Hanging rpms and some surging

[bcware]

I am looking for some insight because I am, yet again, about to give up on tuning this bike.

I can rev through the entire range of rpms, but she tends to hang at 3,000, and slowly fall to 2,000. Sometimes she falls down to 1,500, but rarely or never back to idle at 1,000 to 1,200 on her own. If I "blip" the throttle open fast I can get the rpms to fall, even to idle.

While revving through the range I hear light popping from the exhaust pipes; almost like a quiet backfire. It is noticeable, but not very loud.

I have tried every main jet from 137.5 to 160 in all increments. I have adjusted the needle up and down with most of the different mains. Changing mains does not seem to have much of an effect, surprisingly. With 160 mains and the needle open all the way the bike behaves and sounds almost the same as with stock or only a few sizes up (142.5, 145...).

I have tried the stock air intake with all combinations of jets and needle heights. The stock air intake seems to work better than the cheap emgo pods.

What else could be causing this problem? Why does the behavior remain the same despite dramatic changes in the main jet size and needle height? Could there be a mechanical reason for this problem?

I could not find any leaks by blowing propane on the bike. Blowing gas into the air filters lowers idle, but no change is seen when spraying the intake tract, boots, carbs, headers, etc.

The carb boots and gaskets are brand new. My diaphragms do not appear to have any holes.

Picture included so you can see my current setup.

 

[HoughMade]

Did you change the idle/low speed jets?

 

[drewpy]

are you getting enough fuel into the carbs?

 

[dongrady]

check for air leaks, hoses coming off of intake boots, seal at carb's or even the intakes themselves.

 

[new kid on the brat]

dongrady, thats what the propane is for. i would recheck ignition. change plugs or check for fouled plugs. do you have points? i can't remember your year.

 

[new kid on the brat]

could also be bad wires! my bike ran not good but ok with just a pipe change! you've got another grimmlin somewhere!

 

[new kid on the brat]

hmmm someone said emgo pods enterfere with the vacuum slide port so i just rode my bike around the block and it seemed lean still so just for giggles i pulled the filters and rode again. wow!! broke the tire lose when i grabbed second. on wet road of course!! still a little lean in the middle so i will raise the needle now and try that.

i'm running stock 42.5 pilots
155 air jets
137.5 mains
no filters.
and a 2 into 1 with a kerker slip on.

 

[new kid on the brat]

are you sure your idle circuits are clean? no broken tips stuck in there? lol...... man i am thinking for you.. this sucks cuz i've been there.you gotta be missing something!!!!

 

[hmusket]

Heres a carb guide from the XS650 site, its been quite helpful to a lot of folks. Maybe there'll be something there that will help you out.

http://www.amckayltd.com/carbguide.pdf

 

[bcware]

are you sure your idle circuits are clean? no broken tips stuck in there?

The idle screws are not broken and the pilot jet passage is clear. The idle is actually not bad; a little unsteady, but that is to be expected because I am not synced with a manometer.

i would recheck ignition. change plugs or check for fouled plugs. do you have points? i can't remember your year.

1978. If a plug is fouled a bit is it ruined? Will a wet plug or carbon-fouled plug be permanently damaged, or is this only when exposed to these conditions for a long time? I do have points. Could points, timing, or valve clearances cause any of this? I have replaced plugs 3 times already, but normally I just take a brass brush to them and clean them off if necessary.

are you getting enough fuel into the carbs?

Currently have the floats set to 27mm from highest lip on the bowl. So, if you're measuring from below the gasket it's probably at least 28mm. The "lowest" I have gone is probably 26mm from the same measuring point; which, in proper measuring, is probably more like 27mm.

Also, is it normal to have a little fuel in the carb boots? I can stick my finger inside the boot and there is no fuel towards where the boot meets the engine in there, but there always seems to be a small wet spot on the outer-most lip area after I run the bike for a few minutes. I am referring to inside the boot; fuel is not spilling or dripping out or anything. I had not considered that maybe even rocking the carbs to pull them off might be causing this.

