3.5 to 4k Detonation ... grrrrr ..

[Bullet]

I got 2500km on Blue balls now, the bike runs perfect..

But, I got that annoying detonation thing going on between 3500 - 4000rpm.

Unhooked the vacuum line to the advance for a test run, noise is gone ..

I am running the stock vacuum advance right now, base timing is set 5 degrees.

I was lucky enuff to get a brand new service bulletin replacement vacuum advance a while back ( by accident ) part # 3H5-81683-09-00 as described in Yamaha's TSB from a local ma and pa dealer, box was dusty, and probably sitting in their stock for 30 years, lol .. ..






Here is the difference between the factory installed VA and the TSB replacement part.

At rest, they both have the same arm length ( I measured from base of the advance to the hole in the arm ) 44.85 mm

[IMG][/IMG]

The stock advance comes in full at the 5.9 in/hg with a travel of 7.62 mm

The TSB part comes in full at 3.8 in/hg with a travel of 2.79 mm

pic shows the difference in arm travel at full vacuum.



So there is the difference between the 2 parts ..

I haven't tried the TSB part, but am gonna swap it out after dinner and go for a test run .. will report back ..

[Bullet]

Well, this ended up different ..

I took the original vacuum advance off the bike, and for fun thought I would test it too..

It has a problem .. no leaks, but it would not pull full travel until 8.9 in/hg ..

I checked it when I put it on the bike when I was putting it together, and it was 5.9 then ..

So, long story short .. I put another stock vacuum advance which gets full travel at 5.9 in/hg , and went for a drive ..

No more noise ..


So I will not put the TSB advance on the bike .. no need to now ..

Anyway, for people with the detonation issues, get a vaccum gauge and check when the full travel comes in on your advance .. 5.9 in/hg is the magic number ..

Bullet

[DGXSER]

Great info, thanks!!

[TopCatGr58]

Hey Bullet,

Well, that is interesting. The OEM one pulls almost twice as far as the "FIX" one, so it should provide much more advance, and IIRC, too much advance is what can cause detonation. It sorta makes sense that the FIX one would provide it's full effect sooner with less vacuum, but only 1/2 as much pull/range of advance, so that IF you had a bike that was Detonating under cruise at 3-4krpm, that it was most likely getting too much adv. and so the FIX unit would provide LESS adv. to reduce the detonation!?

And this is what puzzles me with your bike and it's symptoms with your failing unit. From your description, it was requiring MORE vacuum to generate the same amount of advance, and so it was probably actually providing LESS advance at the same throttle cruising input? But perhaps this also contributes to incorrect ignition timing since the cruising A/F ratio is leaner, but it wasn't getting enough adv to take advantage of the leaner A/F ratio?

And with the properly functioning regular Adv. it again was providing the appropriate amount of adv. at cruise to be able to handle the leaner A/F ratio!
T.C.

[Bullet]

Yeah it is weird..

I haven't change the carb setup since I put it on the road. Bike still runs and pulls the same since day 1 ( maybe a little better, since the engine is broken in now)

The original advance I installed when I put the engine together was checked for vacuum and it was at full arm travel at 5.9 in/hg.

The detonation noise started when I had 800 km or so on the bike.

I put the advance I pulled off the bike ( 8.9 in/hg ) on the vacuum gauge all night, and it is holding full travel still at 8.9 in/hg, so the diaphragm is not leaking. Why it went from 5.9 to 8.9 in less than a month is a mystery, I was not a new part, just a good lookin/functioning one I got from a parts engine.

Like I said in the other post, I may install the service bulletin advance for a couple days to see what the power/perfomance difference will be.

[Bullet]

For the spirit of the XS Science, I threw on the Service Bulletin vacuum advance this morning and went for a ride.

I changed nothing else except the vacuum advance. ( carb setup, timing still the same since day 1)

Bike still runs good with service bulletin advance, pulls pretty much the same all the way up to 8k, except there is a minor lag when you crack the throttle at anything above 4k in any gear.

The lag is not huge, but was noticeable compared to the stock vacuum advance which was on the bike had no lag at all. ( crack the throttle and your gone )

I would think back in the day, no one would complain about the teeny weeny lag in order to get rid of the annoying detonation noise, so I guess this was a good trade off as far as Yamaha was concerned.

The 3500-4000 detonation noise is not there with the service bulletin advance either.

So I guess the service bulletin part works as described by Yamaha, but it does suck away some of the throttle snap when using the stock advance..

Gonna put back on the stock advance, I'm kinda missing the instant throttle response..

Bullet

[ManagerMike]

Spark knock, or detonation, in any gasoline engine is more appropriately called "preignition", a condition defined as the air/fuel mixture igniting before optimal time in the cylinder cycle. It can be caused by a number of factors, among them; low fuel octane (not likely), a lean air/fuel mixture which increases combustion temperatures, or more commonly, incorrect ignition timing (to advanced).
This is why the car or truck you drive (assuming it isn't as old as your bike) has what is called a knock sensor. It 'listens' for preignition and therefore allows for the maximun timing advance under varying engine loads and throttle angles. When the knock sensor 'hears' preignition the ECM retards the timing to eliminate the "knock".
Our bikes' engines don't have computers running them so we have to balance these things for ourselves. This post was only intended to offer more info, hope it helps!



































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