sync carbs question and grounding

[grumpyone]

Good evening ;
I have been trying to get my carbs in as good of relation to eachother as possible and ran into a snag.
Rebuilt the carbs and tried the grounding method to balance them. Putting a stump wire in each side and grounding out one cyl at a time and adjusting the needle on the bottom of the carb to best performance,
The cyl on the right when ground it out makes the motor slow a bit and we got the cyl on the left as good as possible but there really is not much change adjusting the left carb screw. When we ground out the left to adj the right cyl it almost and sometimes completely kills the engine and we cant get any improvement on the left carb running by itself . Its like there is an electrical problem doing this or the left cyl is not running and we have checked all other things like clean burning plug since rebuild , valves set and compression is good. It is like you can only do one side this way with the wire ground test and the machine will not let you do the other .
The machine seems to run pretty good the vibrations minimized and it starts rather easily but the test to balance with grounding throws me a curve ball.
Any imput here. :ear
Thanks
Jim and Esther

:nra

[26667]

maybe you have a different model of carb, in which case nevermind, but my idle adjust screw is on the top. On my carbs, the mix screw is on the bottom.

The bike needs to be warmed up (twenty min ride first) as far as I know. I believe adjust idle first. One side then the other, and back and forth till they have the same rhythm, and the proper speed on the tach, then the mix. Sometimes one side is more sensitive and will die during the adjust. Does the stump wire make a good connection? I use spokes cut to about 4 or 5 inches. the nipples are the same thread as a sparkplug. The spokes allow you to take a ride and readjust as neccessary

To adj the mix, turn one side in till it bottoms(gently) then tun it out till the motor slows. Then back in only till it slows, and out again to the middle of those two points.

Put a fan on the motor for cooling if it takes more than a few minutes.

Maybe you know all this stuff, probably someone will have a better idea, but I hope it helps.

[manicmechanic]

Which screw are you adjusting? There are really three adjustments you can make - idle mixture, idle speed/sync, and high-speed sync. The idle mix is the screw that appears to be right in front of the float bowl. I usually get the bike warmed up, then adjust this in/out for a peak reading on the tach, then out just a touch, alternating sides. Then I'll do the idle speed/sync with a TwinMax or mercury stix, followed by the high-speed balance. The last two adjustments use the external adjusters for the butterfly and cables.

[20774]

As mentioned, there are three adjustments - mix, idle speed, and cable tension. Engine must be warm and there must be 1-2mm of slack in the throttle cable tension...these are the knurled adjusters with locknuts on top of the carb where the cable goes through.

At idle, adjust the mixture screw on the side of each carb. Run it in until that side stumbles; run it out until it stumbles again; set at the middle point, maybe a hair towards the rich side (turn out 1/8 turn). Repeat on the other side.

Now with your stubs properly hooked up, short out the left cylinder and see if the right side will run on its own...note the RPM. Let the shorted cylinder clear a bit and short the right cylinder...not the RPM of the left cylinder. Adjust the knurled screw on the bottom of the carb to get an equal RPM when shorted. If the bike dies when shorting, that's OK...note how many revolutions it takes to die. You can adjust the other cylinder to die in the same number of revolutions. In the end, the RPM when both cylinders are running should be 1000-1100. If it's not that, turn each screw at the bottom the same amount to achieve this RPM.

You may want to go back and repeat the mixture adjustments just to make sure nothing has changed.

Finally, you need to adjust the tension in the cables. Have someone hold the RPM steady around 1500-2000...something just off idle. The vacuum is most sensitive at this point. Repeat the shorting of cylinders. Note the RPM the running cylinder drops down to. For the side the runs fastest when running on its own, reduce the cable tension with the top knurled nut. Don't adjust the slow cylinder because you will reduce the 1-2mm of slack (ie, create more tension)...do that too much and you have no slack and you have to start all over. Once you have both cylinders adjusted so they drop to about the same RPM, tighten lock nuts down.

