So confused, 1996 VFR wont draw in enough fuel into engine

As far as I recollect, it is unburnt fuel in the exhaust system igniting. It is more apt to happen on carbureted engines as opposed to an F/I engine. The high vacuum condition that is created when the throttle is closed at a high engine RPM draws a disproportionate amount of fuel through the low speed/idle circuit which doesn't completely burn and ends up in the exhaust system. When enough accumulates to sustain combustion, burning exhaust gasses exiting an exhaust port can/will ignite it.

Most gasoline vehicles mildly backfire when the throttle is closed but we never hear it due to the factory muffler(s) used.

I remember reading somewhere that when an F/I system is tuned to completely stop fuel delivery under high vacuum conditions (throttle closed, RPM "too high"), there is a noticeable delay in engine response when the throttle is opened again, so some amount of fuel is typically delivered just to avoid that condition.

 

It's funny you mentioned that because when I was looking at the Two Brothers Racing exhaust slip on, I noticed that I did not re install the slip on pipe properly to the header pipe. The flanged exhaust coupling was not on correct at all. So, I loosened to coupling and reinstalled it correctly. I had removed the pipe while looking for any exhaust restrictions (rodent nest or what not)

I have not driven it since.

I'm sure I will have it out later today or maybe tomorrow. I got to work.

HRCH

 

I would NOT be changing the main jet or the needle clips for that backfire.

The first thing I would do is check for air leaks on the exhaust pipe to engine connection.

SECOND would be the pilot air screw.
If it is backfiring on deceleration or gearshift change that IS a clue.
It means... that when the engine is allowed to go back to idle for that short time between shifts it is a little rich. AT IDLE. And REMEMBER- first 1/4 of the throttle is controlled by idle jet and air pilot screw.

If it was lean it would backfire on acceleration too. Not the case.

And do not forget to check ONE of the spark plugs from front and rear cylinders when the engine has cooled down.


AND MOST important....ONLY MAKE ONE CHANGE AT A TIME. Wait a couple rides for a solid result. Do not make the mistake of changing something every ride.

 

The PAIR system is still on. I left it alone since there is no benefit removing it. From what I could see it seems to pump air into the exhaust for emission.


HRCH

 

Ok then. That would make sense because as I replied a of post above, I had the exhaust coupler installed incorrectly. I had removed the slip on muffler and pipe so I can inspect the rear of the exhaust manifold for a rodent nest. When I was taking a look at the slip on pipe, I noticed the exhaust coupler wa not installed correctly at all. It allowed the flanged portion on the exhaust pipe to not contact the oring donut (if it has one) or the flanged section of the exhaust manifold. I guess I'm going to road test it again as I have not road tested the bike since I correctly installed the exhaust coupler.

The pilot air screws are all turned 2 3/4 of a turn out from bottoming. I could turn it in about 1/4 turn. That would make it 2 1/2 turns out. I will do that only if the bike still back fires during the up shift. Yes, only one change at a time.

Thank you again for all your help.

HRCH

 

Just a dumb question on the pilot screws. Is it right for lean, and left for rich?
I currently have the pilot screws at 2 3/4 turns out from bottoming out. I'm not sure if the adjustment D shape screws are a fuel/air mixture screw or a pilot air screw. They look like air screws. Not really sure though.

Thanks

HRCH

 

You have got to buy a workshop manual. A MUST HAVE.
Turn the pilot screws out to richen, and inwards to lean.

"the adjustment D shape screws" Do not know what you are talking about.

Have never seen a D shaped screw in my life.
We need a photo.

 

From My understanding Honda only used a D shape mixture screws in the later Hondas with Keihin carbs as I have never seen one until now. In fact when I bought the jet kit, it included the D shape screw tool. My VFR is a 1996 Gen 4.

I do have a factory service manual for the VFR. Thats how I knew how to adjust the valve clearance and double check the valve timing marks. I also had to diagnose a bad voltage regulator. I don't use Clymer manuals as they are too generic. I have a factory manuals for all my bikes. The head of the mixture screw is illustrated in the manual and it looks like a half moon, sort of like the letter D.

HRCH

 

I have not adjusted the mixture screws while the carbs were on the bike. I don't know if I can reach the mixture screws while the carbs are on the bike. I have a special 90 degree carb tool by motion pro which might get in there. I'm not going to adjust the carbs until I test drive the bike. I think the loose coupler on the exhaust slip on may have caused a problem.

Thank you

HRCH

 

Thanks for reporting back on that. It never ceases to amaze me how far they will go to keep you from working on your own bike or car.

I had the change the fuel filter on my ford van and I could not get it off. Found out the dealer wanted $150 just to change the fuel filter! I went to the auto store and was able to buy the special tool for $20.
I change it myself now. Makes me real mad.

 

That is one reason I HATE those frames wrapped all around the upper part of the engine. NO Access for maintenience!!!

STUPID.

 

I'm sure you are correct. I can access the screws from the side of the frame (side fairing off) There is no access plug on the mixture screw. I guess that is why Honda made sure Keihin installed a special D shape head on the mixture screw. This way everyone just can't mess with the mixture screw.

The sun is going down now, I'm going out for a spin. Hopefully all will be ok.

HRCH

 

I took the VFR out for a spin last night. Since I reinstalled the exhaust pipe coupler correctly. the engine still pops/backfires a little while shifting. No better, no worse.

The bike runs so well. I can't notice any hesitation. As I said before, the pull at 3500-4000 rpm in 2nd gear from closed throttle to full throttle is very good.

I then pulled out the two rear spark plugs, they both look the same. I think the spark plugs indicate a little on the lean side still.

Take a look at the pics and please tell me what you think.

Do you think I should adjust the pilot screws first? It's currently at 2 3/4 turns out from full close. Or should drop the c clip one notch on the needle jet? The c clip is currently on the 3rd setting from top of the needle.

Thank you

HRCH

 

Got it running great - Finally

Well,

Once again, I removed the carbs and raised the needle jet (lowered c clip) to richen the engine for mid throttle. The final verdict, she is dialed in perfectly. No more hesitation what so ever, no more flat spot. She is smooth all the way through. And best of all, No more freekin back fire while up shifting.
So, my carb set up is now, 128 main jet, position 4 on the needle jet, 2 1/2 turns out from closed on the pilot air screws. I re sync the carbs and all is well.

My wife and I took the bike out on an 1 1/2 hour ride today and the bike had no problems accelerating and had no back fire problem. I really like this bike, it's not a rocket as compared to my 1998 ZX11, but for comfort and smooth predictable power it's a winner. This 1996 VFR with 30K miles on it rides smoother than my 1998 ZX11 with 4K miles on it. My wife and I went on a 2 1/2 hour run on the ZX11 then we went home and took the VFR. What a difference in ride quality. I won't give up my ZX11 for the VFR, but having both bikes running great, it's a beautiful thing

Thank you to all who helped me achieve my goal. Special thanks to Donald and to tinkerwithstuf.

HRCH