79cc Sportsman Flyer build

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Tom from Rubicon

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Apr 4, 2016
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First run today, started and idled nicely.. On the road to 1/4, 1/2. and3/4 throttle the pull was strong. WOT was a bog. As engine warmed 1/2 and 3/4 throttle were still there nothing above or below those throttle settings.
I Let the engine cool down and pulled the spark plug which was dry and clean.
Test #2 Clip in middle slot. Starts and idles well. Ran reasonably until the engine came up to running temp and started to lose throttle response. but undeterred I headed for the 10/% grade. The street named Hilltop. It got me there. 100 yds up and power is fading fast, I start pedaling (^) and gain another 50 and then the exhaust goes pop! And silence.

Pedaling furiously I make the first wide driveway entrance and turn for home some 600yds away.
All coasting down hill to my shop. 10% pitch will get you going pretty darn fast. Saved some speed rounding the turn to make my drive and and then gravitate to the shop entrance.
Interesting ride, the return bit. Very quiet like Rick ghosting along with electric power. Wished I took advantage if the coast home to site see more than I did.
Engine was still at 148F rolling into the shop with my infrared thermometer. I'll pull the spark plug a see what we will see.

But I have had an epiphany of sorts. One thing I did before I gave the bike any run was route the catch can vent to the air cleaner. Post #357. Every thing begins to change the running characteristics when the engine comes to running temp.
Are the crank gasses effecting air/fuel to such a degree?

Before the hot rod engine was installed. #1 engine was pumping oil from the crank case vent. first to the ground and later to last years catch can. That minimized dumped oil. I have seen enough examples of crankcase vents directed to the intake to give it a go. I have also seen enough Kart engines at the stock angle with just a tiny sort of air cleaner to vent an engine.

From Pat yesterday "I only run 12 ounces of oil in the 79cc and never had a problem."
Seems I have been over filling the engine alot, fearing the designed in splash finger would fail to lubricate the cylinder and top end adequately in a vertical configuration.
What did I know? There ain't a operating manual. ;)
I made changes to the intake system before evaluating the Hot Rod mods of it's self. Fix is on the way. After I mow the grass to please SWMBO. Going back to stock SF config.
You don't have to ask why I drink. but I will tell you.
It kills the pain and allows a pause for thought. Syntax suffers sometimes. But I always post edit.
Tom
 

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Venice Motor Bikes

Custom Builder / Dealer/Los Angeles
Mar 20, 2008
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Tom... It's been my experience that a engine bogging down at WOT is a lean main jet.... To be specific, if it totally shuts off at WOT, that's lean... If it feels very sputtery at WOT, that's too rich.

The fact that it pulled strong up until 3/4 says that your pilot jet is close.

Also... the exhaust popping is another sign of a lean condition.... Try a bigger main jet (& report back) :)
 

Tom from Rubicon

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Apr 4, 2016
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Norm it only just went pop once. Like a short fart or too much gas in the exhaust and a carbon spark lit it.
First time for everything.
"The fact that it pulled strong up until 3/4 says that your pilot jet is close."

Pilot jet may be .030", Tool Makers eye, the main being .086" measured with a Browne and Sharp dial caliper is what the main jet is. Not exactly a flow control orfice for the needle to function in. What is your take on the effect of hot engine gasses from the breather effecting engine performance?
I have thought there might be some conflict between intake pulses and crankcase pulses. Just a thought. The only horse power in my genes had four legs.
Tom
 

Tom from Rubicon

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Apr 4, 2016
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Removed the PCV system from the catch can to the carb. Installed an unaltered air cleaner. Easy to reconnect.
Spark plug was looking lean.
The last run when the engine was gutless and died the needle clip was in center groove.
I think Norm has a correct diagnosis. Lean as in very.
Something I failed to mention about the trial runs last weekend.
Before the Flyer touched the floor, I fitted a piece of green foam filter inside the Hi-Flow and those runs were fine except for climb quality.
I think the filter foam was acting as a half a$$ed choke. But I plucked it out, after the initial runs blaming it on the lack of performance.
There after performance regardless of needle position was in a no load condition a runner, but as soon as more fuel was wanted to supply energy to the engine faltered and stalled. Idle circuit is fine. For those that know what they are doing.
A photo is provided. The Pilot jet body is used to retain the Main jet securing the Main jet face flush to the slide pocket.
As the photo illustrates a .082" gage pin passes smoothly thru the Pilot and Main bores. So in fact both bores/ID's are .083"ID.
On the right side of the parts is the orfice through which all fuel can flow and it has a ID of .028" ID.
Slightly bigger than a gnat orfice. The gate keeper is a #70, after that the needle is flow control.
I have a #68 drill bit which = .031. Give it a go to open the gate keeper? I got another set of parts, and I can peen a gate closed if needed. I think the gnat orfice, is not only being a restriction it is causing flow turbulence. I am and never will be a carb guy. Help a Brother if you can.
Tom
 

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zean

Active Member
Dec 5, 2010
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Excuse me. I appreciate all of the insightful explanations on the details of this build. With permission and if I had the time it would be interesting to write a book on motorized bicycles with a table of contents and a glossary with the thousands of one or two sentence informative and accurate explanations VMB has written over his many years of on the job experience. I apologize if this comes off as a paid for political advertisement. Thank you.
 