Did you change the idle/low speed jets?

Yes, I am 2 sizes larger on the pilot jet. I have stock and 1 size larger on hand as well.




I have yet to check the valve clearances, so I am suspect of this. I could also double check the timing and points.

 

[xschris]

If you have not synced the carbs do this first. If they are off a little it will make a big difference. I would not go any higher than a 45 pilot jet that would be plenty even if you had no mufflers. also do the stock 170 air jet the larger pilot and turned out mix screw will compensate. just my Its always good to start stock and make small changes to only the areas that cause you problems.

 

[bcware]

My 2 up pilots do provide plenty of fuel; I can easily foul the plugs if I turn the screws too far out. The ones I have in now allow me to run at 3.5 to 4.5 turns out or so.

I am going to play with float height. I was avoiding adding more fuel to the bowl because I had problems with flooding and wet plugs when I had the emgo pods on. Now that I can clearly see that they block all of the passages on the carbs I will try again with the stock air box. Low fuel in he bowl could explain my inability to tune with jets and it is easy to test.

 

[bcware]

So, fairly good news for the day; she runs and rides. I took her for a spin around the block and got up to 30, but I can't go anywhere until I get registered tomorrow.

I set the floats to 25mm from the lip on the bowls and dialed down the idle mix screws until my plugs were no longer being fouled at idle. Once I was no longer being fouled at idle and had enough fuel in the bowls I was able to get semi-normal behavior, but there are still a few kinks to work out.

Currently I am running 2 up on the pilots and 145 mains. The revs are falling back down well, but there is popping when the revs fall. The bike also wants to hover at 1,500 rpms despite having an idle set just over 1,000. I can still only get back to my true idle by blipping the throttle very quickly.

After riding around I pulled the plugs and the color looked quite good, but I still need to dial down the idle mix on one side a bit more. With 2 up pilot jets not much over 3 turns out will foul the plugs.

I started to experiment with synching today but I had no luck. I used 5w30 motor oil as a liquid, but the movement was so violent that I could not read anything. Any tips?

 

[new kid on the brat]

have you ever been going down a long mountain pass and noticed that if you down shift and wind your motor up you will get a rappy sound but if you step on the gas just a little you will get loud snaps and pops? this is generally caused by having raw fuel and rich fumes that ignite in your exhaust. i think your pilots are still too rich. no one that i have seen on here runs the size jets that you are trying to run. especially since you are back to stock filters.what do your stock filters look like inside?

and are you just setting your idle screws at 3 turns? and then checking your plugs to determine if the settings are right? how exactly do you set and adjust your carbs?

 

[bcware]

I agree that the pilots are probably a little rich still; I need to do more experimenting. Even with stock air filters my exhaust has a large effect. The way I have been checking the idle mix is by starting at 3.5 turns out, cleaning the plugs, starting the bike and only idling for a few minutes, checking the plugs again, and finally adjusting the respective mixture screw and repeating.

I am definitely open to the possibility that my pilots may have been he culprit when I was having more problems with wet plugs, very slight pooling of fuel on the carb boots, etc. With the emgo pods thrown in the mix problems can be hard to pin down.

I tested 150 mains and only had very slight fouling on one side in addition to the same popping, so it must be the pilot. The other side was actually looking cleaner than I have ever seen it, but I probably need to run the bike at higher rpms for longer to determine if I am dangerously lean.

I just threw in 155 mains last night and dialed down one side a 1/4 turn that appeared to be too rich. If the weather holds out I will take her for a ride and see what happens.

 

[new kid on the brat]

you have to adjust your idle screws by ear! start at 3-4 turns, warm up the bike, raise idle to around 1500 and turn the screws in until the engine slows down, then back them out until the engine just smooths out! i usually run out my screws another 2-3 turns because i have sometimes found another spot that speeds up the engine again but not normally. run the screws back in until the engine slows again , then back out again till it smooths. done! do the other carb now!!

if you get no responce from turning your screws in to the bottom then you have a plugged idle circuit. just a simple test.