Compression test low on BF100 honda mid 80s

[infoanalysis]

I did a compression test on a Honda bf100 which states in service manual that the psi should be over 150 but when I conducted the test I only get about 60psi , but consistent between both cylinders .
Even after spraying oil in each cylinder, as it may have been standing for some time, it still did about the same.
The engine pulls like it has compression and when I sprayed gas into the carbs it did start up with smoke ( that's probably because of the oil in the cylinders.) When I test compression after the start up still 60 both cylinders. Am I doing something wrong with the compression test? is that typical?

 

[GA_Boater]

A motor that's been sitting and showing low compression needs to be warmed up if stuck rings are the cause of the low compression. Spraying fuel in the carbs isn't enough to warm it up.

 

[racerone]

Throttle must be OPEN to do a compression test on that 4 stroke motor.

 

[infoanalysis]

Yes the local outboard shop just told me that Thanks, although it does not make sense since I always thought the throttle dealt with carb not the valves and therefore it wouldn't seem likely that it would effect compression.

 

[infoanalysis]

A motor that's been sitting and showing low compression needs to be warmed up if stuck rings are the cause of the low compression. Spraying fuel in the carbs isn't enough to warm it up.

Thanks, yes that makes sense that the rings might be stuck.

 

[ahicks]

I think you're going to see a big increase in your compression test with the throttle held open. If that holds true, service the motor, get it running right, and go run the hell out of it.

 

[racerone]

Best to do some analysis on the info provided to you..----The carburetor must be OPEN to let air through to the intake valves.-----Then that air is compressed to give your reading.----Best do some reading on how the motor is an " air pumping " machine and how your motor works !!

 

[infoanalysis]

thank you

 

[Scott Danforth]

a motor is an air pump. with the throttle closed, you can not get air into the to pump, therefore you cant get enough air for a compression test.

 

[infoanalysis]

Thanks, that makes sense now that you put it that way.



"He who forgets to change his impellor may find himself a stranded fella"

 

[infoanalysis]

Best to do some analysis on the info provided to you..----The carburetor must be OPEN to let air through to the intake valves.-----Then that air is compressed to give your reading.----Best do some reading on how the motor is an " air pumping " machine and how your motor works !!

Thanks I'l do my best but sometimes its nice just to give a direct answer to a simple question.

 

[racerone]

I think you got a direct answer / opinion about needing the throttle OPEN.------Then you doubted that information with your reply in post #4 I think.

 

[Sea Rider]

Test Compression with a second compression meter and check if both dials 60 PSI. If suspect that's due to rings issues spray liberally a blast of CRC carbon remover through the carb's throat, throats if having more than one carb, follow can instructions. This amazing product should take care of the issue.

Happy Boating​

 

[Faztbullet]

Better do a leakdown test if compression that low....Carbon cleaner wont help much as 4 stroke. and very little carbon.

 

[racerone]

Check valve clearance.

 

[MattFL]

Is that the 10HP Pull start, or 100HP electric start? If it's the pull start, does it have an automatic compression release to make starting easier? If yes, that might explain the low compression.

 

[Sea Rider]

Carbon cleaner wont help much as 4 stroke. and very little carbon.

Don't need to have carbon build ups on the rings for these type of products to release stuck rings if suspect, the product is intended for 2 or 4 stroke motors use, the issue is that most boaters finds to be an outdated and obsolete product which is not.​

 

[Faztbullet]

It only cleans piston tops and valves on 4 stroke as rings and walls are lubed by crankcase oil. Only way rings can carbon up is if it has wear and a oil burner

 

[ahicks]

It only cleans piston tops and valves on 4 stroke as rings and walls are lubed by crankcase oil. Only way rings can carbon up is if it has wear and a oil burner

I would add here that a 4 stroke CAN carbon up if it's never allowed to warm fully. I see this all the time on Honda's run with no t-stats on pontoon boats that rarely run on anything much over an idle.

They're normally fine idling for hours on end, are highly sought after for that reason. But they need to be run warm! Left cold, it's just a matter of time until the compression starts dropping way off. They'll get down below 90-100 psi and still run smooth. It's not until they get down to 75 psi or so that they start complaining with hard to start and erratic running.

Instal a t-stat, service them completely, then go run the hell out of them, and most will come back like nothing happened eventually. You just have to break them in again! -Al

 

[Sea Rider]

Yep, will second Ahicks post. Motors can carbon up due to : Not using a thermostat will make the motor to run much cooler that it should, will lessen top cylinder combustion, incorrect spark plug thermal range and gap will also contribute to increase this carbon up escenario. On 2 stroke motors the combination of both it's a serial killer.

Happy Boating​