sooty plugs

xskev · Posted: July 18, 2009, 1:35 pm Post subject: sooty plugs

Hi Guys,Iv'e got a 1975 XS650 set up-stock motor iridium spark plugs,short megas exhaust system with 34 mikuni round slide carbs 180 main jet 22.5 pilot jet,P-6 needle jets,6F9 needles clip on centre groove,air jets removed and K&N's.i was running 25 pilot jets but the plugs were sooty after a flat out run so i installed 22.5 pilot jets 1.5 turns out on the mixture screw and it's still sooting up the plugs?.I'm using the recommended set up for those carbs so where am i going wrong or is sooty plugs common for a 34 year old bike.The bike is running great but i'd rather have the mixture as spot on as i can get it.
Kev.

schtipp · Posted: July 18, 2009, 1:55 pm Post subject: Re: sooty plugs

how does it perform through the range? nice even pull , break up in the middle? does it start with no choke? couldn't lean it out any more with the mix screws? if it's lean enough at an idle that you need choke to start it then maybe the main jets could go a size smaller. jets are cheap, run an experiment Razz

xskev · Posted: July 18, 2009, 2:14 pm Post subject: Re: sooty plugs

schtipp wrote:

how does it perform through the range? nice even pull , break up in the middle? does it start with no choke? couldn't lean it out any more with the mix screws? if it's lean enough at an idle that you need choke to start it then maybe the main jets could go a size smaller. jets are cheap, run an experiment Razz

Bike pulls like a train throughout the range,revs drop to idle instantly (no hanging idle),needs a bit of choke for a cold start.whats the maximum i can turn the air/mixture screw out without the chance of it falling out whilst riding?.i think the 180 main jets are ok and are not causing the problem iv'e even tried raising the clip on the needles.

Retiredgentleman · Posted: July 18, 2009, 2:25 pm Post subject: Re: sooty plugs

I'm not familiar with VM carbs, but for the BS38 carbs on my bike, I find the "dead cylinder" method is the best way to set the air/fuel mixture screws. No, you should not have sooty spark plugs. Youe plugs should be a nice medium tan colour.
www.dansmc.com/Spark_P...talog.html

5twins · Full Member Joined: Aug 05, 2006 Posts: 1616

deleted

xskev · Posted: July 18, 2009, 4:04 pm Post subject: Re: sooty plugs

5twins wrote:

Float level maybe? If the fuel level in the bowls is too high, it can run rich.

ikuni likes to see between 1 and 2 turns out on the air screws. I think you can safely run up to about 3 turns out before you run the risk of losing the screw. If you do need to go that high for clean plugs, another pilot jet change may be called for.

Jetting wise, you're right in the ball park as far as what everyone runs in these carbs. Sure you're not chasing an electrical problem through your carbs? Do you have a nice strong spark both when the engine is cold and hot?

Float levels should be ok as they haven't been tampered with and the carbs were brand new a few months ago.i fitted a boyer igniton recently so the plugs should have a beefy spark.i checked the ignition timing with a timing light and it's bang on 13 degrees BTDC.

Jack · Full Member Joined: Mar 15, 2005 Posts: 411

irregardless of how you have the VMs jetted to the standard recommended norm,no two engines will perform a like,period! Your heads may not flow exactly,you mufflers may produce more resistance raising back pressure,etc.
The pilot jetting on the Vms is for idling and initial take off up to 1/4 throttle,then the needle jets come into play and so forth. At this point you need to locate the leanest point and work up toward appropriate A/F ratio would be my suggestion. Raise the clip on the jet needle to it's highest point to lean the mixture out and see how the motor reacts but first I would suggest either lightly sand blast or glass bead the plugs first for accurate readings.. On the mains buy a couple leaner jets as they are inexpensive,install and at idle give it FULL throttle in gear and if the motor falls flat on it's face,you'll know it's leaned out to far. Remember with the VM's you make one jet correction for proper mixture,you'll most likely need to back track to make other adjustments accordingly to balance out the mixture for cruising and WOT,just need to find a range that is acceptable for A/F ratio.
Other members may use a different method to dial in their carburetors but this is how I do it and I find a extreme leaner mixture is more noticeable than a rich mixture any day.

schtipp · Posted: July 18, 2009, 8:36 pm Post subject: Re: sooty plugs

man if it pulls that good, starts easy. doesn't break up. a little rich I could live with. runs cooler, better for engine. you must be really close to right where you wanna be. Cool

xsjohn · Posted: July 18, 2009, 8:42 pm Post subject: Re: sooty plugs

Sooty black plugs at some point can induce ring and cylinder wash......may not show up tomorrow but surely will greatly reduce longivity..........

