Vintage Dirt Bike Q & A

Scott,

My suspicion is that you are just not getting a good ground on every kick through while holding the plug to the engine.

I agree with what seems to be everyone ele's assessment that you have a fuel problem brought on by a bad seal between your float bowl needle and its seat in the carb.

Take the carb off the intake manifold leaving the fuel line attached. Drop the bottom of the carb off. (Fuel petcock off and in a well ventilated place with no open flame nearby.) You can move the float up and down but use just ever so much pressure when pushing up on the float. Turn the petcock on and fuel should flow out. Move the float up until the fuel flow shuts off. If you have the float up to where the needle presses into the seat and fuel continues to flow the problem is obviously the needle/seat combo. if fuel does shut off with just a little pressure then the next step is to check your float and the float level. But before we get into that perofrm the above test and let us know.

Loren

Thanks guys. I am feling better about the coil and timing now that I have read John's last post.

FirePig, I will do that this evening. I am sure it is a fuel problem as well. In the few times we kicked it last night the the plugs came out really wet.

As far as the muffler goes, I had it off the other day and it is very heavy. Could this be a sign that it is clogged? I have had several people tell me to put it in a pit and burn it out. I like the idea of taking it to a radiator shop better but we don't have one here (kind of a small town). I am asumming it is safe to do the whole thing because I cannot see a way to get the siencer off. It looks like it is one piece.

The silencer is removed by taking out the screw on the rear of the muffler and sliding it out of the muffler. If you can't see a way to remove it, chances are it havs already been removed and discarded. Most MR that I have seen have had the silencers removed. And placing the mufffler in a fire pit to remove the carbon and gunk is safe for the pipe.

When you remove the float and valve assembly check to ensure there is still spring tension in the little shaft opposite the pointed end or seat. The last one I changed was frozen solid. When you replace it with the new one make sure you adjust the tang before reassembling. Often aftermarket parts are a little different than original. Make sure you change the seat and valve at the same time. DonÌt forget to use the little o-ring on the new valve seat. There should be instructions in the box. The measurement should be taken just when the tang makes contact with the valve before any compression in the spring. There should be a cardboard gage in the kit. Use some aerosol carb cleaner with a little hose and spray in all the passageways to make sure there in no blockage.

I agree with John where he suggested you not use the rest of the jets in the kit. They look nice and shinny but are no replacement for the original Keihin jets. Try not to scratch the originals.

One of the very first things I did was clean the carb. I did notice when I had the seat out was that it had two holes in it. Are those supposed to be oriented in there a specific way? the needles in my carb are in good shape. I do not have a lot of faith in the float valve because of the continuous fuel flow I mentioned earlier. The pin seems to working fine but I am worried about the float setting.

The holes are suppose to be there. There is no orientation for them. Don't worry about the float height. People normally take a running bike and change the float height to make the bike preform poorly. Just put the needle and seat in and go from there. What if everything on the bike is set just right, but the needle and seat are bad. After making adjustment to everything will you know were to set everything back too? Patience my friend, patience. Work on the muffler.

John, That is exactly what I am going to do. I am not going to change or adjust anything until the new valve and seat are in . I will clean the muffler and I might even start sanding the tank and getting it ready to paint, but no engine stuff.

O.K. I did the test that Firepig described. When I raised the float up the fuel float did stop. I did however have to raise it until it was touching he top of the carb to get the fuel to stop, but it did stop. Could this be asign that my float height is bad and could that be causing the flooding? i am going to burn out the muffler this weekend. I will let y'all know how that goes.

I doubt it. You can get a needle to seat and stop fuel, but you are using greater pressure than a float can provide. Almost every old 2-stroke bike I have worked on, required a needle and seat. From all your posts, I am confident that it is the needle and seat. The tang, where the needle sets, should be parrallel with the float cross member but a little higher. As I said in my previous post I wouldn't adjust anything, yet.

Scott:

It's very possible that the spring inside the valve is frozen in the compressed position. This in fact would cause the valve to not shut off until the floats are much higher than they should be. I would not try to adjust anything to compensate. I would recommend you check the shut off level when you install the new one. By the way, I found that putting a small amount of silicon on the o-ring makes installation of the seat much easier. I have damaged o-rings without it. I purchased the silicon for lubricating the o-ring in my scuba regulator. It must be available in plumbing section at the hardware store.

The fire for burning out the exhaust sounds pretty extreme. I know the pipe is a little different on the MT but I removed the baffle and heated with a torch. Everything came off pretty easy then. I then gently worked a thin screwdriver inside the pipe where the baffle starts. I was surprised to see such a narrow opening. All the gases pass through a donut shaped passageway with spiraled partitions. This is definitely the bottleneck area. This gets the gases circulating to separate the heavy particles. Guess this is how the spark arrestor works.

How many miles are on the clock?

I understand what you are saying about the vavle. I did not adjust anything, as I have been told to never ever adjust the tang on the float.

It is not like it sounds to burn out a muffler. You basicly just stick a fireplace match in it and light the oil. It is kind of like lighting an oil lamp and it just burns all the oil out. I have seen it done on silencers off of newer bikes but never a one piece deal like the one that is on this bike.

That part of the pipe is called the expansion camber. The pipe is tuned to send positive pressure waves back to the exhaust port at the speed of sound. This happens until the piston closes the port. This stuffs fuel mixture sucked out back into the combustion camber and in turn helps to create more power. Pipes can be tuned for diferent operating speeds depending on the application. These old pipe designs are primative compared to today.

Not sure if a fireplace match will do much. It takes considerable heat to turn the sludge to carbon. My stock MT pipe is lined with ceramic fiber. I'm not sure what the temperature rating is but if the whole pipe were placed in a wood or coal fire pit you may damage the fiber or the adhesive holding it to the inside of the pipe. I know some people use this method. I may be wrong but I think, for the most part, the pipe is open except for a couple of diverters in the expansion area. The restriction is at or just before the removable baffle at the end of the pipe.

Scott,

You can get anything to seal off given enough pressure. But as John said earlier it should not come up that high. Likely the spring in the needle valve is frozen.

Now one other thing and forgive me if I missed it before, has anyone else been in the carb mucking around? It is possible someone fiddled with the float bowl height adjustment and it's way to high? Well.... first things first.... replace the needle and seat (if it's replace on your carb) and we'll go on from there.

Loren

I have no idea if anyone has done anything to the carb. I have had this bike for 3 weeks and got it from a guy driving by my house that wanted it out of his barn. It has been through some mods though. It has been painted yellow and had yellow fenders put on it. I think I found the wires for the headlight and tail light but I am not sure. There is a spot to mount a speed-o but it is gone to. I was thinking of posting pics of it so y'all could see the before and after. Right now my little girl calls it "The Beast", and my son is in love with it, mostly because he wants a bigger bike .