Topic: Help to analyse compression test results

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Hi Guys. I bought a CB 750 k1 but I know practically nothing about my exemplar so I did let's say a sort of gamble.
The only thing I know is that engine plate and frame plate are very near so the engine is the original one and that in the last 15 years has been stopped without starting up in a repaired garage.

Motorbike has been given to a mechanic that is quite known for his experience on old motorbikes especially Hondas.

Now first impressions from mechanic. Motorbike is very difficoult to start in cold conditions. It is needed to blow a flamable spray into the inlet duct to help starting. Once the engine is started, it restarts easy in warm & hot conditions but on the road is weak and slow to raise of rpms.
The compression test has shown that all cylinders are low in compression and well under the minimum acceptable limits.

The mechanic adviced me that he never saw such results in this kind of engine and is better to make some hundreds km to see if this is due to long time of inactivity (clearances to adjust themselves etc). In few words, prior to open the engine, run some time and at high RPMS to see if the engine come back from these unusual results to a better condition.
See here compression test results (units I don't know what are, I think barg)



Thanks in advance for your advices.


 

[greenjeans]

If it has been sitting, then yes, run it.   It will loosen things up.  Rings have a tendancy to get sticky if they have sat for long periods.    Your carbs should also be thoroughly cleaned if they have sat for a while.   If it were me, I'd run it as the mechanic advised.  Then carbs would come off and be cleaned and cleaned.   Then do the 3000 mile tune up.

[Stev-o]

It is typical that comp readings will be low on a bike that has been sitting, and yes they will come up.
I would put a 100KM on it and have it retested to see if there is any change.

As far as cold starting, what is your procedure? These bikes can be a little picky to start. My K4 likes full choke and no throttle until she fires.

Lastly, have you given it the full 3000 mile tune up as Honda recommends?

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3000 miles tune up? 
Sorry ... what is this ?

Start procedure not yet known... I just made a little visit to mechanic and he explained me that is using the spray.

[Davez134]

3000 miles tune up? 
Sorry ... what is this ?

Start procedure not yet known... I just made a little visit to mechanic and he explained me that is using the spray.

Here, under periodic maintenance...
http://manuals.sohc4.net/cb750k/service_manual/SM750K_6.pdf
It's a must do when going through a newly acquired bike.

[_mark]

3000 miles tune up? 
Sorry ... what is this ?

Start procedure not yet known... I just made a little visit to mechanic and he explained me that is using the spray.

Here, under periodic maintenance...
http://manuals.sohc4.net/cb750k/service_manual/SM750K_6.pdf
It's a must do when going through a newly acquired bike.

Yes of course my mechanic did it and more

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so, in the manual , like my mechanic told, values should be between 10.5 to 12 and I have in hot conditions 9;9;9 and 10.
I hope I can recover the compression even if my values are quite far..

[Davez134]

The good thing is your compression numbers are fairly close across the cylinders. (As in one is not extremely lower than the others) run it for a while and re-test. You said your mechanic is familiar with these bikes and has done the tune up, but are the carbs COMPLETELY clean? Needing starter fluid to start and sluggish acceleration sounds like it could be related to some clogged jets or even float level. If everything else with the 3,000 mile tune-up is in spec, the carbs will make or break how your bike runs. So maybe give them a closer look.

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I'm sure because he completly open and made the revision to them.
98 octane gasoline can help to engine recover phase or is better the common 95?

[grcamna2]

I would think regular fuel is good.I would think that your mechanic could also pour a bit of Seafoam into each cylinder through the spark plug holes and then let the engine sit for at least 24 hours so it could loosen up the piston rings if they are a little stuck to the pistons.

[_mark]

I would think regular fuel is good.I would think that your mechanic could also pour a bit of Seafoam into each cylinder through the spark plug holes and then let the engine sit for at least 24 hours so it could loosen up the piston rings if they are a little stuck to the pistons.

Thank you I will ask and check if seafoam is available in Italy

[grcamna2]

I would think regular fuel is good.I would think that your mechanic could also pour a bit of Seafoam into each cylinder through the spark plug holes and then let the engine sit for at least 24 hours so it could loosen up the piston rings if they are a little stuck to the pistons.

