Dr. Spock's Scooter Baby Oil Formula

[Wheelman-111]

Greetings:

Premix or Pump It's been a controversial subject on and off here. Serious tunas overwhelmingly agree that Premix is the only way to go, while often failing to reach consensus on what ratio.

My objection to that strategy is that the engine in street use gets the same fuel/oil ratio all the time, regardless of operating conditions. Too much for idling at a stoplight, perhaps too little for that big race to the railroad crossing. That and I hate the idea of disabling a system that Honda-san obviously spent a lot of time perfecting. I'm also too cheap to buy a pump plug...

The counter-argument to that is that Hi-Po Mods call off all bets. Engines with 20, 50, even 140% of the original displacement use up to 140% more fuel than Honda-san intended, but the oil pump didn't get the Memo. It still dispatches only the quantity of oil necessary to produce the intended ratio if the engine were a 49cc. (More about this below) Naturally it pumps a lot faster at the new Hot-Rod redline than at the stately 6000 or so in stock form, but not nearly enough more.

When I did my earliest Polini bore Lo! these scant few months ago, I decided to try the compromise. Being math-averse from my earliest days, I decided to go "Purt' Near" and mixed up 50:1 in the premix tank. It oiled very well, and didn't seize, but tuning proved tricky and plug reads were hard through all that grease.

In successive tanks, I tried anything from about 40:1 to 60:1. Never too sure how to match jetting, plug type, interpret reads, etc. On several occasions I had the odd finding of a plug that was too white on one side and too black on the other. Whaddya make of that?! I finally broke down last night and gave it some thought. Scary, I know... Here's what I'm figuring now:

You can't "add" ratios. Especially since one ratio (the pump's) isn't a ratio at all. It's a delivery system, controlled both by engine speed and throttle position. I had to start with an *-umption: Honda's stated Pre-Mix ratio for CR motocrossers is right there on the bottle of HP-2: "Mix at 32:1". So, sez I, figure the pump is set up to deliver something around that proportion of oil for every part of fuel inhaled when the engine's shouldering full load and the throttle-controlled lever is on "full". Thirty-two to one.

But remember, the pump "thinks" it's feeding a 50cc bore. Since I'm running a 78cc, that means (and I'll spare you the calculations, simply because my head hurts too much to re-do them...) the highest ratio the pump can deliver to my Nikasil-encrusted bore is something closer to 48:1. That's at max RPM, wide open throttle, with a marginal air-cooling system. As many have discovered soon after a pump-conserving bore kit installation, this will not Do.

Back to the fuel tank, how much oil do I need to pre-mix to get the ratio back down (or is it Up? ) to the Honda-san-sanctioned 32. Once again, it's a mess of numbers scrawled on the back of a Payment Past Due envelope, but I came up with something like 75:1, or just about 7 ounces added to my 4-gallon fillup. I understand that the relationship between incoming premix and an RPM and Throttle-controlled pump isn't quite linear, but so far I'm running both smoke and seizure-free. And so is my Scooter.

E3MikUniUni.jpg (341.2 KiB) Viewed 36388 times

Thus spake Dr. Wheelman.

[Bear45-70]

I like your think and numbers you came up with too.

[Wheelman-111]

Greetings:

Bear, have you ever had an Oreo spark plug? I don't have a pic, thankfully, but it was bare-* leen on one side - and charcoal on the other. What the flark does that mean??

I think it means over-oiled at 50:1 plus pumpage. And lean in the mixture department, again due to excessive oil. That's what decided me to buckle down and figure out the Premix+Pump ratio thing once and for all. I stickied this thread but will modify when/if I hear from our Learned Colleagues who live in Island States beginning with the letter "H". Or not, depending on what they have to say.

DubH, You home?

[elitedio]

One thing to consider wheelman is that you may want to jet and oil for the worst conditions not the optimum or easy parts of your motors life. Sitting at a light after a hard run might be hard if the oil is not there to keep it lubed.

When I got my scooter going this summer I had purchased a bottle of Klotz TC-2 oil and proceeded to jet the carburettor. It was running good but then I ran out of the oil that I had purchased and went looking and couldn't find the same oil. I did find Klotz oil with a low percentage of bean oil in it, some type of racing oil. I poured the correct amount into a jug and tried mixing it with gas. The first oil would instantaneously dissolve. My new oil formed a stratified layer and was much more viscus. I added a bit of premix to the tank and noted a slower engine. Next time I topped off it was worse. I decided at that point that I need to find a oil that I can get that mixes well and provides good protection. I went to a Honda shop and they had Honda brand in 16oz and Motul brand in one liter for about the same price. Both were synthetics so I purchased 3 quarts of the Motul and drained the tank and float bowl on the scooter. I also bumped up the jetting to a bit richer and adjusted in steps of 5. If I go 10 more on the jetting I get a lot of richness on the pipe (some four stroking) If I go 5 less It doesn't pull any better. Perhaps the best jetting is 2.5 richer than I am but it is very close. I am running 4oz of oil to 1 gallon 32:1. I do think that you might be able to adjust the jetting by more or less oil but have no way of getting there scientifically.

