79cc Sportsman Flyer build

[Tom from Rubicon]

Another thing I did not do, with this high toned AGK fly wheel. Eschew the stock ignition 28 degree TDC timing key for an additional + four un keyed timing advance. I don't think that is it, power wise.
Stock Predator spec's 3hp. I went with a PZ19 which is huge breathing improvement compared to stock.
I am beginning to think these little engines cannot reach optimum torque to accelerate under load, or could it be that green foam filter I but in the high flow filter. Too many variables. I know. One at a time. But at 70. I ain't 50 anymore, nor do I have the strength of a fifty five year old.
Tell what time of year it is. Wood man called today to ask how many cords I would take this year,
Tom

 

[Tom from Rubicon]

Last years gas with stabilizer. Thoughts? For the record. The oil is 10-30 Valvoline straight racing oil. Couple towns East has a quarter mile asphalt track. Slinger Super Speedway | World's Fastest Quarter Mile Oval
Tom

 

[xXNightRiderXx]

Actually, now that I think about it, 3 month old gas can cause some issues. Maybe try with a fresh can? What is the jet size currently in your carb? If it's below #80, that could be the culprit. Using my stock carb and my high flow air filter, I get better power when I use a #80 than when I use the stock #75, and with the upgraded air flow, you'll need an #82 at least. I recommend buying from agk. Their jets are dirt cheap and guaranteed to be accurate in size.

I plan on maybe having an intake machined so I can use the pbhg 19 carb, 19mm ID tapered down to 17mm for velocity. My high flow air filter is a direct fit for that carb. That carb is also a slide, but idk if it uses a needle.

Gotta get the new wheels built first though, otherwise I ain't goin nowhere fast, lol. I don't recommend mag wheels with this engine, they're too heavy, and will retain too much inertia when braking. My brakes are powerful enough to stop these from 20mph in 40ft, so with the lighter wheels I'm going to build, they should be more than powerful enough. They will conform to DOT regulations. Cable driven hydraulic brakes, zoom makes em cheap, but strong. My mtb stops from 20mph in about 30ft, but that's just because it's so light that the rear wheel locks up unless I shift my weight back.

Oh, also gotta get a less restrictive exhaust on this machine, else it won't cope with my intake. I like how quiet and throaty it is, but the expansion chamber is just too large. Sends a big pressure wave back to the cylinder, so I'm not getting proper evacuation. That being said, acceleration is great with my gearing, and it's bound to be even better with a proper exhaust and my custom wheelset.

 

[Tom from Rubicon]

Reading your first paragraph this AM NR, looks like I am going to drill the main jet with a #66 drill.
Pretty much a #83 main.
One has to remember that a stock 79cc Predy carb has a smaller bore than a gnat a$$ and is governed too, to deliver the stated engine parameters.
Hot Rodding the 79cc engines and their requisite PZ19 carbs are not well documented. I'll give it a try.
With a back-up PZ19 arriving Monday, I am fearless. Having a set of drills down to #72 comes in handy.

Taking Mona for our Sunday drive incurred the Eskers that Cty N traverses crosswise between Cty P and Hwy 67 going West.
I cannot imagine a loaded horse drawn wagon traveling this road well. And I tried to get the gutless wonder engine to up these slopes?
I copied this from AGK's website. Seen it years ago, but saw no need at the time. Print it and add it to your folder.
Drilling Main Jets – Affordable Go Karts Thank-You Jamie
Tom

 

[xXNightRiderXx]

I've bypassed the entire governor system. The carb is now directly actuated by the throttle linkage. And yes, I know the carb is bored super small (17mm) for this engine, that's why I'm looking for a 19mm ID intake so I can use the pbhg 19. I can use a piece of 25mm ID heater hose as a collar to attach the carb. I really need a welder so I can just make one, but the decent ones are expensive, and I need a steady supply of argon gas. I would also be able to make my own exhaust, and weld on my own muffler, but alas, I don't have the resources for that.

 

[Tom from Rubicon]

My engines have no governors. Pull the side cover and take it out. plug the external governor hole 1/8 npt. Done.
What is a pbhg 19 ? No matter.
If I can't get this Pi$$ ant engine to deliver the goods a PZ22 is in the line up. Beers are on me when I am satisfied.

