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1985 VT500C
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2 Posts
Discussion Starter · #1 · (Edited)
solved

Link posted below by CSRoad provides everything I need to know about carb tuning, and more. Thank you!

#For refference of others, 85' VT500C is in the high RPM category.

Hey everyone,

This is my first post on this site, so I'll give a little back story and specs after the issue description to fill everything in up until now.

This is where I'm at and what I have changed to get these issues. I have the engine open wide on both ends. Velocity stacks on the upper, open headers at the lower. Moved from 112 mains to 142 and from 38 pilot to 42 pilot with 1 shim on each needle. I already know at this point that I'm still running lean. More jet trial and error to come.

The issue, giving a post rebuild break in ride cruising at 60kmh, at roughly 1/4 to 1/2 throttle operation in both 5th and 6th gear. The rpms surge slightly up and down. Not dangerously so, but its noticable in the ride where extra care is needed to manage throttle application to maintain speed control and occasionally to fix a dip in rpm, throttle application would cause a high surge throwing my head back a bit.

I think I know the issue but I would like to check with the hive mind. Would I be heading in the right direction to say that the issue would be with the venturi. Primarily with the jet needle. Secondarily with the slide. I haven't drilled the slide holes yet possibly effecting interaction with all the extra pressure, and the diaphragm is in excellent condition for a fossil. Maybe an extra shim would do the trick but at this point I'm just spit balling.

Backstory:
I bought a 1985 VT500 in 2020 for a discount due to issues I was willing to repair, or at least attempt, as I have a basic understanding of ICE powered machines. The main issue was a
Blown cylinder. While fixing that I tore the bike down to the transmission to spec the wear and change what needed to be changed and refresh what can be refreshed, eg, cylinder hone etc...

Through the year and with the help of a haynes manual I got it all back together and she starts like new. A beauty to be sure.

Thanks for reading, and thanks for all the advice I've received while lurking.

You are all awesome.
 

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1999 Shadow 750 ACE VT750CD3 Modified
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4,754 Posts
At that throttle range it is beyond the pilot jet.
It sounds like lean surge to me and an additional shim on the needle may well point the way to an improvement.
I am a firm believer in getting the main jet right first as the sizing of that effects the needle jet flow.
Don't drill the slide it will make things worse.

Read this guidance from Factory Pro.
Then click on the appropriate tuning section, I think the VT500c is the only Shadow that qualifies as "High Speed".

Good luck.
 

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Registered
1985 VT500C
Joined
·
2 Posts
Discussion Starter · #3 ·
At that throttle range it is beyond the pilot jet.
It sounds like lean surge to me and an additional shim on the needle may well point the way to an improvement.
I am a firm believer in getting the main jet right first as the sizing of that effects the needle jet flow.
Don't drill the slide it will make things worse.

Read this guidance from Factory Pro.
Then click on the appropriate tuning section, I think the VT500c is the only Shadow that qualifies as "High Speed".

Good luck.
Excellent, thanks a lot!
 
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