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Carb work with vacuum gauges

  • Thread starter Thread starter kungpaodog
  • Start date Start date
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kungpaodog

Guest
I have done a complete tear-down, cleaning, and rebuild of my carbs on my '81 gs650GL. I set the air jet screws to the recommended 1.5 turns out from full close.
I adjusted the air screws a little on 1 and 2 since it seemed to be running WAY too rich (exhaust smelled like gas) until it smelled less like gas and didn't smoke.
I also set the float levels with a caliper (with the gaskets on) to the recommended level, but I think I'll set them with the clear hose method tonight.
I synced all the carbs with a vacuum gauge.
It revs great in my garage, but it still has issues getting past about 5000 rpms on the road. It will, but it requires some ginger throttle control, otherwise it sputters and won't accelerate under full throttle. Once I get past 5000 it revs up to redline just fine, so I believe it is a bad mixture on the idle circuit.

My questions are these:
1) Where should the fuel level be if I use the clear-hose float level method?
2) Can I hook up the vacuum gauges and adjust the air-mixture screws until they give maximum vacuum? I feel like if a carb is too rich or too lean, the vacuum would be less than at ideal mixture, but I'm not sure if this theory is correct.

Any good ideas, gurus of the GS?
 
rough running

rough running

how doo... was the bike running ok before you did the carbs? because the symptoms you describe are consistant with an advance and retard timing problem, my gs1100 has a "bob weight" mechanism on the timing end of the crank and you get the same symtoms if these weights are stuck, also my repair manual states that the fuel level should be in line with the carb body where it meets the fuel bowl, most carbs are set the same way so you cant be far out, hope this helps good luck.....
 
You may find, that after all methods of testing for air leaks at the various connections and finding none, you may need to bump your jet needles up slightly...I did on mine and it is a bone stock engine/air box config. I'd first check all fittings, the intake boots at the head (may be o-rings), and the air box, to see that everything is sealing tightly and all of these areas are ruled out. If they test out perfect, the next thing to try would be to raise the jet needles by one notch.
 
It was running OK before the rebuild, but it was doing the same thing. I hadn't made any adjustments on anything ever, and wanted the peace of mind that the needles were clean, floats adjusted, etc. I haven't looked into the timing yet, though. I'll check that out tonight. Thanks.

Also, what about using the vacuum gauges to fine tune the air mixture screws?
 
...and yes, the boots and airbox seem to be air tight. Can I make adjustments to the needles without rejetting?
 
I don't think you want to adjust your mixture screws based on smell of the exhaust or anything to do with the vacuume readings. Set idle mixture to 1.5 turns out and then tweek slightly to achieve best idle - should be within 1/2 turn of where you are.
 
Thanks! I wasn't even aware that I could adjust the needles! I take it that I pull the clip holding them in place and move them more open, so essentially what is in the main jet is a shorter amount, right?
 
dabe said:
how doo... was the bike running ok before you did the carbs? because the symptoms you describe are consistant with an advance and retard timing problem, my gs1100 has a "bob weight" mechanism on the timing end of the crank and you get the same symtoms if these weights are stuck, also my repair manual states that the fuel level should be in line with the carb body where it meets the fuel bowl, most carbs are set the same way so you cant be far out, hope this helps good luck.....
Click to expand...

That makes perfect sense for some of the symptoms I'm seeing. Sometimes it runs poorly, and then, like someone threw a switch (usually at high rpms) it will run great. Then, sometimes the "switch" gets flicked off and it runs poorly. I'll let you know if it's sticky when I pull it apart tonight. I REALLY hope it's that bob weight, because I REALLY don't want to pull the carbs off AGAIN (4 times and counting...sigh...) Thanks to all for the input!
 
The needles in your '81 probably don't have the extra grooves necessary to adjust them easily. :(
They pretty much disappeared after the '79 models, due to EPA regulations forbidding you to tune the carbs to peak efficiency. 8-[

I have found two ways to get around this:

1) When you remove the needle, you will see a thin metal washer just under the circlip,with a thicker nylon washer below the diaphragm. Switch the two washers, it will raise the needle by the difference in the thickness.

2) Go to your favorite hardware store and get some small washers to add to the thin metal washer. I have found that rivet backing washers work well. Might have to ream out the hole just a bit, depending on the size of your needle.

If you have digital calipers available, measure the washers you are putting in to find four that are close to the same thickness.


.
 
I just made myself a custom extra-long snap ring tool and got out the snap ring to adjust the needle height, and was suprised to see no extra grooves to move the clip to. So I swaped the nylon washer around to the top...and then logged on to see the tip about moving it to the top! Thanks for the feedback... I may need to try adding some custom washers like you suggested. I don't know if I would have thought of that for shortening the needles even further.

And sorry, Dabe, but the timing advance mechanism is nice and lubed, not sticking at all.

With the help of you guys, I'm sure I'll get this sucker running well! I'll update you tomorrow about how shortening the needles worked.
 
I raised the needles by moving the nylon washer to the bottom instead of the top of the clip on the needle, and it's running a lot better, but it still isn't pulling as hard as it formerly has. If I tweak the idle air mixture screws, will this help with higher rpm acceleration? I can't really tell if it's running too rich or too lean. How do fine tune it now?
 
kungpaodog said:
I raised the needles by moving the nylon washer to the bottom instead of the top of the clip on the needle, and it's running a lot better, but it still isn't pulling as hard as it formerly has. If I tweak the idle air mixture screws, will this help with higher rpm acceleration? I can't really tell if it's running too rich or too lean. How do fine tune it now?
Click to expand...

