'99 M750 carb issue

[ducpainter]

I said 22k before but it's just under 21K.

If you've never replaced needle jets...

it's time.

[MotoPsycho]

That's an answer I can live with. Thanks DP. I got it at 10K miles several years ago. Now I'll have to add something else to the list of molested parts. That leaves the seat and motor innards  untouched. If I could only talk the wife into letting me pull the pin on FCR's. I hear they're way more simple.

[MotoPsycho]

Ok. So I replaced my needle jets finally. Now I have the problem of a 4K rpm idle and it appears to be the front cylinder running a little leaner. I did not set the float height to 14mm as prescribed on this board. I haven't unpacked all of my tools yet. I took everything apart replaced the tubes and put it back together. This sounds like a vacuum leak to me. Should I look at the rubber above or below the carbs first?

[bond0087]

A vacuum leak would cause similar symptoms, but I have a hunch that isn't it. For one, if the leak was in the vacuum lines, why would changing the needle jets cause it? A leak between the carbs and intake runner could have been caused by removing and installing the carbs, but that wouldn't be my first guess.

My guess is that the 4K rpm problem is probably the idle speed set just a little too high, which can be solved by simply turning the idle adjustment back, and that the front cylinder running a little lean could be solved by syncing the carbs.

I know what you're thinking -- just a little too high? It's 4 times too high! How is a small adjustment going to fix anything?

The answer lies in the way that the carb'd monsters handle ignition timing. Instead of having a curve that progressively advances the ignition timing with increasing speed, the stock ignition system is basically all or nothing. The flywheel has a metallic bump on it, which looks kind of like a wheel weight but smoother and more permanent. The pick up coils send a pulse as this bump passes by. If the rpm is below a certain point (I think 1300 or 1400 rpm), the ignition system fires a spark at the end of the pulse. When the rpm goes above that level, the ignition system switches to firing at the beginning of the pulse. So the timing is fully retarded below 1300, fully advanced above 1300rpm.

The problem that I think you're seeing is that now that your carbs are healthier, the bike wants to idle a little faster, which pushes the idle rpm just above that threshold level. When the idle is set above that threshold level, because the ignition system moves to full advance, the engine really takes off and settles at about a 3-4k rpm idle.

It could be something else, of course (like a vacuum leak or a problem with the throttle cable), but I have seen this problem come up a number of times on the forum and be solved by a small idle adjustment.

[MotoPsycho]

Could very well be. I had it backed way off initially where it wasn't even touching. It did jump a few times down to about where it would almost die and climb right back up. I think I counted 2 1/2 turns to up the idle and then it just took off. I got a lot of popping out of one muffler too and one was running hotter than the other. The header crossover is still there. It's not individual pipes except before and after the x-over. I got a temp gun to make sure. If it is a vacuum leak that means a lean condition correct? The front header seemed like it was a lot hotter than the rear. Which could explain the difference in muffler heat difference. I know I need to take them back off and clean the throttle return spring and all the little stuff around there. I may go have them dipped for good measure. Haven't decided yet.

[ducpainter]

Are you sure you're turning the idle speed adjustment screw, and not the synch screw?

[MotoPsycho]

Ya. I eyeballed that one while the carbs were out. With them on that screw is way up in there. It's the idle screw for sure. I'm going to hit the headers with a temp gun after a decent warm up to see if I'm right. If I'm having trouble I bet it's the front cylinder. DP, I don't have a problem verifying they are in sync but that means a 30 minute ride to the shop I'd have do it. If I'm going to that much trouble ill just have the carbs gone through all the way and be done with it just for the sake of possibly doing a number on them myself.

After I try the temp gun ill try backing the idle screw all the way off and see if that changes.

[ducpainter]

As suggested by bond0087 the new parts will change the running enough to warrant a thorough set-up of the carbs.

Setting float levels would have been a good idea while you were in there.

It might not be a bad thing to let the shop check things out.

[MotoPsycho]

I totally agree. It's not a duc shop though. Nearest one is 2 islands over in Honolulu. That pretty much sucks. But these guys have more equipment than I do and all the sport bike guys over here go to them.

The float levels would have been nice but I haven't found my vernier caliper yet(still in a box) and I had to go to work. I did what I could for the time I had. That won't happen again. I know everything needs to be cleaned good but I don't have a place to do it yet (carport). I'll just go get it worked on and be done with it. After I fiddle a little bit.

[MotoPsycho]

Started it up and ran for 2-3 minutes and hit the headers with a temperature gun. Front cylinder measured just over 700 and the rear was like 330. Looks like the carbs come back off soon.

[memper]

CV carbs are fairly easy to deal with. You definitely need new emulsion tubes. I suggest the nickel plated ones from Factory Pro.
Cleaning is easy. Remove the bowls and diaphragms and brass jets (carefully). Get a big tupperware and a gallon of name brand Pine Sol. Soak everything for 8-12 hours then scrub and rinse. Pine Sol is rubber safe and really eats old gas, oil, and varnish. Blow out jets with carb cleaner. No carb cleaner on any rubber parts!
Float bowls can be set with a small ruler. Plastic float frame giving you trouble by popping up? Install the float lid with two screws so it hangs off the back of the carb body, thus holding the plastic float frame in position.

Just my .02 if you wanna do it yourself and save some time and money.

[MotoPsycho]

I already installed the FP emulsion tubes. Fairly easy. I just got cable today so I can use my laptop in the carport. Looking at pictures and tutorials on my phone sucks. Thanks for the pine sol tip. The shop I talked to said a half hour shop time to dip and clean them. 

[MotoPsycho]

What's the best way to adjust float levels?

[ChrisK]

check out www.ducatisuite.com, particularly this page: http://ducatisuite.com/jetkit.html Scroll down about 3/4 of the way to where he's measuring float height.

You want to make sure you measure at the first "sitting" point of the floats, and not the second, where they sit down further into the bowls.

[MotoPsycho]

Here's part of the problem. Would this cause a run away idle?