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Loss of power after header grind


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I ground down the header on my 2007 DR650se last week and it seems like I lost bottom end power and some throttle response. I don't know if it messed with the back pressure the bike needed or what. I tried messing with the fuel screw leaning out and richening the mixture but It did not really seem to have much effect on performance. I have it set out at 2 turns right now and the bike runs good but not as great as it did. I read the BST Bible and several other posts, just looking for more advice.

1. bought bike ran pretty good

2. Pulled snorkel shimmed needle 1mm, bike ran great! didn't even touch the fuel screw at this point but I did remove the brass plug.

3. Ground header weld, now it is back to running about as good as when I bought it.

My question is can I get back the performance I had? I wasn't planning on getting a dynojet kit but is that something I have to do now since the exhaust is slightly modified.? Should I remove the shims on the needle?

I also noticed it seems like it is more difficult to start after these mods. Once it's warmed up it's fine but after sitting overnight I have to hold the starter and twist the gas far longer.

Any input is appreciated.

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Hey foxphotog, as another noobie who has read mx rob's stuff, bst stuff, etc., I ended needing higher mileage for my IMS tank, so I have: drilled the slide, removed the snorkel, shimmed the stock needle 1mm aswell, added Jesse's air/fuel mixture screw running at 1.75 out from full hard seat, and a twin air with screen still in the bike, and stock 82db quiet exhaust...I run 50mpg to 30mpg (anzo berrego spinning on jeep and sand trails)...

What exhaust mods are you talking about? (Ground header is it, or fmfq2, or...?)

The circuits are governed more by throttle position than by engine speed...and the air/fuel screw is on the pilot circuit...essential for idling...(so where on the powerband was your performance loss?) more than two turns out and it is recommended to go replace the pilot jet...

I didn't have the air/fuel screw seated all the way, and it vibrated out on a hwy ride from June Lake Loops up to Bridgeport, but pulled over and then ripped the throttle into bridgeport where it wouldn't idle of course, but then I found the whole assembly sitting on the engine case! (Whew)...

Are all the jets stock? Is the plug black now with the header grind and the shim and two turns out on the a/f screw? What elevation are you?

I'm a noobie too and because of CA and stock exhaust, haven't done the airbox mod, or the dj kit, until I get an fmfq2...but need to hear one live...but I did enjoy this link...hope it helps

http://www.iwt.com.au/mikunicarb.htm

I'm sure awesome Eddie S. will chime in sometime...

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I'm not an expert but I can tell you this... when you open up the head pipe by grinding the weld you are allowing more air to go through your engine. That being the case, if you did NOTHING else other than the header grind your bike will be running leaner than it was.

If you think you lost low end power (low RPMS) this is controlled by the needle which controls 1/4 throttle and up until the main jet takes over.. Because you increased your airflow you're going to have to richen it back up a bit. You can either raise the needle or get a jet kit. If you don't want to do a jet kit, stick with your stock needle and start by raising your needle by incriments and see if your power comes back. IE move it up a notch at a time. Or even a half a notch at a time with shims.

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Hey foxphotog, as another noobie who has read mx rob's stuff, bst stuff, etc., I ended needing higher mileage for my IMS tank, so I have: drilled the slide, removed the snorkel, shimmed the stock needle 1mm aswell, added Jesse's air/fuel mixture screw running at 1.75 out from full hard seat, and a twin air with screen still in the bike, and stock 82db quiet exhaust...I run 50mpg to 30mpg (anzo berrego spinning on jeep and sand trails)...

What exhaust mods are you talking about? (Ground header is it, or fmfq2, or...?)

The circuits are governed more by throttle position than by engine speed...and the air/fuel screw is on the pilot circuit...essential for idling...(so where on the powerband was your performance loss?) more than two turns out and it is recommended to go replace the pilot jet...

I didn't have the air/fuel screw seated all the way, and it vibrated out on a hwy ride from June Lake Loops up to Bridgeport, but pulled over and then ripped the throttle into bridgeport where it wouldn't idle of course, but then I found the whole assembly sitting on the engine case! (Whew)...

Are all the jets stock? Is the plug black now with the header grind and the shim and two turns out on the a/f screw? What elevation are you?

I'm a noobie too and because of CA and stock exhaust, haven't done the airbox mod, or the dj kit, until I get an fmfq2...but need to hear one live...but I did enjoy this link...hope it helps

http://www.iwt.com.au/mikunicarb.htm

I'm sure awesome Eddie S. will chime in sometime...

