Can No Air Filters Make Bike Run Like Crap Over 4000 RPMs???

[my77stang]

Im helping a friend with his newly aquired $200.00 special - a 1982 Kawasaki GPz 750

we got the bike running after cleaning the carbs but when you ride it and get to around 4000 rpm it spits, sputters, and wont run worth a crap.

my first thought was his carbs were still funky (looked like someone rebuilt them with a pair of vice grips LOL) so we found a set that came off a recently running bike. we pulled the bowls the "new" carbs and verified their condition (they really were good as new). now after installing these the bike does the SAME THING!!!! we checked the petcock, everything is clean and the bike is definately getting plenty of fuel.

i noticed that if i choke it while riding, it will rev up ALOT higher but still runs pretty crappy. after talking with my biker neighbor, he suggested we get an exhaust on the bike and some air cleaners =) ......well, I modified my old stock nighthawk exhaust and put it on - still same thing. we dont have airfilters yet and we both would like to know what the problem is before he spends any more money on this bike.

btw, although it runs like crap at higher rpms, it starts instantly and idles great

basically, would putting air filters on it make the bike actually run right? it seems like too simple a thing to fix the problem. any help would be GREATLY appriciated.

[ZooTech]

Running without an air filter causes the air/fuel to lean out, essentially starving the engine of fuel - especially at higher RPM's. I doubt it will fix the bike entirely, but it's a step in the right direction.

[my77stang]

thanks for the advice - i kinda figured it would be going in the right direction. but that still kinda leaves me guessin as to what else it could be?!?!?!

i hate troubleshooting LOL

[totalmotorcycle]

I would also suspect it would be an air problem. How are the condition of the carb boots (from the carb to the head)? Are they cracked? If so, the engine might be sucking in air from there and leaning it out too much. The 4 boots are around $30 each. You can try idling the engine and spraying WD40 on the boots and seeing if the RPM changes. If so, they need replacing bad.


Mike.

[my77stang]

while the rubber looked surprizingly good - i will definately check out the ol' wd-40 trick and see what happens (i once saw a guy use carb cleaner looking for a vaccum leak in a dodge van, funny as hell when he came flying outta there with a fireball behind him) LMAO

[mydlyfkryzis]

The Honda CB750's also lean out and get irregular air to the cylinders without the filters/airbox. Seems the air pressure around the center carbs is lower than the outer, causing different amounts of air to be drawn in.

The air filter and plenum box make sure all 4 carbs see equal quantity and pressure air, allowing for more even fuel air mixture .

Bikes with POD filter have the same issue. Unless you have a dyno to play around with, the factory airbox and filter is the best bet.

[my77stang]

nice to see fellow nighthawk people, i ride a 93 hawk 750 myself

we have the stock airbox, however the boots dont seem to fit into the box properly - also theres no air filter so we'll be picking one of them up too i suppose.

all that being said, im still weary if not having the filters could really make it run THAT bad.

[bobdog]

Tey using propane for the vacum leak test. It works way better than WD and is alot cleaner. goodluck Bobdog

[my77stang]

ok maybe this can be retitled to SOLVED or whatever you forum moderators like to do.... the airboxs DID fix the problem, i was shocked but indeed it will rev right up now.

thanks!

Shane