What octane fuel and main jet size should I run in my 1975 135 Evinrude??

[Sparkieboat]

my 135 evinrude has about 140-150 psi on each cylinder. I am going to copy and paste some post from another thread that I unintentionally hijacked. So I am starting this one to help myself and others who may be needing this information.

My question was how can I tell if I need to run higher octane fuel or larger jets. An answer that I got was this one.


its hard to tell but as long as your jetted richer you safer. if your sparkplugs are cardboard brown or darker this is good. without fouling plugs of course. if your jetted on the edge you may not know it until you stick a piston. keep in mind that outboards run at such varied air temp. early spring up here in the north it may be 50F first time i get my boat out. in the summer it could be 90F. look at this picture. my plugs were perfect brown. on a long run across the lake the engine quit. I thought i ran out of gas. never heard anything. the lower right piston detonated and melted. death ash grey. the other pistons were all on the verge of being too hot. too lean, too dry. that brown grey color on the piston top is not good.
if your going to run your 74-76 135 hp i would just run the highest octane pump you can get. 91-93 never use old fuel. never run flat out under heavy load unless you know its rich enough. <fieldset class="postcontent"><legend> Attached Thumbnails</legend> </fieldset>

[Sparkieboat]

The parts list that I saw list several jet sizes for the 135, in 1973 it was a 73C in 1974 it was a 69C, in 1975-76 it was a 61C. I am thinking that I may need to go with a mid to high 60s size jet because octane levels are much lower now than they were then. What do you think?

[Euroski]

What octane were you using when that happened? Just to be safe I would only run Premium...

[EMDSAPMGR]

When that engine came out it was probably made to run on 89 leaded fuel back in the day. Due to the high compression heads and today's ethanol fuels, I'd run 92 high test only-unless you plan on retarding the timing. The stock jets are ok if you feed the engine with the right fuel. If you jet up, it's always possible it will overfuel. Not a real problem for the durability of the engine, but it may not run top rpm if it's overfueling.
Keep in mind it's heat which caused the pictured failure above. The water deflectors in the picture are correctly placed and the water passages around each cyl are clear-look good. So, some of the next questions should be: was the high speed jet that fed that bad cyl restricted in any way? (lean condition) Was the timing too high? was there an internal water restriction in the head, which caused an overheat? (pull off the head water cover and check. ) Is the impeller weak and in need of replacement. Is the pressure relief system working properly in the vernatherm housing? (adequate cooling water when on plane) Hopefully, you've got all these questions covered for your engine. A laser temp gun is really helpful.

[Sparkieboat]

the engine with the burnt piston was not my engine, but someone else posted that on another thread. I am just trying to start a new thread so as not to hijack someones thread. thanks.
The timing sticker on the silencer air cover says 20 degrees. I have a 1974-76 135, I am getting a set of carbs for it and want to jet them properly. I think that 20 is retarted enough dont you?

[hsbob]

if you taking the 135hp motor apart, check the balance of the crank shaft. this was a know problem on the 135. also cut a 1/2" x 1" slot on the bottom of the exhaust flange [right before the bubble] , [between the bottom cylinders]. the 140 jets are larger than the 135.

[Sparkieboat]

Thanks, I had not planned to take the engine apart. I hope that it will run out good like it is. I assume that drilling a slot in the exhaust flange would require me to at least remove the power head from the midsection and removal of the bubble back cover. I am planning on putting this motor on my 1986 procraft 17.5 bassboat. I am hoping to really get some good performance out of it. What would be the max RPMs that you would run this motor without any modifications?

[71V153]

the 140 jets are larger than the 135.

No they're not. The 135 jets are bigger.

[Sparkieboat]

Anyone know what a good set of 135 heads would sell for? what about the intake?

[phillnjack]

I know its an old post but just incase someone wants to know the jet sizes .

Jets on the OMC v4 crossflows went up and down. here below is what they came with from factory.
1973 135hp had 73c jets.. these were the largest jets......... 1974 135hp had 69c jets.. so a drop down of 4 sizes
1975 & 76 135hp had 61c jets,.. a huge drop of 8 sizes down
1977 140hp (the fast one) had 65c jets. so back up just 4 sizes for the 140hp. same jets for 1978, and 1979.
1980 the 140hp jets went up to 67c. and stayed this size until 1985..
.