Compression rate

[Steve Swan]

I have not bubble tested it though I have a tester I made for manifolds and will give it a go. I’m assuming all valves need to be closed for a good read? Also in your opinion the 45psi is low?

i don't know that i am particularly qualified to answer your question about 45 psi compression being too low, however i will tell you what i have been told by recognized experts and also my own direct experience building and riding these IoE engines. fwiw, my understanding is factory compression was rated at 4.5:1 with flat top pistons. I am told the domed piston will raise compression to 6:1. i have built 4 engines, 1-1925, 2-1927's and 1-1929, all with domed pistons; the cylinders are bored to fit these pistons by a recognized expert (who says his new rebuilt engines will read 65 psi of compression; i've not asked him what he does to produce 65 psi). Pistons, front and rear, are fitted with 2 compression rings and a 3-piece expander oil ring.

Re: compression readings, both '27 engines and the 1925 engine all read 45 psi all cylinders after 3-5 heat cycles running the engine down the road anywhere from 75-200 miles. the way i do a compression test, i kick through 4 times with throttle valve open, same exertion like i would normally put on the kick pedal to start. It takes no forceful exertion to kick through or for that matter start a JD engine, so any forceful kicking in an attempt to get a few more psi isn't going to change what is.

I bubble test each engine i build; it is a vitally necessary part of the engine build process given the plumbing design of the inlet manifold system and the fact that the inlet valve housing must also have a seal against its seat in the cylinder. During assembly of the engine, my bubble testing procedure is conducted after both inlet housing assemblies are installed with large ring nut tightened in place and without inlet push rods installed. On assembled engines, i leave the push rods installed, but i remove the push rods from their inlet lever sockets, easy enough to do and then have both exhaust valves closed. of course, the carb is removed; i have a plate i attach to the manifold of which i then connect my regulated 15 psi air supply to.

My rider, "Frank," reads 45 psi ever since the build was finished and up to now with 1,300 miles on his correctly calibrated Johns-Manville odometer. Frank starts without fail, first kick nearly every time; same for the 3 other engines i've built. Frank idles evenly and smoothly and has strong acceleration in all gears on flats and grades; my riding is done in the foothills of the Front Range of the Rocky Mountains, some of these grades are steep and long, Frank manages all grades at any distance in High gear with around a drop in speed of 5 mph, i don't have to shift into 2nd gear and also note i am running an 18t engine sprocket (16t eng.spkt is factory).

so, that's my 2 cents. Do a bubble test so you can rest easy that the AFR is not destructively lean. If you are leaking air past the seat the inlet housing rests against, it will affect your compression. The fact that your psi readings between cylinders are not disparate would suggest each are in similar condition to the other. (attached is a pic of Frank and his plugs' burns at 1,300 miles).

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[chuckthebeatertruck]

Steve, that is a lovely burn on your plugs. Well done. Thanks for sharing thstpiece of inspiration.

Thanks for weighing in with your readings.

I am one of the only in my group of buddies who has prewar experience and they think I’m nuts when I talk about compression being so low. Many of those guys strongly believe a motor cannot start with less than 90 psi. So, the few of us who know better just keep quiet around the campfire and chat amongst ourselves.

I was elated when my latest thumper acquisition gave 50psi after a quick valve adjustment.

[T. Cotten]

...I am one of the only in my group of buddies who has prewar experience and they think I’m nuts when I talk about compression being so low. ....

I'm with you Chuck,..

But with all of these variables that Kitabel mentions, what is the actual compression of a motor, tested the only ways we can? Figure by bore and stroke and head volume?

It fires or it don't, assuming everything else is in order, of course. How low a gauge reading will still fire?

Our Founders needed to eek out every bit of longevity they could, without the tech advances we enjoyed later. Temperature and compression are nearly the same, so naturally the early motors weren't going to push it.
Three to one still sounds scary low to me, but I'm here to learn.

....Cotten

[Steve Swan]

Steve, that is a lovely burn on your plugs. Well done. Thanks for sharing thstpiece of inspiration.

Thanks for weighing in with your readings.

I am one of the only in my group of buddies who has prewar experience and they think I’m nuts when I talk about compression being so low. Many of those guys strongly believe a motor cannot start with less than 90 psi. So, the few of us who know better just keep quiet around the campfire and chat amongst ourselves.

I was elated when my latest thumper acquisition gave 50psi after a quick valve adjustment.

