Can't fix a carb/ignition problem on a 250

[grrl]

I have a 250 motorcycle with 39k miles on it (62k km). It uses dual Mikuni CV carbs and a vacuum fuel pump. The bike often, but not always hesitates/stutters upon acceleration and there are often with wild fluctuations in RPMs when running at 3/4 to full throttle. Until recently, the problem was sporadic. When it first started I worked on it for a few weeks with no success and then one day (mid-gas tank) it suddenly went away. The bike ran great for a couple months but now the hesitation is back with a vengeance and it's much worse than before.

This is what I've done to try to solve the problem:

cleaned the carbs 3 or 4 times with air and cleaner - nothing is visibly dirty, all the jets and screens are pristine

checked the float levels - no problems

checked the gas tank - no rust, no dirt, both screens clean

air filter is clean

changed the fuel filter, spark plugs, coils and wires, cleaned the mounting points for a good ground (I also have a secondary ground on them)

swapped out the CDI, the old unit is fine

battery and regulator are both good

tested the fuel pump and stopcock

the vent on the gas tank is okay


I have no idea what to try next. Any suggestions would be greatly appreciated.

 

[SpatiallyAware]

Are you 100% sure the vent on the cap is ok? Have you tried using a second key to open the cap when this happens?

Double check the fuel filter; most likely it didn't come with one - I've seen them installed backwards etc..

Have you double checked the hoses from the tank?


Have you tried running it in reserve?



It sounds fuel related to me, primarily since it happens 3/4-full throttle.. This is where vent cap/fuel delivery issues tend to show up.

It's also possible something is adjusted wrong with the carbs - they need to be synced too (I'm guessing it's a parallel twin; you can either make your own sync gauges or just hold them up to a light source and fudge it)

 

[SpatiallyAware]

Also have you run the crap out of it a time or two? You'd be surprised how many bikes I've bought that ran poorly until a good strong blast through the mountains... Cruisers in particular..


....also ethanolated fuel goes bad VERY quickly.. Has it sat for a month or so?

 

[PottedMeat]

I have a 250 motorcycle with 39k miles on it (62k km). It uses dual Mikuni CV carbs and a vacuum fuel pump. The bike often, but not always hesitates/stutters upon acceleration and there are often with wild fluctuations in RPMs when running at 3/4 to full throttle. Until recently, the problem was sporadic. When it first started I worked on it for a few weeks with no success and then one day (mid-gas tank) it suddenly went away. The bike ran great for a couple months but now the hesitation is back with a vengeance and it's much worse than before.

maybe the pump diaphragm has a hole in it? temperature related?

i hate carbs

lol @ mikuni's website. carburetors, aerospace, health & beauty, arts & crafts. i guess thats diversification :awe:

 

[Demon-Xanth]

Do you have a vacuum petcock or a straight manual?

 

[grrl]

Thanks for all the responses.

Are you 100% sure the vent on the cap is ok? Have you tried using a second key to open the cap when this happens?

Double check the fuel filter; most likely it didn't come with one - I've seen them installed backwards etc..

Have you double checked the hoses from the tank?

Have you tried running it in reserve?

It sounds fuel related to me, primarily since it happens 3/4-full throttle.. This is where vent cap/fuel delivery issues tend to show up.

It's also possible something is adjusted wrong with the carbs - they need to be synced too (I'm guessing it's a parallel twin; you can either make your own sync gauges or just hold them up to a light source and fudge it)

I've ridden with the fuel cap open, the fuel filter is installed correctly, I've checked all the hoses and I've run it on reserve many, many times.

I'm convinced the problem is fuel related as well, but can't figure out what's happening.

The engine is a V-twin. I haven't synced the carbs, but FWIW, when the engine is running okay it's smooth and sounds good.

Also have you run the crap out of it a time or two? You'd be surprised how many bikes I've bought that ran poorly until a good strong blast through the mountains... Cruisers in particular..

....also ethanolated fuel goes bad VERY quickly.. Has it sat for a month or so?

