Carbs are off, I don't want to miss anything!

[Brian.]

Good afternoon all.

I can't get my 98 M750 to run properly.

Carbs are balanced, and it is happy at tickover, and will even hold a very slow tickover steady; but I don't leave it that low, just a normal speed.

The problem is with slow speed riding, even medium speeds. It is seriously "kerchung kerchung kerchung". At bigger throttle openings it pulls really strongly, 60mph in fifth gear and wind it open, it's great. Anything other than a big throttle opening, it really is a pig. Or a kangeroo would be more accurate.

It soots up the plug in the vertical cylinder. Dry dusty soot, not oily at all.

Fuel consumption appears to high. Mid 30s on mixed rural riding.

I've checked the timing with a strobe, it appears bang on. I've put some of those neon things between the plug caps and the plugs, it appears not to be missing a single beat spark wise.

I've swapped the plugs over, and then the ignition boxes, the soot stays with the vertical plug and it still runs the same.

I've done a compression test, it's within the tolerances, and the two cylinders are very close pressure wise, 4psi apart iirc.

So I've taken the carbs off, only taken the float chambers off so far. Someone has been in there before, I can see where someone has removed the starter jets.

That's all I've done so far.

I suspect pilot jets and or pilot galleries have muck in them causing the bad low speed running, and wonder about the float heights or needle valves causing the sooting up of the plugs? The carbs or not overflowing, it is not smoking from the exhaust, but the ends of the silencers is quite sooty, as you would expect I suppose.

I've done carbs before on various bikes, mostly Keihins on older Jap fours, the Mikunis on my GS750, and Dellortos on Laverdas. I've not done this particular type of carb before. ***The carbs are identical to the ones shown in the Haynes manual, so I am assuming now that they are Mikuni BDST 38mm CV carbs, not as I wrongly described them when I first posted this. ***

I don't want to do all the work involved in taking the carbs off and refitting them, and it still not be right because I've missed something needing attention in the carbs that may be obvious to you experienced Monster owners but I would miss, out of ignorance and inexperience. I really do not want to reassemble the carbs, and reassemble the bike and find that I have missed something out that I should have done

Have any of you had the running symptoms I've described in the past? Is there anything at all that I should check?

Would you go as far as completely gutting the carbs, have them ultrasonically cleaned and rebuild them with all new jets and rubber bits etc? Or would you be more inclined to methodically work through to find the actual fault or faults that is causing this? Though obviously replacing O-rings etc as necessary

My bike has done less than 15,000 miles.

Sorry it's been a long-winded posting, but I wanted to give as much background as I could.

Thank you for reading all this; and any help, guidance, advice anything at all, will be very much appreciated.

Thank you!
Brian.

[Nickj]

Originally Posted by Brian.

The problem is with slow speed riding, even medium speeds. It is seriously "kerchung kerchung kerchung". At bigger throttle openings it pulls really strongly, 60mph in fifth gear and wind it open, it's great. Anything other than a big throttle opening, it really is a pig. Or a kangeroo would be more accurate.

Define slow a bit better, what kind of speed and in which gear.

Evenish money is that the bike feels snatchy when you're in say 2nd 3rd at less than 30 and accelerating.
If it is then there's two things to look at.
Chain tension needs to be very close to just right, also has an effect on selecting neutral when at a standstill.
The gearing is a bit long on most of the monsters, noticable on all and a bit worse on the 5 box 750's. Something like a 15/41 sprocket combo tends to help massively. Chain a bit slack just magnifies it.
If you drop a gear or dip the clutch when it happens and it feels better then as they say that's the badger, it's often called character LOL

If not just clean the carp out of the carbs and bodies

[Darren69]

I suspect pilot jets and or pilot galleries have muck in them causing the bad low speed running, and wonder about the float heights or needle valves causing the sooting up of the plugs? The carbs or not overflowing, it is not smoking from the exhaust, but the ends of the silencers is quite sooty, as you would expect I suppose.

That would be my guess also. Has it just started playing up when it was ok before and unchanged? You say its got flat slide carbs not cv ones? Then it has been upgraded, it's a good upgrade over the standard Mikuni CV's

[Brian.]

