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Message started by batman on 12/08/17 at 13:24:08

Title: Stock cam(s)?
Post by batman on 12/08/17 at 13:24:08

Is it likely that there may be two different stock cams for our bikes? The cam number for 86 to 95 bikes is 12711-24BO5 , but 96 to present is 12711-24B11 this seems to be in concert with the change in the starter gear (anti -kickback) in 96 . Where most cams are balanced having the same valve lift , Lancer states that the valve lift to be .254 for the intake and .244 for the exhaust , and I have no reason to doubt him , but they may be numbers from the newer cam. I wonder if the lift on earlier cams wasn't .254 & .254 and changed to shorten the lift and duration and lesson the chance of kickback to begin with. All this is just CONJECTURE on my part as I have no way of comparing the cams ,so I may be dead wrong and the stock numbers may have just been changed over time.

Title: Re: Stock cam(s)?
Post by verslagen1 on 12/08/17 at 13:30:12

That doesn't make sense to me, kickback occurs on the compression stroke.
http://suzukisavage.com/yabb2.2/Attachments/valve_timing_001.jpg
opening the exhaust early wouldn't make as much sense as closing the intake early.

Title: Re: Stock cam(s)?
Post by piedmontbuckeye on 12/08/17 at 13:32:51

IMO the part number change is due to a real change in the part. When I worked for Honda, they were meticulous about that. Even the smallest change (for example an extra pin hole in the front fork tube to aid hydraulic oil movement) would warrant a change.

I am not sure about Suzuki.

It could be verified by miking the lobes of the two cams to see if there is any differences.

Title: Re: Stock cam(s)?
Post by batman on 12/08/17 at 14:03:12

Versey ,I think your looking at the wrong end of the compression stroke ,the exhaust valve lift is reduced by .010 ,that means the valve overlap is reduced and the exhaust valve both opens and CLOSES sooner which might lower the chance of the fuel igniting in the exhaust pipe and burning back through the cylinder making the piston move backward at shutdown.

Title: Re: Stock cam(s)?
Post by verslagen1 on 12/08/17 at 14:36:57

The piston moves backward at shutdown on the compression stroke to relieve the pressure of the stroke not reaching TDC. If it reaches TDC, then it continues to rotate. If the exhaust valve is open, all fumes will go out the tailpipe.

Title: Re: Stock cam(s)?
Post by batman on 12/08/17 at 15:11:34

And if the piston stops short of TDC ? bad things happen ! AS I stated earlier the exhaust valve with less lift closes earlier LESSONING the chance of the piston being pushed backward , not that it still can't happen.

Title: Re: Stock cam(s)?
Post by verslagen1 on 12/08/17 at 15:31:01

study
http://suzukisavage.com/yabb2.2/Attachments/valve_timing_001.jpg

Title: Re: Stock cam(s)?
Post by batman on 12/08/17 at 15:56:28

OK Versy , I thought the kick back occurred when the piston approached ,but stopped short of TDC on the compression stroke.

Title: Re: Stock cam(s)?
Post by piedmontbuckeye on 12/09/17 at 07:25:53

52415657484543414A15240 wrote:

Title: Re: Stock cam(s)?
Post by SALB on 12/09/17 at 09:15:52

Unless someone can show me that the duration spec on the exhaust lobe is also different(258 according to the diagram), I don't see how the lift would change opening or closing times. My theory is that, for what ever reason, Suzuki was trying to keep cylinder pressure longer, or extend the time it takes to do so. Of course, no one has shown different specs for the two part numbers, and this is all speculation.

Title: Re: Stock cam(s)?
Post by verslagen1 on 12/09/17 at 10:53:38

5F464A4B4240415B4D5A4C444A564A2F0 wrote:

[quote author=52415657484543414A15240 link=1512768248/0#4 date=1512772617]The piston moves backward at shutdown on the compression stroke to relieve the pressure of the stroke not reaching TDC. If it reaches TDC, then it continues to rotate. If the exhaust valve is open, all fumes will go out the tailpipe.

