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Higher Octane Fuel = Timing Issues?

Posted By: ConwayFishy

Higher Octane Fuel = Timing Issues? - 03/07/18 11:29 AM

Could higher octane fuel in a Yamaha Pro 50, 2 cycle engine cause timing issues? When my lower idle kicks in 30 seconds after starting up, my engine will eventually cough and then die. This only happens when its idling. I can run it wide open and its like the day it was made. Thoughts? Opinions? Suggestions?

Motor has minimal hours on it. It was used 8 times per year from 1991 - 2014. The rest of the time it was under a cover inside of a barn. Always winterized and old gas was burned out of it before winter.
Posted By: Pat Goff

Re: Higher Octane Fuel = Timing Issues? - 03/07/18 12:25 PM

Not unless it's gone bad, which doesn't take long nowadays.
Posted By: spankyttx

Re: Higher Octane Fuel = Timing Issues? - 03/07/18 02:08 PM

bad fuel or it's compression testing time
Posted By: spankyttx

Re: Higher Octane Fuel = Timing Issues? - 03/07/18 05:22 PM

could be the idle circuits in the carb are getting clogged, those passages on the yami carbs are tight or it could just need adjusting

you didn't say what rpm's it was idling at before or if it is fresh gas but i would start with a compression test and check the carb idle adjustment, it could have backed off a bit, if that shows to be good, i would run a can or two of seafoam in a couple gallons of gas threw an external gas can at idle to see if it cleans up, if it doesn't clean up and it has good compression, i would lean towards the carb needs cleaning, that's all i got
Posted By: boocat

Re: Higher Octane Fuel = Timing Issues? - 03/07/18 09:19 PM

yeah, I'm thinking the non ethanol gas I can get here that I want to start using is 92. Shouldn't be a problem?
Posted By: V-Bottom

Re: Higher Octane Fuel = Timing Issues? - 03/07/18 10:12 PM

87 Octane is what you need. I heard higher octane E-Free gas will make the engine run hotter.
Posted By: tmd11111

Re: Higher Octane Fuel = Timing Issues? - 03/08/18 01:45 AM

Originally Posted By: V-Bottom
87 Octane is what you need. I heard higher octane E-Free gas will make the engine run hotter.


Actually its the opposite and too high of an octane will lead to excess carbon buildup
Posted By: spankyttx

Re: Higher Octane Fuel = Timing Issues? - 03/08/18 02:36 AM

higher octane burns hotter with less carbon build up
Posted By: Cat finder

Re: Higher Octane Fuel = Timing Issues? - 03/08/18 02:59 AM

Originally Posted By: spankyttx
higher octane burns hotter with less carbon build up

High octane burns cooler, which is why most run it to remedy detonation issues. No need for plus grade fuel mid grade is just fine.
Posted By: spankyttx

Re: Higher Octane Fuel = Timing Issues? - 03/08/18 03:14 AM

lol, better check your facts, it burns hotter and longer than low octane, less carbon build up, high octane is used in engines with higher compression to eliminate pre detonation knock before tdc (top dead center). so low octane will burn out before tdc causing the knock, high octane burns over the top past tdc, eliminating pre detonation knock
Posted By: Cat finder

Re: Higher Octane Fuel = Timing Issues? - 03/08/18 03:44 AM

Its just more resistant to self igniting at a given pressure or temperature. It even has a higher auto ignition temp. That being said it can't burn more hot as it would indeed have a shorter burn rate.
Posted By: spankyttx

Re: Higher Octane Fuel = Timing Issues? - 03/08/18 04:09 AM

higher octane burns slower, low octane burns faster, think of it like this, a low octane fire is like a flash fire, poof and its gone, a high octane fire will be like a bonfire, blazing and hard to put out. which one would be hotter?
Posted By: Curtbass

Re: Higher Octane Fuel = Timing Issues? - 03/08/18 09:36 AM

Spankyttx. You're partly right. Octane is a flame retardant. It burns slower the higher the octane. It doesnt increase the temperature of the flame. Just burns longer. The amount of heptane is what increases flame temperature.
Higher octane 89,91,93 will continue to burn past TDC on the timing issue. So , technically it burns too long for an engine designed to run on 87 octane fuel. Its got the same amount of heptane in most grades of pump gas. So temperature is same , only the burn rate changes by upping octane. In other words , your paying more for the higher octane & not beneffiting from it IF your motor is designed to run on 87 octane. It most likely can cause MORE carbon buildup because its not burning ALL the fuel at the proper time & an incomplete burn will result in more carbon residue being left in cylinder & in ring glands & behind rings on each stroke. Costs more for less efficiency.
Higher compression motors call for higher octane to PREVENT detonation BEFORE needed. Hogher octane slows burn rate down until piston is just before TDC to take advantage of getting more fuel into it before before starting compression stroke & Not igniting prematurely from compression. . Thats the basis of why a higher compression motor produces more power. It burns more fuel on its power stroke. Thats why timing is different on a higher compression motor.
Posted By: ConwayFishy

Re: Higher Octane Fuel = Timing Issues? - 03/08/18 12:26 PM

I found the issue. There was this small plastic box that controls the timing near the throttle control in the engine. Yamaha wanted almost 700 to replace this box the size of a two cigarette boxes. It's black and has 4 little marks on it that mark low idle, high idle, mid throttle and wide open. I simply, slightly, adjusted where the engine idles by raising it just a bit from the lower setting...maybe 1/10 of an inch. The thing really idles smooth in high idle and low idle now....and doesn't die... I also adjusted the carburetors as needed. SO it wasn't the gas, but was a timing issue indeed.

