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How to compression check Honda BF40?
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Larry Brandt



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PostPosted: Wed Sep 22, 2021 6:43 pm    Post subject: How to compression check Honda BF40? Reply with quote

I have the complete BF40 maintenance manual. It tells me to do a compression check, but it doesn't specify how to do it. I have a Harbor Freight compression tester and I'm getting lower numbers than the manual specifies (the factory spec is 212 psi). All six of my cylinders are balanced at about the same psi and I would think it's unlikely that all six would fail in an exactly similar way. I am convinced the problem is pilot error (ie, my error).

I have pulled the kill switch to stop the spark. I pulled the fuel line, although there is likely residual fuel in the system. I disconnected the throttle linkage and wired the carb throttles wide open. The choke is wide open. I inserted the compression tester hose into each spark plug hole and cinched down on it as tight as I could (there's no way to get a wrench on the hose). And I cranked the engine at least two cycles (four revs = one cycle).

What am I missing here? Something in the procedure? I believe it isn't an engine problem.
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tsturm



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PostPosted: Wed Sep 22, 2021 7:29 pm    Post subject: Re: How to compression check Honda BF40? Reply with quote

Larry Brandt wrote:
I have the complete BF40 maintenance manual. It tells me to do a compression check, but it doesn't specify how to do it. I have a Harbor Freight compression tester and I'm getting lower numbers than the manual specifies (the factory spec is 212 psi). All six of my cylinders are balanced at about the same psi and I would think it's unlikely that all six would fail in an exactly similar way. I am convinced the problem is pilot error (ie, my error).

I have pulled the kill switch to stop the spark. I pulled the fuel line, although there is likely residual fuel in the system. I disconnected the throttle linkage and wired the carb throttles wide open. The choke is wide open. I inserted the compression tester hose into each spark plug hole and cinched down on it as tight as I could (there's no way to get a wrench on the hose). And I cranked the engine at least two cycles (four revs = one cycle).

What am I missing here? Something in the procedure? I believe it isn't an engine problem.


Crank for 15 - 20 seconds or so (until the gage quits increasing) then compare between holes. Thumbs Up
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C-Wolfe



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PostPosted: Wed Sep 22, 2021 10:03 pm    Post subject: Reply with quote

Don't quote me on that since I'm pulling that from old memory, but i think the manufacturer spec are at 500 RPM. I'm not sure how fast the starter is turning but i don't think its that fast. You are right about having everything wide open but I will second tsturm about having the engine turning for a bit longer.
I'm not an outboard mechanic so those comment are worth what you paid for it.

What reading did you get?

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T.R. Bauer



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PostPosted: Thu Sep 23, 2021 9:00 am    Post subject: Reply with quote

Not sure what the purpose of the compression check is on good running engines....but when you are doing one it is important that the cylinders are within +/- 5% of each other - at least "ish". And yes, you need to crank until the gauge maxes out. I suspect, if it's running well, you'll have around 180-190 psi if you have many hours on it.
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starcrafttom



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PostPosted: Thu Sep 23, 2021 11:39 am    Post subject: Reply with quote

how far off are you on the psi from what the book states? if all cylinders are the same but low that is a good thing. They are all wearing at the same rate. one being off is a bad thing.
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jkidd



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PostPosted: Thu Sep 23, 2021 11:57 am    Post subject: Reply with quote

Also the engine should be warmed up. Not cold.
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starcrafttom



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PostPosted: Thu Sep 23, 2021 12:10 pm    Post subject: Reply with quote

YouTube is your friend.
https://www.youtube.com/results?search_query=comprision+check+40hp+honda
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starcrafttom



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PostPosted: Thu Sep 23, 2021 12:16 pm    Post subject: Reply with quote

The first video taught me something already. I have only pulled one plug at a time in the past. This guy pulled all of them and only tested on at a time.
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Larry Brandt



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PostPosted: Thu Sep 23, 2021 4:50 pm    Post subject: Compression testing - What I learned today. Reply with quote

Jody Kidd, good point. I am testing these engine cold.

