stuttering at low rpm's

[jan.van.gent]

Hi everyone,


A while ago I rebuilt my carb, since it was overflowing when engine was off (leaking into cylinder) and overflowing when running idle. As expected some of the rubbers were shot.

I used this set: http://www.ebay.de/itm/KR-Vergaser-Reparatur-Satz-Carburetor-Repair-Set-x4-YAMAHA-FZS-600-Fazer-98-03-/162156361779


Since then, the carb is not leaking anymore, everything seemed fine.
However, now the engine hesitates between 2000 and 5000 rpm, especially near the 4000 rpm range. I've set the pilot screws as listed in the manual to 2 turns out.
But I remember when overhauling the carb, the screws were out less than 2 turns (rather 1 or 1.5).


In high rpm's or at high trottle positions everything runs great.
Things I've checked/done so far:
-checked the condition of the TPS (throttle sensor position) --> goes nicely to 5000 rpm, so it's OK
-replaced the spark plugs
-checked and rechecked the pilot screw settings (2 turns out)
-spraying water on the exhaust manifold when idling: they seem to be all quite at the same temperature
-reduce the pilot screw turns out (1 turn out instead of 2 turns out) on second cylinder --> no change notable


When I check the spark plugs, the spark plug on cylinder 2 looks realy black.
Spark plug 1, 3 and 4 look like this:



While spark plug no. 2 looks like this:



Any suggestions on what to do?
Open up the carb again and see if there is something wrong with the pilot screw of the second cylinder? Or could this be caused by anything else?

Thanks guys!

[Bretty]

Swap the spark plugs and leads from cylinder 2 and 3. Give it a good run and then repeat your spark plug check. See if the problem stays with the cylinder (carb / fuelling issue) or moves with the spark plugs/leads

[Gnasher]

It's a fuelling issue, too late now to go into detail will tomorrow.

[jan.van.gent]

Swap the spark plugs and leads from cylinder 2 and 3. Give it a good run and then repeat your spark plug check. See if the problem stays with the cylinder (carb / fuelling issue) or moves with the spark plugs/leads

After replacing the spark plugs, plug no. 2 was black after 2 km's already.
I haven't changed the leads so far...

[jan.van.gent]

It's a fuelling issue, too late now to go into detail will tomorrow.

I agree, looking forward to hear more about it
What to look for/replace/...?

[Bretty]

Swap the spark plugs and leads from cylinder 2 and 3. Give it a good run and then repeat your spark plug check. See if the problem stays with the cylinder (carb / fuelling issue) or moves with the spark plugs/leads

After replacing the spark plugs, plug no. 2 was black after 2 km's already.
I haven't changed the leads so far...

(Ignore me if you already know this but...) Spark plugs 2and3 spark at the same time, but as one is on exhaust stroke and the other is on the compression stroke only one fires. So if you swap the leads over between cylinders 2and3 you can eliminate the problem being with the coil or plug cap. This is a very simple thing to do and worth eliminating before going to the trouble of removing and stripping the carbs again.

An intermittent spark could cause that spark plug to foul like that, and electrical faults can change with rev range as the mixture resistance changes. I'm not saying it IS the problem, just an easy check that is worth doing, before the ball ache that is removing and stripping the carbs again.

[Gnasher]

It's a fuelling issue, too late now to go into detail will tomorrow.

I agree, looking forward to hear more about it
What to look for/replace/...?

Ok, based on your post I’m assuming a few things here so this may not be correct: -

1.The cylinder giving trouble has the carb you rebuilt, or did you rebuild them all?
2.Plus you state everything runs fine at high rpm/big throttle positions, if it was the HT circuit it would usually be the opposite.
The part of the rev range you’re having issues with is just past  pilot circuit, this now points to several possible problems: -

1.Pilot/Main jet damage/blockage
2.Needle height setting
3.Enricher circuit jammed/blocked.

Your leaking crab would have had nothing to do with shot rubbers, but rather worn metering valve/jet, stuck/damaged or wrongly set float height.  When you “rebuilt” the carb what did you do?

