Maybe I'll call it 'Threenix' for a while ;)

[gltriker]

Post#421

*January, 2023 I got a yearning to search for a clear answer to understand the phenomenon of, * backfiring out the throat * of an old school carbureted 4 stroke gasoline motorcycle engine.
The GL1000 engine was my focus. (backfiring into its carbs' air intake plenum)

I discovered sooooo many details about the chemistry of gasoline combustion (deflagration) in a carbureted 4 stroke gasoline engine, my wife was puzzled why I spent so many hours reading on my laptop. anyways...

Although I had participated in hundreds of discussions here in the Forum, I had never held a GL1000 cylinder head in my hands.
an eBay seller filled that order a month later. $15 - with free shipping for this decrepit example
*

20230215_153313 - ebay junk GL1000 cylinder head Fen. 2023.png

*

Eric Heath had made an observation in another member's build thread and my name popped up

It looks like you’re using a long tube/hose from cylinder to gauge. I think Cliff had some info on this causing numbers to be lower than spec, but actually might be close to spec? Anyone?

All right, if I must ... here we go

A proper 12mm cranking compression test adaptor will have its check valve positioned as close into the cylinder head's combustion chamber cavity as possible.
Visualize where the spark plug ground strap is positioned in the spark plug hole when the spark plug is fully installed and torqued into place. That's the same place the 12mm threads testing adaptor's check valve belongs for a Best Practice cranking compression testing set-up
.
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20250426_152340.jpg

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Why ? , what's your point

If a typical generic 14mm to 12mm, unchecked, reducer adaptor is utilized, the all-important check valve is always positioned farther away from the spark plug hole opening into the cylinder head chamber.
The additional unchecked area in the spark plug hole and 14mm to 12mm adaptor is added as if the tested volume of the cylinder head combustion chamber has been increased; resulting in a lower calculated Static Compression Ratio number, witnessed as a lesser pressure reading result on the cranking compression tester's gauge face.
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20200112_113223_1637982760730_resized (2).jpg

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details...details...details
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20250426_150042 (1) - equal length 12mm threads and 14 to 12mm adapter--no check valve.png

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personal observations

Always continue cranking the engine over during the compression test session until the needle climbs no higher on the gauge face.
Forget the ages old advisement to count to 10 compression strokes. Why? I've observed multiple times during the compression testing sessions with my '75 GL1000 engine, as the individual cylinder pressures are reaching higher numbers, the gauge needle will twitch back forth a few psi (5-10 ish) until the needle finally stops climbing on the gauge face.

Then, I release the starter button.

@ericheath-Thursday April 17th
I've longed to perform the testing procedure to measure cc's of the GL1000 cylinder head combustion chamber with an NGK DR8EIX spark plug, fully installed.

Then, after removing the NGK DR8EIX spark plug, perform another cylinder head cc's test with a proper checked 14mm compression tester hose adapted down to 12mm spark plug hole threads diameter to compare the difference in liquid testing fluid displacement volume (cc's) between the correct and incorrect spark plug hole compression testing adaptors.

[3KidsinAustin]

Thanks for explaining! It is my rebuild topic that Eric made the comment on and I didn't really understand. Your picture with the reducer certainly makes it clear!

[gltriker]

I have an old GL1000 cylinder head that sorely needs to be prepped for this project... DONE
but haven't invested in a clear square of chamber covering acrylic sheet...GOT ONE
Also need a graduated burette and stand for the testing liquid... NOT NECESSARILY
They are expensive...here's my workaround though.

A graduated plastic syringe I received in my * swag bag * at our September 2017 NGW Club rally at the Ivy Lea (Lansdowne, Ontario) Campgrounds!!
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20250426_171820 - fabricating transparent combustion chamber cc test containment.png

I will perform and photograph the cylinder head combustion chamber volume testing in the next day or two.

Isopropyl alcohol and food coloring will serve the purpose to measure cc's.
note: 1ml = 1cc perfect
Based upon information I found in the Honda GL1000 Shop Manual, the engine's Static Compression Ratio is 9.2 to 1.
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Screenshot_20250428_113839_Samsung Notes.jpg

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The swept displacement volume of each cylinder is ~250cc's. Dividing 9.2 into 250 gives us an approximate cylinder head combustion chamber volume measurement of 27 ish cc's., with the piston positioned at TDC

The graduated plastic syringe from the most memorable 2017 Ivy Lea Campgrounds rally will finally perform a valuable service. Huzzah!

[Fred Camper]

I have my breath held waiting for the confirmation of 27CC's. Think you will have to consider the area between the piston and the cylinder and the depth down to the first ring and add that in.

[gltriker]

I have my breath held waiting for the confirmation of 27CC's. Think you will have to consider the area between the piston and the cylinder and the depth down to the first ring and add that in.

True, I might have made a better/closer guestimation of assembled engine static cylinder head combustion chamber volume at piston TDC position IF I had sourced a used GL1000 piston assembly, to examine, as well, 2 years ago.

Filled the plastic syringe with 50cc's isopropyl alcohol and blue food color
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20250428_173804 - filled syringe to 50 cc mark.png

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My first test employed the snap-on 12mm hose assembly. The first result was more than 27cc's.hmmmmm
I performed the test of the 12mm hose assembly, 2 more times. All 3 results were 34cc's. okay good enough
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20250428_182950 - 34cc test result with snap-on 12mm compression test hose assembly.png

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moving on...
I installed the snap-on 14mm hose assembly and the gold anodized aluminum adaptor and performed the volume test again. Only twice, though. Both results were 37cc's.
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20250428_192640.jpg

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What we've confirmed with repeatable liquid cc test results, is the 14mm to 12mm step down adapter adds 3cc to the static compression combustion chamber volume at piston TDC.

