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Discussion Starter #1 (Edited)
Hello Everyone,

Okay, here is the story. I restored my 1978 Z50 from my childhood. I sent the motor off to a professional and unfortunately he put in big bore 72cc kit. I got the engine back, put it in the bike and it did not run well. The carb was rebuilt and it has the stock main jet. All top notch!

BUT, with the stock air box or air can plus filter it ran really poorly and rich. I took off the filter and the back part of the housing. It ran much much better. However, it gurgled and had no power at half to full throttle. I kept messing with the air/fuel mixture but would not get much better. I put an after market filter on it last night. When I leaned the air/fuel mixture, the bike would run great just standing there. I could rev it up no problem. I was so excited. When I tried to ride it though, I could not go 20 feet before it ran horribly. It now runs best when the air/fuel mixture is turned all the way in. But the plug is black. When I go to the standard 1.5 turns out, the bike can be revved standing still perfectly, but when I start to ride, it runs horribly. I rode around with the air/fuel turned in all the way and it ran better, but it lacked power. Once I got up to speed though, I could at least rev it out and give it full throttle.

Since it is now 70cc, I know it needs more air, hence the aftermarket filter. Could this be a main jet problem? The stock is a push in #58. I have ordered a #60, #65, and a #70 from Australia. That is the last thing on this carb that can be changed. The guy who did the motor says he really never has to go higher on the main jet. The needle is now set to it's leanest setting but the plug is still black even though it is getting more air (from the new filter) and the needle is full lean. Leaning out the air/fuel makes the bike run horribly when moving.

So lost! Please help a brutha out!
274151
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You are trying to go leaner on the main jet, but you ordered bigger jets..? Do you have smaller main jets to try?
I've never tried running a stock carb on a 72cc engine as you are, soo...no experience to offer.
It makes sense to me that you would need to go up, one or 2 sizes on the main...especially with the new air filter. But you're saying it's rich now. Maybe someone drilled or altered the 58 jet that you have in it??
Is the Oring on the jet good? Did you rebuild the carb?

When you say the needle is full lean...that means the clip is in the top notch...closest to the end of the needle, yes?
 

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Discussion Starter #8
You are trying to go leaner on the main jet, but you ordered bigger jets..? Do you have smaller main jets to try?
I've never tried running a stock carb on a 72cc engine as you are, soo...no experience to offer.
It makes sense to me that you would need to go up, one or 2 sizes on the main...especially with the new air filter. But you're saying it's rich now. Maybe someone drilled or altered the 58 jet that you have in it??
Is the Oring on the jet good? Did you rebuild the carb?

When you say the needle is full lean...that means the clip is in the top notch...closest to the end of the needle, yes?
Thanks for the reply! No, trying to go richer on the main jet since it is now 70cc. I have ordered 3 bigger jets that have not arrived yet. O-ring is good on the main jet. I am mainly confused why, with an aftermarket filter, the bike only runs well with the air/fuel turned all the way in. I could understand having to run the air/fuel richer with the added air but not all the way in!!!!! If I turn it out from all the way in just a half a turn, it is unrideable. It revs fine at wot when in the garage, but when I put in gear and begin to ride, it basically dies. The main is the only thing left to change, but since I cannot even get to half throttle with the air/fuel just a half a turn out, I shouldn't even be using the main at that point. Additionally, the bike is very hot! I check the valves and they are correct at .002. Like I said, all rebuilt carb and new head and cylinder.
 

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These engines ARE hot. To the touch. I wouldn't worry about that unless you're checking temps with a thermometer.

The air fuel screw only affects idle, and up to 1/4 throttle. That's it. You pull fuel from the main jet for everything above 1/4 throttle.

If your plug is black...that's rich...needs to go leaner. Black plug after idling means you need more air at idle...smaller pilot jet, or OUT with the air screw...or both. (I think the screw on your carb let's more AIR in when you back out the screw. Can anyone confirm that??)

If your plug is black after a WOT run, or even half throttle, that's rich...too big of a main jet, or, the needle needs to be lowered to restrict fuel flow thru the MAIN jet.

If ANY of my info is wrong, hopefully someone will pipe in and correct me.

If either of your jets are plugged up...with a spec of dirt or rust or whatever, all bets are off. If the emulsion tube holes are plugged...same.
 

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If you're going to continue running that screen door for an air filter, you're going to have to go bigger on the main jet, to a #60 or #62. You may be getting a false "rich" condition on the plug read due to contstantly giving it the gas to get the bike going and keep it going. You'll want to make sure that your points gap is accurately set at .016" at the "T" mark on the flywheel.

Also, even though the "fuel/air" screw is located to the front of the carb, my past experiences are the same as yours: turning it IN richens the mixture just like an air screw. With a conventional carb, having to turn it all the way IN is indicative of needing a bigger pilot jet, but you may be able to compensate for the non-removable pilot tube with an adjustment to the jet needle clip. Moving the clip down on the needle raises it, and richens the mixture in the 1/4-throttle to 3/4-throttle range, BUT...the pilot jet circuit and main jet circuit overlap into the jet needle range when the bike is running.
 

