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What could be wrong if a bike will only run on choke?

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MarJay
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PostPosted: 17:47 - 20 Mar 2011    Post subject: What could be wrong if a bike will only run on choke? Reply with quote

Hello,

I have recently spent a bit of spare time trying to sort out some of the issues on my FZR400. The other day I changed the oil, filter, spark plugs and coolant. I then tested the bike to get the coolant running through and to make sure I filled the oil to the right level. It didn't run very well, and every time I gave it some gas the engine just cut out.

I didn't worry about this too much as I had lots of other work to do on the bike, but I had it running long enough to circulate the oil and coolant. I then replaced the air filter, adjusted the exup cables and freed off the rear brake caliper which was seized.

So having done all that I decided to take the bike for a ride. I fired it up and it ran better than earlier. I could rev it with the choke on and it died with the choke off. I assumed this was because the bike was cold so I continued with my ride. Once the temperature gauge was showing a reasonable temperature, I pulled over so that I did not kill the bike by taking off the choke if there was something wrong. I took off the choke, and as I suspected, the bike died.

The bike now will not run without the choke on... It ran OK when I picked it up, but I didn't want to ride it too much in the mean time without servicing it.

Any ideas what might have happened to prevent the bike from running without the choke?
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Paxovasa
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PostPosted: 17:51 - 20 Mar 2011    Post subject: Reply with quote

Did you replace like for like (air filter)?
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Livefast123
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PostPosted: 17:54 - 20 Mar 2011    Post subject: Reply with quote

Crap being pulled through the fuel lines partially blocking the mesh or inline filters.?
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MarJay
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PostPosted: 17:56 - 20 Mar 2011    Post subject: Reply with quote

Paxovasa wrote:
Did you replace like for like (air filter)?


It was running much worse before I replaced the air filter. The old filter was REALLY really dirty, and the new one was a pukka one from Yamaha, not even a pattern one.
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Paxovasa
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PostPosted: 17:57 - 20 Mar 2011    Post subject: Reply with quote

Sound like you need to adjust the mixture screw.
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Livefast123
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PostPosted: 17:58 - 20 Mar 2011    Post subject: Reply with quote

Old fuel lines with an air leak somewhere?
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Stelmer
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PostPosted: 20:15 - 20 Mar 2011    Post subject: Reply with quote

If it's not the mixture screw(s) (screw in and unscrew 2.5 turns), I'd check the carb is properly sealed around the intake rubbers and the airbox is sealed too.

If no improvement, i'd clean the carbs.
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Ichy
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PostPosted: 20:31 - 20 Mar 2011    Post subject: Reply with quote

Does it die if you hold throttle at 2k-3k while pushing the choke in?
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jimspeed
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PostPosted: 21:18 - 20 Mar 2011    Post subject: Reply with quote

does it die if you are riding it when you put the choke off?
sounds like the partially blocked air filter was masking the weak fuelling problem maybe Confused
if it rides ok with choke off then its prob just an idle adjustment or carb balancing.
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MarJay
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PostPosted: 21:51 - 20 Mar 2011    Post subject: Reply with quote

Marki wrote:
Does it die if you hold throttle at 2k-3k while pushing the choke in?


No, but as soon as you let off the throttle it dies.
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Ichy
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PostPosted: 22:02 - 20 Mar 2011    Post subject: Reply with quote

Have you tried adjusting the idle speed? Seems like a good start point.
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lihp
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PostPosted: 23:12 - 20 Mar 2011    Post subject: Reply with quote

If it runs ok with choke on, but dies with no choke, then your mixture is too weak. You need to richen your mixture.
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Frost
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PostPosted: 23:40 - 20 Mar 2011    Post subject: Reply with quote

I had something similar, turned out to be a blocked pilot jet.
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MarJay
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PostPosted: 12:46 - 21 Mar 2011    Post subject: Reply with quote

PhilDawson8270 wrote:
If it runs ok with choke on, but dies with no choke, then your mixture is too weak. You need to richen your mixture.


Yeah, good work braniac! I'm aware of that Wink

I want to know why my mixture might have leaned off. If I randomly start adjusting screws on my carbs, I might well just make things worse.

I want to know what sort of things I need to look out for, that might have changed on the bike from me doing the oil, filter, coolant and spark plugs.
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cb1rocket
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PostPosted: 12:59 - 21 Mar 2011    Post subject: Reply with quote

is the petrol fresh?
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Frost
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PostPosted: 13:00 - 21 Mar 2011    Post subject: Reply with quote

Got the same temp plugs as before?
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lihp
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PostPosted: 13:05 - 21 Mar 2011    Post subject: Reply with quote

MarJay wrote:
PhilDawson8270 wrote:
If it runs ok with choke on, but dies with no choke, then your mixture is too weak. You need to richen your mixture.


Yeah, good work braniac! I'm aware of that Wink

I want to know why my mixture might have leaned off. If I randomly start adjusting screws on my carbs, I might well just make things worse.