Your black plug is over fuelling, i.e. too richer mixture which can be any or all of the above.  If you rebuilt all the carbs and it’s not the one that was leaking, it’s still very possible you didn’t assemble one of them correctly,  there's crap in the pilot circuit or you missed something like jammed enricher plunger.

Personally based on what you’ve said and what I know at present I’d go for No 3 first check the plunger is not stuck out, then No 1 as being your problem, it could be something else.  Tell me more if there is any more?       

[tommyardin]

It's a fuelling issue, too late now to go into detail will tomorrow.

I agree, looking forward to hear more about it
What to look for/replace/...?

Ok, based on your post I’m assuming a few things here so this may not be correct: -

1.The cylinder giving trouble has the carb you rebuilt, or did you rebuild them all?
2.Plus you state everything runs fine at high rpm/big throttle positions, if it was the HT circuit it would usually be the opposite.
The part of the rev range you’re having issues with is just past  pilot circuit, this now points to several possible problems: -

1.Pilot/Main jet damage/blockage
2.Needle height setting
3.Enricher circuit jammed/blocked.

Your leaking crab would have had nothing to do with shot rubbers, but rather worn metering valve/jet, stuck/damaged or wrongly set float height.  When you “rebuilt” the carb what did you do?

Your black plug is over fuelling, i.e. too richer mixture which can be any or all of the above.  If you rebuilt all the carbs and it’s not the one that was leaking, it’s still very possible you didn’t assemble one of them correctly,  there's crap in the pilot circuit or you missed something like jammed enricher plunger.

Personally based on what you’ve said and what I know at present I’d go for No 3 first check the plunger is not stuck out, then No 1 as being your problem, it could be something else.  Tell me more if there is any more?       

Looking at Bretty's and Gnasher last posts I am really glad I joined FOC-U, there is so much useful information, knowledge and experience in this forum, and I store it all away for future reference, hoping of course that I never need it.
It might sound silly but when I read stuff like those posts I copy and paste the information into a word document and save it into a 'Fazer Tips' folder on my computer.
Keep the useful info flowing guys.

[jan.van.gent]

Swap the spark plugs and leads from cylinder 2 and 3. Give it a good run and then repeat your spark plug check. See if the problem stays with the cylinder (carb / fuelling issue) or moves with the spark plugs/leads

After replacing the spark plugs, plug no. 2 was black after 2 km's already.
I haven't changed the leads so far...

(Ignore me if you already know this but...) Spark plugs 2and3 spark at the same time, but as one is on exhaust stroke and the other is on the compression stroke only one fires. So if you swap the leads over between cylinders 2and3 you can eliminate the problem being with the coil or plug cap. This is a very simple thing to do and worth eliminating before going to the trouble of removing and stripping the carbs again.

An intermittent spark could cause that spark plug to foul like that, and electrical faults can change with rev range as the mixture resistance changes. I'm not saying it IS the problem, just an easy check that is worth doing, before the ball ache that is removing and stripping the carbs again.

Thanks for the tip. I will try this out for sure, since it is easy as you describe. I do think its fuel related, but it doesn't do any damage to do this quick check.
Thanks!

[jan.van.gent]

It's a fuelling issue, too late now to go into detail will tomorrow.

I agree, looking forward to hear more about it
What to look for/replace/...?

Ok, based on your post I’m assuming a few things here so this may not be correct: -

1.The cylinder giving trouble has the carb you rebuilt, or did you rebuild them all?
2.Plus you state everything runs fine at high rpm/big throttle positions, if it was the HT circuit it would usually be the opposite.
The part of the rev range you’re having issues with is just past  pilot circuit, this now points to several possible problems: -

1.Pilot/Main jet damage/blockage
2.Needle height setting
3.Enricher circuit jammed/blocked.

Your leaking crab would have had nothing to do with shot rubbers, but rather worn metering valve/jet, stuck/damaged or wrongly set float height.  When you “rebuilt” the carb what did you do?

Your black plug is over fuelling, i.e. too richer mixture which can be any or all of the above.  If you rebuilt all the carbs and it’s not the one that was leaking, it’s still very possible you didn’t assemble one of them correctly,  there's crap in the pilot circuit or you missed something like jammed enricher plunger.

