Honda BF50A Compression check

I was reading an earlier thread about carb problems that was very useful. Thanks to that thread I learned alot about inspecting and rebuilding carbs. Also in that thread I recognize that they talked about compression. the gentlemen indicated his compression checked resulted in 190 190 195 psi. I've been doing compression checks and never ever have I reached these numbers. Am I doing something wrong? I know poor compression can be a result of various problems but can someone tell me are these readings the norm for a motor that is in decent shape? Any feedback would be great.

Bill

Come on you pros out there...... can someone respond??? its been several days now and waiting for someone to respond..........

Sorry no one got back to you. Much of this part of the country has just had some enormous storms come through at the end of last week....and there are many small towns near me that are practically gone and many have died. Fortunately, at the shop only one boat got thrown into another.

Normal compression for the 50hp is 199psi at 500 rpm.

If you are not getting anything close to that there is something wrong with the motor or you are not taking compression the correct way.

- Make sure that your battery is fully charged to get maximum speed when cranking.
- Remove all of the spark plugs
- Remove the safety landyard....so that there is no spark.
- Disconnect the fuel.
- Most importantly...Put the throttle in throttle only and advance the throttle to full throttle. If you have a tiller model, you will have to remove the throttle linkage from the carburator on the engine and have someone hold it open all the way.

All cylinders should within approx 10% of each other.

If compression is still too low, then check the timing marks. If timing is good, then do a leakdown test to help narrow down the problem area. You could also have a bad compression gauge.

Mike

Thanks Mike for your response. Sorry to hear that your side of the country is not doing well. I wish and that all turns out alright for anyone affected by the storms there. Here in Hawaii we are experiencing some stormy weather but probably no where near what you are experiencing there. Thanks for the information. I am still an apprentice when it comes to working on these Hondas. Personally I can't afford a mechanic and things in general are expensive here. So I read alot, ask questions and use the basic mechanics skills I have to get by. I am still learning some of the terminology so bear with me because I have a few questions ( I figure the most stupid question is the one I don't ask. So please bear with me).

Question 1: what is the safety landyard?
Question 2: Do I disconnect the throttle linkage to the control box? If I put the throttle in full throttle this would mean the engine would have to be in gear? The motor will not crank over in gear. Or do I just disconnect the linkage from the carb and hold the throttle open by hand?
Question 3: The engine starts and idles fine leaving me to assume that timing is okay. I will check timing marks. What is a leakdown test?

Note: All purpose Compression Kit K-D B3469 is what I'm using to test compression.

Thanks again for your response
Bill the apprentice!

I see Mike hasn't had a chance yet so here goes;

Question 1. Safety lanyard= Kill switch. Or, disable the ignition by disconnecting the coils.

Question 2. Any way you can...YES, disconnect the throttle cable at the carb linkage so that you can hold the throttle completely wide open. Use some wire or vise grips or something if you don't have a helper. You will NOT get good info if you don't hold the throttles open all the way while cranking. Make sure fuel is disconnected as well.

Question 3. The fact that the engine starts and idles well is a TYPICAL indication that the engine is sound. However, if you get questionable results from a compression test, checking the timing is the first step of diagnosis.

Question 3, addendum A. A leak down test is nothing more than putting compressed air to a cylinder "parked" at top dead center compression stroke via the spark plug hole and then determining where the air is escaping. If it leaves via the carb inlet, then you have poorly seated intake valves or bad tiiming. If it exits via the exhaust, then you have poorly seated exhaust valves or bad timing. If it leaves via the crankcase, you have leakage past the piston rings.

An internal combustion engine is, first and foremost, an air compressor. Of course, some leakage is allowed but not much.

Harbor Freight tools sells an acceptable leak down gauge for under $50 but you don't necessarily need a special tool. It makes not putting so much air in the cylinder that the engine cranks easier as well as providing an easy to read leakage percentage assessment aid.

As far as your compression gauge, the only way to make sure it is accurate is to compare how and what it reads to another known accurate air gauge while applying compressed air.

Good luck.

Thanks for the helpful information. I would like to clarify a few things;

If I remember correctly, the timing marks are on the cam pully and crank case (align 2 arrows) What about the flywheel? what am I looking for?

Also top dead center compression stroke means when the respective piston is at the top prior to the intake valve opening to allow gas in the combustion chamber for the spark to ignite?

