Triumphrat

Zarcero · 01-02-2008, 09:05 AM

Can I set the the timing on the Bonnie closer to TDC to compensate for altitude changes? We used to be able to do this on the older bikes and I am wondering how this is now handled on the newer bikes. I have already been met with dismay when asking this question on another board, and was helping someone here could assist me as I am planning on getting a new Bonnie. I'm a re-entry rider, so my new technology IQ is low. On my smaller Nighthawk I can check the timing with a timing light, but can't adjust it. Did this adjusting capability disappear with the magneto? If so, what do we do now to optimize the compression stroke at higher elevations where the air pressure is lower.

Thanks,

Z

bonnieblackinfl · 01-02-2008, 09:11 AM

No, the timing is fixed. You can however advance the exhaust cam 1 tooth, 15*, to increase you dynamic compression ratio. This is a good mod for the '01-04 790 bikes, I do not know of anyone who has done it on '05-present 865 models. The cam profiles changed between the 790 and 865. Perhaps someone can chime in that has done this with results.

Greg

geolpilot · 01-02-2008, 10:30 AM

If you are going to be at high altitude for a sustained period of time you may need to change your main jets to smaller ones because in the thin air, you will be running too rich. I don't know why you would want to change the timing, the problem is mixture.

Zarcero · 01-02-2008, 12:34 PM

Quote:

Originally Posted by geolpilot

...I don't know why you would want to change the timing, the problem is mixture.

Mixture is only part of the problem. I take it you are a pilot. Read The Ravens and the problems they had with their FAC aircraft (Cessna propeller aircraft) in Laos on the Plain of Jars. The problem wasn't solved until the timing was set correctly on their reciprocating engine aircraft. The same is true for autos, motorcycles, lawnmowers and anything else running on an Otto Cycle.

You advance the timing to delay the spark. This allows the piston to compress more air before the spark fires. The additional compression is needed since at the higher altitude the intake air pressure is lower (and hence at a lower density). The additional compression of course gives you a more efficient power stroke.

Cheers,

Mike

mikeinva · 01-02-2008, 01:07 PM

Mike i think you got some things off advaceing the timing makes it fire more before tdc .I have been a mechanic since the late 70s and this is the first i have ever seen anyone advance timing for higher altitude.as air denstey goes down you just lean the fuel out and you will be fine.Unless you are raceing your bike if you get it jetted right on low ground it wont kill it at high altitude. And no these bikes do not have a timing adj. These bike seam to be more forgiveing being rich then being lean.
maybe what you are talking about is leaning the motor out so much that you have to retard the timeing to keep it from detonation but you dont want that on our bikes.

Ben Rawlinson · 01-02-2008, 01:07 PM

Timing is fixed when at full advance but unless you are planning to fly I suspect that you will not encounter any problems.....

CAPTDON · 01-02-2008, 02:30 PM

Unplug the TPS & see how that does? Should do the trick? If not reconnect.

modre · 01-02-2008, 02:59 PM

Quote:

You advance the timing to delay the spark. This allows the piston to compress more air before the spark fires. The additional compression is needed since at the higher altitude the intake air pressure is lower (and hence at a lower density). The additional compression of course gives you a more efficient power stroke.

anyone have a rhyme for enema?

Zarcero · 01-02-2008, 03:30 PM

Quote:

Originally Posted by mikeinva

Mike i think you got some things off advaceing the timing makes it fire more before tdc .I have been a mechanic since the late 70s and this is the first i have ever seen anyone advance timing for higher altitude.as air denstey goes down you just lean the fuel out and you will be fine.Unless you are raceing your bike if you get it jetted right on low ground it wont kill it at high altitude. And no these bikes do not have a timing adj. These bike seam to be more forgiveing being rich then being lean.
maybe what you are talking about is leaning the motor out so much that you have to retard the timeing to keep it from detonation but you dont want that on our bikes.

No it's the semantics of advancing/retarding where you are probably disagreeing with me. The semantics issue has been around since back when we used to do this. By "advancing" I mean we are going to rotate the distributor cap (on a car) so that the tick mark shows that we are closer to TDC. Say for example going from 10 degrees to 5 degrees. This means the spark will fire later, hence we are "retarding" the spark. However, some mechanics use the perspective from when the spark fires, hence they "advance" the firing of the spark by moving away from TDC.

Okay, I can see this discussion is not going to work out well based on the replies so far, and from the quip up in the thread about an "enema". I'll just leave y'all with the link below from Dan, and for those who are interested in the topic you can research it further on your own. In any case my question was answered in that we can't adjust the timing.
http://www.dansmc.com/IGNITIONTIMING.html

Quote:

Now when we increase the spark advance (the point when the ignition spark occurs, at the sparkplug, before-top-dead-center of the compression stroke [BTDC] ) we get more power BUT we also get more heat. There is a point after which we get lots more heat and very little extra power. We want to STOP before we get to this point!

I would read the whole article. You can see even Dan goes back and forth on the semantics/perspective issue. Nevertheless, he does point out we want to get closer to TDC. I would use a timing light as opposed to his suggestion of hearing the revs, and also use a compression gauge. On a bike like a CB250 you can use a timing light, but the ability to advance/retard is no longer there. They must have left the timing window on the bike as a relic.

Oh, and BTW I just got back from visiting the Triumph dealer here in Houston (on College). He reports that the '09 Bonneville will have fuel injection.

Cheers,

Mike

modre · 01-02-2008, 04:10 PM

[QUOTE]You advance the timing to delay the spark.

advance is earlier retard is later...forget semantics and do it this way for universal clarity...it would be short sighted to think we're all stupid and inexperienced.

> This allows the piston to compress more air before the spark fires.

not more air, same air volume under higher compression at the ignition point

> additional compression is needed since at the higher altitude the intake air pressure is lower (and hence at a lower density). The additional compression of course gives you a more efficient power stroke.

the compression at elevation will at best reach somewhere below design specs. I think you're confusing compression ratio with ignition point. your scenerio doesn't change the compression ratio since it's based on ambient to compressed. not an absolute 14.7 at sea level.

stand on your head, it's still a 9.5/1 CR from the bottom of the sea to Mt Everest. If you want more air to compress, look at a blower, shave the heads, or increase the stroke.

playing with timing doesn't change the compression ratio in any way shape or form...at best it only changes the ignition point under compression, and wouldn't gain anything beyond the stock 9.5/1 other than inhibit performance. no matter when or where you ignite the bomb, it pushes back at 9.5/1 depending on the ambient air pressure of your locale.