Timing mark fluctuates

JasonNC

I'm pretty sure I have a vacuum leak, but I'm not able to pinpoint where it is.  The timing mark fluctuates and the engine revs up and down in conjunction with the timing mark movements.  If there is another reason for this problem, I will be most appreciative if one of you guys would steer me in the right direction.

Per

Jason,

I think the timing mark is fluctuating because the engine's speed is going up and down, so the problem isn't with the timing, but with something in the carburetor or perhaps leakage of intake manifold gaskets which is causing the changing of speed. The centrifugal timing mechanism in the distributor is advancing the timing as the engine speeds up, and retarding it as the engine slows down, as it is supposed to do.

Per

Glowplug

One of your best engine diagnosis tools is a vacuum guage.  Go to my Hudson website and select engines from the subjects listed on the front page. Once on that page select the vacuum gauge link.  The resulting page should lead you to the place creating the root problem

SuperDave

To search for a vacuum leak, I use the following. I put a little gasoline in a hand pump oil can. With the engine ideling, trickle a small amount of gasoline at all the suspect connections, IE carb. to manifold, manifold to block, wiper motor switch etc. If the idle changes..you found the leak. Some other spray can like ether, starting fluid etc will work too. Don't overlook the possibility that the advance mechanism may be leaking. I wouldn't recommend spraying combustibles into the distributor LOL.

lostmind

Spraying oil works also , it helps seal the leak , and lasts a little longer , helping to verify the leak. Safer than gasoline , seeing as the exhaust manifold is below the intake.
Works for me.

JasonNC

I sprayed carburetor cleaner on the manifold where it attaches to the block and the engine sped up. I obviously have a leak in the manifold gasket.  What burns me is that this  is a new gasket that was just installed a.few months ago.  

RichardD

Need to comment:  Recommend using a propane/MAP torch with the flex hose to check for the vacuum leak. If you have ever set an 'engine' on fire, you will appreciate this !!! But you still need to be careful (and hopefully have no loose spark plug connectors).

Kdancy

May be the manifold surface is warped and will need a machine shop to "true".