Billgran wrote:
"K Smith" wrote in message
...

This was because
they slow the idle speed by using a very much retarded spark timing,


Ok, Karen, give specifics,

What is too much retarded spark at idle? List the top 3 makes of engines and
what timing they idle at?

Bill Grannis
service manager



Again you distract me from the Why Ficht failed threads & I'm busy
enough, but you have asked a genuine question without abuse so I better
respond, in kind:-)

This is related to the basic problem of getting particularly bigger
multi cyl 2 strokes to idle slowly & smoothly. The problems to be
overcome are;

(i) The exhaust is submerged & notwithstanding some bleed holes, at
idle the exhaust sees quite some back pressure (e.g. see the water the
bleeds spray) & that back pressure is "various" depending on how deep
the leg is.

(ii) Unlike a 4 stroke where they can just shut the butterflies &
restrict the air/fuel going into the motor, the OBs can't because
coupled with the exhaust back pressure they run the risk of back flowing
or spitting at idle. (i.e. if they created a very high vacuum in the
crankcase, with little air & fuel, as the transfer port uncovered the
exhaust would flow back into the crankcase, rather than overcome the
exhaust back pressure & go out there. It is likely to ignite the charge
in the crankcase ) The fix is to keep a fair amount of air/fuel flowing
through the engine even at slow idle.

(iii) Because despite the throttles being against their stops, the
butterflies themselves (or in some a bypass) still allow plenty of air
into the engine which means it will have a very fast idle.

(iv) The fix since very early on is to just keep retarding the spark
timing at idle till a slow soft smooth idle is achieved.

(v) A retarded spark timing has always been known to cause heat
buildup, even in 4 strokes, however with an endless supply of cooling
water & very low temp thermostats, the OBs have pretty much gotten away
with it. Not so the motor cycles, they didn't have access to the sea
water:-) so had to run much more normal spark timing at idle for fear of
overheating & you could see it in there idle's performance, it was
usually lumpy at best & terrible more likely.

As a good starting point use anything later than 10 BEFORE TDC as being
retarded enough to build heat.

You have the manuals as I do, some like merc regularly give a number &
it's invariably extremely retarded at idle e.g. a 200?? 12-15 degs AFTER
TDC which is up to 25 degs later than it ought be, if heat buildup is to
be avoided.

Yamaha ?? The bigger Yamahas are around 5 degs AFTER TDC again well too
late (15 degs) if piston heat at idle is to be avoided.

OMC They don't tell, (that's why years ago you were so totally unaware
that they even controlled idle speed almost exclusively with spark
timing I still can't believe you didn't know that!!) However you can see
how extreme the retarded timing is in the bigger OMC engines if you look
at the pickup point timing, on some even that is still AFTER TDC or very
close to it, now see how much further the system moves in the spark
retard direct after the pickup point has been left???

I should thank you again for at least asking a real question so.....
thanks, however again may I take it you still have no problem with the
substance of the post itself?? if so I'll move on when I get a minute,
busy busy busy, not fair:-)


K.