ktm 640 or 625

Hi everyone. I was looking to get a 06 ktm LC4 640 but I lived in Canada and those are not available so now I looking for the 625 sm. I was looking on the KTM website and it seams like there is a big power difference between those 2 bikes. it said that the 625 has only 25 hp and 40 lbs torques which is far less than the 40hp and 60 lbs of torques of the LC4, is there any mistake???

what do you guys think of the 625 I have a drz 400 and I REALY need more power I'm 6'5 and 220 lbs. I would realy like the 640 but I quest I will have to go for the 625.

Isn't that lower power rating due to a removable carb restrictor? I think it is supposed to limit how far the slide opens on the carb. If so, it's easily removed for full power. FWIW, the 640s are actually 625cc just like the 625 models. Ktm is just being a bit more "realistic" these days.

A deresticted 625 should have plenty of power for you.

The current (2003 to present) 625/640 LC4 engine is (nominally) rated, according to KTM, with all emmisions/sound controls in place at:

40 KW or 53.64 HP @ 7000 RPM

55 NM or 40.56 ft.lbs. @ 5500 RPM

These are US specifications... YMMV


ok lets see if I understand that. The 625 and the¨640¨makes the same power or will after a simple carb modification. which is way more than it said on the KTM web site!!!did I get that right


ok lets see if I understand that. The 625 and the¨640¨makes the same power or will after a simple carb modification. which is way more than it said on the KTM web site!!!did I get that right


The 625 and 640 are the same displacement... the same engine. If you are comparing the 625 SMC to the 640 Adventure, based on the information I have available, the only difference is gearing, intake and exhaust.

The actual peak HP variations between the two might not be as substantial as one might first imagine, as both have similar carburetor bore sizes.

All else being equal, the 625 SMC will in all likelihood produce a bit more peak HP than the 640 Adventure due to the more air-flow efficient FCR41 vs. the Adventure's more emissions efficient BST40.

With identical bikes, one equipped with a FCR41, the other with a BST40, The bike with the FCR will have better throttle response and consequently better acceleration... particularly when starting at smaller throttle openings.

As throttle openings increase, the performance difference will be less pronounced.

The trade off for this is fuel economy and, not that anyone cares... emissions.

The BST40 uses its fuel in a more efficient manor vs. the FCR, which uses it’s fuel delivery capabilities to deliver optimized performance.

I'm not a carburetion expert, and my understanding may have a few gaps which a real carb expert would correct and/or clarify... but I believe I have a pretty good handle on the subject.

Here is, if you care to read it, a brief discription of design and function of the two types of carburetors, which I wrote for another forum several months ago.

The variable venturi, slide type carburetor is represented by the current crop of high-performance and competition 4-stroke carburetors like the Mikuni HSR and TMR series and the Keihin FCR series.

The “size” of the venturi is established by the position of the slide, which in turn is controlled buy the operator’s right hand... and skill level.

The negative pressure or vacuum in the venturi area, and the volume of fuel drawn into it varies with the slide position, engine speed, load and elevation.

When used by an operator who understands the design, how to tune them correctly… and has the skill to benefit, a variable venturi carburetor offers near instantaneous throttle response.

Variable venturi carburetors require an accelerator pump to offset the loss in negative pressure or vacuum that occurs when the throttle is opened abruptly or at too low an RPM to maintain that vacuum.

The air in the column responds virtually immediately, but the fuel, having temporarily lost its high vacuum signal, takes a moment to catch up to the shift in pressure vs. area... an accelerator pump fills that "hicup" in fuel delivery.

The constant velocity carburetor, represented by our BST40 is, in very simple terms, a combination of a fixed and a variable venturi… to a point.

The operator has complete control over the butterfly valve downstream of the vacuum piston or slide, however… the piston itself is controlled not by the operator, but by the balance of pressure in the carburetor.

Negative pressure or vacuum, relayed via transfer ports in the bottom of the piston to the area above the vacuum piston’s operating diaphragm, and positive pressure beneath the diaphragm relayed via an atmospheric port, are what control the pistons movement and position.

