M900 Coughing Issue

[ChrisK]

Hey all, I have a 98 M900 running stock carbs, stock jetting, modified air box with K&N filter, Staintune slip-ons, and CA-Cycleworks Ignition Coils. Recently I've been having some issues with the bike coughing a little at low RPM's when it's not quite completely warmed up yet. It's happened a couple of times in 1st and 2nd in low-speed corners and the sudden drop in speed has me scared I'm going to lay it down for something stupid like this.

I bought the bike back in August and the seller SAID that the carbs had been cleaned, belts replaced, etc. Have no idea if that's actually true or not. I've recently synched the carbs and set the idle screws to where I believe they should be (who knows). Any opinions on what else the culprit could be? Jetting? Valve clearances? Timing belts?

[ducpainter]

With stock jetting and an open air box it sounds like it's lean when cold. I'm surprised it isn't so lean with those mods and stock jetting that it's only when cold, and not the entire throttle range.

Try keeping the choke on a little to see if the 'coughing' goes away.

A jet kit would eliminate the issue if that's the case.

It could be valve clearances, but the issue doesn't usually go away with engine temp...it would get worse as the clearances tighten up due to heat.

[ChrisK]

Okay thank you, I'll try the choke fix tomorrow morning and report back.

[ChrisK]

It seems that keeping the choke cracked helps the issue, but the weather around here really warmed up over the weekend so I didn't have a whole lot of cold weather to test it in. Can anyone else attest that a re-jetting will probably solve this?

Thanks

[koko64]

By modified airbox we are guessing the top is chopped off to reveal the air filter.
DP is correct, take his advice. Yes. Have no doubt.
If you have an open airbox with stock jetting in stock carbs, your bike will be running lean in certain rev ranges.

An exception would be in the lower midrange/off idle if you have worn needle jets which need to be replaced anyway. Lets hope they are because they may protect your motor.

For example standard M900 main jets are usually 140 size. Open airboxes may require up to 165s.
Buy a jet kit to suit an open airbox and new needle jets if they have 10,000 miles on them.

[ducpainter]

He's also running cans.

Like I said originally I'm surprised the thing is actually rideable.

You're probably on the money about the worn needle jets.

[ChrisK]

Okay, thanks for the advice guys.

I am suspicious now though that you say you find it hard to believe the thing is rideable with the current setup. It is VERY rideable, so maybe it does have updated jetting? Obviously the only way to tell is to tear it apart. I feel like the most logical path to take is to take it apart, inspect the jets, then order the new ones if necessary. I'm going to hate doing it though, as it's hitting 70 degrees here for the first time this year, that'll be a hard layup to swallow.   

[ducpainter]

Okay, thanks for the advice guys.

I am suspicious now though that you say you find it hard to believe the thing is rideable with the current setup. It is VERY rideable, so maybe it does have updated jetting? Obviously the only way to tell is to tear it apart. I feel like the most logical path to take is to take it apart, inspect the jets, then order the new ones if necessary. I'm going to hate doing it though, as it's hitting 70 degrees here for the first time this year, that'll be a hard layup to swallow.   

Monsters are lean from the factory, and when you open the airbox and add cans they get even leaner. That's why I made that comment. You stated the jetting was stock, and from my experience that makes for lousy throttle response and running.

It's possible that it was jetted, and you just have dirty pilot jets.

Pull your plugs and see if they're white or brown before you pull it apart.

[ChrisK]

Check that, just ordered the Dynojet Kit, I can return it if I miraculously find I don't need it. So now I'll hopefully be able to get it all done on Saturday. I'll report back, thanks again.

[ChrisK]

Monsters are lean from the factory, and when you open the airbox and add cans they get even leaner. That's why I made that comment. You stated the jetting was stock, and from my experience that makes for lousy throttle response and running.

It's possible that it was jetted, and you just have dirty pilot jets.

Pull your plugs and see if they're white or brown before you pull it apart.

I've pulled the plugs and they appear lean, granted I didn't follow exact plug-check protocol. As I stated in my last post, the kit is now on its way so I'll know more at the end of the week.

[ChrisK]

Okay so in preparation for the re-jetting, it's already been recommended in this thread that the main jets might need to be 165's. Again, I'm running a cut air-box with K&N filter, stock Mikuni's, and Stain-tune slip-ons. What other recommendations do you guys have? What clip on the needle jets, etc?

I've also seen that the recommended idle mixture screw height is 2.75 turns out? Is that what you guys would recommend? That is much lower than what mine are at.

I'll be setting the float height, as I doubt it isn't set at the recommended 14 mm.

[ducpainter]

Okay so in preparation for the re-jetting, it's already been recommended in this thread that the main jets might need to be 165's. Again, I'm running a cut air-box with K&N filter, stock Mikuni's, and Stain-tune slip-ons. What other recommendations do you guys have? What clip on the needle jets, etc?

I've also seen that the recommended idle mixture screw height is 2.75 turns out? Is that what you guys would recommend? That is much lower than what mine are at.

I'll be setting the float height, as I doubt it isn't set at the recommended 14 mm.

I believe my needles are in the middle notch, but I removed the washer. It was too fat even with new needle jets.

My mixture screws are 3.5 or 4 turns out.

I'd recommend you inspect the needle jets with a magnifier. It doesn't take much wear to really kill your fuel mileage and make settings a PITA.

[ChrisK]

Won't the Dynojet kit come with new needle jets?

And I'm pretty certain I remember counting 9 turns out on my mixture screws. BUT, maybe I was counting half-turns, so 4.5 full turns.

[ducpainter]

Won't the Dynojet kit come with new needle jets?

And I'm pretty certain I remember counting 9 turns out on my mixture screws. BUT, maybe I was counting half-turns, so 4.5 full turns.

The jet kits do not include needle jets AFAIK. I don't know if DJ even offers one. OEM and Factory Pro do offer them. They're not cheap.

It includes an assortment of jets...3 sizes...with spare o-rings...needles...and springs for the slides along with a drill bit and some clips and washers. If you can access your fuel screws someone pulled the plugs already.

4.5 may be what yours needs. I think if I had to go that far I'd change the pilot jet.

[ChrisK]

Ahhh I just assumed the kit would come with them because I just installed a DJ kit on my project Virago and that one had needles. But the Duc kit doesn't, here is the link to the spec sheet:



Interesting, so it looks like I could be looking at even more money, yay!........