Butterfly valve not fully sealing causing high idle?

Hello, going to be a bit of a long one just to get the history of this bike written down.

I purchased my '82 XJ750 (or so I thought) last summer. The gentleman I bought it from never rode it and had it just to fix up as a hobby. He rode scooters and the weight of this thing scared him so he never actually rode it during the 1-2 years he owned it. He fixed up the paint, did steel brake lines and a few other things. He did not touch the carbs at all (or so he says but I'm inclined to believe him). It ran well but cut out as soon as I got to 4500+ rpm. I took the carbs off and cleaned the fuel jets and was back in business. I bought the bike to get around and as my only transportation I didn't have the time, nor the skills, to really dig into everything. So running alright was good enough for me.

A month later I rode the bike up to my hometown to visit my parents and on the way there noticed it was struggling to accelerate past 8000 rpm. It was also burning a bit of oil running at, uh, enthusiastic highway speeds for 5+ hours. When I got home I checked the plugs and saw that the bike was running a bit lean. Fast forward 2 months and I had the time to actually try and figure out what was wrong. The fuel levels were not at all in spec so I adjusted that and was good to go. I didn't do the best bench sync however so it was misfiring on cylinder 3 at low RPM's. I was planning on selling the bike to buy a 2008 FZ1 but the day before I was going to do so a taxi backed into it and knocked it over, bending the fork tubes, handlebars, and scuffing up some paint. The local shop I took it to (the only one in town that works on bikes over 20 years old) took nearly 3 months to straighten the fork tubes and touch up the paint around the exhaust. Liittle longer than I wanted but I had my FZ1 so just let them do their thing. Once I got the bike back it sat for awhile until I had the time to start working on it.

Now fast forward to a few weeks ago. Adjusted the valves, only 1 was slightly out of spec and thankfully I had the shims from the last time I checked em. Redid the fuel levels and saw that they may not have been as in-spec as I thought they were when I did them last. To be fair I did them on my knees in my driveway with them resting on a oil catch can, holding them level with one hand while my other hand held the tube and measured them, totally professional. After that was done I put the carbs back on and fired her up. Started quickly with the choke and then ran fine with the exception that the idle was slowly creeping up and eventually sitting around 3000 rpm with the idle screw completely backed off. So I ripped everything back off and decided to dive a little bit deeper into the carbs. I'm planning on selling this bike to a guy who wants to put pod filters and do a whole scrambler build so I'm not concerned with getting it running perfectly, just to a level I deem "good enough". Because of that I don't really want to do a full rebuild when I'm not going to be keeping the bike, as much as I'd like to.

Little piece of information you should know before I continue. During the summer I did the valves on the bike and noticed that the engine VIN did not match the frame VIN. The frame is for an XJ650 and the engine VIN is for an '81 XJ750. So this bike was originally an XJ650 but got a XJ750 engine swapped into it at some point. I knew there was an engine swap but didn't realize it was to a larger engine. I managed to get in contact with the original owner who actually lived only 20 minutes away from me. She was a very nice woman and even gave me the original saddle bags for the bike that her husband had made a custom rack for! Haven't figured how to actually put the rack on yet but that's for another time.

So back to the present. Digging into the carbs more I removed the air jets and realized that they are for an XJ650 not an XJ750. The air jets were #50 for the pilot and #205 for the main. The main fuel jet however was changed to a #120 with the pilot jet being a #43 (Spec if #120 and #40 respectively). There is a K&N air filter in the air box as well as a 4-1 exhaust so I'm not sure if that has anything to do with the change to the #43 pilot fuel jet or for the air jets. If so please let me know I don't want to change em out if they are there for a reason.

The other thing I noticed was that the #4 butterfly was not fully seating, it was touching in the top left and bottom right corners and was showing quite a bit of light around the edges. My thinking, not at all someone who fully knows what their doing, is that despite my bench sync once the carbs heated up at all the #4 carb butterfly expanded and was holding the others open leading to a high idle. When I revved the bike it didn't hang so I don't think it was because of a lean idle. I did adjust the #4 butterfly using the screws and got it to seal a bit better but not perfectly. I only spent about 10 minutes fiddling with it so there may be room for improvement.

Here is a photo of what the #2 or #3 carb butterfly's look like and then the #4 post fiddling.




Still room for improvement with the #4 but that's for later this week.

So finally to my questions. Is it likely that the #4 carb butterfly was causing the high idle? Will the air jets screw things up that badly or be offset by the air filter/exhaust? Any other things I should worry about?

Thanks if you read the whole post but even if you didn't thanks for the help.

