Did my rejetting cause a 30% reduction in 2T oil mix ratio?

fisher1 · Post by **fisher1** » Wed Aug 14, 2013 8:20 pm

I’d appreciate some feedback on why the below is not correct. I’m no expert or engine pro, just trying to see if this makes sense. Thanks.
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Checkout this website
http://www.1728.org/flowrate.htm

1) Click on “flow rate”
2) For Diameter, enter “.92” on the left and “inches” on the right; this is the Stock Stella Main jet.
3) For Velocity, enter “1” on the left and “inches per second” on the right
4) For Flow Rate, leave the left and right blank
5) Then click on “Calculate”

Note that the calculated flow rate is 10.893 cubic centimeters per second.

6) Then change the diameter to “1.05” and re-click on “Calculate”

Note that the re-calculated flow rate is 14.19 cubic centimeters per second.
Do the math and that’s a 30% increase in flow rate!!!!! It doesn’t matter which measure you use as long as it’s the same for both.

So, when I re-jetted by my Stella carb’s main jet from its stock 92 to a 105 I actually increased the fuel flow by 30%! But the problem is that the auto-lube still outputs the same amount of oil, meaning that the 2T fuel-oil ratio is also leaned by a whopping 30% as well.

Please let me know what’s wrong with this thinking, why isn’t this logic correct? Does this contribute to why some Stella cranks & bearings fail?

Thanks.

Stitch · Post by **Stitch** » Thu Aug 15, 2013 2:49 am

Are you premixing?

fisher1 · Post by **fisher1** » Thu Aug 15, 2013 5:22 am

I wasn’t originally. But I’ve now started to add 2T oil to my fuel to compensate for the diluted fuel/oil mix caused by the up-jetting. The auto-lube is still functioning normally.

But I’m not sure exactly how much I should be adding (if any).

If the auto-lube was mixing 2% to the stock setup at speed, then it was adding 2.56 ounces to every gallon (128 ounces) of fuel flowing thru the carb from the original .92 main jet. But when up-jetting increased the fuel flow by 30%, then 2.56 ounces of oil added to 1.3 gallons of fuel flow (166 ounces) is only a 1.54% mix.

Mathematically, another .75 ounces of premix is needed for every 1.3 gallons of fuel to restore a 2% mix (including the auto-lube’s output).

But I don’t know if this thinking is correct. I haven’t read anywhere in the “Bald John” jetting recommendations (on Scooter Central) where any mention is made of 2T oil mix impacts, only the need to up-jet when free-flow exhausts and filters are installed – and I don’t know why that is.

And I wonder if the reduced oil mix impact contributes to the crankshaft and bearing issues that Stellas’ have. I don’t know, so I’m asking for other peoples’ thoughts and help.

A note of caution: I do have a CHT gauge installed, so I know absolutely that my temps are OK. But adding oil does lean out the fuel-air mix, which creates a totally different type of risk. Actually, I don't think any modification should ever be made on a 2 stroke without a CHT or EGT gauge to monitor the temperature impacts.

Stitch · Post by **Stitch** » Thu Aug 15, 2013 11:57 am

The auto lube is supposed to supply the correct amount of oil by rpm. So, yes it changes the ratio of the fuel/oil mix. It shouldn't be a problem though, because the motor should still get the right amount of oil.

fisher1 · Post by **fisher1** » Thu Aug 15, 2013 12:18 pm

I've read that before, and understand it as long as the same amount (or close to the same amount) of fuel is provided by the carb. Meaning, the auto-lube was designed with the original carb setup in mind to provide an average 2% mix, depending on the rpms and throttle setting.

But the auto-lube is completely insensitive to any re-jetting done to the carb. It dribbles the same amount of oil at a given rpm and throttle position regardless if larger jets were installed in the carb and 30% more fuel is being provided to the engine than with the original setup with the smaller .92 jet.

So, how can the original 2% average mix be sustained if the jets are changed to pass 30% more fuel but no comparable change is made to the autolube?