Thank you I will ask and check if seafoam is available in Italy

Another product is called "Marvel Mystery Oil"(just a brand name)that has been available for years here in the USA.
I think the original piston rings may be stuck/frozen a bit into your pistons w/ old carbon,etc. and both these products do help to 'melt'/soften-up that carbon and cause the piston rings to move again freely in their grooves;you will need to allow the product to sit for a while(best to pour it in to a warm engine)to do the most good.I would think it best to do a few oil changes w/ a lighter weight engine oil to fully wash-out any old debris from inside the engine also.

[bochnak]

Can you provide more info on the "thing" in that image you posted? I have never seen one of those and am curious.

If you want to see where the compression is going, perform a leak down test. I have a video on that here:

Matt

[_mark]

Can you provide more info on the "thing" in that image you posted? I have never seen one of those and am curious.

If you want to see where the compression is going, perform a leak down test. I have a video on that here:


Matt

Hi Matt, my Mechanic I saw uses this one: http://www.samstagsales.com/motometer.htm







and also my brother in law that is a cars' mechanic says that such devices with the paper graph are better.. but I don't know why.. I will investigate.

[bochnak]

Can you provide more info on the "thing" in that image you posted? I have never seen one of those and am curious.

If you want to see where the compression is going, perform a leak down test. I have a video on that here:


Matt

Hi Matt, my Mechanic I saw uses this one: http://www.samstagsales.com/motometer.htm



Thanks for the info. That is one expensive tool! Mine was $40 and serves me well.



and also my brother in law that is a cars' mechanic says that such devices with the paper graph are better.. but I don't know why.. I will investigate.

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here you can see how the measuerment with such device is performed. When you slide up the paper on each measurement you can compare on the same sheet all the cylinders.

it's a professional tool

[bochnak]

So if I'm reading the graph correctly, you have 103-110psi across all 4, correct?

[DaveBarbier]

That's a cool little tool. I've never seen anything like that before. You say that it might be in Bar(g) so to me it looks like your number 1 cylinder semi cold is 8.4 bar(g) which would be 121.8 psi. #1 hot is about 9.2 bar(g) which would be 133.4 psi. If that's the case, it doesn't seem so low to me, even for a motor that's been sitting.

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Correct, I have   9.2 barg in #1, 8.9 barg #2, 9 #3 and 10 barg in #4  in hot conditions

under the periodic maintenance manual linked in this 3d and as stated by my mechanic, the pressure should be from 10.5 (minimum) to 12 barg

so my engine is weak .. correct ?

thanks regards

 

[DaveBarbier]

According to the manual and your readings, yes it's weak. But as others have said, riding it and/or using seafoam or MMO should help bring those numbers up. No need to be concerned yet. Looks like you'll have to saddle up and get some riding in .

The poor acceleration and throttle response is another issue. I would trust the guys on the forum when they say it points to carb issues. As long as the 3k mile service has been performed (which you say that it has).

[flybox1]

You are trusting the adjustments your mechanic made are the right ones.  Blind faith?  I/we don't know your mechanics abilities.
Around here, there are very few of us who would trust a mechanic to do anything to our bikes.

I agree with a few of the previous posts.
You need to ride your bike (I'd ride at least a few hours) be sure the rings are free and doing their job.
Your choice to use a fuel or oil additive.  Id just opt to change oil after a hundred miles or so.
As DaveBarbier mentioned, if all maintenance items were done (correctly), poor throttle response would point me to carbs(bowl fuel level/dirty jets/mixture)

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Thanks all. I don't have tryed motorbike yet.  My mechanic told me something about " ...is not pushing/going up at high revs"
I will ask him better about this (if I correct understood...).
Today I tried only to run the throttle while motorbike engine was on (I don't have insurance yet to run on the road).
Engine is fast to go up revs but if I open too fast the throttle, it shutdowns, is this normal?
 

[flybox1]

Its not a fuel-injected bike.
Twist too fast and you can make the bike cough.  More likely if its not tuned properly.

[Stev-o]

It depends on how fast you are twisting the throttle.

Get insurance!  Where in Italia are you?  I've been there 3 times.....

[_mark]

It depends on how fast you are twisting the throttle.

Get insurance!  Where in Italia are you?  I've been there 3 times.....

Como, north of Milan.

I will update You about updates. Thanks and Bye.