Sorry, Another vac advance unit question

[andy graham]

In between hooning around this weekend been checking a few things over on the car
Either years of smoking have killed my lungs OR my advance unit has failed
Removed the unit and attached a section of tubing and sucked has hard as I could, no hissing or signs of a leak, bit the link arm didn't budge one mm , will move if you push the arm with a bit of force
Would you expect to be able to move the arm by sucking on the tube ( without oneway valve )or has mine died. What would be the symptoms on the engine running if it was time for a new one ??
Andy

[GreenV8S]

andy graham said:

Would you expect to be able to move the arm by sucking on the tube

Yes. The thing that usually fails is the diaphragm, you get an air leak but no vac advance. From the sound of it the diaphragm is still OK on yours but something else has failed internally. You need a new one. When you fit the replacement, be careful to hook it on to the base plate.

[chassis 33]

Also make sure the two screws holding the unit to the side of the dizzy are secure when you replace the vac. unit. If it falls off your timing goes all over the place and, as the 'charged Griff racer at Mallory found out last weekend, it can detonate your engine.

Regards
Iain

[andy graham]

Thanks for the response, seem to remember seeing RPI selling them quite reguarly on eBay. Old unit tight in place and actuator link connected after refiting, noticed a slight pinking on run afterwards, I guess there is just enough movement on the mounting holes on the vac advance unit to allow the timing position to shift slightly, so I'll be praying for bad weather until new unit and timing light arrive
What would the effect be with a failed vac advance on general running/performance of the engine??

[GreenV8S]

andy graham said:

What would the effect be with a failed vac advance on general running/performance of the engine??

Depends how it fails. If it's just the diaphragm you won't get any advance, so it'll run slightly hotter at idle but that's all - generally safe.

From your description yours has failed/stuck with some advance on which will give you excessive advance at full throttle and can cause detonation - generally very unsafe.