Topic: 1978 750 Compression Question

[Dan-O]

I recently bought a 1978 750K with 12K miles on it. I did a cold wet compression test and all cyl's tested 140-145. Question is, what was Honda's spec's and what is normal for a cold test?

I tried looking up info before posting and got tired of reading the thousands of pages not answering my question. So Sorry if this has been asked/answered before.

Thanks in advance,
Dan

[WHALEMAN]

Does cold-wet mean you added a small amount of oil to each cylinder before the test? How much oil? Did you do a dry test before you added the oil? If so what were those numbers?

[NitroHunter]

From the 78 Honda manual:
Cylinder compression pressure: 185 (+/- 14psi)

[TwoTired]

http://forums.sohc4.net/index.php?topic=39971.msg411927#msg411927

[Ecosse]

Excellent discussion (link) on compression testing, thanks again TT. Can you clear something up for me? Assuming the readings are accurate and not alarmingly low is the next most important factor continuity of readings? Put another way; as long as the readings are close to one another a lower than 'spec' number shouldn't be a matter of worry. Again, being sure one has a proper gauge and such.

[Dan-O]

Does cold-wet mean you added a small amount of oil to each cylinder before the test? How much oil? Did you do a dry test before you added the oil? If so what were those numbers?

Yes, I sprayed a small amount of WD-40 in the cyl before cranking it over. Also I cranked it with no plugs or gauge attached before taking the test. I have a gauge like someone else posted a pic of here, with a quick disconnect.

I did a dry test first just by kicking the motor over with my foot and got 85Lbs per cyl,  but after spraying WD-40 in the cyl's I used the electric starter gaining to 140-145 Lbs.

Plans are now to attempt to start it, warm it up and take another test. According to the guy I bought it from, it hasn't ran in 15 years. It was garage kept and evendent.  I've removed the motor from the frame, cleaned it and cleaned the carbs. I've set the points, timing and checked the valves. All appear to be in order.

[TwoTired]

Excellent discussion (link) on compression testing, thanks again TT. Can you clear something up for me? Assuming the readings are accurate and not alarmingly low is the next most important factor continuity of readings? Put another way; as long as the readings are close to one another a lower than 'spec' number shouldn't be a matter of worry. Again, being sure one has a proper gauge and such.

I don't know how to take this question...  As in the last paragraph of the very first post I referenced is....

The numbers can still be meaningful even if they are lower.  All cylinders should still be within 10% of each other.

Is there another way I should word this?

Anyway, compression tests are for finding out if something bad happened to a cylinder or two.  Engines *can* wear out and have all their cylinders actually have low but equal compression.  But, most frequently one goes bad before others.  If indeed you have a blown head gasket, two adjacent cylinders will have the same low compression reading.  A leak-down test is a superior test...if you have the apparatus, an air compressor, and a means to hold the crank in a fixed TDC position.

If you have a cold cylinder/exhaust, and good spark, a compression test might save you from endless fiddling with a good carb, if it shows a big difference in readings w.r.t. the other cylinders.

Cheers,

[Ecosse]

Sorry about that TT, it's not you... it's me. Careless oversight.

[TwoTired]

Sorry about that TT, it's not you... it's me. Careless oversight.

I was taught by my work managers that reading an entire report was unlikely, even distasteful .  They would read the first paragraph and the last.  The first is supposed to tell what the report is about, and the last was the summary or whatever relevant data was supposed to be gleaned from the report.  The rest of the report was only supportive info, read by peers who disagreed with the summary and wished to find holes in the methodology or some way to discredit or invalidate the report.

Not all manager would admit that, of course.  But, you could easily figure that out during staff meetings by bringing up some point made in the middle of the report that was ditributed.  Used to tork me off.  I'd be assigned the work needed for the report an spend a week writing and making sure the report was indisputable, only to find out 90% of it was never read.    I was able to change some of those reading habits by embedding symbols, drawings, or pictures within the report.  I found out it didn't matter what was put in as visual interest.  People would the read the adjacent text just to figure out if there was connection to the symbology presented.

Anyway, I put the essential fact (that you missed) in the summary paragraph out of habit, I suppose.  However, I speculate the picture was too detailed, drawing the eye away from the summary.  The picture probably needs to be placed near where it is discussed in the main body.
It's just that the forum format mandates placing it at the end of text, and I have no control over that.

So, it's not your fault, really.  You were tricked by the human condition, and technology foibles.

Cheers,

[Ecosse]

Now TT, you're just trying to make me feel better.

Kidding aside you're observation about human behavior is fascinating. I'll be sure to pay closer attention in future. Top to bottom.

[Dan-O]

I fired the beast up tonight. First time it's ran in 15 years. Purred like a sick kitty that'd been eatten beans. Short of not putting in new plugs, carbs out of adjustment, accelerator pump not working and it being mounted to a engine stand, the thing actually ran pretty good.

Don't know what everyone else uses for spark plugs, but I put a set of Denso Iridium's in my 05 Suzuki C90 back two years ago and noticed a huge improvement in power and performance. SO, I ordered a set for the CB750 at the tune of $11 each +shipping.

Can't wait to get her back together again!