Carburetor Tuning

Again, thanks for the help guys. Yeah, this post has really turned around.

Going back to the original problem, I ended up ordering the 'correct' carburetor (PHM40) off ebay. I really don't like messing with carburetors and would like to use to the stock jetting and carburetor maybe with a few tweaks (air mixture screw and needle placement). When I was playing with the air mixture screw the other day trying to get it to idle somewhat decently, I noticed that the pipes were glowing red and immediately shut off the bike. That was at two turns out (leaner), so I immediately turned the screw all the way back in and decided to stop messing around with this carb. This is one of the reasons I still think the carburetor/jetting is the culprit (lean conditions) as far as hard starts and popping during idle. I wish someone had a jetting configuration for a 38mm carb on a 600 but I couldn't find any specs. If you know anyone that wants a VHSB38 cheap, let me know.

Paul - Honda's use a distributor ignition system up til 2000 (a very common car to work on as a mechanic), and about half of them can still be adjusted by spinning the distributor. Either way, many other makes (mainly Japanese) used adjustable timing up til mid 90's, which seems to be the majority of cars I work on at my shop. I've only ever needed a static timing light because all these cars have multiple marks on the crankshaft pulley to line up. Usually there is a TDC mark (white), a couple marks between, and then the appropriate timing mark for idle speed (red).

My spark plug is a brand new platinum NGK. I have to wait for the carb to come to time the bike at the idle (still doesn't idle reliably) but I could atleast check the maximum advance for now. I have a feeling it's probably going to be a bit too advanced as it is hard to start.

Thats funny about the glowing pipes, but it doesn't surprise. After working on a Husaberg and Husqvarna alot recently, they don't seem to be ordinary machines like I'm used to working on. The last time I saw an exhaust manifold glow was on a turbocharged charger (I bit more acceptable).

Man I wish I had these settings before I bought the new carburetor, especially cause it didn't come with the plunger for the cold start so now I have to source that also. Those specs are virtually identical to what came in the VHSB38 when I bought the bike, yet still I could never get it too idle, start easy, or not pop during close throttle deceleration. Perhaps my ignition system (or not being able to turn the engine) is the real problem....

While I'm at it, do you guys have good specs for the PHM40 with an exhaust system (Big Gun if it matters)? I minus well try a setup that someone knows works well already. I have quite a few different jets laying around now so I can play around for a while. Also, is there a huge difference between the K35 and K51 needles? I only have K35's laying around. While I was able to find their geometric differences, I couldn't find much information of how big of the difference between running conditions.

For reference from another post:

K35 = 2.50 - 1.40 - 43
K51 = 2.52 - 1.45 - 45

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A bit off-post, but perhaps interesting anyway: the glowing pipes are nothing to be overly concerned about. It's just a consequence of using reasonably thin-wall, stainless steel primary-pipes. Stainless has realatively low thermal conductivity cf. other exhaust materials and you can make the wall thickness very small because of its high corrosion resistance => low thermal mass and inertia => rapid heating during blow-down => a very surprised lee30bmw!!!

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Yeah makes sense to me. I'm actually working on a giant thermodynamics problem for my senior project right now in school, so I'm pretty familiar with this stuff. Generally things that are more reflective (stainless) do not conduct heat via radiation as well as something that is say, black. I figured the pipes could heat up quickly because of their low mass, and therefore thermal capacity, but like I said earlier, it's just not something you're used to seeing when working on 'normal' cars and motorcycles. The fact that I was chasing a lean problem amplified the shock when I saw the glowing pipes. I didn't really feel like melting a piston at that point....

The dealer I bought my '96 FE501 from said that the Delorto needles were kind of like black magic. Now, if I remember right, the K35 was used with the 38mm flat slide on the 501/600 but when they went back to the 40mm round slide the 501/600 used the K51 (my '02 FE650 had a K51) but the 400 continued using the K35. It also seems like the 400 came with the flat slide 38mm carb in '97/'98; the 501/600 used the 40mm round slide those years, It seems kind of weird that yours has the 38mm but the jetting sounds right. Somewhere I think I still have the brochures for '96, '98, and '99 and if I could find them it would confirm this. I may do some digging through my motorcycle stuff....

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I have the manual for the 98 models. Here's a pic of the jetting chart:


It looks like you were right about the needle usage, as the 98 501/600 claim to have a PHM40 carburetor with the round slide. It says the 400 has a PHM38, but doesn't mention whether it's a flat or round slide. I would be willing to bet it is a flat slide like you said as the 400's are the only bikes using the K35 needle. I imagine a combination of all these things on the 400 (smaller diameter carb, different jetting, etc) were all done in an effort to set the powerband a little lower to make up for the smaller displacement.

BTW, the advantage of the flat slide carb is that it takes less energy to twist the throttle than with the round slide as there is less friction (less metal to metal contact) than with the round slide. These big 4 strokes create a lot of intake vacume. My 650 took considerably more wrist strength to open the throttle than did my 501 which was probably a good thing!

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You're right about that. I was surprised by the weight of the round slide compared to the flat, and it's definitely noticeable at the throttle.

