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Discussion Starter #1
Hello,

I have a quite old Yamaha 550 Maximum inline 4 and it seems to be missing when the RPMs kick up (put put put) when at 3K or above RPMs.

I pulled the spark plug and noticed it is very very black at the inner plug side. The inner walls had this black filmy substance similar to what was on the spark plug. Three of four are operating normally.

I replaced the plug and still issues persist. I pulled the coil wires and switched them side to side and the same cylinder has issues. Cleaned the carbs to see if maybe a fuel hole was blocked or something, no issue really there. Still the weird backfire issue at high RPMs.

This is a very old bike, it has its issues but I feel this can be fixed. My only worry is something inside the top end needs to be cleaned out. I'd rather not do that myself.

Any technical know how?
 

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Female Rider
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Urban, you might try posting in the Motorcycle Repair section. I am not a mechanic, but do you have a heat gauge? I was having trouble with my 1985 Yamaha Maxim 700 and when we checked the temp of the pipes one was a lot cooler. We ended up cleaning the carbs and found that one had gunk in one of the small jets. Once we cleaned that out they were all the same temp and my bike ran great again.

Good Luck to you, I really hope you find the problem and it isn't something major.
 

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Discussion Starter #5
Thanks MONI we will check that. By chance who is a mod who I can have this loved to the repair section?
 

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Female Rider
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Dodsfall and Eye_M_No_Angel are the 2 most active Mods. If you look at the home page, right after the Off Topic section you will see a box titled View Forum Leaders. You can click on one of them and send them a private message.
 

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ZAMM Fanatic
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Alright Urban, alright. Gonna cost you lunch....

Misfires are commonly caused by ignition breakdown and/or a lean condition

So the first thing is to determine which. Swappijng the coil wires was a good first step.

Now try revving the motor while spritzing LOTS of starter fluid in it, see if it will rev correctly with added fuel. If it does, you know you have a fuel starvation problem.

Questions:

Did you "clean" the carbs or rebuild them, put a kit in.

How long was this bike sitting before you tried to get it running? Did you replace all the old, stale fuel? If not, do so now.

If the bike has sat more than 2 years plan on a complete carb rebuild. AND install an inline fuel filter so crap/rust from the gas tank doesn't contaminate your fresh rebuild.

What year is this bike? Has it got points? Have you tried touching them up a bit, with an emery nail file, then adjusting them to spec?

What is the recommended spark plug gap on this bike?

Is the choke working properly?
 

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Discussion Starter #9
You don't even give me a day? I am a student and only let myself check forums when appropriate. It's not even my bike its a buddies, so when I get back to you it will be when we can both meet up and try for a fix.

Also, I've been a member since July, have I not?
 

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Discussion Starter #10
Alright Urban, alright. Gonna cost you lunch....

Misfires are commonly caused by ignition breakdown and/or a lean condition

So the first thing is to determine which. Swappijng the coil wires was a good first step.

Now try revving the motor while spritzing LOTS of starter fluid in it, see if it will rev correctly with added fuel. If it does, you know you have a fuel starvation problem.

What is starter fluid and do I just drop it in the gas tank? We've been riding the bike for a couple hundred miles and have cleaned the carbs at professional place twice... It was free but that didn't seem to be an issue.

Questions:

Did you "clean" the carbs or rebuild them, put a kit in.

Cleaned twice via family moto shop.

How long was this bike sitting before you tried to get it running? Did you replace all the old, stale fuel? If not, do so now.

Twenty years :). New fuel was added after a full drain and seafoam added to the tank now in appropriate amounts.

If the bike has sat more than 2 years plan on a complete carb rebuild. AND install an inline fuel filter so crap/rust from the gas tank doesn't contaminate your fresh rebuild.

We will put in a fuel filter, nice idea. As for complete rebuild, it has been done.

What year is this bike? Has it got points? Have you tried touching them up a bit, with an emery nail file, then adjusting them to spec?

What the heck are points :eek:? It's from the 70s so it is old as the soil beneath our feet. Do you mean valves?

What is the recommended spark plug gap on this bike?

Four brand new plugs, the plug in the problem cylinder has to be replaced weakly to fix the missing issue. They come out charred super black. Not oily, but black as night inside where the spark plug sits. I tried to start the bike with the plugs out of the hole but the spark was not visible on each cylinder.

Is the choke working properly?
Yeah, it isn't stuck open or anything weird like that.
See responses above. The big issue is that black crud behind the spark plug inside the top end of the engine. I really don't want to pop open the top of we do not have to. I appreciate the response.
 

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What is starter fluid ?

What the heck are points ?


Starter fluid is ether in a spray can. You spray it into the carburetor throat, or snorkel coming from the air cleaner. It works "like" gasoline but is 10X more flammable. Used on cold mornings to start trucks, and to diagnose engines not getting ENOUGH gasoline.

Points are what cause the ignition...plugs to fire. Points do that by INTERRUPTING the current to the coil. The coil, like a sleeping dog, does NOT like to have it's supply of current interrupted and hurls a small lightning bolt as payback. That goes down the plug wires and fires the spark plugs. Check wikipedia.

Any inductor --- a coil, miles of telephone wires along poles, the wire windings in an electric motor...has INDUCTANCE. Current flowing likes to CONTINUE flowing....and when you break the circuit, either by throwing a switch, points opening, pulling the AC plug out of the wall...it creates an arc as the magnetic field around the wires collapses...in a brief, but failed ATTEMPT to keep the current flowing. That's what inductors do.

You obviously have very noble intentions, helping your buddy, but you need to find someone who knows a bit more about motorcycle mechanic'ing to help both of you. At least get you started.
 

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Discussion Starter #12
What is starter fluid ?

What the heck are points ?


Starter fluid is ether in a spray can. You spray it into the carburetor throat, or snorkel coming from the air cleaner. It works "like" gasoline but is 10X more flammable. Used on cold mornings to start trucks, and to diagnose engines not getting ENOUGH gasoline.

Points are what cause the ignition...plugs to fire. Points do that by INTERRUPTING the current to the coil. The coil, like a sleeping dog, does NOT like to have it's supply of current interrupted and hurls a small lightning bolt as payback. That goes down the plug wires and fires the spark plugs. Check wikipedia.

Any inductor --- a coil, miles of telephone wires along poles, the wire windings in an electric motor...has INDUCTANCE. Current flowing likes to CONTINUE flowing....and when you break the circuit, either by throwing a switch, points opening, pulling the AC plug out of the wall...it creates an arc as the magnetic field around the wires collapses...in a brief, but failed ATTEMPT to keep the current flowing. That's what inductors do.

You obviously have very noble intentions, helping your buddy, but you need to find someone who knows a bit more about motorcycle mechanic'ing to help both of you. At least get you started.
We will try the starter fluid issue first. Ty.
 
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