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
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
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.
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
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.
I know, this is a big topic too. I'll richen it up for this weekends race.
72 six-day
79 KTM MC80 250
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.
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
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
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
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.
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
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
Needle Jet not needle or main jet. It is what the needle runs through.
Larry P
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.
"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."
Garrett - if you don't check out the carb on this bike ASAP you just may DNF soon and kill your points standing completely.
Never EVER EVER sand or polish on internal carburetor parts. you may have caused your own problem here. Adding a little oil to the fuel will not change a thing about the problem that you are having, but it may cause more problems.
A quick look through the carb is not major surgery. Get someone to help you. Go through it carefully & inspect what you have - don't force anything apart or together. Set the float height so that the floats are square to the base of the carb just as the float needle begins to seat. Put it back together slowly & carefully EXACTLY as it was before except the float height. Again, don't force anything.
Now - make a list of the parts that appear to be worn and order them. Sounds like you will at least need a new needle if you cleaned it with emery cloth.
Go race and have fun. Make new notes of how the bike performs with the float level set correctly.
When the parts come in pull the carb again & replace them. It gets easier every time you do it.
Go ride the bike, make notes, and begin the process again.
Let us know if you need more help.
Good Luck!!
Clark
Right on clark
Somebody earlier mentioned about the float level I bent the one of the floats when I pulled the carb apart to "clean" it. I just bent it back, but didn't really check it. It was "that looks good" ( I know I gonna get it on this one)
After this race, I'm gonna print out these notes and go thru the bike and check everyhting out.
G
72 six-day
79 KTM MC80 250
Good luck Garrett on your race, Several years ago I was having trouble with my bike loading up, it started great and ran fine but if I lugged the bike it would load up.. I rebuilt the Bing carb, which included a new slide, new jet needle and needle jet, all new rubber pieces, and gaskets, wow what a difference! In my conversation with Bing agency, they suggested that I replace the needle jet and jet needle every year. The reality is that you may not be able to see any wear, but that does not mean it is not there..
As for the pistion, The stock piston will wear like iron.. I still have the stock piston in my 76 mc5 I did replace the rings, If the bike is making alot of noise Ie. piston slap then yea it might take further look, the easy way to do this is to take the piston off carefully remove the rings and slide the piston back in the cylinder. slide a feeler guage between the skirt of the piston and the cylinder wall, check your manual for theses spec. now the rings are a different matter, first slide the ring back in the cylinder making sure that it is square to cylinder, and check for end gap. again check your manual.. and the last and most important, with the ring still in the cylinder, lift the cylinder up into the light so that you can see through it, what you are looking for is any light that shines between the ring and the cylinder if there is, it is time for a new ring... My Mechanic tells me that it is best that once a year, I pull the top end off, give the cylinder a quick hone and then if everything is fine, replace the rings... never put new rings into a cylinder that has not been honed, No do not use emery cloth this can cause illregularities in the cylinder wall, a quick cross hatching of the bore is all that is needed, this will keep the barrel fresh and actually prolong the life, because all of the cross hatching will catch oil keeping things moving nicely.
Be thankful you don't have a four stroke, By the way I went nearly a hundred moto's with out a DNF and it still runs great,, These KTM's you can't hardly break them with a sledge hammer...
Thom Green,Still crazy after all these years!
76' 250 MC5 (orginal owner)74'
250 hare scrambler (project bike)
Race Update. My main competitor Tom Chavez (AMMEX)was back in action today. We both have missed the same # of races and I think he has 1 more DNF than me. 2 moto's to my 1. Anyways, I was thinking about just cruzeing around to get points, but that didn't set well with me. I psych myself up to run it like I stole it. Go For IT!
1st Moto, No mamby pamby on the starting line and got the holeshot of all classes, a yz400 ducked underneath me and a Modern open bike went around on the outside after the 1st turn. I said screw it, Hold that throttle wide open and took the moto win. Me and Tom were neck and neck the whole way, He mis-counted the laps and pulled off and ended up getting 3rd
2nd moto. I nailed the holeshot again and held onto the lead of all classes until the Yz400 and Modern open bike went past me on the back straight. Once again me and Tom were racing neck and neck. I passed the yz400 on the 2nd lap and then the bike started acting like it was running out of gas. Zoom Tom and the 400 flew by me. I put it on reserve and just cruized for the overall.
The bottom line I have a month until the next race and am going to tear down the carb and pop off the head.
I pulled the pug after the race and it was 3/4 black 1/4 tan. Track elevation was 4000 ft. Fuel mix, still 40 to 1. I left my oil at the house.
After the Race, Tom stated your bike looks and sounds like its running pretty good me. I know there is something wrong here.
Again thanks to everybody for pointing me in a direction.
G
72 six-day
79 KTM MC80 250
Saw the pictures on facebook Garrett. Tom is #54 I guess? What is he riding? Looks like a CZ. At least the engine.
Tom Chavez is # 54 and is Running a bike he bought as a kid. It is a AMMEX. It was a bike Designed by Don and Gary Jones, built and assembled in Mexico. It really is a neat Motorcycle. Apparently it has Yamaha Components in the Motor and Maico parts in the transmission. Cylinder castings look kinda like a cross between a CZ and Maico. I can't believe the story behind that bike and the whole AMMEX story. Also Tom is friends with the Jones Family and gary helps him keep it running. Its got a Kawi pipe that Gary told him how to make it work.
Apparently there is only 3 complete bikes known to exist. 2 of them are in collections that very rarely get ridden.
There is a Desert myth out here, that there is a house/garage/warehouse that has 10 of them in crates. The guys from Texas that "pick" vintage bikes say they know where they are, but won't let on.
I got to admit the guys from Texas do a good job of finding bikes out here.
g
72 six-day
79 KTM MC80 250
Amazing story. I have never heard of that brand. But it isn't so unlikely actually. In Italy there are numerous small brands that are almost unknown to the outside world.
I hope you get your carb right and keep racing your bike.