Flat slide blues but getting there...

I have had these carbs three months now and have had issues from day one. Wouldn't idle and a high speed misfire. On idle the revs would also raise bu 1000rpm without any input! I have had the carbs off about a dozen times. I did have a slighty warped boyer stator plate and evidence of it shorting out behind as the coils solder connections can foul the timing recess.

I always had one plug completely sooty (black) and the other much lighter.

I absolutely convinced myself that it was the boyer. I checked the battery voltage, the resistance of the trigger wires to the boyer, I swopped the plugs, renewed them three times, I swapped the colis, remove all the jets, checked them, measured the needles relative to the base of the slide to check that they were the same distance into the carb knowing the needle clips were in the same position, shifted the needle clips up and down more than a whores drawers, checked the resistance of each boyer pick up coil and then in series.

This week I had been through everything so I bought a new boyer digital and fitted it this morning - no difference

Today I did make progress. I used the gunsons colortune to set the tickover ending up with the screws one and three quarter turns out (not the quarter advised). Suddenly the plug soot was gone. I have never seen carbs so sensitive as these - that is the take home point here. Basically a turn of the mixture screw will drastically alter the plug colour. They were so bad they were fouling up completely and wouldnt run at speed.

So after all this and a new boyer I now have a reliable tickover, light tan plugs in the midrange but still a high speed misfire that is around 80-90mph and then pops and crackles up the exhaust after it is shut off. Is that running lean on the mainjets would you think? Has anyone fitted mainjets larger than 150's on an 850? Perhaps this has a slightly hotter cam?

Fred, I assume by flat slides you are talking about the JSmotorsport carbs Jim here sells?

or someone else's flat slides?

If they are Jim Schmidt's, then all you have to do is call him and talk with him about it.

He is a terrific guy who stands behind what he sells

Yes 1up3down. I have had about twenty emails between us so far. I just sent him an email. I think the problem is that these carbs are a bit demanding and you have no idea what state of tine you are fitting them to unless you built the engine which I didn't.

but still a high speed misfire that is around 80-90mph and then pops and crackles up the exhaust after it is shut off. Is that running lean on the mainjets would you think? Has anyone fitted mainjets larger than 150's on an 850? Perhaps this has a slightly hotter cam?

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Fred, I have an 850 with a freshly rebuilt motor and hotter cam.

If you are popping on deceleration, that is definately too lean on the air screws as it goes to slow idle circuit

I had that same issue and simply closed the air screws until it was gone

Interesting your comment about the air screws, yes they are sensitive, and yes I do have mine about 1/8 of a circle turn out from fully seated. If yours have to be one and 3/4 turns out then they seem way to lean, hence the popping

turn them both in to about 3/4 of one turn out and see if that gets rid of the popping, if not then in some more

my motor finally stopped popping on deceleration when I got to 1/8 of a turn out from seated

Fred

You need to "hear" the idle lope and adjust the mix screws for best smoothness (fastest idle and then re-adjust the idle speed screws etc). 1/4 turn at the idle mix is only a suggestion and a starting point. Some carbs work at 1/4 turn and others at 1-1/2 turn. Maybe the color tune will work for you but the wandering idle speed suggests that the idle mix has been off. Yes the idle mix will effect the plug color drastically. The idle has to be right first - smooth & even with no wandering and well behaved when the throttle is quickly shut. The idle mix has to be within 1/4 of being correct or it will be too lean or too rich and nothing will work right. Hearing the idle run smoothest by adjusting the mix is easy for some and almost impossible for others. If the idle mix is off then the idle speed will have to be jacked up to keep it nunning and then it will wander all over the place.

Your high speed misfire is another misterious matter. One other customer had this problem because of restrictive mufflers - the bike ran great with straight pipes and so he had to lean the mixture for the mufflers. Maybe try removing the mufflers as an experiment. Anyone else have this problem? - it seems to be very rare.

PS
I use 160 main jets in my 750 but my head has had port work.

FastFred said:

I did have a slighty warped boyer stator plate and evidence of it shorting out behind as the coils solder connections can foul the timing recess.

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I'm guessing this "evidence" of shorting was perhaps only visual evidence?

The coil retaining band soldered joints on the rear face of Boyer stator plates often touch the rim of the points housing, however the band is just a mechanical retainer that anchors the coil securely to the stator plate, so the bands are not part of the pickup's electrical circuit and should be completely isolated from the coil windings.

Thanks LAB for clrifying that - it was exactly what I meant but I am sure you are right as I checked the resistance across there today and got zero so it is just an anchoring.

Anyway I do have the mixture screws right at my settings and JIm is right (unsurprisingly) that if the idles arent right then you have no chance with the rest as the plugs will foul. I only worked that out today after months of fiddling. Going to get bigger mains tomorrow.

1up3down - I am talking about poppimg and crackling on partial throttle say half, not on the idle circuit. The mains are too small I am sure. Thanks for help all.

