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

[lhanscom]

Keg analogies, gotta love it.

[Anonymous]

I guess if your talking kegs, any leak becomes significant!!

[earlfor]

Re: any ideas why my bike

No problem in taking a while to get back.........no hurry.......
I understand your point and by the numbers, things look good.
However....:-)
quote ...(But, the leak is allowing 1/5 = 20% more air to get into the cylinder. Rpm goes up and the mixture leans out. ) Leaner or richer mixtures may each cause either an increase or decrease in rpm dependent on what the initial stoichiometric ratio is. So an induction leak can cause either an rpm gain or loss. If someone is unaware of the leak, they are (unknowingly) setting the pilot and air screws to compensate. This will work in idle range because you have mixture adjustments available. When the engine is running in higher ranges, there arent any handy adjustments.

quote...(So, whether it is a high vacuum situation or a low vacuum situation is really a moot point. It is the relative rate through the openings, determined by the relative sizes of the openings, that is important. )

Agreed a particular velocity is the product of a given volume through a set opening in a specified time and it would consequently appear that a small induction leak would not be critical to the overall high flow requirements of upper rpm operation. That sounds perfectly logical and I would accept it..........except.......:-)
I have spent quite some time with vacuum guages and carbs and have observed guages showing that when there is an induction leak (depending on leak size), the engine will "go flat" at some point (commonly) between 4 to 6k rpm and the only way to increase rpm when the engine initially goes "flat" is to reduce throttle. The guages will show an abnormal decline in vacuum for the offending cylinder which will not be in alignment with the normally operating cylinders and if throttle is further advanced, vacuum approaches zero on the cylinder with the induction problem. The exhaust for the cylinder with the leaking induction will be very weak. If we adhere to the area ratios, this shouldnt happen if the leak accounts for only 1 or 2% of the total volume, but I have observed that it does. Admittedly, I have no way of measuring precisely what percentage of the intake volumes the leaks did account for.

Earl

[Don Lobacz]

I think that the symptoms point more to a lack of fuel than an overabundance of air, especially at top end. Perhaps the mains, emulsion tubes, needle jet or jet needle were not completely cleaned. It may also be the pilot system since that continues to supply critical fuel after the high end system has kicked in. Also, since it sputters and dies after attempting full throttle runs, it could well be a fuel delivery problem caused by the petcock or fuel line. Several people have posted that they had to clean their carbs several times before they got them right just because they overlooked critical areas. Keep trying things - there are plenty suggestions to go around.

[Anonymous]

A friend of mine complained of a "flat spot" on his Gold Wing between 140kmph and 170kmph. The problem was that the vent on his gas cap was plugged, starving the carbs for fuel.

[Anonymous]

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)?

[earlfor]

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

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)?

[Don Lobacz]

It may also be the pilot system since that continues to supply critical fuel after the high end system has kicked in.

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


Earl, I think you covered everything except the flux capacitor conundrum.

[earlfor]

:-) :-) :-) :-) 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

Earl, I think you covered everything except the flux capacitor conundrum.

[Don Lobacz]

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)

[nabrams]

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.

[Anonymous]

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?

[dgeorge]

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

It probably got so deep he had to get out or drown I know that it had my brain hurting trying to keep up with the two of you. 8O

[earlfor]

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

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.

[Dave_A]

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.

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..