Fuel 87 89 or 91 octane?

[Johnny 5]

I have been running 91 octane in our buggys with 145 psi compression and am wondering if it is too high causing carbon build up. Last weekend when the motor on the 183cc buggy was apart with about ten hours on it to replace the case gasket it had a lot of black carbon on the piston and head.

The float level is good and fuel/air screw at 1.5 turns out 125 main with a 35 pilot. Uni filter with a 6 inch intake tube and stock muffler with opend up tail pipe.

It runs good but always has black soot in the tail pipe. My 155cc red buggy with 122 main 34 pilot same intake and muffler set up is clean almost on the lean side.

[x-bird]

black soot in the tailpipe is usually an indicator of running too rich.

[Johnny 5]

I am wondering if back east if your gas has more ethanol and that is why I see some buggys with large main jets. My stock 150cc with a uni ran awesome with the 114 34 jetting. I tried a 120 and it fell on its face, went with a 117.5 with the uni set up for best results. I see some people running 130 mains on stock motors and it dosnt make any sense how they would run without choking from too much fuel.

[MASTERBATES]

Mine is happiest with 140 main and 38 pilot. I have also been blending my 10% ethenol 87 with PC1 brand octane booster. It runs really smooth. I have been keeping an eye on my cylinder with a harbor freight snake scope camera. These cameras are worth every cent of the $100 price tag.......also my buggy calls for 91 octane fuel. Its just hard sin southern Illinois to find a pure based gas, so that's why I chose to take this route

[SLESTAK75]

The huge difference in jet sizes has to do with elevation. Elevation affects not only the amount of O2 in the air but also the density of the fuel spray. Higher elevations need larger jet size. The black deposits are from running too much fuel in the a/f mix.

[Masteryota]

Agreed, soot and carbon are a dead giveaway of a rich condition, and if running too rich, it can actually hurt the performance as well. Lean runs hot, in both ways, more power, and more heat, however, if the compression is too great and running lean, you might start melting pistons. If you want to try something before changing the jet, then try a different heat range on the plug. Given the driving conditions of most buggies, you would want a 'colder' plug, but since you have some soot and carbon build up, I might try a 'hotter' plug, and perhaps iridium, if you don't already have it. Try google-ing your plug number and heat range, you might get lucky and have them at your local auto parts store, I just picked up a set of 'colder' iridiums from advance for my sport bike, to my surprise, in stock.

[Masteryota]

Originally Posted by SLESTAK75

The huge difference in jet sizes has to do with elevation. Elevation affects not only the amount of O2 in the air but also the density of the fuel spray. Higher elevations need larger jet size. The black deposits are from running too much fuel in the a/f mix.

Its actually the opposite, less air, less fuel, and also less power. Take this in mind as well, if, lets say, you wanted to compete in one of the buggy bash's drag races, it would be a great idea to rejet for the locations elevation, or have a stand by carb with the jets ready to go. Oh carburetors, now you know why I love fuel injection, lol

[SLESTAK75]

Originally Posted by Masteryota

Its actually the opposite, less air, less fuel, and also less power. Take this in mind as well, if, lets say, you wanted to compete in one of the buggy bash's drag races, it would be a great idea to rejet for the locations elevation, or have a stand by carb with the jets ready to go. Oh carburetors, now you know why I love fuel injection, lol

I thought this was the case but I have noticed from comments made here that people at higher elevations often post larger jet sizes. It seems that it would be opposite as air is thinner the higher you go. Am I missing something??

[SLESTAK75]

Then again I may just have read it backwards. Wouldnt be the first time.

[Miamieddie]

Use 91 octane you can't go wrong using good gas.. you can go wrong using cheap gas. Use cheap gas for your lawnmower, weedeater, edgers, etc. Not on your buggy.

[MASTERBATES]

hey i lucked out! checked the website and found a gas station on the way to work that sells pure 92 octane....score

[Masteryota]

Originally Posted by Miamieddie

Use 91 octane you can't go wrong using good gas.. you can go wrong using cheap gas. Use cheap gas for your lawnmower, weedeater, edgers, etc. Not on your buggy.

I especially would not use cheap ethanol blended fuel in my lawn equipment, I don't know about you, but I hate having to work on my mower or trimmer before I can use it, and I see the effects ethanol have on both, as I have had to sink $$$ into both just from ethanol. Since I switched, my mower started on the first pull after sitting for 5 months.

[Miamieddie]

I agree with you 100% masteryota I was just stating a point not to use cheap gas on the buggy. I'll admitt one thing I do use regular gas on my lawnmower though but never on my buggy nor my truck.

[jmansracerocket]

its funny I use 93 octane and still have the stock pilot jet and main jet that came with the carb and they both don't have #'s on them but they work the best and Im still running rich because I to get black soot and carbon build up but im hoping with my new head the mixture will be better.

[Johnny 5]

Thanks for all the reply's, I was thinking that the 91 was not burning complete because of the 145psi compression. The Honda civic I did A compression test at work the other day had 215psi and runs on 87. I will still use the 91 because carbon is better than a blown up motor.
I finished putting the 13/43 gears in after having to grind the case open for the 43 tooth gear that is about 1.5mm larger in diameter. I also replaced the bad output shaft bearing that was easy to get out but was hard going in. I was not getting it in straight and thought the case was going to crack, but with patience it went in. Way smoother when the shaft is rotated, the old bearing was rusty and full of sand.

Will get it running after work today and post results from the 3.08 to 3.30 ratio.

[rich1]

The compression ratio is what determines the correct octane . General guidelines for all 2 and 4 strokes are as follows ( Turners Tuners hand book)

Compression Ratio / Octane #

8.0- 8.5 ..... 87-89
9.0-9.5 ..... 90-91
10.0-10.5 .... 91-93

Most GY6 motors are in the 8.5 to the 9.5 for a stock motor. If you dont know what it is then you have to CC the head .

[jpman]

Around here most gas stations advertise ethanol free fuel. And my new rototiller ( Briggs motor) forbids using fuel with ethanol.

[rich1]

Originally Posted by Masteryota

I especially would not use cheap ethanol blended fuel in my lawn equipment, I don't know about you, but I hate having to work on my mower or trimmer before I can use it, and I see the effects ethanol have on both, as I have had to sink $$$ into both just from ethanol. Since I switched, my mower started on the first pull after sitting for 5 months.

Sta-Bil now makes a gas addative for E10 based gas for 4 strokes. Ive tried it and it works good. I have not seen a bad effects that E10 causes with mixed gas 2 strokes. However all the mixing oils for 2 strokes already contain a gas stablizer.E10 does not seem to cause any problems in fuel injected engines either.EPA is now pushing to make us use a new E15 based gas .Consumer Reports tests however have shown fuel system damage .