Calling James Dean about Diaphrams

11 replies to this topic
  • Fullbore4

Posted August 29, 2008 - 01:05 PM


Can anyone give info on how long an AP diaphram lasts and what effect it has as it ages. I put a new one in and it squirts a little too much. Was tempted to put the old one (3 months old) back in.

Also James Dean is there any way to verify the low throttle studder from the ignition and is a Dynatek a good replacement for an ignition?

  • Charles De Mar

Posted August 29, 2008 - 01:11 PM


:worthy: :busted: :thumbsup:

  • William1

Posted August 29, 2008 - 02:24 PM


Compare the old to the new. Is the rivet the same length? Diaphragms do not affect timing, only volume of squirt/duration. Shorter rivets squirt for a longer time.

We have no idea about what bike/year, what carb (stock or otherwise), what is your current jetting, any other engine/exhaust mods, where you ride (altitude and temp). I'll guess you have a WR but to help any more, we need to know what we are talking about

  • Fullbore4

Posted August 29, 2008 - 02:52 PM


My jetting data is on the last page of the jetting database.....its an 06 WR450....I have done the Red Beard mod and I would put my bike up against any other 450 made including CRF450R. I beat two of them hands down and it runs better than my buddy's tricked out KTM 525 EXC up to 60 mph. My only gripe is it has a low throttle position stutter like its getting too much gas. JD pointed out it could be the ignition. I dunno.

  • Thumper_Bloke

Posted August 29, 2008 - 07:27 PM


I love my Dynatek ignition. Has 4 curves. Starts really fast, no surging with advance switched on curve 3.

  • Fullbore4

Posted August 29, 2008 - 09:58 PM


Have you tried making your own curve for that Dynatek with their software? As old as I am (50), I enjoy wrenching and seeing how well I can make a bike young buddy who has an 07 CRF450R wants me to go through his carb as he's dissapointed in it since he rode mine. Being hard to get at it makes it tougher. Thing is like JD mentioned elsewhere, it depends on how one rides and many other factors so the definition of "runs great" is kind of subjective. The diaphram is one weak link since it stretches with use. I don't notice a stutter when on a track. I might try putting my old diaphram back in becasue I increased leak jet size at the sams time as putting a new one in. Its fun trying.

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  • clark4131

Posted September 02, 2008 - 04:00 AM


Did you post this in the Jetting forum?...SC

  • Fullbore4

Posted September 02, 2008 - 07:45 AM


No I haven't.......that would have been a better place I guess.

  • James_Dean

Posted September 02, 2008 - 01:59 PM


Make sure the diaphragm is the same length on the bottom stud. There is usually some stretching and fatigue over time, but changing a leak jet size or altering the timing screw setting will be more significant in most cases.

I don't know if the Dynatek ignition will reduce the common WR450F ignition stutter at low throttle/low speeds. I have not tested one to compare results on the dyno.

Diaphragms with different length studs below, and the longer stud will bottom out first, shortening the pump stroke.
Posted Image

Leak jet effects below, and the shorter stroke diaphragm is the lower curve (less fuel when twisting to full throttle).
Posted Image

The leak jet is located in the bottom of the carburetor float bowl for those that may not know about the part. The leak jet bleeds back a portion of the fuel pumped from the diaphragm cavity, returning the fuel back to the float bowl instead of squirting it into the motor's intake port. A bigger leak jet is bleeding off more fuel, and therefore is making leaner jetting.
Posted Image

  • Fullbore4

Posted September 02, 2008 - 03:03 PM


In my case, I've ground the diaphram stud off completely because I wired the AP linkage where some put an o-ring. JD, I know EFI is coming down the line but don't you think you could make an AP device much better than what we have to work with? With the linkage wired the diaphram I replaced, with just 3 months of use, was stretched out completely. The rubber part is much larger than the new one (same part number - with stud ground off) that I put in.

Also if I may hi-jack my own thread.......what will we gain and lose by going to EFI because I read long ago it will be hard to get instant throttle response with EFI like you can with the current carburetor.

  • James_Dean

Posted September 03, 2008 - 02:51 PM


If you really wanted to improve on the accelerator pump action, you could replace the pump cam and follower with the solid linkage that Honda came out with in 2007. The linkage no longer pulls with a flexible link, but pushes instead and has a roller-cam follower.

The accelerator pump has the advantage of instantly pumping as much or more fuel than needed, whereas fuel injection will pause to sense the drop in pressure before increasing the fuel supplied. The FI won't bog, but also seems smooth (or sluggish) and still favors slipping the clutch or downshifting.

See the Honda accelerator pump linkage differences below-

(Roller cam on right)
Posted Image

(Newer CRF pump linkage on right)

Posted Image

  • Fullbore4

Posted September 03, 2008 - 05:14 PM


I guess I don't understand why Honda didn't also change the other end of the accelerator pump and convert to a positive displacement, piston/cylinder arrangement instead of the diaphram. It seems to me the actuator cam, the inherent diminishing pumping action of the diaphram/stud and the leak jet are all redundant? I know I'm probably missing a lot.


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