Running a Rotax Engine with Pre-Mix Gas: Why Do People Do It?

[Seadoosam]

I change the plugs usually once a year. Yes if it idles for a very long time the plugs do get oily a bit but a little bit of WOT cleans them off. There is a decent amount of smoke at low speed but i am usually moving fast so it doesn't affect me.

 

[gerallison]

Do your plugs ever foul? Not much smoke at low speed? There are a lot of people who run 50:1 with positive report.

Personally, I think it's a little light for WOT blasts but I haven't tried it myself, still using injection.

As far as running both injection and pre-mix 50:1 would save your engine if injection failed, not sure if plug fouling would be a problem.

The BR8ES seems to run pretty cool in my 951 and will foul in a season, so I'm testing BR7ES this year. I don't have any results to report yet but so far no problems.

So far I am still in the breaking-in process, and no problems with plugs, been using BR8ES also without a problem, starts right away! So I will try after break in 50:1 and check for plugs!! As I mention before, I do not want to remove the oil injection system but as well, I want to have that extra layer of safety, rather buy or clean plugs than replace engine parts like I've been doing !!

 

[mikidymac]

I think you are worrying too much. Since you replaced the small oil lines and everything is working fine I wouldn't bother with the 50:1 premix. However if it makes you sleep better it won't hurt anything but your wallet.

 

[Sportster-2001-951C-Stock]

Yep, I'd start with 50:1 and see if the BR8ES can live without fouling. It's a lot of oil so you may notice some top-end RPM loss or maybe plugs fouling during WOT

Everything else matters too, to avoid detonation. Carburetors are critical, if you have hesitation or lean surging then suspect the carburetors and look for fuel line air leaks by low pressure checking.

http://www.contactmagazine.com/Issue54/EngineBasics.html

 

[Sportster-2001-951C-Stock]

Yes if it idles for a very long time the plugs do get oily a bit but a little bit of WOT cleans them off.

Good to know it works. 32:1 is the "all-purpose" ratio I've settled on for an idiot-proof safety factor.

The reason I went back to 32:1 occurred the day I seized my chainsaw while quick cutting of a tree branch following a storm. Granted, the pre-mix was probably a bit stale and had been in the shed a little too long but that happens.

And I've noticed my engines seem to run better at this ratio. Today's oil is great oil but there is no miracle oil IMO.

Basically I agree with this guy, he was one of the top bike mechanics:

http://www.bridgestonemotorcycle.com/documents/oilpremix6.pdf

 

[Frank_Hassa]

I found this post in another thread from "seadoosnipe" and thought it would be a good addition to this thread.

Oil injection. You are not doing yourself any favors by premixing this motor. It was designed to run injected, and premixed will cause fouling of your plugs. The reason: Put your thinking cap on. When you premix, lets say, 40:1, then that means, your engine is always burning that oil, at that ratio. In reality, that engine does not need that much oil to operate. When you are at WOT, then your oil injection system is putting in about a 40:1 mixture. But, as you slow down, your ratio changes. By the time you come down to idle, your oil mixture could be 1 part oil, 60:1, because at idle, you don't need as much oil. That is why you are smoking at idle, and probably, fouling plugs. I have 2 GTX's, one XP, and 2 Challenger jet boats, and all run the 787, and not one of them is premixed. Two of my craft are running the original engine, because it's always been taken care of, running direct oil injection, to keep from gumming up the cylinders.

The post also gives some good information on how the carbs work at various stages and how to adjust them, and why.

 

[mikidymac]

Sounds like what I said. The oil injection system is just fine if maintained properly.

 

[Sportster-2001-951C-Stock]

My best guess based on oil and fuel consumed, my 951 operates on average around 65~70:1, that's about half of the ratio I'd use typically for pre-mix.

I disagree running pre-mix will result in gumming the cylinders, running 50:1 [MENTION=37162]WOT[/MENTION] might cause that considering the injection ratio is probably more like 32:1 or so?

Fouling - The BR8ES plugs run conservatively cool in my engine.

 

[Sportster-2001-951C-Stock]

Sounds like what I said. The oil injection system is just fine if maintained properly.