xsjohn

5twins · Full Member Joined: Aug 05, 2006 Posts: 1616

xskev · Posted: August 3, 2009, 3:41 pm Post subject: Re: sooty plugs

Does anyone know why it is recommended that the air jets are removed from these carbs?.is it not the case of the more air intake the carb gets the more fuel it uses.maybe thats why i'm getting sooty plugs.i'm also running K&N off set conicle air filters so it's drawing plenty air through them anyway.
Kev

Jack · Full Member Joined: Mar 15, 2005 Posts: 411

as to why air jets are supplied VMs using the primary type needle jet is a good question as air jets are more commonly used to regulate the A/F ratio through the bleed type needle jet but in desperate needs,if you're encountering extreme lean conditions of A/F ratio in the mid range ,you can opt the usage of an air jet for correct mixture.This is not your case,just take the A/J out and go for a test ride,.then check your plus again for coloration.

xskev · Posted: August 3, 2009, 5:40 pm Post subject: Re: sooty plugs

Jack wrote:

as to why air jets are supplied VMs using the primary type needle jet is a good question as air jets are more commonly used to regulate the A/F ratio through the bleed type needle jet but in desperate needs,if you're encountering extreme lean conditions of A/F ratio in the mid range ,you can opt the usage of an air jet for correct mixture.This is not your case,just take the A/J out and go for a test ride,.then check your plus again for coloration.

I have the air jets removed already.i'm going to re install them to see if it makes a difference.i checked the float heights and they were ok.i have 175 main jets now so i'm going to try them with the needles at full lean and work from there.

Jack · Full Member Joined: Mar 15, 2005 Posts: 411

I don't know how the VM's react cold but with the way you have the jet needles set,if starting out cold from the initial take off,if that motor gives any indication of a missing feeling,you're to lean on the jet needle setting. Mikuni's react differently when hot so keep that in mind when tuning them,at least my flatslides do. Also when the motor is nice and hot, make some adjustment to the idle-air screw for the pilot jet to see how the motor responds idling and on take off.

xskev · Posted: August 25, 2009, 2:05 pm Post subject: Re: sooty plugs

Found the problem i think..the bike is going through a hell of alot of engine oil.the oil rings to the pistons are damaged or the valve stem oil seals are leaking.the thing is the bike dose't give any smoke out the exhausts,i'm running short Mega's and the outlet ends are kinda sooty. looks like its time to strip the top end down.

Retiredgentleman · Posted: August 25, 2009, 2:20 pm Post subject: Re: sooty plugs

xskev;
You may have oil being sucked from the camchain tunnel into the cylinders. That is what I found on my engine. The PO did not have the head torqued down properly. The engine ran fine but used a really large amount of oil.

You could try re-torqueing the head. Remove the top stay on the engine. 30 ft-lbs on the 8 large 10 mm studs. 14 ft-lbs on the 4 bolts on top as well as the 2 under the plugs. If no improvement, then time to pull the engine for a top-end re-build.

xskev · Posted: August 25, 2009, 2:42 pm Post subject: Re: sooty plugs

Retiredgentleman wrote:

xskev;
You may have oil being sucked from the camchain tunnel into the cylinders. That is what I found on my engine. The PO did not have the head torqued down properly. The engine ran fine but used a really large amount of oil.

You could try re-torqueing the head. Remove the top stay on the engine. 30 ft-lbs on the 8 large 10 mm studs. 14 ft-lbs on the 4 bolts on top as well as the 2 under the plugs. If no improvement, then time to pull the engine for a top-end re-build.

Thanks for the info,my engine is running fine too and as i say is going through a large amount of oil.from the the top mark on the dipstick to the bottom mark on a 60 mile ride.Here's hoping your advice solves the problem.

xskev · Posted: September 13, 2009, 3:10 pm Post subject: Re: sooty plugs

Well i ended up taking the top end off the motor.no wonder i was burning oil the PO had made a pretty bad job of checking stuff before rebuilding it.i found that the barrels have a finish like a sheet of glass,the valves had not been lapped in properly one intake valve actually had a dent on the face and the valve springs where in upside down Confused

.
New valves and seals,piston rings,gasket set and oil filters on order,Man that big bore kit looks nice........but maybe on the next project Smile

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