Carb Balance

[davidpdx]

    It looks like the rains have set in for a while here so time to do all those jobs I have been putting off. I have never been satisfied with the way my bike idles and borrowed a twin-max from a friend to check the balance. Can I hook it up to the spot where the pulse air system was removed from or is that on the wrong end of the carb? I have a 1984 with what I think are original carbs and am wondering if there is another place to hook this tool up? Thanks, David

[Barry]

In this pic you can see the mixture screw and the vacuum port.  The Vac port is the smaller of the two.   This is a RH carb. on the LH carb the positions are reversed i.e. vac port is on the inside rather than the out side.  


If you have never done it before consider removing the carbs and holding them up to the light with the throttle stops backed out. This will enable you to check if the butterflies are centered in the venturi and properly closed.  One of mine wasn't and I had to slacken off the securing screws to adjust it. Even if the screws are peened you can usually slacken them enough to do this. Look for a very fine even ring of light around the butterflies. They are open such a tiny amount at idle that getting them both the same is a good baseline to start balancing carbs from.

[flybot]

I do the butterfly's sync by feel. At idle, if one piston is working harder than the other, it makes an obvious imbalance. Do the adjustment at around 1200 rpm to start. Pick a side and slowly turn idle set screw to closed and then open. You can easily hear and feel when one cylinder is being lazy or doing all the work. Leave the set screw in the middle, which also feels the smoothest. From there, adjust up or down on both sides equal amounts to set idle speed and fine tune sync once more at the desired idle.

Make sure your valves are adjusted (this is important), and engine fully warmed up.
EDIT: Also make sure the cables are equal in slack on both sides. That and valve adjustment are two sort of hidden stumbling points with carb syncing.

Ive never used the vacum set up before. Im sure it works. But it assumes everything is equal on both sides beyond the carb throat and through the exhaust vallve. Ive always done a carb sync by feel, on cars and bikes and never had any problems.

[Bob_Roller]

You can use the ports where the pulse air system hoses go onto the carb bodies .

I've used a Twinmax on my bikes and really wasn't satisified with the result .

I made my own oil filled manometer a few years ago and that's all I use now .

Use whatever method works best for you .

As has been mentioned, perform a valve clearance check and get the engine thoroughly warmed up before doing the carb synch and also have a fan or fans blowing air over the cylinders while performing the synch if it takes you longer than 5 minutes or so .

Here's a picture of a simple homemade oil manometer .

http://i1178.photobucket.com/albums/x361/128as0050404/Picture063-1.jpg

[Ed Miller]

The Airheads have tech days up there in the Portland area pretty often, if you want an audience.  Who may actually be of help, that is!  I don't get out to those much lately.  

[Luca]

Rainy days aren't the best for carb synching.  Like Bob said, you need the engine fully warmed up.  A twenty mile ride will do the job.  Tuning the carbs on an engine that's not fully warmed up usually results in a high idle (once it does fully warm up).

If you use the twinmax I'm pretty sure you will have to invert it to work on an airhead.  They only pull somewhere around 5"hg at idle.

BTW, if you don't have screws for plugging the vacuum ports (where the pulse air line was), you can get them at the dealership.  It's a pretty odd size.  Rubber caps work, but they tend to dry out and crack pretty quickly.  If they start leaking then even the best tuned engine will idle poorly.  I put rubber caps over my screws to both keep the dirt away and provide some extra insurance against a screw backing out.

[davidpdx]

Thanks for all the advice. I am hoping to be able to do it this weekend. It has been right around freezing here for the last few days so engine overheating has not been a problem but does anyone know what the max allowable engine heat is? I thought I would stop at Harbor Freight and pick up one of their infrared thermometer. They say every new job is an excuse to buy a new tool.  

[Barry]

does anyone know what the max allowable engine heat is?

Not a problem I think I'll ever have but if I did and was concerned about the health of the engine I would measure oil temperature or by inference oil pressure. Oil pressure is quite high on airheads something like 20 PSI per 1000RPM so if it dropped below 10 PSI per 1000 RPM I would start to worry it was too hot. If the oil pressure light comes on even at idle then it's too hot.

I'm not sure what an infra red thermometer would show as I expect the heads will be much hotter than oil temperature.  It would be interesting to see if both sides are the same temperature.

[Bob_Roller]

I've never seen a max engine operating temp in any manual .

I have an infared thermometer from Harbor Frieght, got it about 4 months ago .

I did check cylinder head temp after the commute home on, my '81 R65 .

70 mph 115 km/h on urban expressway, air temp 115 F 46 C, cylinder temp 325 F 163 C and oil temp was 275 F 135 C .

I do have an OEM oil cooler on the bike and it dropped the oil temps about 40-50 F  20-25 C .

I have a temp indicating oil dipstick on the bike that has been tested with calibrated test equipment at work and it is within +5 F accuracy .  

[davidpdx]

I took the bike out and got it warmed up this afternoon and hooked up the Twin-max. It looks like my carbs are as close to being balanced as I can get them but adjusted to idle mixture screws out about a 1/4 turn which seems to make it idle smother. Changed out my spark plug wires and caps for plastic instead of metal and it got me thinking. If it is a wasted spark system where both plugs sparking every time, could I hook either coil to either plug with same result?

[Bob_Roller]

You have two ignition coils, or one dual output coil ??

But the answer you're looking for, it doesn't make a dfference which coil output goes to which sparkplug .

[luckyd20]

Piggy backing off this thread, I made my own manometer see here:

and my carbs are way out of sync. Seems like my right carb seems to be doing all the work. I can adjust the idle mixture screw on the right and hear it adjust the idle. But when I adjust the left side there seems to be no change at all. In my manometer, the water is completely emptied into the left container.

What do I need to do to fix this?

[Barry]

The mixture screws do not have any direct impact on vacuum and should not be used to attempt a vacuum balance.

It's best to set the mixture screws to the default basic settings of 1/2 turn out from gently closed. Then do a vacuum balance using the throttle stop screws at the factory specified idle speed which for a 79 is 800-1000RPM.  1000RPM is best. Expect the throttle stop screws to be very sensitive with the smallest possible turn of the screw having an impact on the vacuum. This can make it seem difficult to zero in on an exact balance if you a are starting from a position of a long way out. The factory basic setting for the throttle stop screws is 1 turn in from the butterflys being fully closed but that is usually far too much and I start at 1/2 or 3/4 turns in.  Most important is that you strive to get them both exactly the same before attempting a vacuum balance.

Then you can tune the mixture screws for fastest idle and go back if necessary to reset the idle speed with the throttle stop screws. The idle mixture is also very sensitive to the smallest turn of the mixture screws even if it doesn't seem so. Make small adjustments and wait a few seconds before judging the effect.

If one of the mixture screws is still unresponsive then something is amiss. I would check the carb for air leaks, blockages or damaged O rings on the idle jet or mixture screw.

[Bob_Roller]

LuckyD, my first guess, would be you have a clogged idle jet .

The opening at the end that goes in first, if it is clogged, it's hard to determine, the hole is so small, even when it's open, it's hard to tell .

[flybot]

+1 on that ^^^^^