Choke issues suspected help please

Yellow0206

Yellow0206

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So my 77XS400D project (made it into an OEish cafe racer) is largely complete and the bike is looking great but now am running into issues after a few rides.

Ive owned it less than a year and it was running well but not perfectly - had a high RPM miss.

I have cleaned the carbs twice and tried some jetting changes to fix the miss, but have a bigger issue now - the bike will start, choked or unchoked very easily but will not settle into an idle- stays at 3k rpm even with choke off - then dies after its warm and will smoke from exhaust on attempts to restart but not start - the smoke appears to be from unburnt fuel and i suspect a very rich mix and given that it starts unchoked and stays idling high I am thinking it may be that the choke circuits are remaining open.

What are your thoughts, experts and does anyone know how to rebuild the choke system and are parts available? I have found very little info on either.

Thks in advance for any help
 
Sounds like you over jetted it. Have you synced the carbs with a manometer? Stock air boxes on the bike? Make sure the idle mix screws have not broken off into the carb bodies. If you have aftermarket mufflers your going to have tuning issues. Sticking to stock or close to stock will make things much easier and your bike will perform much better.
 
Hi Chris and thanks for your response - rhe only jetting change i made was (by memory) changing from like 145 main to 142.5 so quite minor - also I am running stock air filter setup and in terms of mufflers i do have set of shortys but this issue occurs with shortys or stock. It such a “dramatic” issue that I am not thinking its just jetting - what do you think about what I said re choke? Im going to take carbs off again soon and am just preparing - I have not broken the bank the 2 times i cleaned - I just did (broke the bank and rebuilt) this to my (new to me) CB1100F so Im feeling more confident now....but the choke in the XS CV is not the more typical butterfly type so trying to understand it and wondering if I will be able to find the presumed rubber part that seals it internally...
 
If you have a choke leak the way to test is to raise the diaphragm slide all the way and put your finger over the venturi hole, the oval hole at the front of the carb mouth. If the slide fall fast there is a leak in the choke system or a bad diaphragm. The chokes system almost never go bad. What do the plugs look like? Have you set the motor valves,synced carbs, timed the ignition, set and cleaned the points, make sure the ATU/governor is clean and oiled, compression test the pistons? Lots of things that can cause issues.
 
Thks for choke info - bike was running well and issues started cropping up largely after i used regular gas which has ethanol and after bike sat dor a month or so with that gas - I discovered the availability of “rec” gas (no ethanol) locally and cleaned carbs for second time but bike has had this start but then die when warm issue since - of course it can be many things that goes without saying but im trying to isolate the issue. Ive inspected points and cleaned them - I have not got into the valves, I have tested compression which was at spec... i am pretty sure this is carb related as it seems highly unlikely bike would start well run well but then die (before its super hot which theoretically could cause a heat soak issue on electrical components) based on valve adjustment or electrical issue....I am unaware of the existence of a governer and dont know what an ATU is?
 
ATU is the mechanical advance unit. I would check everything that could be checked as I stated before to rule everything out. What was you cold compression numbers?
 
xschris said:
ATU is the mechanical advance unit. I would check everything that could be checked as I stated before to rule everything out. What was you cold compression numbers?
Click to expand...
Dont remember exactly - they were good though.

Took the plugs out again, quite black and wet - a high idle can also be an air leak - but I'm pretty confident thats not it - I was remembering that once before when i worked choke a little revs dropped - The choke plunger(s) sticking seems high likelihood - I'm gonna try that first.
 
Have you checked float height? Should be 26mm without the gasket. What pilot jets do you have? I would use 42.5 with the mix screws a 3 turns out. What is your jet needle set at? I would set it at the third from top or the middle. From pics of your bike you posted it looks like the bike had a set of mac mufflers on it before you put the short ones on. Those where not stock. The stock ones would have been much different.
 
Actually the longer ones i have may be from an XS360 - they do not look like aftermarket pieces - I am at 42.5 I believe and at proper float height however i don't think a couple of points one way or the other on the jets matters much until the bike will not die when warm - I have a bigger problem and will get the carbs off later this week and test my choke sticking fix theory after I finish up some work on the CB1100F - i rebuilt the 4 carbs on that thing including deracking them and got em back in and it started up and runs well - a friend is going to help me sync them with colortune so then I will also later sync the 2 XS400 carbs....
 
My buddy has 7 bikes ranging from 70s to 90s, is an engineer and has been working on bikes since 1985 so Im thinking he knows what he is doing I think he has the colortune and a manometer both but thks for the added input....
 
Awesome!! With his knowledge of bikes and engineering your issue should be solved in no time.
 
Well actually he's my Honda friend and the CB1100F.net forum is great too but looks like you are a guy really trying to keep this forum going - which is appreciated but I would just ask you to be aware that not everyone wants to run a stock bike and that not everyone is an imbecile - your comments seem to imply both a little bit - lol - but again your watching the forum and attempts to help are appreciated xschris
 
I don't try to imply that anyone is smart or stupid. I just try to answer questions on a model of bike I have owned for over 20 years and have five of. I have run them stock and very modified.I only convey what has worked the best for me and others over the 10 years of this forum. If running a non-stock bike is someones thing, that's ok with me. Most topics have been covered many times in the past and can be found in this forum. Just use the search function. The users manual has most of the technical data already.
 
OK Chris thks have a great Sunday and enjoy the upcoming start of the riding season - Im headed back to my garage trying to get both bikes ready!
 
Rechecked compression - 155 and 150 - when i took off carbs i relooked at intake manifold rubber boots and I dont think i inspected them carefully enough when i did carbs - they are quite dry, some cracks, and when i was starting bike earlier today it was running a consistently high idle, so now I am thinking I have leaks through those boots - hopefully a fresh set will cure the ills.....
 
Update: compression actually 170 and 175 with WOT -issues largely resolved - petcock was leaking and floats were not adjusted properly - rebuilt letcock and adjusted floats starting idling and running well. Been riding a bit everyday - its still cold though 45-50 is still too cold really! At higher revs like 7-8k its not pulling strong or real cleanly - i think its rich on the main jet as i had these issues before but worse in warm weather and it was better when cooler so since cooler air is denser charge more oxygen leaner i think I am rich? Any thoughts?
 
Colortune is really only for checking the idle mixture. I have one and Ive used it once before and its useless on a motorcycle when you can adjust the idle screw while youre on the bike at a stop light.

Colortune is useful for the old MGB I had.

Screw in the idle mix screws all the way, if the bike stalls youre fine. If nothing happens, then the pilot is too big.
 
New Haven Mike thks for chiming in - my issues are no longer with idling / low speed /pilot circuit and also on my carbs i have the semi fixed idle mix screws anyway (with the plastic pcs on them)

My issues are at revs above 6k when i open throttle a lot/quickly i get some “waah” not feeling good clean power.....
 
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