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Boy this one sure has me stumped..

Bought my 3rd bike recently, a 1977 Yamaha xs750 and it runs. However:

Problem: Bike starts up no problem with choke on, even in this cold December weather. But after letting it run for awhile to warm up, the RPM's slowly start to rise until it gets to an idle of about 2,300. If I take it out for a spin it gets much worse. After a few miles or so it will immediately rev even higher after pulling in the clutch or putting it in neutral. It really freaks me out and I'm sure it is approaching redline revs. It probably rev's to about 4-5,000 without giving it any throttle. I don't know the exact rpm's because I only recently threw a tachometer on it (it didn't have one when I bought it).

Here's what I have done:
The carbs have been completely taken apart and cleaned thoroughly. There was a lot of gunk but overall everything was still in good shape. While I was at it I rebuilt both of the petcocks, cleaned the inside of the gas tank, and put in new fuel filters.
I tried to check if there was a vacuum leak in the carbs by spraying around them with carb cleaner and there is no leak at the carb boots, but I did notice that there was a change in idle when I sprayed around the intake boots. It only made a change in idle when I sprayed with carb cleaner though, and not when I tried it with WD40. So I'm not sure if there's an issue there or not.
When it idles or when I'm riding it I do notice that there are sometimes backfires/popping noises going on.
I have not messed with the idle screw yet.
I have not adjusted the ignition timing yet.
I have not yet adjusted the jet needle in the carbs.
At first glance I do not see anything that is obstructing or messing with the throttle cable.
The oil is just about black and needs to be changed but I'm waiting for a new filter to come in the mail.


Bike specs:
1977 Yamaha xs75 3-cylinder DOHC.
Exhaust is choppped about halfway from stock (it came that way when bought...)

Thanks guys.
 

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Carb cleaner is highly combustible, WD, not so much. High or hanging idle is usually an air leak, often at the intake boots, so that may be your problem. Look closely for cracks, and that the carbs are fully seated.

Just to clear up definitions, by intake boots, I mean those that bolt to the head. The other boots connected to the carbs I call air filter boots. Which did you spray to get a change in idle?
 

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Discussion Starter #3
Carb cleaner is highly combustible, WD, not so much. High or hanging idle is usually an air leak, often at the intake boots, so that may be your problem. Look closely for cracks, and that the carbs are fully seated.

Just to clear up definitions, by intake boots, I mean those that bolt to the head. The other boots connected to the carbs I call air filter boots. Which did you spray to get a change in idle?
Sorry you're right I might have called them the wrong name.

So, I sprayed both the intake boots (that bolt to the head) and I also sprayed the filter boots. It was the filter boots that changed the idle when I sprayed them. But I am confused because if the WD didnt cause an idle change and the carb cleaner did, then dont you think that some of the carb cleaner is just getting sucked into the actual filter and going into the engine? Which would be normal.. These are new aftermarket air filter pods by the way. The point of spraying boots to test for a vacuum leak is to just temporarily fill in those potential cracks to notice a change in idle sound, right? So if it was a vacuum leak wouldnt it have the same result with carb cleaner and also WD?

Thanks
-jh
 

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Make sure the boots on the pods are not diverting air flow away from the air jet holes that are in the throat of the carbs. This is a fairly common issue, where the new boots are just a bit narrower than the stock filter boots, and disrupts the mixture at various points in the rpm range.

Leakages around the air filters wouldn't normally cause a big change in idle characteristics, unless the filters are rather restrictive. But, yes, changing the mixture with another 'fuel' will affect the idle, if it is getting drawn in.
 

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Being an older bike, and not running for awhile, did you overlook the obvious? Lube the throttle cables, and check for some slack in the cable.

The only reason I suggest this is because I went thru the same thing awhile back. 2500 to 3000 rpm at idle. Made me go crazy until I discovered I had a very tight throttle cable.

Popping and back firing? Timing rarely slips, but you have know idea what the PO did, so check it. Also check the plugs, coils and plug wires. After I replaced my plugs and wires, my backfiring went away.

Ain't old bikes fun?
 

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JaronHall... I'm sorry if this question sounds stupid, but you gave a really nice, detailed description of the problem and what you have and have not tried. But I read your original post twice and have to ask... Are you turning off the choke once the engine warms up? I ask cuz you said it starts up fine with the choke on in UTAH in December, but you never said you turned the choke off when it warmed up.

