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Post Info TOPIC: Engine bogs down under load.


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Engine bogs down under load.
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G'day, I have a 2010 (Australian) TTR250 that has about 600kms (370mls) on it.  The bike has only been ridden very lightly and is almost as new. 

The bike  started first go after a four month lay off but would bog down as soon as any throttle was applied. I suspected the fuel may have gone off so I drained the tank and carby and replaced the fuel with some fresh stuff. Same result.

I then checked the air filter and found that the foam had started to break down and appeared to be reacting with the filter oil I had used. I bought a new air filter and installed it. Same result.

Next I checked the spark plug. It was black, indicating to me that the bike was getting way too much fuel. The bike was running perfectly before the four month lay off. I replaced the plug, same result.

After seeing how rich the plug was indicating I then suspected the choke was not shutting all the way off or the float level was way off. I removed the carby and found that the choke was not the usual butterfly type but was more like a fuel bypass and appeared to be functioning properly. At this stage I thought it may be better to get professional help so I contacted our local Yamaha dealer and he agreed to go over the carby. When I picked it up he said they could find nothing wrong with it but it may still be the problem. I asked if they checked the float level and he said it was fine. He then said he has the same bike on his farm and it had the exact same problem and the only way he could fix it was to replace the carburetor. I asked him what was wrong with the carby and he said there was nothing wrong that could be seen, it just wouldn't work. ????? He then said he had another TTR250 come in and the only way he could get it to run correctly was to replace the carby. He also said they had a brand new four wheeler with the same carby and it also had to have the carby replaced to get it to run.

I went home with a lot less confidence in our local Yamaha dealer, changing a carby because you can't fix it does not sound too professional to me. I did a search to see if anybody else had to change the carby to fix a problem but came up empty.

When I got home I refitted the carby and attached a clear tube to the drain valve to check the float level. It was showing the float chamber to be completely full, so much for the dealer checking it. I again removed the carby and then removed the float to check the needle and seat. There was a thin coating of gum that was preventing the needle from sealing, I removed it with a cotton tip and some Brasso and soon had the needle sealing perfectly. I then removed all the jets and blew them out with compressed air as well as all the air passages. I reassembled the carby then connected the tank to check the float level. It was now at about 9mm from the top of the bowl which is right where it should be. I then put the bike back together and started it up. Fired first go as usual, revved OK in neutral but as soon as I put it under load in gear it bogged down again.

This is where I am up to now, if anyone has any suggestions I am all ears. If I can't sort it out I will have to transport it to the next nearest dealer which is over two hours away, I don't have enough faith in the local dealer to take it there.

Cheers, Magilla.

 

 



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Does it bog down while stationary or on the move or both?

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The Mancunian wrote:

Does it bog down while stationary or on the move or both?


 

It is fine in neutral but breaks down under load when in gear.



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I had a TRX850 once that seemed to do a similar thing. Tried all sorts for ages and couldn't get it going right. The fuel line from the tank to carb was in a protective spring type conduit to keep it away from the cylinder head. What was happening was that, every time the tank was lifted and dropped the actual fuel line was kinking ever so slightly but you couldn't see it. There was always enough juice getting through to fill the bowl up, for idling and feathering the throttle, but not enough got through when rode properly. So, have a look see at the pet**** and fuel pipe, and the breather in the cap.



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The Mancunian wrote:

I had a TRX850 once that seemed to do a similar thing. Tried all sorts for ages and couldn't get it going right. The fuel line from the tank to carb was in a protective spring type conduit to keep it away from the cylinder head. What was happening was that, every time the tank was lifted and dropped the actual fuel line was kinking ever so slightly but you couldn't see it. There was always enough juice getting through to fill the bowl up, for idling and feathering the throttle, but not enough got through when rode properly. So, have a look see at the pet**** and fuel pipe, and the breather in the cap.


 

 

Thanks mate, the tank breather is fine and the fuel line is only about 6 inches long with no kinks and no blockages.



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Will the TTR rev out in the lower gears? If so, the issue would seem to be a blockage in the the pilot jet. If it doesn't then it's possibly a main jet/needle problem. 

The pilot jet is used up to about 1/4 to 1/3 throttle and then the main jet takes over.

Try running the bike, after you have warmed it up, with the airbox cover off. Does it go better (over rich mixture) or worse correct or lean mixture)?

Can't believe the dealer suggested swapping out the carb no

Brian

 

 



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TTRfan wrote:

Will the TTR rev out in the lower gears? If so, the issue would seem to be a blockage in the the pilot jet. If it doesn't then it's possibly a main jet/needle problem. 

The pilot jet is used up to about 1/4 to 1/3 throttle and then the main jet takes over.

Try running the bike, after you have warmed it up, with the airbox cover off. Does it go better (over rich mixture) or worse correct or lean mixture)?

Can't believe the dealer suggested swapping out the carb no

Brian

 

 

Same in every gear, splutters and dies, just won't rev out.  I pulled all the jets out and they are all clear. Plug indicates it is running very rich. I will give it a try tomorrow without the airbox cover.

 


 



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Got a mate who can lend you a carb?

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The Mancunian wrote:

Got a mate who can lend you a carb?