[gltriker]

I was looking at the wrong edge of the rubber plunger when I loaded the syringe with the test liquid.
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20250426_171820.jpg

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That first photo in the previous post where I marked 50cc contents on the side of the syringe, in actuality may have been only 48... maybe 47 or 46cc ..
Be that as it may, I still confirmed the volume difference between the 2 individual compression testing adaptors was 3cc.

And, after I composed the previous post, I went to ebay looking for a used GL1000 piston and discovered the top of the piston has a clearly domed profile and valve relief pockets on the edge of the piston dome, as well
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Screenshot_20250429_073902_Chrome.jpg

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To be continued

[Fred Camper]

The domed piston volume will need to be calculate in Math, or in an assembled state at TDC, but Math seems easier and less error prone.

[gltriker]

Understood.
I have thought about how I discovered the unchecked 14mm to 12mm gold anodized adaptor adds 3 cc's to the original cylinder head combustion chamber volume.

With the engine completely assembled and the piston set at TDC am I correct the calculated combustion chamber volume is, indeed, 27cc?

If yes, adding those 3cc gained with the unchecked aluminum adaptor to the 27cc volume figure we now have 30ccs to test with the 14mm hose reduced to 12mm spark plug threads with the unchecked generic adaptor.

250cc ÷ 30cc = a Static Compression Ratio of 8.33 to 1.

Is my assumption copacetic?

[Fred Camper]

Yup and that is a measurable distance. No problem with the adapter if using it to see the variation across your 4 pistons but not to compare to someone with a 12mm gauge.

[gltriker]

Done

[gltriker]

Thanks for explaining! It is my rebuild topic that Eric made the comment on and I didn't really understand. Your picture with the reducer certainly makes it clear!

You are very welcome
I thank ericheath and you for motivating me to finally investigate and prove out my reasoning with, *Science* some Math, too

[ericheath]

I’m wondering about how the 9.2:1 compression ratio is calculated. Is it static or is it dynamic? I don’t know, but I thought someone mentioned that a few years back. You would think that dynamic is more important to establish octane levels needed for safe operation.

Per the 1000 pistons, my measurements had them 0.040” below the deck. The valve pockets are considerably deeper. Interestingly, the 1200 pistons were 0.040” above the deck. Don’t anyone take this as gospel, my calipers are pretty beat up.

[gltriker]

I’m wondering about how the 9.2:1 compression ratio is calculated. Is it static or is it dynamic? I don’t know, but I thought someone mentioned that a few years back. You would think that dynamic is more important to establish octane levels needed for safe operation.

Per the 1000 pistons, my measurements had them 0.040” below the deck. The valve pockets are considerably deeper. Interestingly, the 1200 pistons were 0.040” above the deck. Don’t anyone take this as gospel, my calipers are pretty beat up.

Hey there Eric-
In this case, the 9.2 to 1 number is a Static Compression Ratio calculation that would be accomplished with a *suffocated* engine. Suffocated? Yes, it can't 'breathe'.
The Math requires the intake and exhaust valves are in place in the cylinder head, but a camshaft is not in the picture.

Dynamic Compression Ratio is calculated dependent on how a camshaft manages the opening and closing events of the participating cylinder head's intake and exhaust valves. There's an amazing link down below

A few years ago, (2018!? ) after Nixon had advised readers to increase their engine's intake and exhaust valve lash spec an additional 0.002", and possibly gain an additional 10 maybe 20 PSI cranking compression reading could result, I had posed a similar question about octane requirement.

In his reply he said:
"Compression ratio and compression values are not directly related, neither are compression values and required octane, and neither are compression ratio and required octane." https://www.ngwclub.com/forum/viewtopic ... 8&start=15


I'd never researched the description of Dynamic Compression Ratio until I began my January 2023 quest for, "backfires out the carburetor throat" phenomenon enlightenment. Very frustrating to find all answers about that phenomenon were automotive focused and not what I needed to know. Finally, I came across a short article buried in a Mikuni carburetor website that appeared to be focused to the Harley-Davidson crowd. Regrettably, I haven't been able to locate that article in the Mikuni Corporation media again. (Nixon puzzled if it hadn't been authored by a fellow named Joe Minton. )

anyways... back to subject of this post

A few days later, I happened across an amazing website, Grumpys Performance Garage. GET READY !!
https://garage.grumpysperformance.com/i ... ssion.727/

Keep in mind this link to Grumpy's is from 2008.

Some of those many sub-links are no longer available.

[gltriker]

My GL1000 Burn victim has been patiently waiting for operational reincarnation since Oct 6, 2023

https://www.ngwclub.com/forum/viewtopic ... 71#p821171
https://www.ngwclub.com/forum/viewtopic ... 72#p821172

Last summer, I had no idea how I would manage to get an *if and when removed from frame* engine lifted off the hydraulic motorcycle jack, onto an elevated work bench.
Eventually, I spotted a freebie FB Marketplace listing for a hydraulic patient lifter. Rated for 400lbs, Eureka!! and both chains with hooks were included, as well
Quickly contacted the listing party and my 2cd youngest son picked it up for me.
It rolls effortlessly and unexpectedly rolled out of our garage heading down our paved driveway towards the busy road. I caught up with it on my mobility scooter
Removing the rear axle/swingarm as a rolling assembly is not too difficult. Unscrew the swingarm pivots and remove both sides lower air shock retaining bolts.
I made a 'parking space' for the axle/swingarm assembly against an adjacent wall, then rolled it there and chocked the tires. That was accomplished around 2 weeks ago, Will get back to the project on Tuesday.

20250514_133922 - Freebie Marketplace patient lifter repurposed to engine picker.png

[CYBORG]

Nice find Cliff. I can see many uses for that in your shop