Backfiring VFR800 Vtec 2004

After a rebuild (not the engine) of my 30k mile VFR800 Vtec 2004 and fitting a new stainless exhaust I have a backfire when I gently roll off the throttle and then gently open it. It isn't just a gentle backfire, but a fairly loud violent backfire, making riding a real pain. I have checked for exhaust leaks and there don't seem to be any. The butterflies in the inlet were slightly sticking open, but after resetting the throttle return cable the backfire is still there. Could it be a problem with the O2 (lamda) sensors? :grumpy:

 

Did the bike behave this way before you molested her?
How is your sparks?
PCIII or V can sold the problem :wink:

 

.... also have you removed the PAIR valves?

 

Is this a full exhaust? A full exhaust will cause a lean condition. That would require a fueling computer like the Power Commander.

What was the extent of the rebuild, aka, what was opened up/touched/etc? Any problems before this, or this rebuild was just for fun and add-ons?

Do you see a flame out of the exhaust during the pop?

Is there any liquid gas in the exhaust? Any raw gas coming out of it as it runs?

There are no warning lights on, is that right?

 

As stated the bike was mechanically a full rebuild, but the engine was untouched. The bike was originally mine, but my son had been riding the bike for the past couple of years 365 days per year as his sole transport, so the salt and grime of the UK had got to the bike, so I stripped it down completely, got everything powder coated, reprayed, stainless fastenings, and replaced things which were totally rusted away such as the exhaust guard behind the riders right lower leg (see picture). I kept the original VFR triangular silencers as they were in remarkably good condition but bought a Delkevic set of down pipes and collector (I have to say that I wasn't impressed with the fit of the Delkevic pipes to the cylinder heads and I had to get them "warmed" and bent to get them to fit snug). Change of plugs with Iridiums going back in, change of rotor, stator and rectifier/regulator as the connector from the rotor to rectifier/regulator had well and truly been fried (see photo) , which I believe is a common fault. New air filter. Nothing on the engine either on this occasion or previously has been changed.

The bike has about 37k on the clock, has had regular oil changes and basic maintenance, has not backfired prior to the rebuild. With not touching the actual engine my initial thought were air leaks on the exhaust, but can't feel anything either with the engine hot or cold. My second thought was the O2 (lamda sensors) and wrong fuel mix, as I believe there as been some controversy about them being wired the wrong way round in the past, had I got them back the wrong way? I have tried with alternative sensors and replacing them with "eliminators" with no improvement.

The backfire doesn't seem to be there when you first start the bike, but once you are under way and "roll off" the throttle, say coming into a corner, and then slowly open the throttle, the backfire is when the throttle is just beginning to open. I haven't noticed any flame out the exhaust. No warning light showing.

Not sure what "PCIII or V can sold the problem" or "also have you removed the PAIR valves?" means but I am fairly mechanically adept and will try anything (in the middle of rebuilding a Yam GTS1000 with hub centre steering atm).

 

Jikoku, beautiful yellow bike!

"PCIII or V" = Power Commander hardware version III or version V. These are fuel controllers. They interrupt the ECU signal and modify the amount of fuel delivered.

If this problem is due to a lean condition from the exhaust change, then a fuel controller will allow you to make the mixture richer as needed. If you can borrow one, you can plug it in, try a stock map or also modify the fuel rate using your windows laptop, and see if it fixes the problem.

The evidence that it takes a while for the backfire may point to two things: One, a leak appears once the pipes are heated and change shape. But you seemed thorough with the inspection so that may mean number two, a lean condition will rear its ugly head when the engine is hot.

The PAIR valve is an EPA device named for "pulsed air injection". It injects air into the exhaust so that unburnt fuel in the exhaust will ignite. This valve tends to make a popping noise on deceleration. It may do this due to age, or it may do it as a result of aftermarket exhaust changes. This is not a backfire as you know it, merely an annoying "pop". Some people disable the PAIR valve to eliminate the popping noise.

Did you tune the starter valves? I highly recommend this. The off throttle to on throttle response, and slow speed maneuvers can become jerky as the vacuum among the four cylinders goes out of synch due to age and wear. You can do this easily with a four-column manometer or four separate gauges.

 

Knight explained the PAIR valve well ... while not likely the issue on its own, it can contribute.

Knight has very good advice - and agree with him on the exhaust.

 

Hi Thanks for the prompt response.

It is a definite "backfire" rather than a "popping" sound. My oldest son runs a Ducati 750SS as well as a special that he has built and the Ducati pops under decelaration and it is nothing like that. What are the "starter valves"? The only thing we have checked and amended is when we took off the air box, we noted that the butterflies on the injector inlets were not quite closing when the throttle was closed and the return side cable was sticking about 2mm before the stop with the adjuster screw. We traced this to a slight restriction on the cable run for the return throttle cable and cleared that so that the close throttle cable ran free and sat tight up against the stop. All 4 butterflies are moving in unison. Are the "starter valves" the butterflies mentioned above? Can you point me to a thread on the "starter valves" If they aren't the butterflies and if you think that might be an answer.