What I got on the starboard engine was right around 130 +/- a bit in either direction, cold, on all three cylinders. My sources say that as long as the pressures are balanced those are very acceptable numbers. I've only checked the starboard engine so far, but because the pressure I initially got was so far from the "212 psi" cited in the Honda spec, I figured I'd better talk to the dealer and other knowledgeable sources to make sure I was doing the compression check correctly.

Apparently my procedure is correct (except for jkidd's comment about being a warm engine). My engines are cold. I am using a brand new Harbor Freight compression meter, nothing rocket science about it. The hose fitting that screws into the park plug hole can only be hand tightened, and while it has an O-ring on it, I presume the main seal for the compression is the result of the threads on the fitting in the spark plug hole. The choke is wide open. I disconnected the remote throttle linkage and wired the throttles wide open. I pulled the kill switch to make sure there was no spark. I disconnected the fuel line, although there is likely residual fuel in the system. I made sure to crank it enough turns that the one-out-of-four-turns compression stroke is solidly made.

What puzzles me about Honda's spec is this. The maintenance manual cites 212 psi for these engines. It also cites a 9.2 to 1 compression ratio. Given that ISA standard atmospheric pressure is 14.7 psi, and Honda's compression ratio of 9.2, the calculated result is 135.24 psi. That's almost exactly what I am seeing on my gauge. (Where did Honda get their 212 psi?)

My purpose for checking the compression is to determine whether these engines "have a good heart." That is, whether they are worth spending money on to get them running top notch. I just bought the boat, and my experience and 'sense' is that the engines are decent at heart; that the problems are readily fixable. They run smooth up to ~2100 with some finessing the throttle and choke, but beyond that they act as if they are being starved of fuel. They also don't idle reliably. The boat sat for three years, probably with fuel deteriorating in the carbs. An ultrasonic carb rebuild is my next step.

I'd love to launch this C-dory in the Willamette River and run it fast down to the Columbia, with the rest of the world beckoning. Stay tuned.
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T.R. Bauer



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PostPosted: Thu Sep 23, 2021 9:14 pm    Post subject: Reply with quote

Yes, all plugs must be pulled. I'd be looking at how close those numbers are to one another and the condition of the plugs more than anything. Also - quench matters. Hondas are tight in that area and I think that impacts static compression ratio and something not considered in your calculator. Call a Honda tech and ask what he normally see with the 40s. Valve adjustment matters too by the way, as does valve timing. I see more than 130 (ish) with my 45s. Also, somebody said 500 rpms....if true no way your starter is hitting that.
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jkidd



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PostPosted: Fri Sep 24, 2021 6:42 am    Post subject: Reply with quote

From my Suzuki manual
1. start engine and allow to warm up.
2. remove all spark plugs.
3. install compression gauge into plug hole.
4. disconnect safety lanyard.
5. disconnect remote control throttle cable.
6. move and hold throttle in the full open position.
7. crank the engine to get the highest reading and repeat for each cylinder.
8. reinstall all the parts.

My book says 185 tp 242 psi.
My pictures are here http://www.c-brats.com/modules.php?set_albumName=album3298&id=IMG_6246&op=modload&name=gallery&file=index&include=view_photo.php

My readings were low because of 4300 feet altitude. If you add the 12 percent back in there good. You should have a good battery when doing this.
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Larry Brandt



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PostPosted: Fri Sep 24, 2021 11:01 am    Post subject: Interesting results. Reply with quote

Jody, thank you for the checklist on compression checks. Your list should be pinned for easy access to anyone doing a compression check. Before I started this project I would have been grateful to see this info.

The one thing on the list that I did not do was run the engines beforehand. I may go back and do it the "Suzuki Method." Perhaps I will accidentally learn to play the violin along the way? Or I may accept the cold readings I got as encouragement and go direct to a carb overhaul.