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Discussion Starter #12
These engines ARE hot. To the touch. I wouldn't worry about that unless you're checking temps with a thermometer.

The air fuel screw only affects idle, and up to 1/4 throttle. That's it. You pull fuel from the main jet for everything above 1/4 throttle.

If your plug is black...that's rich...needs to go leaner. Black plug after idling means you need more air at idle...smaller pilot jet, or OUT with the air screw...or both. (I think the screw on your carb let's more AIR in when you back out the screw. Can anyone confirm that??)

If your plug is black after a WOT run, or even half throttle, that's rich...too big of a main jet, or, the needle needs to be lowered to restrict fuel flow thru the MAIN jet.

If ANY of my info is wrong, hopefully someone will pipe in and correct me.

If either of your jets are plugged up...with a spec of dirt or rust or whatever, all bets are off. If the emulsion tube holes are plugged...same.
So, I rode it yesterday and there is carbon on the plug but not really on the electrode. It looks nice and brown. The needle is set to the middle. So, to recap: Needle in the middle, aftermarket filter, upgrade to 70cc, only able to ride with the air/fuel turned all the way in. I can give it full throttle when riding but it is weak and slightly surges a bit.

If I put in a #60, #65, or #70 main, then it should richen everything up and then I can turn out the air/fuel to then lean out the engine. Right?

The pilot is pressed in so I don't want to mess with that. Carb is super clean and I have blown it out several times.

Thanks again for all your input!!!

Glad to hear these run hot because I was worried about blowing my baby up!
 

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Discussion Starter #13
If you're going to continue running that screen door for an air filter, you're going to have to go bigger on the main jet, to a #60 or #62. You may be getting a false "rich" condition on the plug read due to contstantly giving it the gas to get the bike going and keep it going. You'll want to make sure that your points gap is accurately set at .016" at the "T" mark on the flywheel.

Also, even though the "fuel/air" screw is located to the front of the carb, my past experiences are the same as yours: turning it IN richens the mixture just like an air screw. With a conventional carb, having to turn it all the way IN is indicative of needing a bigger pilot jet, but you may be able to compensate for the non-removable pilot tube with an adjustment to the jet needle clip. Moving the clip down on the needle raises it, and richens the mixture in the 1/4-throttle to 3/4-throttle range, BUT...the pilot jet circuit and main jet circuit overlap into the jet needle range when the bike is running.
When you say the points gap, do you mean the valve clearance? I checked that and it within spec. If not, I will check out some videos. Got a gap gauge yesterday! Thanks! And yes, the filter does look like a screen door!! Lol! I would love to run the stock box and filter but it really runs poorly when I do that. When the jets come in, I will try to put it back on with a stock filter, but I am pretty sure what will happen!
 

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Well I'm thinking...
If you want to end up with the stock air filter, you should mount it up before you start jetting. Jet to your end goal instead of wasting time jetting with the screen door.

If you're going richer...install your stock air filter, and move your needle clip to the bottom notch...full rich...your bike should run better. Adjust your AF screw. Run the bike...down the road...when it starts acting up, shut the key off, hold the shift pedal down, and stop to look at your spark plug. That will give you a idea of what's going on when your motor starts running poorly..."plug chop" kinda.
 

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Typically, I run a jet with the needle clip full down rich, then full up lean...see which is better. Then if necessary, change jets and try again. Or maybe the jet is the correct size and you just need to raise/lower the clip a notch or two.
 

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Discussion Starter #18
Well I'm thinking...
If you want to end up with the stock air filter, you should mount it up before you start jetting. Jet to your end goal instead of wasting time jetting with the screen door.

If you're going richer...install your stock air filter, and move your needle clip to the bottom notch...full rich...your bike should run better. Adjust your AF screw. Run the bike...down the road...when it starts acting up, shut the key off, hold the shift pedal down, and stop to look at your spark plug. That will give you a idea of what's going on when your motor starts running poorly..."plug chop" kinda.
Yeah, I tried all of that before I bought the new filter. I ran horribly. Then I took the filter out, along with the backside of the box. Ran much better but still not great. Hence, the aftermarket filter. Once I get the bigger jets, I will just play around with putting the original stuff back on. It definitely works better with the new filter so I am going in the right direction. But, mainly have to wait until the jets come in. If that doesn't work, I will be at a dead end.

I mean, upping the cc's by 20 should mean that I need more air. Right???? Anyway, the bike was basically unridable with the stock air box and filter.
 

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Yes...more air because of more CC's. But you have to add more fuel to go with that air. You need the air/fuel ratio to stay the same...more of both. The new air filter is making your bike leaner...right now...than the stocker would. So if you want to be richer...right now...with the same jet...the stocker should help some, by restricting how much air your engine can pull.
 

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Discussion Starter #20
Yes...more air because of more CC's. But you have to add more fuel to go with that air. You need the air/fuel ratio to stay the same...more of both. The new air filter is making your bike leaner...right now...than the stocker would. So if you want to be richer...right now...with the same jet...the stocker should help some, by restricting how much air your engine can pull.
Yeah, I hope the new jets help out. But would the new filter force me to go all the way in on the a/f mixture. That is the part that seems extreme to me.
 
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