Thanks, Chuck, for your kind words. Whilst i have been associated with so-called antique bikes for 5 decades, besides being self-taught, i am always ever-mindful that most of what i think i know is built off of one old timer who took me under his wing and who knew more than i'll ever know in my lifetime. i know my way around basics and i am also indebted to the likes of yourself, Cotten, kitabel and countless others on this forum as i read and learn from various thread topics here for the sake of learning; never too old to be curious.

my H-D experience is pretty limited, 1970, i owned a '52 pan that was chopped for less than a year. around 1981, i bought an inventory of parts from the family of a deceased old timer friend, there was a '72 Sportster project he had not finished that i did and rode not more than a few miles to discover that era Sportster was not for me. So, buying JD parts beginning 2013, was my real introduction which became an intimate relationship with the year 1927 as the project moved forward and would have stalled numerous times were it not for the fine fellows on this forum and a world wide community of JD lovers. Having no riding experience with a JD was pretty spooky and uncertain in the beginning, but more time in the saddle has certainly made my heart grow extremely fond of the IoE marque, the first generation of factory twins. Anyway, i blather. The bottom line is i've learned a lot from guys such as yourself that helped me get my '27's going and the other two that i helped my local buddies get going.

i guess what i am trying to say, is the first time i performed a compression check on my engine, the 45 psi could have knocked me over like a feather, as i have un-American motorcycle fancies where 140-175 psi is what i was used seeing. i read Kitabel's link and also reflected on your comments; it makes sense these IoE design engines don't have much compression as they were originally designed to move down the road at 35 mph, not average 100+ mph like board tracker racers which i would bet have significantly more compression than 4.5:1...

attached another pic of my plug burn, i sure was happy to see both plugs look like that.

Chuck, what single gives you 60 psi? (i know your motorcycle tastes are eclectic).


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[chuckthebeatertruck]

I brought home a late 1940s guzzi gtv 500 earlier in the year. Basically a prewar bike Guzzi was forced to keep building for a few years after the war. Supposed to be 4:1 and I got 40 on a few kicks. Loosened the valves and hit them a few times to loosen any carbon. Dripped a little oil on the stems and brushed grime off the springs. Reset and viola just over 50 on ye olde gauge. I suspect there’s a healthy crust on that piston and chamber. It’s got enough there to prevent Mrs Chuck from smoothly kicking it without use of the deco lever. All on 50 psi.

The lowest I have ever witnessed was a hit and miss. There was essentially no compression and you could just flick the flywheel to start it. Ran for hours at thresher shows running a mini saw mill. My father, a master Porsche/vw mechanic said it shouldn’t run. My grandfather laughed and told him he better go tell the motor.

I adore early machines because they are so different from our modern expectation of what we will find.

oh, and I swear I once saw just under 60 on a 350 rudge, honest ;-)

[chuckthebeatertruck]

For anyone reading these posts and wondering what we are on about: next time you are at antique store keep an eye out for Dyke motor manuals and similar books. These are wonderful books for understanding prewar motor tech at the 30,000 foot level. The tech is both odd and way more advanced than many realize.

I recently paid ten dollars for the 12th edition (1920) of Dyke’s automobile and gasoline engine encyclopedia because it has color plates for Harley and Indian models and a whole section on aero engines due to all the WW1 surplus. it is 936 pages.

[Steve Swan]

Chuck, thanks for the tip on those books by Dyke. i'm bidding on a 1927 manual, the information looks fascinating and the pictures high quality. Dyke's must have been every motorman's go-to resource during that era. Enjoyed reading your comments on your post-war Guzzi. The other thing that goes along with low rpm, low compression engines of that era is low octane gasoline. Compared to these so called "low performance" factors, racers that were running high compression, alcohol alcohol powered engines were in another universe.

[kitabel]

More of "why CCP isn't "CR X ATM":
Since the intake valve is still open when the compression stroke starts, less than the full displacement is captured
Compression heats the charge, raising the pressure

Although they act in opposite directions, they don't completely cancel each other out because neither of them is linear

[T. Cotten]

More of "why CCP isn't "CR X ATM":
Since the intake valve is still open when the compression stroke starts, less than the full displacement is captured
Compression heats the charge, raising the pressure

Although they act in opposite directions, they don't completely cancel each other out because neither of them is linear

Great points, Kitabel!

So what do you feel is the least CCP that should fire and run?

....Cotten

[kitabel]

I wouldn't have guessed that 40 was enough!

[duffeycycles]

My brothers early BMW starts & runs at 40.You can "kick" it over with your hand.

[Steve Swan]

I wouldn't have guessed that 40 was enough!