Also, I replaced the bolt that actually stops the throttle control when its wide open. I guess its kind of a control bolt that keeps the thing from going past that last little tic mark on the black box. Since I raised the lower setting to stop the coughing, I put a bolt in there with a slightly larger neck and in wide open the throttle stops at that last mark where it's meant to instead of going 1/10 of an inch past.

Thanks for the replies. I'll run 87 octane fuel in it from now on and avoid mid grade. See how that works. Also threw a can of sea foam in the tank to help get any gunk out.
Posted By: gary purdy

Re: Higher Octane Fuel = Timing Issues? - 03/08/18 12:36 PM

Definitely will have more carbon build using higher octane. Seen personally on all kinds of motors. Midgrade fuel is what most outboard manufactures recommend. Timing and operating temperature effects the fuel burn also.
Posted By: Cat finder

Re: Higher Octane Fuel = Timing Issues? - 03/08/18 01:24 PM

Thank you for the intelegent conversation. If you think about it like this... Given all are equal volume,temp and pressure it would burn more cool due to nothing changing. Resistance is the factor in the equation. Burns cooler slower and more deposits are built up because of said reasons. Any engine I've had always ran more cool by bumping up the octane to reduce ping.
Posted By: Gamblinman

Re: Higher Octane Fuel = Timing Issues? - 03/08/18 02:41 PM

Originally Posted By: Curtbass
Spankyttx. You're partly right. Octane is a flame retardant. It burns slower the higher the octane. It doesnt increase the temperature of the flame. Just burns longer. The amount of heptane is what increases flame temperature.
Higher octane 89,91,93 will continue to burn past TDC on the timing issue. So , technically it burns too long for an engine designed to run on 87 octane fuel. Its got the same amount of heptane in most grades of pump gas. So temperature is same , only the burn rate changes by upping octane. In other words , your paying more for the higher octane & not beneffiting from it IF your motor is designed to run on 87 octane. It most likely can cause MORE carbon buildup because its not burning ALL the fuel at the proper time & an incomplete burn will result in more carbon residue being left in cylinder & in ring glands & behind rings on each stroke. Costs more for less efficiency.


Higher compression motors call for higher octane to PREVENT detonation BEFORE needed. Hogher octane slows burn rate down until piston is just before TDC to take advantage of getting more fuel into it before before starting compression stroke & Not igniting prematurely from compression. . Thats the basis of why a higher compression motor produces more power. It burns more fuel on its power stroke. Thats why timing is different on a higher compression motor.


Excellent explanation. Thank you.
Posted By: Curtbass

Re: Higher Octane Fuel = Timing Issues? - 03/09/18 04:33 AM

Cat finder. Download the app on your phone called
puregas.org
It gives locations of some places that sell non ethanol fuel. It lists what octane rating at those places also.
My reasoning to drive 40 miles one way is twofold. FIRST: i dont have to deal with ethanol issues by doing it.
SECOND: i fill boat (56 gallon tank) 5- 5 gallon gas cans, & pickup. The pickup gets an extra 9/10ths of a mile per gallon on mileage. Ethanol itself is an octane booster & MOST people don't even know that.

So i normally buy 100 gallons at a time, but it lasts me longer & i dont have ethanol issue costs on top of it all.
Contact your Congressmen & Senators & let them know your AGAINST ethanol. Theyre actually starting to raise it to 15% ethanol with future increases to 20%. Theyre KILLING the outboard industry. THE main reason engine manufacturers are soon to phase out higher horsepower 2 stroke motors & developing their 4 stroke models. EPA & Ethanol producers are killing the industry. Period.
Good luck
Posted By: Cat finder

Re: Higher Octane Fuel = Timing Issues? - 03/09/18 03:41 PM

Curt. I miss the days of non-ethonal gas. When the switch was made I noticed a loss of 40 miles per tank. It is what it is I guess.
Posted By: Curtbass

Re: Higher Octane Fuel = Timing Issues? - 03/09/18 04:42 PM

Yessiree... I UNDERSTAND the reasons for Ethanol push, BUT, forcing EVERYONE to no choice for specific needs is what gets my hackles raised.
The same ones that tout ethanol sure WONT like it when they are forced to an electric only group that will try to take away THEIR choice of fuel!!! They'll be squeeling(sp?) like a pig caught under the gate then. HAHAHAHA
Posted By: ChuChu1

Re: Higher Octane Fuel = Timing Issues? - 03/10/18 12:22 AM

Originally Posted By: ConwayFishy
Could higher octane fuel in a Yamaha Pro 50, 2 cycle engine cause timing issues? When my lower idle kicks in 30 seconds after starting up, my engine will eventually cough and then die. This only happens when its idling. I can run it wide open and its like the day it was made. Thoughts? Opinions? Suggestions?

Motor has minimal hours on it. It was used 8 times per year from 1991 - 2014. The rest of the time it was under a cover inside of a barn. Always winterized and old gas was burned out of it before winter.


Have you changed your fuel filters?
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