Carb tuning help

[balky1]

Hi! This all started in another post but I think it's time for a new one.
I have (or had) perfectly running 1985 FJ. Then to fix a leaking carb I changed float valves and seats, at first only in that one but then in all four. And I messed a bit with floats to set them according to manual. I set the mixture screws to 2 turns out and floats to 21.5 mm. It starts good on full choke and idles but when I open the throttle it first stumbles then revs. And a backfire follows. When I constantly rev it around 2-3 k RPMs it backfires constantly through the exhasut. Searching here told me it is lean so I turned mix. screws until 3 turns out, same thing. Also tried lowering the float to as low as 20 mm and getting it higher to 24-25. Same thing. All jets are clean, carbs are clean. This is killing me.
One of the fuel pilot jets is a bit messed up, though. But orifice is intact, only top part of it where screwdriver slot is is bad. Do you think that could be the issue? I also changed the fuel and vacuum lines. Hoses have the same inner diameter but are a bit different looking from the outside. I really don't think that is an issue.
Float valves and seats are from Keyster and they do look a bit different in hight, but measuring it total, it is the same like OEM. And they are 2.3 mm in dia. I'm thinking now to give it a last try and change the O-rings on old ones to put them back in.
Carbs are not synched, but since the bike rode well before carb disassembly I didn't think it is necessary. Am I wrong?

[Antonn3]

YES.. re-sync the carbs... any time you touch the carbs.
A trick I was told by Randy.. for a starting point when setting your mixture screw, with filters off, (set mixture screw all way down.. lightly) reach inside thru air intake with a finger nail contacting the very tip of the needle, back off on the mixture screw until you lose contact with the needle  tip. This compensates for manufacturing tolerances . I found  carbs 2 and 3 where out by a half turn each, this way.
Good luck

Tony

[balky1]

YES.. re-sync the carbs... any time you touch the carbs.
A trick I was told by Randy.. for a starting point when setting your mixture screw, with filters off, (set mixture screw all way down.. lightly) reach inside thru air intake with a finger nail contacting the very tip of the needle, back off on the mixture screw until you lose contact with the needle  tip. This compensates for manufacturing tolerances . I found  carbs 2 and 3 where out by a half turn each, this way.
Good luck

Tony

Thanks Tony!

I did that with finger nail on the screws while the carbs were down. Mine are also different. So I'm sure now they are set the same. I'll try with synching.

Ivan

[balky1]

Sooo, still with the same issue. After your reply Tony I was off to synch them just to find out they are perfectly aligned.  Difference at most for thickness of the needle on the gauges. I got them down again and tore them to pieces. Thoroughly cleaned, seems like air gets out everywhere it should when I blasted them. In the mean time I checked the compression in the cylinders. Almost perfectly the same in all four. Although a bit lower from the minimum value, measuring around 7 bar. But I'd say that's faulty gauge more then the engine since bike rode perfectly. got minimal compression increase when I poured some oil in a cylinder. So I ruled out engine as a cause definitely and now I'm focusing only at the carbs.
There is still the possibility that one or more is so heavily clogged that i couldn't clean it, but I doubt it. Problems started when I put new valve seats and needles in. They do look a bit different, but the hole size is correct at 2.3 mm. So I somehow doubt it's their fault, maybe the float height thingy and I will try to give it more fuel, measuring the fuel level in the bowls today since only float height measurement gave me nothing.
I'm still suspecting that striped pilot fuel jet is giving me the headache and this is where I would like to hear your experience. It's screwdriver slot is messed up, with the orifice intact and clean. Could that be the sole cause of low (and rough) idle (air/fuel mix screws are 2 turns out, floats at 21.5 mm) and stumbling on opened throttle? Diaphragms are intact. Same happens with/without the airbox. So I'm guessing if the fuel level measuring gives me nothing it could only be that pilot jet or heavy clogging that I can not fix. Thoughts anyone?

[JohnnyW]

Did you manage to get anywhere with this?
I had an overflowing bowl after the bike had been stood for a good while, so decided to strip the carbs. I am no expert, but no idiot either, and I have taken days over them. I replaced the needles and seats with parts from Yambits UK, cleaned everything to the n'th degree, blew everything out with a compressor etc.
I rebuilt them, then fitted them today - It's awful.
At first it wouldn't start at all, and then, after jumping the battery (as I had flattened it) I managed to get it to run of a sorts, but it will only do so with the choke on. It fires up and revs hard to around 3500/4000 momentarily, then comes back to 2k-ish, but is hunting, spluttering etc, not idling really. If I take the choke off, it dies immediately.
I thought I had bench synch-ed OK, and thought I'll just go for it and try a vacuum synch, even with it running so bad. Problem is, there is NO suction registering on my Carbtune! Now that had stood for years too, but is mechanical, and I can raise the bars by sucking on them, so wtf have I done so wrong?

I am bamboozled.

Now I have got a second hand old tank on it, (the old one rusted through, which was the start of my woes), and whilst I had no issue with the fuel filter when it was flooding the bowl before, could this be the issue, fuel starvation? I am gonna test it off the bike tomorrow, but I'm getting fed up now.....

Any ideas as to the symptoms?

[FJmonkey]

JohnnyW, what year is your FJ? How far down did you strip the carbs down? If you did not strip all the way down and use an ultra-sonic cleaner then you might need to clean them again. Sitting will cause all kinds of problems for these carb bikes. The new gas is crap!!!

