Poor Power <2500RPM Adjusting Carburetors and Valves?

[robbiecampbellsoup]

Hi everyone,

I've recently gotten into the world of owning a bike (in general, this is my first bike) and owning an XS400. A 1983 Maxim to be specific. She's a lovely color - note the bluing on the Mac 2-in-1 exhaust the PO put on - this means it's running lean?



So far this is what I've done;

1. Tried to ride the bike home, but it died on me, kept loosing spark, stay running below 2500 RPM, would stall out when I used any lights etc.

I ended up partially solving this by determining that the battery was old, and the charging circuit was not working at all. I ended up repairing the old stator (all the connections were corroded off underneath the stator cover) and re-doing the wiring harness - and voila, the charging system is back to life. There is still something RPM related happening below 2500 though.

2. I changed the oil. It was pretty clean, but a bit low. I've not determined if the oil is making it's way up to the cam area, I still need to do this. I'm a bit scared as the bike kind of sounds like a sewing machine, between my crotch, which is unnerving.

3. I tried adjusting the idle mix screw to help with the low power at <2500 RPM - I first turned the screw all the way in, then set it to 2 turns out. Bike didn't want to start well, and it felt the same. Then 3 Turns, bike still didn't want to start very well (I had to give it throttle while turning the engine over, without enrichment as it was already warm). I then ended up going for a 40km ride with a friend, and found out some other things.

a) The power below 2500 RPM is heinously bad. Trying to accelerate up a hill, I couldnt let the motor drop below 2500, else i had to downshift. It just bogs down completely.

b) When Idling, it sits around 1200 RPM, if I blip the throttle, it drops down to 800-1000 and almost sounds like it will die. My strategy for start/stop has been to pre-advance the throttle a bit before the light changes, rev up to ~3000rpm and let the clutch out slowly then.

c) Highway driving around 100km/h had the bike around 5-6k RPM, it had pretty poor power above 6K RPM. Entering on the on-ramp, it would pull nicely between 3000 RPM and 6000 RPM, and then taper off. I never had it above 8000 RPM.

d) Throttle position does not matter if the bike is below 2500 RPM - it always bogs down.

What I'm planning to do in the near future:

-Sync the carburetors
-Adjust/inspect the valve tappet clearance (this is shimming correct?)
-Do a throttle map thingy where I go up a hill at full throttle 3rd gear, stop and check plugs at the top, rinse and repeat at 3/4, 1/2, 1/4 and idling - I was told this will give me some indication as to whether the engine is running lean/rich at these throttle positions, which can then tell me if my jetting is off/on etc.
-Adjust the idle mixture screw - I don't know how to do this properly


Can anyone offer any advice on the order in which to do these? Or whether I'm even pointing in the right direction??

Thanks so much!

-Robbie

 

[JaredK]

Welcome to the forum! I'm in a similar place, picked up my Maxim as my first bike last fall and learning a lot about keeping these things running well. I don't know a lot but I'll say what I know (or at least what I think I know...)

Have you checked for air leaks? This is your first step, and probably most important. Get some starter fluid and spray it around the carb boots (both the air intake side and the manifold side). If the idle changes, up or down, you've got leaks. Also, check your vacuum line going from your petcock to left carb boot, and the plug on the right hand side carb boot. A leak here will leave you lean (and possibly without enough fuel from the pectock!). You can switch the petcock to PRI to bypass the vacuum switch on the petcock and make sure you get enough fuel, although you've also got a fuel filter so you should be able to see this.

These bikes aren't great under 2500 in general, but yes you might be running a bit lean on the idle mix. Mine is currently set to 3 1/2 turns out, although I'm running a stock air filter and exhaust setup. The lack of power on WOT (above 7k) for you might mean you need to rejet, but cleaning the carbs is a good start. The bluing could indicate it's a bit on the lean side and getting too hot.

Addressing the leaks and cleaning carbs should be done first - any tinkering with jet sizes (and idle mix) will be a waste if it's compensating for other issues, and probably inconsistent.

To check to see if the oil is getting up top to your cams, there's a small bolt on the right-hand side near the top of the engine. You let this out while the engine is running and shut off the bike once you see oil come out. Check the manual (download it from this forum). The valves are a bit noisy anyway.