If I assume that you are turning off the choke and the bike still increases rpm as it gets hotter, then I agree with the other members. There's an air leak somewhere.
 

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Choke

To follow Eye's question.
Is your choke like the one on my XS11, that has a lever that gets pulled away from the bike?
On mine, it is a two position lever. All the way out for cold, once the engine has warmed it gets moved in to the next position. My bike will rev high in this position.
It is like, all the way out is all choke. The next position is a bit of choke but mostly fast idle. So Eye's question is valid if your choke is like that.
If it is a cable controlled choke like my other Yamaha, then I just push it back a bit at a time as the engine warms. And I would also check that cable to be sure the choke is returning properly.

Unkle Crusty*
 

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WHoa... I just re-read this thread, and paid particular attention to Jaronhall's 2 posts. He has cleaned the carbs and done everything to improve fuel distribution. BUT, in his first post, he states that the bike came with the exhaust cut off (No mufflers). In his second post, he states he installed aftermarket air pod filters with no airbox.... Could we be looking at a re-jetting issue here??

WintrSol... what are your thoughts on what I'm thinking??
 

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Thoughts

I noted the pod filters.
But, on my XS400 with stock filter and air box, with no filters, and later with EMGO pod filters, the idle did not change.
The WOT power did change. My next job is to make velocity stacks. I have heard mixed reports on pod filters on diaphragm carbs.
And definitely, at least the main jets will need to be changed with the filters, and the pipes.

Unkle Crusty*
 

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No mufflers and pod filters often creates a very lean mixture; I suppose that, if the combustion chambers got too hot because of this, the idle would hang after a long enough ride. Time to have a look at the spark plugs.
 

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Discussion Starter #11
JaronHall... I'm sorry if this question sounds stupid, but you gave a really nice, detailed description of the problem and what you have and have not tried. But I read your original post twice and have to ask... Are you turning off the choke once the engine warms up? I ask cuz you said it starts up fine with the choke on in UTAH in December, but you never said you turned the choke off when it warmed up.

If I assume that you are turning off the choke and the bike still increases rpm as it gets hotter, then I agree with the other members. There's an air leak somewhere.
Yes I remembered to turn off the choke ;) And to answer the other question, yes it is the kind of choke that you pull away from the bike with two settings.
Also, I did buy new air filter pods to put on the bike, however it already had pods on it when I bought it, they just weren't in the best shape.. so I dont think the pods have much to do with it..

UPDATE!!:
So I took out the spark plugs and they were as black as could be. This of course might mean it has a rich air fuel mixture, but since the bike is new to me I figured I would just get new plugs anyways. So I got new plugs today, set the gap, put them in, fired up the bike and it idled high again as expected, but when I took out one of the brand new plugs it was black again. And it only ran for like 30 seconds. So I am definitely running a rich mixture...
Today I also decided to check out the ignition timing just to make sure it was all set up correctly by the PO. The timing was in fact a little off, and the gaps were not set correctly. There was also some corrosion on the points so I sanded them down and set the correct gap. HOWEVER, right before setting the points I accidentally broke off one of my header studs... so I cant start the bike without the exhaust on and expect to get some good results... so it looks like I gotta wait for that header stud to arrive.. I did see inside the header though that there was a lot of dark black soot, which is another sign i'm running rich...


Can improper ignition timing make an engine run rich or produce running rich symptoms?
 

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No, timing will not make an engine rich; over-filled carburetors will, or a blocked air filter, stuck choke, blocked air jets ...

One note: you should not use sandpaper or emery cloth to clean up points, that's what point files are for. Sandpaper can leave grit embedded in the surface, which makes for poor performance.
 

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Floats

Jaron.
Your carburetors are probably similar to the 1100 engines of 78 to 83.
They have a site XS11.com and a bunch of guys that are good on the technical stuff. I have read reports there, that some of the floats and needles do not work correctly. In fact my XS11 had new floats installed by the PO. And they would leak. Carburetors is a common subject for the 1100 and the 400 cc bikes.

Too rich would normally result in lumpy running. An air leak is usually the culprit with high revs. But I would check the choke to be sure it is closing properly, the floats again, and a check for air leaks. We can assume that the bike did run properly at some point. So something has changed by itself, or something changed during repairs.

Unkle Crusty*
 
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