Jump to content

Idles fine but dies upon throttle


ajc99z
 Share

Recommended Posts

The bike hasn't been running to my knowledge for 4-5 months. Prior to my purchase the previous owner, a friend, said he cleaned the carbs. Basically, she'll start up fine and idle around 1500-2000 rpm (full choke) but upon going to 1/2 choke or giving her any throttle, she dies. I assume it's due for a carb adjustment of some sort, but I need a point in the right direction.

Thanks!

Link to comment
Share on other sites

Start by getting a Colortune kit and check your air/fuel mixture.

Link to comment
Share on other sites

if i had to guess, I'd say you're getting way too much air and not enough fuel. take the carbs off & blow everything you can find out with shop air, wouldn't hurt to check the float levels.

easiest way to see what is happening during idle is to just take the spark plugs out. are they white? tan? black? wet and black? that means, very lean, happy, rich, omgwtf not firing at all rich.

could be an air leak maybe? take a look at the plugs, that'll give you a start. maybe feel (close not touching) the exhaust pipes to make sure that both sides are running.

I'm no expert, I'm just (finally) starting to get mine tamed. Now its a voltage regulation problem for me :unsure:

Link to comment
Share on other sites

if i had to guess, I'd say you're getting way too much air and not enough fuel. take the carbs off & blow everything you can find out with shop air, wouldn't hurt to check the float levels.

easiest way to see what is happening during idle is to just take the spark plugs out. are they white? tan? black? wet and black? that means, very lean, happy, rich, omgwtf not firing at all rich.

could be an air leak maybe? take a look at the plugs, that'll give you a start. maybe feel (close not touching) the exhaust pipes to make sure that both sides are running.

I'm no expert, I'm just (finally) starting to get mine tamed. Now its a voltage regulation problem for me :unsure:

I had this on my sr500,i cleaned the carb out to find the main jet was blocked.it would idle on pilot jet,but die on throttle.

hope this helps

Link to comment
Share on other sites

The bike hasn't been running to my knowledge for 4-5 months. Prior to my purchase the previous owner, a friend, said he cleaned the carbs. Basically, she'll start up fine and idle around 1500-2000 rpm (full choke) but upon going to 1/2 choke or giving her any throttle, she dies. I assume it's due for a carb adjustment of some sort, but I need a point in the right direction.

Thanks!

If its like mine, your pilot jet(s) are blocked and your floats are set wrong.

Link to comment
Share on other sites

  • Moderator

Idles fine but dies upon throttle.....

1st thing that ALWAYS comes to mind........dirty or plugged Pilot Jets........cleaning 'em should make quite a diff. ;)

Link to comment
Share on other sites

Got her running again after work today. Turns only the left cylinder is firing. The left spark plug tip is completely black. The right spark plug is fairly clean and smelled like gas, makes sense huh.

Went to Autozone and got some replacement plugs and carb cleaner. Looks like the carb holders (rubber boot between carb and engine) are dry rotted and may be letting in air. Not 100% sure as i haven't pulled the carbs yet, that'll happen tomorrow. Appreciate all of the tips, will keep everyone posted.

Andrew

Link to comment
Share on other sites

Got her running again after work today. Turns only the left cylinder is firing. The left spark plug tip is completely black. The right spark plug is fairly clean and smelled like gas, makes sense huh.

Went to Autozone and got some replacement plugs and carb cleaner. Looks like the carb holders (rubber boot between carb and engine) are dry rotted and may be letting in air. Not 100% sure as i haven't pulled the carbs yet, that'll happen tomorrow. Appreciate all of the tips, will keep everyone posted.

Andrew

Hey Andrew (I'm over in Athens, by the way)..

I'm still betting you've got carb problems. My bike strangely seems to idle and rev fine on only my right cylinder when its cold... still haven't pinpointed that problem yet, but there's no on-throttle dying even if I pull the plug and everything for my left cylinder.

Anyways, I'm new at this... but the pilot jets control only the idle, right? Seems like those might be OK for you. But worth looking at and cleaning if you take the carbs apart.

The round slide, followed by the needle, needle jet, and then main jet will control fuel when you rev it. The round slide seems pretty stout, but the diaphragm that controls its operation could get a tear in it and cause it not to operate right. There are lots of pinholes in the needle jet that clog up easily. The main jet is a pretty big hole that wouldn't clog too easily, and mainly just limits the flow at full WOT.