Thanks again for all the helpful info. I'm glad I was able to find this forum.

Bill The Apprentice.

Please keep in mind that I do not/have not worked on a BF 50 A. This is coming from my CD manual. Are you working with a manual? When you start getting this deep, I strongly urge you to buy the Helm Inc. shop manual for your outboard. One SCREW UP takes out your motor. Preventing one SCREW UP pays for the manual 50 times over. Ok, I'm back down off the box.

You are correct about the cam pulley and the engine block having the two arrows and aligning them.
On the side of the flywheel is a "T" and what looks like another mark resembling a vertical line like this: l

There are a couple of "engine lifting eye mounting bosses" sticking up on the block. the one on the port side of the motor has a small mark that is used to align the flywheel mark. I can't determine as yet if you align the "T" or the "l" but for a quick check to see if the belt has jumped time, close should be good enough. I believe the "T" is for valve timing and the "l" is for ignition reference but I may be wrong. I would have to do some more study to make sure or ask Mike. He sounds as if he might be a wee bit busy right now but, rest assured, he's monitoring this and will straighten us both out when he gets the time.

You are incorrect about the TDC however. Yes, the piston IS at top dead center when the cam is beginning to open the intake valve but TDC compression is where you need to be. Here's why:

4 strokes right? Intake...compression...power...exhaust....ok?
Let's start with power.

The spark has just occurred and the piston is forced down turning the flywheel. Obviously, both intake and exhaust valves are closed. the piston reaches the bottom and starts to head back up so we go to....

Exhaust. The cam opens the exhaust valve and the burned gasses exit the cylinder. As the piston reaches the TDC of the exhaust stroke (the TDC you describe in your scenario) the intake valve begins to open a couple of degrees before the exhaust valve is fully closed. This is called "valve overlap" and is engineered into the cam so that a "scavenging" effect takes place where the inrush of fuel/air mix pushes out the last of the waste gasses.

So, you can see that, in actuality, the cylinder isn't completely sealed on this stroke even though the piston is at TDC. That's why TDC compression stroke is always emphasized as the place to set and adjust valve timing and clearance.

Intake...After overlap takes place and the piston comes up over center, the exhaust valve is fully closed and the intake valve opens all the way to allow the mix to come in. As the piston reaches the bottom of this stroke, the intake valve closes and as the piston goes over BDC (bottom dead center) the engine begins...

Compression...Now you can visualize the piston coming up and both intake and exhaust valves are completely closed. They will remain closed until the engine has completed it's power stroke or for nearly a complete revolution of the crank.

Understanding this "theory" stuff sometimes clears up some mysteries for a guy. I hope I presented it clearly enough.

Got it! I do have a manual and understand most of the reading. It an old pdf. version (Honda Outboard Motors Repair Manual 1976-2005 from www.Readmanuals.com

I will double back and check timing and try to do a compression check. Thanks again for all the support. Truly appreciate you and Mike's input and ways of explaining things. I will keep you updated.

Bill The Apprentice.

Thanks for jumping in, Jimmy. The only thing I can add is a picture of the specific pivot linkage that to disconnect for the lever on the bottom carburator. See #2 in this picture. Prying gently with a small needle nose pliers behind this linkage will pop it right off. http://www.boats.net/parts/search/H...LRA VIN# BAZS-3300001/THROTTLE ROD/parts.html

Mike

What compression reading would you expect if the timing has slipped, is it slightly down or way down?

Mdoherty,

I do not know. I don't look at it that way. If the compression is off in any way, the easiest thing to check is the timing. If that is ok, then a leakdown test will help locate the problem.

Mike

Okay, I have done an accurate and complete compressioin check on both BF50A motors. After warming the engine, removing all spark plugs, landyard wires, and disconnect fuel line, my compression for the first motore read;

#1chamber=155 psi
#2 chamber=155 psi
#3 chamber=155 psi

Second motor;
#1 chamber=155 psi
#2 chamber=160 psi
#3 chamber=155 psi

Prior to checking compression I check timing alignment and both motors are right on the mark (align the T on the flywheel to the arrow like pertrusion on the hoist bracket mounting post and the arrow mark on the camshaft pulley with the arrow mark on the crankcase).
I know Mike indicated that 199 psi is the norm. My readings don't indicate this however the manual indicates that achieving a consistant reading is important with an allowable amount of variation between chambers. Given my readings I'm within the allowance. However, can't help but think that my engines have lost some of it's compression due to hours used and year/age (which I don't have the VIN tags nor the hours of operation). Keep in mind motors run pretty decent (some shake but not much at 950 rpms).

