Honda VFR 750 RC36- Extremely high fuel consumption?

Hello once again ! I will try to put this as short as possible. Thank you for your patience =D.

So I recently bought a Honda VFR 750 RC36 1990.. The bikes look is modified (I am finishing what PO started)I think the only related to my"problem" in this could be the 190 rear tire. The engine is as far as I know complete stock (except the wings slip on).

So here is the thing, the bike starts from the first crank,even if it sits for month, no choke needed.Works like the perfectly healthy bike (i noticed a little knock sound while idleing,but I think it is because of too low idle (it is about 800rpm, even lower when hot) or maybe the clutch bearing (since the sound almost completly go away when I engage the clutch). If you rev it a little in neutral the throttle reaction is amazing and fast..There is a issue with temp. gauge.. The gauge never goes up past 1/4 no matter how long the ride is,and it is obvious that the bike is warm (vent/cooler starts while waiting on the traffic lights,huge amounts of hot air coming from a bike, so I am not suspecting a thermostat)

While riding: 2 things I noticed: 1st there are no quiet detonations/backfireing like on "normal bikes with slip on" but while engine braking there pops a LOOOUUUDDD BOOOOMM.. The loudest I heard on the street bike ( and I had a few "poper bikes").

If I accelerate little more aggressively and then let off the throttle the pop would be even louder.

2nd. I was riding it like a granny all the time so I decided to give it a little speed. So I opened the full throttle in 1st, the bike jumped to 10 000rpm and kinda stoped there , shifted it to 2nd opened full throttle bike jumped again very fast to 10 000rpm and just stoped there.. No more power producing from there, it didnt sound like the limiter, it just stand there powerless and couldnt pass that 10 000rpm..

And now the problem: While riding it granny style , low rpms etc for entire gas tank. the best I could get is about 22MPG , or about 60 miles per tank ( while gauge got to red reserve mark i tanked it to full, and got 60 miles to that same gauge position).

While looking at the consumption experience from my other bikes and from other vfr owners this is brutal.. And gas is very expensive here.. The bike is probably running rich ,but there are no hard gas smells or something.

So as this is my first vfr, does this bikes have some known issues (except R/R) to be related with consumption , or should just classic carb detail clean, new air and fuel filter , carb sync, spark plugs do the trick?

Thank you!

22 is really low.. should be closer to high 30's, low 40's for miledge. Check the sprocket sizes - I think it should be 16t front, 43t rear.. if they are stock sizes, then get the carbs cleaned and make sure all cyl are firing.

It could be the choke is out of adustment as it seems odd that you never need to choke it.

This sounds like exactly what another member is going through. If you haven't seen what I wrote there, here it is:

http://www.vfrdiscussion.com/forum/index.php/topic/71912-80-miles-on-a-full-tank/page__st__30#entry846786

Check the carbs, sounds like you're running rich. Check your jets, they might not be the correct size for the mods you have. I had the stock jets in mine and I was getting crappy gas mileage. Changed jets, carb sync, and viola, 45 mpg. Not saying that your fix is going to be that easy, just giving you a direction to look. Good luck.

Thank you so much for the answers people!

@ "Burns" : IDK but gears do feel a little short.. I dont know how it should be since I never ridden one before..

Do you know approximately how much rpms does engine rev on 100kmh (60mph) ?

@ "FROSTIE" : I was thinking the same,it makes sense but since its pretty hot outside (about 30- 35 celsius or 303f) it doesn't seem too odd that it starts normally..

@ "afpierce489" : Thank you soo much for the reply, will check jet sizes.. Im afraid that someone didnt bore them maybe.. did gauge sensor give you any other trouble besides false reading?

The problem here in my country will be geting the jets if they are too big, there is no way that I can get them anywhere here.. In that case Im going to look online for them..

I'll start my list.

1) The 190 rear tire. Can you explain why you have a 190? I can not think of a conversion where you would have that large of a tire on there. A 180 with a different wheel yes. Stock is 170. What is the rear wheel on your bike?? Did someone put the wrong size tire on there. I'd change it to the correct size soon.

2) Your idle is too low. Raise it up to 1200rpm.

3) Check all of your ignition coils and your grounds for them. I had low fuel mileage before. I was only getting 25mpg going out to Colorado. I found that the ground wire that bolts on the tank hinge bolt came off. I also found a wire on one of my front coils had come off. So I was only running on 1 or 2 cylinders.

