Bike ran fine, now BARELY running. Help please?

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AaronKyle432

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Hey everyone, I need some assistance.

Here is the backstory: I rode the bike 55 miles 8 days ago. Ran perfectly fine, other than it backfiring quite a bit when I let off the throttle. Started the bike 5 days ago and let it idle, started and idled perfectly fine too. All that I have done to the bike is cut off the exhaust right behind the foot pegs, and stock air boxes. Before the exhaust was cut off, the carbs were synced.

Now today, I go to start it and it won't start. It won't start will full choke, or half-choke either. It will only start with no choke, and if I have the throttle at about half way, and even then, it has a hard time revving up, but won't idle at all, even with adjustment of the idle screw. Took the carbs and took them apart. Blasted the holes and parts with air, but didn't soak them in carb cleaner or anything. Put them back on and still no progress at all. I sprayed some carb cleaner into the carb with the air box off and it idled fine and ran good for about 15 seconds, then went back to not running.

I took the plugs out and they were solid black. I bought some new plugs that were 1 step hotter. Bike fired right up and idled fine. So I took off on a ride. Started off fine, but quickly went downhill. It started to act like it was being starved of gas. I had it at full throttle and it was only going 40mph and barely chugging along. Sounded very bad too. Got home, and it turned it died. Tried starting it and it wouldn't even start unless i had the throttle open again. I took the plugs out and they were just a light brown/tan color. Not black, so I'm kinda stumped on what it could be now?

Do you think it could be the carbs need to be cleaned really well/synched again/re-jetted from the exhaust? Ignition timing? Something internally? Help please. Thanks
 
You need to rejet. You really need some backpressure also. maybe some shorty mufflers. Go up a size on tye pilots, 45. I would start with a 140 on the mains.
 
Any time you change the back preasure or air intake by obstructing air flow or free flow you will need to make jetting changes, needle adjustments, and air mixture adjustments. My guess is your extramly lean. I wouldn't run hotter plugs in these old engines.
 
Let's see those plugs (picture of the black ones please).

You might be totally fine moving forward, but you have some work ahead of you before you can get back to riding. Depending On how your plugs look, you might be worse off, though....

...By cutting off the muffler, you have caused your bike to run leaner relative to its previous tune. Lean means hot. Hot can spell engine wear, and at a much accelerated rate. I'd have to say that 55 miles might have been enough to cause damage to your top end and potentially cylinder walls and piston rings. Black plugs usually indicate oil in the combustion chamber, which can be caused by worn rings or cylinder walls, or perhaps a worn valve allowing top end oil to seep past (not closing all the way). My worst case scenario would be that running your bike hot for 55 miles has caused engine damage that lead to your plugs fouling out on oil, and eventually, a break down.

If the plugs are not clearly roasted, then you might be OK, in which case, as Bamabill said, you will need to rejet. Search the forums for jet or jetting or exhaust jet. Your search should lead you towards a solution to your lean issue.
 
Here are a picture of the plugs. First picture is of the plug that I rode about 55 miles on. The 2nd picture is of the one step hotter plug I got and rode about 4 miles on.

Also started up the bike this morning and it idled fine, and reeved up fine. Didn't act like anything was wrong, but I didn't ride it. Could have been because it wasn't completely warm, or I wasn't putting strain on it from the carb from riding it.
11909305363_97571110f4_c.jpg

11909014885_3ac8cff029_c.jpg
 
Your engine is definitely running lean by the color f the one plug at least on one side.The other plugs color doesn't look that bad,except down by the insulator on the plug looks like too much fuel or oil burning.
 
You may have gotten away with it, confirm it with a compression test.

Now, I would advise you to reject your carb. You will need to go at least one up pilot and main. My guess would be two up main, one up pilot. However, could be a good idea to buy a 3 or 4 sizes up from. Where you are to dial it In.

