More jetting questions....

[skiracer]

Hey guys,

I'm back with more jetting questiond...gee what a surprise, right???

Okay, resolved the sucking air issue, bike was running good.  It seamed to be running a bit lean, so I increased my Golden Spectro oil mixture from 52:1 to 40:1.  The plug had a very light brown color to it.   The bike sounded better.  I did some open trail riding, 3rd, 4th gear stuff, nothing too hard, or too fast on the Monday before the RR.  The bike rode, and sounded good.  Went to the RR, and ended up with a mild seize while going down one of the roads.  Came home, took it apart, and found the bottom ring had seized to the piston, cylinder was okay with a slight hone. (Motor only had about 25 hours on it) New piston from Al, Wisco, increased my main jet from a 290 to a 310.  Did some break in time, and then went easy riding in 55 degree weather.  Bike did fine.  However, the plug is light brown, as it was the last time I went trail riding.  I need to get this bike running richer!  At this point, should I increase the main jet further, or increase my oil mixture.  As a further note, there was plenty of oil on the piston, and bearing when I took it apart.   Any advise?  Thanks, James

[454MRW]

Keep in mind, that the more oil you include in your gas/oil mixture, the less gas is able to pass through the main jet, so although you are getting enough oil, you may be still lean on gas. Mike

Michael R. Winter
I enjoy rebuilding and appreciating Pentons!
1974 250 HS Pentons-1980 KTM 175-400'S

[firstturn]

You failed to indicate if this was a heat related ring problem?  In other word was the discoloration (heat marks) on the piston and cylinder and what did the plug look like after this mild seizure.  Also, remember that you can have heat build up on either end of the spectrum of too much oil or too little....think of it like a bell curve as the top part is good lubtication and any where you drop off it on either side of the curve you will have heat.

Ron Carbaugh

[linglewn]

On your "easy" runs and maybe on the RR run down the dirt road, you probably weren't running on the main jet unless you ran extended time at full throttle. I would try raising the needle or putting in a larger needle jet. More oil is probably not the solution.

Nelson Lingle
73 Jackpiner
71 DKW 125

[skiracer]

Ummmmmm, lots of information!
 Ron, the piston was black, except where the ring froze, and it scraped the cylinder wall(about 1.5" wide, and about 1.5" long.)  There were just a couple light scrapes on the cylinder wall between two port holes.  It appeared that there was a lot of oil coverage on both the piston and cylinder.  When I was taking it apart, my hands got covered with oil residue, and the crank had a nice coat of oil on it too.  The plug was a very light brown, not white or gray which would indicate too hot, correct?
Nelson, yes, on my "easy" rides, I was not using any full throttle, as I was on the dirt road in the RR.  That is why I am leaning towards a main jet issue.  If I change the neddle setting/jet, isn't that more of a mid range setting?
Thanks for your input.

[tomale]

skiracer,  typically 40:1 with Golden Specto is a good place to be. Most people never get on the main jet all that often. what this means is the the needle jet and the jet needle have more control than most people realize. most people ride mostly on the mid range which is controled by the needle jet/jet needle combination. so the easist thing to do is to move the needle up which will enrichen the mix in the midrange....

Thom Green,Still crazy after all these years!
76' 250 MC5 (orginal owner)74'
250 hare scrambler (project bike)

[firstturn]

James,
  First of all listen to Nelson and Thom on the needle jet adjustment and think about what Thom said about the main jet used only when you are totally wide open.  OK with that out of the way let's look at some of the issues.  I am not there so I am not sure of what ports you are talking about...
"the piston was black, except where the ring froze, and it scraped the cylinder wall(about 1.5" wide, and about 1.5" long.) There were just a couple light scrapes on the cylinder wall between two port holes."
Intake, exhaust or transfer ports and if transfer ports which side standing at the back of the bike looking to the front?  Also, is this where the pin for the rings are? I am also not sure of proper ring fitting to the piston as I have gotten a mixed bag of piston ring combination on bikes I have bought or piston sets I have purchaed off ebay.
  I hate to make a guess without having answers to the above, but I would be sure the rings are fitted well on the piston and you have no high spots on the piston to make hot spots in the future.  Next I would try and make a base line of what jetting and needle setting you had when this problem occured and start again and raise the needle(lower the clip) to get a richer setting go out and do some "easy ridding" and check your plug.  You may go up to settings on the needle to get a rich plug reading.  Let us know what is happening.  I hope all of this helps....if not we can visit via a phone call.

Ron Carbaugh

[skiracer]

Thanks guys!  
Thom, you are right, it is not often that we get into the range of the main jet.  However, with the gearing I have on my bike (12,52) I had the trottle twisted pretty good going down the road.  
Ron, looking from the back of the bike, the damage on the piston was above the wrist pin on the right side.  As far as the rings being a proper fit, I would think that if there was a problem with them they would not have lasted through 25 hours of use, would they?  This motor had just had a complete rebuild of both the top and bottom ends, and this carb is brand new (3 hours use prior to the RR) I will change the needle setting do some riding, and check the plug.  I will let  you know the status from there.  Thanks again for all the help.  James

[firstturn]

James,
  Thanks for the post.  All I can say at this oint is the damage may have happened when you had the air leak?  I do not know how long you ran it prior to repairing that issue.  I have some projects today and will give it more thought this evening.  Thanks for having such interest in getting this old war horse running right.

