"Coughing" in the airbox?

[Devilman]

Hey guys / gals. Just been doing a service on a mk2 1200 Bandit, no real issues anywhere (except that front camcover bolt, what a fecker!) however I decided to put the tank back on quickly just to fire it up after doing tappets etc.. 

The bike sounds a bit... "wooly" if ya know what I mean, the idle drops really low once a little warm and if trying to hold a really light throttle opening (say holding the bike at 2k rpm in neutral) it seems to hunt around a little.

As far as I know there was no known running issues with the bike beforehand (although to be fair, not my bike so how good my mate actually is at spotting such a thing... who knows lol)

I decided to whip the plugs back out and take a look, heres how they are... I snapped 2 pics just for better reference.



Looks a bit like #4 plug (right hand one) is somewhat lean compared to the rest? Any thoughts / opinions / ideas to test or look for?

Tappets have all been set to "firm" .006" inlet and "firm" .008" exhaust, and no audible tapping or anything of the sort so I dont think its anything I have done during the work on the bike, but would really like to hear what the rest of you peeps think and any suggestions you may have as to a cause (or a fix)

Cheers all

[screwriverracing]

Have you balanced the carbs?
Cheers SRR

[Devilman]

No, hadn't go that far yet as the tappet's were only 1thou or so on the tight side, so figured such a small adjustment wouldn't have upset the sync that badly?

I'll go grab the piece of paper I had the clearances written on and see what they were on each cyl before I adjusted them, might give a clue as to why #4 plug seems cleaner than the rest.

Back shortly

[Devilman]

OK so heres a list of the tappet clearances prior to adjusting them...




Exhaust

1 2 3 4
 .007"   .006"  .006"   .007"  .007"   .006"  .006"   .006"
(tight)




(tight) (loose) (tight) (tight) (loose)
 .004"   .005"  .006"   .006"  .006"   .007"  .006"   .005"
1 2 3 4
Inlet
All tappets were adjusted to a "slightly tight / firm" .006" on inlets and .008" on exhausts.

Looking at cyl #4 it does seem to have undergone the largest change in total clearances, so maybe it has upset the sync more than I thought it would?

Any thoughts?

[katana]

I don't 'like' tight clearances - I always shoot for middle of the range and for racing use open / top end clearances as it actually improved power a bit. Reading plugs after a short / non loaded run is always dubious so wouldn't just rely on that.

[Devilman]

Jun 12, 2015 8:14:35 GMT katana said:

I don't 'like' tight clearances - I always shoot for middle of the range and for racing use open / top end clearances as it actually improved power a bit. Reading plugs after a short / non loaded run is always dubious so wouldn't just rely on that.

Aye I don't like it when the clearances are on the tight side either, which is where most of them were as you can see from the numbers...

Everything is set towards the "top-end" of the clearances as recommended by several people (including Dale Walker iirc) as the common trend with these motors is for the clearances to tighten up a bit over time anyway.

Reading plugs after such a brief run may not be 100% ideal, but I don't think I would call its results "dubious"... its still indicitive of the state of burn in the cylinder under those specific conditions (which also happens to be where the "cough" occurs) thus it's gotta be of some valid indication that #4 plug is cleaner / lighter than the other 3


Gonna have to do some carb balancing I think, hopefully that will sort out the slight "cough" / low rpm hunting. 

[quazi]

yep. as a general rule if it's coughing back through the carb/airbox it's lean, if it's popping out the exhaust it's rich. But I'm sure you knew that already.

I'd be looking at a balance now you've set the valve clearances, and have a general nosey around the hoses and connections/intake condition etc.


Those plugs do look lean from right to left, a gradual darkening as you go across.

[yoshi]

have to agree with quazi ,have a look at the vacuum pipe its easy to get it kinked when refitting the tank worth at try yoshi

[Devilman]

Yup, spent some hours sorting out balancing "by ear" (piece of tube to each bellmouth and compare the sound of the "suck") and it seems much better now..

I had not actually realised that these mk2 1200's run BSR36 carbs, not the BST36's of the GSXR / early bandit and their pilot settings are hugely different. BST's factory setting is around 1.5 - 1.75 turns out, while the BSR's setting is 3 turns out.

Interesting at 3turns out, she rarely coughs into the airbox, but the exhaust "smells" rich, which seems odd?

Out of interest, is getting a vac pipe onto #2 carb on these with the push/pull cable setup etc.. looks a total fecking nightmare, any tips?