It works great, sure cuts down on oil consumption when it's not needed and pours it on when it is needed..

I disagree pre-mix causes gumming though, it's the opposite if anything. Also, pre-mix helps protect carburetors from corrosion.

Drawbacks:

Sometimes during storage, the oil can separate from the pre-mix and stratify, all kinds of weird things can happen in a period of months. Bean oil pre-mix for instance, rapidly loses lubricity just a few days after mixing.

A dry carburetor means there will be no lubrication, not the case for injection thus less chance of damage by some degree, depending.

In my experience it cannot be over-emphasized to only use fresh fuel in marine engines due to the heavy constant load, high duty cycle and this includes pre-mix, which can be more difficult to deal with once it sours or needs to be consumed before souring. Say you siphon pre-mix into your car for instance, in order to get rid of it before it goes stale. Will the oil damage the emissions system? I expect it might.

 

[Frank_Hassa]

When you premix, lets say, 40:1, then that means, your engine is always burning that oil, at that ratio. In reality, that engine does not need that much oil to operate. When you are at WOT, then your oil injection system is putting in about a 40:1 mixture. But, as you slow down, your ratio changes. By the time you come down to idle, your oil mixture could be 1 part oil, 60:1, because at idle, you don't need as much oil. That is why you are smoking at idle, and probably, fouling plugs. I have 2 GTX's, one XP, and 2 Challenger jet boats, and all run the 787, and not one of them is premixed. Two of my craft are running the original engine, because it's always been taken care of, running direct oil injection, to keep from gumming up the cylinders.

I think it mathematical terms and see a relationship between "x", the engine rpm and "y" the rate of oil consumption. But it's not a simple, and direct relationship. If it were, then the ratio of fuel to oil would remain static, say 40:1 at idle, and 40:1 at WOT. But he's not saying that. He's saying the opposite. That at idle the ratio is lower (more oil to fuel) and at WOT the ratio is higher (less oil to fuel). Varying between 40:1 to 60:1 between idle rpms (1,500, I think) and WOT (5000?).

So mentally I have this graphed out as an exponential curve, and not a straight line. Then I read this:

My best guess based on oil and fuel consumed, my 951 operates on average around 65~70:1, that's about half of the ratio I'd use typically for pre-mix.

Which is a broad "overall" statement and does not address the idea that the lower the engine rpm, the less oil is used.

First I'd like to know if Seadoosnipe is correct, that the fuel-oil ratio changes based on engine RPM, and if it's true, how do they do that? I assume they don't have some kind of electronic monitoring circuit which senses engine rpm and senses oil output and adjusts the output based on engine RPM, but if not that, then how?

Second, it makes sense to me that pre-mixed gas is more likely to foul plugs simply because there's more of it. Maybe some people do not idle for long periods of time (Sportster), but other people do, and those people may have plug fouling problems while others do not. Also I assume that how well the engine runs is going to affect plug fouling. The extra oil from pre-mix gas in addition to a poorly-running engine might make plus more likely to foul than (for example) Sportster's perfectly tuned and immaculately maintained engines.

(I have a bent piece of a guitar's g-string to clean those tiny orifices in my carbs, btw.)

 

[mikidymac]

This is getting too complicated.
Premix is safe time tested and fine for the skis at 32:1-40:1 and remember to still supply the rotary valve on 787 and smaller engines.
Oil injection on Seadoo's is the most reliable in the industry and will save you money in oil.
Either is good either will work.
I love my oil injection.

 

[Sportster-2001-951C-Stock]

What I'm saying is, using the oil injection my average consumption is much less than if I used pre-mix b/c the oil injection is variable. There's no oil practically, at idle and it goes up from there to at least 40:1 maybe even 32:1 at WOT. It's difficult to measure since there's no flow sensor so all I can do is measure average consumption.

I don't run WOT for appreciable amounts of time thus average consumption is considerably less than 40:1 or whatever maximum would be.

Fouling plugs is a function of plug temperature, the BR8ES was chosen based on the factory engine tune to give a reasonable self-cleaning temperature without creating a hot element (plug glowing red) that might cause pre-ignition. Excessive plug temps don't cause detonation, they case pre-ignition, which means ignition occurs before spark occurs. Detonation occurs when the fuel stops burning too early.