 

[rugbywarrior89]

You will need some specialty tools. First and foremost, JIS (japanese industrial standard) screw drivers. They are very similar to philips but are machined differently and will save you from stripping many screws on the bike.

As far as carb specific, you will need a vacuum line for syncing the carbs. There are 100 different ways to make your own for less than $5. Personally, I took about 3 meters of clear tubing that fit the nipples on the rubber boot connecting the carbs and engine, and zip tied it to a bar from my weight set in a U-shape and filled it with transmission fluid. Put each end of the hose on each nipple and start the bike. Adjust the carbs until both sides of the line have equal amounts of transmission fluid. Just be careful not to suck the fluid into your engine. There are other set up to prevent that but I find mine works just fine as long as I'm careful. Troll around for other ideas you like.

Lastly, I ground the sides off of a flat head screw driver so I could get my jets out easier. I was using a micro flat head but some of the jets and needles were in so tight that it bent the head so I just sacrificed a normal screw driver for the greater good.

You obviously know how to solder so you are +1 there. Bullet connectors are a big no no on this site.

Common Issues: Dirty carbs/air leaks, old stator/field coil wires, dry rotted carb boots, bus fuses (convert to blade type ASAP), broken petcock (some rebuild, I prefer to replace with non-vacuum petcock), voltage regulator/rectifiers.

 

[robbiecampbellsoup]

Thanks for the advice guys. I'll fill you in on my progress this weekend.

Also, on a side note...

What is this thing? It's missing from my bike. I have the blue plug, but this is gone.

 

[rugbywarrior89]

That is a relay, which is pretty important. There are two of them that go on the underside of your bike. They look similar but I don't think they are (someone correct me if I'm wrong). One is a headlight relay and honestly, I'm not sure what the other one does (ignition maybe). I assume it's the other one since your headlight is working. On my bike they fit interchangeably, but one relay will operate my headlight and the other won't. Not sure why though. XSChris tends to have random parts like those laying around. Become his friend, he can hook you up with parts you won't find anywhere else FYI. Does your e-start work? Maybe that's it.

Keep in mind I have an 82' SOHC, the Maxim DOHC have some major differences so I might be incorrect on the specifics but it's definitely a relay.

Also, is there a reason that ground wire is not connected in the background?

 

[robbiecampbellsoup]

I stole the picture from someone else My ground is connected to the battery there.

Everything on the bike *seems* to work fine (with the exception of the issue discussed in this thread.. and the brakes).

E-start, lights, high beams, brake lights, turn signals, charging circuit, horn all work.

There's just an empty plug..

 

[16VGTIDave]

The missing relay is one of 2 "safety" relays. One is to kill the ignition if the kickstand is down and the transmission is in gear. The other is a starter interlock - the starter won't operate unless the transmission is in neutral or the clutch lever is pulled.

Now, for the standard advice to all newcomers - clean your carbs before anything else! Fully disassemble and clean them. When you are confident that they are perfectly clean, do them again! Once you are tired of cleaning them, and will swear on your life that they are impeccably clean, do them again! I'm not kidding...

There are plenty of threads here with suggestions on how to properly clean carbs. Read them and learn. An ultrasonic cleaner is the best option, but boiling in a mild acid like vinegar or lemon juice also works.

 

[robbiecampbellsoup]

Well - when I put the kickstand down in gear, it definitely doesn't kill the ignition. Remind not to do that in general.

Fine. I'll clean my carbs this weekend.

 

[Lou Ranger]

... Fine. I'll clean my carbs this weekend.

Dave is right, this will definitely help a lot. Be careful that you take out any and all rubber bits before you boil the metal parts. Didn't happen to me, but a number have found out too late that the seals, rings etc. don't boil well.
When I first got mine, the carbs were perfect and it was almost scary how it would pull all the way up to 10,000.

But power below 2,500? . Accelerate from 2,500? . .Up a hill?
There is none. . . . . . . . . . . It can't . . . .. . .. .. .. . . . . It will stall and roll backwards down the hill!

Best advice I got when I first complained about exactly what you describe? If you want to roll the throttle from 2,500, sell the bike and buy a V-twin and learn to like the 6,000 rpm red-line.
This is not quite a 100% rule if the displacement is larger. But with 400 ccs, once you get the carbs clean, just enjoy the hell out of the 5,000 -10,000 range.

