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Old 12-16-2012, 09:05 AM   #71881
Mambo Dave
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Originally Posted by Kommando View Post
Why not just add preload, by using longer spacers? Keep your springs intact. It's easier to revert back to stock or to sell.
Selling isn't a worry - most americans are frickin heavier than I am, so they'll need the extra stiffness.

I've read up on suspension enough to believe that preload is preload - it's either set right or it isn't. While it may change the spring rate a fraction of a percentage, it really isn't supposed to, and it doesn't make up for spring rate changes.

Adjusting preload with the end-goal of a quick and dirty rate change is sort of ... well, it just doesn't seem right.
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Old 12-16-2012, 10:56 AM   #71882
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Quote:
Originally Posted by Mambo Dave View Post
Selling isn't a worry - most americans are frickin heavier than I am, so they'll need the extra stiffness.

I've read up on suspension enough to believe that preload is preload - it's either set right or it isn't. While it may change the spring rate a fraction of a percentage, it really isn't supposed to, and it doesn't make up for spring rate changes.

Adjusting preload with the end-goal of a quick and dirty rate change is sort of ... well, it just doesn't seem right.
On progressive-rate springs, like the DR's stockers, a change in preload changes the spring rate.
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Old 12-16-2012, 10:58 AM   #71883
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Originally Posted by LexTalionis View Post
Have over 18k miles (you can do the conversion math to kilometers) on my OEM carb, haven't opened it up since I adjusted the fueling at around 500 miles by installing a washer and backing out the A/F screw.
As shimming the needle clip preloads the slide spring beyond standard, it results in lowering the slide rather than raising the needle (except when the slide is against the stops). I would not shim the needle clip unless the goal is actually to lower the slide for a given intersection of throttle angle and rpm. Note that the additional preload from shimming the needle will also make the slide come off the stop at a later point in terms of rpm and throttle position (i.e. more velocity will be required to get it to come up off of the stop).

The most proper way to refer to the idle mixture screw (aka pilot screw) on a BST carb is as a fuel screw, as it does not adjust air and fuel simultaneously. There are things called air screws on carbs where you adjust the idle mixture by changing the quantity of air being bled into the pilot circuit. The reason this is important is that a fuel screw makes the idle mixture richer by screwing out and leaner by screwing in, whereas an air screw makes the idle mixture richer by screwing in, and leaner by screwing out. Fuel screws are usually on the downstream side of the slide, and air screws are usually on the upstream side (although I have seen occasional exceptions).

Regards,

Derek
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Old 12-16-2012, 11:05 AM   #71884
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Originally Posted by blackcap View Post
seems i keep tearing through slides, slide guides and emulsion tubes on the BST. starting to feel like this set is done after only about 10 000km and the set before that was only about 20 000km
Have you been increasing the lift hole area, or leaving it stock? I assume this is a single lift hole model? Make sure you are preventing the ingress of dirt through the carb vent (the 1/2" {~13mm) steel elbow on the back of the carb, just under the diaphragm).

Regards,

Derek
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Old 12-16-2012, 11:15 AM   #71885
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Another chain question

Hi guys,

Quick question. I'm on the road in southern Argentina and can't replace my chain for a few days. I adjust at the tightest spot, of which there shouldn't be one after only about 10k kms. When I'm on the loaded bike the chain is quite loose. Better loose than tight, I know, but does anyone have any suggestions on other courses of adjustment? When should the damn thing be adjusted? Loaded? Loaded, with my fat ass on it as well? Unloaded? On side stand? Upright under its own weight? Rear wheel off the ground?

Will a sticky point chain ever "unstuck"? It's at 4+2 on the adjuster cam now.

It's all cleaned up, lubed and awaiting your collective wisdom.

Cheers!
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Old 12-16-2012, 12:49 PM   #71886
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Originally Posted by blackcap View Post
i dont know anything about the TM40 or how it operates.
Slide carbs will always have a loss in bottom end to lower midrange of the rpm range at the larger throttle openings because the air velocity (and therefore the quality of the metering) is proportional to the engine rpm and inversely proportional to the throttle opening. Slide carbs require you to learn throttle control. When tuned properly (I say "tuned properly" because it is possible to mask many problems with an overly rich mixture), you cannot open the throttle to WOT or near WOT suddenly from low rpm without significant hesitation (its even possible to stall the engine if you dont back off). A properly tuned accelerator pump will help this, but is not likely to completely mitigate the problem. On the other hand, CV carbs allow you to open the throttle all the way from low rpm and will pull smoothly when tuned correctly, because the height of the slide automatically attempts to maintain a consistent velocity.

The ability to open the throttle to WOT from low rpm without hesitation with a CV carb does come with a price, and that is in a comparative lack of responsiveness in those areas where the combination of throttle position and rpm does not cause too much of a loss in in intake velocity with the slide carb.

Another area where the CV carb has an advantage is in the ability to change the fuel delivery curve based on rpm via the needle shape. On a slide carb, you can add or subtract fuel via tuning a circuit responsible for a given throttle position, but you cannot change the shape of the delivery curve in terms of rpm. For instance, if you had a lean and a rich spot at different rpm at a certain throttle position, you could fix the lean spot while making the rich spot even richer, or you could fix the rich spot while making the lean spot even leaner. You could not fix both. Unfortunately, these types of scenarios happen quite frequently.

