Tuning my fizzer

[riffer]

Hi guys. My fizzer has started to feel a bit rough in the last couple of weeks, and I'm hoping one of you guys will be able to point me in the right direction and hopefully save me some time.

Symptoms:

Really rough idle, needs a lot of choke to stop it from stalling, won't idle below about 2500 rpm.

Seems a bit "woolly" on the first bit of the throttle, but once it gets to 4000 its like it clears its throat and gets going.

Stalls a bit at lower revs.

A smell of petrol from the bike.

I'm thinking it could be seals in the carbs stopping a perfect vacuum, however I'm open to any possibilities.

I haven't checked the plugs yet. Or the Air Filter.

I've had the bike for 7000kms now, done and oil and filter change and adjusted and oiled the chain regularly has been my maintenance so far.

The bike is a 1987 Yamaha FZR750RT 2LM, with 48,000km on the clock. Doesn't seem to be any camchain rattle, so I've ruled that out at present.

Am I on the right track?

[wkid_one]

Running rich?

[Slingshot]

Sounds like my problems...which I think is caused by crap in the carbs, I'm stripping mine down this weekend to give it a good clean out so I'll let you know how I get on.

[merv]

Yeah could be crap in the carbs but if it smelling of fuel probably one or more are suffering from too much fuel usually caused by float needle problems i.e. worn or crap in them allowing too much fuel into the bowl. However, that wouldn't tie in with needing the choke on which implies the opposite - running too lean. Does it have CV carbs and could you possibly have a split diaphragm in one of those? Sounds like you need to have a thorough look at all the carbs to see what is wrong. Symptom of split diaphragm is usually a gutless engine slow to pick up speed.

[riffer]

Symptom of split diaphragm is usually a gutless engine slow to pick up speed.

It's not exactly gutless - after 4000rpm it pulls well. I'm just finding that I'm using more throttle than I usually do now. They are CV carbs I think - Mikuni BDS34's.

Sounds like shit in the carbs though. Over Xmas the bike sat in the garage for two weeks and I think this may have caused the problem.

[riffer]

Is it possible that the petrol smell is caused by petrol going back through the breather tubes?

Which would indicate float level problems I guess...

[pete376403]

A mate with a 1200 Trophy had a problem similar to this (Wgtn M/C couldn't fix it, either). Was a split diaphragm in the vacuum fuel tap, was leaking fuel down the vacuum pipe into the inlet manifold.
Gas mileage was crap, low end running was uneven and in spite of the obvious over fuelling, gave the impression of running lean (?!)

[riffer]

A mate with a 1200 Trophy had a problem similar to this (Wgtn M/C couldn't fix it, either). Was a split diaphragm in the vacuum fuel tap, was leaking fuel down the vacuum pipe into the inlet manifold.
Gas mileage was crap, low end running was uneven and in spite of the obvious over fuelling, gave the impression of running lean (?!)

Pete - sounds like I may have to bring the bike over to show it to you mate, seeing as you're a stone's throw away. I'll pm you about it...

[wari]

A mate with a 1200 Trophy had a problem similar to this (Wgtn M/C couldn't fix it, either). Was a split diaphragm in the vacuum fuel tap, was leaking fuel down the vacuum pipe into the inlet manifold.
Gas mileage was crap, low end running was uneven and in spite of the obvious over fuelling, gave the impression of running lean (?!)

I've had the same problem before tiday ...

[Slingshot]

The smell of fuel could be caused by a stuck float or needle valve letting to much fuel in and therefore fuel hitting the overflow and pouring out.
If you can see the bottom of the carbs you may be able to see the fuel coming out. Start the engine and see if you can see it.

I took my carbs off last night and it took about half an hour, that including removing the tank & airbox.

The carbs themselves came apart pretty easy too.

[Motu]

How do the carbs attach to the head? with stubs is most common....check those,often they are bonded to an alloy flange.Mine were no longer as one piece,so rebonded them with indusrial strength urethane,good as new.The symtoms are as you describe,poor idle but good top end.

[merv]

OK if its got a vacuum fuel tap I wasn't thinking of that but as Pete says that could be a problem too.

Sitting the bike for 2 weeks shouldn't hurt - usually got to be parked up for months before it really causes a problem - except for that issue I have talked about before with this modern unleaded fuel. Then the problem is hard starting unless you drain the carbs first but once drained once the bike runs fine and the carbs aren't crapped up with dried fuel. As I say that takes months not weeks.

[gpercivl]

I'd check the plugs and air filter before doing anything else...a partially blocked air filter can give all the symptons you've described plus lead to plug fouling.

[riffer]

I'd check the plugs and air filter before doing anything else...a partially blocked air filter can give all the symptons you've described plus lead to plug fouling.

I think you're spot on there. I pulled the plugs on Saturday. What a prick of a job to get to those inner plugs! Anyway, managed to change them without stripping the plugs, and put some new ones in.

Plugs are completely carbon fouled. Bike is running a whole lot better - in fact I did my first ever off-the-throttle wheelie on Saturday road testing it!

So I'm picking up a new air filter today (Maindstone Yamaha didn't have one on Thursday) and I'll put it into the bike on the weekend.

The bike seems to pull harder and cleaner now, particularly off the throttle, however the idle is still dodgy (varies between 2000 and 5000). Still, I will withhold doing anything to the carbs until the air filter is changed.

[Slingshot]

How long has it been since the plugs were last changed? And did the problems start all of a sudden or have they been getting progressivly worse.

The reason I ask is that there maybe something causing your plugs to foul up, if that's the case you may have just fixed the symptom not the cause.