Carb Needle Shimming Advise..

I've been putting the first miles on my re-awakened '87 VFR700. Bike was nice to begin with, and low mileage (just turned over 5000). Carbs were well-cleaned and I swapped the pilot jets over to #40's, fuel screws out 2.5 turns. Has a K&N air filter and a slip-on system I made that includes a cross-over pipe. Valves have been checked & adjusted and new / proper NGK's installed. Anyway, it starts, idles, accelerates, and generally runs great. However, I've started to notice some "surging" while at steady highway speeds. Thinking that it may be lean. Reaching out to the collective wisdom here, but what are the thoughts as to shimming the needles to correct this issue? Any other recommendations? Thanks!

 

probably can be tuned out.

what do the plugs show ? your custom exhaust will need some tuning. could try a washer, sure. size 0.5mm. you could also try increasing fuel screws about 1/2 turn out and note any improvement.

could try #5 step up in main jets too.

 

I admit that I am not an expert in carb tuning. That being said, if it is running lean I believe you need to shim the needles so that they are shorter, thus allowing more fuel to be pulled up through the main jet at the same throttle opening. Does the bike hesitate to accelerate? If so, that could also be a sign of a lean mixture.
MC Garage has a short video that may help.

One way to check lean/ rich burn is to pull a spark plug or two and check the color of the residue on the electrode area. Light brown similar to a toasted marshmellow is ideal. Darker is rich, and white is lean.

 

My VFR was running very well at idle, accelerating, under load, and while varying the throttle input (such as on most back-roads). Only issue was a steady cruise/steady throttle, 50-70mph range, then it would surge . Plugs looked god to me, light brown/ tan, and consistent for all 4. I just finished shimming the needles, adding shims that resulted in .5mm additional lift to the needle. Will take it out for a run in the next day or 2 and see if it made a difference.

 

if needle shimming doesnt fixit...............

at part-throttle openings in that range an idle jet one step larger might help, just a bit richer in that rpm range at the top of idle jet effects. or trying a slightly richer mixture screw setting maybe less than 1/2 turn.

if its normally #38 go to #40.

tuning is experimentation and guessing, sometimes alot

 

Surging indicative of intermittent starvation of fuel.

 

I did install #40 pilot jets to replace the #38's when I cleaned / re-sealed the carbs last fall. Seems to run well in the range mainly under the "control" of the pilot circuit. Starts quickly and needs minimal choke.

 

I'm pursuing this as a lean-condition. I believe that fuel flow to the carbs is ok due to it pulling/accelerating very well, even when the throttle is opened during the surging. Thinking that it would just bog if fuel delivery was insufficient. Not saying that I'm right. Other areas (other than richening the mixture) I might consider?

 

what mods on ur bike ?

 

K&N air filter that was in it when I got it (good shape, clean, oiled). Slip-on pipes I made (mimics the stock design. Stainless mandrel bent pipes with a cross-over pipe. Overall diameter is a bit larger than stock). Pilot jets bumped up as mentioned. It was a CA model, and I removed the extra emission plumbing, plugging the ports and air-inlet's in the front cylinder head. Otherwise, fresh valve clearance check, plugs, carb synch, oil, coolant, etc. Just turned over 5000 original miles.

 

Try shimming as you first mentioned, that could make the difference.

Do you have the snorkel still in place on the air box lid?

Also, you seem to have done the de-smog properly, just make sure the plug/cap on the one vacuum spigot on the carbs is proper.

BUT I feel it is time to up the mains unless you happen to come across some adjustable needles with a proper length/taper/profile.

 

Does the snorkel being removed have a drastic effect on this bike?

 

I did a back to back test on one of my 86. It feels very "flat" in the mid-range to me without it, if not properly compensated for. Twisting the throttle just made more noise than expected progress. I did not like it, put it back in and the bike ran stronger and linear again.

Another one of my 86 has it removed, but that bike is properly jetted for it, and a 4-2-1 exhaust.

Putting the snorkel back in would be a good test for improvement. Or partially taping the hole.

 

How many '86 500s do you have?

 

We are talking about 86/87 VFR700/750 here, of which I have four. I have two 86 500s

 

I was careful with the necessary port-plugging during the de-smog. Couldn't help but observe the apparent "crudeness" of the emissions equipment when compared to the rest of the bike. Ugly stuff! Anyway, the airbox snorkel is still in place. My needles are now shimmed, but I haven't had time to give it a ride and see what difference it made. I agree that if the surge is still there, I'm going to up the main jets. Any recommendation as to how rich I should try/main jet #?

 

You could easily go up a size, or two, and be good with your mods.

For reference, one of mine has a K&N, no snorkel, 4-2-1 off of stock headers with Tyga muffler and I am running #125 FR and RR with shimmed stock needles. Runs fucking perfect, and has for over 6 years.

 

Finally got to get out for a quick ride. Still found a bit of surging remains, but reduced from what it was prior to shimming the needles .5mm. Since it's running so well otherwise, I'll probably finish out the summer this way, then pull the carbs this fall and bump the main jets up in size. Happy to take any other advise in the mean time. Certainly appreciate all the help!!

 

At least you saw improvement and know which direction to go. It will clean right up.

You could try 1/4 to 1/2 turn out on your pilot screws if they are still under 3 out for some possible minor additional improvement too in the mean time. You stated you went up a size, so you should be nowhere near 3 out right now.

 

Have the mixture screws out 2.5 turns, with the #40 pilots. I probably will try 3 turns and see how/if it effects the surging. Right now, it starts easy and needs very little choke enrichening. I did order #122 main jets today...