Fzs600 low rpm stutter/bog down.

[Levvers]

Hi Fellow Foc-ers! (Is that alright to say?!)

So I have been using your info for the past few week helping me rebuild a 2001 FZS600 that had been off the road for 5 years.

It’s been a fantastic learning curve so far, but now I’m struggling…

It doesn’t like being in a low rpm and pulling away.

1st gear pulls away okay and the engine seems happy,
2nd gear, below 3,500rpm it judders quite viciously,
3rd, 4th, 5th, 6th, it bogs down below 5000rpm.

It would likely judder in the higher gears at a lower rpm, but I wouldn’t be in them at any low speed.

It doesn’t Idle without the idle adjust screw/butterflies opened a bit, but even then at 1250rpm it surges every couple of seconds.
Pilot adjust screws are set at 2 turns, though have tried at 1.5 turns and 2.5 turns which don’t make too much difference to idle speed.

Above 5000rpm the engine really sings now, and pulls constantly well especially after the work I’ve done below (previously it even bogged down at 7-8000 rpm), but none of the jobs have rectified the low speed rpm judder.

The clutch does seem to make a chatter on idle.

It does smell like a rich mix on idle, I.e too much fuel?

Soooo,

Following reason other people’s advise on here,

I have stripped the carbs,
cleaned/rebuilt/balanced/jets cleared/all pipes flushed. (Balanced visually when carbs removed and with a 4way vacuum gauge)
New orings fitted on main jet and float needle and float needle valve (remembered the 4 little screws hold the valves)
Fitted new air filter (though not k&n)
Fitted new iridium spark plugs even though originals were brown and gapped well.
Replaced cracked carb/engine rubbers
Emptied as flushed fuel tank.
Checked caps for sparking and they do all spark well or at least sound like they do, and don’t arc in the dark.

I have a new fuel pump (not yet fitted) as I thought it was fuel supply when it bogged down under high rpm.

What have I missed? Fuel/air mix? Float level? Clutch issues?

Any help would be greatly appreciated as the bike is fab, just not round town and don’t wanna have people look down on the Fazer as I rev the nuts off it and feather the clutch at 25mph through town trying to stop the judder!

Many thanks!!

[Gnasher]

From what you've posted and if  I've read you correctly i.e. the bike isn't idling or running below 5k properly.

This is pointing to 2 main areas: -

1. pilot circuit/jets
2. choke or enricher plungers not returning properly, it only takes one and there's 4 one on each carb.

It could be all the above which will make correct balancing impossible.  With balance it's not just about them being all the same they must pull the correct level of Hg, which is datum controlled/set by the pilot bleed screws.     

[Levvers]

Gnasher! The one and only, who’s responses have already helped me a lot!! Thanks

So the choke/enrichers x4 were cleaned and checked to the best of my ability for movement/blockages when the carb was off.
The pilot system was as flushed out/cleaned at the same time, I thoroughly enjoyed seeing if I pushed air through a hole seeing where it would come out!! Then the pilot screws then set to the 2x turns as per factory instruction.

The vacuum balance is pretty even, though maybe low?

Please see attached image…

[Gnasher]

Yes a little low which would answer why it's not idling properly and the pilot circuit is restricted.  Your symptoms are classic air/fuel issues of the pilot circuit, something isn't right somewhere, the mixture is either too rich restricted air flow or too lean restricted fuel or an air leak too lean.  you mentioned you've changed the intake rubbers are they seated correctly? Is the intake rubbers from the air box to the carbs correctly seated both sides and done up?  Is the air intake restricted, I've had bikes in where a rag placed under the seat has been sucked into the air box?

Have you seated the diaphragms correctly, when revved do all the vac needles move together or is one or more slower? 


Check the fuel level in each float bowl are they in spec? 

If after riding if you open the tank is there a rush of air?  If so try running the bike with the tank cap not click shut so it's slightly open, if it idles properly check the tank breather pipe, this is one of the 2 that are connected under the tank, the other is the fuel cap drain.   

Bypass the pump and run fuel straight from the tank see if it will idle and run below 5k, it it does it's the pump.

Give that lot a go see where you get.

Failing that it's got the be either a choke plunger not fully returning or the pilot circuit is restricted somewhere.     

[BBROWN1664]

The only thing I would add to teh above is check the airbox isnt blocked and the idle speed is between 1150 and 1250.

Other than that, I dont think its been mentioned above, check the TPS setting.

[Levvers]

Appreciate the feedback so far!

I guess my main question is what am I missing with vacuum? I mean, I now understand vacuum better, more vacuum means more air/fuel pulled through, hence why the lack of vacuum would cause a stutter (some things take time to sink in!)

So,

No obvious fuel issue from the tank as breather hose blows through and there’s no gasp as you open the tank. I ran the engine with the fuel valve closed to drain the filter/pump and watch the pump fill up again and it did so pretty proficiently!

The air box is clear without obstruction and the new (white plastic, metal grilled paper finned) filter has done maybe 150 miles.

I tried the pilot screws down to 1 turn though they seemed to toughen the idle so I’ve turn the pilot screws to 3 turns out which seemed smoother, and then loosened the idle screw off to get the idle back to 1150-1250 rpm, then checked the balance and the TPS. TPS is at 5000.

I then ran the engine and sprayed engine start and WD40 around all the seals/connectors to see there was any surge in power/leaks allowing the spray to get in, and there was no obvious difference (just the constant little surge in revs off idle that happens which can also be seen in the video below) I did this also when I refitted the cleaned out carbs, as had seen that on the forum here

The Balance and throttle response is as below in attached video.

After rebalance and increasing pilot screw turns, I did go for a little ride, and there is some improvement to the low Rev stutter, but still evident. 

I have still to check float bowl levels, but finding a plastic flexi tube has been a bit of a challenge today!

Is there anything blatantly obvious/simple I’m missing to create more vacuum? Does a video of the engine running give anything away?

Again, many thanks for your time and responses.

[BBROWN1664]

that doesnt seem too bad to me.

BTW - 3500 revs is a bit low to be trying to accelerate hard from. Try keeping the revs a bit higher.