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Jan 23rd, 2013
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Higher Octane Fuel = Timing Issues? #12661794
03/07/18 11:29 AM
03/07/18 11:29 AM
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Northeast Texas
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ConwayFishy Offline OP
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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.


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Re: Higher Octane Fuel = Timing Issues? [Re: ConwayFishy] #12661837
03/07/18 12:25 PM
03/07/18 12:25 PM
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Seadrift TX
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Not unless it's gone bad, which doesn't take long nowadays.


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Re: Higher Octane Fuel = Timing Issues? [Re: ConwayFishy] #12661962
03/07/18 02:08 PM
03/07/18 02:08 PM
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Denton Tx.
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spankyttx Online content
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bad fuel or it's compression testing time


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Re: Higher Octane Fuel = Timing Issues? [Re: ConwayFishy] #12662311
03/07/18 05:22 PM
03/07/18 05:22 PM
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Denton Tx.
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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


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Re: Higher Octane Fuel = Timing Issues? [Re: ConwayFishy] #12662750
03/07/18 09:19 PM
03/07/18 09:19 PM
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Hill Country
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boocat Offline
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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?

Re: Higher Octane Fuel = Timing Issues? [Re: ConwayFishy] #12662836
03/07/18 10:12 PM
03/07/18 10:12 PM
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V-Bottom Offline
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87 Octane is what you need. I heard higher octane E-Free gas will make the engine run hotter.


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Re: Higher Octane Fuel = Timing Issues? [Re: V-Bottom] #12663139
03/08/18 01:45 AM
03/08/18 01:45 AM
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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

Re: Higher Octane Fuel = Timing Issues? [Re: ConwayFishy] #12663213
03/08/18 02:36 AM
03/08/18 02:36 AM
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Denton Tx.
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spankyttx Online content
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higher octane burns hotter with less carbon build up


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Re: Higher Octane Fuel = Timing Issues? [Re: spankyttx] #12663240
03/08/18 02:59 AM
03/08/18 02:59 AM
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Dallas, Texas
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Cat finder Offline
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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.


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Re: Higher Octane Fuel = Timing Issues? [Re: ConwayFishy] #12663260
03/08/18 03:14 AM
03/08/18 03:14 AM
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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


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Re: Higher Octane Fuel = Timing Issues? [Re: ConwayFishy] #12663295
03/08/18 03:44 AM
03/08/18 03:44 AM
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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.

Last edited by Cat finder; 03/08/18 04:03 AM.

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Re: Higher Octane Fuel = Timing Issues? [Re: ConwayFishy] #12663308
03/08/18 04:09 AM
03/08/18 04:09 AM
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Denton Tx.
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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?


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Re: Higher Octane Fuel = Timing Issues? [Re: ConwayFishy] #12663391
03/08/18 09:36 AM
03/08/18 09:36 AM
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Grand Prairie Tx
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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.

Last edited by Curtbass; 03/08/18 09:40 AM.

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Re: Higher Octane Fuel = Timing Issues? [Re: ConwayFishy] #12663433
03/08/18 12:26 PM
03/08/18 12:26 PM
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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.


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Re: Higher Octane Fuel = Timing Issues? [Re: ConwayFishy] #12663444
03/08/18 12:36 PM
03/08/18 12:36 PM
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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.


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