Fuel Reversion...Revisited

MrSki · 11-18-2009, 01:13 AM

So after a Y 2009 summer of various engine builds and pragmatic experimentations, I have compiled a matrix of engine component configurations with their resultant affects around fuel reversion or “carburetor standoff”, as so coined.

All my testing was performed on the Kawasaki 550 (and some 440) twin cylinder 2 stroke platform. The fuel reversion topic on these motors has been previously discussed here and met with a lot (A LOT) of controversy. I am responding back with actual test data and results which can not be refuted in any way. The cause/effect, the before/after, the results of the work are laid out here for your benefit…ie no cost to you.

When I first asked what was the deal with this fuel reversion, I was told that a 440 full circle crank was the reason and that I must remove this component to ever get the 550 motor to run right. In essence use only the “t-bone” webbed 440/550 crank. After about 5 minutes of contemplating this I concluded this was bogus advice, at least 95% bogus as will be discussed later. Well most of the engines used today in PWC applications use full circle cranks. Even back in 1978 & 1979 when Kawasaki used 440 full circle cranks, well they sold a lot of boats with them in it…hmmm, it must have worked.

Taking a long hard look at about 15 years of my 550 motor builds and tuning notes, I started to see some trends. This latest 550 full circle crank motor was not the first time I saw fuel reversion out the carbs…no! I have seen it for about 12 years but didn’t realize what I was seeing.

Side note…fuel reversion is the raw air/fuel mix coming backwards up and out of the carb(s). There are really 2 instances of this so let’s distinguish. Sometimes when you first hammer the throttle, you may see a spitting of fuel (actual droplets of gasoline) emitting up and out of the carb. This is not the reversion I’m talking about. This is more from an incidental pressure lag when first cracking the throttle plate.

The reversion I’m talking about is a higher RPM phenomenon where at about 6,000-6,500 RPM, a slight fog will develop above the carb(s). Typically the fog gets thicker with increased RPM. In extreme cases, the fog will lift up and be visible 4 or 5 feet above the carb(s). This was what I saw with the 550 Westcoast cylinder/440 full circle crank motor.

We sit around and talk about 2 stroke theory up down and sideways. But until you really get into the bowels of it does it start to make sense. So a lot of this is coming together here, not just from a text book stand point but also the pragmatic application aspect.

Fuel reversion. It is the result of an exhaust system geared towards a high RPM range, aggressive exhaust height porting and a pump load that exacerbates the heat load in the exhaust system. This is the base issue.

There are other factors that add to the fuel reversion tendency. So if you have a perfect storm of the above, a full circle crank with less available “open” crank case volume or weak reed petals or dual carbs…will all make managing fuel reversion that much more difficult. But none of these secondary factors alone are the root cause of fuel reversion.

I studied some 4 stroke instances of fuel reversion. Needless to say for some of us, this affect has been around since the sixties when radical cams cam out in drag racing applications. Dual plane intake manifolds acting as cross over tubes under the throttle plates emerged to help “balance” the intake side of the induction system. Long velocity stacks on top of carbs or fuel injectors were developed to contain the escaping A/F charge. This is nothing new kids.

With the 4 strokes, 2 leading theories emerged as the cause for reversion. First, an overly aggressive intake/exhaust cam duration where the exhaust pressure blasted up the intake. Second, when the intake valve closed, the raw A/F mix rebounds off the slammed door and ricochets back up through the carb.

Personally, I’m not too sure about the second theory. There is a lot of mass in gas atoms. I don’t think they like to defy gravity so easily. But the first theory makes a little more sense since the exhaust blast has some power or thrust behind it. And in our 2 strokes, we have the same effect…even without a cam shaft.

It’s well know how the modern 2 stroke expansion chambers scavenge the cylinder of burnt exhaust gases and pull so hard on the cylinder through the open exhaust port that once the transfer ports are exposed, this draw will pull on the carb(s) even when the piston itself is not “sucking” on the carb. It’s estimated (perhaps it’s been measured) that about 1.5 atmospheres is drawn on the cylinder from the expansion chamber. This is quite an incredible claim however since on this planet, there is only one atmosphere. Anyone ever see more than one? After that, there is no pressure at all. There is no such thing as a negative pressure…just degrees of pressure. Anything less than one atmosphere of pressure often referred to as a vacuum. I seem to be digressing a little so to clarify, the expansion chamber can drop pressure in the cylinder even lower than just the sucking action of the piston. It is a dynamic reaction from a static piece of hardware (the pipe).