I guess the biggest thing that I could add with this is that you need to maintain the exact same lubrication oil so that you maintain the same fuel viscosity.

Some types of twosmokes run more oil than 30:1 Particularly small engines as in model airplane. I think I recall seeing 10-20% castor oil in the fuel mix. It has been a lot of years ago.

Have you ever read the section on spark plug reading in the Jennings book. He has a lot of insight there that has nothing to do with chocolate brown insulators.

Wheelman, Keep up with your interesting posts. Bear, Try and stay steady, teach those who need help. Maintaining friendships helps to keep us young.

[Wheelman-111]

Greetings:

Thanks for your considered remarks, EliteDio. Yes I'm familiar with plug read charts as well as Gordon Jennings' Two Stroke Primer. This half-and-half number had me flummoxed. Maybe it's an oil-pooling thing due to the angled plug insertion on the SA/16 head?

Anyhow, no I do not plan to adjust my jetting by messing with oil ratios, sorry if it sounded that way. I also agree that you have to provide for lube and cooling during the worst of times, not the best. That's why the idea of conserving the pump is so attractive, yet it's complicated. The math I'm talking about is to make sure the pump is there delivering all it needs to deliver with this bigger, hotter, higher-revving little pepper-grinder. Since there's no purpose-calibrated pump that fits this 78cc, that means adding a little some other way. My post is about determining "semi-scientifically" how much to add to run safely while not wasting any oil. Then test and confirm or blow up. Gotta give MadDog something to mock...

So far this formula's worked well. I will continue to share good, bad, and calamity alike.

[Bear45-70]

Greetings:

Bear, have you ever had an Oreo spark plug? I don't have a pic, thankfully, but it was bare-* leen on one side - and charcoal on the other. What the flark does that mean??

I think it means over-oiled at 50:1 plus pumpage. And lean in the mixture department, again due to excessive oil. That's what decided me to buckle down and figure out the Premix+Pump ratio thing once and for all. I stickied this thread but will modify when/if I hear from our Learned Colleagues who live in Island States beginning with the letter "H". Or not, depending on what they have to say.

DubH, You home?

Yeah, my 2 liter racing V-6 did it on all six plugs with 25 and 18 to 1 oil mixtures ( I never ran less that 25 to 1). The intake side was nice and brown and the hot exhaust side was white. Fattening up the mixture (4, 6 and 8 jet sizes) only made the intake side darker and the exhaust side stayed white. The explanation was mostly in engineers gooblie goop and I didn't really understand anything past "It has to do with air flow in the cylinder and the exhaust pulse back into the cylinder." But I know the exhaust side was the white side because all plugs were indexed.

[Wheelman-111]

Greetings:

Thanks, Bear that makes perfect sense. I have cut down the oil considerably and now it looks like I may need to jet up a bit, even though my temps are pretty good.
Were you indexing for performance or some other reason? Where can I score some 14mm indexing washers? I thought those Mercs were inline, not V6es.

[Bear45-70]

Greetings:

Thanks, Bear that makes perfect sense. I have cut down the oil considerably and now it looks like I may need to jet up a bit, even though my temps are pretty good.
Were you indexing for performance or some other reason? Where can I score some 14mm indexing washers? I thought those Mercs were inline, not V6es.

We were indexing for performance. On the dyno the indexing was good for up to 8 horsepower. The last engine I ran was a Mercury 2 liter (122 CID) carburated in the Champ boat class. A 16 foot boat and the limit on the engine was 126 CID, carburated only, and boat, motor and driver had to weight 1000 pounds after the race, other than that have a good time. At 132+ mph it does make the adrenalin pump pretty good. A friend and fellow racer did the engine work with me helping. By helping the engines were free to me and I learned a ton because we had one of the engineers that designed the engines for Mercury as a consultant. In my first year I finished 2 in the Nation in the class. I only won 2 races but never finished lower than fourth, consistency counts in a points race. My 1st, 2nd, 3rd and 4th boats were all driven by the 99.9 CID in-line engines. The first two were stock engines and the other two were modified classes.

On cutting back on the oil and ending up lean, most 2 stroke oils have power enhancers in them. So less oil can make you lean if you jetted at a higher mixture.

I have no idea where to get indexing washers of 14mm plugs.

[Wheelman-111]

Greetings:

Quoth Bear:

On cutting back on the oil and ending up lean, most 2 stroke oils have power enhancers in them. So less oil can make you lean if you jetted at a higher mixture.

Just to clarify here: Normally more oil = a lean mixture. Less oil = richer mixture. The volume of non-volatile oil displaces fuel in the mix: Adding oil is like subtracting fuel, so that there's relatively more air.

I think what Bear means is that "most 2-stroke oils" have additives that offset this rule-of-thumb, and contribute some combustible substance or catalyst. I'm not saying I agree completely but just putting Mr. Bear's remark in context.

In my case, I just bolted the Mik TM24 on as received, with a 130 main. The carb became part of my "system" that included what I now believe was excessive premix (50:1) along with an active pump. The plug either appeared rich (during break-in) or Calico-lean (once I started adventuring into WOT runs more often). I now believe the rich appearance was due to oil and in fact my jetting was too lean, the telltale pale hidden by a coating of burnt oil.