So if you run rich fuel or oil your plugs tend to be deposited with hydrocarbon soot. I've found they're more susceptible to a rich fuel mixture than oil but that perception might relate to plug heat range.

The reason heat range is important is if the plug is a cold plug it's more liable to foul, too hot it might cause pre-ignition.

The oil ratio is variable by mechanical means, as the throttle moves there's a cable or linkage that varies a valve in the oil injection system. That's why we always recheck the mechanical adjustment of the injection valve after making carburetor adjustments, idle speed specifically.

I'm pretty sure I've posted articles explaining what and why, how detonation occurs and how to select your plug heat range. Seadoo did a fairly good job of juggling the variables thus if you set it up to spec it's close to dead on with some safety factor built in.

Once the calibration of these variables drifts, your safety factor goes out the window though. Let's say, the small passages in the carburetor are partially restricted for instance, the fuel charge will be lean of fuel and might stop burning too early in the compression stroke and there you go, detonation begins to occur.

Detonation can burn holes in the pistons due to heat increases, pressure spikes occur nad the excess heat cannot travel into the culinder walls through the oil fim quickly enough thus aluminum is lost from the piston or the oil film on the piston fails due to heat, scuffing occurs and ring lands are coked with carbon from the oil. Compression is lost, ring seal is poor and blowby increases, heating the piston and damaging the oil film further.

This is why a worn 2-stroke engine will seize, every 2-stroke will eventually suffer this fate if not rebuilt once compression is compromised beyond the safety factor.

One of the least of your worries is having too much oil, the inverse is true. The downside of having too much oil are cost of the oil, cost of replacing plugs (select appropriate heat range to reduce fouling), and oil spooge all over the transom, is what I call it. Even having a rich fuel mixture can cause spooge, which simply is hydrocarbon carbon soot from the exhaust.

I have some antique engines I run occasionally that burn 15:1 non-detergent motor oil, these don't suffer from fouling in any significant way. I'd say the plugs run hot enough to avoid fouling.

 

[Sportster-2001-951C-Stock]

So mentally I have this graphed out as an exponential curve, and not a straight line. Then I read this:

First I'd like to know if Seadoosnipe is correct, that the fuel-oil ratio changes based on engine RPM, and if it's true, how do they do that? I assume they don't have some kind of electronic monitoring circuit which senses engine rpm and senses oil output and adjusts the output based on engine RPM, but if not that, then how?

Sportster's perfectly tuned and immaculately maintained engines.

(I have a bent piece of a guitar's g-string to clean those tiny orifices in my carbs, btw.)

I think the curve fairly linear until you get into the extremes, in those regions an exponential function would fit well.

I think seadoosnipe is correct that 40:1 at low speeds will be more susceptible to fouling the plug, assuming the plug is too cold, plug temperature in my seadoo seems conservatively cool but I've also manipulated the fuel mixture somewhat toward the rich region, I was trying to get on the richer side of the factory tune.

I wouldn't call my tune perfect from the perspective it's my observation the plugs tend to run too cool for sluffing the soot completely, which is why I'm testing a hotter plug this season. I wanted a wider safety margin but the main point was to eliminate the lean hesitation issue, which was accomplished in spades. Honestly, I did clean those three holes but I'm sure they're still restricted, and this was the root cause of the hesitation. Before boring them out to a larger diameter I tossed the same jets that were used in the 1989 951 and wa-la, the hesitation was gone. Yeah, arguably it might be slightly rich but not by much, I like the slight hint of 4-stroking b/c this tell-tale sound tells me there's plenty of fuel.

It's not so rich the throttle is unresponsive like you'd expect if the mixture was so rich it sputters a second or two while clearing out excess fuel once acceleration throttle is applied.

Remember, this is a 951, this engine and carburetors are considerably different than yours so what works for my 951 certainly won't apply to a rotary valve motor.

Design of the carbs is very similar though, which is why I'm here explaining why if your engine is lean to concentrate on those small holes drilled in the bore.