And this dynamic process has a rebound effect. Once the initial “pop” of the piston clearing the exhaust port opening on the down stroke hits the tail cone of the exhaust pipe, it returns a wave back towards the cylinder. It is precisely this return wave that is most likely the key contributor to fuel reversion back up the carb(s). This return wave is one of the keys to making 2 stroke power. As the initial pull on the cylinder from the pipe removes burnt exhaust gasses. It also pulls in raw A/F mix from the carb(s) into the exhaust pipe. The return wave pushes that raw A/F back into the cylinder and supercharges it just before the piston closes off the exhaust port on the up stroke.

So it is the timing of this dynamic exhaust action that creates or steals horsepower. And the timing can be manipulated to give you low end or high end power ranges. Longer pipes take longer to return the supercharging wave and this makes more power on the bottom end since lower RPM’s take more time to rotate the piston around – it’s a better timing match. At higher RPM, the piston shows up a lot faster to close off that exhaust port so you need a faster returning wave – a shorter pipe will do this since the distance shorter. It is simply and matter of distance (length) with a relatively constant speed of the exhaust waves.

All this background is necessary to explain what is happening with fuel reversion. Sorry if it’s just review for you.

So then, the motors I have seen fuel reversion with had all the right combinations of physical geometries to create the phenomena. High exhaust porting lets the initial pop wave out early. Once it’s heading down the exhaust pipe, nothing is slowing it down. It leaves early, it will return sooner.

All these builds had a relatively short exhaust pipe. As mentioned, the shorter pipe will return the wave sooner.

In addition, I found that a restrictive exhaust aided significantly to fuel reversion. More restrictive, the hotter the gasses inside get and the faster the exhaust waves travel. It has been documented that cooler gasses travel slower and vice versa.

Restrictive exhaust systems could result from the use of water boxes, rear exhausts that just take longer to remove the gasses from the boat and, what I discovered, simply a motor just hanging up at a given RPM, not really making any more power but burning more fuel and generating more heat.

Stick a restrictive flow 550 pump behind a high RPM power motor, and you will hit an RPM wall that the motor just can not over come. If you just lay on the throttle at this point, more fuel and air dump into the motor but no more RPM, or horsepower or boat speed will result. The additional air and fuel consumption just dumps into the exhaust as heat…and…the returning wave comes back even sooner.

So these above factors all come together in that perfect storm to create fuel reversion. All these factors add up to having a return wave hit the cylinder when the transfer ports are open and blasts right up and out of the carb(s). The same affect seen on 4 strokes with radically overlapped cam lobes.

MrSki · 11-18-2009, 01:14 AM

Going to a 440 pump, which is an axial flow type as opposed to a mixed flow 550 pump, removed any affinity for a RPM wall to develop. 440 pumps offer so little resistance that my 550 motors just romped straight up from idle to 9,000 RPM without hesitation. One 440 motor I built up with extremely high exhaust porting and a supper short PJS2300 pipe showed no signs of fuel reversion using a 440 pump.

Back to the full circle crank…well with the second theory from the 4 stroke world that the closed off intake valve allows A/F to ricochet off the closed door and shoot back up the carbs. There could be something like this going on with a full circle crank since compared to a T-bone crank, there is always a huge hunk of rotating web facing the carb(s). If the piston draw is so string on the upstroke and immediately reverses on the down stroke, maybe ricochet is occurring. Personally, I’m not buying it.

I was able to eliminate the fuel reversion to any level I could detect on that Westcoast 550 motor with the 440 full circle crank through intake manifold cross over tubes, stiffer reed petals and spacers underneath the reed cages. I left the exhaust porting and shorter pipe alone and just concentrated on the intake “band aides”. And I retained the 550 pump. An RPM wall did develop here as well but I overcame that with the right carb and exhaust water tuning. The final set up did “work” but all the efforts to getting it just right further proved that aggressive exhaust porting, short exhaust pipes and 550 pumps were they key contributors to fuel reversion. Meaning, the band aides stopped the negative results of reversion (A/F contained within the motor) but the root causes still remained.

My pink 1992 550 had very similar fuel reversion symptoms as this Westcoast motor. It has a Kawi reed cylinder with exhaust height porting nearly identical to the Westcoast cylinder, and yes, a T-bone crank. This boat also runs a 550 pump. I left the intake and pump alone and stopped the reversion with a longer exhaust pipe – Jet Power ½ pipe – and slightly stiffer Boyesen dual stage reed petals.