[JohnnyW]

Yeah, I did the full strip, the complete dismantle, carb cleaner and compressed air. I didn't use an ultrasonic, but did steep the parts for hours in the cleaner, then blew them out methodically,

I do have a few theories having gossiped with my brother tonight (a racer), so back to the drawing board tomorrow. I will report back......

It's a '93 3XW

[FJ_Hooligan]

You can't sync them when the choke is on.  Air coming through the choke opening ruins the vacuum signal at the intake vacuum port.

You've got to get it to run with the choke closed to sync.

[JPaganel]

You can't sync them when the choke is on.  Air coming through the choke opening ruins the vacuum signal at the intake vacuum port.

Sorry... What?

As far as I know, the "choke" on Mikuni carbs is an enrichment circuit, plungers open into a fuel circuit and add fuel to the mixture.

Where is this openening the air would come from?

[Pat Conlon]

Thru the choke vents....those 4 little hoses on your carb's are vent hoses. When the choke is on, they draw in air to mix with the fuel thru the choke circuit.
This air/fuel mixture bypasses the throttle plate.

[FJ_Hooligan]

The choke circuit dumps out on the engine side of the throttle plate.  The majority of the air for the choke (I refuse to type enrichener multiple times and everyone knows what I'm talking about when I say choke) passes underneath the slide diaphragm.  The air Pat is referring to is a small amount of bleed air added to the fuel side of this circuit.

If you pull the diaphragm out and look at where the plunger is, you'll see that the plunger acts as a sliding throttle plate that controls the amount of air from the airbox that is allowed to mix with the air/fuel.  The needle tip of the plunger is metering the amount of air/fuel allowed into the circuit.  It's really like a mini carb circuit that's jetted very rich.

Anyway, if the choke is on lots of air is flowing through it and that will spoil the vacuum seen at the sync vacuum port.

[JohnnyW]

You can't sync them when the choke is on.  Air coming through the choke opening ruins the vacuum signal at the intake vacuum port.

You've got to get it to run with the choke closed to sync.

Yeah, I know that. Was just a bit desperate. That explains my vacuum gauge not working correctly! Got up at 4am this morning with it all wooshing around my head, and have been in the garage ever since. Have got it all off again, and have checked my work. I cannot see any obvious mess-ups.

I do seem to be having a problem setting No2 butterfly at a 'needle's width' gap. Even with my throttle stop adjuster fully wound in, it needs to be a couple of mil more to achieve a gap that would allow a sewing needle through (as suggested by some). How much of the little holes above the butterfly would you expect to see at bench synch to be a starter for a decent idle speed? How much an initial gap? Weird how stripping it down has affected the throttle stop!?

[Urban_Legend]

Sometimes when they are close, but not quite, synced, it works better if you take them (all of the butterfly's) way out of sync, and bring them back in.

[JohnnyW]

Nope, no joy when I re-bench synch'd it and fired it up again.

Weird thing this time is that downpipe 2 and 4 (looking from seat) got hot, but 1 and 3 didn't, so that kind of eliminates a coil problem (clutching at straws here), but I suspected my new plugs (*clutch, swipe*), so put my old ones in 1 and 3. Tried again, and this time 1 and 2 stayed cold, so didn't fire at all. Not electrical I think.

What the heck can I have done during the cleaning to f**k this up so!?

We have a great word here in Scotland to sum up my mood today after countless hours wrestling this bugger back in, airbox etc.

Scunnered I am. Scunnered.

[balky1]

Nope, no joy when I re-bench synch'd it and fired it up again.

Weird thing this time is that downpipe 2 and 4 (looking from seat) got hot, but 1 and 3 didn't, so that kind of eliminates a coil problem (clutching at straws here), but I suspected my new plugs (*clutch, swipe*), so put my old ones in 1 and 3. Tried again, and this time 1 and 2 stayed cold, so didn't fire at all. Not electrical I think.

What the heck can I have done during the cleaning to f**k this up so!?

We have a great word here in Scotland to sum up my mood today after countless hours wrestling this bugger back in, airbox etc.

Scunnered I am. Scunnered.

Hi Johnny!

I'm still nowhere with this problem. After changing the seats and needles with Keyster ones and new pilot fuel jets, thorough cleaning with carb cleaner, cooking the carbs, compressor cleaning etc., nothing. When I spray the cleaner, it comes out everywhere it should. I ruled out starvation problems by eliminating the tank when testing, giving it the fuel right into the line. New plugs just in case, but  I don't think it's electrical since bike ran great before removing the carbs. Strange thing happened last time I took the carbs off and put them back on. It ran pretty good and reacted well when I opened full throttle from idle, but I needed to sync them with gauges since they were all striped down to bits. And I managed to sync them perfectly in less than 2 mins. Then it started all over again. First my idle got higher to 2.5k, I got it back down and same shit happens. It idles but when I open the throttle it sputters, chokes and usually dies. But if I manage to get it to about 2-3k RPMs, it backfires through exhaust and runs badly in general.
I'm thinking that maybe the rubbers or only O-rings between carbs and head are leaking. But I don't know how would I be able to sync them perfectly if that's the case. I also changed fuel and vacuum lines with the new ones with same inner diameter. Hoses are a bit thicker in general since the walls are thicker than the old ones. But I don't think this is an issue. I actually gave up on this and found a good mechanic. I lost a ton of nerves and it's better to give it to someone with far more experience than I have. Next I could do is to push the whole bike into the sea.