I can't imagine why you'd feel a need to disable the oil injection, the system works great as designed so just make sure it's in top shape and calibrated properly.

If when you do get yours back together and in the water, don't be surprised if there's some lean hesitation. But avoid running ti that way due to it will detonate and can be damaged if run long enough that way. The low speed circuit has kicked many butts in these hallowed halls so you're not the first guy to have this issue (assuming you do), it's the nature of how these carbs were designed and materials they were made from. Remember, they work great when those holes are clean as new and the calibrations are set to the factory specification.

So yes, stick with the oil injection b/c it just works well and will simplify your efforts of getting that machine running correctly.

And pre-mixing does work, but there's no need to go that route if you're willing to put a little effort into setting up the injection as intended.

 

[gerallison]

Wow, thanks guys!! I guess I will stick to it as soon as I am done with all the premix I still have in the tank. Yes I am running rich, plugs are oily black, but I am cleaning them with carb cleaner, engine seems to be running fine, just a little under power, I am still breaking it in, so I do not want to go full throttle just yet, I have one last question. I had this waverunner in my beach house at sea level, I decided to bring it to higher altitude (5,046 feet above sea level) So far I have not done anything to the carburetor! What are your suggestions. The XP started from the rebuild right away, as soon as gas kicked in. This weekend same thing! (I cleaned the plugs before starting it) I have never messed with the carburetor before! It's been some time sine I replaced jets or do anything about it, I ride dirt bikes and mine is FI since I don't want to deal with that anymore. This carburetor seems to be very complex, never seen something like it!! Should I take it to a professional or I can do it myself!!

 

[Sportster-2001-951C-Stock]

At 5,000ft, the fuel mixture will be somewhat richer than it was at sea level, there's less air at altitude. It depends on how well it's running, if it's hesitating and lean-stalling then you should rebuild the carburetors ASAP.

Lean stalling is not a result of high elevation, it's a result of the carburetor being dirty inside the small passages.

At high elevation you may discover it's too rich and power will be much less if much too rich, plugs will foul easy (fuel+oil make soot, to much of either will foul plugs with carbon soot), listen to the sound of the engine, if it's making a rich 4-stroking burble and takes time to clear out or won't rev quite to normal speed, it is rich.

If the engine lean stalls, hesitates and falls on it's face like it's running out of fuel then the carburetors are likely dirty inside and should be disassembled and cleaned internally (D&C). Also the fuel tank may be empty or a fuel filter is clogged, or an air leak in the fuel line prevents fuel from reaching the engine.

Before you can judje by the spark plugs, you must have a reasonable amount of oil, too much oil will make the plugs dark. You can only judge by how well the engine is running (performance) in the case there's extra oil, the plugs will be sooty even if there's not enough fuel due to oil is heavy and doesn't burn completely.

I hope this explanation is easy to understand.

I'm afraid if you take this to a professional maybe he doesn't have enough experience with the Mikuni carburetor to understand what needs to be done, it might be a big waste of money if he doesn't know. You will need some tools and patience, you must pay close attention during disassembly and cleaning. You might consider buying a used set of carbs for cheap, for a practice disassembly and maybe spare parts.

 

[gerallison]

Thanks for all the help. So far seems it is running too rich, does not hesitate in low, have no problems to start, takes some time and a lot of smoke to take off, (specially when cold) also I feel it too heavy or not as fast as before, not sure it it's just me, extra oil, or that I am using it in fresh water, since I am used to sea water! If it's running to rich, what should I do? Can I adjust the hi speed adjustment screw?

Last month I bought a 2001 crashed XP for parts. I bought it all, engine, all electronics, carburetor, propeller, etc... for extra parts so I have an extra carburetor in a box, maybe I'll check it so see if it's in better condition than the one I have running. Later I will post some pics of how the plugs came out from using it this weekend!!

Thanks for all the help!

 

[Sportster-2001-951C-Stock]

At this altitude, did you already test your WOT RPM? It should be almost 7,000RPM, 6,800 is pretty good.

And first, you should confirm the carburetor mixture adjustment screw is 1.5 turns open for low speed idle mixture adjuster (top of carburetor) and 0 turns open for high speed mixture adjuster (underneath carburetor), these are the factory recommendation.

