Carb sync vac level

I put the carbs back on today and when I was trying to sync the vac on all 4 inputs would drop almost off the scale of chacals meter as I was syncing. What causes that? I turned the choke on a little and the rpm jumped to 4k or so and so did the vac reading. I got done with the readings about 1/3 the way up the scale. I got them all even and it ran like crap. 2 turns out on the mixture screws. The plugs were starting to clean up from totally black after I took a 10 min ride. I tried to listen and tune the carbs till the bike sounded like it ran good. Closer to 3 turns out on all of them. Re sync and now runs like it did last year. I hope it will start cold tomorrow.

 

Vacuum pressure is directly related to engine speed. What was your RPMs when you were getting the low reading? If you were below 1K, that is probably the culprit. The vacuum pressure is pretty low once you get below 1K.

 

It was just above 1k on the tach.

 

Yea 1k is too low try 1500 to 2K

 

I disagree. I just synced LtDave's 900 at about 800 RPM and it pulled better than 8 inches of vacuum. Almost every bike I sync pulls 7-11 inches, even at low rpm.

If you aren't getting more than 5 inches something is wrong. Even one cylinder not firing will drop the readings very low. A fouled plug will do this as will one mixture being way off.

On your 82 I suggest you start your tuning with the mixture screws 3 turns out. 2 turns is not enough for the fine threaded screw they went to in 82.

 

MiCarl, What is the reason for the decrease in vacuum in the intake manifold of a misfiring cylinder? I don't get that... :?:

I thought that intake manifold vacuum was a mechanical only event and was affected by RPM and the restriction of the butterfly upstream. I don't understand why the vacuum pump of an engine would care if the cylinder fired. It's still going to draw the same intake charge volume whether it burns it or not isn't it?

What am I missing? :?

Burnarr

 

It's not just the misfiring cylinder that has less vacuum, it's all of them.

Imagine all 4 cylinders working at 1000 RPM. Now kill one of them. To keep the engine turning 1000 RPM the remaining 3 cylinders will need 1/3 more fuel to pick up the slack. To get that fuel the butterflies need to be opened more. So, engine pumping same amount of air (1000 RPM) but the butterflies are wider open = less pressure drop=less vacuum.

 

Whew... That's better. Sounded like you were drawing a direct cause and effect relationship between a misfiring cylinder and low vacuum and that didn't make sense. :lol:

A misfiring cylinder will cause your RPMs to drop, and in order to make up for that RPM drop, you need to open the throttle a little, which in turn decreases vacuum levels on all cylinders.

Are we talking the same thing now?

 

I deffer to your experience. I use a vacuum gauge and use 1500 to get more stability in the reading. It sure sounds like he is getting lower vacuum - I understood even the carb sticks needed about 1500 rpm. I don't mind being wrong its how I learn

 

And you may have to start at 1500 rpm. Unless the sync is half way decent at the get go it may not run much below that.

But as you bring them in line you can turn the speed down - and they may come back out of line. Then tweak them and go down some more. Once you get close to 1000 rpm you can bring the colortune into the mix.

By the way - if you sync them at 1500, then work your way down to 800 and sync them there they should still be synced at 1500. The measurements are just more accurate the more the butterflies are closed.

 

That makes sense - I have a better restrictor for my gauge now I will have to play with lower RPMs.