Chuckthebeatertruck has a valid point about these early turn-of-the-20th century engines being in a class all of their own when it comes to the compression psi necessary for these engines to start and run all day, every day, during their lifetime. What Chuck is offering is some perspective on what era of engine we are dealing with. I don't have the in depth understanding that Tom and Kitabel have of valve timing, duration, etc, but i do have firsthand experience with the old hit-and-miss 1hp to 6hp engines that were used on the farms and elsewhere in the industries. a 6hp hit-and-miss engine will run all day long, probably firing once or max twice per minute when not under load. the 6hp engines had a piston roughly 5 inches in diameter and maybe a 6-7 inch stroke, and could be started by hand, it only makes sense, when considering everything that was learned back in those early times was new and durability, reliability and longevity for consumer use was important. Not to say there wasn't tremendous sophistication in a variety of designs, Glenn Curtiss, the Cyclone that pushed technology forward, but these sophisticated and complex designs didn't put bread on the table and milk in the ice box. The IoE engine was the earliest, technologically and commercially successful design for the first nearly one third of a century, hundreds of thousands of internal combustion reciprocating gasoline engines were produced. These engines, by the inherent nature of their design in these early times, HAD to be low compression to survive the primitive environments they operated in. Imagine retail public riding a Curtiss V8, his W engine or the Cyclone on dirt trails, mud paths or where roads did not exist. i guess what i am trying to say, is the level and sophistication of technology in that early era was inherently limited by a number of factors; human understanding of what was to be accomplished under the limitations of what heavy machine works could produce, metallurgy, the intended use of the end-product, etc. Case in point, until the transverse 4 cylinder Honda CB750, began mass production in 1969, the aforementioned limitations kept any 4 cylinder engine in any configuration out of reach for widespread, daily use from the retail buying public. Prior to the CB750, Indian enjoyed a 16 year uninterrupted, albeit somewhat production run. Honda was not concerned that their SOHC four cylinder engine would be technologically successful, Honda was concerned that their engine would be commercially successful. Heavy machine works technology and metallurgy had evolved enough to meet the desire for performance, high retail demand and reliability sufficient that anything even remotely similar to the outdated low compression, low rpm hit-and-miss or IoE engines had outlived their usefulness.

[T. Cotten]

..... I don't have the in depth understanding that Tom and Kitabel have of valve timing, duration, etc,..

Valve timing, duration, etc., eludes me too, Steve!

That's why I asked Kitabel.

....Cotten

[kitabel]

Engines of that size are easy to start for another reason: flywheel diameter and mass.

[gharper]

Back to the original problem: Your great running J was shut off one day and the next it won't start. I think you're barking up the wrong tree chasing compression ratios. These engines will run with very low compression! My reference is Brand X, Excelsior which is also an IOE design, similar to Harley. My first X project was a 19 X full electric model, engine previously worked over decades ago by one of the old timers in the club.. It was stock compression at 4.5/1 and had a battery ignition system, so guaranteed HOT spark at any RPM. There is also a compression release tied into the kick starter, lifting the intake valves for the first 1/4 stroke of the starter. I could literally start that motor with my hand, pushing down on the starter crank! At one point I pulled both intake cages out and redid the valves and guides, they were old originals, worn quite a bit. I re-installed them and started it up for a test ride. Started perfectly, super easy, but..... Going over 20 mph it just slogged along, could barely get out of it's own way! Looking closer, I discovered a MAJOR leak in the front intake cage / nut assembly. Like HD these are a bear to get sealed up properly and a source of constant concern, front especially. After I re-sealed that up and did a leak test, it started just as easy and then had POWER!

My point here is even a massive air leak will still allow the engine to start and run! As RPM goes up, then the air leaks quickly become a huge issue!

So, stop fretting about compression ratio, you have plenty!!! Interesting comments, science and math from all, but not necessary here! Look at the other two variables, fuel and spark! If you're running a magneto, pinch the spark plug gaps down to .015". This is contrary to what the book says but will make starting much easier! The reason here is the inherent weak spark from a mag at low RPM. If it has a small gap, the spark can jump that much easier at low RPM, making a good hot spark and you're off. Yes, it's fine to run them that way down the road. Fuel can be another issue too. I've discovered these motors are pretty particular about idle settings and need a rich setting to start easily. If you choke the motor, CLOSE the throttle and kick it over once or twice, there should be fuel dripping out of the carb intake. If not, fuel may be your issue. I've had this issue before, motor runs perfectly, shut it off, 15 minutes later, it will not start!!!! No fuel... perhaps the float stuck and all the fuel boiled out, fuel off, who knows? But, restarting the process would always get it going.

In 2018 I prepped the 24 Big Chief and sidecar for the Cannonball. It ran like a top, until..... I hauled it to Davenport. That thing was a BEAR to start! It was embarrassing to kick it 20 times with a crowd watching. They probably thought "this guy is an idiot, and he expects to do the Cannonball when it won't even start!!" Things got worse in Portland Maine.... Obviously the issue was no fuel, because of the change in altitude. I had richened the idle setting up a couple notches thinking that was enough. Once started, it ran perfectly and would idle smooth and slow. I called Howard Wagner and asked for his advice. He asked what my starting procedure was, I explained what I was doing. He chuckled and said, "Hmm, after all these years I have to explain to you how to start your motorcycle?" He explained the process I now use all the time: Full choke, throttle closed, 1-2 priming kicks. Choke half open, slight throttle, ignition on and kick, should take right off. It did!!! He explained to me something I never thought about: The idle and priming circuit works with the throttle CLOSED! This allows all the fuel to be pulled through the idle circuit, which (one way or another depending on carb model) is richened up by lifting the low speed needle. If the throttle is opened, even a little bit during the priming kicks, much less fuel is drawn in. Still, I opened the idle circuit up quite a bit to get it adjusted for altitude.

So, back to basics, you have compression, now insure you have fuel and spark. If you have all 3, it WILL go!