I want to know what sort of things I need to look out for, that might have changed on the bike from me doing the oil, filter, coolant and spark plugs.


Carb jets may have become dirty, or worn.

My R6 developed running issues, and found 2 mixture screws 2 turns out, 1 was 3.5 turns out and the 4th was almost 5 turns out.

The bike had ran fine for the first 2 months after I bought it, but gradually got worse for some reason.

I'd pull the carbs, and clean and check them all then check all adjustments are correct.
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Ichy
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PostPosted: 13:25 - 21 Mar 2011    Post subject: Reply with quote

What is wrong with just reseting the idle speed?

There are a lot of assumptions that the mixture is weak, but based on what, it won't tickover without the choke? A choke richens the mixture and causes the RPM to rise.

How about this for logic. The airfilter may have caused a small restriction in the airflow. This caused the mixture to richen and the revs to rise. To compensate the previous owner adjusts the idle speed to drop the revs. New filter, no restriction, revs drop, bike doesn't tickover.

It may be way off the mark but its a 20 second adjustment. if it doesn't work then no loss and you have ruled out another thing. You'll need to adjust the idle speed to set the mixture correctly anyway.
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MarJay
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PostPosted: 13:39 - 21 Mar 2011    Post subject: Reply with quote

Marki wrote:
What is wrong with just reseting the idle speed?

There are a lot of assumptions that the mixture is weak, but based on what, it won't tickover without the choke? A choke richens the mixture and causes the RPM to rise.

How about this for logic. The airfilter may have caused a small restriction in the airflow. This caused the mixture to richen and the revs to rise. To compensate the previous owner adjusts the idle speed to drop the revs. New filter, no restriction, revs drop, bike doesn't tickover.

It may be way off the mark but its a 20 second adjustment. if it doesn't work then no loss and you have ruled out another thing. You'll need to adjust the idle speed to set the mixture correctly anyway.


I'm going to try that Marki, I haven't got around to it yet though. I'm aware that the problem was much worse before I changed the air filter, and changing the air filter appears to have improved it slightly.

Before I changed the air filter the bike would not rev even on choke, but afterwards it would rev on choke but die off choke.
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Ichy
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PostPosted: 13:44 - 21 Mar 2011    Post subject: Reply with quote

Ah, see now this changes things slightly. I didn't realise from the first post that the bike had problems before you changed the filter.

Personally I would go with Frosty and give the jets a good clean. Fitting an inline filter wouldn't hurt, 10-15 minutes and they cost around 2.

Any other symptoms? Which bike is it by the way?
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MarJay
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PostPosted: 13:53 - 21 Mar 2011    Post subject: Reply with quote

Marki wrote:
Ah, see now this changes things slightly. I didn't realise from the first post that the bike had problems before you changed the filter.

Personally I would go with Frosty and give the jets a good clean. Fitting an inline filter wouldn't hurt, 10-15 minutes and they cost around 2.

Any other symptoms? Which bike is it by the way?


Its an FZR400RR 3TJ from 1989. The guy I bought it from (a local back street dealer) claims to have cleaned the carbs before I got it. It was working fine the day I picked it up (back in February).
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G
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PostPosted: 13:56 - 21 Mar 2011    Post subject: Reply with quote

Try covering a bit of the airbox by hand and see if that changes things.
Had a bike that would only run ok with that - after trying lots of other stuff, fresh fuel sorted it.
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Robby
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PostPosted: 14:01 - 21 Mar 2011    Post subject: Reply with quote

Any rust or crap that could have been carried down from the tank can clog up an idle jet. The fact that the carbs were cleaned before you go it suggests that they needed cleaning; dealers don't clean carbs out for fun.

My money would be on crud in the tank blocking something. Personally I would an inline filter and fill it with optimax or similar then give it a hard ride with the choke off, in case that will shift stuff. Rise the idle too.

If the idle jets are blocked and the idle is wound up to compensate, the revs will die down slowly. Rev it, if the revs fall back sharply and continue idling then I would say it's fixed. If they take too long to settle back down then I would clean the carbs.
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MarJay
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PostPosted: 14:14 - 21 Mar 2011    Post subject: Reply with quote

Robby wrote:
Any rust or crap that could have been carried down from the tank can clog up an idle jet. The fact that the carbs were cleaned before you go it suggests that they needed cleaning; dealers don't clean carbs out for fun.

My money would be on crud in the tank blocking something. Personally I would an inline filter and fill it with optimax or similar then give it a hard ride with the choke off, in case that will shift stuff. Rise the idle too.

If the idle jets are blocked and the idle is wound up to compensate, the revs will die down slowly. Rev it, if the revs fall back sharply and continue idling then I would say it's fixed. If they take too long to settle back down then I would clean the carbs.


He cleaned them because the bike had been stood for 8 years. He put a new battery in, and cleaned the carbs... but did not change the oil, filter or air filter. He was selling it trade anyway and just wanted to get it running.
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lihp
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PostPosted: 14:51 - 21 Mar 2011    Post subject: Reply with quote

How do you know they're set correctly anyway?
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