Vacuum Advance Modification

[IanDMacDonald]

Timing @ idle:

http://db.tt/651xrfIx

[IanDMacDonald]

@3k, I have it at 35 degrees. close as i can get.

[IanDMacDonald]

For some reason, I could not get it started, not sure if I ran it out of gas or whatever. Ended-up foolin' with it some more and got it running. I'll have to re-adjust it, as it chugs a little at light throttle opening.

[TopCatGr58]

The bolt is not placed in the advance slot, it's just in the hole in the cover where the vacuum hose used to be. The centrifigul advance slot was resricted by me to have more initial advance. The compression was up a bit also with 171# compression test average. With the vacuum attached there was too much advance. When I sold it it got 44 mpg without vac adv.

Hey Scott,

At least I'm assuming you're Scott? Even though Yamaha says that stock compression at Sea Level is supposed to be 142psi +/- 14, but Crazy Steve has measured stock machines with much higher levels, and not from excessive carbon buildup. So he suspects that the Comp values were understated by Yamaha for some reason??

After I did the Wiseco little(1179cc) big bore kit on my 81SH and after breakin, it attained ~180 psi across all four. Because of this increased compression and concern about detonation/knock and burning holes in my new pistons, I use only Premium octane in my machine. I still have the stock vac. adv. pot connected/functioning. I have the 4Ro style TCI which has the cent. adv. curve programmed into it, but with the vac. adv. it is still supposed to be able to attain up to the maximum 52 degrees adv. at the cruising range/rpm/throttle input. With my premium fuel I haven't experienced any knock/detonation.

My question is were you concerned about knock with the higher compression and using regular octane, and that's why you retarded or limited the total amount of vac. advance?? Did you actually experience knock prior to doing the Vac. Adv. Mod??

Having the total amount of adv. limited to ~35 degrees is right at the maximum most power adv. amount as stated by Yamaha, and as has been stated, when the throttle is cracked WOT, the vac. adv. that could have been as high as 52 degrees will retard/release and drop back to the ~35 degree point which provides the POWER adv. point, from ~3500-8k rpm.
This is why folks can get rid of their vac. adv. pot and not loose any actual POWER, but can loose fuel efficiency/mileage when they are cruising because they will not be able to get the higher timing adv. that takes advantage of being able to burn more of the fuel over a longer compression/power stroke cycle.

T.C.

[crazy steve]

The SG I refurbished and sold a few years ago had 180+ compression across all four, and ran great on regular....