That's what my small brain doesn't understand. LoL

Stitch · Post by **Stitch** » Thu Aug 15, 2013 1:24 pm

Ok. Yes. It changes the ratio if you add more of one or the other.

Stitch · Post by **Stitch** » Thu Aug 15, 2013 1:41 pm

I don't know how else to explain that the ratio is irrelevant, and that the auto lube supplies sufficient oil for the motor at the rpms it is running at.

jimmbomb · Post by **jimmbomb** » Thu Aug 15, 2013 2:16 pm

So... as long as its getting the -predetermined amount of oil to the cylinder. ...it doesnt matter how much fuel?... as long as the cylinder is being lubed? ?? Correct?
.

BuddyRaton · Post by **BuddyRaton** » Thu Aug 15, 2013 5:43 pm

jimmbomb wrote:So... as long as its getting the -predetermined amount of oil to the cylinder. ...it doesnt matter how much fuel?... as long as the cylinder is being lubed? ?? Correct?
.

Yes...the volume stays the same. The worry isn't so much the cylinder...with oil starvation the bearings will go first

jimmbomb · Post by **jimmbomb** » Thu Aug 15, 2013 8:49 pm

BuddyRaton wrote:
jimmbomb wrote:So... as long as its getting the -predetermined amount of oil to the cylinder. ...it doesnt matter how much fuel?... as long as the cylinder is being lubed? ?? Correct?
.
Yes...the volume stays the same. The worry isn't so much the cylinder...with oil starvation the bearings will go first

So the predetermined (from the oil pump) amount of oil is ok for the piston/rings and cylinder....
BUT is it enough for the bearings too ie: no matter which jet??

and also....

just read this from a nearby thread...

"""topic25367.html"""

Quote in part...August 5th "Too much fuel will wash oil off the cylinder walls. When looking for a good safe tune, I look at the piston crown. http://vintagesleds.com/bs/index.php?topic=65633.15""

I hope this doesnt veer off of the OP's main question, but I think there is cause for concern within the topic..

fisher1 · Post by **fisher1** » Thu Aug 15, 2013 9:31 pm

I don't know how else to explain that the ratio is irrelevant

I don't know how to either. Really, I think the reason that can't be explained - is because it can't be true.

Every 2-stroke engine I know of has a recommended fuel-gas ratio specified by the manufacturer in order for the engine to survive - whether it be for bearings, rings, piston wash, whatever. That ratio is not expressed in terms of oil per RPM, or oil per unit of time, or oil per fill-up.

It's always expressed as a ratio of fuel volume to oil volume. Always. If I google a search on "optimum 2 stroke fuel-oil ratio" or something like that, I can't find a ratio that isn't associated with fuel volume and oil volume.

So, if richer jets provide substantially (30%) more fuel volume to the same old unchanged oil volume outputted by the auto-lube that was designed anticipating the stock smaller fuel jet, I don't understand how that can possibly be still maintaining the 2% oil volume - fuel volume ratio specified by the manufacturer.

Isn't it the same as making a gallon of 3% premix for an older VBB Vespa or a chainsaw, and then adding an extra quart-plus of raw unmixed fuel into the can and thinking it's "OK". That in essence is what I was doing - but I can't figure out how it's OK.

fisher1 · Post by **fisher1** » Thu Aug 15, 2013 10:25 pm

I don't mean to sound so emphatic..... I'm just trying to get to the bottom of this. Sorry if I sound that way.

I can add oil to my fuel as a premix to augment the auto-lube because with a 105 main AND and up-jetted SI-Pilot as well, I know that I will not lean out the fuel-air mix and ..... I also have a CHT gauge to keep a watch on temps.