Also confirm there isn't too much oil, you should try a proper normal ratio instead of double feeding. Using injection I almost cannot see any oil smoke with my 951 consumption rate is 1 gallon oil for each 65~70 gallons gasoline approximately. Confirm your oil injection pump control cable is properly adjusted to the reference mark or use only pre-mix.

At 5,000ft this engine will not be able to make full horsepower thus probably the fastest and best way to improve performance at high altitude is by installing an impeller with lower pitch, the good impeller dealer can advise which one to use for 5,000 ft.

Last, you can try installing smaller jets by 0.92 of original size. Mikuni offers sizes available in increments of 2.5, and factory low speed size was 75 so 0.92 is two sizes smaller thus would be change from 75 to size 70 low speed.

Mikuni part number for low speed jet size 70 is N-100-606-70 (last digits are size) Remember, this can be dangerous if the ski is returned to sea level, it must be reversed.

I think you must make some adjustment for 5,000 feet, it's quite high.

 

[gerallison]

Small question, what is WOT?!

 

[Sportster-2001-951C-Stock]

Oh yes sorry, WOT means Wide Open Throttle, full power. You can check this briefly to see if engine is making full potential power. If your engine is new you should break it in first.

If the engine cannot make 6800RPM but stops at some lower speed such as 5700RPM this means it might be a result of high elevation.

I do expect the high elevation will have a big effect and it's best to install an impeller that has slightly less pitch due to horsepower at elevation will be lower and this will help obtain near 7,000RPM WOT

Consult with a good impeller shop, one who has experience for this model ski at high elevation.

 

[LawnDart]

The lakes I go to are 4200 to 6000 ft. On my machines, the low speed screw has enough adjustment to compensate for elevation. The high speed is where the problem lies, as stock jets are too rich when the screws are turned all the way lean. As long as it doesn't foul plugs you may be able to live with it..........the boat will just lose some top speed and responsiveness 3/4 to full throttle.

 

[gerallison]

This weekend I was more focus on smoke and power, plus I had kids all over me so I could not see the top revolutions, I may check this weekend, or not sure if what's missing are the RAVE valves not kicking in !! Need to ride it with new plugs and fresh fuel, I still have 20 Lts of fuel with mix plus half tank!

 

[Sportster-2001-951C-Stock]

My 951 seems to be programmed to engage the RAVE valves at over 5200 RPM, this makes a big difference if they're not working.

Also, there's a water valve on your exhaust water box that is exhaust pressure sensitive, if this one isn't working the power will be effected. As exhaust pressure is increased, this valve throttles lower the expansion cone pipe injection water. If you remove the cap on top, there's a spring inside that keeps the valve open in the down position and as the pressure increases the rubber diaphragm bellows is lifted up against the spring and slows down the pipe injection water flow.

Sometimes some sand or weeds can get caught inside the water box pintle valve area restricting the movement. The pintle shaft should move up and down easily and the rubber bellows diaphragm shouldn't leak water, sometimes the metal clamp will rust and poke a hole in the ribber diaphragm bellows, if the clamp is broken the inner diameter of the diaphragm can leak. You can replace the metal band clamp by substituting a small size plastic tie-wrap.

 

[Frank_Hassa]

My 951 seems to be programmed to engage the RAVE valves at over 5200 RPM, this makes a big difference if they're not working.

How DOES the machine know when it's time to engage the RAVE valves. Is it just the air pressure and the bellows and the springs all being "tuned" in some way?

 

[Sportster-2001-951C-Stock]

In the case the 951, the air pressure to lift the RAVES is controlled by a slave solenoid, the MPEM engages it. The pressure originates from a check valve connected to a nipple on the PTO cylinder lower case under the PTO carburetor.

For the 787 carb engine the pressure comes up through a passage in the cylinder from the lower case and operates as you said, there's no electrical control. So adjusting the spring pressure by turning the knob in the cap is more critical.

 

[mikidymac]

The carbed 951's also have a throttle position switch in the throttle cable that adjusts the timing at 80% throttle. If it is not adjusted correctly you will not hit 6,800 rpm either.