Here's where I don't completely agree with The Bear in my specific case: Since cutting back on premix ratio (to 75:1) my operating temps appear to have actually decreased a little. This suggests that subtracting oil richened the overall mixture. Furthermore it ran better and cooler in the midrange when I richened my needle. This is also consistent with lean-ish mixture, though evidently not catastrophically so. I plan to jet up a size next week and put the needle back where it was. Then test, test, and test some more. Hopefully someone learns from my mistakes.

[elitedio]

A couple of points:
Do you think that oil dribbling into the intake port mixes as well and rapidly and goes to critical areas, main bearings, rod ends etc as quickly as oil that is suspended in the fuel? Seems like a lot of fooling around and potential for weirdness.

You really don't have control of the viscosity of your fuel and hence mixture when you start adding or subtracting oil to the fuel in small amounts. Tuning decisions become muddled. Do I change from 60:1 to 75:1 to richen the mixture?

To simplify and make things work; run the same oil and gas mixed to the same ratio each time and vary the jetting to tune.

[Wheelman-111]

Greetings:

Quoth EliteDio:

To simplify and make things work; run the same oil and gas mixed to the same ratio each time and vary the jetting to tune.

On this we agree and perfectly. My thread is about what premix ratio to use. I was having precisely the problems to which you allude, methinks because I had been using way too much oil in the premix @50 or even 40:1. The yield was something in the 20:1 range once the oil pump added its *. Unlike The Bear's Hi-Po engines, this humble scooter wasn't producing the CC temps to handle that much oil, hence funky bicolor plugs and tuning madness.

Recently someone posted an article - maybe by Jennings again? - where some well-funded amateur actually did some scientific testing on oil mixtures in controlled (semi) scientific manner. Testing oil ratios down to like 10:1 on two Suzuki two-strokes on two dynos, starting with new pistons and jugs each time, running WOT (after break-in) to failure. Conclusion was engines get stronger the more oil you add, until plug fouls, provided jetting corresponds to those oil-rich ratios. Bear's teens:1 ratios in those slab-6 Mercs seem to support that. Trouble is I don't want to be fishing for spark plugs on a daily basis. Racing isn't street riding. We spend to much time idling to run 15:1 all the time.

Now I'm thinking 75:1 is plenty for a pump-equipped BBK. My calculations suggest the net is around 30:1 when the engine needs it most. A meltdown may prove me wrong. I'll keep at it.

[mousewheels]

Good discussion. Formula and sample table here for convenience. Note all figures assume the oil pump puts out 50:1 at 'full tilt'. 50:1 was used because it matches Mr Wheelmans calculation of using 75:1 to get to a net 30:1 ratio.

Premix calculation:
pre-mix ratio = 1/(1/desired ratio - 1/oil pump ratio)

Desired ratio: 30/1
Oil pump ratio: 50/1 {put in your value or assumption here}

Solve for pre-mix:
pre-mix = 1/(1/30 - 1/50) = 75:1

[Wheelman-111]

Greetings:

Pay no attention to Mr. Mousewheel's outwardly elegant mathematical solution to the dilemma that caused me massive migraine the other night. Though written by someone who obviously was paying attention in in Math Class, it is fatally flawed:

Note all figures assume the oil pump puts out 50:1 at 'full tilt'. 50:1 was used because it matches Mr Wheelmans calculation of using 75:1 to get to a net 30:1 ratio.

Y'see, anything based on "Mr. Wheelman's calculation" is likely to be as wrong as a Britney Spears/PeeWee Herman wedding. I never paid much attention in Maf, and most of my "calculations" involved fingers (and toes.)

Sinceriously thanks a lot Mouse. But I thought I had it solved where the pump max ratio was 32:1. My Maf's worse than I thought.

[mousewheels]

But I thought I had it solved where the pump max ratio was 32:1. My Maf's worse than I thought

Explanation: Both our calculations based upon your original assumptions. But the spreadsheet makes no attempt to guess at the original oil pump ouput or derate for bore size. The spreadsheet starts with your derated figure of 48:1 rounded up to 50:1. That's the 32:1 at the oil pump (OEM bore) that you calculated to be 48:1 after bore kit and re-jetting. (Below quote is the source).

But remember, the pump "thinks" it's feeding a 50cc bore. Since I'm running a 78cc, that means (and I'll spare you the calculations, simply because my head hurts too much to re-do them...) the highest ratio the pump can deliver to my Nikasil-encrusted bore is something closer to 48:1.

[Wheelman-111]

Greetings:

Uh, er... now I get it. I think. I mean I'm pretty sure.

I guess several different spreadsheets would need to be configured, based upon bore size and the Root Assumption of Honda-san's baseline pump output. I'm sticking with mine of 32:1 because that's the magic formula for race engines, and a stock scooter at WOT probably approaches that level of output and oil demand. Knowing Honda's tendency to provide "more than enough" - though tempered by EPA pollution considerations.

You think I'm theoretically close with my 75:1 choice, Mouse?