But how do you explain the 9.2 to 1 compression ratio, and that my numbers coincidentally(?) came out as calculated? [Sea level pressure 14.7 psi X 9.2 compression ratio = 135.2.] If the engines had been run, would the hot cylinders be contributing to the compressed high reading?

My results *cold* were all in the 130-135 range, balanced across all six cylinders. Those are encouraging results, but not the numbers that you found on your Suzuki. And I am at sea level.
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jkidd



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PostPosted: Fri Sep 24, 2021 11:50 am    Post subject: Reply with quote

My guess would be it isn't calculated that way. If I do it that way mine would be 9.8X14.7=144.06.

The motor needs to be warm so the oil coats the cylinder wall and the rings seat against the cylinder wall. When you first start up that is why you might see blue smoke because you might have some blow by until the engine is warm.

You can also squirt some oil into the cylinder and retest and see what that gives you. There could also still be a ring problem. I had a service manager tell that a motor will probably run all the way down to 100 psi. Having balanced numbers is good It tells you it is all worn the same.
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T.R. Bauer



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PostPosted: Fri Sep 24, 2021 7:48 pm    Post subject: Re: Interesting results. Reply with quote

Larry Brandt wrote:
Jody, thank you for the checklist on compression checks. Your list should be pinned for easy access to anyone doing a compression check. Before I started this project I would have been grateful to see this info.

The one thing on the list that I did not do was run the engines beforehand. I may go back and do it the "Suzuki Method." Perhaps I will accidentally learn to play the violin along the way? Or I may accept the cold readings I got as encouragement and go direct to a carb overhaul.

But how do you explain the 9.2 to 1 compression ratio, and that my numbers coincidentally(?) came out as calculated? [Sea level pressure 14.7 psi X 9.2 compression ratio = 135.2.] If the engines had been run, would the hot cylinders be contributing to the compressed high reading?

My results *cold* were all in the 130-135 range, balanced across all six cylinders. Those are encouraging results, but not the numbers that you found on your Suzuki. And I am at sea level.


I think you're getting parts of it, but missing the whole picture some. You're numbers are likely fine. Parts are designed to seal better when warm. That's why it says to do it warm because they expand. Your compression formula does not include quench.....which in way simple terms includes the head gasket and somewhat the combustion shape and piston shape. It's possible to set it up so the piston goes past top dead center and it does a lot for engines as long as you don't get too crazy with it and have the right shape of the compression chamber. Outboards run it to the cutting edge from what I've seen, but I doubt any run that crazy. Also, you're starter probably turns like 175 rpms, not 500 rpms - when it runs slower gas leaks out - a lot of times the cam timing even overlaps at lower RPMs leaking out compression. Have you even measured compression on a high compression engine with a giant cam - I'm thinking no.....the compression and vacuum at idle is crap and that is why at 1000 rpms because both valves are open at the same time! That is why they run terrible there. Are they high compression engines? Absolutely....that is why they are running on gas you can't buy at the store. Throw a gauge on one and laugh....it's pathetic. Then there is dynamic compression.....as you can see this isn't nearly as simple as it seems. My 45s have very similar readings to JKidds by the way. Screwing with your carbs will do nothing to compression readings also if you were wondering.
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Larry Brandt



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PostPosted: Fri Sep 24, 2021 7:55 pm    Post subject: Reply with quote

That's an informative answer, T.R. You sound like you're experienced in auto racing. Yours is the kind of insight I was seeking, the WHY of compression numbers being what they are. Thank you for that.

I fully appreciate that the carbs have nothing to do with compression (except that for the compression check the choke and throttle should be open to allow air to enter). My purpose for the compression check was simply to validate that the engines were essentially good at core before I spent any time or effort on rehabilitating them.

What I failed to do is warm the engines before I checked them. Nevertheless, the fact that I got consistent readings across all six cylinders I took as a good omen, and so am proceeding on to the next step: improving the fuel flows.

Thanks again. Enjoying the discussion. And learning along the way.
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