The exhaust mod was the one where lots of guys pull their stock header pipe off to find that the wel on the inside sticks out and needs to be ground down, this is the mod that I did. I plan on keeping the stock exhaust.

elevation 1024 ft

the jets in the carb are stock, the power loss seems to be right off of throttle. I do a lot of wheelies and it just doesn't quite feel like it had the low end torque it had when I pulled the snorkel and shimmed needle 1mm. How much further can I keep shimming the needle before having to buy a notched one?

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I wouldn't do more than three washers-how many washers = 1mm? I think I used .040 washers english system...I knew about the header grind, and was intending to do that but only own a dremel for grinding...which might work but just slow but precise...so no aftermarket pipe, mid pipe, etc...

I think at one time, I did have two washers on the stock needle...but once I gummed up the carb after neglect, I just pulled it, cleaned it, added Jesse's fscrew, etc, and put it back to normal...with one washer

Perhaps it is just as stated above...as we know more air means more fuel required...shim until you see the difference, check plug...also, I can't imagine that grinding the header down is equal in cfm to replacing a pipe for example...a clear example of a modification that requires main jet adjustment/airbox mod

I don't feel like I have helped that much...but it's nice that people are listening:bonk:

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I don't feel like I have helped that much...but it's nice that people are listening:bonk:

Thanks for the advice, I might can tinker tomorrow, Right now there are two small flatwashers that added up to 1mm raised.

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It does sound too lean.

You've leaned it out slightly more with the header grind so it needs to be a little richer on the mixture screw.

Turn the idle up too high and then wind the mixture screw out to best revs and then back in half a turn.

Wind the idle back down to where it was.

The header grind shouldn't be noticeable from a riders perspective.

It's more of a purist thing. "It shouldn't be like that so let's remove it and feel better."

I felt better when I did mine.

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It does sound too lean.

You've leaned it out slightly more with the header grind so it needs to be a little richer on the mixture screw.

Turn the idle up too high and then wind the mixture screw out to best revs and then back in half a turn.

Wind the idle back down to where it was.

The header grind shouldn't be noticeable from a riders perspective.

It's more of a purist thing. "It shouldn't be like that so let's remove it and feel better."

I felt better when I did mine.

I would really reccomend that if you decide to try two different fixes, do ONE at a time. Save yourself the headache and don't try them both. I would say try Nordies advice first (its the easiest) and see if it helps. If it doesnt do it move on to the needle.

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I would really reccomend that if you decide to try two different fixes, do ONE at a time. Save yourself the headache and don't try them both. I would say try Nordies advice first (its the easiest) and see if it helps. If it doesnt do it move on to the needle.

Cool thanks guys. as soon as this 100 degree weather lets up I will try this.

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Don't assume you need richer jetting just because you removed a potential exhaust restriction. It's possible you lost some torque right off the bottom if your weld globe was significant. Back pressure is not your enemy for low end power... if you really want to loose low end power run out and buy a FMF over-sized head pipe. I'd stick with a pilot setting that allows the smoothest idle in the most closed down position possible plus about a 1/8 or 1/4 turn more. The pilot circuit just doesn't need to be any richer than that and certainly does not need a larger pilot jet on the DR650. You can loose throttle response from going too rich on your pilot circuit. Also remember high air temps mean less dense air and less performance.....

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Don't assume you need richer jetting just because you removed a potential exhaust restriction. It's possible you lost some torque right off the bottom if your weld globe was significant. Back pressure is not your enemy for low end power... if you really want to loose low end power run out and buy a FMF over-sized head pipe. I'd stick with a pilot setting that allows the smoothest idle in the most closed down position possible plus about a 1/8 or 1/4 turn more. The pilot circuit just doesn't need to be any richer than that and certainly does not need a larger pilot jet on the DR650. You can loose throttle response from going too rich on your pilot circuit. Also remember high air temps mean less dense air and less performance.....

Im confused. Why would you lose torque just from removing the weld build up? And why wouldnt more air mean conversly more fuel? If its running really well, then you let it breath better and get more air and power drops, I cant figure out why that wouldnt mean you need more fuel after it got leaned out a bit. And Im not arguing, Id really like to know the deal. What am I missing?

wait I just reread your post... so you're saying you WANT back pressure for torque?

