Jump to content
Sign in to follow this  

Question about off idle bog

Recommended Posts

Hi, pretty new here but I've been lurking around for a bit.

I have a DRZ400s with a fcr-mx 39, non TT version. I get a bog mainly when I'm in second, and go to do a wheelie. So mainly around 1/4 to 1/2 throttle I'd say. First gear lifts the wheel very easily. I was at clip three on the needle, switched to the second and it seemed to be a bit better. When I take the airbox cover off, it does get a little better so I figure I'm running rich. It idles very well, and top end seems better but still improves without the airbox cover.

Current jetting is:

155 Main

45 Pilot

EMN clip two

40 leak jet

Merge Racing AP spring

About sea level

Around 45F during the day

I've done William1's pilot jet test, and I know I need a smaller pilot because with the fuel screw all the way in, the bike continues to run. That wouldnt affect my problem while riding, right?

I've checked the AP squirt, consistent flow for around a second. I also adjusted and rode a few times, and found a half turn in on the screw seems best. I know I have to adjust again once the jetting is perfect.

Do you think I should change to a smaller main jet because of the airbox removal?

Also the bog affects from basic cracking the throttle up 1/2, so would the needle clip need to be changed again? Or maybe a different needle?

I was at a 35 leak jet but figured a higher one would help a little, but it was tough to tell.

Any hints would be great, sorry for the long post but I figured the more info the better.

Share this post


Link to post
Share on other sites

Be sure the idle speed is set low too. If the idle speed is too high, you will be slightly on the needle too, which can confuse things. It also matters if the carb has a CE/ACV (coast valve). If that is not working right, it can make it too rich at idle. But none of matters at all at 1/2 throttle. Airboix door on makes no matter at idle, only at WOT. If it is better with the door off at WOT, you are too rich on the main.

40 leak jet should be fine. You need to recheck the AP timing. Try turning the timing screw 1/2 turn CW. Bog is all AP, not needle, pilot or main.

Share this post


Link to post
Share on other sites

Yes, my carb has the ACV. How can I check if it's working/should I just try and clean it out?

I'll redo the pilot jet test and lower the idle speed a bit, as I don't have a tachometer to accurately check.

Also I'll play with the ap screw a bit more tomorrow. I had it set to just missing the slide when the carb was out but I know you can also adjust while it's in.

Thanks for the response.

Share this post


Link to post
Share on other sites

Easy test is to simply clamp off one of the vacuum lines (if it is external) or disassemble and carefully inspect.

Set the idle speed as low as you can without it stalling, then adjust the fuel screw. Once properly set, remember a fast idle is much better for the engine than a slow one. A well adjusted bike will be able to idle below 1,000 rpm but it will hammer itself to death. One thing you'll notice is when the idle speed is high enough, some of the metallic sound will go away and it will sound 'softer'.

Visually setting the AP assumes you have microsecond vision. So visual is good for initial (as is the technical method of adjusting with feeler gages and .8mm drill bit blank). But often, slight adjustment is needed by riding, tweak, ride. The entire AP mechanism is not a finely machined series of parts, just some stamped metal and a screw. A little finesse is needed.

Share this post


Link to post
Share on other sites

True, I think I had my idle a little high when testing with the fuel screw.

I'll mess with that and the AP screw tomorrow some more to see what I can figure out.

It's so close, because even with a stock pipe I can see a huge difference with this carb over the Mikuni.

Share this post


Link to post
Share on other sites

I can't really seem to get it exactly right. I've been playing around with the screw for a while and seemed to find the best of the settings I've tested, but it still seems off. I tried half turns in each direction until I was 1.5 turns each way, and the setting I was initially at seemed the best.

I have a small track at my house, and sometimes when I slow for a corner then gas it in second I'll stall out. That would be the AP timing correct? The mikuni carb wouldn't give me a blast of power, but it also wouldn't bog/stall in the same corners.

Could there be something else affecting my carb that would cause this? Larger leak jet?

I'll check the diaphragm a little later to see if it's worn out, but I vaguely remember that even with the Merge Racing spring the black lever wasn't immediately trailing the screw. Almost a little lag as if I had the stock spring.

Should I try the O-ring mod even with that spring installed, because it seems like it isn't doing enough.

Share this post


Link to post
Share on other sites

First, comfirm you have a strong squirt, Pull off the carb boot, wear safety glasses, engine off, look in the carb and have someone twist the throttle hard. You should see a really strong squirt. If you do not, you may have a clog or crapped out diapghram or a bad/gunked check valve.

You can try a smaller (smaller is richer) leak jet. You may want to close off the leak jet entirely. Simply take a new, spotless leak jet and solder the hole closed. Most big bore (meaning the 450 MX and larger) bikes have the leak jet blocked off entirely.

If you did the spring, a oring will do nothing.

Share this post


Link to post
Share on other sites

Well I took off the carb again today. I found two things related to my pilot adjustment. One, I found my washer for my fuel screw sitting on my work bench, cleaned and put that back in in the correct order on the screw. Also, I took out my pilot air jet and realized it was clogged pretty bad. Could be just me but I think this solved why my fuel screw bottomed out with no effect.

Onto the bog. Blew compressed air threw the check valve and it came through the other side so I'm guessing that should be fine. Also air through the holes in the bottom of the AP, everything seemed fine, the diaphragm looks brand new. I switched back to a 35 leak jet, not sure why, haha.

It's hard for me to tell what a strong AP squirt is. To me, mine looks strong, but I don't really have any reference. Anyways, when I put it all back on it bogged heavily, I re-adjusted the AP screw even though I checked clearance before and got the best I could but still has a little bog. I guess I'm just getting greedy here, because in first gear there's no bog at all, just in lower second.

Share this post


Link to post
Share on other sites

Yeah I'd say it squirts about four feet, most. I might just pick up a new diaphragm. This one looks good but I have nothing to compare it to, might be a little flimsy.

And on the short shifting, I ride the same as I did with the previous carb, so I don't think I'm too low.

I'll just keep working at it, I'm sure it's something simple. Thanks a lot for all the help though.

Share this post


Link to post
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Reply with:

Sign in to follow this  

×
×
  • Create New...