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honda b100 low power, compression?

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oldf2504x4

New member
Hi just got a b100 honda and It does not seam like it has much power. I had it on my 10.5 foot flat and it will just plane with me aboard and it picks up speed very slowly and tops out at 13 knots . With 2 aboard it is only about twice as fast as rowing. I checked the compression today and it has 80 psi on top and 100 psi on the bottom what is normal compression for this engine? is there a common problem with these engines like head gasket failure or something to do with the valves? This is my first four stroke motor and I like how cheep it is on gas but if this is all the performance I can expect I am not impressed. I would have thought this thing would fly on my small boat as it is now it isn't any better than my friends 4 hp mariner.
 
Compression should be 10.6 kg/cm² (151psi) @ 600 rpm.

You may not be pulling the starter quit 600 rpm but compression should be relatively close.

Be sure to take it with your throttle wide open. The easiest way is to have someone hold the lever on the carburator to the full throttle stop. Disconnect your safety landyard, if you motor has one...to kill the spark.

There seems to be quite a difference in the compression of the two cylinders. It could be head gasket failure, but it could also be the valve clearances are not adjusted correctly.

There could be carbon built up on the rings.

If you are going to try to do this yourself, you should get a Honda Shop manual to step you through it.

So...I know that was a lot.

First....recheck compression.
Second...shoot some oil in each cylinder, rotate the engine a couple of times, then take compression again. If there is much of a change, it may be rings or carbon build up.
Third...Adjust the valve clearances. Get manual for this.
Fourth...Be sure you are using NKG spark plugs....should be DR5HS. It is best to stay with NGK. Other brands are ok, but NGK's work better. I have no techical reason for it...only experience.
Fifth....buy some Sea Foam, and follow directions on can. Use a very concentrated dose to run engine. I think it gives instructions to decarbonize the engine. If not, I saw a good writeup within the last two weeks on this Honda Forum.
Sixth...pull the head and check the head gasket. Again, I highly recommend using the Honda manual as a guide. The good news is...you do not have to pull the powerhead to pull the head.

Mike​
 
Thanks for the info. I will be doing all the work myself so I will try and get a manual. What is sea foam? I've never seen it is there an alternative I could get at a auto store or garage? When you said carbon on the rings did you mean valves? also does honda still have parts for these motors or will I have to look for aftermarket parts? I am thinking if the rings are worn out I might as well rebuild the whole engine then I know it should last for a few years anyway.
 
Almost any auto parts place caries Sea Foam. Heck, even the few convenience stores that are around here carry it, as well as the Food Lion. Yamalub Ring Free is a more expensive alternative. Mercury also markets a decarbonizer.

Folks around here swear by Sea Foam, especially the commercial fishermen and crabbers.
 
Sea Foam is tried and true. It is also recommended and used in thousands of automotive shops around the Nation.
 
OK I found some sea foam now what? Do I just add it to the gas and oil? the directions I thought were a little vague on amounts to add. also could I pour some in the plug holes and let sit maybe turn the engine slowly to let it get up around the valves?
 
Actually, you could spray it in the cylinders using a squirt bottle and then stuff the end of a rag or paper towel into the plug hole to provide some resistance to create a SMALL amount of compression.

Pull the engine over rapidly like you're trying to start it and that would force some of the liquid up around the valves and around the rings before your makeshift plugs blow out. Remember, never put the spark plugs back in with any liquid in the cylinder. You could bend a valve or worse.

NOW, here's the important part: Let it sit for a few hours after treatment and then squirt a few drops of oil in each cylinder to lube the rings, pistons and cylinder walls. You don't want to start the engine with the oil washed off of the cylinder walls. Pull the the engine over again to clear the cylinders of any liquid and then reinstall the plugs.

Or, you could go a little higher tech and use the BG Products "Injecta Flush" system.

http://www.bgprod.com/products/fuelair.html Works great on cars.

You can use about 10 ounces of Sea Foam in a three gallon gas load to clean the fuel system. I like to be conservative when adding it to the oil. I recommend 1 ounce and then do a short cycle oil change and treat again. However, your crankcase is not causing your run quality issues, You purchased a low compression engine and the Sea Foam in the fuel and an upper cylinder treatment MAY help. Possibly...perhaps. Worth trying anyway.

If you only did a DRY compression test to get those results, you may want to do a WET test to see if it is a piston ring issue just as Mike has already pointed out. I would do this before any treatment so that I could go back and check it again and see if I gained any compression. Good luck.
 
If my recollection is correct, the can of Sea Foam will have directions for using a concentrated mixture for decorbonizing. If not, go to their website or do a google search on something like "decarbonizing with sea foam".

Basically, buy, borrow, or swipe a 5 or 6 gallon outboard gas can that will mate with your pump-up bulb. Mix the Sea Foam and gas as directed, and run it through your engine. It will smoke, burp, and cough a lot. You'll likely see lots of black smoke and rough running that will start to smooth out. Then, as jgmo suggested, let it sit for several hours. Then crank it up and run it again. Repeat until all the gas is out of the portable tank. You may need to repeat the procedure a second time.

