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one spark plug lean...one rich

4674 Views 8 Replies 4 Participants Last post by  4everrunning
I just installed an Area P full exhaust on my 08 250R as well as a Dynojet kit. I was hoping the recommended settings would work but...of course not. It is running poorly below 4500 RPMs. I have now moved the needle lower and it seems to be running fine now.

However, that is not what this post is about. After removing the spark plugs, that were used with factory settings, one side appears to be running lean while the other running rich. I have not tried to sync the carbs as none of that has been touched. I do know that the factory settings for the fuel/air screw is different for each side. Could that be the reason one side shows rich and the other lean? Should I have the carbs re-synced?

Any advice would be appreciated.

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Have you checked the gap for both plugs and make sure they're within specs?
I did. what happen was about a year ago I went to start the bike and it back-fired and then would not start. I took it to the place where I bought it since it was only a month old and then said that the sparkplugs were fouled. They replaced them and it has ran fine ever since. When I pulled the plugs to check them while I was installing the exhaust and jet kit I checked the gap and both were in specs. One was just a little lean, maybe even closer to perfect but the left side was running rich. Not enough that I would really be concerned with. It was just a little on the black side.
It wouldn't hurt to re-sync the carbs. If it's just a small difference, and the engine is running fine, I really wouldn't worry about it. The gap maybe a little off on one or both of the plugs. I would imagine that even the index of the plug (read: the way the electrode points) would make a small difference in plug color.

If the bike has over 1000 miles or so on it, and the carbs have never been re-synced, I would say that a re-sync is going to do it. The initial service on the bike calls for the carbs to be re-synced.

The carb work during the first service, from what I gather, is really the only critical thing besides the oil change. Every bike tech I talk to says that the valves are almost always in spec, but the carbs almost never are.
I think we finally got the bike working. We took the carbs off again and visually checked the sync and they appear to be exactly in tune. We upgraded the main jet from a 98 to a 100 and after moving the needle up and down a bit we found that needle position 2 seemed to fix everything. It ran much better. At needle position 3 and 4 if run like crap below 5000 RPMs but after moving to positon 2 it seemed to run great. I am going to ride it a few days and then check the plugs again to see how they are looking. If it still is not right then I am just going to take it to a shop to be adjusted.

Thanks for all the help,

Hopefully you've properly synced your carbs but I'm not exactly clear you have based on what I've read here...

Syncing the carbs is the final adjustment you will make to the carbs(really only one carb is adjustable) after you have made any changes to your bike i.e. exhaust or jetting changes...

Syncing involves the adjusting the vacuum pressure that the carbs draw while the engine is at idle... Synchronizing the carbs balances the pressure so that they are functioning within the same vacuum pressure range thus equalizing the amount of air/fuel mixture that is drawn into the combustion chamber.

Hopefully this is what you adjusted as the final step in the process of the changes you made as this will make for a happy bike and a happier rider... :eek:hyaaa
After pulling the carbs again I did check the sync and from just looking at it and using a pencil to test they are in sync. We finally ended up moving the main jet to a 100 (for no real reason since the higher rpms have always been good). We then tried the needle on the 3rd and 4th slots and though they would idle fine there as soon as you started moving you could tell it was missing. We then moved the needle to the 2nd slot and it runs a lot better. The only problem I am having now is that sometimes below 4800 RPMs it still appears to miss. I can speed up slowly all the way to higher rpms and it will never miss. The problem mainly seems to appear when I have reached like 5500 and have to drop back down to say 4500 and then you can tell a slight miss. Any idea?

Did you use one of these... or something equivalent...?


If not I'm not sure how you would use a pencil to sync the carbs especially if you have taken them off the bike...

A carb sync tool such as the one linked above is used to measure the vacuum pressure drawn by each carb and allow you to tune them to each other thus synchronizing the carbs...

This is done with the carbs on the bike warmed up and idling...

Did you preform this type of procedure...?

If not then the carbs are not synced...
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A friend used that method to make sure the butterflies were opening at the same time. So, we did not use any special tool to determine sync.

I have been working with Area P, the company where I bought the exhaust, to help determine the problem. They believe it has more to do with the air/fuel and needle positions than anything else. They have given me some tips for setting the air/fuel mixture to try and resolve the issue. I am going to take it back apart this weekend and see what I can do. I will update this thread once I have put it back together.

From what I have noticed from riding now is that the problem occurs from 0 to 1/4 throttle and then it is fine. However, I think it is running rich now up top since I changed the main jet to 100. The problem only occurs when I am trying to maintain a speed from 0 to 1/4 throttle.

Thanks for all the responses,

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