Thanks

 

As the VFR is an ex-USA bike I assume it has the EPA PAIR valve that you mention, which wasn't fitted to UK bikes, so again if you can point me in the right direction to remove that as well!!

Thanks

 

To be clear I just threw in the starter valve synch because of your thorough rebuild. This is just a gem to add on to everything else you did. This has nothing to do with the problem at hand but is just a standard maintenance item. If this was never done you should see improved throttle response and slow speed smoothness.

Note the vacuum lines that bypass the butterflies and go right into the cylinders? This is an idle control circuit. If you open the throttle just a crack, you will see the butterflies do not move. All air from idle to 4% throttle goes through the starter valves into the cylinder. The starter valves are adjustable and are meant to be tuned at major service intervals. First off you can youtube this, "vfr800 synchronize starter valves." There is a good video for 6th gen, even though your bike is 5th gen. Then you can search this forum for text versions. Also this procedure is in the Factory Service Manual. You remove the airbox, connect manometers to the vacuum hoses, and turn the starter valves until each cylinder reads according to the FSM.

For the problem at hand modifying the fuel is your best bet. I mention borrowing a PC just in case it isn't the issue. However, realize most exhaust changes that involve better flow (everything but a muffler change) tends to run lean and require more fuel. So it is a good bet that lean fueling is the issue.

 

I just noticed this post. I did a google for "disable PAIR" and that tends to provide better results for this site than the forum search does. Here is a link for doing this on the 5th gen bike but you may read through and others can comment on any changes for your 6th gen:

http://vfrworld.com/forums/showthread.php/35929-How-To-PAIR-removal-for-5th-Generation

 

Don't know if this will help as the frame is mostly aluminium, but when you put it back together (after powder coating) Did you clean all the parts that earth's connect to? Could be an electrical glitch caused by something not earthing properly.

 

I didn't powder coat the actual alloy frame as the surface seemed ok, only ferrous parts (subframe, stands, various mounts etc) and the the ali fork yokes and sliders etc. I sprayed all push connectors with a contact cleaner and worked the connectors a couple of times to get any oxide off the faces.

 

It all is beginning to make sense now. I have checked my rebuild log and I renewed the front PAIR valves when I took off the PAIR valve cover to get it cleaned up, but the rear cover was protected from the elements and I didn't take it off or renew the valves. This would probably give an uneven balance between the back and front PAIR valves and combined with the new Delkovic exhaust pipes and collectors sets up a situation that is confusing the ECU to run rich. Does this sound feasible? I think I am now commited to removing the PAIR valves and solenoid. At the same time I would probably dump the EVAP canister and related solenoid, is there a good link to removal the only one I have found wasn't too clear on what to actually do.

 

While I do not have your vehicle I would be surprised that any vehicle has multiple PAIR valves. They generally inject air into the entire exhaust, not per side of the engine. My research shows that this is the PAIR valve for your bike:

http://www.hondapartshouse.com/oemparts/a/hon/5053f5b2f870021c54be7bc7/air-injection-control-valve

BTW this site and Partzilla are great sites to decipher the entire bike.

 

Removing the PAIR valve is very easy.

Here are the instructions: http://www.vfrdiscussion.com/forum/index.php/topic/72327-6th-gen-pair-valve-removal/

I used block off plates and removed all the guts (save weight) - some guys just stuff marbles into the tubes and disconnect the solenoid that operates the air injection.

This is a very inexpensive mod, but I am leaning towards fuel map on this.

 

The valves I am refering to are the reed valves shown as 10 on the attached blowup
( http://www.hondapartshouse.com/oemparts/a/hon/5053f5b1f870021c54be7bb2/cylinder-head-cover ) and their covers. The pipes labelled 2 and 3 on your blowup connect to the covers over the reed valves, if you look closely you can see the continuation lines from the tubes in your blow up go to the reduced scale engine front and rear heads. There are 2 reed valves in the front cylinder head and 2 in the rear one and are the ones shown in Jeff Barrett's link for removal. The ones I replaced on the front cylinder head were totally gummed up and I assume the rear ones would be the same. I don't know what the implications are of running one new and one old gummed up set, probably gives a mixed up reading on the respective O2 sensors.

 

Discussed this with my son and we have spotted the replacement cover plates on ebay

http://www.ebay.co.uk/itm/PAIR-Valv...110859?hash=item3cd8c954cb:g:RPoAAOSwnipWXr2~

It will probably take us a couple of weeks to get the parts and do the work, but I will report back on the result.

 

Those are them! I don't know that they'll be the entire solution - but I have removed them from both of my bikes.

Reduces weight and the bike doesn't feel as 'snatchy' at low throttle with this done along with removing the snorkel and doing the flapper valve mod.