Container Egg Plants are doing their best. Tonight I made It. sausage loaded egg plant halves, topped with fresh tomato sauce. Covered with parchment paper and sealed with foil. 300F for an hour. Served with crusty It. bread and salad.
Going to see my Doc. for a blood sample Monday. Plan is to arrive by Flyer, while avoiding the most difficult Esker pitches.
Tom

 

[xXNightRiderXx]

Lol, my bad. It's a phbg 19. Dellorto. I might just go with this one, since I already have an intake that will bolt onto my engine and uses a clamp on carb.

dellorto PHBG AS 19mm clamp style

this is the clamp style 19mm PHBG Dellorto carburetor with three jets (main, pilot, idle), pull style choke, mixture screw, and endless tuning opportunities thanks to a wide variety of needles, throttle slides, jets, etc. 19mm - clamp style- pull choke

www.treatland.tv

It has mixture screws, it has a pull choke, I can use my current air filter, and this site gives you the jets that come with it. Yes, it's for a two stroke, so I'll have to run it a little lean, but it's got a needle, so I can adjust the main mixture there. I'll probably bring the idle and main jets down a bit, but keep the starter jet. I'll definitely buy it from somewhere else though. I'll also grind down my intake so it smoothly ramps in, instead of being a square edge for the AFM to ram into.

I'm waiting until I have the new flywheel delivered before I remove the governor. I'll have to remove the flywheel anyway to get that thing out, so I might as well do it when I'm ready to install the new one. Less waiting with a nonfunctional engine. It'll also give me the chance to inspect my engine for wear. Say, is there any way I might be able to install a stator under the agk flywheel? If not, that's fine, I'll just find a way to install an alternator. I realized that my electrical system almost drains the battery in an hour of use.

 

[Tom from Rubicon]

Side cover is on output side. The case is solid on flywheel side. Removing the flywheel is not a must do.
Tom

 

[xXNightRiderXx]

Maybe not, but it is easier to remove the governor in one whole piece and then tap the hole with the flywheel removed.

 

[Tom from Rubicon]

New PZ19 carb. arrived. Plan was to just modify the main jet and install it in the bike mounted carb.
Into the shop and unmounted the float bowl and it sure didn't look like the Vids. Should have taken a photo.
Vid has one unscrew the Pilot Jet and then unscrew the Main Jet underneath which is numbered. Not this carb.
Big change!
Designed for ease of robot manufacture and assembly has arrived. In this carburetor, the Main Jet is turned straight on two ends with a symmetrically centered seating flange. It slip fits into the carburetor body and is retained by the Pilot Jet. Brilliant!
Only just dissected the new carb. today to get in my mind the scope of my next move. Pin gages to find out the existing
Jet orfice, and go from there. I really like this carb jetting design.
New Mains shouldn't cost more than a buck retail, and I can make my own in my shop. More tomorrow with pitchers.]

Sunday came another treat arrived. A Burt Munro bio.
One Good Run: The Legend of Burt Munro
In about sixty pages already.
Tom

 

[Tom from Rubicon]

.
Before I reassembled the new PZ19 carb with just a dial caliper, I measured the ID of the main jet .084".
No need to bring out the pin gages, and no. I have not checked plug color.
Just for sh!ts and giggles I dropped the float bowl from carb #1 and the two carbs are twins. Same pilot/main assy..
Same main bore ID.. #2 needle in middle clip slot for a base line. I am hoping carb #2 behaves differently.
Tom

 

[Venice Motor Bikes]

.
Before I reassembled the new PZ19 carb with just a dial caliper, I measured the ID of the main jet .084".
No need to bring out the pin gages, and no. I have not checked plug color.
Just for sh!ts and giggles I dropped the float bowl from carb #1 and the two carbs are twins. Same pilot/main assy..
Same main bore ID.. #2 needle in middle clip slot for a base line. I am hoping carb #2 behaves differently.
Tom

I'm sure you're measuring that main jet wrong!

The way I measure jets that aren't number stamped is to insert a micro drill bit into it & reference the drill bit size.

 

[zean]

Drink two beers or one 16 ounce malt liquor, put the bike on the street, let it warm up good and get on, then turn the throttle all the way. If you don’t have to hold on for dear life, try the 212. I’m just respectfully saying.

 

[Tom from Rubicon]

This model PZ19 main is a slip fit bushing and not stamped retained by the pilot jet. I did not give the pilot jet a hard look.
Just a reminder Norm, this is a 79cc Predy. Both mains I measured were .086" ID. Don't make sense, I know.