I've not had to do this on my Honda engine, but certainly have done it several times on my old two-strokes, as well as some friends motors. And many years ago it revived an old GM 235 in a 56 Chevy that everyone believed was on its last legs.
 
Ok here is where I'm at I checked the compression again at WOT and was basically the same as first time. Then I put some oil in the cylinders and the compression went up about 10 or 15 psi. so I added some sea foam to the cylinders and now I'm letting it sit for awhile and will run the engine and check compression again later today. About the 5 or 6 gallons of gas, it would take a week to burn that much gas in this motor LOL. I haven't even burned one 2.5 gallon tank ( original honda tank) since I've had it in the three or four times I've had it out on the water.
 
hey everyone I finished the sea foam treatment with little improvement so I went straight to the tear down today. I think I found out what happened to this engine some of the water passages were blocked with solifided salt water and must have overheated it but the pistons, rings ,and cylinders show no signs of over heating or any scars. so I'm thinking that the problem is with the valves leeking compression. what do you guys think? so I'm going to replace the rings, head gasket, and valves (or can I reseat the valves)? does any one know if these parts are still avaliable? model is b 100 made sometime in the 1980's I think. any one got parts for this motor I need the bracket the starter cup bolts to the two coils bolt on to it too.
 
just finished rebuilding the motor today new valves,rings, and head gasket and guess what compression still the same 100 psi (give or take a few psi ). I set the valves to specs in manual 0.002" to 0.004" . I just don't know what to do with it any more any one have any ideas? could valve timing be off? it seems straight forward to me line up the T on flywheel with mark on recoil bracket and dot on cam pulley with dot stamped into block? any one with any ideas please respond it doesn't matter how crazy they are I'm willing to try anything now. The only thing I can think of trying tomorrow is put a new gauge on my compression tester.
 
Yep - Make sure your compression tester is working correctly. If you have adapters, try it on a realtively new auto with a known compression PSI. Hope you are using the screw-in type with a rubber seal and followed Hondadude's direction to make sure the throttle is wide open. Make sure the engine turns over 10-15 times during the cylinder test.

Going back on my auto rebuild experience, what about the valve seats. Did you inspect them for cracks and gouges? Did you seat the valves into the seats using a grinding compound to seat them fully? Did you inspect the cylinder walls for cracks and gouges? Did you mic them for roundness?

If yes to all of the above, then you may have a cracked head. That means a rebuilt replacement.
 
no I didn't seat the valves because I don't have any compound right now. I'll try to find some tomorrow and seat them . will this really make that much difference? not saying your wrong or any thing. I don't have much experience with rebuilding four strokes and I didn't realize it would Thought it was more for performance engine building or something like that. I'll try to do a leak down test first and listen for air escaping from around the valves before I do any thing else. Is it better to seat them by hand or with a drill seen it done both ways on youtube.
 
At this point Hondadude or JGMO probably need to jump in here, but here are my thoughts.

Yes, proper seating can make a big difference. It's been MANY years since I replaced valves in an engine and the last time I did, it was by hand using a crank-type rotating tool made specifically for that purpose. But I think a variable speed drill will work fine as long as you have a suction cup that will hold the valve head securely and you keep it at low speed. The key is to make sure you have an even abrasion ring around each valve and each seat.

A couple of other things occur to me...

Could you be leaking compression around the spark plug ports? I assume you installed new plugs and the new plug seals would likely cover that issue. But you should check the plug ports thoroughly and make sure they are clean and there is no possibility the plug seals are not sealing properly.

Since this engine may have overheated - when you had the head off, did you check to make sure it was not warped? A metal straight-edge run across the head in several directions will test that issue - no gaps whatsoever. A warped head could explain the low compression and the difference in compression in each cylinder.

Finally, somehow you will need to determine whether you have a crack in the head. It's unlikely since you have low compression in both cylinders, but you should thoroughly clean and inspect the head for cracks.
 
problem solved! when I set the cam pulley on the motor after rebuilding it I had it set one tooth off . I aligned it where the previous owner had it set. it was set to a small line not the stamped dot ( it was covered by some grease) so I set the cam up right and tried it and it would idle excellent but would sputter and backfire when giving it throttle. so I set the points right and when I tried it it was like a new motor. It still doesn't have the compression of a new motor but it might improve as the rings and valves seat. it has about twice as much power now and I am very pleased with it. so long story short if I had cleaned the bit of grease off that was covering up that dot and set the points I could have had the motor running good weeks ago and saved all the trouble and time and 150$ in parts. thanks for all your help guys now I can finally enjoy this great little motor.
 
Your compression has probably improved now that everything is in sync.

I am surprized you did not see the timing problem if you adjusted the valve clearances.

However it happened, glad things worked out.

Mike
 
I actually set up the valves with the head on the work bench. I think I missed the timing issue because I was in shock of being ripped off and was frantically looking for mechanical damage and when I got that in my head I wouldn't be happy until I tore it apart and had a look at it. anyway I'm not a professional mechanic and it was a learning experience for me. Not only did I learn a lot about these engines but I learned to always test an engine on a boat not just on a stand when you buy it.
 
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