am attaching information about what someone has said about this condition and what they have done.I have also attached a pic of the chain in my bike. Maybe they are trying to sell modified machines at a very high price. Motorcycles are only original once. They can be restored many times

Tom

pruebas


Primary Chain.jpg
 

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Jerzy,
I am not suggesting that I wanted to make any modifications.
Well.I got it wrong then,but my way of thinking is when one wants to investigate, then is up to something,doesn't it?






I was only inquiring to see if anyone had done this.
I am doing it now.

Maybe they are trying to sell modified machines at a very high price.
One can always try to sell but without the buyers there will be no sales.What I think is that there are still some good people wanting to help others and share own experience.

Motorcycles are only original once. They can be restored many times.
One can put a new bike in museum or own bedroom to keep it original, or ride it and keep improving things as many times as needed to own liking in order to bring it to today's standards.

Cheers
Jerzy
 

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Hi Tom,
Now since the primary chain tensioner came to existence in my Benelli's 750 sei engine, I can proudly show the photos of it.
Because any modern motorcycle engine will have one(primary chain tensioner) I think this is a great improvement to any engines that don't have it.
IMGP1935.JPG IMGP1936.JPG IMGP1938.JPG IMGP1939.JPG IMGP1940.JPG
Cheers
Jerzy
 

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Great work, Jerzy! :clap:and great pics to see clearly this area.

As you mention I still suspect that the noise in my SEI comes from the primary chain. Seems to come from the rear sprocket (the opposite from the crankshaft one),it sounds like if the chain was jumping over the sprocket -only at low revs and high torque-. The problem is to have a look there... :doh: Is there any way to check this sprocket without needing to disassembling the whole engine?.

Cheers.

Carles
 

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I'm resurrecting an old thread here, but I did a recent job that's relevant to the discussion. Being a mechanical engineer and classic motorcycle enthusiast, a friend asked me to give an engineering assessment of his proposal to install a primary chain tensioner on his 900 Sei. Here is part of my response to him:

Most (if not all) bikesthat use inverted tooth type primary chains (Morse, Hy-Vo, etc.) don’t have tensioners.


Pre-tensioning of inverted tooth chains is generally not necessary, especially with short chain runs typical of motorcycle primary drives. A tensioner is only considered necessary when there is a need to eliminate backlash on reversible drives, or if there is a large distance between sprockets (as with a cam chain for instance). Neither of those conditions exist in a motorcycle primary drive.


A rule of thumb is that the drive should be designed with shaft centre distances such that the sag in the chain is less than 1% ofthe distance between sprocket centres. That’s when the chain is new. It’ll get a little slacker as the chain beds in and wears during service. 2 or 3% sag is quite OK. But if the chain drive is noisy because it’s slapping all over the place (more than 3% sag), you need a new chain rather than a tensioner.


So, I wouldn’t recommend a tensioner. It may quieten a drive that’s noisy due to the chain being worn out. But that’s not a really a good thing. You could end up leaving the chain in service too long, resulting in carnage if it eventually breaks.
 

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Another pont that hasn’t been touched on above is wear on the crankcases. When I acquired my 654 it had done less than 10,000 miles but it had dropped a valve so I had to rebuild the engine. When I took the crank out there was evidence that the primary chain had been in contact with the lower crankcase. I don’t expect that the chain could ever wear right through the engine casing but it would be nice to prevent it from touching.
 

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Yes. Good point Richard. I hadn't considered that because I wasn't aware it was possible for the chain to hit the crankcase. It probably won't do any real harm, but grinding bits of aluminium off the inside of the engine is not an ideal scenario! If that's happening with a chain that's within wear limits, then I'd conclude that the designer had not allowed sufficient running clearance.


Poor design is a bit of a disappointment to me because I'm a 900 Sei owner myself. I probably should have declared that from the outset. I also have a Kawasaki Z1300 that has two inverted tooth primary chains (the drive is split by a jackshaft between the crankshaft and clutch).