I've run it hard on many occasions hoping to 'clean it out' and most of my riding is at highway speeds on hilly roads. I ride through the winter so the tank gets emptied every couple of weeks at the most. The last two tanks I used some gas treatment as well.

maybe the pump diaphragm has a hole in it? temperature related?

i hate carbs

lol @ mikuni's website. carburetors, aerospace, health & beauty, arts & crafts. i guess thats diversification :awe:

The problem doesn't seem to be temperature related - it sometimes acts up when cold and sometimes when hot - and it has run fine in both conditions as well. I disassembled the pump and the diaphragm is in good shape as is the vacuum hose. Plus there is no fuel leaking down the hose into the cylinder.

I hate carbs too. I now hate them more than electrical issues.

Mikuni's site *is* pretty worthless.

Do you have a vacuum petcock or a straight manual?

Straight manual. Just today I bypassed it but the problem didn't go away.


EDIT: I also did a quick vacuum test around the carbs with WD40 and there was no speeding up or hesitation when I sprayed it.

 

[JulesMaximus]

What kind of bike is this?

How does the engine run with the choke on?

 

[grrl]

What kind of bike is this?

How does the engine run with the choke on?

It's a Hyosung GT250. I haven't used the choke in a while, but normally it will stall if I put the choke on.

 

[JulesMaximus]

It's a Hyosung GT250. I haven't used the choke in a while, but normally it will stall if I put the choke on.

So you don't even need the choke to start it cold? It sounds as though the bike is running rich. Is there any oily buildup on the inside of the exhaust?

 

[grrl]

So you don't even need the choke to start it cold? It sounds as though the bike is running rich. Is there any oily buildup on the inside of the exhaust?

No, I need full choke to start the bike in the winter. I thought you were asking about once the bike was warm and running. The plugs are tannish, so the mix appears okay. As far as I know there is no oil in the exhaust, but I have looked down the can recently.

 

[JulesMaximus]

No, I need full choke to start the bike in the winter. I thought you were asking about once the bike was warm and running. The plugs are tannish, so the mix appears okay. As far as I know there is no oil in the exhaust, but I have looked down the can recently.

I'm talking about when the engine is cold, not the weather outside. You should still need some choke to start a cold engine...even when it's warm outside.

 

[grrl]

I'm talking about when the engine is cold, not the weather outside. You should still need some choke to start a cold engine...even when it's 100 degrees outside.

I've never needed to use choke on any of the bikes I've owned when it's warm outside.

 

[JulesMaximus]

I've never needed to use choke on any of the bikes I've owned when it's warm outside.

Well, maybe not 100 degrees but certainly anything below say 60. I had an old Honda and pretty much always had to use some choke to get it running when the engine was cold.

 

[grrl]

Well, maybe not 100 degrees but certainly anything below say 60. I had an old Honda and pretty much always had to use some choke to get it running when the engine was cold.

I usually only need the choke below 40 or so, but I'm going to recheck the float levels tomorrow just to be sure.

 

[SpatiallyAware]

Needing to use the choke varies extremely widely from bike to bike - even with the same bike different carbs act different ways in different conditions.

 

[hanoverphist]

I've never needed to use choke on any of the bikes I've owned when it's warm outside.

depends on the bike. my dads old 45 knuckle needed choke regardless of the temp. hell, in the winter he would sit there with a blow dryer for 10 minutes warming up the cases before he kicked it over. my old honda cb750 would need a bit of choke for the first minute or so, regardless of the temp outside. my friends honda shadow never needed choke at all, but he would have to goose it a few times to get it to stay steady on colder days.

 

[Zenmervolt]

Is this a CV carb? If so, is it possible that something in the CV setup is screwy. At 3/4 throttle you're mostly dealing with the needle taper jet in the CV slide with the main jet only just starting to come into play. (This is, of course, general and not specific to your model. Models' tuning varies from one manufacturer to another.)

If these are CV carbs, make sure that the diaphragm (the one in the carbs that is used to adjust the venturi size based on air pressure and vacuum) is functioning properly and that the air passages are free and open. You may need to physically check the diaphragms for cracks if the air passages are OK.

ZV

 

[grrl]

The bike has CV carbs (Mikuni BDS26). The diaphragms are in good shape and working properly.

Because of some things JulesMaximus asked, I rechecked the floats yesterday and found the fuel level was too high on the back carb. The height appears to creep up overnight, so I will need to change the needles. Luckily, gas wasn't filling the cylinders. I adjusted both floats and the bike is now running much, much better, but I still need to dial it in. Unfortunately Hyosung's manual gives the wrong float height. They want 17mm, but it's physically impossible to do that.