Yamaha Starbike Forum banner

2003 vstar 1100 starts and idles, but dies when throttle is applied

22804 Views 43 Replies 8 Participants Last post by  Valley Wrenchin
This should be an easy fix, but it's got me pulling my hair out (what's left of it) The bike is a friend of mines and I told him I would be happy to work on it for him. The story behind it is this, he was selling it after buying a new bike. So he went to fill the tank when someone called to come look at it and on the way, apparently he heard a pop(I'm guessing it backfired) add he was going up a hill, and then he lost all power. After getting the bike to my house, and looking into to it some, he didn't lose electrical power, the bike just bogs down when you twist the throttle.

I figured it was a definite carb issue, so I ordered some kits, removed the carbs, tore em down, soaked them, blew out all ports an ran carb cleaner through each one and reassembled them. After reinstalling, I still have the same symptom. I have removed the carbs 3 times now to make sure I didn't miss a port or something and have yet to fix it. I decided to go a different route and check everything that out U could think of. I installed new plugs, even after checking to make sure that both cylinders were firing and getting hot. Installed a new fuel filter and tried a different pump to make sure it wasn't a fuel pressure issue. Nothing has seemed to work.

After stepping back, scratching my head (reading a ton of forum posts) and thinking about it a little more, the issue happened after he was already riding and came on instantly. If it would have been dirty carbs, I would think it would have happened when he started it up, not while out riding. So I really do think it's not the carbs anymore, but maybe ECM related? What do you think? Anything else I might be missing here or not thinking about?

I hope to get this bike back on the road for him before spring!!!
1 - 20 of 44 Posts
A few things to check:
Fuel filter, replace, it's cheap. Something similar to this works fine.

Pilot jets, there are small holes on side of jets that must be open.

Either one will cause issues like you are having. The pop could have been a lean condition. The bike will idle with just the smallest amount of fuel being supplied. It's not uncommon for these issues to be quick to manifest. Could be some water in fuel. How long did bike sit? Keep us informed.
See less See more
Yep, thought I mentioned it, I did replace the fuel filter before testing the fuel pump. As far as pilots, when rebuilding the carbs, all jets were replaced, not just cleaned. Doesn't mean that there isn't something spok in the circuit, that's why that was my first guess, and the reason for checking it 3 times so far. The bike actually didn't sit for a period of time, it was his first bike that he put some miles on and then decided on purchasing a street glide.
You did mention fuel filter, I just missed that part. Possibly the backfire blew the sync port caps off causing a very lean condition. If it were mine I would carefully spray some brake cleaner around carb area. If idle increases you have a vacuum leak. Another Carb test, spray carb cleaner / brake cleaner into intake with air filter off. Does it rev more than it did? If it does the carbs still need some work. There are many ways to complete the testing I suggested, but they work if safely done.
Another thing that happened to me once. The hose going to the carbs had collapsed internally. it was letting enough fuel thru to idle but not enough to supply during acceleration.
KCW, going to steal one of your normal suggestions ?, replace air filter if not white. These bikes do not like dirty air filter. Many have solved simular issues with just a new air filter.
Quick question, was pilot jets replaced also or just mains? You said you bought "kit". It's uncommon for pilot jets to be included in "kits". Sample of most kits, no pilot jets included.

See less See more
Great suggestions, will certainly work on it this weekend. Yes both the main and the pilot jets in both carbs were replaced. I did not replace the air filter yet, it does look clean, but that can be deceiving so I will do that as well.
  • Like
Reactions: 1
he was riding, the bike went pop and now its not right

ditto vacuum leak

check the compression

make sure the air intake box is not split open

check the position sensor on the carbs, it tells the ECM how much to advance the timing
I did check compression and it checked good, I also did an ohm check on the TPS and it checked good as well. I will be checking for vacuum leaks tomorrow. I've checked all hoses previously for cracks and attachments and didn't see anything obvious, but I will check with carb cleaner while running, see if that shows anything. Thanks again for all the advice.
  • Like
Reactions: 1
Propane from an unlit torch also works, and doesnt spray a mess all over the carbs and tubes.
Propane from an unlit torch also works, and doesnt spray a mess all over the carbs and tubes.
Brake cleaner dries in seconds and actually cleans what it's spayed on. Plus it's something anyone who works on their vehicle should have, so it's a free test. Just my thoughts.
Ok, so unfortunately I haven't had time until just now to do a little work. I used a propane torch and went all around the boots, the carbs, both sides. No increase in RPMs at all. I did it first with about 1/4 choke on, then again with no choke, not that this makes any difference, but just want to be clear on the steps I am taking. Also with the choke on a bit, I can twist the throttle but after a couple of seconds it still bogs and dies. With no choke, it dies immediately. The airbox is attached, and no signs of damage at all, but the filter assembly is not attached. It seems to have no effect with it on or off, I have read these bikes are very finicky about having all the parts on though.
Repeatedly had the sync port plugs (vacuum caps) pop off, it was usually after a hard acceleration from partial throttle to full throttle already at speed. I don't remember a pop or anything, but things are loud and I wear full face. The sure sign of it is when you slow down to exit or pull up to a light...feels and sounds like you are running on one cylinder. After you get up in speed, it would run fine (mix would be enriched). I took the extra steps to always carry an extra cap or two, and zip tie them on.
I actually still have sync gauges hooked up, but even before attaching the hoses the caps were on, and still had the same issue. Good advice though. Happy for all suggestions!!
you said you changed the fuel pump with no improvement

