Good Compression

[nc dawg]

I have a B-75 Honda 4 stroke that will not crank
I can't get it to even hit a little by spraying carb cleaner directly into the cylinder or into the carb.it's getting a good spark on both plugs so my only conclusion is low compression.My question is, what is good enough compression to crank and run...also i can't seem to find out what year it is the only number that i can find is B75 2007149 every time i give this number to a boat store they always say that this number is no good....any help with these two issues will be greatly appreciated

 

[Johny25]

I would think 85-90psi in all cylinders would be enough to fire over on a 4 stroke. As long as they are all within 15%. But I also know 4 strokes like higher compression than a 2 stroke. I also don't know jack squat about Honda's and not very much about 4 stroke outboards lol..... :lol:

 

[shawnfish]

good effin luck trying to figure out the year... theres a number front of the block at the base that is supposed to indicate year but i never had any luck. i got a steal of a deal on a bf50 that was immaculate besides a few minor parts, after 3 months of trying to get an answer on what year it was i said to hell with it sold it!!!! dont know what i thinking to buy a honda anyways this is AMERICA JACK!!!!!! no, i was thinking put 200 bucks or so in it and making a few grand to buy a boat but in the end i doubled what i paid so i was happy.........

p.s. i checked every number i could find on the motor and when i tried to match them they didnt exsist....

 

[nc dawg]

i hear ya,we checked the compression and it had like 60/65 this may be the problem..i know what you are talkin bout Shawn ...this motor is immaculate but it wont hit a lick

 

[Johny25]

Like I said I am no expert in 4 strokes but from watching and reading enough posts on them I can assure you that those numbers are way to low for a 4 stroke with a displacement of a 75hp motor. I would guess 150psi and above would be where you want to be with that size motor. I have seen posts that state normal readings are from 170-220psi on these type of engines. So my guess is that you found the problem

 

[nc dawg]

im sorry this is a B 7.5 hp Honda.the decal says B75,,my apologies for not wording my post a lil clearer..

 

[Andrew04]

Make sure spark plugs and wires are not switched, if this happens it'll have spark but never fire. Compression seems low, but may be acceptable. My next guess would be trouble in the valvetrain, and that would explain the low compression as well as not firing. Good luck!

 

[nc dawg]

K..i never thought about the plugs wires being crossed.thanks I'll check that and get back to ya.. i wonder how much would it would cost to rebuild this motor..prolly more than its worth.idk

 

[Johny25]

If you do the rebuild yourself it isn't as bad but if you take it to a mechanic you might as well leave your first born with him also. way more cost effective to pickup another motor.

 

[nc dawg]

i hear ya Johny..i,'m gonna do it myself ..have a machine shop bore it out, idk whats the norm 30 over.buy new rings/pistons i guess lap the valves kinda like rebuilding a go cart/lawn mower motor sound like a good challenge to me unless it to expensive.

 

[Johny25]

I'm guessing it will cost you $300 plus in just parts, maybe more? I don't even know where to find powerhead kits for honda's though. I am just shooting in the dark on this post and maybe I will learn something my self from this :lol:

.30 is usually the norm, that is if you have to have it bored. Maybe the walls are good still and just a honing is required?

 

[nc dawg]

Ok if the walls are still good and i can get by with a good honing ,then what, get some over size rings.surely that should'nt be too expensive..

 

[Johny25]

Depending on the clearance specs for that particular engine you may only have to get rings after it is mildly honed. If it is bored or honed to much then you will have to get oversized pistons. I have no idea what the clearance specs of that motor would be though? And you are supposed to be putting pics up as you do all of this

 

[nc dawg]

i hear Johny..when i get all this figured out I'll post plenty of pics.prolly gonna be a winter project.i just got a 9.9 merc running good so i got a lot fishin/huntin to do till then

 

[nc dawg]

Ok..i cheked the plugwires and swapped them.still doesn't hit..so i'm still trying...thanks guys for the help ..

 

[nc dawg]

guess what guys...i started tinkerin with this honda again and found some points and condenser ...did'nt have a clue ..well i'll keep yuns posted...

 

[turbotodd]

check the valve clearance.

Compression psi can be astrayed some. Many of the Japanese outboard manufacturers use an auto decompressor which reduces cranking compression. 65 psi sounds just about right for an auto decompression mechanism....but check the valve clearance first.

Also have you REPLACED the spark plugs? I've seen more than a couple where they'll fire outside the motor...but put them under pressure inside the engine and they won't fire a lick. Replacement usually fixes that

I know not much about Honda's but you don't bore any of the Jap stuff .030". You bore them typically .50mm which is roughly .020" inch. SOme of them can't be bored at all. Plated cylinders. In that case, you replace the block-or the cylinder if it comes off of the block like many of the single cylinder bikes.

 

[nc dawg]

thanks turbotodd..I'll check valve clearance too..changed plugs still no good fire/spark gonna work on the points and condenser..my friend that is helping me ...well he has been tied up a lot so maybe he'll be able to help me more after Sept.again thanks for the help.