80 G Vac advance question

[thewiz]

Finally have my G running good BUT the vacuum advance unit makes an awful hammering sound at low rpm which goes away once the rpm increases. Have tested it with a Mighty Mite (holds vacuum) ... even exchanged for another one but it still makes noise. Bike has some VM33's, K&N insert, and a Super Trapp on it but otherwise stock .... only other info I can add is: when I got the bike it didn't run due to bad PU coil wires ... I had another timing plate layin' around so I fixed it and put it on in place of the one on the G .... looked the same although it came from a 79 motor .... any ideas anyone?

[planedick]

Could be????

Sounds like you have the hose in the wrong place. It goes on the nipple on the #2 carb, not on the manifold nipple....

[DiverRay]

The other thing to do is try spraying some silicon lube on the moving parts. I would check the vac hose location first, though.

[pggg]

With VM33's the vac unit's probably hooked to a carb boot, screw a pilot jet up into the hose as a damper

[thewiz]

Ok ...

Everything you all have said makes good sense .... when I first got the bike, the vac advance was hooked, as PGGG and the rest of you suspected, to direct manifold vacuum, but the first time I pulled the carbs off, I discovered the ported vacuum port tucked in tight between the 2 and 3 carb ... capped. So I used it instead of a boot when I put it back together. But .... the hose that was hooked to the carb boot originally did have a small restrictor in it ... I figured it was put there to make up for the fact that it was hooked to direct mfld. vacuum instead of ported vacuum .... but probably not so, eh? .... I have a 15 pilot jet layin around ... think that might work? Thx for the input too .... I'll try that on Thursday when I'm off.

[pggg]

The pilot jet, half a mm, drilled out

[AlanB]

Hi Thewiz,

If you have stock carbs, and are using the vac port on no2 carb body, you should not need a restrictor in the line; there should already be one in the port!

If the carbs are not XS, or if the port in the carb has been altered (heard of several!) you will need a restrictor in the line.

To test if this is the source of the noise, it is OK to run with the Vac unit disconnected, so long as you cap all of the vac ports on the engine/boots/carb.

AlanB