I mean no offense, but in my opinion, you're intent on building a racing kart engine for a mini bike. Apples and oranges. I have Tillie's on both. My advise is install a 4 degree offset key to back your ignition timing from the built-in 32 your billet flywheel has that was engineered to run @ a continuous high rpm in narrow power bands on racing engines to a more acceleration friendly 28 degrees unless you plan on racing your mini bike on an oval. Might go 30 on an engine for a broad torque curve build, depending on application, but never 32 on anything not on a track. This is static timing. No advance mechanism like a car. 32 is 32 at idle and at max rpm. You're killing your low and mid range torque with too much advance. I have a similar mini bike build to yours, live in the hills of Eastern Kentucky, climb the hills, and hit 45 on the straits with a PVL flywheel/ 28 degrees advance and the TCT carb, which I believe has increased port velocity (numbers anyone?), which pairs well with the increased piston dwell time provided by the long rod and the velocity killing hemi head. These engines can have issues with low port velocity and it can unintentionally can be made worse. Flow is important, but not at the power killing loss of velocity. Torque rules. Push your gear, don't drag it along until you hit top end at 7000+ sustained where the billet flywheel was designed to perform. Clean up your ports. Polish the exhaust only. Get a pipe to match that cam and install a filter kit. Look at your timing and gear ratio. Read up on port velocity and how to make torque. End of advice. Good luck and happy trails.
No offense taken. I'm on this forum looking for information/recommendations/advice which I appreciate. I'm not an expert on these engines nor have I flowed the heads or dyno'ed any of these engines. The components that I used for this build were recommended to me by several of the primary mini bike/go kart engine builders/suppliers. In fact, the approach that I took was recommended by the engine supplier as they told me that is what they do to the Tilly engines for their Trailmaster MB200-2 mini bikes.
With that said, I believe that some of my issues are from the TAV, gearing and the Tilly carb. You can see in my above posts performance issues that I was experiencing. The Tilly engine in Stage I configuration (no governor), while riding, would not exceed 4200 RPMs @ 34 MPH on GPS. Also, had issues with the carb. With the upgraded carb jet, the engine wouldn't initially start. When it finally did, it ran extremely rough. After cleaning the carb and reinstalling the jet 3 times, it finally started to work. Pretty easy to install a jet! So, I was suspicious of the carb. Gearing issue is kind of obvious as I started with a 38 tooth rear sprocket. With the TAV, because of the shorter Tilly PTO, the engine supplier sent me a new driver unit and belt. Belt doesn't matchup with calculation for correct sized belt based on driven pulley diameter and bolt to bolt centerline spacing between jackshaft bolt and driver PTO bolt. Belt is approximately 1/2" too long.
In for a penny, in for a pound! So, I ran the mini bike the other day for a short time with the built-up engine and Tilly carb. When I originally ran the Stage I engine, I was guessing it topped out at around 4,000 RPMs. It was actually 4200 RPMs with a tach. I'm guessing now with the 50 tooth rear sprocket, I'm guessing it topped out at round 5000 RPMs. I'll install the tach again after I finish installing the Mikuni 22 carb.
Comments:
1) I'm not having an issue with low or mid-range torque. Off idle, I can easily spin the 19" rear tire on dry grass.
2) Sparkplug looks a little lean. I killed the engine while running, no idling, to check the plug.
3) Engine noses over at top end.
4) Without a muffler on the header, engine is gutless. Put a muffler on the header and with some back pressure, engine runs fine.
5) Engine supplier told me this is the best flowing mini bike/go kart engine head I could buy without having the head machined. This statement was based on their dyno testing. They claim 10hp in stock form for this Tilly engine.
6) I have several 800-1000 foot straightaways where I can get on this engine.
My plan is to finish the Mikuni 22 install and then test again, weather permitting.
Tilly!Tilly! - what mini bike are you running, specifics with gearing, TAV?, etc...? My Trailmaster weights around 165 lbs.