Pat replied to my email and said go counterintuitive and drop the needle to lean it out. Nothing to lose and much to gain.
Wednesday is the day.
I need to completely clear my work bench and rid it of mouse sh!t.
Tom

 

[Tom from Rubicon]

Drink two beers or one 16 ounce malt liquor, put the bike on the street, let it warm up good and get on, then turn the throttle all the way. If you don’t have to hold on for dear life, try the 212. I’m just respectfully saying.

Kinda what Pat is hinting at. I know the good of a 212.
If I still lived on the Illinois prairie, this thread would have withered away. I am not a whiner, but I'm doing it.

The few times I got half tuned up and went for a proficiency run, I was happy to get home and not bloody.
Tom

 

[sportscarpat]

Hi Tom,
Didn't catch this thread early on. I had two similar 79cc bikes in the shop, one hot rodded and one stock. The difference was so minor it was was hard to tell but the stock internal motor may have had an edge. The horse power is so low (how low is it?) that I felt the heavy valve springs I was running actually used up more horse power than it added. Extra drag.
I only run 12 ounces of oil in the 79cc and never had a problem. 12 is what the engine manual recommends. At 16 ounces you may be causing drag on the crank.
If you want more bottom end and are not worried about top end change the gearing. I had a 100cc race bike and the bigger rear sprocket I ran the faster the bike would go. This seems backwards but the little engine simply could not push through the gearing to reach higher rpm. Finally did some gearing changes and got that little motor to spin over 9000 rpm.
You mentioned running an aluminum flywheel. A quick test would be interesting. Throw on the cast iron flywheel and see if it makes a difference.
I have tuned so many carbs over the years but do not consider myself an expert. When a motor bogs at higher rpm I can convince myself it needs more fuel. I always find the opposite. Bigger jet makes the problem worse. Going to a smaller jet always improved the situation. This is of course assuming the carb is correct for the application and not to big or too small. Check plug color.
Finally, I will say that tuning a small engine to do big things can be very challenging. Even for a simple single cylinder push rod four stroke.
Pat

 

[sportscarpat]

I'll throw two more items out.

Pilot jet, needle adjustment, main jet. If you haven't read up on this you should. There is a mikuni guide on line that is a good read and explains it well.
The pilot jet handles from zero to around 1/4 throttle.
The needle adjusts mid range fuel flow from about 1/4 to 3/4 throttle.
Main jet is 3/4 throttle and up.

Gearing. Take your bike and point it down hill. See if it will push through your problem rpm zone and even reach full rpm. If it does, but can't push up a small hill you are geared too tall. My guess is your gearing is fine if you haven't changed it from before the carb swap. Have you tried your old carb with the new engine mods? If the problems go away its the new carb. If the problems remain, its something else.

 

[Tom from Rubicon]

Timed out in Word, fooked again.

 

[MotoMagz]

Hey Tom, I did a lot of jet and pilot swapping in the past. It is very time consuming and can give great results. I didn’t enjoy this part of the build it peed me off. So my new mission was to find the best out of the box carb.I found what has worked for me and a few others with built Up Or non built up motor.………….https://www.vmcchineseparts.com/products/chinese-pz25-carburetor-hand-choke-version-11 They ship super fast too.

 

[Tom from Rubicon]

Thanks for the reply Pat,
My gearing is stock from the Bully clutch to the 44T drive sprocket mounted to the sprocket adapter. I have enough speed on the flat and moderate down slope.

“You mentioned running an aluminum flywheel. A quick test would be interesting. Throw on the cast iron flywheel and see if it makes a difference.” I weighed the billet ACR flywheel and the stock iron one. Three pounds even for both. If there is any difference it is the strength of billet magneto magnets which are much stronger.

“I have tuned so many carbs over the years but do not consider myself an expert. When a motor bogs at higher rpm I can convince myself it needs more fuel. I always find the opposite. Bigger jet makes the problem worse. Going to a smaller jet always improved the situation. This is of course assuming the carb is correct for the application and not to big or too small. Check plug color.”

Wednesday I pulled the spark plug and it was sooted up. Stock spark plug gap was .020! Now opened to .032. I am sure the short spark is part of my problem. I also set the C clip in the leanest needle slot.

“Have you tried your old carb with the new engine mods? If the problems go away its the new carb. If the problems remain, its something else.” The old carb is what I started with, clip in middle notch and the would not take a load, which is when I set the needle to it’s richest setting and performance was slightly better. I’m thinking the root cause of lack of power was the short spark. I’ll find out today.
Tom