 

carbs should be jetted for the 750 motor
XJ750 Models:

Years & Models: 1981-3 XJ750 Maxim, Midnight Maxim, and Seca USA models
Carb Manufacturer: Hitachi
Carb Series: HSC32
Carb Model ID: 15R00 (Maxim, Midnight Maxim)
Carb Model ID: 5G200 (Seca)
Main FUEL Jet Size: #120
Pilot FUEL Jet Size: #40
Main AIR Jet Size: #80
Pilot AIR Jet Size: #225
AIR COMPENSATOR Jet Size: #36
Starter FUEL Jet Size: #40 (non-replaceable, in float bowl)
Main jet NEEDLE ID: #Y-13
Main needle JET Size: #3.2mm (the main needle JET is also known as the "power valve" or "emulsion tube").
Fuel Level in float bowls: 3mm +/- 1mm (.118" +/- .039")
Idle RPM's: 1,050 rpms

go back to a oem style paper filter

you will also want to look at the slide needles to see what motor they are for Y-13 is the 750 Y-10 or 12 is 650

the exhaust may require a jet increase on the mains and pilot fuel jets

 

Yeah I was thinking that's what I'd have to do, probably end up getting jets for the stock bike as well as bigger sizes for the exhaust and compare. The slide needles are Y-10 so yeah someone definitely didn't do the conversion right, or at the least just did the fuel jets but I'm still not sure why they decided to go with a #43 for the pilot. Could they have tried to keep the smaller air jets and kept the fuel jets in order to compensate for the exhaust and intake? Sorry for my lack of knowledge but what changes between slide needles? Why change back to paper as opposed to just getting bigger jets? Also, besides the jet issues any tips for the butterfly's? Thanks and sorry for the long list of questions lol

 

I'm thinking that the combination of the Intake, Exhaust, richer needle, and smaller air jets kinda cancel each other out to an extent. The bike was running alright, maybe a little lean so a slightly bigger air jet could fix that. The only major issue at the moment is the butterfly I believe. I think I'm going to try and get the butterfly sitting right and see how it runs.

 

I think once you do a good sync on her she will run better. Keep up the great work and posting the good pics.

 

Regarding the butterfly valve, when I was reassembling my carbs, I noticed that the valves have a 'right side up' to them. If I put them back in upside down, it didn't sit right. Rotate 180 degrees and all was fine. It was easy to tell front vs. back, and I'd backed all the parts together to the correct carb body, so I know those weren't issues.

No idea if that's what's going on, but it's a 5 minute experiment that doesn't require disassembly. Just don't lost any screws..

 

It may be as simple as loosening the screws and open an close the throttle to let the butterfly find its way back and retighten the screws.
The jetting will probably need to go up one size on both once you have the correct air jets installed.

 

This was my first time ever touching the butterfly valves, I did take the #4 out and flip it 180 degrees to see if that was the issue. It ended up sitting even more not right haha.

The photo of the #4 butterfly is post loosening, opening/closing, and re tightening. I'll probably go back and see if I can get it closer as I only fiddled with it for about 10 minutes. I know this may be a naive question but what is the difference between going up a in the fuel jets and going down in air jets? My understanding is that more fuel = richer and less air also = richer. Once I get the butterfly sitting right I'm going to see how it runs, if its still not running right I'll probably end up getting all the right jets into it.

Would filing the holed in the butterfly's a bit bigger for more adjustment cause any harm as long as the screws still fully cover the holes?

 

your pilot jet is the proper size for Canada also notice the change in air jets for Canada
also different piolet air jet for you
I missed that you are from BC

Years & Models: 1981-3 XJ750 Maxim, Midnight Maxim, and Seca Canadian models
Carb Manufacturer: Hitachi
Carb Series: HSC32
Carb Model ID: 15T00 (Maxim, Midnight Maxim)
Carb Model ID: 5H200 (Seca)
Main FUEL Jet Size: #120
Pilot FUEL Jet Size: #43
Main AIR Jet Size: #80
Pilot AIR Jet Size: #195
AIR COMPENSATOR Jet Size: #36
Starter FUEL Jet Size: #40 (non-replaceable, in float bowl)
Main jet NEEDLE ID: #Y-14
Main needle JET Size: #3.2mm (the main needle JET is also known as the "power valve" or "emulsion tube").
Fuel Level in float bowls: 3mm +/- 1mm (.118" +/- .039")
Idle RPM's: 1,050 rpms

 

if you can move the left side of your butterfly up it will give you the cresent light you want to see at the bottom . you may need to adjust the idle screw in the back of the carbs to give you the room to move them.

take a look at the carbs in this link notice the distance from the large hole (enrichment port) in upper left compared to yours.
https://www.xjbikes.com/forums/threads/owww-my-eyes.112790/

 

you will be doing a final adjustment of the butterfly to just expose the front small hole of the 2 holes close to each other deepest in the throat