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Any ideas why my bike looses power at freeway speeds (cont)?

  • Thread starter Thread starter Dave_A
  • Start date Start date
Earl,

Re: EMPIRICAL ENIGMA

Hmm. That is an interesting observation, Earl!

The flattening of the engine acceleration (especially if the engine is under load) is a common one and can be caused by one or more of a number of problems. The unilateral drop in vacuum for the offending cylinder, I suspect, is due to Inefficient burn in the combustion chamber, resulting in reduced velocity of flow of gases through the head, combustion chamber, and exhaust system. This, again, may be affected by one or more of a number of tune parameters. If the vacuum disparity, at the initial flattening is remedied by correction of the induction leak, without an inadvertant altering of any of the other tune parameters, I am inclined to suspect the vacuum leak to be rather large or the stoiciometry of the mixture to be very critical at this induction velocity. I would like to understand this further and would be open to any suggestions on this. Yeees. Vvvery interesting!!

I am sure, since first making this observation, you have given it considerable thought. If the ?keg model? doesn't hold up, based on properties of fluid dynamics, what is your present working hypothesis of this phenomenon??

'This, Dr. McCoy, appears to defy logic?

Junkman

P.S. Was the engine under load and what kind of carbs were you testing (CV or Mechanical link)?
 
The carburetors were Mikuni VM26mm (mechanical) and all vacuum measurements were taken without the engine being under load.

As for my hypothesis, I suspect the leak introduces a secondary venturi pattern blending with the primary pattern and causing a cyclic induction throughflow mismatch. The intake stream is not a constant flow, but one that stops and starts. It may be similiar to the pressure wave timing on a two stroke tuned exhaust. If the pressure distribution is out of time on a two stroke pipe, it can easily result in the engine developing less power than it would wihout any pipe at all. Perhaps the pressure wave resonates at the wrong frequency. I believe induction tuning and mixture in high rpm ranges is critical. Here's an example of how slight an amount I have found to be critical.
I tuned up the 750 a couple weeks ago. New plugs, new points and condensers, cleaned air filter, set float bowl levels,new fuel lines, new fuel filter, static and dynamic timed it for both pairs of cylinders and synchronized the carbs. I also checked voltage levels at different points in the ignition wiring. (you know how those cranky connectors get corroded and give problems..:-) ) The bike started easily, idled well and ran normally. I took it for a test ride. I have to travel about 15 miles to reach
"my road" where traffic is scarce and I can let the bike stretch its legs.
The bike was well warmed up, running fine without any kind of miss or hesitation in any range and power was smooth and steady. Top speed
turned out to be about 110 mph. Something definately wrong.
Came home, rechecked everything. Nothing out of spec. I pulled the plugs out and saw that #1 and #4 were buring light brown while #2 and #3 were lighter (maybe a dark tan). I removed the carbs and turned the fuel pilot screw out on #2 and #3 about 3/16ths turn. Put everything back together and went for another ride. I believe we all accept that the amount the idle range fuel pilot screw contributes to the overall mixture is slight. I found that opening that screw by 3/16 turn on only two carbs resulted in a gain of 12 mph in top end speed. The bike now runs 122 mph and that was the only change I made. So, it appears the mixtures for optimum top end performance are critical.

This probably doesnt agree with fluid dynamic theory either. :-)
:-) Seriously, I expect that all my findings do agree with theory but there are variables not being allowed for.

Earl





Junkman Frankenbiker said:
Earl,

I am sure, since first making this observation, you have given it considerable thought. If the “keg model” doesn't hold up, based on properties of fluid dynamics, what is your present working hypothesis of this phenomenon??

'This, Dr. McCoy, appears to defy logic”

Junkman

P.S. Was the engine under load and what kind of carbs were you testing (CV or Mechanical link)?
Click to expand...
 
It may also be the pilot system since that continues to supply critical fuel after the high end system has kicked in.
Click to expand...


Hey, didn't I say that?! Where's my thunder?


Earl, I think you covered everything except the flux capacitor conundrum.
 
:-) :-) :-) :-) Well Don, its pretty easy to state what I have tried and found that does work. Its a tad more difficult sometimes to figure out why it works. :-)

Earl



Don Lobacz said:
Earl, I think you covered everything except the flux capacitor conundrum.
Click to expand...
 
Earl, I'm just funnin' ya. We take our shots when we get them, and the opportunities to send them your way are pretty rare. Seriously, though, I think that the 'seasoned' people in here generally agree on one main point: our Suzukis are well-designed, very efficient machines when they are 10/10ths right. Indelicate, ill-advised or ham-fisted tinkering will probably only cause problems, and then you'll be stuck running with the Harleys (110mph vs 122mph thanks to microscopic quantities of gas, for example). When we give advice, we are trying to make our best guess over the whole range of possibilities (mixture, spark, timing, compression, etc...), but nonetheless it still is long-distance troubleshooting. Bear with us, we're not charging anything for the apprenticeship, but we do ask that you pass it on when you become the expert. We think that you'll agree: THE MORE YOU KNOW, THE MORE YOU'LL APPRECIATE YOUR SUZUKI!

