After proudly dialing out 90% of surging on my bone stock 96 DR650 ( now jetted to 152.5, Uni filter,  and opened airbox)  it's time to attack what seems to be the last carb problem:  Heavy surging when bike reaches fuel reserve.

Fuel starvation due to clogged reserve filter or restricted flow - right?  OK let's test it.  Next time stumbling happens near reserve fuel level, I simply go WOT on the freeway, the bike immediately loses all stumbling and smoothly soars to top of speedometer.  Doesn't that rule out fuel starvation?

Nonetheless I decided to remove BOTH in tank filters (see photo below) , and the carb inlet filter, and add one new inline filter.   DONE.  Excited for the test ride.

And... (drum roll)  The bike runs horribly.   Now it stumbles at cruise throttle all the time now, no matter what the fuel level!!

What did I do wrong?  How could petcock affect surging at cruise throttle on DR650 when there is clearly plenty of fuel??

Bummed I Broke Something,

-Michael

(but grateful for advice )

It could be possible that you broke some dirt, or something loose and it found it way into the carb. It all flows downhill. I had the same problems with mine and I went to the raptor style petcock and a TM40 and all the surging went away. Only thing is, with the raptor style, you have to remember to turn it on and off. Could it be possible that the vacuum going to the petcock is not consistant, therefore causing the fuel flow to "surge"? If vacuum goes away, so does fuel flow, no fuel flow, or slow fuel flow = cutting out and running crappy Just some ideas--Good Luck

Good point.  I should take it off an clean it out again, though it's been very clean inside, something from the filter might have found it's way into the bowl.

You got me thinking.  Might the pulsing of the diaphragm cause a pulsing of the fuel in the float?

It just seems so odd that opening up the fuel flow made the problem worse -- again -- only surging happens at cruising throttle level, and there is clearly no starvation.

Edited by MichaelS, 17 June 2012 - 09:24 PM.

that surging is common with cv carbs and opened airboxes,
all that extra air its sucking overwhelms the spring in the carb,and you get the slide bouncing up and down,
easiest cure is put the airbox back to stock,
another solution,though its a bit hit or miss,is cut 3 or 4 coils off the spring ,then stretch it out half an inch,makes the spring slightly stiffer and reduces the slide bouncing,
make it too stiff though and the slide wont rise as it should,bit of trial and error

 paint it black, on 18 June 2012 - 12:16 AM, said:

Very helpful.  If I get the surging back, I'll play with the spring.  

The issue at hand seems to be related to the petcock (I know sounds strange) as the ONLY  time the bike misbehaves now seems related to when the fuel level gets to reserve level .

We've ruled out starvation.

By opening up the line between the tanks and bowl, the problem is now worse. How could that possibly be?

I'm tempted to restrict the lines to see if that makes a difference..

I'm also considering a vacuum ... regulator?  That smooths out the vacuum pulsing that may be going to the petcock diaphragm

It is just baffling that reaching fuel reserve level could have any impact on carb other than starvation, which we've ruled out.


Perhaps I'm just mad that I made something worse.

Edited by MichaelS, 18 June 2012 - 02:06 PM.

Its a very common problem with the dr, just get a raptor and problem will be fixed.

 ManiacalChicken, on 18 June 2012 - 03:19 PM, said:

Or if you don't want to spend that much, get a Raptor petcock.

Thanks for the Raptor solution.   No doubt this has solved folks problems, and it might solve this one too.

Nonetheless, inquisitive minds want to know why!  Guess I'm more curious than concerned about fixing (which will happen eventually)

What is it about the stock petcock that, besides starvation, could possibly affect running performance?

Stock petcock tests fine with vacuum (no leaks) and the flow is immense now. What might the Raptor do differently?

Here's a strange theory:

Since the vacuum comes off the top of the CV... and the slide is doing a bit of bouncing...  some pulsing vacuum on the diaphragm is translated through the fuel hoses to cause a pulsing in the float bowl.  Perhaps all the filters I just removed used to dampen the pulsing?  Geeze, that seems like a stretch.

Anyone got a better theory than that stretch?

-Michael

I went out and took my old vacuum petcock apart. The only think I can think of is that you have some junk in the reserve circut. As for running performance, Ponder this, The bike is low on fuel and through normal rideing some air gets in the line. Possibly your floats are set a little low ( low fuel in the bowl) now add a shot of air to that = surging and cutting out. This is all an idea, not saying this is true. Its the best I can come up with without tryin to over think things. Get a raptor petcock and dismiss it as one of lifes little mysteries....

 MichaelS, on 18 June 2012 - 07:37 PM, said:

What the Raptor petcock might do differently is fill your engine with gas when you forget to turn it off.
Something to try, next time you hit reserve, use PRI instead.
The petcock vacuum is a separate passage, it doesn't care what the slide is doing. Here you can see the brass tube it uses.

I agree with CraiginDenver, The only setting on the stock DR petcock that does not need vacuum is the PRI or Prime setting, this setting is there so you can prime your carb after rebuild. That being said, if your problem is caused by vacuum, using the PRI setting should remove it.

Well, I found the issue...

When I opened up the fuel lines, I left (or knocked) the choke cable connector loose at the carb.  

Following Wyomedic's advice, I went to open up the bowl to check if replacing the stock filters loosened up some garbage that ended up in the bowl (or float valve).  

I noticed the choke cable connector was sort of hanging there.   Went ahead and cleaned out the carb bowl and jets (were spotless and clear) and tightened up the choke connector, and we're back to normal

Next test point is when I hit reserve level.  

Now, Where shall I ride the next 120 miles?

Thanks for all your help!

On the road,

-Michael

I am having the surging problem on my '09 DR650SE.  I bought it last weekend with 66 miles on it.  It has been sitting for about 2-3 years.  not sure when he bought it.  But the surging (missing?) happens at cruise mostly low-mid speed range.  accelerates smoothly.  sputters when I let off the gas clutch engaged or not.  I was hoping that draining the tank, filling with fresh gas and new plugs would solve it. Just in case I put some Lucas fuel system cleaner but so far no luck.  Was hoping not to have to take a part the carb so soon!