Yes, I evidently have some kind of multi-cylinder motorcycle disorder, but it hasn't yet reached its zenith because I don't have a Honda 6 (although I'd like one).


There are a number of parameters that effect drive chain efficiency (whether roller or inverted tooth type). Everything else being equal, efficiency is greatest when there is zero slack, but also zero pre-tension (pre-tension loads the bearings, increases friction and shortens chain life). Spring loaded tensioners obviously depart from the "zero pre-tension" condition. So if I were to do anything to prevent the chain hitting the cases on my own bike, I'd be more inclined to install chain guides rather than a tensioner. A guide could take the form of an adjustable, but non spring-loaded tensioner. Similar to that fitted to Laverda primary drive chains.
 

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From my own experience must say that I found a considerable improvement installing the primary chain tensioner. For me, it worths the effort absolutely. The chain, at least in my 750 SEI, and others I have seen, has a considerable play, making some knoking noise when idling. Some play ever must be there, to prevent major failure, so the main aim to install a tensioner is not to increase the tension on the chain but to avoid it having too much play at low revs. And that is what the tensioner does perfectly. And yes, I have seen pictures from SEI engines wich has been scratched by the chain contact by an excessive slack...
Furthermore and not least, we must not forget that the tensioner comes from the Honda CB 650 four (with an identical engine design like the Benelli in that area). In the japanese bike is an original part, so is clear the Honda enginers put it there for any reason...

Carles.

I'm resurrecting an old thread here, but I did a recent job that's relevant to the discussion. Being a mechanical engineer and classic motorcycle enthusiast, a friend asked me to give an engineering assessment of his proposal to install a primary chain tensioner on his 900 Sei. Here is part of my response to him:

Most (if not all) bikesthat use inverted tooth type primary chains (Morse, Hy-Vo, etc.) don’t have tensioners.


Pre-tensioning of inverted tooth chains is generally not necessary, especially with short chain runs typical of motorcycle primary drives. A tensioner is only considered necessary when there is a need to eliminate backlash on reversible drives, or if there is a large distance between sprockets (as with a cam chain for instance). Neither of those conditions exist in a motorcycle primary drive.


A rule of thumb is that the drive should be designed with shaft centre distances such that the sag in the chain is less than 1% ofthe distance between sprocket centres. That’s when the chain is new. It’ll get a little slacker as the chain beds in and wears during service. 2 or 3% sag is quite OK. But if the chain drive is noisy because it’s slapping all over the place (more than 3% sag), you need a new chain rather than a tensioner.


So, I wouldn’t recommend a tensioner. It may quieten a drive that’s noisy due to the chain being worn out. But that’s not a really a good thing. You could end up leaving the chain in service too long, resulting in carnage if it eventually breaks.
 

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My 500 LS also suffers from what seems to be a similar horrible rattle that seems to be coming from the same area, around the sprocket, pulling the clutch in makes no difference.
It is noisiest at tick over, I also suspected the hy-vo chain, it really is a very worrying clatter, as the rpm rises above 2500 it is quiet.
I have started it up with no clutch plates fitted, the noise ceased completely, no matter how slow the engine runs, so that rules the hy-vo chain out, there must be something in the gearbox or clutch, so, I have fitted new clutch basket bearings, and tried the spacer in various positions, to no avail.
I have looked on a Honda 500 forum, and this fault is well documented, it is thought to be gearbox 'chatter' caused by uneven running between the cylinders at tick over, caused by incorrect settings of carbs or points, this sounds plausible, though I have very carefully set everything up and double checked, I have a dodgy camshaft, and skimmed cylinder head of unknown amount, the other unknown quantity I have yet to try, are the advance/retard springs.
Unfortunately, I cannot experiment further, as the cam chain adjuster broke causing a lot of damage that I cannot put right, because I also broke my humerus in 3 places it turned out, so I'm out of it this summer.
 
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