(applause, general cacophony)
 
I have to agree with Earl regarding the counter-intuitive behavior differences based on tiny air leaks. My GS450 ran significantly better at mid-to-high RPM ranges after I sealed some tiny air leaks between the air box lid and the air box. It's really weird that such small differences in air flow can make such a large impact on performance.
 
Earl,

I am inclined to agree with your hypothesis. KUDOS!!

I am sure also that the numbers would crunch if we just had them.

Sometimes, when I turn the key and press the starter, I marvel at the amazing sequence of events that is beginning to take place under my own scrawny and undeserving ass.

I would not have sent this thread off onto this tangent if I had not thought you were trying to tell us that: "If you have 20 oranges in the basket and 2 fall out (high vac), it doesn't make as much difference as having 3 oranges in the basket and having 2 fall out" (my quotes). My apologies, I misunderstood! Sorry for the food analogy but its getting close to lunch.

Junkman

P.S. I wonder what ever happened to Dave?
 
P.S. I wonder what ever happened to Dave?
Click to expand...

It probably got so deep he had to get out or drown :D I know that it had my brain hurting trying to keep up with the two of you. 8O
 
Yeah, its the little stuff that'll get you every time. With our old bikes, I am beginning to think there is a strong probability that about half of the carb tuning problems newbies have are due to
trying to tune a bike with an induction system that is outside design specs (leaks). Very few new owners even know there are "O" ring seals on the flanged induction rubbers and that they dry out and leak with age. You're right, seal the leaks, even if they appear to be small and you cant imagine them making any difference? They DO! :-) :-) :-)

Earl



nabrams said:
I have to agree with Earl regarding the counter-intuitive behavior differences based on tiny air leaks. My GS450 ran significantly better at mid-to-high RPM ranges after I sealed some tiny air leaks between the air box lid and the air box. It's really weird that such small differences in air flow can make such a large impact on performance.
Click to expand...
 
nabrams said:
I have to agree with Earl regarding the counter-intuitive behavior differences based on tiny air leaks. My GS450 ran significantly better at mid-to-high RPM ranges after I sealed some tiny air leaks between the air box lid and the air box. It's really weird that such small differences in air flow can make such a large impact on performance.
Click to expand...

Lid???

It's supposed to have a lid????

My air box is just a black plastic box, with a black foam filter held in by rectangular white plastic 'grid' inserts. It did not come with a lid, and has a rectangular opening in the top.

Someone also drilled a hole in it, in the round plastic part just before the carb boots. I plugged that hole with electrical tape.

And aside from the nifty physics stuff (some of which I get, and some I let fly right over my head), I'm still here... Thanks for the help..
 
Yes, it should probably have a lid. Put duct tape over that top access hole and see if it makes a difference.
 
earlfor said:
Very few new owners even know there are "O" ring seals on the flanged induction rubbers and that they dry out and leak with age. You're right, seal the leaks, even if they appear to be small and you cant imagine them making any difference? They DO! :-) :-) :-)

Earl
Click to expand...
If your bike has an air leak, wouldn't that show up on your plugs as a lean mixture?
I seem to be having the loss in power between 4-6K rpm at highway speeds SOMETIMES, and not always at the exact rpm each time. Turning on the choke does nothing, but letting off or giving it throttle brings it out of that slump. Pulled the carbs, they are squeaky clean, floats are fine, and they have been synced. Number 3 plug sometimes soots up, and the other ones are fine. The bike has a Jardine header and el cheapo pods. It has a Boyer Bransden ignition and mechanical carbs. It ran fine for YEARS, but now is acting weird. :? It's off the road for a few more months, so I can't do any road tests. Ideas?
 
earlfor said:
Very few new owners even know there are "O" ring seals on the flanged induction rubbers and that they dry out and leak with age. You're right, seal the leaks, even if they appear to be small and you cant imagine them making any difference? They DO! :-) :-) :-)

Earl
Click to expand...
If your bike has an air leak, wouldn't that show up on your plugs as a lean mixture?
I seem to be having the loss in power between 4-6K rpm at highway speeds SOMETIMES, and not always at the exact rpm each time. Turning on the choke does nothing, but letting off or giving it throttle brings it out of that slump. Pulled the carbs, they are squeaky clean, floats are fine, and they have been synced. Number 3 plug sometimes soots up, and the other ones are fine. The bike has a Jardine header and el cheapo pods. It has a Boyer Bransden ignition and mechanical carbs. It ran fine for YEARS, but now is acting weird. :? It's off the road for a few more months, so I can't do any road tests. Ideas?
 
As it stands now

As it stands now

I am investigating the 'what, it's supposed to have a lid?' angle...

I aquired a lid and a new filter from ronayers.com, installed them, and am now wating for good enough weather (i.e. no snow or rain on the roads) to go for a test spin...

Unfortunately, I dropped the bike on it's RH side cover (that I had left on the ground next to it) while trying to get it on the centerstand... So I'm looking for a new sidecover, and I'm not about to pay BikeBandit $50 for one... In the mean time, the old one is getting jigsaw-puzzled back together with epoxy...
 
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