My WR400 carb doesnt look like the one in the manual.. What is this..


7 replies to this topic
  • cjard

Posted September 24, 2009 - 11:45 AM

#1

At first glance it looked like it should have something electrical connected to it, but thinking about it further, it might be a hot start valve that is missing the pull out knob? Why is it not attached to the carb like the manual/videos i've seen?

Also, is there any deviation from the dicumented starting procedure for a bike that is configured like this? I think i'm following the steps in the video accurately, but it's still a pig to start (havent had it running yet, after rebuild - believe I have timed everything up correctly, have spark, next step to check that fuel is reaching..)

Cheers

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  • jayh300

Posted September 24, 2009 - 12:06 PM

#2

that is the correct carb for a 400 and that is a remote hot start with the knob gone... atleast that is the way my yz400f looked.
as far as starting procedure, does it still have the manual decomp lever ?

  • GCannon

Posted September 24, 2009 - 12:13 PM

#3

Clean the Carburetor that will make it start better. Adjust the valves and clean the air filter while your at it.

  • cjard

Posted September 24, 2009 - 03:28 PM

#4

I guess I'll pull it in bits again this saturday if I don't get any joy in starting it.. Thanks for the feedback guys.

ps; is it possible to buy another hot start knob or should I just clamp something on it? Maybe I can form another one out of a blob of epoxy resin..

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  • cjard

Posted September 24, 2009 - 03:34 PM

#5

ps; yep, it has manual decomp.. I'm kicking until it's solid, decompressing, pushing the kick "one notch" further (maybe that part is taking it over top dead? I havent looked on the timing sight window), releasing decomp, returning kicker to top and kicking through..

I had a question about that actually.. Let's assume that kicking until we hit compression, we have charged the cylinder with petrol/air mix, but too much to compress easily.. so decompressing lets some of this escape.. this means that when we kick, there is less to compress (easier) but still enough for a small explosion that drives the engine sufficiently into proper 4 stroking power..

OR

is it the case that we find compression, decompress, go over top dead/timing, miss our spark, release decompression, then a through kick takes us all the way through exhaust stroke with enough momentum to complete a proper intake stroke (rather than a partly depressurized one)

I'm most curious to know which of these is the case.. It may help me to look for the timing marks to know if I missed the spark, if the method is the first one..

  • jayh300

Posted September 24, 2009 - 05:33 PM

#6

for the most part, you could live without a hot start. the nipple on the intake port could be capped to make sure the hot start is not partially open, esp with the knob gone... it would rule out one problem.
you are kicking it right, push kickstarter to tdc, decomp, one notch farther, reset kick lever and kick. on my 400 like that it was no throttle at all.
cold, pull choke, put hand on brake master cyl to avoid twisting while kicking. 3-4 kicks and it would lite right up...

  • ncampion

Posted September 26, 2009 - 07:19 AM

#7

I had a question about that actually.. Let's assume that kicking until we hit compression, we have charged the cylinder with petrol/air mix, but too much to compress easily.. so decompressing lets some of this escape.. this means that when we kick, there is less to compress (easier) but still enough for a small explosion that drives the engine sufficiently into proper 4 stroking power..

OR

is it the case that we find compression, decompress, go over top dead/timing, miss our spark, release decompression, then a through kick takes us all the way through exhaust stroke with enough momentum to complete a proper intake stroke (rather than a partly depressurized one)

I'm most curious to know which of these is the case.. It may help me to look for the timing marks to know if I missed the spark, if the method is the first one..



The later is the case. You can confirm this by looking at the timing marks after you have "decompressed". Check this thread: http://www.thumperta...ad.php?t=827834


.

  • grayracer513

Posted September 26, 2009 - 09:30 AM

#8

.. Let's assume that kicking until we hit compression, we have charged the cylinder with petrol/air mix, but too much to compress easily.. so decompressing lets some of this escape.. this means that when we kick, there is less to compress (easier) but still enough for a small explosion that drives the engine sufficiently into proper 4 stroking power..

This is how it's supposed to work. Running the engine up against the full compression stroke, then reducing the amount of compression you must develop with your leg by moving the lever about one inch farther, which cuts the compression stroke by about 1/3. Then you drive it through from there, getting your spark and ignition of the fuel charge, starting the engine. Less fuel is released than you think, though. The engine is only rotated about 30 degrees.

This is what the auto decompression on the later bikes does for you; it cuts the compression from somewhere around 150 degrees of crank rotation to about 85-90, allowing the engine to generate about 120 pounds of compression as opposed to 200.

OR...is it the case that we find compression, decompress, go over top dead/timing, miss our spark, release decompression, then a through kick takes us all the way through exhaust stroke with enough momentum to complete a proper intake stroke (rather than a partly depressurized one)

Nope. But that is how things were done back in the day of the Matchless and the BSA Goldstar, the old school thumpers with 50 pound crankshafts that could carry the engine completely through a compression stroke, once they were set in motion. The YZF doesn't have enough intertia for this technique to work well.

If the proper technique is used, most problems with starting a YZ400 or 426 result from an improperly set up idle circuit (fuel screw adjustment).

http://www.yamaha-mo...troke_vid_a.mpg

http://www.yamaha-mo...troke_vid_b.mpg




 
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