-----------

If this is the scenario, what causes the backfire? Wouldn't there be some sort of explosion in the cylinder while the exhaust valve is open? Or...is there enough gasses sent into the exhaust pipe to cause the explosion there?
[/quote]
Technically it's afterfire, and it's an accumulation of gas in a hot tail pipe where oxygen is sucked in between puffs. On shutdown it's a little different as you have unburnt fuel and ox being pumped thru by the cylinder just waiting to get the right combination of hot pipe and fuel/air mixture.

Title: Re: Stock cam(s)?
Post by Dave on 12/09/17 at 17:34:55

It is my believe that the event that can break the starter gear and case on early bikes occurs when there is a strong explosion after you shut of the ignition.

I believe that there are 2 potential causes of the explosion:
1) As the engine is coasting to a stop and the fuel/air is being pushed into the exhaust....it can be ignited. If this explosion occurs as the engine is on the exhaust stoke....the valve is open and the pressure can potentially push the piston down and cause the engine to rotate backwards.
2) It is also possible that when the ignition is turned off and the engine is coasting down to a stop.....that the engine loses momentum right after an intake stroke as the piston is on the compression stroke. If there is almost enough momentum to take the piston very near to TDC without actually crossing over....the fuel/air mixture will be compressed and ready for ignition - and I believe it is possible that the hot engine could ignite the fuel/air without a spark. NOTE - while I acknowledge the chances of this are somewhat remote....I do believe it is possible and could happen - and it is more apt to happen when a bike is really hot or running really lean.

Both of the above situations are not common - most times the afterfire in the muffler occurs a second or two after the engine has come to a stop....however if the exhaust valve is open the fire and pressure in the exhaust could go into the cylinder and push the piston down. I also believe on rare occasions it is possible for the fuel mixture to ignite in a hot engine on an "incomplete" compression stroke. Maybe?

Title: Re: Stock cam(s)?
Post by Ruttly on 12/09/17 at 21:48:40

Being that in stock condition these are lean running bikes and the slower reacting vacuum carb both of these scenarios are correct. I'm thinking the belch of power/backfire is mostly a carb & exhaust issue and then made worse by the false spark. However , the combustion chamber is loaded with sharp edges that cause preignition when heated by a lean condition add a slow reacting and a restrictive exhaust and a false spark , it's the perfect storm to spin the crank backwards hard enough to break stuff. When the exhaust backfires it's loud like a gun or small cannon. When a engine is forced to spin backwards it's the same forces involved but trapped inside the engine and it can bend a connecting rod. I can't blame it on cam timing , after installing the vm36 & header my bike has never backfired , just a tiny lean popping while jetting , oh and removed all the sharp edges in combustion chamber !
My 2 cents.

Title: Re: Stock cam(s)?
Post by piedmontbuckeye on 12/10/17 at 06:11:39

Great discussion ad very informative!

Title: Re: Stock cam(s)?
Post by batman on 12/10/17 at 10:55:22

I don't want to beat a dead horse, but the number 2 scenario that Dave gave is what I was thinking would cause the most damage to the side case and starter gear train of bikes 95 and older. When look at the valves springs ,rocker arms and cams there are only change in numbers for the rockers ,(which I believe is the change in the material used where the rocker rides the cam lobes (hardface),and the cam so that if there is a difference it must be in the cam grind .I don't have them to compare .If I'm wrong and there is no difference, my plate is set knife and fork at the ready ,and please remember I like my crow well done! ::)

Title: Re: Stock cam(s)?
Post by Ruttly on 12/10/17 at 11:27:31

You are close in 96 they started using the torque limiter instead of a solid gear and this has protected the starter & drive gears from that nasty back spin at shutdown & during a huge backfire. It's possible the cam changed but I just don't buy it , I'm sticking with stock carb & exhaust and more
The carb it to blame. I have the webcam standard grind a little more lift & duration as well as the vm36 & header which may account to not having any backfiring. And that false spark and sharp edges in combustion chamber have always bothered me. I'm glad I unknowing bought a 96 for the torque limiter. As for the rest of it every motor you build you learn some thing new therefore you next motor reaps the benefits of all the previous engines and this engine has 15 to 20 engines ahead of it. I'm never in a hurry to build one , it's always better to take your time and make it right , attention to all the little details is a must and incorporating all you have learned.
Steak Tonight ?