Personally based on what you’ve said and what I know at present I’d go for No 3 first check the plunger is not stuck out, then No 1 as being your problem, it could be something else.  Tell me more if there is any more?       

I rebuilt the lower end of all 4 carbs. Thus replacing the neelde valve, the needle O-ring and the needle valve seat and the float chamber gasket. I believe one or more of the neelde valves were not closing properly, + the float chamber gaskets which were quite brittle ended up in leaking carb(s).

Would a blockage not rather cause a fuel shortage instead of to much fuel?
What do you mean by enricher circuit? Is this linked to the choke? Or? What to look for in the manual (http://foc-u.co.uk/index.php?action=downloads;sa=downfile&id=8) ?

Thank you very much for the input!!

[Gnasher]

Looking at Bretty's and Gnasher last posts I am really glad I joined FOC-U, there is so much useful information, knowledge and experience in this forum, and I store it all away for future reference, hoping of course that I never need it.
It might sound silly but when I read stuff like those posts I copy and paste the information into a word document and save it into a 'Fazer Tips' folder on my computer.
Keep the useful info flowing guys.

Glad you find them useful, that said having the info is one thing skill/experience is another.  I've had many bikes brought to me by owners who have "had a go" based on internet forum information, you wouldn't believe what some people will do!


I'm not saying don't do anything but as with most things "it's easy when you know how!"       

[Gnasher]

Would a blockage not rather cause a fuel shortage instead of to much fuel?

No not if it's air.

What do you mean by enricher circuit? Is this linked to the choke? Or? What to look for in the manual (http://foc-u.co.uk/index.php?action=downloads;sa=downfile&id=8) ?

The bike doesn't have a choke in the real sense, when you pull the choke lever out, look closely at what happens, it pulls out 4 plungers, they can stick open even when you push the lever back in.  This cuts off air at low RPM through the pilot circuit, thus enriching the mixture, so when you rev the bike when it's warm with a stuck open plunger it's over fuelling.       

[jan.van.gent]

Would a blockage not rather cause a fuel shortage instead of to much fuel?

No not if it's air.

What do you mean by enricher circuit? Is this linked to the choke? Or? What to look for in the manual (http://foc-u.co.uk/index.php?action=downloads;sa=downfile&id=8) ?

The bike doesn't have a choke in the real sense, when you pull the choke lever out, look closely at what happens, it pulls out 4 plungers, they can stick open even when you push the lever back in.  This cuts off air at low RPM through the pilot circuit, thus enriching the mixture, so when you rev the bike when it's warm with a stuck open plunger it's over fuelling.       

OK now I understand. I took a good look at it. When I push the choke lever, all 4 plungers move properly. But perhaps the one of cyl2 is faulty on the inside? I will open up the carb next week and have a good look at it.
Should I open up the top half of the carb to do this? Or is it an issue which can be solved from the lower half?

Could I screwed this up when overhauling the lower half? Or is this a coincidence? (or maybe my manipulating/rotating/... the carb some debris got at the wrong location...)

[Gnasher]

The plungers or stater plunger to give them their proper name, don't move that much and it only takes a few mm, try pushing each one back with something.   They unscrew from the body and can be stripped they have a plunger, spring, oring and body, mostly the oring fails and moisture gets in casing them to seize.  You don't have to strip the cabs to get at them, just remove the tank it's fiddly but possible, or just remove the carb bank.           

[Disorderlypunk]

looks like a re-rebuild is definatly on the cards - as with any carb rebuild make sure you have a nice clean area ready to seperate each carb into its own area - go through each piece checking all jet holes are clear etc etc (grandma this is how you suck eggs) - make sure you check your float levels on each carb it is amazing how often this is overlooked (including by myself as it is a final step and im impatient) it never hurts to throw a dose of redex through just before a rebuild (if it runs at least) and get a few miles to let it do its stuff
while carbs are off it would be a good idea to check your inlet rubbers (even though its a rich mixture problem you might as well)
and also ask around your area if anyone has a carb balancing kit (wont hurt to check and balance as you can loosen screws while they are off and make it easy)