Cheers guys

[wsn03]

Can I suggest you check your diaphrams on top of the carb - they do go / split, mine did on my Mk1 at only 8000 miles, small hole, totally messed the job up. Easy to check. Also the pilots at least on the Mk1 are so tiny that if left standing a while it's the first place the fuel goes sticky, a blocked pilot will cause all manner of issues, and its so easy for them to get blocked.
I keep fuel stabiliser in mine every winter now, and always fire her up every 2 months to keep the fuel flowing through the pilot.

[Devilman]

Jun 18, 2015 9:26:15 GMT wsn03 said:

Can I suggest you check your diaphrams on top of the carb - they do go / split, mine did on my Mk1 at only 8000 miles, small hole, totally messed the job up. Easy to check. Also the pilots at least on the Mk1 are so tiny that if left standing a while it's the first place the fuel goes sticky, a blocked pilot will cause all manner of issues, and its so easy for them to get blocked.
I keep fuel stabiliser in mine every winter now, and always fire her up every 2 months to keep the fuel flowing through the pilot.

Thx for the info, I don't *think* its carb tops / diaphrams, as I originally discounted them due to the issue only seemingly apparent when on the lightest of throttle openings (imagine holding the rpm at approx 2k, in neutral) and from previous experience I have found that diaphrams / carp tops typically exibit symptoms when "transitioning" on the throttle.

I suspect it's a combination of the carb balance was out a bit and also the bike's got an end can fitted and I doubt it's probably had any jet changes to suit and since these mk2 bandit's run tiny pilot jets (for emission's BS) that it really didn't like it.


I had an old 750 that was a pig to ride through 4-5k and that was down to bad slides (had those DJ restrictors in them) etc.. so no doubt it can cause a lot of very annoying running problems, but usually seems to be higher up the rpm / throttle.


Cheers

[wsn03]

Jun 18, 2015 15:58:03 GMT Devilman said:

Jun 18, 2015 9:26:15 GMT wsn03 said:

Can I suggest you check your diaphrams on top of the carb - they do go / split, mine did on my Mk1 at only 8000 miles, small hole, totally messed the job up. Easy to check. Also the pilots at least on the Mk1 are so tiny that if left standing a while it's the first place the fuel goes sticky, a blocked pilot will cause all manner of issues, and its so easy for them to get blocked.
I keep fuel stabiliser in mine every winter now, and always fire her up every 2 months to keep the fuel flowing through the pilot.

Thx for the info, I don't *think* its carb tops / diaphrams, as I originally discounted them due to the issue only seemingly apparent when on the lightest of throttle openings (imagine holding the rpm at approx 2k, in neutral) and from previous experience I have found that diaphrams / carp tops typically exibit symptoms when "transitioning" on the throttle.

When my diaphrams went the symtoms were stuttering on ever slightly opening the throttle....... it was slight and annoying. If I wound her open fine, but if I just touched the throttle during very light cruising there was like an annoying missfire affair going on.
It was a hole the size of a pin hole in my diaphram.

If I get a problem I have to inspect every possible cause for piece of mind, it's always interesting what you find when you do!

Read more: old-skool-suzuki.proboards.com/thread/5788/coughing-airbox?page=1#ixzz3dUqm0ymN

[Devilman]

Jun 19, 2015 8:58:27 GMT wsn03 said:

Jun 18, 2015 15:58:03 GMT Devilman said:

Thx for the info, I don't *think* its carb tops / diaphrams, as I originally discounted them due to the issue only seemingly apparent when on the lightest of throttle openings (imagine holding the rpm at approx 2k, in neutral) and from previous experience I have found that diaphrams / carp tops typically exibit symptoms when "transitioning" on the throttle.

When my diaphrams went the symtoms were stuttering on ever slightly opening the throttle....... it was slight and annoying. If I wound her open fine, but if I just touched the throttle during very light cruising there was like an annoying missfire affair going on.
It was a hole the size of a pin hole in my diaphram.

If I get a problem I have to inspect every possible cause for piece of mind, it's always interesting what you find when you do!

Read more: old-skool-suzuki.proboards.com/thread/5788/coughing-airbox?page=1#ixzz3dUqm0ymN

Intersting, thx for the followup.  Was surprised to read that a pinhole in a diaphram caused such issues at real light throttle, I guess anything is possible though  

Bike seems to run pretty good now considering the balance was sorted "by ear", My only troubles now are with the shitty 6pot tokico's on it lol

[wsn03]

You're welcome. I can do the by ear thing, but for these multi cylinder oil engines I find precise balance is everything, no amount of experience will match a gauge I've found. Strongly advise you to buy a set - I use carbtune, and balance at 2000 rpm