Tom from Rubicon

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Apr 4, 2016
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Drilled the jet with the # 68 .031"and set the clip in center slot. Started out good but once the engine heated up gutless under any load. Down slope it goes. No pop and stop. Once the engine cooled a bit I checked the plug. White.
Just ordered a 30 pc. set of # drills 47-81, .0785"-.013" . Come tomorrow, Twenty bucks.

Tom
 

Tom from Rubicon

Well-Known Member
Apr 4, 2016
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Rubicon, Wisconsin
Excuse me. I appreciate all of the insightful explanations on the details of this build. With permission and if I had the time it would be interesting to write a book on motorized bicycles with a table of contents and a glossary with the thousands of one or two sentence informative and accurate explanations VMB has written over his many years of on the job experience. I apologize if this comes off as a paid for political advertisement. Thank you.
Somebody aught to write a book zean , I have thought to my self. When I get this SF80 running as I want it to run that I will print every page of this thread. High lite all spec's and mechanical details to create a Owners Manual for the SF80.

From my very first ride running the 79cc Predy and PZ19 carb it always was weak running hills. I had been so used the performance of a 49cc Huasheng that it never occurred to my that the PZ19 was under jetted.
Farthest thought in my mind. What I did think was replicating a AGK Stage II would be the Holy Grail. With my machine tools and metal working skills I was sure of success of my engine performance goals. Everything is golden now but the bloody lean running carb. zean, your reference to VMB alludes me. Google was not my friend either.

Tom
 

Tom from Rubicon

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Apr 4, 2016
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Rubicon, Wisconsin
Miss spoke # micro drills come Monday.
Today a cool front made mowing Two acres pleasant and domestic tranquility prevails.
Time to finish reading.
The World's Fastest Indian by Roger Donaldson
Great read. Roger's prose is so spot on the NZ syntax complementing Bert's own quotations. The book is so much better than the film for filling in so much of what came before the 1962 run and after. His exploration of most of the USA.
New York, city he tried to meet Mea West because she was quoted as saying, "come up and see me sometime when you've got nothing on". He failed to be introduced. Tried to be admitted to the White House to meet Pres. Eisenhower. same result.
Great book.

Tom
 

Forbidden Tuna

Active Member
Sep 3, 2019
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Kings Mountain, North Carolina
Miss spoke # micro drills come Monday.
Today a cool front made mowing Two acres pleasant and domestic tranquility prevails.
Time to finish reading.
The World's Fastest Indian by Roger Donaldson
Great read. Roger's prose is so spot on the NZ syntax complementing Bert's own quotations. The book is so much better than the film for filling in so much of what came before the 1962 run and after. His exploration of most of the USA.
New York, city he tried to meet Mea West because she was quoted as saying, "come up and see me sometime when you've got nothing on". He failed to be introduced. Tried to be admitted to the White House to meet Pres. Eisenhower. same result.
Great book.

Tom
The film version is probably what started my vintage motorcycle bug. I should pick up the book and give it a read!
 

Tom from Rubicon

Well-Known Member
Apr 4, 2016
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Rubicon, Wisconsin
CML Supply Mini Micro Drill Bit Set and Pin Vise Chuck 31 pcs
Great selection of drills for adjusting jet size. Not!
Mostly too big or too small with multiple pairs instead of progressive sizes. My intention was to use a # 65 drill =.035".
Up form the previous #68 drill = .031". This great selection of paired drills are <or> than what I wanted and this may be OK.
The engine has been running lean. The only drill size that came in this set I deem usable is a pair of #60's = .039".
So I have a jet now drilled .039" loaded into the carb. slide needle clipped in middle groove. All is set for a test run Tues.
.
I am not now afraid of over sizing the carb jet. Guys having over sized a jet, just filled it with solder and began again.
No worries. By the way, the bloody drill set came with a pin vice. Pin vices? I have three sets.

Tom
 

Tom from Rubicon

Well-Known Member
Apr 4, 2016
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Rubicon, Wisconsin
The engine now takes 1/2 - 3/4 throttle cold. Still not responding to WOT. But with the .039" jet, it runs much better. Guess I'll drill the jet with a #57 .043" and see what happens. Pat suggested to go to a higher tooth drive sprocket. Currently a 44T. I'll see if my local supplier Lynne Rietzke has a 46T cheap.
Looking back at my first test run post, where I described bogging under load but it ran pretty well Aug.13 post#438.
I did not mention that on that initial run, I had green air cleaner foam stuffed into the high flow air cleaner. Acting as a choke. Makes sense now. Test run again. tomorrow.

Tom.
 

Forbidden Tuna

Active Member
Sep 3, 2019
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That's really odd that you're having lean issues. Maybe I got lucky but when I bought a pz19 carb and intake adapter from affordable go karts it just seemed to work alright. I think all I had to do was adjust the clip placement on the needle. Maybe they already put in a proper sized jet when I bought it? My two gear ratios that I played with were 13:1 and with my belt drive trial it was 15:1. Governor removed and heavier valve springs I think I got my RPM up to 5000 or so before the valves started floating or felt like it, but that was judging by ear.
 
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xXNightRiderXx

Active Member
Jan 12, 2017
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Boise
Well, I may just have to reevaluate my jetting options. I've got a #78 in it now, but the stock was #65. With this carb, I might have to do a #75 or #72. I can readjust if I need to with the dellorto.
 
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