An attempt to stop a GL1000 from smoking

[cfairweather]

When we were discussing the compression numbers, I forgot to mention that I live at an altitude of over 6000 feet. Recently, I read that compression tests will be lower, but I don't know just how much it affect the readings.

[gltriker]

When we were discussing the compression numbers, I forgot to mention that I live at an altitude of over 6000 feet. Recently, I read that compression tests will be lower, but I don't know just how much it affect the readings.

Nixon had posted a photo of a compression gauge test result of a recently (May,2018) rebuilt high compression engine.
I think he took that reading at Prescott, AZ.( Google says elevation there is less than 6000 feet above sea level)
He stated whatever the reading was on the gauge face. it would be 200psi when the compression tester gauge reading is, "altitude corrected."

Is it reasonable to believe Honda's standard procedure for testing their motorcycle engine cranking cylinder compression pressure is presumed to be performed at sea level? ideal Barometric pressure? ideal Temperature range? etc, etcetera, et cetera

google informs me the elevation in my neck of the woods is a measly 381 ft above sea level.

[Track T 2411]

Generally speaking, atmospheric pressure at sea level is 14.7 PSIA (absolute). At an altitude of 6000ft., it is 11.77 PSIA. My non-math brain figures this is about 80% of sea level pressure, so the compression numbers should be about 80% of 'ideal'... and this is all from my little noggin, lol!

[Track T 2411]

Simple explanation... 10:1 compression ratio times 14.7 PSI = 147 PSI. 10:1 X 11.77 would yield 117.7 PSI reading... plus variations due to temperature, high/ low pressure weather systems, etc, etc...

[dontwantapickle]

From some guys that race on the salt flats...
These are pretty standard conversion ratios for compression numbers at different elevations compared to sea level:

4000' .8881 = 88.81%
5000' .8617 = 86.17%
6000' .8359 = 83.59%
7000' .8106 = 81.06%

Of course,
everything affects density altitude numbers. heat, humidity, etc. so I would consider these numbers as
a pretty close ballpark, but not gospel.

[cfairweather]

So, my reading of approximately 140PSI on the freshly rebuilt GL1000 is adjusted to about 168 using a correction value of .836. Now, I have some concerns about my 750 with high compression pistons. It registered about 169PSI, so it is adjusted to about 202PSI which is pretty high. I have a stock camshaft in it, but it has not pinged or had any issues here in Cheyenne. If I rode it at to sea level, it might ping, I don't know.

[gltriker]

So, my reading of approximately 140PSI on the freshly rebuilt GL1000 is adjusted to about 168 using a correction value of .836. Now, I have some concerns about my 750 with high compression pistons. It registered about 169PSI, so it is adjusted to about 202PSI which is pretty high. I have a stock camshaft in it, but it has not pinged or had any issues here in Cheyenne. If I rode it at to sea level, it might ping, I don't know.

Don't worry... be happy!

read posts 28 and 30
viewtopic.php?t=63468&start=15

[redglbx]

Cedric, great job ! On the compression thing, I wouldn’t be to concerned about it, I think it will run fine w/o any detonation. When I redid my 76’s heads years ago where i miled .040in off of them as well as advancing the cam timing 4deg I had a cranking compression of 200-210psi, I initially ran premium fuel in it and then stepped down to mid grade and then ultimately 86 grade with no noticeable spark knock, I tried to make that happen by whacking the throttle open at very low rpm to lug the engine and see if it knocked,,, it didn’t .

But I’ll add that I learned years ago that when I built hot rod car engines that I would deburr things in the engine, pistons and heads. I radius edges and corners so there are no sharp edges for hot spots to form and cause preignition. So if you’re are worried about your 750 just run mid grade in it, being an aluminum engine helps with all this as well because it dissipates heat and hot spots better.

So anyway,, great job !

[cfairweather]

Steve and Cliff- Great information, so I will not worry about it. This engine is very strong for a CB750 and enjoyable to ride. All three of my CB750s run incredibly well, but you can tell a difference with the one with high compression pistons. It is the 1970 K0, which is my favorite. I don't need three of these, so I will probably sell one in the next year or so.

[cfairweather]

After putting a couple hundred miles on the bike, I have started seeing some minor smoking upon startup. Not bad, but it is starting to act like a "normal" GL1000. Just when I thought it would never smoke ...

[Fred Camper]

Give us an update at 2000 miles. There remains hope it is better.

[gltriker]

Cedric-We all take comfort in knowing you "built" the Engine, Correctly.

Have you performed additional cylinder compression testing since the Engine rebuild ?

[cfairweather]

Cliff- I haven't done a compression test recently but I am sure it is fine. Thanks for your confidence in my ability to do the job right and I am certain it is a good engine even if it turns out to be a "normal" one. So far, just a little smoke when I start it.

[redglbx]

Personally,,, I think we put to much emphasis on the compression numbers,, me included.. My 76 had 135-145 psi from new . I expressed my concern to the dealership that something wasn’t right but they just poo-poo’d it that things were fine. My 76 stayed at that 135-145 for many years, never getting better or worse until I had a blown head gasket and I lapped the valves and milled .040in off the heads, Before this I had some new keyways EDM’d into the cam gears to allow me to advance/retard the cam timing for better performance.

This was about the time that I had been running the 76 on my friends Dyno to verify my results. But unfortunately my buddies shop shut down before I got to run it with the milled heads. So we all know that more compression equals more power and efficiency, the hot rodder in me made me do all this by the way !

So at this time my 76 had around 205-210psi cranking compression vs the oe 135-145psi (Honda spec is 171 +/-2psi) ,,, So did I get a huge increase in power ? Don’t know ! It seemed to accelerate more effortlessly but the cam advance to +4 deg actually made the bike feel slower (it wasn’t) but on the dyno it showed the torque varied only 2lbs from peak from 2200-6800 rpm,, just constant acceleration, no peaks, on the cam/pipe sensations,, just constant acceleration.

So what’s the point of this ramble ? Well my friend Ed has a 76 as well with a lot of miles and has had 1 cylinder with 85psi in it for many years and not a great deal more in any of the remaining 3,,, but it is one of the sweetest running GL’s you’ll ever experience,, she just rolls down the highway effortlessly, yes she uses a little bit of oil but she has a fair amount of miles on her. I used to bug Ed about rebuilding her because 85psi ! Really ? But she runs soooo nicely just leave her alone and enjoy her ! That’s the point here, Ed just smiles !

Yes more compression will give you more power & efficiency but really is a gl1000 gonna whoop up on a new bike ? Probably not so just enjoy that smooth effortless power and smile ! That’s my .02

[Sidecar Bob]

Ah, but does your '76 smoke? Or Ed's?

I think I said this before but the '79 engine I had in Mr.H for 21 years smoked about half the time. It was occasionally embarrassing but it didn't need the oil topped up between changes so I stopped worrying about it.