Converting Decomp lever to Hotstart?

5 replies to this topic
  • Traction12

Posted July 14, 2005 - 03:32 PM


I'm installing the 03' 450 cam into my 02' 426 soon and will not longer be needing my decomp lever. :applause: (Awwww Yeeaa!) While researching the swap I noticed that someone mentioned turning their decomp lever into a hotstart lever. I'd like to save a few bucks by doing the same, if possible. If it's not something worthwhile, or can't be done, I'll be posting a wanted add in the classifieds soon! Has anyone done this before? Thanks!

  • Chills

Posted July 14, 2005 - 03:50 PM


Hey Traction,

While it might seem worth wild to convert over your current Decompression lever to a hot start, I have read that people have issues with getting the setup for the new Hot start to seat properly into the carb and seal. I personally have used the decompression lever a few times to clear the cylinder out when it becomes hard to start, so it does come in handy. Also, the plug that replaces the decompression mechanism has also had some issues sealing in the hole that becomes vacant from the mechanism. I persnally have done either, other than the 450 cam mod, and while it does seem much easier to hold a lever in on the handle bars rather than reaching under to pull the hot start out, I honestly didnt want to deal with any potential problems that might arise from the decomp. plug, or the new hot start sealing in the carb...

Just my .02 cents

  • offroadnut

Posted July 14, 2005 - 04:35 PM


do you mean you can switch cams and still keep the decomp lever and assy. in place?

Visit the ThumperTalk Store for the lowest prices on motorcycle / ATV parts and accessories - Guaranteed
  • Vibeguy

Posted July 14, 2005 - 05:34 PM


If you decide to remove the decomp lever be sure to install the TT decomp plug, the stock plug from Yamaha will come out.

  • Chills

Posted July 14, 2005 - 06:18 PM



The decompression mechanism itself does not interefere with the cam, it inserts on the side of the cylinder head extending in to the right exhaust valve bucket, The actuating arm for decompression is kinda like a dowel pin and the part of the dowel actuator that extends over the edge of the valve bucket is cut is shaved to create a half moon effect to that the flat part hovers over the valve bucket, when you pull in on the decompression lever, it rotates the half moon dowel over the valve bucket pushing down, thus opening the exhaust valve slightly to allow pressure to escape.

With the 450 Cam, the decompression mechanism is located in the shaft of the cam. The cam has a spring loaded mechanism that operated a little nub extenting from the cam and making contact with the left valve bucket near the cam lobe. Because it is natually in the extented position, when the cam rotates at slow speed, the timing is set to allow the little nub to slightly open the exhaust valve, not interefering with any other operation, once the revolutions of the cam increase, centrifical force overcomes the spring actuation of the decompression mechanism on the cam, and retracts the little numb from making contact any more with the exhaust valve bucket.

Therefore, there really is not NEED to remove the decompression level, but some do. If you do remove it, you will need a decompression plug in order to fill in the hole that where the mechanism itself is removed from on the side of the cylinder head...

Hope this helps explain it a little and how they work...

  • grayracer513

Posted July 15, 2005 - 04:09 PM


You could convert the decomp lever to a hot start, BUT, it makes a rather large hot start lever, and you would need some sort of aftermarket cable and plunger assembly.

If you want a bar mounted hot start, I'd say get the Dr.D. I have one on the 250F, and it works well. You can get water into the plunger bore via the cable and cause it to stick, so either re-grease it regularly with WP grease, or just run it for a few seconds after washing to slurp out the water.

do you mean you can switch cams and still keep the decomp lever and assy. in place?

Why yes you can, and you can even put a manual decomp kit on your '03 or later YZF if you want to. I have no idea why you would, though.

Your options are several:

>Take the decomp shaft out, and replace it with Yamaha's fine OEM bore plug. Be sure to paint it a bright color so you'll be able to watch it fly out of the bore. Start a pool about when it will come out and keep half the money. Try to make it hit a race official.

> Remove the shaft and see if you can find a steel cup plug at an auto parts store that fits properly.

>Take the shaft out and use One of These They work very well.

> Cut the lever off and use it for a plug. Cut off BOTH the outer lever and the half round extension that overhangs the valve lifter. (CUT BOTH if you do this!)

> Leave the lower lever in place without the cable, etc. Just make sure the coil return spring at the head remains in place to keep the lever out of mischief. The disadvantage is that it will be in the way if you need to adjust the right exhaust valve.

> Leave the whole thing just as it is. Same disadvantage as above, plus the weight, and it's pretty much completely useless with auto decompression.

Related Content


Thinking about Yamaha 250... by Arctic Pride

Dirt Bike   Special Interest Forums   Pro Racing
  • Hot  491 replies

yz250x vs 450 4t comparisons (preferably desert riding) needed by JakeNorthrupYZ450F

Dirt Bike   Make / Model Specific   Yamaha   Yamaha 2-Stroke
  • 9 replies

Need help by Goforaride

Dirt Bike   Dirt Bike Technical Forums   Suspension
  • Hot  68 replies

YZ450F 03 Sparks driving me crazy by SirAttard

Dirt Bike   Make / Model Specific   Yamaha   YZ 400/426/450
  • 5 replies

Is the 2016 GYTR Yamaha YZ450F too fast? by Fryboy

Dirt Bike   Special Interest Forums   Pro Racing
  • * - - - -
  • Hot  241 replies

Join Our Community!

Even if you don't want to post, registered members get access to tools that make finding & following the good stuff easier.

If you enjoyed reading about "" here in the ThumperTalk archive, you'll LOVE our community. Come join ThumperTalk today!

The views and opinions expressed on this page are strictly those of the author, and have not been reviewed or approved by ThumperTalk.