Hi Fred, the other customer Jim is referring to is me, I found some good looking mufflers on TradeMe (NZ EBay? and thought as they looked like std mufflers would give them a try at a recent track day.
I had to wind the mix screws out from approx 1/4 to 1 1/2 even to get it to run low down.Put it on the track only to find that I couldn't get the bike to rev over 5000 and it missed and farted through the rev range. Dropped the clip on the needles from slot 2 to 3 and again to 4 only to find it made it worse!!
Finally put on the scruffy straight through mufflers(noisier), needles back to slot ,2 mixture back to approx 1/4 turn out and voila back to performing at its best.
Also remember if you have an air leak in your exhaust system anywhere it will back fire (pop) especially on over run and throttle off.(Shouldn't make too much difference to tuning though at idle)
My next step is to try some different jet sizes with the first mufflers, am running the ones supplied by JS, but have ordered to sizes down just to try.
haven't had the oppurtunity to fit and test, towards the end of July should see that done. Then try with the second set of mufflers (the ones that didn't work!!!)
REgards Mike

Ok. This should sort it. Fitted new silencers yesterday and 160 mains today. Road test tomorrow. If this doesn't work I am putting the bike in the dustbin :roll:

Hi Fred
Not long to sort it out before Mallory, or do we just keep a lookout for the Commando popping and farting round the circuit. :mrgreen:

Hi Snakeyboy. Yes it should be good in the pissing rain! What sessions are you in? I am in 24 & 12 group 3.

I am taking plenty of jets and tools!!

hi Fast fred, in my above comments, one thing I didn't do was adjust the timing, which in hindsight i could've done to see if a couple of degrees either way may have made a difference.
On the strength of that, am off to a dyno close to home , armed with three sets of mufflers that I have (including the noisey ones that the bike runs well on and are fitted) and a handful of jets and a timing light to see what/how I can get the best out of the bike.(wont be for a week or two until I am back in NZ)
Current timing is 31 degress BTDC at 4000 RPM (Pazon EI)
Will post results
Regards Mike

Thanks Mike. I retarded mine the other day but no difference to the misfire so i put back to 31degrees. Cheers.

Oh well...Went out tonight confident the new silencers, boyer and 160 mains would cure the top end misfire, No joy - just the same

The only thing left is the coils and plug leads. they are fairly new but I just had the strobe on it again and it was misfiring up the silcencers at around 5000rpm. I had swopped the coils previously but I was really looking to get rid of the gross difference of the plug colours which I now know was the pilot jets and set-up. Perhaps one of the coils is breaking down at speed and I had just transferred it to the other cylinder when I swopped them but by the time I got home I was just checking the effect of the 'pilot settings' on the plugs?

I am just lost now with this problem,

FastFred said:

I am just lost now with this problem,

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Put the old carbs back on?

Difficult as I sold them. MarineCommando offered to loan me a set but I have left that too late for Mallory...

FastFred said:

Difficult as I sold them. MarineCommando offered to loan me a set but I have left that too late for Mallory...

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Were any of these symptoms present before hand? What else did you do to the bike when adding the new carbs?

Not to be redundant, but what all have you done to the carbs as far as troubleshooting is concerned?

The jet block was an issue for me and the pre shaped o-ring needs aligning. If the jet block is not set right, the needle can bind and actually keep the slide from returning completely to the home position
see here: http://www.mikesxs.net/product/48-9024.html

160 mains won't make much of a difference in my opinion. Have you modified the output side where the little casting are to take a line for a balance tube? I drilled them with a 3/16" drill and pressed in a 1/8"id piece of brass tube. This makes idle adjustment much easier. I had a 35 pilot jet in the right and a 40 in the left to make idle proper and leanness/richness balanced.

To say the least, float bowl removal is fussy.

I had to treat these carbs individually (needle clip, pilot jet etc.,etc) to work.

FastFred said:

Hi Snakeyboy. Yes it should be good in the pissing rain! What sessions are you in? I am in 24 & 12 group 3.

I am taking plenty of jets and tools!!

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Hi Fred,
I am in 28 but confused how you are in 24 and 12 as twelve is group B. Hope you sort the misfire but you could always try my clean running Amals.

pvisseriii, I had a few issues with the float boawl gaskets after removing the bowls for a check& clean, rubber Oring gasket wouldn't fit back in, always seemed too large and kept popping out of the groove no matter what i tried( i have worked with o rings all my carreer and still do) had to nick 2 mm out and reglue to make them fit. Checked with jim, advises to warm in the sun prior to fitting.!! I ended up getting acouple of spares , is that jet block on the back of that float bowl gasket?
Regards Mike

Brooking 850 said:

pvisseriii, I had a few issues with the float boawl gaskets after removing the bowls for a check& clean, rubber Oring gasket wouldn't fit back in, always seemed too large and kept popping out of the groove no matter what i tried( i have worked with o rings all my carreer and still do) had to nick 2 mm out and reglue to make them fit. Checked with jim, advises to warm in the sun prior to fitting.!! I ended up getting acouple of spares , is that jet block on the back of that float bowl gasket?
Regards Mike

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Yes, I recall tucking it in with a small screwdriver as I tightened the float bowl to the body.

The jet block is in the float chamber, screwed to the bottom of the body and has the same oring issues.

MikeXS has all the goodies for these carbs.