Originally Posted by Darren69

That would be my guess also. Has it just started playing up when it was ok before and unchanged? You say its got flat slide carbs not cv ones? Then it has been upgraded, it's a good upgrade over the standard Mikuni CV's

APOLOGIES Darren! My mistake, I wrongly thought they were flat slide but they are actually the same as the ones in the Haynes manual, so I now assume that they are Mikuni BDST 38mm CV type.

Sorry for causing confusion! I have corrected the original posting.

Yes, it's always been like this, I assumed that it was just simple carb balancing that was needed. That has given it a much better tickover, but it's had no noticeable effect on the really rough running.

[Brian.]

Originally Posted by Nickj

Define slow a bit better, what kind of speed and in which gear.

Hi Nick,
In a 30 limit, it's not happy in 3rd; if there become even a slight upwards incline and I open the throttle as per normal it it does what I can only describe a a violent "kerchung kerchung" snatching, if I change down to second to get over it it only really does anything like smoothing out if it's accelerating. Which in a 30 limit is obviously no good.

I went out on it yesterday with some friends, some of it very rural Shropsire roads and villages, it was a swine. Once out on the open road, accerating with a decent throttle opening it was great. Trying to hold it a steady speeds, 30, 40, maybe 50 it just jerk jerk jerks. I'd have thought beyond character by a long way.

I'll check the chain tension, I have done it and not done many miles since but I'll check it again.

Thanks for the replies all

[Darren69]

Sounds like it's not getting enough fuel. Is it any better with the choke on a bit?

[Brian.]

Originally Posted by Darren69

Sounds like it's not getting enough fuel. Is it any better with the choke on a bit?

Thank you Darren; I’ve not tried that, because it keeps sooting up the vertical cylinder I was thinking along the lines of too much fuel getting through tbh.

Maybe it’s short of fuel on one, too much on the other?

[Darren69]

Actually is the choke coming off both carbs fully? It was a common issues with Dell Orto the plungers would not all go up and down equally/fully.

[Mr Gazza]

This could be a float height/operation issue, or perforated diaphragm.

Another thing to check would be the triangular grey plastic boxes that sit in the frame either side. It is important that these are present and that the relevant pipes are correctly placed in them and are in good condition, free of splits and leaks.
The boxes provide a place to draw still air from for the pipes, but I'm damned if I can remember what those pipes do and why they need still air except that if they don't it will run rough, though probably more at higher speeds than slow and definitely worse in windy weather.

[Darren69]

Originally Posted by Mr Gazza

This could be a float height/operation issue, or perforated diaphragm.

Another thing to check would be the triangular grey plastic boxes that sit in the frame either side. It is important that these are present and that the relevant pipes are correctly placed in them and are in good condition, free of splits and leaks.
The boxes provide a place to draw still air from for the pipes, but I'm damned if I can remember what those pipes do and why they need still air except that if they don't it will run rough, though probably more at higher speeds than slow and definitely worse in windy weather.

Sounds like an Italian bike, it depends on the weather

[motomartin11]

I too have had loads of problems with my '96 M750. They worsened when I changed the fuel tank as the original was leaking. Mine had exhibited poor low speed running like yours but not as bad.

It was always happiest under acceleration-the harder the better and was somewhat difficult to ride in town, especially commuting. Carb set up will depend on what pipes / cans and air box combination you are running. It turned out my carbs were on standard settings but running Sil cans and an after-market air filter.

After changing the tank mine became worse and worse and my mate has spent the winter going through it big time at great cost .

The replacement fuel tank was extremely rusty inside despite having been repainted to look very smart externally.

Consequently although the carbs were ultrasonically cleaned several times over the winter and the tank flushed many times-even to the extent of my mate sending me a video of his hose at full pressure through the filler and with the water escaping at pressure , each time the tank and carbs were put back on the misfire / poor running would re-occur due to the rusty debris from the tank.

The fuel filter was re-newed several times , the fuel pump was also rebuilt and the vacuum tap was changed to a manual type.