Unless you can monitor your temps, I'd be careful about adding extra oil or doing anything for that matter. ( I once spotted an air leak in the exhaust clamp several months after installing a new muffler because the temps went up 25 degrees for no reason. )

BuddyRaton · Post by **BuddyRaton** » Thu Aug 15, 2013 10:27 pm

The 2% rule is ancient history. If premixing...yes it is important to get the percentage close. In many places in europe you used to be able to buy 2% premix. Piston ported Vespas such as a GS150 or SS180 require 5%.

A Stella uses an injector delivering a metered volume that increases with rpm.

The Stella's owner's manual states that all engine parts are lubricated by the injector (page 5). The system is also explained on pages 13 and 14. So NO it is not lways expressed as a ratio of fuel volume to oil volume. . If you google a search on "optimum 2 stroke fuel-oil ratio" or something like that, of course you will get results refering to percentage...Duh...you got what you asked for.

In the Stella manual there is NO mention of a percentage. The injector system is pretty much bullet proof.

If you are premixing it is a completely different story.

On my modified P200 I will be running the injector, just removing the return spring so it stays fully open at all times.

fisher1 · Post by **fisher1** » Fri Aug 16, 2013 1:18 am

The Stella's owner's manual states that all engine parts are lubricated by the injector (page 5). The system is also explained on pages 13 and 14. So NO it is not lways expressed as a ratio of fuel volume to oil volume.

OK, I get that - I really do. And I appreciate your reply & info.

Here's the "but"....... but doesn't the design and engineered flow of the Stella auto-lube anticipate the stock Stella carb & jetting?

How is it that the correct oil mix is provided to the engine by the same unaltered auto-lube mechanism regardless of how the carb jetting is altered and the flow of raw fuel increased?

Are you saying that changing / leaning the oil mix ratio by 30% due to up-jetting doesn't effect crankshaft bearing lubrication? Thanks!

Stitch · Post by **Stitch** » Fri Aug 16, 2013 2:43 am

fisher1 wrote:
I don't know how else to explain that the ratio is irrelevant
I don't know how to either. Really, I think the reason that can't be explained - is because it can't be true.

Every 2-stroke engine I know of has a recommended fuel-gas ratio specified by the manufacturer in order for the engine to survive - whether it be for bearings, rings, piston wash, whatever. That ratio is not expressed in terms of oil per RPM, or oil per unit of time, or oil per fill-up.

It's always expressed as a ratio of fuel volume to oil volume. Always. If I google a search on "optimum 2 stroke fuel-oil ratio" or something like that, I can't find a ratio that isn't associated with fuel volume and oil volume.

So, if richer jets provide substantially (30%) more fuel volume to the same old unchanged oil volume outputted by the auto-lube that was designed anticipating the stock smaller fuel jet, I don't understand how that can possibly be still maintaining the 2% oil volume - fuel volume ratio specified by the manufacturer.

Isn't it the same as making a gallon of 3% premix for an older VBB Vespa or a chainsaw, and then adding an extra quart-plus of raw unmixed fuel into the can and thinking it's "OK". That in essence is what I was doing - but I can't figure out how it's OK.

Are you trolling? Or is this a serious question?

fisher1 · Post by **fisher1** » Fri Aug 16, 2013 3:12 am

Yes, I'm being serious - but maybe I need to just "drop it" and let it go.

Sorry for the hassle, I just can't figure out how a diluted mix lubricates bearings as well as stock undiluted one does.

But, no matter - thanks - I'll let it go.

Stitch · Post by **Stitch** » Fri Aug 16, 2013 3:20 am

It probably doesn't , but its still adequate for the application it's in.
You might be able to get more specific answers if you ask very short technical questions, and then expand on them once the info starts flying around. I think stuff gets lost in translation.

stASH · Post by **stASH** » Fri Aug 16, 2013 1:08 pm

It sounds to me like the oil injector in the stella is calibrated to supply a specific volume of oil to the engine per rotation, and that volume is the appropriate amount to protect the engine. So, one drop of oil each rotation is what the engine needs. Even if more fuel is running through the carb, the engine is still getting the one drop of oil that it needs with each rotation.

