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Bog (yes I know, yet another post about the same)

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Just give me a second before sending me off to read all the previous posts :) (trust me, I have been there).

  1. Picked up a used bike about a month ago, bike ran ok'ish but was hard to start.
  2. Changed the pilot from a 50 to a 45, bike ran better and was easier to start. And with some adjustment of the fuel screw no bog.
  3. Installed a JD jetting kit, bike ran great and pulled like a freight train! No bog at all!
  4. The suddenly, the bike started bogging. It's only a problem right of idle, if it gets up in revs there is nothing.

Video: https://goo.gl/photos/4t5xVVXQRfa8Cqjb8

I do not believe the temperature changes across our practice rides are big enough to explain that big a change. I have tried 25, 38, 42, 45 and 50 pilot jets without much change. Also changing the fuel screw does little to the bog.

Any ideas on what to try? Could my idle adjustment affect this?

 

 

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One of the first things I would recommend is a R&D flex fuel screw. It's a must to really get that fuel screw tuned correctly, especially when trying different pilot jets out. If your adjustments to your fuel screw are not changing your nature of your idle, I would propose the possibility your pilot circuit is not performing properly for one reason or another, especially if the symptom came on suddenly. Although it is generally accepted that a 45 pilot is a normal mod, my experience has just not taken me there. A 50 pilot just seems way rich unless your in sub zero temps I suppose. The highest pilot I have used is a 42 and even then the fuel screw adjustment was less than half a turn out. I have said before that many will oversized a pilot on an FCR carb before considering reducing the size of the leak jet which will strengthen and increase the squirt generated by the accelerator pump. I can prove this to be true on my bike as I have an adjustable leak jet(R&D powerbowl) and can generate a lean bog by dialing in a larger leak jet setting, then eliminate it by reducing it. I am running a 38 pilot jet which many will tell you is too small, but works perfect on my set up.

Yes your idle adjustments can affect your quick throttle blips as the idle adjustment also moves the accelerator pump cam and changes your acc pump timing! If the acc pump squirt is hitting the throttle valve before it can move out of the way, you can definitely get a bog when you hit the throttle.

Of course an air leak is always a possible culprit as one may find themselves increasing jet sizes to find balance against a small air infiltration somewhere without realizing it. Check your carb boots, tighten clamps etc. A bad throttle valve gasket won't help any as well. If you have sprayed carb cleaner around the throttle valve without removing this small rubber gasket, it will get destroyed.

Check your valve clearance. Improper valve adjustments can disguise themselves as carb problems, particularly with starting and bogging, and present different issues at different operating temps. Check them cold and set to the larger end of the clearance as the clearance will reduce as the valves wear(IMO).

Hopefully something here will be helpful.

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What year? Pre 2012 benifits from the o-ring mod. 2012 and later has the redesigned accelerator actuator and doesn't need it.

The bike has a big carb and a little motor. It is very sensitive to moisture in the fuel or the pilot gumming up if left sitting with fuel in the bowl.

Sent from my SAMSUNG-SM-G920A using ThumperTalk mobile app

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Thank you both for your replies :)

wchefd: I am at zero elevation and temps around 18C/65F, so that is not extreme either way. After getting a tachometer I took a crack at it again yesterday. I started by adjusting the timing of the AP, then with an idle of 2100rpm and a 40 pilot jet I got the bog almost eliminated :) To be honest, I don't know what leak jet is installed. The guy I bought it from had changed all the other jets, so it is quite possible that it is non-standard. I have ordered both the R&D flex screw and the power bowl to get some tuneability, because tilting the carb for all changes gets old very fast. I have not checked for air leaks, so thank you for that. An air leak would explain the sudden change is behavior.

ohiodrz400sm: the bike is a 2009 so it has the old carb. The previous owner has installed the o-ring mod, but with a piece of wire instead of an o-ring...

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I've been thinking about trying a 40 pilot. If you and the prior owners have been twisting the carb sideways over the tabs for 8 years, do yourself a favor and pick up a new boot and clamps. The boots are cheap money and can leak around the tabs and raise hell.

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I tried a 45 pilot first, but when turning the fuel screw I got a rising idle speed as I turned it all the way in. As far as I have read, that means the pilot is too rich. So I changed to a 40, and found max idle speed with the fuel screw 1.5 turns out. I do not have much experience with tuning carburetors, but as far as I can read/youtube that means that it is adjusted correctly?

