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Discussion Starter #1
so i had a little issue with a strong misfire that would lead to stalling out after. it seemed like it would only happen when the engine was hot or when i hit high revs while riding and then let off. there were a few occasions where i would try to start it again and then the battery would almost be completely drained. i ended up going all out replacing the coils, the ignition (dyna s), and eventuallly the entire engine as i wanted to have more spare parts on hand or to rebuild so when one engine messes up or needs a rebuild i would have one to drop in. well after all of this was done it seems like the misfire hasnt got any better. i pretty much chainged everything but the carbs which have been cleaned, the wire harness, and the rectifier. would the rectifier cause a misfire for any reason. im looking for any ideas as to what may cause this to happen. the riding season is almost at its end so any help would be halful at this point.
 

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Fuel tank cap vent
 

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Discussion Starter #3
actually on the 78 there is a door that covers the cap. it isnt out in the open like the 550s from the previous years. there is a hole in the tank under that door next to the gas cap and it is clear at this point.
 

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quote:Originally posted by photoputz83

actually on the 78 there is a door that covers the cap. it isnt out in the open like the 550s from the previous years. there is a hole in the tank under that door next to the gas cap and it is clear at this point.
yeah? really? you are confusing the fuel door drain for the tank cap vent?

1) the fuel door doesn't seal airtight over the cap. Least it shouldn't

2) your cap should still have a vent in it otherwise it is a sealed container.

3) learn the parts of fuel tank and how it works.
 

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when you say miss fire what do you mean?
 

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Discussion Starter #6
the drained battery and misfire would not be an issue with the fuel or supply. i am getting enough fuel to the engine as i am running a little rich at idle and almost perfect at 3/4 to full...
quote:Originally posted by Geeto67

quote:Originally posted by photoputz83

actually on the 78 there is a door that covers the cap. it isnt out in the open like the 550s from the previous years. there is a hole in the tank under that door next to the gas cap and it is clear at this point.
yeah? really? you are confusing the fuel door drain for the tank cap vent?

1) the fuel door doesn't seal airtight over the cap. Least it shouldn't

2) your cap should still have a vent in it otherwise it is a sealed container.

3) learn the parts of fuel tank and how it works.
 

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Discussion Starter #7
the engine runs fine in normal conditions or atleast it did. the misfire from what i am feeling is a shudder and then refusal to spark / ignite the gas. the it seems to start happening when i push the bike pretty hard, and after i let off is when things all start to go downhill. i know it isnt the coils, or the ignition as it is timed correctly, and both are new. i do however suspect something electrical which is why i was wondering about the rectifier.
quote:Originally posted by jaguar

when you say miss fire what do you mean?
 

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Discussion Starter #8
oh and i ran it with the gas cap off and it did the same thing.
quote:Originally posted by Geeto67

quote:Originally posted by photoputz83

actually on the 78 there is a door that covers the cap. it isnt out in the open like the 550s from the previous years. there is a hole in the tank under that door next to the gas cap and it is clear at this point.
yeah? really? you are confusing the fuel door drain for the tank cap vent?

1) the fuel door doesn't seal airtight over the cap. Least it shouldn't

2) your cap should still have a vent in it otherwise it is a sealed container.

3) learn the parts of fuel tank and how it works.
 

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Connect multimeter to battery terminals.
Tape multimeter to middle of handlebars.
Go ride, check voltage when misfire happens.
 

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Not saying your misfire has to do with the vent, just saying you identified and checked the wrong parts.

The cap has a vent so your petcock can feed fuel without creating a pressure problem. The gas door is there to prevent the bike from dousing you in gasoline in case of a front end accident. The hole near the gas cap is just in case you spill fuel filling the bike up or you ride through a rain storm and need a place for the water that collects under the door to drain.

Misfires are caused by two things fuel or spark. Running it with no gas cap eliminates it being a vent problem.

Have you checked your spark advance mechanism? Might be hanging up.
 

