Carb issues and/or petcock?

smurfy71

smurfy71

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So last week I left the petcock on prime and gas got in the case. I put new needles and seats in the carbs, gave them a good clean and put them back in. One ride later, it's running like crap. It won't start with the choke on and it's bogging in the midrange.

It also seems the gas is always flowing even when not on prime. I had disassembled the petcock, cleaned it up and put it back together but that doesn't seem to be helping.
 
When on prime the petcock just flows freely; if your petcock is still leaking when NOT on prime it needs to be rebuilt or replaced. Most people seem to screw up the rebuild process so I always recommend replacing the unit.

Second, check your float height. This is most likely the culprit.

Third, while the carbs will normally prevent flooding via the floats/needles, they are not designed to do this. You must take the bike off prime.

Fourth, did you change the oil now that it has gas in it?

Fifth, bogging usually means you're too rich. Again, check the float height. Make sure you are measuring it properly. The procedure is described in words in the manual and most people get it wrong. There are pictures of the procedure on here. Also, the Haynes manual has some mistakes in listed float heights for certain models.

Sixth, if your bike is running too rich and flooding using the choke is going to make things worse because it adds even more fuel.
 
So it was just the once I left it on prime, and yes I changed the oil. I will read up on adjusting the floats I have the Yamaha service manual, and look for a new petcock.
It just struck me as odd that after changing the needle and seat, it started running like crap, it was fine before.

Thanks for the info.
 
By changing the needle and seat you changed the fuel level.
The replacement parts are different than the ones you took out.
Anytime you change parts you always need to check adjustments of the associated system, in this case the float level. The big mistake most people make is not removing the float bowl gasket, you measure from the sealing surface to the float. Also check both bulbs of the float. The bracket between them can be bent or twisted.
Leo
 
I have been too busy to get at this recently, but was planning to attack it today or tomorrow. I have been having difficulty finding a good write up on adjusting the float level. can someone point me in the right direction? Also, what should the float height be? From what I've read, 20 mm is correct?

The space between the 2nd and 3rd line I have measured at 20 mm. Please tell me if I am doing this right.

ImageUploadedByTapatalk1438531242.526620.jpg
 
Last edited:
I once heard the tip to softly blow on the fuel hose connected to the carbs when you adjust the float level. That way you surely know if it closes at the position you measured.
 
According to This tutorial float level should be 22.4 mm for the Mikuni BS34. This way is easier to measure without special tools from Yamaha to read fuel level (as you probably have read in the shop manual).

Turn the carbs upside down, gentely blow trough a fuel line and slowly drop the float on the needle, the height should be 22.4 mm when the flow blocks. If not, adjust.
 
Finally back together and running. I put a shut off valve on the gas line between the petcock and the filter. I couldn't find a good petcock and this was easier than rebuilding mine.
 
I am not 100% certain for the Seca, but the SOHC, This is what I did to measure the fuel height. I made it with more things than I needed, (I didn't need the Tee, or vice grips. Live and learn.
full
 
Taking them out would be a bench test. on the bike, is the preferred way to test the fuel level as of the float capability. provided you have the test port under the carb bowl.
 

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