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Discussion Starter #1
Hello,

I need some help. My 1978 Yamaha XS750 starts and idles fine, but when I touch the throttle, even a little, it revs high and doesn't want to come back down. It will also start to pop from the muffler. I assumed this must be a vacuum issue, stuck throttle, or dirty carb. I have replaced the intake boots and bought and installed a carb repair kit. I have also thoroughly cleaned the carbs. I checked the throttle cable and nothing is pinched and moves as expected.

While taking one of the carburetor diaphragms out, I noticed the edge cracked on one. The main portion of the diaphragm is fine (see attached picture).

Would this possibly be the cause of the high rev that won't come down and slow throttle response? Or, do you believe it is something else?

I want to get someones opinion before I spend the money on a new carburetor diaphragm.

20201025_210847.jpg
 

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Ace Tuner
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Would this possibly be the cause of the high rev that won't come down and slow throttle response?
Yes.

There could be other problems but that is a bad diaphragm. You should check the other diaphragms for pin holes while you're at it.

S F
 

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Discussion Starter #4
I have fixed the diaphragm and checked the others as well. I won't start the bike until tomorrow to find out it that turns out to be the problem. I re-cleaned the carb and soaked areas in carb clean and put a compressor to the areas. This seems like a carb issue. I'm not sure what else it could be. I can give an update tomorrow to see if I got lucky.

Otherwise, I'm definitely open to new possible issue/solutions it could be if this doesn't work. (There were multiple holes in that diaphragm that I couldn't see until I put a flashlight to it).
 

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Discussion Starter #5
So to provide an update. The bike now runs fine in idle for about 30 seconds to a minute then when it starts to warm up, it revs to 6,000 rpm before I kill it. (It did idle fine before I fixed the diaphragm). I also only let it idle for a few minutes tops before trying to touch the throttle. I've tried adjusting the butterfly valves in every way but it always does the same thing as it warms up. I plugged the vacuum ports on engine for the petcock and replaced the intake boots, so I dont think vacuum leak (I could still be wrong if there is something I'm missing). I cannot leave the engine running long enough to sync the carbs using my sync tool or spray starting fluid near it to see if there is a vacuum leak im not seeing.

Could this possibly be a timing issue? The bike only shows 15K miles but it could have rolled over and I don't know it.

Also steam comes rolling slowly out of the far right cylinder after I turn the bike back off.

Any help would be greatly appreciated. I plan on dedicating tomorrow to do everything I can to fix it before the weather gets worse.
 

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Could this possibly be a timing issue?
Refresh my memory. Ignition points, yes? I can't remember the exact type of carburetors either...
Sure sounds like a lean mixture or a vacuum leak but running ok on cold idle makes me think maybe not a bad vacuum leak.
Possibly maladjusted carburetor slow speed (idle) tuning?
Are the slow running speed adjustment screws in front of the slide/diaphragms (toward engine side) or behind the slides (air box side)?

S F
 

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Discussion Starter #7
This bike has a CDI not points. The type of carburetor is a Mikumi (see below picture for reference). What do you mean "slow running speed adjustment screw"? Are you referring to the idle screw that moves all three butterfly valves? It is located on the air filter side. The air inlet screws are on the engine side. (I am not overly experienced. I restored a 2-stoke 250cc engine previously and that is it) I got this to step up my .

16041116190405681273734215756979.jpg

16041116640398088899691726728147.jpg
 

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What do you mean "slow running speed adjustment screw"?
Thanks for the pics.
The slow speed running screws (fuel mixture screws in this case) are the brass head screws with springs located on the top front of the carbs near the spigots that plug into the intake manifolds. I asked location to figure out if they are fuel or air screws. Those are fuel mixture screws.
How do you have them set? Adjustments are "X" number of turns out from lightly seated. A 1978 model, I don't know what bench setting would be. Anyway, turn them out for a richer mixture. In for leaner. Going to eat, will check back later.

S F
 

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Discussion Starter #9
I had read two different amount of turns online (may service manual I bought is worthless in regards to any carb adjustments). I had them at 2 and half turns out. I then moved to 1 and a half turns out and I think it was worse. I will go more turns out to try and add more fuel and help remove the running lean issue.
 

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Discussion Starter #10
I just wanted to provide an update. I've got it running! And with pods!

