0 vacuum

[Anonymous]

Ok I need to sync. my carbs but I was just wondering if anyone can help me. For some reason when I punch the throttle wide open the sync gauges jump strat to 0 vacuum. any ideas on this one fellas.

[Billy Ricks]

If you have non CV type carbs it's because the wide throttle opening slows down fuel mixture velocity.

[mcconnell]

They're supposed to. The vacuum is measured between the engine and the butterfly and the reading on your gauge shows the difference between the measured area and surrounding atmospheric pressure.

The engine is trying to pull air in (actually the air upstream of the butterfly is trying to push it's way in, but for the sake of discussion I think imagining the engine trying to pull the air in is easier to understand) and the butterfly won't let it, thus creating a negative pressure area. This negative pressure is what lifts the mercury on the glass tube manometer or, in your case, causes a negative pressure reading on the gauge. When you punch the throttle open, you eliminate most of the vacuum as it is now closer to the pressure of the surroundings. Then there is nothing to hold the mercury up, and the vacuum reads near zero.

[Anonymous]

the problem is that it goes beyond near zero and completly zero's out killing the bike.

[earlfor]

If you have VM type carbs, its likely the initial position of your slides is set too high. The mechanical slides are throttle position priority. With the throttle closed, do the slides rest at the lowest possible position, or is there a large gap?

Earl

the problem is that it goes beyond near zero and completly zero's out killing the bike.

[Anonymous]

Why are you punching the throttle wide open while trying to balance your carbs?
Just connect the gauges, start the engine and balance the carbs, no throttle punching necessary. Just a couple of gentle blips to see that they stay balanced.

On slide type carbs, going WFO really quickly may well stall the engine.

[Anonymous]

Sorry they are CV carbs and the bike dosen't quit run right to begin with so I was seeing what they did when I gave it gas.

[daveo]

balance them 1st at idle, then gently ease up to and re-check at 3,000 rpm

[Anonymous]

OK I have them balanced and it still runs like crap. Better but still pretty bad. After a lot of work cleaning connections and testing wires I have come to the conclusion that my Ignition box is shot. It fails the test in the clymer manual. So I tried a set of boxes from a honda sabre knowing they wouldn't work but just to see what it did. It cleaned the bike up at idle and got rid of that rich gas smell but of course it doesn't work at all going through the rpms becouse of the differences in the boxes, however the cleaning up at idle and the failing of teh test in the clymer manual have lead me to a very certian belife that the ignition box is shot. Any ideas on what to do or where to get a box????

[Anonymous]

Remember that synching you carbs is the LAST thing you should do when setting a bike up.
Fix all other problems and carry out all service actions (plugs, valves etc.) first.
Good luck with your hunt for a box.

[earlfor]

Moreso than that, even touching the carbs is the LAST thing you do when setting up the bike. If everything else isnt right, adjusting carbs only makes matters worse and disguises the real problems.

Earl

Remember that synching you carbs is the LAST thing you should do when setting a bike up.
Fix all other problems and carry out all service actions (plugs, valves etc.) first.
Good luck with your hunt for a box.