An accelerator pump is not needed in a CV carb because in theory, velocity thru the venturi is held at a constant and high negative pressure. If there is no abrupt loss of that high vacuum signal, then fuel delivery to the venturi is also constant and determined only by jet and needle sizes... hence the name, constant velocity or CV.

The operator can send a pressure “signal” by closing or opening the throttle, but if the signal is inappropriate for the manifold pressure at the time, the vacuum piston serves to “dampen” the engines response to that signal while maintaining that all important high vacuum.

To put it another way, if the operator is unskilled at matching RPM, load and throttle position, the constant velocity carb will dampen his or her mistakes… to a degree.

The CV carb is actually more efficient than a variable venturi carb in the sense that it uses less fuel to produce a response, but due to the damping characteristics, that response may not be as quick to occur as some would like. In a performance application, the delay, no matter how short, is unacceptable.


Hey thank you!!! That answer to all my question

Hey thank you!!! That answer to all my question

I've already invoiced you for tech services. :banghead:

Just to echo what has already been said. The 625 has a restricted FCR carb. Remove the throttle stop and its far superior to the BST40. Offroad the BST40 sucks! The BST40 is also quirky (stalls,weird idle or no idle, etc) on road but a lot of fiddling can get it sorted out (kind of)

Recently two freinds of mine had BST40 carbs on their 01' Duke II and 00' LC4 dualsport respectively and both bit the bullet and bought FCR41 Keihin carbs for the KTMs and are extremely pleased.

I had a 2002 DUKE II with a BST40 and now have an 04 625SMC with a FCR41 (derestricted carb, modded airbox, Hard parts airbox lid, jetted etc) .... its night and day. Best bang for your buck IMHO!

I also replaced the BST 40 carb on my 00 Duke II with a FCR 41. I dont think the BST 40 is a bad carb, it performed well for me after rejeting and did give better gas mileage, but the gain in performance by switching was astounding. Money well spent.

I do agree that the FCR is a better carb for performance but w/ proper set up the BST has been working great for me. I have put 12,000 miles on my LC4 since I bought it. That make 20,000 miles overall. All I had done was rejet it, open up the air box up and drill out the slide. The stop was already removed, so I don't know what a stock LC4 is like. For me this is a great compromise between mpg and fun factor.

Just to get back on topic (since the original question has nothing to do with CV vs FCR carbs), the reason the SMC is underated according to KTM is simply the slide travel limiter on the slide. Try dyno'ing a bike at 1/3 throttle and see what happens...The proper part to fix this costs like $4 from Sudco or you can modify the stock slide stop screw with a dremel. Get a 625SMC and don't look back (they both have the same motor anyway)!

While you're talking carbs, can I pile on a question?

I acquired my first KTM about three weeks ago; a 2005 640 Adventure. It stalls when I close the throttle, even when warm, unless I use some choke to keep the idle up to about 1700rpm.

What idle speed do you guys use?

I'm thinking I'm needing some carb work.

Welcome and congrats on the 640 purchase. :ride: Check out Advrider.com. There is a great how to index for the LC4 640 bikes. It's in the Thumper section at the top. Good luck. :crazy:

Hi there people, I have a 06 625 sxc with JD Jet kit, air box mod and full staintune Race exhaust, run on 100 octaine fuel only, I also run full soft mx tyres and a 49t rear sprocket. Its just had a 1000k service how much hp should I expect? Ps it can be qiet a handfull in tight heavy sand but a real blast on the open Pine trials....

Why do you need to run 100 oct. fuel? I get by just fine with Premium 93. I've got 21,200 trouble free miles on mine. These are great bike. :crazy:

The only reason for the fuel is ease of obtaining it, there is only a marginal difference with the 98 octaine premiume being on pump. I also ride with WR450s and 525s and some times in need all the punch I can get, But i figure get good on this and have a challenge...

My 625 smc was loads faster the my old 640. Reved higher, more top end power. Its lighter aswell. different switch gear clocks etc. Alloy rear subframe.

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now