So now that the carburetor is in (with the aforementioned jetting, probably a bit on the lean side but enough to get me going for now) I was able to get the bike running a lot better. It no longer pops during idle and deceleration, and can actually idle. The carburetor didn't come with the choke/cold start plunger setup, so I had to plug the hole for now which simulates the choke being on. I realize this means I won't be able to set the idle speed yet or play around with the mixture screw yet. The hard to start situation is still there, but I am going to wait to ask anymore questions about that before checking the ignition timing properly. However, I do have yet another question....

The previous carb's mixture screw was an 'air mixture screw' meaning turning it out goes lean, turning in goes rich. You could follow air jet on the intake side of the carb to the screw:


The new carburetor that I just installed (PHM40) seems like it has the mixture screw that adjust the fuel flow instead of air:


Pleasure ignore the plug where the choke plunger is supposed to be...

You can follow the passage from idle jet to the mixture screw and then to the orifice in the 'after slide' portion of the carb, the part that receives manifold vacuum. Am I correct about this, or am I just not able to see the passages correctly? I wanted to know for when get the choke plunger so I can start tuning the idle mixture.

After all the helps you guys have been, I would be glad to do that. I don't have a scanner though. If you don't mind, I could take high res pictures (as I often do with my textbooks so I don't have to carry them) and make it into a .pdf file for you.

The mixture screw on this carb is a fuel screw - turning in leans and out richens.

Another advantage to the flat slide 38mm over the round slide 40mm on the 501/600 is better throttle response as a result of increased intake velocity. I have ridden a friend's '98 FC501 after getting off my '96 FE501 and my bike felt as though it had more power and definitely picked up rpm up faster. The '98 motor felt almost sluggish in comparison. On the other hand, it was a bit easier to ride in tight technical stuff....

BTW, can't you use the choke assembly fron the 38mm carb?

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Well maybe the previous owner switched to the 38mm carb for that reason. Either way, the PHM40 is in and is running alot better now.

The choke assemblies are different from carb to carb. The 38mm carb has a much longer assembly that is held in with a screw while the PHM40 carb has a choke assembly that has the threads integrated into it's body.

No problem.

Just got home from school and instead of doing homework like I should, I decided to throw the choke lever and see if I could make any progress...

I got the bike to fire up super easy when cold! It idles steady now and doesn't pop at all during deceleration. I rode it around and seems to run really well. I was able to play around with the idle speed and mixture screw to make it idle the best I could.

I brought it into the garage and shut it off to make sure I could get it to start up warm also. I wasn't able to get it to start, so I pulled the plug to see what was going on. It definitely looks like I'm running on the rich side even with the pilot screw turned all the way in. The spark plug was had developed a little bit of soot even after a 2-3 mile ride. On top of that, the plug was really wet. Once I cleaned it off, I was able to get it to fire up again. To remind you I'm running:

190 Main Jet
DR270 Needle Jet
K35 Needle (clipped on the highest position because of the undersized main jet. Remind me: is that position #4 or #1?)
45 Start Jet
48 Pilot Jet

Given that the manual states the FE600 it supposed to have a 40 PJ, that's most likely my problem. It makes sense that because it will start cold but then have a tough time restarting warm. I have a 33, 35, and 68 (ha) PJ. The needle position could be too high also, but I'm not getting any rich bogging at any throttle position. What do you guys think?

Looking at different carb tuning charts on the internet (the ones that show fuel flow vs. throttle position from each portion of the carb), most of them show the needle position affecting up to about 3/4 throttle, with a diminishing effect. Even on the husaber owner's doc it states that you need to move your needle based off changes in MJ size. In the end, it probably depends on the carb design (slide length, needle length/taper, etc) itself, as once the slide is pulled up far enough, the needle should be fully removed from the atomizer/main jet and therefore no longer have an effect. All main jet at that point.

Becoming an old man at the age of 26, I've learned my lesson running an engine too lean, and don't want that experience again. I was worried that with a supposedly undersize MJ and exhaust system, the engine might start to lean out under higher loads, and hence wanted to move the needle up more to correct for that to atleast richen the 1/2 - 3/4 throttle area.

Now after reading about Husaberg's, hearing other's jet settings, and playing around with this bike, I totally agree that these things must come pretty rich from the factory. Just like the MJ, I was afraid going to the 35 PJ would be too small (from the way too large 48 PJ) as the manual asks for a 40. I finally got the to start easy hot or cold, idle, and not pop during decel! I also moved the needle clip to position two (from position four) and didn't notice any power change, so I'll probably leave it there unless the spark plug says otherwise. Thanks again for the help guys

I find it funny that with an exhaust system and leaner jetting than factory spec (smaller PJ with the screw tuned out ~1 turn, smaller MJ) that the bike runs great. Perhaps having the K35 needle helps a bit being skinnier than the K51.

I still gotta check the ignition timing, but the bike has plenty of power, and most importantly starts EASY so I don't expect it to be far off. What do these engine redline at? I need to set my Vapor unit according....

Looking at the factory jetting chart, I'm starting to wonder what the differences are between all the different 600's. The Enduro and Motocross 600 call for a 172 main jet. I wonder what they do to get away with that. Perhaps a smaller venturi with a larger pressure drop (event though the manual says 600's all have 40mm carbs)