Anyways, I'd first look at the fuel level to double check that first, plus its easy to check without disassembly. You could just be trickling just enough fuel in there to idle and draining the bowl instantly as soon as you rev it. I'd just hook a piece of tubing to the bowl drain, and string it up the side, open the drain, and watch the fuel level in the tube to make sure it stays at a decent level. (as was recommended in my last thread)

Link to comment
Share on other sites

looks like a good tool but i'd have a hard time justifying the $60 expense. hopefully some carb gurus can chime in with educated guesses.

Can be had for cheaper.

Also, it's far cheaper than paying a mechanic with the right equipment to precisely set your mixture.

Link to comment
Share on other sites

Got her running again after work today. Turns only the left cylinder is firing. The left spark plug tip is completely black. The right spark plug is fairly clean and smelled like gas, makes sense huh.

Went to Autozone and got some replacement plugs and carb cleaner. Looks like the carb holders (rubber boot between carb and engine) are dry rotted and may be letting in air. Not 100% sure as i haven't pulled the carbs yet, that'll happen tomorrow. Appreciate all of the tips, will keep everyone posted.

Andrew

Well, first problem is only one cylinder is firing. Did the new plugs help?

If the left side is completely black, she's running too rich, no air leaks on that side likely haha. A couple things affect how rich/lean the bike runs at idle, easiest thing to check/adjust first is the float level. New idle mixture screws can be purchased from mikesxs.com (look for parts for the 650, our shared carb is the BS34).

Doesn't sound like you have an air leak in the boots, but wouldn't hurt to coat them in silicone as a short term improvement.

Carb cleaner will get rid of some varnish, but if you have any ports plugged what you're probably going to need is access to shop air. Nothing like 60 or 80 PSI blasting through a port to knock out anything crudded up in there!

Don't leave out any details of what you've looked at or changed, anything could be a clue.

Is your battery fully charged, and the bike's charging system working? If the battery is a bit low, perhaps the right side is having hard time firing reliably.

Link to comment
Share on other sites

Hi everyone, I'm the new kid on the block. I bought an 81 XS 400 Special II last week to use as a learner bike. (I take my MSF next month but have been riding on a learners permit for almost a year) It had been a swap meet find by the previous owner who had done little with it outside of knowing that it had ran the last time he had fire her up. We had similar problems with it in that once we got her running she was only hitting on one cylinder. My husband (yeah, I'm a lady rider) tore into the carbs and the left pilot jet was plugged solid. After removing it, he blasted it with carb cleaner and carefully reamed it out with a thin guitar wire. That problem was solved. We still have a multitude in the wings waiting to be worked on. It's running unevenly at high RPM (anything above 4500 rpms) and has some electrical issues that are probably minor in nature. We are going to be checking the diaphragm for problems next.

Otherwise, this has the potential of being a truly great bike to learn on as I am a petite gal and I will post some pictures in the future of the restoration process as I do all my own restoration, body work, painting, customizing, etc.

Oh, btw, I'm a 55.5 year old nurse from north eastern Missouri, USA. We have 8 Yamahas in our stable. One RD350, One TTR-125, two SR500s, one XT500, one Virago 700, one RZ350, and the newest, (new to us) the XS400 Special II.

There are also a few Hondas lurking around the place, three VTR 250s, two scooters and a CRF 150.

What can I say, with the exception of the dirtbikes, we are suckers for hard luck classics. ;)

Link to comment
Share on other sites

Well, first problem is only one cylinder is firing. Did the new plugs help?

If the left side is completely black, she's running too rich, no air leaks on that side likely haha. A couple things affect how rich/lean the bike runs at idle, easiest thing to check/adjust first is the float level. New idle mixture screws can be purchased from mikesxs.com (look for parts for the 650, our shared carb is the BS34).

Doesn't sound like you have an air leak in the boots, but wouldn't hurt to coat them in silicone as a short term improvement.

Carb cleaner will get rid of some varnish, but if you have any ports plugged what you're probably going to need is access to shop air. Nothing like 60 or 80 PSI blasting through a port to knock out anything crudded up in there!

Don't leave out any details of what you've looked at or changed, anything could be a clue.