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It's not necessarily back pressure but velocity (back pressure builds velocity). Velocity increases flow in the exhaust just as it does in the intake tract and helps draw the charge out of the cylinder. Obviously there is a point that this logic runs out of steam and flow. Head pipe diameter and length has a huge affect on engine tuning. It's possible that the weld restriction was providing enough velocity down low to be noticeable at right off idle engine speeds. Head pipe diameter and length have more to do with the performance/powerband of the DR650 than any of the aftermarket exhaust systems (mid-pipe and muffler) because they are mostly big enough (2" mid-pipe) to drop velocity to the point they are not contributing to flow. The exception is if there is a restriction added... like a quiet core. Take the GSXR system for instance... even though it is not a straight through system it has huge volume and provides no resistance to the stock head pipe tune/flow. Then look at the FMF-Q2 with it's curved pipe sound reducer/spark arrester. On the dyno it actually produced more HP than the GSXR system... but any gains were way up at over-run where the extra velocity (back pressure) provided the perfect draw. Mostly useless for the DR650's low to mid range powerband.

Now to the fuel.... just because you increase flow does not mean you need more fuel... sometimes it's quite the opposite. Remember... it's a carb working off of velocity and pressure drop to draw fuel out of the float bowl. If you increase flow you will also draw more fuel with that extra flow. Take our recent dyno run with NY T100's DR725 and Web 245 cam. He had one of my FCR-MX's on his bike before the engine work. He removed my std 155 main and installed a 160, which is two sizes bigger, following the "I must need more fuel logic". On the dyno his AFR was way too rich to produce max power and a clean over-run... we went back to the 155 main and power/over-run went up when the AFR got back to where it needs to be.

So the morale of the story is if you decrease velocity in the intake tract (huge useless carb) you might well need bigger jets.... but it's not a given that velocity changes in the exhaust tract require jetting changes. Of course there are exceptions to any logic. ๐Ÿ˜

Edited by mx_rob
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+1 on Mx-robs' explanation. i bought the FMF power bomb head pipe and my idle mixture went rich requiring leaning it way out. In fact i still need to go with a smaller AP nozzle but have been too busy with a new job to get one ordered. So i obviously noticed immediately that the pipe was too big, although i will say that way up around red line it just keeps pulling and that might be a combination of the 190 cam, hc piston, TM-40, etc.. I'm planning on cutting a straight piece of the old exhaust and machining a 'torque cone' or anti-reversion cone, putting it in place and seeing how it runs.

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WOW... thanks a lot for the explanation. I love learning new stuff and that is great info.

SO let me make sure I have it straight in my head. The header weld may increase velocity of the air going through the exhaust thereby actually pulling more air/fuel behind it through the carb, whereas removing the weld will open it up BUT slow the air down because of the increased volume it can go through.

So you open it up and it can move more air, BUT at the cost of the air moving slower. And air speed in the header can have a direct influence on the airspeed going through the carb and into the piston?

Wait, I think I just had an epiphony (lmao)... opening it up allows a potential for more air flow, but no matter how much you open it the piston still only moves X amount of air. And now that air is moving slower because of the increased volume. You would have to increase the volume of AIR to match the increased volume of pipe to maintain the higher air velcocity and therefor the amount of air the piston can pull through the carb.

Wow this is heavy... ๐Ÿ˜

OK and to the OP... I didnt mean to hijack your thread but I thought it might be good info for you too. I'm not sure what to tell you now. haha

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WOW... thanks a lot for the explanation. I love learning new stuff and that is great info.

SO let me make sure I have it straight in my head. The header weld may increase velocity of the air going through the exhaust thereby actually pulling more air/fuel behind it through the carb, whereas removing the weld will open it up BUT slow the air down because of the increased volume it can go through.

So you open it up and it can move more air, BUT at the cost of the air moving slower. And air speed in the header can have a direct influence on the airspeed going through the carb and into the piston?

To clarify, drawing the maximum charge out of the cylinder leaves room for more incoming charge at a given RPM.

Wait, I think I just had an epiphony (lmao)... opening it up allows a potential for more air flow, but no matter how much you open it the piston still only moves X amount of air. And now that air is moving slower because of the increased volume. You would have to increase the volume of AIR to match the increased volume of pipe to maintain the higher air velcocity and therefor the amount of air the piston can pull through the carb.

๐Ÿ˜ This is the general theory and why bigger does not always equate to better.... unless we are talking CC's :cheers:

Wow this is heavy... ๐Ÿ˜

OK and to the OP... I didnt mean to hijack your thread but I thought it might be good info for you too. I'm not sure what to tell you now. haha

Yes, please excuse the :ride:

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