Need a new piston, I'm at a loss?

[garrettccovington]

How can you tell if you need a new piston?
 I bought my 79 KTM last Nov.  I had the Moto-plat rebuilt, rebuilt the carb (sorta)more like cleaned it out. and the bike fired right up.  So long as I'm on the gas it Hauls A.  The problem is,  I'm not a very good rider, which means I'm not on the gas that much.  The bike is 100 percent original. right down to all the decals on the frame, rims, bars, etc.  I had to put on new side ,rear fender, re-covered the seat and rebuilt the forks.  Thats IT!  If I drop off "the pipe" it burbles pretty bad.  So long as I rev it, it runs good.
When I took of the Mag cover it had very little mag rot. Closer to none and the transmission was completly full of oil.  It doesn't leak a drop, I've played with the fuel mixture. I just can't seem to get that burble out when I'm off the pipe.  It idles, It starts right up, snap the throttle and it revs right up.  The bike could get the holeshot, If I wanted it to, but I don't want to be in everybody else's way at the track. Actually, I just like riding around the track more than riding out in the desert and its safer.  I decarbonized the pipe and that helped alot.  This guy at the track said I needed a new piston and rings, cause it "started right up and idled".  I've only fouled 1 plug in 10 months of riding.  Never took off the head or the left side cover, shifts fine,  Clutch is stiffer than all get out, but doesn't slip.  Do I need a new Piston?  Any opinons?  
Who knows, maybe its been ported and needs to rev out.

G

72 six-day
79 KTM MC80 250

[Larry Perkins]

Garrett,

If it has the Bing carb replace the needle jet(it looks kind of like a little funnel) as they ovalize with time and are no longer the size they once were.  Also if the slide has a smiley face worn into it very deep replace it.  Money says this will fix your woes.  Bing Agency should have both.

A compression check will tell if you need a piston but I don't think that is the deal.  A new and properly routed clutch cable will probably help on the stiff clutch.  Al has them or Terrycable will.

Larry P

[dirtbike]

Adjust the gas level in the bowl. It seems as the level is to high in the carb. Maybe the valve doesn't close properly either. That causes the engine to bludder until it gets going. It can be jetting issues too of course. The problem is that it probably gets to much gas, to rich mixture.

What do you mean, played with the fuel mixture? Have you played with the premix ratio? That won't help much. 4% is what I use and I do not have to change connecting rod since the lower needle bearing doesn't wear.

[garrettccovington]

Thanks for the info.  Got a new clutch cable from Al, Still stiffer than all get out.  Of couse it could just be week me.  As far as the premix I use 40:1 to 45:1  Maxium Super M with Sunoco Leaded race gas.  I liked VP cp12 but I can get he Sunoco for 1/2 the price.  I'll take a look at the carb again.

Thanks for the input.
G

72 six-day
79 KTM MC80 250

[dirtbike]

There is nothing more controversial than an oil discussion but my opinion is that, even most (all) hi tech syntetic oil manufacturer claims 40:1 or 2 - 2,5% to be sufficient, I don't agree. The problem is not the cylinder or piston. It's the lower connecting rod needle bearing. A lot of people claim that 2% is sufficient, yet they change the connecting rod now and then and never give it a thought.

[garrettccovington]

I know, this is a big topic too.  I'll richen it up for this weekends race.

72 six-day
79 KTM MC80 250

[dirtbike]

Hi!
Not to mark words but things could be misunderstood further on. I assume that you mean more oil when you wrote "richen" but that is not really the same thing. When you richen you re-jet the carb, or raise the needle in order to give it more gas-mixture in comparison to the volume of air. If you add more oil to the premix you will infact leaning out the mixture since there will be less gas.

So the terms lean and rich reffers to the ratio between air and premix. If you run more oil you will increase the premix ratio but run a somewhat leaner carburation.

[Larry Perkins]

I seriously doubt that the condition described is caused by gas/oil mixture and it takes a bunch of change to effect running of a motor that you feel.  Gas/oil ratio change does effect more long term the wear factor but very few people could tell the difference "seat of the pants" between 40:1 and 60:1.  