I have little emgo shorties on my bike, take the edge off of an open header, but probably not a lot mor restrictive than open header, but still quite loud. I am two up main, one up pilot, with the idle screw just about one full turn out,
 
I already ordered new plugs. 45 pilot, and a 140 and 145 main jet to kinda trial-and-error on which one works best. It's just weird how it started up fine this morning and idled perfectly, but it wouldn't even do that yesterday. I'm just still stumped that it will start up and idle fine, I'll take off for a ride and it'll ride fine for about half a mile, then it starts to kinda sputter, especially in mid-range, then about 3 miles into a ride, it's having the hardest time holding 40mph while I have it pinned, then it won't even idle when I come to a stop after the ride
 
I took the carbs apart today. Everything looks alright, but the Air/Fuel mixture screw in the left carb is broken off. The tip is stuck down where it is barely visible, but it is stuck where I can't dislodge it. I don't feel like that would be my bike issue, because it has probably always been broke, but ran fine. Could the vibration of the bike with really short exhaust have messed something up?

Waiting for the new jets and then will reassemble the carbs and see if it doesn't run like a piece of crap
 
Broken tip happens when the mix screw is over tightened. You need to fix this to properly tune your bike. As it is currently, the idle mixture is set as LEAN as it possible can be. This in conjunction with improper jetting for your exhaust system can, as stated before, cause real long term damage to your bike.

Weak mid range would also indicate lean mixture, and at the midrange, it's the mains that need to be providing more fuel.

I know you must be excited and exhilarated working on your bike, but you really ought to fix your lean issue before you continue to ride, or risk much more work than you currently have on your plate.

Cheers and good luck
 
Sounds like your on track with the jets you have ordered. Like Tombo said, go back and order new idle mixture screws. I didn't realize mine were bad till I changed them out, I ordered new ones with my jets. I'm running stock filters without the boxes and Emgo shorties, 45 pilot jets, 140 mains. My air adjustment screws are at 2.25 turns out.
You should be able to get that tip of the screw out, use compressed air in the passages and see if that doesn't blow it out, or a can of carb cleaner with the straw and shoot it up the passages.....it has to come out. Bet it's a huge part of your problem.
I doubt the vibration would have much to do with it. You should be fine if your running just the headers, but you would want a mount if you attach a muffler....bet it is scourching loud!
 
Thats a broken tip brah.

I had the same issues, my bike always ran and then i discovered the tip was busted and thought it couldn't be the problem since it was running the whole time.. I was wrong, just like you're wrong lol

That tip is one of the issues, you might have got away with it with mufflers on, but now you have cut them off and took away the backpressure, and its sucking your engine dry and you need to get that tip out and replace it to get the proper fuel/air mix to the engine.
 
Be extremely careful when removing the broken tip,most people try and drill them and end up ruining the carb body.I would do a search or seek good advice on how to remove it.Until the broken tip is removed from the carb you will continue to have tuning issues.The broken tip explains why the one plug was so clean like as mentioned that carb was starved for fuel.
 
A dental pick, available cheaply at surplus stores, may also work to push the broken tip out. Just don't try using an offshore manufactured pick, get the good ones. Dental picks come in handy for many things, not just picking at your teeth!
 
Does anyone have a link where I can buy one of the air/fuel mixture screws? I have a '77 XS400 also.

Thanks for all the help guys, I appreciate it. When I get all the parts and get it back assembled, I'll definitely clarify what seemed to be the problem
 
NEED SOME HELP!

I rejetted my bike, went 1 up on the main, and 1 up on the pilot jet. Also fixed my pilot screw for the A/F mixture at idle, BUT IT'S STILL BOGGING DOWN! It will start up with full choke, warm up, I take it off choke, then it will idle for like 30 seconds and then die. I rode it down my driveway like 100 yards and back and it was still doing the same thing as a few weeks ago. Take off fine, about mid-range throttle, it would bog down and sputter, and then with full throttle, it was barely going. And it was doing that in all gears. I took a picture of the spark plugs after that little trip down the road and this is what they looked like. Left plug is left cylinder, right plug is right cylinder. Possible carb sync problem, or what? So confused.
12014132836_0311663424_c.jpg
 
The lighter plug on the right makes me suspect your timing is off. Do you have points ignition? The color change on the strap should be on the bend, not at the tip.

I would re-time the bike.

Of course you're fouling out on the left plug, but you need to deal with the timing first.
 

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