...can't get consistant idle setting...

[Zoup2nuts]

I apologize in advance for the long post, but I didn't want to leave anything out that might be of help.

The bike is running the stock air-box, stock header (non-Exup), with a slip-on Vance & Hines slip-on.



When I got the bike, it was from a lady who was recently divorced and had not ridden the bike much in Colorado (6000') elevation, but had moved from California. She compensated for altitude (in her words) by slightly engaging the choke circuit to increase the rpm's. The total movement was probably about 1/8" (at the starter plunger) out of nearly 3/4" of travel. That was a hit or miss thing and often times it would close by itself and then the bike would stall at lights, etc. If you could hold the throttle on about 1500 rpms, the bike would move away smoothly without any major glitches right to red-line in both the needle and main circuits.

Not having ridden an FZR400 before, I don't have a base-line to compare its performance to, but it seemed to be ok to me.

I found the idle adjusting knob and it seemed to have no effect when turning it clock-wise and then after several 1/4 turns, it jumps to 4k rpms. I brought it back down and again, no effect the first few 1/4 turns and then again it immediately dropped to 1000 rpms. That's when I decided to have someone look at it.

I had a hard time finding anyone around here to work on the bike and finally tracked down a guy who's family had owned a Yamaha dealership years ago and seemed to know what he was talking about and assured me he could make it right. I asked him to put new plugs, flush the radiator, and sort out the idle issueI dropped it off and he started working on it on a Sat. morning and about 4 pm I get a call saying he can't get the bike started and had run the battery down. He had taken the carbs apart enough to see that it had 87.5 main jets but hadn't determined the pilots were stock or couldn't tell me if the needles were stock. This bothered me, but I was more concerned about the non starting. I told him I'd bring over a shop manual (on cd) and he could use it. When asked about his starting proceedure, he admitted to cranking on the throttle after it didn't fire after a few seconds, which probably flooded the thing. In the end, it was his hooking up the plug wires in the wrong order that kept it from firing. I get a call at 7:30 the next morning and he said it was running and he would continue to work on the idle issue. I go over at 1 pm and he's given up, charged me for the parts and about 1 hour's labor and says take it home. It's now that he admits he's really better working on dirt bikes and only took the job due to a need for the money.

Now, when I ride home, the rpm's hang up when cutting the throttle, but not as badly as they do now. So the hanging up part has developed since his working on the bike and I'm thinking its related to improperly putting on the carbs or the air-box as its now exererbated by my taking things apart and re-hooking them up.

The throttle cables seem to be hooked up properly but I noticed they don't "snap" back down by merely letting off the grip, but do return fine when you twist the grip. I have checked free-play and if anything there's a scosh too much, but that should let the throttle shut completely not hang it up.

I did find a small washer on my parts table when I got done that I think came from the first carb I dismantled, and probably belongs on the needle underneath the "plug" and is probably leaning out that carb a bit as that needle is dropping down the thickness of the washer making it a bit leaner (but how much?)...

Also, my carbs still have the brass covers on the air-fuel mixture screws. I think I'll drill them out Friday but not change anything yet, but could at a later time without removing the carbs if I wanted to start messing with
them. Again, it was running ok with the current jets and settings before we took it apart. I want to get it back to that base-line before changing much.

I'm considering getting a jet-kit, but don't want to invest any more until I can at least set the idle.

I was considering pod filters, but from what I've read they would mess up the mid-range and I'd have to re-jet. I'm not opposed to rejetting, but don't want to have to buy 3-4 sets of jets to sort through finding the right setting for 6000' altitude.

I had to do that with my '72 R5c, but changing jets in that one takes about 10 minutes and is a breeze.

I realize this is long-winded, but wanted to throw everything in in case something struck a chord.

[ragedigital]

Well, you definitely gave all the details...

Here is my assessment:

- The bike is running too lean at idle. That is why the RPMs are hanging.
- I don't believe you need a jet kit to solve this problem. Most jet kits correct mid-high level performance.
- You will probably need new emulsion tubes as well as mains and needles to get the best performance out of the bike. I'm guessing that none of this has been done since the IMS (Idle Mixture Screw) covers have not been removed. But, once again, for the idle issue, I don't believe they're necessary at this point.

Solution:

- I don't have a 400, but on a 600 we have the ability to adjust the throttle tension by the throttle grip and also where the cables attach to the carbs. Correct this first, so they snap back. Be careful not to bend the cables.