Any feedback on my motor compression and what kind of life spand I may have with these motors before I have to rebuild the cylinder head.

I have done a leakdown test yet because I still have questions on how you check for air leaking through the crankcase with the motor assembled. Any help on this would be helpful as well.

Thanks again for all the support.....

I meant to say I haven't done a leakdown test yet. Sorry for the typo's

Last question first....during a leakdown test, listen for a crankcase leak by removing the oil fill cap or removing the crankcase breather hose that goes from the valve cover to the muffler cover on the front of the carburetors. The oil cap is the easiest The leakdown is going to tell you more about the life of the engine than compression.

The compression still sounds a bit low...you did not mention that you advanced the throttle to full throttle. If you did not, that should bring you closer to 199. And...yes the 199 rpm is at 500 rpm, which is a lot faster than cranking speed, so you might not get to it...although most 50's that I work on do get very close.

Next....it is good that they do not shake much at 950 rpm. The idle in neutral should be 950rpm plus/minus 50. A carburetor vacuum balance would probably smooth that out.


Best thing that you can do to prolong the life of the engines is to change the oil, oil filter and gear lube per the maintenance schedule. Also, adjust the valve clearances at the appropriate intervals.

Mike

Thanks Mike for responding so quick. I did have the throttle wide open. I forgot to mention that. Also prior to everything I mention, I did check valve clearance by following the manual (align #1 timing marks on flywheel and camshaft pulley, then turn 120 degrees to align #2 and so forth). Should the gauge be snug or should the gauge slide between the rocker/valve stem smoothly? Manual calls for .005-.007 for intake and .008-.010 for exhaust. I went in between (.006 for intake and .009 for exhaust). I changed oil, gear oil, plugs, gas and oil filter prior to running checks.

I know the book calls for synchronizing carburetors in a body of water, and my motors are currently on stands and not on my boat yet, and I don't have a test tank, is there any other way you know of that I can balance the carburetors given my situation? I really don't want to put them on my boat until I'm absolutely sure I'm ready to use them. Mechanics here think that the difference is minimal (testing in a body of water vs. a water ears attach to the water inlets on the leg).

Your experience has been very helpful and beyond what the manual describes. Again any personal advice and/or experience is greatly appreciated.

Thanks again,

Again I forgot to mention, When doing a leak down test, now I know where to check for crank case leaks, I am assuming for intake valve leaks I would check the intake ports. Is it best to have the intake manifold off or can I listen for leaks via the carburetor muffler?

Again thanks much,

feeler gauge wil just fit in the opening.

balancing the carbs on the hose is better than nothing. Make sure of the motor is warm. and most importantly, make sure that you cover the water pick up that is just above the prop with some duct tape...... so the motor does not overheat

Mike

Your compression results seem ok. Not great but ok. Take heart in the fact that you have less than a 4% difference between BOTH ENGINES and that being with 15% usually allowed for one. I would be concerned if they were down in the 130 to 140 range but see them run all the time in 120 psi territory.

If you do a "wet" compression test, I'll wager your pressures will come up substantially and will prove that the engines just have a bit of compression ring wear. I think you'll be ok when you get the carbs right.
Good luck.

Thanks guys for the feedback. I will keep you posted on my progress.

Also forgot to mention, what is a wet compression test?

Same as a "dry" except that you squirt a small amount of engine oil into the cylinder being tested prior to installing the compression gauge. This oil spreads out and provides a little better cylinder wall to ring seal and, if compression comes up substantially, you will know that the reason the dry readings were low is due to piston ring wear. Wear on all components is normal and as long as the engine starts, runs and has decent power, you can expect to get several more seasons of use before any type of overhaul.

Thanks for the info.

Oky, to update you all, my engines are mounted, gauges all working, and back in the water fishing. Did 2 trips this weekend and engine performed okay. Wanted to thak all of you for all your advice and expertise. This forum has broaden my horizons and save me a ton of money in mechanic's labor. Actually what I would spend in labor I was able to buy some important tools.

Thanks guys.

I Updated!!!!!!!!


I have a newbie...

I want to gather more information from here......

Glad to hear you had some success. Boating can be FUN!!.....sometimes.