4) If your ignition wires are good, check your air filter and fuel filter.

5) Check your sprocket size. You might have to count the number of teeth on them. I believe stock size is 43 teeth.

6) If you are still having problems, you will have to take the carbs off and check the jet sizes. Stock sizes are 125 in the front and 128 in the rear (or 125 all around. Don't remember). If there are any other sizes in there, put it back to stock.

7) Try syncing your carbs. It's not too hard to do, but you might burn your hand a little bit if you are not careful.

8) If it is cheap, you might want to replace your Thermostat. It sounds like it is bad, but I haven't ridden my 96 in a while to remember where the temperature usually holds. IIRC, it should be at 1/2, not 1/4 from bottom.

POST PICS!!!

Check your mileage against GPS, your ODO could be modified from its stock kilometer reading to read in miles, causing calculation error.

sudco.com has an excellent selection of carb parts and jets. They have a minimum order dollar figure to ship, but I'm sure with a new set of pilots and mains, that won't be an issue.

- My temperature gage was running low. I knew that everthing was fine, but it was just reading low. I opened it up and there was corrosion all over the sensor. Tried to get it out, but destroyed it, so I replaced it and everything started reading fine.

- The stock jets on a 1990 Canada model should be #130 for main jets, and #40 for slow jets. I found (with my mods) that this was making it run rich and I was only getting about 25 mpg. I changed the jets to smaller ones (like Yoshi mentioned) to #125/128's and now it's running fine.

So today I finished the fairings, it is the beta beta version but will do for now, for registration and temporary riding.Then I got to the engine part ,since I didnt have the time, and lately I have problems with tools this is what I did..

First I checked choke since many people suggested it ,the cable is ok, I loosen it max, no problem on the choke lever, and on the carbs when i turn it off it goes all the way in.. So externally speaking the choke seems to be fine..

I lifted the tank and checked the 2 spark plugs on the rear cylinders ( Didnt have the time to play with the front ones), first the spark plugs where probably screwed in only by hand -.- , there where no force whatsoever,and the metal o rings on the spark plugs where not compressed at all (like on new plugs :S)..

plugs are obviously new, since the electrode looks like new, and the plastic around the electrode is white but not due to lean burning but white as from factory brand new white.. I guess that the situation with the other 2 plugs would be the same.. Picture of plug ( on the picture looks much worse and darked)

http://imageshack.us/photo/my-images/15/20120802002.jpg/

The rear sprocket is stock (43T) ,but I didnt checked the front one.. I get about 100kmh (60mph) by clock in 6th at about 4000 + - rpm.

Next I checked the air filter.. And boy oh boy,it was full of everything, oil,dirt,and the up side was completely black. I cleaned it as much as I could and dryed it (temporary solution till I get the money to make full service). Removed the airbox snorkel, and went for a spin.. By opening full throttle bike was stalling and jerking alot. But strangely after a while it started to work better and better.. After a while I could open the full throttle whitout stalling but it was lacking some power. The backfireing was more occasional and quiter. But still would pop a loud booomm after some time.. And this is what doesnt make sense to me since I made it run lean without the snorkel.. Got back home, put back a snorkel and went for a little longer ride..By opening a full throttle there is alot of power and the bike is now hitting the limiter( at about 13000rpm).After 10T rpm there seems like there is some extra power, something like turbo spooling and hitting after 10 lol. But the consumption is the same, maybe got better but just by a bit. And I Iooked behing to the exhaust when I was expecting a boom and when it poped it was a huuuugeeee orange flame lol..So this is definitely a jetting, filter,syncing sort of thing and will keep you informed with everything I do on the bike ,and I will for sure post a solution when I find it..Need to earn some money to play with this things + priority now is to register the bike.

Sorry for the long post, here are 2 pics of the bike since you asked.. Front fairing is from VTR sp2 and the rear tail was already on the bike and I dont know from what bike it came. Maybe you do?

[image: 20120803001.jpg]

[image: 20120803002.jpg]

Popping under decel may be due to clean air (from either poorly blocked off PAIR ports or other exhaust connections) entering the exhaust system:

http://www.mikuni.com/tg_backfires_in_exhaust.html

I wish my bike shot fire out the back :sad:

...and the proper size tire for that rear wheel is 180. Ditch the 190 when you get a chance.