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Author Topic: Gray smoke spit from carbs  (Read 3027 times)

AgentX

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Gray smoke spit from carbs
« on: August 16, 2012, 05:10:32 AM »
In brief, what does gray smoke spitting from the carb indicate?  (Besides my mechanical ineptitude.)  Happening sometimes on start, sometimes while deaccellerating to a stop and then stalling.

Causes/fixes?  Thanks.

ace.cafe

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Re: Gray smoke spit from carbs
« Reply #1 on: August 16, 2012, 09:29:49 AM »
Lean mixture on the pilot circuit.
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AgentX

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Re: Gray smoke spit from carbs
« Reply #2 on: August 16, 2012, 09:46:45 AM »
Lean mixture on the pilot circuit.

Interesting, thanks!  I will go to a 35 pilot and see how that works.

I have another probably related situation:  When I engage the enrichener/choke, the bike will not start, or it dies if already running.  But it starts pretty nicely without the choke when cold; you do have to give a little throttle just after you kick it over to get it to keep going, though.

I thought this meant I was too rich on the pilot--that the enrichener was giving an over-rich mix which would not start, and that the regular mix was decent, if not perfect, for starting.  Obviously, I was wrong.  Any thoughts on what this indicates, or how to fix it?


ace.cafe

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Re: Gray smoke spit from carbs
« Reply #3 on: August 16, 2012, 11:42:05 AM »
If you are needing to go to a 35 pilot, and don't need the enrichener, then maybe it needs to be slightly leaner instead.
I may have jumped the gun on this before having all the facts.
I'd keep the 30 pilot, and use the air bleed screw to try to get it dialed in by screwing the bleed screw out until it behaves better.
Then re-adjust the idle speed with the big screw when the bike is heated up, and that should help. If you adjust these things when it's hot, then it doesn't die at stop lights. Then you use the enrichener for starting.
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AgentX

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Re: Gray smoke spit from carbs
« Reply #4 on: September 08, 2012, 03:49:59 PM »
I am still baffled by my bike.

When I run through the procedure for setting proper idle with the bike nice and warm, the air screw is always near-fully out (leanest).

Yet I have the gray smoke spitting problem reasonably frequently, and the bike still won't start with the choke on.  I put in jets just over stock size owing to my use of a K&N filter and open header with a short megaphone muffler, minimal baffling.  I also installed a new spark plug at that time.  It was black and looked a bit oily when I pulled it out today after 80 miles or so.

I figured it HAD to be a vacuum leak causing this, but I tried spraying WD-40 and couldn't find anywhere that made the engine race.  Could it be something else?  Ignition timing off, or bad coil??

ace.cafe

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Re: Gray smoke spit from carbs
« Reply #5 on: September 08, 2012, 05:04:56 PM »
It's possible that you have some worn things in your engine, which might cause it.
If you had leaky valve seats, that can cause spitting back thru the intake. And leaky valve guides can cause oily spark plug.

And an exhaust leak at the head joint might cause a lean mixture condition and spitting, so check your header pipe fit into the head for any leaks or visible carbon showing around the header pipe joint to the head.
« Last Edit: September 08, 2012, 05:07:39 PM by ace.cafe »
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AgentX

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Re: Gray smoke spit from carbs
« Reply #6 on: September 08, 2012, 05:19:39 PM »
It's possible that you have some worn things in your engine, which might cause it.
If you had leaky valve seats, that can cause spitting back thru the intake. And leaky valve guides can cause oily spark plug.

And an exhaust leak at the head joint might cause a lean mixture condition and spitting, so check your header pipe fit into the head for any leaks or visible carbon showing around the header pipe joint to the head.

thanks--will check as best I can.  The cylinder head rebuild by the shadetree jackass I paid to do the initial work sure isn't precluding any of that.

How would you check for a leaky valve seat or guides?

I've been regularly looking for soot at the exhaust header join, though, and not found any.

barenekd

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Re: Gray smoke spit from carbs
« Reply #7 on: September 08, 2012, 05:41:34 PM »
Your timing could be a bit retarded.
The carb sounds quite rich enough.
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ace.cafe

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Re: Gray smoke spit from carbs
« Reply #8 on: September 08, 2012, 07:22:44 PM »
thanks--will check as best I can.  The cylinder head rebuild by the shadetree jackass I paid to do the initial work sure isn't precluding any of that.

How would you check for a leaky valve seat or guides?

I've been regularly looking for soot at the exhaust header join, though, and not found any.

Normally to check for leaky valve seats, you check your compression with a compression gauge first. Then if compression is low, it's either valves or rings. Check for rings by adding a few ccs of oil into the cylinder thru the spark plug hole, and check compression again. If compression raises substantially more, it's rings. If it doesn't, it's valves.
If you have no compression gauge, you can remove the head, and put it on the bench with the combustion chamber facing up, and pour some light oil or ATF into the combustion chamber enough to fully cover the valves. If the oil or ATF leaks past the valves into the ports, your valve seats aren't sealing, and you need a valve job.

Regarding guide leaks, many Bullets don't have valve stem seals, but some of the later models do. If your guides are worn, or stem seals have gone bad, oil drips down the guides and gets into the cylinder, and leaves oily deposits on your spark plug. Oil in the combustion chamber is generally not good, and can be a cause of detonation in some cases.