Ron Carbaugh

[skiracer]

Thanks guys!  
Thom, you are right, it is not often that we get into the range of the main jet.  However, with the gearing I have on my bike (12,52) I had the trottle twisted pretty good going down the road.  
Ron, looking from the back of the bike, the damage on the piston was above the wrist pin on the right side.  As far as the rings being a proper fit, I would think that if there was a problem with them they would not have lasted through 25 hours of use, would they?  This motor had just had a complete rebuild of both the top and bottom ends, and this carb is brand new (3 hours use prior to the RR) I will change the needle setting do some riding, and check the plug.  I will let  you know the status from there.  Thanks again for all the help.  James

[Ed Chesnut]

James:

It seems pretty clear to me that you are lean on the main jet.  There is nothing like an extended period of full throttle riding on a road to reveal the problem.  Note that when I say "extended period of full throttle", I mean anything longer than the longest straight on the grass track at the RR.

Your first jetting rule ought to be: Find Rich, work toward lean.

Right now you are lean, working toward rich.  Not good.  No.

As has already been mentioned, when you changed your pre-mix ratio from 52:1 to 40:1, you actually leaned out the gasoline:air ratio, making the leaness worse.  Yes there is more oil in there to (hopefully) help stave off seizure -- but the real problem is lean jetting.

You are at 310 now.  Beg, borrow or buy a 350 (and 340 and 330 and 320).  Put in the 350.  Is it blubbery rich at full throttle?  Good.  You've found rich . . . now work toward lean.  Drop main jet size one step at a time until the full throttle richness just goes away.  There, you have found the proper main jet for that altitude/temp/humidity.  Did I mention Find rich, work lean?

I will sound wayeeeee more cautious than many advisors, but I prefer a nice chocolate brown plug.  A light brown/tan plug just makes me nervous in a two-stroke engine which is being asked to do road work.

Ed

Keep the rubber side down!

[skiracer]

Ed and Ron, thanks for your time and brain power!!!!  Sooooo, if changing my gas ratio to the 40:1 was not the right thing to do, shoudl I return to my original 52:1?  I don't have the higher number jets, so I will have to order them.  I will get back to you in a few days with my results.  Thanks again.  James

[Larry Perkins]

James,

I think you still have some air issues.  On that bike you should not have to go beyond a 320 main jet, a 55 pilot jet, and a 285 needle jet.  40 to 1 is a good ratio but you should not seize at 50 to 1 even.  Too many times people keep messing with jetting and there is a mild air leak at the base gasket, intake, or crank seal.  I can't imagine you should have seized even at 290 on that bike if all is right in the air department.  My 2 cents worth.  Also needle position won't be the difference between seizing or not.  It will just help you fine tune what is already close.

Larry P

[Ed Chesnut]

Given Larry's information (which has wayeeee more familiarity with your exact engine/carb configuration than I've got), confirming that ALL air leaks have been cured is the best idea.  I have to admit that I ASSUMED you have an engine with no air leaks.  Once you KNOW there are no air leaks . . . find rich, work lean.

Ed

Keep the rubber side down!

[john durrill]

James,
The difference in your oil to fuel ratio is small ( unless your oiling bell curve is very sharp like Ron hinted at ). Take a look at this FAQ from Golden Spectro's web site
 FAQS

Question:

This may be a ridiculous question, but I would like to know the correlation between air/fuel ratio and oil/fuel ratio rich/lean mixture. Is it acceptable to lean out the oil/fuel ratio from that of the manufactures recommendation to compensate for rich running conditions. I use Golden Spectro 2stroke oil in my '99 KDX 200. Manufacturer recommends 32:1,I run the Golden Spectro at 40:1 and sometimes leaner and I find this to be acceptable by reading my spark plug. Please, could you inform me if I'm off track?

Answer:

Its not a ridiculous question and you are right on track. Golden Spectro is ideal at 50:1 in the KDX 200 and the affect on fuel mixture is as follows:

A change from 32:1 to 50:1 oil fuel mix = 1% in percentage of oil present. 50:1 = 2% oil and 32:1= 3% oil

So, at 50:1 you are 1% richer on FUELfollow that? 1% less oil = 1% more fuel

Mikuni jets are metered in percentage increments. So, a 170 main jet is 10% leaner than a 180 main jet. In order to compensate exactly for the oil ratio change, not accounting for air density, you would need to go leaner 1%. If you have a 180 main you would have to scrounge around for a 179 main jet. Good luck! They don't make them.

More important though is what happens in the low and midrange because we can feel the bike running a little richer there when we go leaner on oil. Well, some of us can. So, the best course of action is to turn out the air screw a 1/4 or 1/2 turn to clean up the bottom to compensate for changing from 32:1 to 50:1. THAT'S ALL!



Eds advice to starting rich and working lean is sound advise.
 Larrys trying to keep you with a full head of hair with the advise on checking for air leaks chuckle chuckle!!!!!!
  I think you missed something IMPORTANT! in Ron C's note. Check the ring end gap. If it's too big ( even with a new bore and little time ) blow by from the rings could damage the oil film on the cylinder walls and cause a seizure.
 I have seen more that one engine seize like you described. when backing off full throttle to 3/4 or so and the needle needle jet combo could be lean at that throttle setting.
 Are you running a Mikuni or a Bing for a carb?
john d. the curious