Need to get this machine dialed in!!

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yooper trooper

yooper trooper

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Feb 28, 2020
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Just made a few changes on my 750 roadster and just can't seem to get it right!! It's a 1973 750 I picked up in a trade for a Suzuki ATV. Bike wasn't shifting right so I opened up the tranny and replaced a few parts, works great now, also replaced the clutch pack, that works now too. Installed a TRISPARK ignition and right now have it static timed at 28 degrees BTC, also installed a 42 mm Mikuni. The bike starts right up but won't take any throttle until things warm up and even then I can't snap the throttle open without it bogging down momentarily.
 
42 seems big...usual Mik is 34mm.
Be sure to review the layshaft bearing issue in the tranny if not already addressed. And note the timing scale on primary can be out of reality...need to check its read out with a degree wheel on the crank.
 
Carb's too big. To use that size of a carb you need one with an accelerator pump. Even then, on a street engine, it's too big to snap the throttle open and not expect it to bog until the RPM is up in probably the 3500+ range or more. It's not a workable carb on a stock engine.
 
Wowee that's a large carb, better on a BIG twin. HD or something. Too much juice.
 
220 main jet needle set in second slot from the top. Here's a little more info..bike starts on second or third kick, choke on. I immediately turn the choke off as soon as it starts. Seems to idle ok but don't even think about twisting the throttle until things warm up and even then it needs to be nursed. Also, the spark plugs soot up really fast!! Really fast!!
 
New carb? Clean? Try dropping the needle or adjusting the air mix screw, out to lean.
There is a ton of info out there on tuning Mikuni carbs.
 
When I had single Amal on my 850, had a stumble right off idle. Would stall at lights etc. Turned out to be worn needle jet. Not sure if Mik's can suffer similar.
 
You need to decide whether the motor gasps, or if it loads-up as you open the throttle. If it is too lean, it will probably gasp. If it is too rich, it will usually still respond as you open the throttle. If you have the correct needle and needle jet, you should be able by lowering the needle one notch, to induce a cough as you ride the bike. When you get that, raise the needle one notch and you should get good throttle response. If your carb is jetted to the optimum, you should probably have to feed the throttle on with a moderately fast but smooth action, rather than just snapping it open. That way you get the quickest acceleration of your machine.
 
Tornado said:
When I had single Amal on my 850, had a stumble right off idle. Would stall at lights etc. Turned out to be worn needle jet. Not sure if Mik's can suffer similar.
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When you buy a second-hand bike, the first thing to replace is the needle jet. My friend bought an RZ350 Yamaha and managed to find replacement needle jets on the web. The carbs are very different to any other RD or TZ. He took the bike to the Broadford Bonanza and saw off a lot of other very fast bikes. It is probably as fast as a TR3 production racer. When tuning a motor, there are two things - ignition timing and the needle jets - more important than most other things - particularly with two-strokes.
 
If you want to learn how to tune a motorcycle and you have plenty of money - try tuning a two-stroke on methanol.
 
Both of my 750s have single 34’s. You can only leave the choke on a few seconds. When cold I always set the idle screw up a couple full turns till motor warms up. I would look for an air leak between carb and head. Also try moving the timing up.
 
Mwood7800 said:
Both of my 750s have single 34’s. You can only leave the choke on a few seconds. When cold I always set the idle screw up a couple full turns till motor warms up. I would look for an air leak between carb and head. Also try moving the timing up.
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That pretty much describes where I'm at...still wondering why the plugs soot up so fast. That kind of sez it's running to rich. As far as timing goes, I'm at 28 degrees BTC static. Maybe bump things up a bit like 32?? I do get some backfiring when cold but very little.
 
Any choke more that 3/4 seconds and plugs black. I set my timing to where there is no kickback.
 
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