1980 Honda BF75 average speed?

[Tarinazo]

I was working on a Honda B100 for the past few weeks, once I had it running and ready to hit the water... I traded it for a BF75! I use a 1436 jonboat for fishing, just me (175 pounds) and my son (100 pounds or less, skinny 15 year old), the B100 I had was a long shaft, this BF75 outboard is a short shaft and is in a much better overall condition than the B100, it seems hardly used, runs on the first to third pull and idles with no problem... pee streem seem narrower than the B100 but it's a steady stream of water, I'll check the impeller anyway. Thing is I never got to use the B100, within two days of having the B75 I took it for a spin and noticed it was slower than my Mercury 6hp (87 two stgroke). Is this normal? My 6hp tops at 16mph, BF75 tops at 11mph... same boat.

 

[Sea Rider]

Is the BF75 a 7.5 HP-OB ? Is the boat rated for a short or long shaft OB ? With under powered OB's max speed depends entirely on OB/tramsom height, trim, proper deck weight distribution and prop pitch.

Happy Boating

 

[Tarinazo]

It's a 7.5hp OB, the jonboat is rated for a short shaft OB. Height of the cavitation plate is 1" below my boat's lowst point, OB's trim is set so that the cavitation plate is alligned with thw boat (at best, not perfectly).
My jon has a fishing deck on it, it's not that heavy fully equipped... I know this factors affect the performamnce of the engine but my concern was it being slower than a 6hp 2 stroke.

 

[Sea Rider]

If wanting to count with the max power out your 7.5 HP-OB while powering a large boat, firstly need to know the factory min-max wot rpm range. Check OB tech specs for that.

Secondly install an induction tach, go for a wot spin as usually will be loaded and check achived max wot rpm on calm flat no wind water cond, Check if the wot rpm achieved is inside its factory rpm range.

If found revving towards the min side need to drop at least 1-2 pitches from current one for OB to rev towards middle/max wot rpm range, combo will gain some more speed. For that to happen OB must be running 100% healthy.

If a 6 HP-OB performed much better probably was better propped than the 7.5 HP-OB or the latter is begining to have prop issues ?

Happy Boating

 

[Tarinazo]

Induction tachometer... off to amazon!

 

[Sea Rider]

Induction tachometer... off to amazon!

If plan buying one would recommend the Hardline Hour/Tach model # HR-8061-2, compact and easy to install. Follow installation procedures. Can install it on the arm next to the trottle grip. Report max wot rpm achieved to go from there...

Happy Boating

 

[Tarinazo]

Before getting a tach, I went ahead with a general maintenance on it, I took off the carb to give it a good clean... I know: if it ain't broken, don't fix it, but since this is a new to me outboard, I guees it's a good task.
Took off the carb and it seems new, no varnish, no scuffs, no residue marks nothing, brand new... even then I'll clean it.
Took off the intake manifold: NEW! perfect gaskets, valve ports clean as a whistle.
Took off the valve cover... you guessed it: new!
The motor seems hardly used, I know it's been used because of some carbon buidup on the exhaust channel wish I'm guessing is more of a carb adjustment issue, and that's exactly where I'm going at with all this... the carb's mixture screw is 1-7/8 turns out from fully seated, I've read that it most likely has to be around 2-1/4 turns BUT! I've also read that on some carbs 1-7/8 turns is correct so how do I know wich setting applies to me? my carb has no low speed jet screw form the outside, just a small ball bearling type of plug. OB's SN: 1201769.

 

[racerone]

I am sure there is a low speed mixture ( pilot ) screw.----There may not be a high speed mixture screw.-------Make sure it is running on both cylinders too.

 

[Tarinazo]

How do I know if I have the 2-1/4 or 1-7/8 turns out of the mixture screw carb?

 

[racerone]

The high speed jet is fixed no adjustment.-------Adjust the low speed mixture screw so the motor idles nicely and accelerates smoothly

 

[Sea Rider]

Most outboards are factory adjusted at sea level, if that's your case and OB is idling, revving smoothly leave it there. That adjustment is only good for idle, ralenti rpm once passing that throttle setting non adjustable fixed jet will take over..

Happy Boating

 

[Tarinazo]

Yes, I'm at sea level... 6 meters under sea level actually. And yes, the engine idles smooth and starts between the second or third pull so I left the mixture crew at 1-3/4 turns out from seated (it was not at 1-7/8 as I stated before, sorry, I'm a metric guy so...).
Yesterday I assembled tha manifold and carb back to the engine and then took the lower unit off for inspection, again everything looked new but with lots of carbon residue... powdered black everywhere. One rubber fin on the impeller is partially broken, ripped but still in place so I guess I'll be needing a new one, gaskets came off in one piece. No body corrossion anywhere.

Because of the carbon issue, I proceeded to do a valve adjustment and noticed that on TDC, the intake valve had no clearance, exhaust valve had some movement, but the intake valve. Same thing with the other cylinder: intake valve tight and exhaust valve had little movement.
I left every valve at .003" clearance while at TDC.

 

[Tarinazo]

Next up: compression test!

 

[racerone]

Not uncommon for early Honda motors to have burnt valves,----Do you agree now that your motor has a FIXED high speed jet with no adjustment ?

 

[Tarinazo]

Yes, I was confused between the high and low speed jet, the carb on the B100 had an access to clean the jet, not to adjust, from the outside. this one doesn't

 

[Sea Rider]

If having some form of carbon build-up worries you, buy a can of CRC decarbon product, remove carb's air cover, once OB is well warmed up spray several short consecutive blasts through the carb throat. Probably will not remove all the already hardened carbon formed, but OB will idle and run more smoothly throughout the whole throttle range.

Happy Boating

 

[Tarinazo]

I'm not worried about the carbon buidup, I'm more worried about why does it have so much of it if everything inside and out of the OB seem new, barely used. I normally use Sea Foam in fuel of my motorcycles and OB's as a fuel stabilizer more than a decarbonizer. Today I'll do the compression test and on Sunday I'll give it a spin to see if adjusting the valves helped with the lack of power.

 

[Tarinazo]

Did a compression test today with positive results, it's a HB compression meter so I guess readings are not too precise, even so, both cylinders showed to be in the hundreds.
top cylinder 115, bottom cylinder 110

 

[racerone]

These 4 strokes are generally a bit higher I think.-----Does it have compression release on it ?----Have a couple of the ancient Honda 7.5 myself.----Maybe run a compression test someday.

 

[Sea Rider]

All motors have carbon buid ups issues inside them including their exhaust systems. 4 strokes much less than 2 strokes no matter how well are tuned or taken care of, it's the nature of the internal combustion process of the beast.

Happy Boating