- Before you begin tuning, you will need to verify that the carbs are seated properly on the instake boots and are tight. Make sure that the airbox is positioned properly as well and is tight. Drill out the IMS brass covers.

- I would buy/borrow/steal an Infrared Heat Thermometer with a laser pointer. Once the bike starts and if you can keep it idling around 1500-2000 RPMs, measure each header about 1"-2" from where it connects to the header. Find the lowest temperature and adjust each other carburetor to meet that temperature by adjusting the IMS. You screw them counter-clockwise to "richen" them up and clockwise to "lean" them out. While adjusting the screws, your RPMs may drop. This is normal and means it's working. You can always raise the overall idle level by adjusting the idle knob.

By adjusting your idle properly, your bike should start without the choke or very little.

[Zoup2nuts]

Thanks for your assessment, I agree now that its a lean condition causing the rpm's to hang up. Before I did the carb clean, the rpm's weren't hanging up, I just couldn't get the idle knob to do anything to set the idle above 1000 rpms. I'd turn and turn it and nothing until boom, it would jump to 4 k rpms. I'd then turn it counter-clockwise and nothing would happen and all of a sudden it would immediately drop to 1000 rpms.

You mentioned an adjustment at the carb-side for cable tension. Mine seems to have one adjuster at the grip to set free-play. I've played with this and it doesn't seem to make a difference...but when I let go of the grip the throttle doesn't snap back like I'd expect it should. The throttle bar seems to have enough spring to it so it should. I didn't change any of the cable routing when I cleaned the carbs.

Also, the rpms hanging aren't nearly as evident with the air-box off. The minute I put it back on and they hang like crazy.
I'm suspecting the center two boots on the air-box might not be seated properly and air might be getting in those two. It would be nice if you could get to them better or at least see in there better with the air-box on.

Going to the carb sync, I can see a slight crack in the far right butterfly and the one to the left of it, while the first two are completely closed, so I know the sync is off. I'll pull them off Friday and bench sync it (didn't read about that until after I did the cleaning).

In retrospect, when I removed the carbs, I loosened the lower screws and removed the boots, should I have done it the other way around?

Right now, I'd like to get the rpm's not to hang and then I will go from there. Also, should I be continuing this on a PM so not to stretch out the thread? (want to be politically correct on here)..

I do appreciate you thoughts as each one sparks an idea, hopefully we'll hit the right one...Bob

[ragedigital]

Keep it in a thread... that way everyone else can pitch in to help.

Are you saying that you took the intake boots off the cylinder head? If so, no you do not need to do that. Just undo the hose clamp at the top of the boot holding onto the carb.

[Zoup2nuts]

no problem, yeah I did take the boots off...I'll do it the other way next time.

I'm going to recheck the cables, I think I might have too much free-play now, but that shouldn't make the situation worse...

[Zoup2nuts]

Ok, I took everything off for a second time and found a crack in the casing housing the slide. It had been repaired with J B Weld ( it was done and sanded well before), so air would have been entering.

Also had the last two butterflies not closing completely (just a smidge) the last time.

At midnight I went to Walmart's and got some J B Weld and after I finished with everything else, welded it back together making sure it was air-tight. I didn't figure I had a Ghosts chance in Hell, but tried anyhow.

I also drilled out the brass plugs on the air/screws and (3) gave me no trouble. They were set at 1.5 turns out. The fourth one (same carb that was busted) screwed in tight and at 3.0 turns locked up and I ended up flattening the screw slot, so I set the other (3) at 3.0 and wished for the best.

Mid-afternoon the next day I put the assy back in and at first it really didn't want to start (without eyther), but when it did it sounded great. The rpms dropped right down and I was able to set a 1250 rpm idle. I put the air-cleaner back on and warmed it up and it didn't change.

The power delivery was way better than before and has more power in all circuits (especially from 3k and up) Before it wouldn't pull strong below 5k.

So, I might have struck on the right combo by accident...but pleased as hell anyways.

I'll be on the lookout for a back-up carb assy as the J B Weld might not hold forever. The broken carb is the one that the throttle bracket is screwed onto, so I made sure not to re-hook that up until I had the throttle cables hooked and will do the same if taking off again so as to not put pressure on that area.

Thanks for all your replies

p.s. the emulsion tubes looked like they were new, and I checked float height and all were at 22 mm...