The throttle pull on the TM40 is also quite a bit heavier than on the BST40.


Regards,


Derek
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Old 12-16-2012, 12:54 PM   #71887
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Quote:
Originally Posted by Kommando View Post
On progressive-rate springs, like the DR's stockers, a change in preload changes the spring rate.
If the fork is far enough into the travel, the rate becomes the same as it was again.

Regards,

Derek

motolab screwed with this post 12-16-2012 at 01:05 PM
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Old 12-16-2012, 01:04 PM   #71888
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Originally Posted by Mambo Dave View Post
I've got to imagine that since the extra range of the springs is being substituted for firmness, that it becomes a tiny bit harsher for a given outcome than would buying new springs that are just as long as the stock ones, but made thicker.
It makes no difference whether the rate is achieved with a shorter spring or bigger wire. Rate is rate. The only time you have to worry about a shorter spring is if the travel is so long that that the spring could become coil bound before the fork would normally bottom (not a common issue).
Quote:
Anyway, I can't quite see how cutting my springs to save money would be such a bad idea since I'm not creating a Baha racing machine here. The last bike I bought springs for showed me that aftermarket springs were often shorter, anyway, than the originals.
Just make sure you cut them evenly so the rates are the same, and that you close the ends and grind them so they are square and flat when you are done.

Regards,

Derek
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Old 12-16-2012, 01:36 PM   #71889
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Quote:
Originally Posted by motolab View Post
It makes no difference whether the rate is achieved with a shorter spring or bigger wire. Rate is rate. The only time you have to worry about a shorter spring is if the travel is so long that that the spring could become coil bound before the fork would normally bottom (not a common issue).Just make sure you cut them evenly so the rates are the same, and that you close the ends and grind them so they are square and flat when you are done.

Regards,

Derek
Thanks man - like, evenly within... say... 2mm or 4mm? or are we talking precise here?

I take it the PVC i should buy will be schedule 40... anyone happen to know the size?
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Old 12-16-2012, 01:55 PM   #71890
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Originally Posted by Mambo Dave View Post
Thanks man - like, evenly within... say... 2mm or 4mm? or are we talking precise here?
I wouldn't worry too much as the rates likely varied to some degree to begin with. If you wanted to do a really good job, you could measure the rates after cutting and then once again after closing the ends and grinding. Oh, and a belt sander will do a far better job getting the ends flat than a grinding wheel.

Regards,

Derek
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Old 12-16-2012, 02:41 PM   #71891
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Originally Posted by SprintST View Post
Hi guys,

Quick question. I'm on the road in southern Argentina and can't replace my chain for a few days. I adjust at the tightest spot, of which there shouldn't be one after only about 10k kms. When I'm on the loaded bike the chain is quite loose. Better loose than tight, I know, but does anyone have any suggestions on other courses of adjustment? When should the damn thing be adjusted? Loaded? Loaded, with my fat ass on it as well? Unloaded? On side stand? Upright under its own weight? Rear wheel off the ground?

Will a sticky point chain ever "unstuck"? It's at 4+2 on the adjuster cam now.

It's all cleaned up, lubed and awaiting your collective wisdom.

Cheers!
The pic shows where you want your chain to still have some free play. It's where the chain is at its tightest, whether the bike is loaded or not. I use a ratchet strap over the seat to pull the bike down until the three points are in line. Lay something over the seat so the strap doesn't cut the cover. Depending on how long the kickstand is, be careful the bike doesn't tip over (high side).

Thanks to BergDonk for finding the pic.


Quote:
Originally Posted by Mambo Dave View Post
.............................

I take it the PVC i should buy will be schedule 40... anyone happen to know the size?
Stock fork spring spacer: 37mm diameter (1.465") - 40mm tall (1.58")
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Old 12-16-2012, 03:32 PM   #71892
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[QUOTE=ER70S-2;20263761]The pic shows where you want your chain to still have some free play. It's where the chain is at its tightest, whether the bike is loaded or not. I use a ratchet strap over the seat to pull the bike down until the three points are in line. Lay something over the seat so the strap doesn't cut the cover. Depending on how long the kickstand is, be careful the bike doesn't tip over (high side).

Thanks to BergDonk for finding the pic.


Thanks ER70S-2
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Old 12-16-2012, 04:03 PM   #71893
Mambo Dave
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Originally Posted by motolab View Post
If you wanted to do a really good job, you could measure the rates after cutting and then once again after closing the ends and grinding.
Regards,

Derek
Wow! what a great idea! Not sure where I'd get the measuring apparatuses, but a damned fine idea!

Quote:
Originally Posted by ER70S-2 View Post

Stock fork spring spacer: 37mm diameter (1.465") - 40mm tall (1.58")
Thank you!
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Old 12-16-2012, 04:09 PM   #71894
ER70S-2
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Originally Posted by SprintST View Post
Thanks ER70S-2
Quote:
Originally Posted by Mambo Dave View Post
Thank you!
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Quote:
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"BTW, I don't do style. It's a dirt bike, not some girlie dress-up thing." -
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Old 12-16-2012, 04:50 PM   #71895
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Originally Posted by motolab View Post
Just make sure you cut them evenly so the rates are the same,
Derek
Why? The forks are going to be connected by the axle and will move up and down synchronously.
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