Carb slide does not go back down

[LeBouc]

Hi all. I am having a problem with the slide of one or both carbs; it does not go back down when I close the throttle and grab the clutch - the engine spins at 3,500 rpm until I stop and let the clutch out in first gear (in rare cases, it will do it by itself). On the bench, both pistons go down when I raise them with my fingers and let them go and they close at the same speed, arriving at the bottom at the same time. My idle is set to 1100 and my pilot screw is set at 1 1/8 turn back from fully closed. I have made sure that all jets and passages are thoroughly cleaned with carb cleaner. Anybody has an idea on where the problem could be? Thanks

 

[CaptChrome]

Might be a lean condition/air leak at the carbs. If the bike is idling normally and you blip the throttle to send the revs to about 5000rpm or so, does the engine "hang" at the higher rpm and take a bit to come back down to idle, or do the revs return to normal immediately? Hanging revs are an indication of the idle mixture being lean. This could be due to:
1. Idle jets are too small or your idle mixture screw is set too far in (try backing the screws out to 3 turns out and see if things change). If opeinging the idle mix screws up help, you should re-adjust those.
2. There is an air leak from the carb holders, the vacuum nipples or lines from the carb holders or from the butterfly shaft seals on your carbs. Changing the shaft seals at this point should be done on carbs this old anyway, but do an air leak check on this area to determine the cause.

 

[LeBouc]

Might be a lean condition/air leak at the carbs. If the bike is idling normally and you blip the throttle to send the revs to about 5000rpm or so, does the engine "hang" at the higher rpm and take a bit to come back down to idle, or do the revs return to normal immediately? Hanging revs are an indication of the idle mixture being lean. This could be due to:
1. Idle jets are too small or your idle mixture screw is set too far in (try backing the screws out to 3 turns out and see if things change). If opeinging the idle mix screws up help, you should re-adjust those.
2. There is an air leak from the carb holders, the vacuum nipples or lines from the carb holders or from the butterfly shaft seals on your carbs. Changing the shaft seals at this point should be done on carbs this old anyway, but do an air leak check on this area to determine the cause.

Thanks for the info. I looked at a video on Youtube that has taught me that when the idle does not come back rapidly, it means that the pilot screw in set too lean. So I unscrewed it up to 3 1/2 turn, back from the fully in position, and now the rpm goes down as soon as I shut down the throttle. So problem solved! Thanks again!

 

[LeBouc]

Thanks for the info. I looked at a video on Youtube that has taught me that when the idle does not come back rapidly, it means that the pilot screw in set too lean. So I unscrewed it up to 3 1/2 turn, back from the fully in position, and now the rpm goes down as soon as I shut down the throttle. So problem solved! Thanks again!

 

[LeBouc]

Well, I hought that the problem was solved but it's back again!!!! So I looked at the spark plugs and noticed that one was light gray while the other was dark brown. So I removed my carbs again but noticed that the tube that is connected from the petcock to the right carb boot was damaged; I figured that it could be the source of the problem so I changed it and went on the road for a check; it was better but not perfect. So the carbs went out again and re-cleaned the passages and jets. While reassembling, I noticed that the two diaphragm springs were not the same lenght so I tought that one could resist more than the other to the lifting of the slide that is pushed up by the vacuum created by the downstroke of the engine. To test them, I used long screws barely screwed in for the cap of the carb (with the rest in place) and measured the gap between the top of the carb and the cap which was pushed up by the spring. One was almost 20 mm and the other was less than 9 mm! I think that it means that one cylinder will automatically run leaner than the other since the springs do not offer the same resistance to the upward movement of the slide. I looked on the net to find a pair of new springs but the part with the correct number (3f7-14933-00-00/) isn't available anywhere. I tried stretching the smaller one until I could have the same resistance on both side (it worked) but now, the engine runs ultra lean until 5,500 rpm where the main jet is almost totally in control. So it looks like my diaphragm springs are now too strong and does not let the slide (and the needle) raise enough to provide the correct amount of fuel! I will try compressing it and heat it with a heat gun to see if I can arrive at something better suited to the carbs! If you have any other idea for me, go,go,go!

 

[avemachina]

Hey friend!

I bet you can find a used slide spring for a Mikuni BS34 carburetor on ebay--they were used in XS650, XJ900, and FJ1100s as well as XS400s, so not that rare--approximately the same length as one of your existing springs. That way they'll be the same length and both cylinders will have equal resistance. Then you can adjust your idle and balance/adjust your carbs.

 

[LeBouc]

Hey friend!

I bet you can find a used slide spring for a Mikuni BS34 carburetor on ebay--they were used in XS650, XJ900, and FJ1100s as well as XS400s, so not that rare--approximately the same length as one of your existing springs. That way they'll be the same length and both cylinders will have equal resistance. Then you can adjust your idle and balance/adjust your carbs.

Thanks for the info. When I search for carbs details, I saw that Yamaha have used the same carbs for different bikes. But some parts are changed to provide the right combination. It's obvious to me that the vaccuum created by a 325cc cylinder will be stronger than the one created by a 200cc one and, thus, the jets as well as other parts like the spring have to be calculated in consequence. But, anyway, I brought my springs back to the lenght of the smaller one and it helped. I also lower the needle and raised the fuel level in the bowl 1 mm because at idle, the mixture was lean while at higher rpms, the mixture was too rich. Now, it's not perfect but it's rideable and not dangerous. I also noted that I have some holes in the exhaust on one side and it can create a problem in the mixture so I am right now making sure that the silencers can do their job efficiently. Then I will go back to correct the fuel mixture.

 

[OleDoc2]

Kawasaki also used them on the kz750&1000

 

[OleDoc2]

Check ebay