Do a plug chop at the point that it seems to fade/bog out. This will tell you if you're still running lean or rich. Lean = white plug. Rich = black plugs.
 
Dave8338 said:
Do a plug chop at the point that it seems to fade/bog out. This will tell you if you're still running lean or rich. Lean = white plug. Rich = black plugs.
Click to expand...

OK, I give up. What's a plug chop? I tried to search GSR for instructions, but no luck. Thanks!
 
kungpaodog said:
OK, I give up. What's a plug chop? I tried to search GSR for instructions, but no luck. Thanks!
Click to expand...

Run the bike (after completely warmed up) out on the road and when you reach the RPM/speed where you notice that you're having the bog condition, run it there for a few seconds and then hit the kill switch and pull in the clutch. Take out the plug wrench that you'll need to bring with, and remove the plugs one at a time and note the color (you may want to bring along a note pad so that you can record which plug is doing what). This is the most accurate way of reading plug color as you will be doing it at the exact moment in time, that the problem presents itself.
 
kungpaodog said:
OK, I give up. What's a plug chop? .... Thanks!
Click to expand...


INGREDIENTS
2 loin cut pork chops with pocket
1 small egg, beaten
2 tbsp all-purpose flour
1 tbsp vegetable oil
1 tbsp butter
1 tbsp onion

...Things get a little complex now, so pay attention. We need to do what's called a "plug chop". This is where you pull the clutch, kill the ignition, and pull to the side of the road (carefully of course) WITHOUT ALLOWING THE BIKE TO IDLE. Still using your mechanic gloves, remove a plug and look at the color. If they are black and sooty, you are running rich on the pilot jet and need to reduce the amount of fuel by installing a smaller jet. If they are clean white, you are lean and need to give the engine more fuel by using a larger pilot or turn the mixture screw out a 1/4 turn. If they are brown no change is required. To aid in tuning, when you go in with the mixture screw, you get a leaner mixture. Out you richen the mixture. As a general rule of thumb the idle mixture should be 1-1/4 to 1-1/2 turns out when you have selected the right pilot jet. Repeat your test with fresh plugs for 1/2 throttle. Here in order to make a change, we can add fuel only with a stock needle by shimming with tiny washers under the needle. Every shim you add richens the midrange. The Dynojet tuner kits are equipped with a needle that has grooves in the end to adjust the position with a clip. Remember the higher on the needle the clip is the leaner the midrange becomes.

Install the appropriate jet you have decided upon (or make needle change), and a new set of plugs and redo the test. Repeat this until you have the correct light chocolate appearance.

The main jet is your cruising jet and needs to be tested above 3/4 throttle. It requires nice stretch of open road, or better yet a drag strip. Repeat the same type of test as before at 3/4 throttle and full throttle, doing your plug chop. Make changes accordingly. An additional test that is handy is to get up to, fourth gear at about 4000 RPM. Open the throttle all the way then immediately let off the throttle about 1/8. If the engine slows just a bit, the jet is close, if it seems to pick up speed or RPM; the main jet is too lean. If it hesitates or stumbles, the main is too rich. Change jets and test again.

After you have the jets close, I just ride about 50 to 75 miles observing the performance of the bike at different speeds. Poor acceleration, pinging, knocking, surging and popping or spitting through the carburetor suggests you're still lean. Black smoke, sooty exhaust, smell of unburned fuel and a rough idle suggest too rich. When I get back from the ride, I pull the plugs once more and see what I've got. If they are tan, light chocolate and the bike runs good, I'm done.


Full version here.

Tony.
 
Seeing how I don't think anyone answered your questions about the vacume guages... yes that should work just fine (i was going to adjust my carbs the same way, but then i bought an inductive pickup digital tach (on my new multimeter) and figured that would be easier/more sensitive).

Before adjusting ANYTHING, it is very important that you get your float levels adjusted properly as this changes the fuel metering for all of the circuits. If i remember correctly, the fuel level (measuring using a fuel gauge) should be 4mm below the split of the bowl and top of your carb, although this MAY differ for your particular model.
 
Nightmare said:
Seeing how I don't think anyone answered your questions about the vacume guages... yes that should work just fine (i was going to adjust my carbs the same way, but then i bought an inductive pickup digital tach (on my new multimeter) and figured that would be easier/more sensitive).

Before adjusting ANYTHING, it is very important that you get your float levels adjusted properly as this changes the fuel metering for all of the circuits. If i remember correctly, the fuel level (measuring using a fuel gauge) should be 4mm below the split of the bowl and top of your carb, although this MAY differ for your particular model.
Click to expand...

Thanks, Nightmare. I think I may need to readjust the float levels AGAIN, since it doesn't seem to be running right yet. I tried the plug chop (low rpms and mid) and the 3 cylinder was rich, the rest looked ideal. When I get some warmer weather I'll do the high rpm plug chop. I still want to know why I can't get it over 65 mph in fifth, when it used to be able to do 90 mph in fifth up a hill! Other than that, it seems to be running pretty well, but I'm really starting to think about an EFI conversion...grumble...grumble.
 
Just to make sure you aren't wasting your time, plug chops are only useful at throttle positions and not as much at RPM range, check your plugs at closed throttle (idle), 1/3 throttle (Pilot circuit), full throttle (Main circuit). This page is useful for seeing which circuit is affected by which throttle position and I hope it helps you...

http://www.650motorcycles.com/CarbTuning.html
 
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