Then it's time for a test ride.

[sumran]

1) Make sure your float levels are right. If they are off significantly the system doesn't work properly.
2) Make sure you have slack in the throttle cables, as Kurt mentioned. If you don't have slack, your idle adjustments will be meaningless.
3) Check to be sure each cylinder will run at idle with the other grounded out. It should run at about 500-600 RPM's with one cylinder grounded. Adjust idle speed screws as needed to get in this range. Check again for cable slack. You will probably need to adjust the idle speed again.
4) Now you can adjust the mixture. It sounds like you know how to do that, but some of the other steps are frustrating your progress.
5) Check your idle speed adjustments after the mixture adjustment. Verify cable slack.
6) Adjust throttle cables to sync carbs.

Valves have to be right before any of this is done.

Good luck.

[sgborgstrom]

To all of the above I'd add:

How complete was your carb rebuild? If you replaced the o-rings on the throttle shafts and did not get the plates back in the right place ( centered in the tube) you will never get the carbs work properly.

[grumpyone]

Thanks Much for the great tips. I guess this proves that the simplest things confuse the most. Or not able to see the forest for the trees :doh
I have maintained everything I have ever drove or rode all my life and I cant believe I was forgetting the idol screw. I was trying to set the idol with the needle on the bottom instead. :doh :doh :doh
Will take all the great tips and go for it again when I get a break in the gun work today and Esther isnt looking :bolt
Thanks again will keep you posted.

Jim and Esther

Plays with guns for a living

[tghsmith]

go to the airheads website and look at making a carb balancer, the differance and ease of tuning makes it worth your time and the 5 dollars or so it takes to make one of these up. set the carbs up the best you can ride the bike(15 minutes min.) and have everything waiting , balance the carbs ride again and you will be amazed ( exspecially if your riding a post 81 light flywheel bike)

[Isamemon]

quote
go to the airheads website and look at making a carb balancer, the differance and ease of tuning makes it worth your time and the 5 dollars or so it takes to make one of these up. set the carbs up the best you can ride the bike(15 minutes min.) and have everything waiting , balance the carbs ride again and you will be amazed ( exspecially if your riding a post 81 light flywheel bike)

I agree with this 1000000%
I was able to balance carbs that were so way out, an electronic balancer had fits
so I built the $5 balancer and got them right on
then for fun hooked up the electonic balancer, it showed no difference
the $5 was as good as the $80 balancer

[DennisDarrow]

For me, carb sync is all about the throttle cables. This is where I get rid of virbration and really dial in the 4k RPM sweet spot. It also effects that little bit of transition when letting out the clutch leaving a stop or very slow u turn when feathering the clutch.

The balance rig for me, is the way to go to achieve this. Yes, it helps set the idle. Some folks go with a slightly higher RPM on the one cylinder running when disconnecting the other and grounding it. I prefer to get the idle set so that when it is correct and I disconnect the plug that it just chugs 3 or 4 times and then dies. Results in an idle speed that is around 8 or 9 hundred. Others set up the idle so that when the plug is disconnected it runs at a few hundred RPM; but is the same as the other. The result is the same just a bit higher idle.

Again, the most important thing for me, is to get those throttle cables adjusted so that they are in sync or pulling each carb the same.

Make sure, before you start adjusting that you go for a ride of 10 miles or so. You will get entirely different results from adjusting carbs with a "cold" engine.

Dennis

[Isamemon]

I agree with Dennis
getting the carbs right is only part of the game, cables have to be pulling the same,
otherwise balance at idle will do little on the road

[tghsmith]

thats the final step in a good carb set-up, take it up to 5k and hold it there(10 or 15 seconds) the levels in the balance tube should be adjusted so they hold even and don't drift(all done with the cable adjustment) the single tube method is around 10 times more sensitive than merc sticks, (i use my old mercs to do the first tune before the clean it out and get it ready to do it right test ride, the single tube can be a bit touchy untill the carbs are close.