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Discussion Starter #11
yeah i ended up takeing the new ignition off and everything was pretty crisp when i would advance it manually and then let it spring back. i thought of that at first but put it on the back burner until lastnight.
quote:Originally posted by Geeto67

Not saying your misfire has to do with the vent, just saying you identified and checked the wrong parts.

The cap has a vent so your petcock can feed fuel without creating a pressure problem. The gas door is there to prevent the bike from dousing you in gasoline in case of a front end accident. The hole near the gas cap is just in case you spill fuel filling the bike up or you ride through a rain storm and need a place for the water that collects under the door to drain.

Misfires are caused by two things fuel or spark. Running it with no gas cap eliminates it being a vent problem.

Have you checked your spark advance mechanism? Might be hanging up.
 

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Discussion Starter #12
hell the misfire is bad enought that i dont need to ride. it happens when i have it sitting and just play with the throttle a bit. ill have to try that again when i get off work.
quote:Originally posted by mlinder

Connect multimeter to battery terminals.
Tape multimeter to middle of handlebars.
Go ride, check voltage when misfire happens.
 

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do you understand what the rectifier does?

have you started from the beginning?
what are your compression numbers?
you claim you are running rich in one sentence and fine in another. what do the plugs look like? are they the right ones?
dyna S and stock coils? leads are tight?
have you sampled the fuel?
what jets are you running for your nifty big bore?
pods?
cam timmed right? how do you know?
has it ever run right?
fresh battery? is it the right one for the bike?
 

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Is it a misfire or a hesitation?

What are you doing when it happens. If you are grabbing a fist full of throttle very fast and the bike hesitates and then Takes off you might just be dealing with the effects of not having an accelerator pump on your carbs.
 

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Discussion Starter #15
Yes i understand what a rectifier does, and i do know that they can cause a misfire. the compression numbers are right where they should be with the different cam and the and pistons, within 5 psi. i didnt say that i was running lean at any point, i said it leans out, not that it runs lean. plugs are a nice tan no white and not carboned up at all, and yes they are the right ones for the bike. the whole ignition is dyna s and the coils are dyna as well. stock mains were 92 currently 110, velocity stacks on normally but uni pod filters on now. the timing is 2 degrees advanced which is specified by the company that made it. yes it has run right for the first five years that i have owned it. on the old engine i logged 13k no issues until a couple months ago. the old engine tested out fine for compression and while performing a leak down test. the battery is correct and is a gel battery, less than five months old. the fuel is fine. 93 octane from shell and a 20% mix turbo blue for a combined octane rating of 98, and yes the jet requirements have been figured out to keep the bike from running too lean from the turbo blues nature to burn hot.
quote:Originally posted by jaguar

do you understand what the rectifier does?

have you started from the beginning?
what are your compression numbers?
you claim you are running rich in one sentence and fine in another. what do the plugs look like? are they the right ones?
dyna S and stock coils? leads are tight?
have you sampled the fuel?
what jets are you running for your nifty big bore?
pods?
cam timmed right? how do you know?
has it ever run right?
fresh battery? is it the right one for the bike?
 

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Discussion Starter #16
it happens when opening the throttle even slightly but usually starts after a highway ride when i start to ease off the throttle. after that starts to missfire it tends not to matter when or how much you touch the throttle.
quote:Originally posted by Geeto67

Is it a misfire or a hesitation?

What are you doing when it happens. If you are grabbing a fist full of throttle very fast and the bike hesitates and then Takes off you might just be dealing with the effects of not having an accelerator pump on your carbs.
 

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Kinda lossing me with this old engine new engine stuff...
Don't care at all about the old engine.
After this "build" has it run right?

What dyna coils are you running...ohms?
 

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Discussion Starter #20
3 ohm coils and yes it ran great for the first 300 or so miles. im pulling 13.2 volts at the battery as shown by my fluke meter. not the battery at all. even after i hit the starter button a few times it was still at a solid 12.6. im going to break down the carbs again tonight and see if i cant find anything. wrong or loose. then re sync. ill post the findings
quote:Originally posted by jaguar

Kinda lossing me with this old engine new engine stuff...
Don't care at all about the old engine.
After this "build" has it run right?

What dyna coils are you running...ohms?
 
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