86 VFR 750 Carb Tuning Advice

Hey guys,

I'm looking for some advice on how best to tune this bike.

The back story on this bike is that it used to be raced in the 90's when it was yellow. I purchased the bike in 2002 and for the better part of the last 10 years she's been sitting in storage.

I bought a parts bike in 2019 that was RWB and stripped both bikes to the frame building one solid bike. The carbs that came with my bike were gummed up and like an idiot instead of cleaning them I just pulled the carbs off the parts bike and threw them on, believing they were in good condition.

I put the original carbs on the parts bike.... and then sold it. So I have NO IDEA what modifications had been done to my original carbs.

Fast forward a couple of months and I discover that the parts bike carbs were in fact dirty as all hell so I've cleaned them and then took the bike for a ride.

I'm experienced in carb rebuilding, cleaned the carbs correctly, set the float correctly, etc but the bike doesn't pull like she used to. Don't get me wrong. It runs PERFECTLY! But she used to scare me and now it doesn't and DEFINATELY does not have the same overrun burble it used to.

I'm thinking that a solid start would be to shim the cv diaphragm needles with the v4dreams washers. Do you guys just run 1 washer on each?

I was also thinking about adjusting the air mixture pilot screw to get her a bit more rich. It's not going to be tracked but more than anything I want this thing to sound nasty with lots of pops on deceleration. Just gotta get a bit more unburnt fuel into that exhaust. haha

The bike has a 4-1 hindle exhaust, a k&n filter with an otherwise stock airbox and snorkel and as far as I'm aware all internals are stock.

I understand I'm a child for wanting the bike to burble. lol


Porn pics as a treat for getting to the end.



Not me, not my bike.. but you can hear some of the overrun burble / popping from this guys bike to give you an idea of what my bike USED to sound like on decel

 

Get your carbs back out and inventory what you have. 1 shim washer is plenty, your not gonna make huge gains with that mod. Go up one size on the slow jet to a #40. Do it. Start with your fuel screws at about 1-3/4 turn out from lightly seated, adjust out as needed. I would go up a bit on your mains with that exhaust and K&N. Don't go crazy, but on one of my similar set-up 86 I'm at 125 and it fucking runs perfect. You likely don't HAVE TO up the mains, but the other items are HIGHLY recommended. And don't worry, plenty of satisfying exhaust things going on, mine sounds ridiculously good and runs and carburetes perfectly. But I do have a different muffler than you.

If you don't like how I think that VFR sounds like, then do something stupid with a shitty muffler.

 

read the tuning instructions included with both factory pro and K&N/dynojet carb kits, both available online.

 

Thanks so much for the responses gentlemen!

Do you have a clip of your exhaust? And when you say "Start with your fuel screws at about 1-3/4 turn out from lightly seated" are you referring to the D shaped air mixture pilot screw?

Hey squirrelman, I've been looking everywhere for them. Do you have any leads on where I might find the Factory Pro tuning instructions? I have found that the Factory Pro carb kit I need is a CRB-H07-1.0

I believe I've already located the Dynojet ones: https://www.officine08.com/catalogo/documenti/20190610165451.pdf

 

i think it's been common knowledge that the jetkits available for the '86/'87 700/750 don't do much and are hard to tune. maybe it's time for you to do a hot compression test to check for sub-par performance.

your mixture screws don't do anything above about 3500 rpm, and decel popping is a sign of leanness.

why not try a high speed plug cut ?

 

Hold on, hold on. I am talking about FUEL screws (they are NOT air screws), but the 86 (or 87, 88, 89) does not have "D shaped" fuel screws. They are flat blade, both with the epoxied caps and without. Please elaborate and provide pics.

 

Are you talking about one of these guys?

https://v4dreams.com/uploads/3/5/5/2/35528301/img-4850_1.jpg




Or this guy?

 

The last pic is the fuel sceew on a 86. Dont even know what you were linking to above.

 

So the air/fuel mixture pilot screw. Gotcha

Interesting that you say, "1-3/4 turn out" I think stock is 2-1/2 turns out. I'll play around with what works best on my bike.

I'm gonna get the 125's and 40's, shim the needles and call it a day.

Thanks for input boys.

I did some more thinking about this all last night. I think I likely had more pops with the old carbs due to an air leak at the T-vacuum tubes and out of spec floats. And she's likely less jumpy due to the fact that I had a 47T rear sprocket on the 86 rim. When I converted it to the F3 rim I used the stock cbr sprocket which is a 42T.