 

[robbiecampbellsoup]

hahahah thanks Lou.

I think then that I still need to do some tuning, as the throttle response, not under power, from idle is poor - it will bog *not* under load as well, just trying to rev the engine.

Would you say that it would be typical, if you were moving say 5km/h (say in a slow moving traffic) then needed to accelerate, that you would clutch in, rev, then release to get it to be able to accelerate? Or should you simply be able to accelerate? I want to get my expectations in check!

 

[Lou Ranger]

No, I wouldn't say that should be typical - unless you're in 2nd.
Even with the 17T front sprocket, once rolling in 1st, it would accelerate smoothly - but NOT fast - until you hit 2,400 or 2,500. Then it would really go.

Driving in traffic, in town, I would usually shift around 3,000 and keep the revs in that range until there was clear space in front.
If yours is not accelerating smooth and fast right up to 10,000 (and REALLY taking off from 7,000+) then I suspect cleaning the carbs will fix that.
And, at the same time, it will fix the low speed problems too.

 

[16VGTIDave]

Second gear, engine idling to creep along with traffic, open throttle and accelerate away. Slowly at first, but picking up as the RPM's rise. No drama. But, I've changed my sprockets to have the engine revving higher than stock, when most people reduce the RPM's. If you want to cruise down the road with your engine going "potato-potato-potato" like a big v-twin, then you should have bought one! Our 400's like to rev, especially the DOHC's.

All that said, when everything is in good working order, these bikes are very easy and civilized to ride.

 

[robbiecampbellsoup]

Second gear, engine idling to creep along with traffic, open throttle and accelerate away. Slowly at first, but picking up as the RPM's rise. No drama.

This definitely does not happen for me. I apply throttle like that and it just bogs down.. performing worse than if it was just idling...

-Robbie

 

[16VGTIDave]

That would be because your carbs are probably (very) dirty. There are a number of tiny passages in the carbs that can get plugged with varnish or corrosion. There is only one solution, as previously mentioned...

 

[robbiecampbellsoup]

I can't wait to prove everyone wrong when I open my carbs for the first time and they are spectacularly clean.



...hahaha right

I'm going to clean the carbs this weekend -and will post photos to prove such things.

 

[rugbywarrior89]

They probably will be clean...but don't forget the Golden Rule:

Now, for the standard advice to all newcomers - clean your carbs before anything else! Fully disassemble and clean them. When you are confident that they are perfectly clean, do them again! Once you are tired of cleaning them, and will swear on your life that they are impeccably clean, do them again! I'm not kidding...

 

[robbiecampbellsoup]

Here is perhaps a silly question.

Say I clean the carbs (I'm going to do this this weekend, I swear), re assemble, sync, check for air leaks and everything is gravy, but I'm still getting the low throttle RPM issue.

One might say it's time to increase my pilot jet - I'm assuming that if I want a richer mix after assuming that my circuit is too lean (assumption stems from poor idling, poor throttle response below 3000 RPM, and when throttle is blipped while idling, RPM dips and motor sounds like it may die)... can I simply use a numbered drill set, or a reamer to increase the size of this wee brass thing? Or is the orifice hole non uniform?

Actually.. upon searching a bit more it appears this may be a terrible idea - but I'm going to pose this question anyways, in the hope that I'll learn something new.

 

[xschris]

If you have a set of micro drills you can. The set I have goes from 30-160.

 

[robbiecampbellsoup]

Oh great! Thanks for the confirmation of my assumption Chris.

I'm not sure what sizes of drills I do have. Are the pilot numbers simply the diameter in milimeters *100?

 

[rugbywarrior89]

You could but for $6 or so, you could just get new jets. mikes has them but you may want to add it to a more extensive list as his shipping prices are kinda high. This way you could also get a couple different sizes. I know on mine, I had to go up two sizes on the pilot (42.5 to 47.5) because of my pods. Of course you have a completely different bike than me so yours may be different.

There is also a jet kit on ebay where you tell the guy your operating conditions and elevation above sea level and he will size them for you for $50. People have had lots of success with that.