have you pulled the fuel line to the carbs to see if there is flow and pressure into them?

you checked the throttle position sensor for the correct reading. Did you sweep the throttle and watch the reading change, and can you verify the cable / wires are connecting all the way from the position sensor to the ECM? If the sensor is working but the signal is not reaching the ECM, the timing will not advance when you hit the throttle.

Is there a way to see if the exhaust is plugged? These bikes do have catalytic converters in the factory exhaust. I had a cat fracture and turn sideways inside its can on a Saturn year ago. The car would not accelerate, it was like someone stuffed potatoes in the exhaust pipe.

Intake, exhaust, fuel, spark, timing, compression... if they are all good the bike has to run. Its not optional :^)

The only other thing I can think of is, if you are running the bike and you crank the throttle and it dies, pull the plugs immediately and look at them. Are they wet? Black and wet? bone dry and white... They should tell you something.
See less See more
Thanks for the reply KCW, yes I put the lines in mason jars and got good flow out of both sides. Fuel is definitely getting to the carbs.

You are correct, I checked the ohms reading, but not if it's getting to the ECM. I think I have the electrical schematics so should be able to check to see if it's getting back to the ECM, however I don't know how I can see if the adjustment is being made by the ECM.

I can look up into the exhaust, not sure if I can see the cat or not. However, I could probably pull the exhaust temporarily to see if there is a change.

Lastly, I've looked at the plugs, they didn't seem horribly black and wet, somewhat wet. They certainly weren't a nice tan color. Yes this shouldn't be too difficult, it's the computer's that throw a wrench in my works. I don't have a good way of seeing what the ECM is doing, can't find that dang OBD2 port ?
Another issue that came up once before (couldn't find link) was spark plug wire and boot issues. The previous issue was a misfire condition and turned out to be where the plug wire went into boot there was not a good connection stopping full voltage flow. Might be worth checking.

See less See more
If you took the quarter sized timing cover off the left side cover and put a timing light on one of the spark plugs, you should be able to see a timing mark at idle (or make one with a sharpie) and see if the timing moves when you bump the throttle.

I dont think an OBD2 reader would tell you much since its not fuel injected. I guess it might tell you the timing angle, Ive never tried that.

If the plugs are wet the carbs are rich, which does not make sense because you have to engage the choke a little to keep it running. Rich plugs means they are getting flooded, which could still be a carb issue, intake or exhaust flow issue, or the timing is so far off the mixture does not burn completely before the valves start to open.

You said early on the owner was riding the bike and then something happened, which would be a good thing, usually a single point failure is easy to isolate. Is there any chance the owner tried to fix the bike, messed with the carbs, left parts out, screwed something up, and (like many people) is too embarrassed to tell you what he did? It happens a lot, there are many motorcycles for sale that are "not running needs some TLC..." cheap because of this.
See less See more
Good idea on the timing suggestion!! When I get home tonight I will give it a shot. I will also take a pic of the plugs after bogging it down and try and post those.

I use a bluetooth OBD2 and with the Car Gauge Pro app, you can see what every sensor is doing, voltages, just about everything. You can even do buzz testing of injectors, which really helped me out on a diesel once. Anyways......

I don't think the owner asked on it, when he had the issue he was actually going to fuel the bike up because he was selling it, and someone was coming to look at it. He called the guy told him what happened and said he couldn't sell it like that. Then he asked me what I thought, and I told him it was his carb/carbs not knowing if the was more than one at the time. Months went by and I asked him if he'd had time to look at it and he said no, plus he wasn't sure of his ability to do it. So I said don't worry, I will come pick it up and take care of it. And now here we are....

I have another friend coming by, possibly today, to give me another set of eyes on it also, sometimes we get tunnel vision and can't see the obvious. I will keep ya posted!!
See less See more
1 - 20 of 44 Posts
This is an older thread, you may not receive a response, and could be reviving an old thread. Please consider creating a new thread.