Tuning//Syncing issues

[NYSingh]

I've had this bike for over 5, and it runs and is fun to ride, but I don't think it's running quite like it should.  I've had the top end done by Chuck, installed a Vape, boost bottle, etc, but still not running primo.  So I can only conclude it's my inability to tune the thing properly or sync the carbs properly that's the problem.  This being my first and only 2 stroke, I don't have the experience to know what it is supposed to feel like or sound like.

And so I turn to 2sw for help.  I recorded a video of the bike running and my efforts to sync the carbs as explained in my Haynes manual.  I'm hoping that someone here can watch and say "your problem is that you did X, you should be doing Y" or "if you listen at 2:02, you can tell it's running rich/lean" or point out something else I'm doing wrong that'll help me get the bike dialed in.

Setup:

• RD400C
• Chuck stage 1 with TSR boost bottle
• Dave F mod w brass ball removed, airjet drilled to 2mm
• Vape ignition @ 1.9 BTDC
• Main: 200
• Pilot: 35
• Needle Jet: P2
• Need: clip at 2nd position from the top
• Air screw: 1.25
• Slide: stock, synced so the bottom of the "dot" as viewable through the hole on the carb body is align to the bottom of the view hole at full throttle

[m in sc]

double check the float levels.  I mean, from a cold start it sounds pretty normal.  actually, motor sounds really good. 

try it at 1 1/8 on the airscrew. anything between 1 and 2 turns is ok.   also, its on the side-stand and un-ridden so it doesn't mean much until its fully warm (ie pipes fully warmed up.. they aren't here) and ridden then checked, preferably more upright.


dotn worry about the cyl temp, especially 30 degrees,  except for while being ridden, thats what matters.   thats my take.

[Striker1423]

The Carb Windows and dots on the slides are there to sync them when lined up in the middle of the window. Not the bottom of the window.

The bike stalling... well, choke is only there to initially richen the motor. With choke off, the bike no longer feeds from the choke circuit. If you continuously run it on choke it over-richens the mixture and the bike stalls. Mains and Pilots... Might need to go a bit bigger on them given the work that's been done to the bike. Air screw setting. Anything under 1 turn out usually indicates a lean pilot. Try adjusting this at 1/8 turn increments back in.


Regardless of my suggestions, whatever you do, make sure you change ONE THING at a time and record the results. This, of course is assuming your basics of carb setup (fuel level, clean carb, no leaks past needle, etc) and sync are done FIRST.

[Alain2]

What is the problem with the bike? How does it ride? The engine sounds very good to me.

[1976RD400C]

I think you have to take it for a ride and "clear it's throat", warm it up, and then see how it sounds. Does it sound ok as it accelerates?

[NYSingh]

The Carb Windows and dots on the slides are there to sync them when lined up in the middle of the window. Not the bottom of the window.

Is this one of those situations where the manuals are giving bad advice? The Haynes and Yamaha Workshop both say bottom of window, but I imagine the more important thing is that they're even.

[NYSingh]

A few questions:

• Is the process I went through to sync the carbs accurate or did I do anything wrong there? And did each side sound right individually?
• Should I be tweaking the airscrews along with the idle screw?
• If so, should I do them one cyl at a time like I did for the sync?

double check the float levels.   

I did that right before I took that video when I cleaned out both carbs and reassembled - almost spot on 23mm from gasket surface to top of float (gasket removed).

What is the problem with the bike? How does it ride? The engine sounds very good to me.

I think my biggest issue is that it seems unresponsive in terms of power.  It revs easily enough and will get to 9k no problem, but it doesn't have much oomph below 7k.  I know that's to be expected with chambers meant for racing, but even still, seems on the weak side.  When it does hit the powerband, power comes on but still seems subdued.  There's a bit of hesitation around 8k-9k but still accelerates through.  Even in 1st/2nd at 8-9k, the bike doesn't feel like it wants to lift the wheel at all. I've read that improperly synced carbs can cause poor acceleration so wanted to verify my sync process to eliminate that as the cause before I move on to jetting changes.

Does it sound ok as it accelerates?

It sounds pretty good on the pipe at > 50mph.  Below that it sounds, i know how how to describe it, but just 'not quite right'.  At some point in the coming days I'll try to record a video operating the bike to give a better picture of how it sounds through the gears.

I went for a ~20 min ride today (half highway, half city streets) and below is what the plugs looked like after.  I know it doesn't tell much since I've been on 2-3 rides with these plugs, but to give a general sense of the state of tune.  Left still seems richer based on lower cyl temp, the less regular 'pop pop sound of the exhaust, and spooge coming out the header (even after filling the flange up with ATV), but plug doesn't really look rich to me. It actually died at a stop light, but noticed that the choke flipped on during the ride, so need to fix the choke lever spring so that doesn't happen.  Funny how if the bike is warm at all the choke almost acts like a kill switch.

[Striker1423]

Well color me dumb I guess. I think I'm wrong on the carb windows. I remember seeing these on some sleds and they were always synced to the middle of the window. But, both books describe the sync the same way as you show... then I'd run it how Yamaha is asking you to. 

[NYSingh]

Sorry, one last thought:  as I was reassembling the carbs I noticed that on the left carb, the little pip in the body inside the emulsion tube that's supposed to align the needle jet so it only fits in one way was missing or broken off.  Could this be causing fuel to get sucked through? I wouldn't think so because the washer below the main jet should seal the tube but just trying to think through the possibilities.

I took a video of me reassembling the carb for reference - you can see what I'm talking about around 4:18-26 (if you happen to watch more than just that part, I apologize for my swearing particularly when trying to assemble the choke lever - didn't intend to share this out when I filmed it!):
https://photos.app.goo.gl/umNakcXde91qVeCfA

[scully]

Sorry, one last thought:  as I was reassembling the carbs I noticed that on the left carb, the little pip in the body inside the emulsion tube that's supposed to align the needle jet so it only fits in one way was missing or broken off.  Could this be causing fuel to get sucked through? I wouldn't think so because the washer below the main jet should seal the tube but just trying to think through the possibilities.

I took a video of me reassembling the carb for reference - you can see what I'm talking about around 4:18-26 (if you happen to watch more than just that part, I apologize for my swearing particularly when trying to assemble the choke lever - didn't intend to share this out when I filmed it!):
https://photos.app.goo.gl/umNakcXde91qVeCfA

It should be fixed, but as long as you're sure the needle jet hasn't turned/spun misplacing the needle shroud. That would definitely disrupt fuel flow.

[hayduke]

Try fiddling with the air screws, once idle is set you shouldn't need to mess with it. I used to carry a long screwdriver with me and I'd try 1/8th turn increments, it can make a noticeable difference is the 'crispness' of the motor. It's also help figure if your jetting is too lean or rich based on how far in or out the screw is.

Also you shouldn't need to use the choke more than a couple minutes on a cold start.



PS your bike sounds really good

[m in sc]

yes, you absolutely need to fix the pin holding the emulsion tube in. this will cause issues as it will misalign the air jet hole and if its missing will suck fuel right into the jet tube.

and yes, bottom, (or top) of hole with the marks on the slide.

[scully]

yes, you absolutely need to fix the pin holding the emulsion tube in. this will cause issues as it will misalign the air jet hole and if its missing will suck fuel right into the jet tube.

and yes, bottom, (or top) of hole with the marks on the slide.

Marc, it looks like the pin is still there in the video, just needs to be pushed in and then staked to keep it from backing out again.

[Alain2]

  Personally I would avoid spinning the engine at 9k until you figure it out, you should first be able drive and cruise around normally between 5-6k rpm.

[m in sc]

i admittedly didnt watch the carb video.