How worried should I be about a lean condition?

Hey all,

I bought a '93 VFR750 a couple of weeks ago and I'm concerned about a possible engine issue. The bike has a little under 19,000 miles on it, and had a Two Brothers exhaust installed when I bought it. The previous owner bought the bike with the exhaust and only put about 300 miles on it, but I don't know how long the owner before him had it on.

The bike is clearly running lean; surging between 4-6k rpm, burbling and popping on deceleration, and occasionally backfiring if I let off the throttle fast enough when I shift. Part of it may be the fact that the previous owner was in Connecticut and I'm in New Orleans, so running at sea level might have something to do with it. I'm planning on replacing the TBR exhaust with a stock one, but I'm concerned about potential engine damage. As I said, I don't know how long it's had the exhaust on and it clearly wasn't re-jetted for the exhaust; I don't know enough about motorcycle engines to know how bad potential damage could be, so I'm hoping someone on here might be able to give me some guidance.

Thanks!

 

I have my doubts that just switching to a two brothers exhaust leaned out your jetting so much that it is causing an unsafe condition.

This lean condition can also be caused by air leaks, or improperly tuned carbs. Fuel with water in it can cause backfiring. Backfiring can be caused by both an improper air/fuel ratio, as well as, improperly timed ignition. In very extreme cases it can be that the exhaust valves aren't completely sealed when the compression stroke fires.

In your case, my first guess would be your carbs might need a little tune up and the timing needs to be verified. Once you know these two things are where they should be, then set about the task of adjusting the carbs, I would do this with the assistance of a dyno and EGA, that way you know you are getting it right.

1. No air leaks, cracked vacuum lines, carb insulators, etc.
2. Carbs are clean and functioning properly
2. Verify timing
3. Fuel is fresh and verifed clean
4. Clean air filter
5. Check and replace spark plugs
6. Re-jet carbs if necessary, utilizing EGA

.

 

Uhhh, parts of CT are at sea level too, as its largest border is with the Atlantic Ocean, technically called Long Island Sound in that part. ;-)

weird tie in fact, the guy who taught me to ride, and sold me my first motorcycle, when we both lived in -- CT, now lives in New Orleans!

 

NCB has it covered, but if you can find the idle mixture adjusting screws, you could easily dial in an additional 1/2 to 1 turn counterclockwise to see if that helps. Your carbs could be partially clogged, so a can of SeaFoam might improve things.

Also, why not inspect your sparkplugs' color to confirm or contradict your suspicions ??

 

Thanks so much for the detailed reply, I don't know if I have the resources to do all of that at once, but good to know either way. The condition isn't affecting the entire powerband, and the bike still rides ok; it idles fine, runs well on the low end, stumbles and surges from 4-6k, and then is fine the rest of the way. It's only on deceleration that I really notice anything. Do you think switching back to the stock exhaust would take care of the problem well enough? Also, how would I check for an air leak?

 

to check for air leak, spray carb clean around the carbs and intake boots.

the rpm's will drop ir you find a leak

 

May I suggest a new fuel filter?

If the bike sat for a while the filter may have become clogged. Cheap part too! Easy to replace

 

Not the pipe

No, and here's why.

When the owner bought the pipes he probably also got a jet kit. You will have an indication of that if the covers on the fuel mixture screws have been removed. If so then you can gently turn the screw in till it bottoms and count the number of turns to see where you are set, then turn it back out.

My guess is that the fuel mixture screws are opened way up, ~3 turns or more, in an effort to compensate for the lean mid-range condition.

Is the bike popping and burbling on deceleration, or backfiring flames out the pipe?

Does it just bog down from 4-6k and then clear up above 6k rpm?

If you apply the choke lever does the problem go away or get worse or no difference? Try it with partial choke and full choke. If the problem goes away with the choke then you probably have a clogged pilot circuit and need the carbs cleaned. If it gets worse then you are running too rich--dirty air filter, floats set too high, stuck vacuum slide, etc.

Overall it just sounds like a good carb cleaning is needed, especially if it has a jet kit, to verify it was re-assembled correctly.

Carburetor Cleaning + Specialty Motorcycle Services

 

I had a TBR pipe on my 900; everyone I took it to could not get the carbs set correctly, so it always ran lean. Made it a little more difficult to start in cold conditions, but I really liked the cackle that it made. It had 24k miles on it when I traded her in, and the pipe was on there since new. Never had any issues, other than the occasional backfire/pop that it did. Never did any damage to the engine.

Just enjoy the noise, and enjoy the light show at night as well. It just burns the baffle out of the pipe is all.

 

I have a similar question to think one. My 1990 VFR with 54k on it has a stock pipe (anything but stock is illegal in Maine now) but has a K&N filter. I have no idea if the previous owners put a jet kit in, but since I have been regularly getting 50+ mpgs, I figure not. It back fires on hi-reving deceleration, but I have NO other issues with it, other than a very recent drop in MPGs (I think tire pressure, since its getting cooler and I have not checked them in a bit). Should I be worried about anything with this setup?

 

I get 32 to 37 mpg, I am nice to my stuff but must run faster on the super-slab. My bike has 76,000 and I recently got a V&H slip-on for it. Bike has stock jetting with a K&N air filter and rides about the same, similar sound too!

After living with the bike for 17 years, if it farts, I know :smile: I do have replacement float needles and some other parts that I plan on doing in the near future. Plan on shimming my needles for shit and grins and see how it runs. (shimmed the EX and it made a difference on top) I never had an issue between 4000 and 6000 grand, just a buttery smooth power band. You need to service the carburetors and do a proper carb-synchronization with a manometer. Fuel mixture screws on this bike I believe are turned out 2.5 turns or so, but its easy enough to finger fuk it anyway, Cheers BTW carburetor balance and fuel mixture screws only come into play at a cunts hair off of idle anyway.

I did take all that pollution crap off the bike, De-PAIR or what eva youss call it ; )