Am I running too lean/hot?

[Ryan843]

A few weeks ago my GT 90 build was maxing out at 22mph (with a 32T sprocket) and sputtering for a while until the air filter fell off while riding one day and suddenly it could do 36mph+. So I cleaned the air filter and then bought a Malossi E12 air filter which is what I've been using for the past week. I'm not used to a bike going this fast and I'm making sure the mix isn't too lean.

I bought an IR gun and the hottest I've seen (once parking the bike and measuring within 10 seconds) was 315deg Fahrenheit after a 10 minute ride at full throttle (72degrees outside), measuring the cylinder body because the cylinder head is cooler than the body for me. Am I safe cruising at full throttle for extended periods of time or is it running too hot?

It responds best to 1/4-1/2 throttle until I'm around 17mph, then I can go full throttle and it'll climb to 36+mph, is this normal?


I calculated the RPM at 38mph (the fastest it's gone) and it's about 6500RPM which I've read is an acceptable RPM for cruising

 

[FOG]

I'd be looking at your spark plug before you listen to me, but, FWIW. I ride a 360 Yamaha with a temp sensor under the spark plug and when fully warmed up I routinely see 350 degrees. Continuous miles of uphill on the gas will go over 400.

But again, I'd be lookin' at the plug just to be safe.

 

[indian22]

FOG's right read the plug...350's not real hot on these after pushing the China girls hard. Be careful 38 mph is very high prolonged cruising speed for tires and hubs normally used on kit bikes and most of the kit bikes you "read about" won't run 38mph if you push them off a cliff!

Your case inducted, reed valve engine is a far cry from the typical kit motor Ryan and I'd think it able to cruise at 6,500 with a 32 t. as 11,000 rpm plus with a pipe & 40 tooth is quite probable.

Rick C.

 

[Ryan843]

I'd be looking at your spark plug before you listen to me, but, FWIW. I ride a 360 Yamaha with a temp sensor under the spark plug and when fully warmed up I routinely see 350 degrees. Continuous miles of uphill on the gas will go over 400.

But again, I'd be lookin' at the plug just to be safe.

I didn’t know about spark plug temp sensors. Could you get me a link to buy one that’s compatible with our bikes spark plugs?

FOG's right read the plug...350's not real hot on these after pushing the China girls hard. Be careful 38 mph is very high prolonged cruising speed for tires and hubs normally used on kit bikes and most of the kit bikes you "read about" won't run 38mph if you push them off a cliff!

Your case inducted, reed valve engine is a far cry from the typical kit motor Ryan and I'd think it able to cruise at 6,500 with a 32 t. as 11,000 rpm plus with a pipe & 40 tooth is quite probable.

Rick C.

If something were to fail from cruising that fast, would it likely result in wrecking the bike doing 38mph or would I probably be able to safely park it?

Also, as of yesterday, every time I start the bike when it’s cold, there’s a loud pop. Any idea what that could be and is it a red flag?

I just got my older bike running again today. I have a new appreciation for how my GT90 build sounds.

Now that my bike can keep up with traffic on most of the roads around me, it’s sooo much fun! My wide crank set is arriving in the next 2 days so I can finally add my SBP x chamber. I’ll post new pics once that’s done

 

[FOG]

I didn’t know about spark plug temp sensors. Could you get me a link to buy one that’s compatible with our bikes spark plugs?

Google Trail Tech temp sensor and they do have different size rings for different sized plugs. I'm guessing China Girls have a 10mm thread on the plug, but I don't know that .....

 

[indian22]

Ralph it depends on what part decides to fail at that speed, where your at, heavy traffic, in a curve etc. & how lucky you are at that moment. Catastrophic failure is best addressed before it can occur. Most of us like to bump our bikes up occasionally to the max speed for a few seconds, but going way beyond pedal bike speeds of twenty five mph for mile after mile places a great strain on the bicycle frame components themselves, even though the engine is just loafing along & capable of much more.

Your engine should really come to life through the mid range with the addition of the expansion chamber getting you on the pipe quicker. That said if your riding on average quality hubs, wheels, frame , fork and tires sustained speeds of thirty plus will at some point cause a failure. Using low quality parts... going fast is a really bad idea to start with. Bicycle hub, loose bearings, and tires get really hot even at sustained speeds of twenty plus mph even in rather cool weather, much worse on hot days. So high quality tires and wheels are a great investment to start with. If you want to ride a fast bike safe take time to build a safe bike first and that means learning what's required to ride faster safer.

I've started upgrading the bikes I want to cruise on at sustained speeds over thirty mph, to sealed bearing hubs, Maxxis tires and disc brakes front and rear. Just an example for a quality tire brand, not a recommendation. I'm pleased so far with these changes. I took a 3 day, 500 plus mile ride this Fall on my e-hybrid China girl with this combination, so I covered a lot of ground at sustained speeds of around 30mph and highest speeds around forty according to gps charting. 36t final sprocket so even at forty the motor wasn't working hard. It was cool weather but I stopped and checked the hubs several times after long stretches and felt no sign of overheating. Same with the tires, which are 60 psi Holy Rollers...for paved road and gravel use. The Maxxis Hookworm is a better choice for paved surfaces and cruising at speeds.

Keep safe. I really like the Gt 90 motor you've built.

