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YamahaRacer

Boyesen Accelerator Pump Cover?

83 posts in this topic

Hey all,

I saw in one of my MX mags this month that Boyesen has a new accelerator pump cover for our bikes. It supposedly eliminates ALL bogging and throttle response problems. Anyone tried one or heard anything??

Thanks!

Chris :)

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We installed them om my YZ 426F and John's RMZ 250 last month. Super well made parts;works as advertised. The suggestion on the screw out 1/4 more worked well. I would recommend them to any FCR carb user. Do a search here for a pretty good thread on some members taht are using them. The advice on getting some time on it to get the air out before trying to accelerate hard in a corner or especially a jump was good thinking.

Bill

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We installed them om my YZ 426F and John's RMZ 250 last month. Super well made parts;works as advertised. The suggestion on the screw out 1/4 more worked well. I would recommend them to any FCR carb user. Do a search here for a pretty good thread on some members taht are using them. The advice on getting some time on it to get the air out before trying to accelerate hard in a corner or especially a jump was good thinking.

Bill

are you using the stock leak jet or #40 with it?

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Mountainman,

I am still stock on the leak jet with no problems what so ever on overall jetting. I did go three steps leaner on the main back in 2003 to accommodate the DCM and smooth out the power. Works beautifully for me at sea level and in humid southeastern conditions.

Ga426,

I got ours from Boyesen so I paid retail. When I got the covers I could not find them at the usual on-line stores. I imagine they are getting out now so maybe someone knows of a good price.

Bill

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Mountainman,

I am still stock on the leak jet with no problems what so ever on overall jetting.

Bill

i'm wondering how it would work with the #40 lj or if that would be too much of a good thing?

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i'm wondering how it would work with the #40 lj or if that would be too much of a good thing?

Can't you just install a smaller leak jet to accomplish the same thing?

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#40 is the smaller lj ,and yes it solved most of it but every now and then it still bogs (usually at a bad time)

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I run a #40 leak jet in the summer. I took the stock unit and silver soldered it shut for a true #0 leak jet. Runs great in cold weather! :)

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Can't you just install a smaller leak jet to accomplish the same thing?

I purchased the $89.95 pump cover (plus $8 shipping = $98) to make an evaluation of your exact question.

The answer is yes, a smaller leak jet will do the same thing. (The instructions also say to turn the fuel screw out 1/4 turn more and set the accelerator pump timing screw to stock.)

The pump cover is designed to refill the cavity with 2 ports flowing fuel upwards toward the diaphram, lifting the diaphram to add more fuel (only slightly). This might add 10% more fuel to the cavity which actually does seem to push more out the nozzle. I made some tests, and checked the duration and distance fuel squirted with an FCR carb and throttle off of the bike. The Boyesen cover change was compared to lowering the leak jet by #10 and #20 (#70LJ with Boyesen -vs- #60LJ and #50LJ with stock pump cover).

The leak jet lowered by #20 (#50LJ with stock cover) was pushing far more fuel than the Boyesen cover with #70. Both the duration and distance of squirt was greater with the #50LJ,

The stock cover with leak jet lowered by #10 (#60LJ) was nearly identical to the Boyesen cover with #70. The amount and distance the fuel squirted was nearly the same after repeatedly snapping the throttle open, both fast and with a brief delay between snaps.

If you want to get the same effect, just lower the leak jet by #10. Remove the float bowl and it's the little jet in the bottom.

Yamaha leak jet part numbers are-

#70- 4JT-1494F-15-00

#60- 4JT-1494F-11-00

#50- 4JT-1494F-07-00

#40- 4JT-1494F-03-00

#35- 4JT-1494F-01-00

(cost is ~$12 for these)

Honda leak jet part-

#55- 99108-MEB-0550 (cost is ~$8 for this)

Sorry to those of you who may have already purchased one. I spent the money on the cover and full range of leak jets too.

I hope this helps,

James

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Great job as usual JD we were under the same impression but you took the time to prove it! Thank You! :)

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JD,

I guess I knew better and should have waited until you reported your findings on this. Oh well, I will settle for the fact it didn't hurt anything but my :) wallet. At least I have some unseeable "Bling" now :p

Thanks for your research and the details. I am printing your post out and putting it in my binder with the other knowledge items from you and the others here at Thumper Talk. :)

Regards,

Bill Barnard

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I purchased the $89.95 pump cover (plus $8 shipping = $98) to make an evaluation of your exact question.

The answer is yes, a smaller leak jet will do the same thing. (The instructions also say to turn the fuel screw out 1/4 turn more and set the accelerator pump timing screw to stock.)

The pump cover is designed to refill the cavity with 2 ports flowing fuel upwards toward the diaphram, lifting the diaphram to add more fuel (only slightly). This might add 10% more fuel to the cavity which actually does seem to push more out the nozzle. I made some tests, and checked the duration and distance fuel squirted with an FCR carb and throttle off of the bike. The Boyesen cover change was compared to lowering the leak jet by #10 and #20 (#70LJ with Boyesen -vs- #60LJ and #50LJ with stock pump cover).

The leak jet lowered by #20 (#50LJ with stock cover) was pushing far more fuel than the Boyesen cover with #70. Both the duration and distance of squirt was greater with the #50LJ,

The stock cover with leak jet lowered by #10 (#60LJ) was nearly identical to the Boyesen cover with #70. The amount and distance the fuel squirted was nearly the same after repeatedly snapping the throttle open, both fast and with a brief delay between snaps.

If you want to get the same effect, just lower the leak jet by #10. Remove the float bowl and it's the little jet in the bottom.

Yamaha leak jet part numbers are-

#70- 4JT-1494F-15-00

#60- 4JT-1494F-11-00

#50- 4JT-1494F-07-00

#40- 4JT-1494F-03-00

#35- 4JT-1494F-01-00

Honda leak jet part-

#55- 99108-MEB-0550 (cost is ~$8 for this)

Sorry to those of you who may have already purchased one. I spent the money on the cover and full range of leak jets too.

I hope this helps,

James

James, so a "2003 YZF450 - with the JD Jetting kit" - go with the #50 leak jet for optimum fuel performance, correct? Does this cure all bog completely? Does the accellerator pump screw need to be reset, and to what?

Was there any other benefit to the Boyesen vs stock pump cover except visual?

thanks for testing this before I purchase... :)

David

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Ga426owner,

Dropping to a #50 leak jet should be a good setting for most riders. The accelerator pump timing screw doesn't need to be touched.

The jetting kit will give improved throttle response in all conditions, not just when the throttle is snapped. Most YZ450F's don't bog if the fuel screw is set carefully (part of the Boyesen cover instructions to turn it out 1/4 turn).

Thanks,

James

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I wired the black accelerator pump actuator to the adjustable screw and had favorable results.

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Thanks James! I was just about to buy the cover. You saved a lot of riders money by doing this test and reply. :)

Matt

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well my question still stands,as it doesn't look like you tried the cover together with a smaller jet. i have the #40 lj and was wondering if they would compliment each other or it would just be like taking away another 10 from the #40??? JD if you have time could you try that scenario :) ....thanx bob

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Has anyone tried the leak jet that James is talking about? Does it really make a difference?

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I have the #40 in there right now and it removed a lot of the bog. It will still hesitate if I go from low rpm to WOT but all that would give me is wheel spin anyways. I would say the #40 leak jet solved about 90% of my problems. I think adjusting the the AP timing screw a bit will get me that other 10%.

Total cost ... $12 leak jet and personal time.

I wouldn't waste your money on any of those aftermarket AP covers. It's not going to give you anything changing the leak jet won't.

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