 

 

Local dealer said he would swap a carb from one in his shop but if that works it still doesn't tell me what is wrong with mine.



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The Mancunian wrote:

Does it bog down while stationary or on the move or both?


 I just checked it again and if you crack the throttle open quickly it will bog down in neutral as well as in gear. If you crack the throttle slowly in neutral it will rev out. If you crack the throttle slowly in gear it is fine until about 3000 revs then starts to cough and splutter and struggles to pull up a very slight rise in third gear.



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Tried the bike with the airbox cover off and made no difference, also tried with the air filter removed and it was the same.

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My bike started doing exactly the same thing yesterday morning preventing me from getting out on the lanes. On a quick inspection, it seems the float is sticking. On a brighter note, my carb kit arrived yesterday ten minutes after I pulled out. That's the way it seems to go eh hmm



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I thought I would post the conclusion to this problem in the hope it may save others the frustration I have had.

After pulling the carb apart about six times and finding nothing wrong I eventually took it to the local Yamaha dealer. He disassembled the carby and found nothing wrong so he soaked it for two days in a solvent even though the carb looked like new inside and out. This also never cured the problem so he approached Yamaha and they agreed to replace the carby under warranty even though it was 4 years old. The consensus is that there is a problem with the casting on some of the TTR250 carbs that is allowing extra fuel to be drawn in causing the engine to flood. The dealer has had 3 other similar carbs with the problem. A new carb was fitted and the bike now runs like new again.

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Wow, that's an unexpected and unsettling result.  Can you post the exact P/Ns of the affected and replacement carbs?  This might help others to quickly identify whether this is a likely source of their problem (and whether they already have the "good" carb), since without a P/N or other physically identifying detail it would be very hard to isolate this mfg defect as the root cause.

Dave



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Magilla wrote:

The consensus is that there is a problem with the casting on some of the TTR250 carbs that is allowing extra fuel to be drawn in causing the engine to flood. 


Having had a good look at a dismantled TTR carb, I really can't see how this can happen confuse

The flow of petrol into the float bowl is regulated by the height of the float, If this was allowing too much petrol in then you would get flooding but this is easily rectified by adjusting the float level.

The only connection between petrol and intake are the pilot and main jets. The jets meter the fuel so that even if there was a flaw in the body of the carb to which they fit, the engine can't draw more fuel than they allow,

I guess there is also the diaphragm pumper and the choke mechanism that pass petrol into the intake but I can't work out how they could have a casting fault to allow too much petrol through.

Not to cast doubts on your Yam mechanic's diagnosis but what am I missing?

Brian 



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dlowenst wrote:

Wow, that's an unexpected and unsettling result.  Can you post the exact P/Ns of the affected and replacement carbs?  This might help others to quickly identify whether this is a likely source of their problem (and whether they already have the "good" carb), since without a P/N or other physically identifying detail it would be very hard to isolate this mfg defect as the root cause.

Dave


 

I don't have the old carb so have no way of finding the part number. It would appear that not all TTR250 carbs are affected.



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TTRfan wrote:
Magilla wrote:

The consensus is that there is a problem with the casting on some of the TTR250 carbs that is allowing extra fuel to be drawn in causing the engine to flood. 


Having had a good look at a dismantled TTR carb, I really can't see how this can happen confuse

The flow of petrol into the float bowl is regulated by the height of the float, If this was allowing too much petrol in then you would get flooding but this is easily rectified by adjusting the float level.

The only connection between petrol and intake are the pilot and main jets. The jets meter the fuel so that even if there was a flaw in the body of the carb to which they fit, the engine can't draw more fuel than they allow,

I guess there is also the diaphragm pumper and the choke mechanism that pass petrol into the intake but I can't work out how they could have a casting fault to allow too much petrol through.

Not to cast doubts on your Yam mechanic's diagnosis but what am I missing?

Brian 


 

I have had the carb apart more times than I care to remember and also can not understand where it is leaking. The Yamaha mechanic suggested it was porosity in one of the channels but that was as far as he could go. The fact is that every part of the carby was removed and checked many times, the float valve was replaced in case it was an intermittent problem and gaskets were replaced even though the original ones looked perfect, all to no avail. Before taking it to the Yamaha dealer I had a talk with another bike shop mechanic that specializes in high performance machines. He wanted to have a look at it but said he had also seen cases where the carby had to be replaced to fix the problem so I took it to the Yamaha dealer. The new carb was just put in with no adjustments and the bike runs like new.



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I had a similar bog 1/4+ throttle after bikes set for a couple of months this summer. I cleaned carbs with spray carburetor cleaner and compressed air, new rubber parts, with no luck. Using spray carb cleaner had worked for me for years, not this time. We have varying amounts of Ethanol in our local gas, eats rubber and lots of varnish build up. I was about ready to remove top and check timing chain.

Gunk Carb Cleaner (with Basket) - My solution was a gallon of Gunk Carburetor cleaner (with basket, $20 at Auto Store). I soaked bowl, jets, check valve, and carb for 1 hour. Rinsed and cleaned Gunk off parts with fresh gas and reassembled. Both of my TTRs now run great - No Bog. Note, you need to remove all rubber (plastic - check valve, needle, throttle valve...) parts, Gunk will dissolve.

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