Rick C.

 

[Ryan843]

I took a 3 day, 500 plus mile ride this Fall on my e-hybrid China girl with this combination, so I covered a lot of ground at sustained speeds of around 30mph and highest speeds around forty according to gps charting. 36t final sprocket so even at forty the motor wasn't working hard. It was cool weather but I stopped and checked the hubs several times after long stretches and felt no sign of overheating.

Dude 500 miles?! That's awesome! I'd be worried something on my bike would fail while being that far from home. So far it hasn't had any issues (knock on wood) and I've been very impressed by the reliability of it compared to my other motorized bikes. My dad knows far more than me when it comes to bike parts so I'll be asking him what to buy when I upgrade my hubs and tires and such.

My wide crank came in today so I'll be adding that and the x chamber tomorrow hopefully.

I LOVE this bike. I still look forward to the ride when I need to go somewhere.

 

[indian22]

Ryan that's the kind of enthusiasm we like to hear! Post several good photos of the motor & bike when you get the pipe and wide crank on. We love photos!

Long rides require a great deal of preparation. The bike I rode has thousands of miles on it and many modifications along the way. I trust it to get where I'm going and back because I know it's built correctly. It's a hybrid electric front hub, with a 66cc China girl that makes about 5.5hp...dependably. I've built 66cc motors that had a lot more power but weren't fun to ride day in and out unless they were ridden flat out and that's not the way I like to ride anymore. I'm an old guy lol. This one always starts and doesn't, too much, mind going 10 or 15 mph in town with a 36 or 38 tooth road gearing, but 44t around the city would probably suit it better, but I use the electric for the really slow going and all starts and take offs so even a 32 tooth on this would work in town. I still wouldn't ever have to pedal.

Photo taken along the way during the 500 mile ride. Only problem encountered was a broken pedal chain and some cold weather on the last day.

If you ever need help finding good wheels or hubs with sealed bearings and disc brakes front and rear PM me. Glad to help.

Rick C.

 

[Ryan843]

This bike just took some steroids. I added the expansion chamber and I was so not ready for how big of a difference it made. Before the upgrade I could manage about 23mph going up this steep hill with my 32T sprocket but after installing the x chamber it was doing 35mph!! The bike bolts forward when you twist the throttle. I haven’t tested top speed yet because traffic is bad right now, but considering it was doing 35mph up a hill, I’ll easily be beating it’s previous top speed of 38mph on flat ground.

This thing just got way too fun. Now I need to upgrade the hubs and tires so I can take advantage of the speed increase

 

[Ryan843]

3 min video of my first extended ride with it:

After I cut the video I went back home and reached 40mph since I could tuck my head down

 

[Ryan843]

As of about a week ago the bike suddenly has horrible compression. Little to no power going up hills and it’s useless below 20mph. I took the head gasket off and see there’s a single line running down the cylinder wall that’s deep enough to catch my finger nail. There’s a little bit of scoring on the other side of the cylinder body but you can barely feel it.

I can think of two things that might’ve caused it:
1) I was dumb and filled the bike tank with gas and then added oil so they may not have mixed well enough. I did that 3-4 times recently and the bike did lose its power/compression after filling the tank up like that
2) I JB Welded the inside of the main jet and drilled it out to make the mix leaner because I couldn’t find a proper jetting kit anywhere, so one day suddenly the bike will only ride at 3/4 throttle, any more makes it bog down. The next day after riding for 15min at 3/4 throttle it loses power/compression right before I get home. I get home and measure the temp to be 365°! Took the jet out and redrilled it and suddenly it runs well at full throttle again. So maybe some of the JB Weld somehow came loose and blocked part of the jet?

Any idea how to get the cylinder body and piston without paying $150 for the entire rebuild kit?

 

[Venice Motor Bikes]

There are plenty of ads on the sides of this page that sell pistons & cylinders.

 

[Bikeguy Joe]

Never use JB on a jet.
Solder them, then drill.

A LEAN run will not usually cause a score like you described, but losing a retainer clip or other foreign object will.
Check everything thoroughly.

 

[Ryan843]

Never use JB on a jet.
Solder them, then drill.

A LEAN run will not usually cause a score like you described, but losing a retainer clip or other foreign object will.
Check everything thoroughly.

Yeah I’m definitely not using JB Weld in the future. I drilled the hole again in the jet and it’s no longer running too lean, so that was part of the issue. Pretty sure the wall scoring was due to failing to mix the gas and oil well enough which I’m now much more cautious of

It still runs right now and gets up to about 37mph, it just doesn’t have the same power that it used to. Still planning on buying another rebuild kit for my GT90 and swapping the cylinder body/piston out for the new one since right now you can turn the engine over pretty easily just by pushing the bike across the ground.

I don’t need my bike at the moment since I’m working from home right now and I’m saving for a car, so it might be a while before I spend another $150 on the rebuild kit

 

[Bikeguy Joe]

It runs?

"If it ain't broke and you mess with it long enough, it will be."

 

[Ryan843]

It runs?

"If it ain't broke and you mess with it long enough, it will be."

Yeah it runs, but doesn’t go as fast and it’s really slow accelerating under 20mph. Ima mess with it til it’s back where it was before because that bike was way more fun