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AP question

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Should the accelerator pump nozzle be adjusted so that the stream hits the needle, or go straight into the throat of the carburetor?

The reason for the question is because while getting the bike running again, i noticed that the AP stream was not going directly down the throat of the carb, so i remembered someone saying it should hit the needle, so i set it that way and now have a slight hesitation when opening the throttle that wasn't there before. In all fairness i changed internals, but i can't see how that would affect the carburetion.

Thanks in advance for any help.

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i remembered someone saying it should hit the needle,

TM40?

I believe that it should "hit the needle" dead on for the DR650.

Jeff from Pro-Cycle or MX_Rob may be able to verify. 👍:worthy:🤣

Edited by Bob Onit

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It is important that the nozzle’s direction be correctly

set. The nozzle is held in position by the friction

of its O-ring seal and can be turned quite easily with a

pair of long nosed pliers. Rotate the nozzle until the fuel

stream strikes the needle. This ensures an even distribution

of fuel between the front and rear cylinders. Nozzle adjustment

should be made with the engine stopped and with a

minimum of pump strokes to avoid flooding...... 👍

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Ok, thanks MX_rob. I guess i'll need to look elsewhere for the source of the hesitation.

AP timing is the likely culprit. Too early and you get a rich mixture momentarily and a hesitation. Too late and you don't get the needed enrichment when things go lean during the initial throttle valve movement and a hesitation. When all other components of the AP circuit are functioning properly timing is the key to snappy throttle response. 👍

Check to see if there is a gap between the AP rod and the adjustable lever.... if there is you may be experiencing a lean hesitation.... if the rod is contacting the lever already you could be dumping too much fuel initially. Backing out the adjusting screw on the lever increases contact with the rod and starts the pump squirt sooner when the throttle is moved. I'd try setting the adjustment so the lever just contacts the rod and go from there...... if it still hesitates try going richer first (backing out the screw) and then try going leaner to see how the bike reacts. Of course all of this depends on the pilot circuit and needle being in proper adjustment. The AP can't make up for deficits in these circuits.🤣

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You got it right on MX-Rob, i backed off on the AP timing and it has cleaned it up considerably. I didn't realize that it was that sensitive, but it is. I'll be driving around with a pair of needle nose in my pocket stopping and tweeking the screw every few miles until i get it tuned just where i want it.

Thank you again for the info and the help.

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You got it right on MX-Rob, i backed off on the AP timing and it has cleaned it up considerably. I didn't realize that it was that sensitive, but it is. I'll be driving around with a pair of needle nose in my pocket stopping and tweeking the screw every few miles until i get it tuned just where i want it.

Thank you again for the info and the help.

:applause: Yeah, it's extremely sensitive actually... at least to get it as close to perfect as possible. An 1/8 of a turn on the FCR-MX AP timing can be the difference between🤣 and 🙂 not only does the FCR have the AP timing adjustment and travel stop but also a bleed jet to vary the squirt volume based on how quickly you whack open the throttle.... then there is the AP spring tension........ So many things to consider when making adjustments... but worth every second when you arrive at the "holy sh*t, this things runs awesome!" moment. :jawdrop:

Hang in there..... the more time you spend tweaking (and learning) the better your bike will run..... and you get to go ride at the same time. 👍

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Older and Slower what did you do with your eng problem? go back to stock? just wondering what you came up with.

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That timing adjustment screw will move out of adjustment on its own. You will need to put a glob of silicone or something on it when you get it right. I have the TM-40 and I have to keep fixing it because I have not had the time to "glue" it in the sweet spot which I am not sure I have found. Jeff at Procyle said the gap should be about the thickness of a popsicle. Just under 1/8".

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Ok dirtmonkey, thanks. I had the arm almost contacting the rod, maybe .005" gap. Obviously not enough. Thanks for the tip on siliconing it into position.

Bergman, well it's been fun, but i had it back together stock and it ran fine ... just like before with the cam in the slightly retarded position that it came from the factory in. Bought another piston (different vendor), talked at length with the people at Web cam about the problem and gave them my measurements from doing the clay on the piston with the cam in its various positions. What they think and my gut feeling about the whole thing is that the factory doesn't have the timing marks in the correct position on the fly wheel. Anyway, i pulled it back apart and put in the new piston and the 190 cam, but this time in the position that lines up advanced. The factory cam position is very slightly retarded and one tooth advanced is very noticeably advanced. And it runs. Not having a degree wheel, and not really wanting to tear the motor down yet again, i am leaving it like it is and seeing how it runs. I did make a post asking if anyone makes adjustable cam gears and i got no response. I also did a web search and come up with nothing.

So that's where it is for now. The next project is the YZ fork swap. thanks you again for all of the info on the bearings, that helps a bunch.

Cheers.

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Should the accelerator pump nozzle be adjusted so that the stream hits the needle, or go straight into the throat of the carburetor?

Yeah, Mikuni say the stream should hit the needle but it's my opinion that info is a holdover related to installation on a H-D Sportster. On the Harley if the stream misses the needle it ends up spraying onto the back wall of the manifold. Hitting the needle gives the AP spray a chance to make the required 90 degree turn to get into the Sportster cylinders.

On the DR650 it doesn't matter much as long as the stream is pointed more or less toward the intake port.

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im a little confused. the stream is supposed to just miss the slide and hit the needle?

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Yeah, Mikuni say the stream should hit the needle but it's my opinion that info is a holdover related to installation on a H-D Sportster. On the Harley if the stream misses the needle it ends up spraying onto the back wall of the manifold. Hitting the needle gives the AP spray a chance to make the required 90 degree turn to get into the Sportster cylinders.

On the DR650 it doesn't matter much as long as the stream is pointed more or less toward the intake port.

I was wondering about that line in the manual.... makes sense with that explanation. Yeah, the FCR nozzle shoots from the right side but aims the shot basically down the middle of the venturi without hitting the needle.

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