Most of the time these spitting and dying symptoms are from poor tuning. But then can be from engine problems. Work on trying to get good tuning first, and if you can't get it tuned right, then look at engine issues that I mentioned.

« Last Edit: September 08, 2012, 07:53:30 PM by ace.cafe »
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Bullet Whisperer

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Re: Gray smoke spit from carbs
« Reply #9 on: September 08, 2012, 08:37:00 PM »
It would be worth checking that the pushrod adjusters are set correctly, as they might be too tight. Make sure you can spin the pushrods easily at TDC on the compression stroke.
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Arizoni

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Re: Gray smoke spit from carbs
« Reply #10 on: September 08, 2012, 10:39:40 PM »
If my sparkplug is black after a 80 mile ride I assume either the fuel/air ratio is too rich or my sparkplug is too cold of a heat range.

Of course, once I thought I was wrong but I was mistaken.  ;D
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Kevin Mahoney

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Re: Gray smoke spit from carbs
« Reply #11 on: September 09, 2012, 04:15:59 AM »
This is shade tree mechanics at it's best. It was shown to me years ago by an old hand at the factory. I use it but never admit to it. If the compression of your bike is good enough, a reasonably sized person (not me) should be able to gently stand in the kick starter at the top of the compression stroke and the kick starter should hold you. Crude but effective

AgentX

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Re: Gray smoke spit from carbs
« Reply #12 on: September 09, 2012, 09:07:34 AM »
Kevin, that's what I've been told too, but I've never seen a 350 that could support the weight of a grown man for more than a few seconds...might well be the fact that everyone's bike here is clapped out to some degree.  :)  Mine won't hold me past 2-3 seconds, but it's only got around 275 miles on new piston/rings/cylinder and it feels very good by local standards.

Ace, no idea if I can find a proper gauge locally.  I know they exist but dunno if they're any better than the Harbor Freight cheapies found in the US.

I began to suspect this was an ignition/timing issue, so I went and replaced the contact breaker plate, with new points and condenser.  The old auto-advance unit seemed much smoother than the new one I had, with nothing disconcerting going on, so I left it in place.

I tried ping timing per Snidal and settled on about 2/3 of the way advanced.  I never got a clear pinging like a spraypaint can being shaken (heard that on my UCE 350 once); at best when I was on or past 3/4 throttle on the hill I heard a vague rattle underneath the engine and valve clatter.  Bike also became really hard or impossible to start around that setting, so I backed off a bit and seem to have found the sweet spot.

But it didn't solve the spitting problem and idle remains lumpy despite my continued best efforts to tune it smooth.

Question, though, since the guy who put the bike together did many suspect things...if I'd been unable to make it ping when fully advanced at the contact breaker plate, what then? Can you just break/re-set the taper and start again?  That was my only guess.
« Last Edit: September 09, 2012, 12:18:49 PM by AgentX »

ace.cafe

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Re: Gray smoke spit from carbs
« Reply #13 on: September 09, 2012, 01:23:48 PM »
You can re-set the taper and move the points plate to start over again, but it seems to me that you are probably in the neighborhood.

In some cases, being too rich can cause a spitback too.

Part of this problem is because there is so much overlap on these stock cams, and it is ridiculous for such a mild output engine. There is no reason at all for these cams to be opening up the intake at 50 degrees before TDC, and I have seen that on plenty of these stock cams. Even the factory spec of 40 degrees before TDC is too much for this low-revving low output engine.

Having the intake open while there is exhaust pressure still in the cylinder is a recipe for reversion. If there is any unburned fuel in that exhaust, and a hot spot sets it off when the intake valve is open, it spits back thru the carb.
It can have this unburned fuel in the cylinder if there is a misfire, or if it's too lean, or sometimes if it's too rich.
« Last Edit: September 09, 2012, 01:26:31 PM by ace.cafe »
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AgentX

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Re: Gray smoke spit from carbs
« Reply #14 on: September 09, 2012, 03:56:52 PM »
Thanks for the explanation, Ace.  Makes things clearer yet more complicated, I guess...

I will try going down to the stock jetting and see how the plug reads; will be careful not to get overly lean, but it doesn't seem like I'd be in the danger zone given the current setup is only 30/105.  When I got it, it had a really big pilot jet (looked hand-drilled and I had to ez-out it anyhow, destroying any markings) and the stock 90 main jet, and didn't give me these problems.  I will keep my eye out locally for a 28 pilot and 95 or 100 main which seem rare as hen's teeth.

Maybe my muffler isn't as free-flowing as I thought.

Given your explanation and the fact that this started when I went richer, the answer now seems easy.

Still don't know what's up with my excessively lumpy idle, but I think maybe it comes down to a fairly sketchy alternator...I think either the rotor or the crank stub itself is just off true.  It wobbles a bit and I think that definitely affects its output; getting it to run without contacting the stator is a real chore. 

I am going to try using a Boyer powerbox to see if the capacitor smooths things out; found one lightly used for a great price at Hitchcock.
« Last Edit: September 09, 2012, 03:59:43 PM by AgentX »