I kept moving the butterfly valves around and got the idle back to normal and normal throttle response. Whats weird is the valves are at all different openings. I'm putting the carbtuner pro on it to get all three cylinders balanced. I think it was a combination of the air-inlet screws, the butterfly valves, and the needle in the carburetors. (I didn't have the tool to replace the needle and it finally came in the mail).

So I think you were right that it was just running way to rich because of the pods.
 

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I just wanted to provide an update. I've got it running! And with pods!

I kept moving the butterfly valves around and got the idle back to normal and normal throttle response. Whats weird is the valves are at all different openings. I'm putting the carbtuner pro on it to get all three cylinders balanced. I think it was a combination of the air-inlet screws, the butterfly valves, and the needle in the carburetors. (I didn't have the tool to replace the needle and it finally came in the mail).

So I think you were right that it was just running way to rich because of the pods.
pods will make your bike run LEANER not rich. when installing pods and open headers you should install larger jets. go to jetsrus.com .
 

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Discussion Starter #12
pods will make your bike run LEANER not rich. when installing pods and open headers you should install larger jets. go to jetsrus.com .
Yep, I agree. I misspoke. I moved the needle up one notch on the carb piston and it seems to be alright. I plan to look at the spark plug this weekend to see if there is any black residue.
 

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YOU NEED TO REJET THE CARBS. check out jetsrus.com. that site has helped me a lot. determine the stock jet size then go bigger. i forget the calculation. you can also drill-out your existing jets but you will need to know the exact drill size. put your needle back to stock position then rejet and go from there.

i got a free yamaha 550 and it started to run but then would sputter and bog. i figured out that someone installed TOO BIG of jets. i got the right jets and it ran exactly as fast as an evo harley. lol... a little 530cc bike and the harley guy couldn't pass me.
 

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One of the bad things about drilling jets is that a drilled jet can fool the next guy, or maybe you if you forget.
A selection of jets would be less likely to cause even more confusion. ;) And yeah, you'll need to tune (re-jet) for the pod air filters.

S F
 

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One of the bad things about drilling jets is that a drilled jet can fool the next guy, or maybe you if you forget.
A selection of jets would be less likely to cause even more confusion. ;) And yeah, you'll need to tune (re-jet) for the pod air filters.

S F
Wot SF said. Do not drill the jets. Get a selection of sizes. Main jets first. UK
 

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Discussion Starter #16
Fair enough, I've noticed a bog while it was cold, but got better as it warmed up. This is the first bike I've ever owned, so I have no real comparison. I will order a jet one size up right now.

I had read online conflicting information, so I went the easier route. I have heard from different people that just moving the needle up one spot will solve the problem. On the XS650 website is where I had read this. Regardless, it isn't too hard to change the jet so I will change it out and see if it is more powerful than before.
 

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Fair enough, I've noticed a bog while it was cold, but got better as it warmed up. This is the first bike I've ever owned, so I have no real comparison. I will order a jet one size up right now.

I had read online conflicting information, so I went the easier route. I have heard from different people that just moving the needle up one spot will solve the problem. On the XS650 website is where I had read this. Regardless, it isn't too hard to change the jet so I will change it out and see if it is more powerful than before.
You will get more conflicting info on jet sizes than most anything else. It often takes several sizes to get it right. Drilling a jet ruins it, and ruins your systematic approach. There is tuning advice on line by the pros. Diaphragm carbs can be a pain. Even spelling diaphragm is not easy. They do not crack open very well compared to earlier Mikuni type carbs. Raising the needle will get you about one size change in jets. They are listed as 117.5 120 122.5 125 127.5 etc. Not sure if that gap sequence is the same with larger jets, or with other brands. that is the sizing for my XS400. I use foam pod filters and aftermarket mufflers.

Bog when cold, and improving when warm, is a tuning clue. A bit lean. UK
 

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Generally pod filters will greatly increase air flow meaning you might need to make some fairly extreme jetting changes. However, I do not know how restrictive the original air box was on your 1978 so I don't have a good guess on jet recommendations.
When changing to a free flowing air induction system on many of the newer carbureted machines it is not uncommon to end up with main jets that are somewhere around 5 ~ 6 steps larger than stock. (At sea level).

S F
 
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