Is your battery fully charged, and the bike's charging system working? If the battery is a bit low, perhaps the right side is having hard time firing reliably.

Got the carb torn apart today. Main jet on the firing cylinder was clogged, which i think explains the "die upon throttle" symptom. Both main nozzles were dirty but it didn't appear that any of the tiny holes were clogged. One of the needle valve o-rings was almost completely worn away. Haven't been able to remove its counterpart just yet, can't find my needle nose pliers.

Think i'm going to breakdown and purchase some new carb holders. There was some minor cracking on the inside of one of them. $39 is the best deal i can find for the pair, including gaskets.

I didn't test the new plugs before breaking down the carb. But when the carb is reinstalled, ill install the new plugs and swap coils in order to determine if they are part of the issue.

Not sure why i'd need new idle mixture screws? To be certain, these are located under the butterfly spring assembly with the heads facing one another (making it a PITA to adjust)?

The battery isn't fully charged. I was kick starting it, figured i let it idle long enough to get a charge, but i'm picking my battles one at a time.

Link to comment
Share on other sites

Not sure why i'd need new idle mixture screws? To be certain, these are located under the butterfly spring assembly with the heads facing one another (making it a PITA to adjust)?

No, the mixture screws are on top of each carb between the slides and the carb mount, and on 80 and newer they are under a brass plug which needs to be drilled out to get to them. I would suggest changing them, along with the o-rings that go around them as they are probably pretty gone and idle mixture tends to not adjust right without good o-rings.

Link to comment
Share on other sites

No, the mixture screws are on top of each carb between the slides and the carb mount, and on 80 and newer they are under a brass plug which needs to be drilled out to get to them. I would suggest changing them, along with the o-rings that go around them as they are probably pretty gone and idle mixture tends to not adjust right without good o-rings.

1/4" drillbit did the deed perfectly for me if I remember correctly. Nothing I did could get my right cylinder to idle correctly, always way too lean. New screws and bam, nice light tan sparkplug.

Might want to double-check you have 14.5 or 14.7 or whatever the recommended voltage is across the battery terminals while you're idling. My voltage regulator died, I wasn't getting a charge and it idled progressively wierder and wierder as the battery slowly died before I figured out what was going on.

Link to comment
Share on other sites

1/4" drillbit did the deed perfectly for me if I remember correctly. Nothing I did could get my right cylinder to idle correctly, always way too lean. New screws and bam, nice light tan sparkplug.

Might want to double-check you have 14.5 or 14.7 or whatever the recommended voltage is across the battery terminals while you're idling. My voltage regulator died, I wasn't getting a charge and it idled progressively wierder and wierder as the battery slowly died before I figured out what was going on.

Got them drilled out yesterday. As Cy predicted both o-rings were gone. Adding 2 new screws to the list. Hoping the keep this rebuild under $40, we'll see.

Thanks for the tip on the voltage regulator. Sounds like that will be a good place to start.

Link to comment
Share on other sites

Ahh, I fed you incorrect information :)

Mine only has about 12.7V across at 1250RPM, at 2500RPM you should have between 14 and 15VDC. Apparently at idle it doesn't really charge teh battery, just keeps it from discharging.

Make sure you find your correct float height. Diff sources have slightly diff measurements for diff years, mine being an 81 special II (XS400H) has a measurement between 25.7 and 27.2 mm depending upon which source I read if I remember correctly, I just set it for 26mm and man did it make a difference. New battery, correct float heights, new idle screws = wonderful bike :)

The way I've read to set the idle screws is to back them out until the bike stops increasing in RPMs, then turn them back in until the RPMs drop just a hair, then turn them back out just a tiny bit, apparently leaves them a little on the rich/safe side at idle. Do that for both sides, dropping the idle via the adjustment screw as necessary, until both are dialed in nicely. Mine has a little burble, maybe something is just a hair rich, but I figure better a hair rich than a hair lean. Plugs look happy, bike runs happy, so I'm happy ;)

Link to comment
Share on other sites

  • Moderator

set mine at 26mm and it was too rich. so i got them at 28mm. Haynes wanted then at 32mm which was way too lean

Link to comment
Share on other sites

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.

Guest
Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

 Share

  • Who's Online (See full list)

    There are no registered users currently online

  • Recently Browsing

    No registered users viewing this page.

×
×
  • Create New...