85 VF750F Rattling/Throttle Hesitation

So I have searched the forums extensively and found many good threads about rattling cam chains, tensioners, and valves. But I figured I would finally post my problem on here and see if anyone can help me out a little more before I decide to take the bike somewhere.

Upon starting, the bike seems to run just fine. However, once the bike gets up to normal operating temperature you can really hear a noise that basically sounds like a chain running around inside the motor. When the bike is warmed up, it has an issue at around 3-5k rpm. If I am on the throttle maintaining 5k and I then get off the throttle and come back on after coasting down to 4k, the bike just kinda...dies. It runs still, just no power, no acceleration. Kind of like it wants to pick up and go...but then hesitates a couple times, then goes. This is especially noticable when shifting from 3rd down to 2nd to take a turn in town, after coming out of the turn I try to get on the throttle, and it hesitates, sometimes I can get it to drop 3k rpm. However, the "colder" the bike is, the less noticable all this is. After starting the bike and letting it idle a bit, the bike does not do this hesitation, only when it is up to a good temperature. Further, it does not do this every time. I can say that it does it on probably every curve that I slow for, and maybe 1/2 the time if I am on a straight street.

Other than the throttle hesitation, the bike runs great. No oil leaking, no smoking, exhaust does not smell to rich/lean. The PO took good care of it. The bike accelerates nicely up to what ever speed you want to do, quickly if you ask it to. I can not hear any knocking, just that annoying rattling.

I am not sure what this would be, as this is my first bike and have limited engine mechanical skills. I am leaning towards saying the CCT is in need of replacement, allowing the chain to skip and flop around loosely. I want to think that, because I love the bike and won't admit it was flawed in oil system design.

 

admit it and you will set your mind free young grass hopper.......first, do the oil mod you'll be glad you did.... second, clean out your carbs.....you may have a clogged or fouled jet....third. change the plugs.........then lastly check for an air leak.......cold= contraction of the rubber=tight. heat = expansion= plyability of the rubber= leak possibly...

good luck

Crusty

 

Sounds like a lack of fuel. I take it the rattling from 3-5k rpms is only in that range and only when warmed up? If so it sounds like detonation. Which is a bad thing. You can knock the rod bearings out of it. Almost sounds like your pilot jet/circuit is partially plugged. Definitely a lack of fuel someplace because of the hesitation. That's why when cold it runs fine the fuel is more dense. Is the choke on then? Could be a partially plug fuel filter which won't let the bowls fill with fuel, plugged jets/cicuits, float setting(I doubt it). What's the history of the bike in the last year or two? Been setting someplace over 8 months? The cam chain rattle usually is at idle or just above. Not just only between 3-5k rpms. What octane are you running? If 87 then try 89 to see if rattle goes away. If it is a lack of fuel the noise may not go away. Have you checked your manifold clamps for tightness? Try tightening the both clamps on each carb. They could be letting in air which will lean out the fuel ratio after the carb. If the carbs have never been off and rebuilt which includes soaking in a cleaner then they most likely will have some kind of gunk in them. Which will limit the amount of fuel the motor needs.

The bad part is you most likely will not find a shop to work on your carbs. Most places don't have anybody that has any knowledge of carburetors. They are old school. The only thing fuel injection has over carbs is they are more forgiving in extreme climate changes and easier to change the fuel curve.

There are several guys on this board that rebuild carbs, including myself. You may want to find someone close to you to do a rebuild on them the proper way. Changing jets and cleaning fuel bowls is not the way to go. It's a little more indepth than that.

 

Thanks for the replies, guys.

I found out what the rattling is, it is a loose exhaust pipe. I feel embarassed for riding it in such condition! I finally took my helmet off and reved it up high, it is louder than my diesel pick up!

I talked to a couple mechanic shops in town here and they figured it sounds like dirty carbs/jets. Especially since there is aftermarket exhaust on the bike, they figure it was just never really set up for it. So, the one shop will take a look at that for me.

I am unsure of the history of the bike. All I can really say is that the last two PO's seemed to have taken care of it. It has been rode the last 2 years at least. The person before me was a 17 year old kid, but his family was big in to bikes, mostly dirt bikes, but there was a fancy VMax in the garage. So I assume they would have taken care of it. The owner before was actually the owner of a Honda (car) dealership. So I would assume they got it taken care of as well.

As far as octane levels go, I have been trying all different kinds. It seems to run the exact same on 87, 89, and 91.