BuddyRaton · Post by **BuddyRaton** » Fri Aug 16, 2013 1:49 pm

Here is some information on DelOrto main jet sizes. First off...Italians use the metric system. There is no way going from a 92 to 105 is going to increase your gas flow by 30%. You would be so rich that it would never even start.

The jet size is related to diameter and for dellorto it is close to tenths of millimeters. Here are some examples of DellOrto jet sizes and diameters

•40 is a jet with a 0.38mm hole
•41 is a jet with a 0.39mm hole
•42 is a jet with a 0.40mm hole
•43 is a jet with a 0.41mm hole
•45 is a jet with a 0.43mm hole
•46 is a jet with a 0.44mm hole
•47 is a jet with a 0.45mm hole
•50 is a jet with a 0.47mm hole
•53 is a jet with a 0.49mm hole
•55 is a jet with a 0.51mm hole
•57 is a jet with a 0.53mm hole
•59 is a jet with a 0.55mm hole
•60 is a jet with a 0.56mm hole
•62 is a jet with a 0.58mm hole
•63 is a jet with a 0.59mm hole
•65 is a jet with a 0.61mm hole

After that you need to take this information and plug it into your flow calculations.

fisher1 · Post by **fisher1** » Fri Aug 16, 2013 3:01 pm

There is no way going from a 92 to 105 is going to increase your gas flow by 30%. You would be so rich that it would never even start.

OK, then what is the increase? That's the crux of what my original post was trying to understand.

Because if the calculator below is used, then the actual flow difference between a .46 diameter jet on your chart (half of a 92 stock jet) is compared to a .53 diameter jet on your chart (a bit more than half of 1.05) then the flow difference is 33% !

There's a big calculation difference between just the diameter of a hole and the AREA of a hole. A 2 inch pipe is only twice as wide as a 1 inch pipe, but will flow FAR more than twice as much because 1) it's AREA is far more than twice as much and 2) the impact of fluid dynamics.

http://www.1728.org/flowrate.htm

As to your comment that it would be too rich to run ...... the optimum air-fuel (vapor) mix is about 15-1 or .066. Increasing the smaller fuel component by 30% only results in 15 - 1.3 or .086. Richer, yes - but not as big of a calculation difference as one would expect because of the far larger air volume component.

BuddyRaton · Post by **BuddyRaton** » Fri Aug 16, 2013 3:38 pm

The table you linked provides calculations for water in a pipe and really has nothing to do with carburators.

I'm not going to argue...it's your scooter...do whatever you want.

These things were designed to run on the crappiest 2T oil available 30 years ago. I think some people have way too much time on their hands and way overthink oil.

As far as your original question...use your injector. If your going to kit or mod it...change your crank and use your injector.

Out

Howardr · Post by **Howardr** » Sun Aug 18, 2013 6:42 pm

Well, I can't really help you with your question. I will add this, though. I modded my Stella and the best mechanic I know, was concerned (like you) that the injector would not be able to keep up with demand. Our plan is to eventually install a P-200 injector. In the meantime, I run the stock injector AND pre-mix the fuel to 1%. It may be overkill, but I don't want to risk my motor.

That's my $.02

Howard

fisher1 · Post by **fisher1** » Sun Aug 18, 2013 8:46 pm

I run the stock injector AND pre-mix the fuel to 1%. It may be overkill, but I don't want to risk my motor.

I agree with you and your mechanic.

Also, I was at the races this weekend and asked the guys who run 270cc micro-sprints (2-strokes) if they still ran the automatic mixers on their engines - none do, they all pre-mix after they modified their engines.

They also told me that my thinking that up-jetting richened the air-fuel mix to @ 15-1.3 was incorrect because of the added air flow thru the carb that the less restricted exhaust creates. Their opinion is that the up-jetting is needed to restore back a typical @ 15-1 mix because of the lean condition created from the exhaust change.

Meaning that more air volume and fuel volume is flowing thru the carb into the engine - but the oil volume is unchanged and thus lean.