There were ignition issues as well as a result of the poor fuelling and I had to replace the ignition pick ups with new ones from Electrex World and get new igniter boxes from Germany. The wiring was in a very poor state resulting in rewiring of parts of the loom including a new coil harness ,cleaning or replacement of the majority of the electrical connectors, repair and replacement of the reg / reg wiring etc etc etc..!He also sent me a video of the sparks breaking down -hence the new ignition parts / wiring referred to above.

On top of all this somehow the crank needed a repair at the ignition end as the securing nut had come undone damaging the crank end.

More work was needed but I won't go into that now!

I'm told my bike is now running sweetly using a remote fuel tank BUT I had to get the original fuel tank welded up and he hasn't been able to repaint it due to the poor and lengthy winter cold / damp weather.

My bike was returned to me after Christmas but as I don't have a useable fuel tank it's been off the road since last September when it started running so erratically I couldn't ride it safely .

I'm not sure how much of the above will be relevant to your bike but it does go to show the effects of what can happen over time and what you may have to resort to to resolve historical issues which may have never been fully tackled previously. I wish you good luck in resolving your running problems and fingers crossed they won't be as bad as mine.

[Brian.]

Originally Posted by Darren69

Actually is the choke coming off both carbs fully? It was a common issues with Dell Orto the plungers would not all go up and down equally/fully.

That's one of the first things I checked Darren, it would have been a nice easy fix too. But no such luck, they were both shutting fully with a bit of slack in the cable.

Thanks for the suggestion though.
Brian.

[Brian.]

Originally Posted by Mr Gazza

This could be a float height/operation issue, or perforated diaphragm.

The float height/operation is something I did wonder about Mr Gazza, even though there's no signs of overflowing. And I was going to check the diaphragms as a mattter of course, but I don't know what symptoms faulty ones would give.

Originally Posted by Mr Gazza

Another thing to check would be the triangular grey plastic boxes that sit in the frame either side. It is important that these are present and that the relevant pipes are correctly placed in them and are in good condition, free of splits and leaks.
The boxes provide a place to draw still air from for the pipes, but I'm damned if I can remember what those pipes do and why they need still air except that if they don't it will run rough, though probably more at higher speeds than slow and definitely worse in windy weather.

Now this is interesting, because the triandular box on the left has no pipes leading to it at all, the one on the right has one - a breather pipe which I put in there because it looked like that was where it should go, based on photos of other bikes. And the similar breather pipe from the other carb is incomplete.

So this could be at least part of the problem, although the worst problems in my bike's running are at low speed....

But that is definitely something that I must correct when I put it all back together.

But thanks for the reply

[tridentperu]

I had a similar issue with my M600 - very poor running at small throttle openings, running rich, sooting plugs, jerky running, only happy on big throttle openings, exactly what you describe.

I got very very good at taking the carbs off/ and back on again. I cleaned everything (several times), set everything according to the book, fitted new float kits, diaphragms were good - did it improve? No.

I won't bore you with the whole saga but in the end I bought a used set of carbs to try them, but they weren't in any fit state to use, so I kept them for spares.

What was immediately apparent when comparing parts from both were the needles - the needles from the spare carbs were much thicker (visibly) than the ones from my carbs, totally different in fact. I did a load of research and it turned out the the numpty who owned it before me (I bought it cheap it as a non runner) had fitted dynojet needles but left the original Mikuni needle jets in....Doh. Fuel was pouring through it as soon as you touched the throttle!

I fitted the std needles from the spare carbs and, hey presto, smooth running was achieved. That was a couple of years ago now and the bike has run great since, all I have done recently is weaken the mixture a bit on the pilots as it was running a bit rich on the smallest throttle openings, you could tell by minor fluffing and snatching low down - now, all sorted again.

And yes, the grey boxes are important as they allow the vacuum part of the carb, that control the diaphrams, to draw still air when the bike is going. If the pipes are left open to the elements, the diaphragms can go all over the place and upset it even more!

Iridium plugs have helped prevent plug fouling too.

Hope this helps,

Jim