Vac advance chatter

[dntemp_ca]

The vacuum advance mechanism on my 78E chatters at certain rpm's. It's a very unnerving sound if you've ever heard it. I've read all the posts on this topic and have been fighting it since I got the bike two years ago. This year I installed new carb boots and cleaned & synced the carbs. Chatter is somewhat less, but still unacceptable. I've also tried new heavier vacuum hose. It is hooked up to the proper nipple. The vac diaphragm holds suction and the mechanism moves freely (ie: not sticky). There are only about 28,000 kms on the odo. The bike performs very well with the vacuum hose disconnected and plugged with a small metal screw in the end. I have a couple questions:
1) I've read that there is also a centrifugal advance that operates in addition to the vacuum advance. Does this unit function thoughout the rpm range?
2) What are the risks of running with the vacuum advance hose disconnected and plugged?
Comments?

[Ken Talbot]

Usually, chattering happens when the vacuum line is attached to one of the manifold nipples rather than the nipple on the body of carb #2, but you say it is connected to the correct nipple. Do you have a vacuum gauge you could connect to the nipple on carb #2 just to see what is happening there? If there are no wild fluctuations, that might point to some sort of problem with the centrifugal mechanism or some other mechanical linkage.

[uk1.1s]

Chattering advance

If you've checked the posts then you must have seen my plea for help in one of my here ...about last Oct.

I too suffered from a terrible chatter which had me phoning all sorts of people, emailing Merriam Cycles (pretty difficult to diagnose over the email - so no slight on them ...they tried to help and offered sound advice) and asking help from everywhere.

Bike was running over a year before I posted here and it was probably Ken and some others who pointed me right.

The vacuum in direct connection to the cycles of the cylinder (no. 2) are not damped - this will cause the vac unit to chatter if connected. To damp the vac cycles you need a much smaller hole than the brass take-off pipe from the rubber carb manifold. Yamaha in their wisdom put another take-off on the metal carb body of no. 2 carb just beside the rubber manfold take off. This has a restriction at the bottom to damp the effect of the vac cycles. What I did was mix up the vacuum tap 'octopus' connection (on the rubber manifold of No. 2) with that from the vac advance. So I got chatter AND a slight fuel flow problem from the fuel taps (difficulty starting sometimes).

I was CERTAIN that I'd got this right - so do have another look and make sure.

As for running without the advance, I tried this for a while as I was fightened that the advance unit would get damaged. Found that at lower revs to about 2.5k the bike bogged down and didn't pull too well.....then it ran OK, but not as 'crisp' due to a single 'advance' position. Also found it drank fuel at a higher rate....didn't calculate it, but the only long run like this was expensive!

As a final thing to share I did the following that didn't help (in my case)

1. Replaced carb diaphragms

2. Bought a second hand vacuum unit

3. Fitted new rubber hoses

4. Checked the carb manifolds for leaks - and taped them with
electrical tape temporarily just in case

5. Stipped the carbs

6. Tried to fit my own restrictor (experiments of 0.6mm - 1mm drilled in a plastic plug) within the vac unit hose.

....but it was definitely my poor memory on where the hoses fitted that had caused the trouble.

[dntemp_ca]

Thanks for the ideas. Yes, the vac hose is indeed hooked up to the proper nipple on the #2 carb body. I'll try to find a vac gauge and test that nipple. I'm a little reluctant to take the vac advance mechanism apart since I don't have a timing light yet. But I guess that'll be a next step.

[jkgrubbs]

I had the same issue, with poor running until 3000. The #2 carb had trash in the nipple hole, plugging the vac, and we all know how painful that can be.....

[MartyA]

I marked everything when I took it all apart, so I could put it back in the exact same place. Just a line across the advance ring to the motor. If I recall, the vacuum advance will come out without impacting the timing. Just don't lose that tiny "C" clip on the end of the arm. The centrifugal advance comes out also, and can be cleaned and put back in without changing the timing. I don't have any 'chatter' with my vacuum unit, but it does bounce a lot.

Yes, you can run without the vacuum advance, should not hurt it at all, but you will miss that neat snap in acceleration as it suddenly advances an additional ~15 or so degrees. If you try this and get little acceleration, the centrifugal advance needs cleaned.

My centrifugal advance was gummed up after I bought the bike to the point it would not advance for the full travel. I took it out, apart, sanded it with carb cleaner and 200 grit, sprayed a little Lithease, wiped it down and reassembled it. 7th year no problems. And if it advances too soon, meaning the springs are worn, you will run out of advance at early RPM;s and will have a real dog then.

Clymers says the centrifugal unit is tested by blocking the vacuum and running the RPM up to 5,200 (use a real tach, not the bike tach) and the timing plate should read 36 degrees (Specials are 31 degrees, poor thangs).

[Jdwags]

When I had this problem about a year ago, it never seemed to be solved, whatever i did... ( had a bunch of problems going on)


Then Mike Hart thought of somethign.. It had a different air filter on it.

He suggested I put the stock back on it for now and just try it.

Not only did it solve the other problems I had, It instantly took care of that chatter... Like it was choking for lack of air flow ..

and sucking back and forth on that vacum.

[dntemp_ca]

Thanks guys. More things I can try. Hmmm, I'd had never thought to check the air filter, Wags. I put a UNI on the bike when I got it and soaked it with the requisite filter oil. I threw out the old stock air filter cause it was toast. The UNI doesn't seem restrictive. I washed and re-oiled it this spring. Regardless, I'll check that out. Take care.