One 550 set up I have seems to defy some of this explanation above. PJS 550 cylinder, T-bone crank, Dual 46 mm Mikunis, Westcoast full pipe with just 2 rings left on the tail cone (super short). This has a 440 pump that takes a 20 pitch prop to keep it from cavitating. Now the extremely high exhaust porting of this T-3 cylinder and the short exhaust pipe would normally be prime candidate for fuel reversion. I know the 440 pump and no RPM wall reduces the heat load within the exhaust so that helps. But I also run this PJS rear exhaust with a little baby water box up front that offers no real resistance. (It is a very loud ski.) I think I just got lucky on this set up by trimming enough corners off the fuel reversion contributors to have something that works as a high RPM motor. I haven’t looked too hard but I bet I could find some reversion…at least at 8,000 + RPM.

I have created another document with a configuration table of all my testing and additional notes to supplement all this here. Well I’m not sure where all this will lead on a public forum but at least I have tried to set the record straight with what I have found out by empirical and methodical testing and results measuring and documentation. Have at it.

the WaTeRhAwK · 11-18-2009, 01:53 AM

Quote:

Originally Posted by MrSki

Fuel reversion. It is the result of an exhaust system geared towards a high RPM range, aggressive exhaust height porting and a pump load that exacerbates the heat load in the exhaust system.This is the base issue.

lmao, really genius!?!?

if that's the case then how in the world did you end up coating everything in your garage with oil/gas mix, with no load to the pump? maybe 440newbs' cat was over there messin' around and got his thing stuck again......

I love ya jeff, I really do.

MrSki · 11-18-2009, 01:23 PM

Honestly Travis if you intend to try and nit pick everything in this thread to find some minute fault or detail that shows I'm incompentent, I'll disprove every last bit of it and continue to expose your own personal ineptness in the process.

Exacerbate means to make something worse, to aggravate or intensify. I did not say pump load causes fuel reversion, just makes it worse under the right set of conditions...the perfect storm. As previously discussed, the higher heat in the exhaust returns the wave sooner.

So if a prevalent condition exists (that means pre existing, already established) where fuel reversion will result, adding a huge pump load and creating a RPM wall will make the reversion much worse once you get the pump in the water.

This is what I observed with no other changes than running the motor with the pump dry and then backing the trailer into the lake and running it with the pump adding more load.

the WaTeRhAwK · 11-18-2009, 06:23 PM

I'm not nit picking and I don't believe you're incompetent. I just think you get in a hurry sometimes and throw stuff together while trying really hard to not look like it.

Hurricane249 · 11-18-2009, 06:40 PM

That sure is a lot of effort to put into ancient engines that very few people care about. No offense Travi.

MrSki · 11-18-2009, 07:47 PM

Quote:

Originally Posted by Hurricane249

That sure is a lot of effort to put into ancient engines that very few people care about. No offense Travi.

Yeah. I noted this much in one of the files attached. But the effort wasn't to go figure out the ins and outs of fuel reversion. The real effort was getting a few motors to run right. But nonetheless, some people are still farting around with these motors and have sent me questions on how to manage this same fuel reversion affect so hopefully it will reach those who need it.

'Crockett · 11-18-2009, 11:02 PM

I'm wondering why you posted this in Harry's Forum . . .

You have not addressed him with a question, the topic is old and outdated news to him I'm sure . . . . . surely you didn't think you were going to teach him something did you ?

the WaTeRhAwK · 11-18-2009, 11:43 PM

guys, have you seen the picture that jonnyx2 is using for an avatar? that is jeff, the rest of that picture that you cannot see is all of the electrodes coming through the ceiling from the lighting rod on the roof. they are attached to his ski (picture a scene from "the burbs"

) ..........................

MrSki · 11-19-2009, 01:38 AM

Quote:

Originally Posted by 'Crockett

I'm wondering why you posted this in Harry's Forum . . .

You have not addressed him with a question, the topic is old and outdated news to him I'm sure . . . . . surely you didn't think you were going to teach him something did you ?

Yeah I asked Harry about this some months back...April or May this year. Travis can give you the link to the thread since he seems to post it in about a third of his overall posts...I guess he has the URL memorized or something.

So Harry either didn't want to deal with the topic or he really couldn't answer the question. I'm not sure if he's learned something from my report, refreshed his memory or it's just to passe a topic to waste his time on.

Nonetheless, the reversion phenomena is real. More and more people are asking about it so it's not just me. And as such I wouldn't say at all that it is old and outdated. Particularly since no one else could pose any cause/effect results around it nor any potential solutions...at least not as posted on PWCToday that is for sure.

But as I mentioned, as more and more applications move to conservative 2 stroke geometries and just add displacement to make power, most people will not ever achieve the perfect storm scenario as described. That's OK by me. I'm not trying to revolutionize the industry or anything. This is an old school scenario that I've perhaps reincarnated with my tinkering but if I can help any other tinker folks then that's what the info is for. I'll try and send some of this to some old school tuners I know of - Dan Lamey, Greg Beaver...See what they say.