Do yourself a favor and take out the needle jet and look at it.  I bet it is worn enough you wull see it is not a round hole anymore. You may be able to see that part of the opening has less metal than it once did.  New they are uniform. Over the years I have seen this a jillion times and the description is most always what you have said, Garrett.  Also look for the smiley face on the slide.  I can't give any better advice on Bing carbs.  If nothing else you will have eliminated what is a common prob.

Larry P

[wfopete]

So you have a 30+ year old bike but don't know much of it's history prior buying it.

The problem(s) could be all over the place, from adjustment to parts replacement.  

Go with adjustments first (carb/fuel mixture ratios) then consider going deeper.  Larry is right, a careful carb inspection is in order & a compression check is a priority.  

Start out cheap and free and go on from there.  Do one adjustment at a time.

10 months of riding equates to how many hours?? If the motor has never had new rings/piston, YOU might have enough time on the motor alone to justfy replacing them but that may not be causing your burbling issue. Do the free/cheap stuff first.



Pete Petrick
175 Jackpiner
Slow but Good

[Steve Minor]

I might suggest too that you write down all existing conditions. Then:
(1) make one adjustment at a time (in agreement with Pete) and write down what you changed.
(2) test the bike and if that didn't solve the problem, change it back to exiting conditions and move on to another possibility.

It might take time to sort it all out, but you'll get it sooner or later. The important thing is to keep a shop journal of what you did. It might come in handy in your future or another POG member's future.


Steve Minor
Wilmington, NC

[dirtbike]

There are 2 issues brought up. The first, if you need a new piston, cannot be answered just like that. First, if you have good compression which should be quite obvious when cranking it over, then you don't, in respect to this issue. The piston can, however have many hours and tiny cracks that grows each time you ride her. If you don't have a clue, get a new piston and a fresh bore.

The second, is that it's getting a to rich mixture. It can be worn jets, but it can also be wrongly adjusted gas level in the float bowl. I have had exactly those problems many times. The biggest problem is if you have a worn carb with oval holes in the needle jet and to high gas level, everything at the same time, which is likely.

[joe novak]

Garrett,  Compression check is necessary, and free.  If you do not have a gage, someone will have one available for your use.  Can someone give proper compression readings to Garrett?   30 yr. old main seals get hard, especially on the mag side.  My suggestion is the same as other POG members.  Do the compression check so you know where you are.  Good compression does not always equate to a good piston.  Pistons fatigue over time and cycles, and skirts crack.  It is sort of easy to clean a BING, but hard to clean all the small orifices.  I like to soak carbs in carb cleaner, and blow with high pressure air until I feel that the openings are flowing well.  Replace worn carb parts.  Joe

[garrettccovington]

Well for this weekend, I'm just gonna add a little oil to the fuel.  I think Larry is onto something there with the needle and the main jet and also maybe the float isn't set correctly.  When I cleaned up the carb.  I took a burnishing cloth to the needle to get it "clean".  After this weekend I'll have 6 weeks before my next race.  A compression test, a good look over the carb is in order.

If I wasn't in the middle of "The American Retro-Cross Over 50 Novice, Air Cool, Non-Linkage WORLD CHAMPIONSHIP"  I'd tear the bike apart today.  My main Competition is a great guy on a AMMEX and we are neck and neck in points.  The 1st one to DNF loses.  So I've just been finishing and luckin out with some wins.

  I'm gonna look into what everyone has suggested after this weekend.

Thanks
G

72 six-day
79 KTM MC80 250

[Larry Perkins]

Needle Jet not needle or main jet.  It is what the needle runs through.

Larry P

[dirtbike]

I had the same problem with my maico 400. When i hunted the problem down I tried the KTM 400 carb without altering any jetting. The thing is that the bike ran quite good.

It's easy to think that an engine of equal size would require more or less the same jetting? or similar at least or what do you think? I'm thinking of trying a Husky Mikuni 390 on my maico or KTM.

I don't expect much more than some fine tuning afterwards.