I will get the boots, no need to chase jetting issues and risk engine failure (in case it sucks in sand) just because of two small pieces of rubber :)

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13 hours ago, wchefd said:

One of the first things I would recommend is a R&D flex fuel screw. It's a must to really get that fuel screw tuned correctly, especially when trying different pilot jets out. If your adjustments to your fuel screw are not changing your nature of your idle, I would propose the possibility your pilot circuit is not performing properly for one reason or another, especially if the symptom came on suddenly. Although it is generally accepted that a 45 pilot is a normal mod, my experience has just not taken me there. A 50 pilot just seems way rich unless your in sub zero temps I suppose. The highest pilot I have used is a 42 and even then the fuel screw adjustment was less than half a turn out. I have said before that many will oversized a pilot on an FCR carb before considering reducing the size of the leak jet which will strengthen and increase the squirt generated by the accelerator pump. I can prove this to be true on my bike as I have an adjustable leak jet(R&D powerbowl) and can generate a lean bog by dialing in a larger leak jet setting, then eliminate it by reducing it. I am running a 38 pilot jet which many will tell you is too small, but works perfect on my set up.

Yes your idle adjustments can affect your quick throttle blips as the idle adjustment also moves the accelerator pump cam and changes your acc pump timing! If the acc pump squirt is hitting the throttle valve before it can move out of the way, you can definitely get a bog when you hit the throttle.

Of course an air leak is always a possible culprit as one may find themselves increasing jet sizes to find balance against a small air infiltration somewhere without realizing it. Check your carb boots, tighten clamps etc. A bad throttle valve gasket won't help any as well. If you have sprayed carb cleaner around the throttle valve without removing this small rubber gasket, it will get destroyed.

Check your valve clearance. Improper valve adjustments can disguise themselves as carb problems, particularly with starting and bogging, and present different issues at different operating temps. Check them cold and set to the larger end of the clearance as the clearance will reduce as the valves wear(IMO).

Hopefully something here will be helpful.

Well said, wchefd! When I set up my CRF a few years ago I followed a path similar to the one you describe: I started with a Flex fuel screw and QS3 for the adjustable leak jet, and a bunch of experimentation to learn many of the same lessons. One of my mentors was TT denizon weantright who counseled: tend to leaner on the pilot, richer on the main (this is what Honda did in '08—went down one on the pilot, and up one on the main, with a different needle, which people with '07s find helpful in their quest for clean jetting).

Since I don't race but ride tight single-track, I was looking for clean throttle response and useable torque from right off idle, so I went one step further with a flow-splitter (an FMF Snap in my case) and found what I was looking for. It's worthwhile for anyone who needs good partial-throttle response down low. Another thing about the flow splitter is that it makes makes the pilot circuit much less sensitive, so when the weather and/or altitude changes, you don't have to tweak the pilot screw as often, and the pilot jet almost never. Do all the other good stuff first though to get a proper baseline.

 

Edited by Old Plonker
left out note about altitude changes
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15 minutes ago, Old Plonker said:

Well said, wchefd! When I set up my CRF a few years ago I followed a path similar to the one you describe: I started with a Flex fuel screw and QS3 for the adjustable leak jet, and a bunch of experimentation to learn many of the same lessons. One of my mentors was TT denizon weantright who counseled: tend to leaner on the pilot, richer on the main (this is what Honda did in '08—went down one on the pilot, and up one on the main, with a different needle, which people with '07s find helpful in their quest for clean jetting).

Since I don't race but ride tight single-track, I was looking for clean throttle response and useable torque from right off idle, so I went one step further with a flow-splitter (an FMF Snap in my case) and found what I was looking for. It's worthwhile for anyone who needs good partial-throttle response down low. Another thing about the flow splitter is that it makes makes the pilot circuit much less sensitive, so when the weather and/or altitude changes, you don't have to tweak the pilot screw as often, and the pilot jet almost never. Do all the other good stuff first though to get a proper baseline.

 

I was skeptical about the Snap before I understood how sensitive the Pilot circuit is on these bikes. Its good to see that I am not the only one who believes in the leaner pilot jetting. If this were a trail bike, I would probably have a 40 or 42 in there to mello/smooth out the combustion characteristics. Consider that a CRF250R uses a #40 pilot jet(when they had carburetors) and you come to the realization that the size of the pilot jet itself (jets in general) is only one consideration in the quest for your perfect combustion. I read once that if you want to visualize combustion, think about how an acetylene torch works.  The more air you add, the tighter and hotter the burn becomes. Add too much air and its  and it pops ie. backfire and decel pop. The more fuel you add, it cools down the combustion and produces a slower burn. Add too much fuel and you got black soot generating from the flame ie. black smoke from the exhaust and sooty plug. 

IMO, the reason  much larger bikes can use the same size pilot as a smaller, is the engines ability to create more vacuum and pull more fuel through the pilot. I believe this explains why the 150r is so finicky at that off idle to 1/4 as the engine is trying to create enough vacuum to pull the fuel through the pilot. At the end of the day, it would appear as though the air flow and vacuum dynamics play more of a role than the jets themselves!

These observations add much credibility your opinion that the Snap is a worthwhile investment, which is why I am ordering one right now.     

 

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