But I don't know if the impact is 30%, 20%, or 10% - it's like Buddy Raton correctly says, the formula isn't an exact fit. But, the main jet's size measurement is simply the size of it's hole - so ....... I donno.

The guys at the track also said they run an oil mix of @ 3% and some run more - but they are racing, not running stop and go street work.

Bottom line is that I think there's cause to be concerned & the more information different people can add the better our understanding will be.

BuddyRaton · Post by **BuddyRaton** » Sun Aug 18, 2013 9:03 pm

fisher1 wrote:Meaning that more air volume and fuel volume is flowing thru the carb into the engine - but the oil volume is unchanged and thus lean.

Lean refers to the air fuel mix. It is obvious that you refuse to believe that the volume of oil is unchanged.

You are using flow equations for water. Did you remember to include the Hazen Williams friction factor?

You are soliciting advice from guys that run race bikes. If you were building a race bike that would be a good source, you're not.

If anything adding oil to your gas is leaning out your mixture...more oil volume...less gas volume per stroke = leaner running condition. Lean running is the main cause of engine seizures.

The fact that nobody on scooter central wants to touch this question should also be an indicator.

You are going to believe what you want to believe, keep asking the same question until someone give you the answer you want regardless of facts. Good luck and enjoy your delusions of a 30% increase in fuel flow. Ride with the clutch covered.

fisher1 · Post by **fisher1** » Sun Aug 18, 2013 10:47 pm

Lean refers to the air fuel mix. It is obvious that you refuse to believe that the volume of oil is unchanged.

I'm referring to the resulting lean fuel/oil mix, not the fuel/air.

We know the Stella auto-lube was designed for a stock Stella. But with the less restrictive exhaust & filter many of us now use, it's not stock anymore. Significantly more fuel is being ingested because of the larger jetting and thus the oil mix is reduced. That condition cannot be helping the bearings, which many people are already having trouble with.

But what is the actual increase in fuel flow? I think (like you) that the 30% calculation of added fuel is high, but that's what the calculation says - and I don't know another formula for fluid flow.

P.S. Since I'm running a 1.05 main and a larger SI Pilot as well, my fuel-air mix is just fine (not lean) with the added oil that I pre-mix, and the 300-310 degree temperature readings from the CHT confirm that.

Sorry if I've created a hassle, I'm settling in on the same strategy as Howard's mechanic and just adding 1% premix and be done with it.

Thanks,

Stitch · Post by **Stitch** » Mon Aug 19, 2013 2:25 am

We run 1% over the injector on our 177's. my temps are good, it happily runs up to 7500rpm's and runs just fine wot for an hour at a time (rider wants a break, not the scooter).

Before someone lunges at me, this is not a magic bullet. Every collection of parts has different requirements, please check with your doctor to see if premix is right for you.

BuddyRaton · Post by **BuddyRaton** » Mon Aug 19, 2013 12:28 pm

fisher1 wrote: But what is the actual increase in fuel flow? I think (like you) that the 30% calculation of added fuel is high, but that's what the calculation says - and I don't know another formula for fluid flow.

Sorry if I've created a hassle, I'm settling in on the same strategy as Howard's mechanic and just adding 1% premix and be done with it.

Thanks,

Dude...do you even read the responses?

THE FORMULA YOU ARE USING IS FOR WATER IN A PIPE, NOT JETS IN A CARB.

And in case you didn't read it the other times

THE FORMULA YOU ARE USING IS FOR WATER IN A PIPE, NOT JETS IN A CARB.

You started out with a completley wrong assumption and are now stuck in your own little world.

You ask for advice and then argue using the same stupid assumption.

Either you really don't get it or this is a troll that did a good job of hooking me.

And people wonder why some stop offering advice.

Some people do add an additional 1%. I'm not a believer that it leans out the mixture. At WOT the auto lube is delivering close to 3%..adding another 1% gets you to about 4%. Really the worst that will happen is you will foul a plug.