Inliners International Forums General Discussion Engines 250 idle question

installed new points, set gap with vac advance unhooked engine idles smooth at 800 rmp. when i connect the vac advance it jumps to around 1800 rpm. also when i attach the dwell meter it is reading zero, any thoughts???

Skirk, dwell is independant of advance and should remain the same with it connected or disconnected. You have to have dwell or the engine would not idle @800; check your meter. The rpm increase when the vacuum advance is connected is normal; that MUCH increases is not. When dwell is adjusted timing is affected....I suspect you have too much base timing for the current dwell setting (which may or not be correct).....Fats

I had this happen too and thought it was a bad dwell meter.
But, it was bad points - severely burned. The car would run but would not register dwell on the meter. New points, problem solved. Same gap setting.

The dwell can change slightly as the vac advance works, since the vac adv moves the plate and it is not exactly concentric. Pretty close though so dwell should not change much unless you have a lot of vac advance.

My experience is that if you have a good "hot" mechanical curve on the 250, say about 34 total initial+mechanical all in by 3000, then it will only want another 10 or 12 degrees of vacuum for cruise which is not very much movement. If it gets "rough" when the vac advance kicks in, you might have too much vac advance.

DeuceCoupe,

WOW....I've found another person that knows how an ignition system works. If I didn't know better I'd think I had written your answer. Your suggestion for a "curve" will cover 95% of the engines out there. The other 5% will usually have me scratching my head and looking for an answer.

Ron

Here is my opinion on this problem. the vacuum advance on my engine connects to a port that is not full vacuum. it could be that you are hooking your vacumm link to full vacumm and that is why its advancing so far.

In my setup the port that the vacuum advance connects to on the carb leads to a very small hole inside the carburetor throat. when the butterfly valve is shut at idle this vacumm port is not exposed to manifold vacuum, then as you open the throttle slightly the butterfly valve gradually exposes the distributor connection to the manifold vaccum and starts to gradually pull on the advance mechanism.
There sholud be no advance during idle and only advance when you open the throttle. Usually that vacumm hose connector is just above the level of the throttle butterfly. if you are using any other vacum source , that is most likely your problem.

I would check your timing with the hose off and plugged, then hook up the vacuum advance and adjust your carburetor mixture screw(s) for proper mixture, and then the idle speed adjustment for rpms.

Ported vaccuum is a result of smog regulations and unless your vehicle has to undergo emmissions testing full manifold vaccuum is the way to go.

May I suggest reading this article on distributor tuning http://www.hotrodders.com/forum/distributor-tuning-theory-part-1-a-59033.html

I don't know how many here remember the California mandated NOX modifications ('70s?) for all cars '62 and newer. Basically it was a kit that consisted of a plug and a sticker, the vacuum advance was disconnected and plugged, the timing retarded, and a 'Certification' sticker was applied. The engines ran like crap and overheated, but the Air Resources Board could pat themselves on the back because now the older cars ran just as bad as the new ones with retarded cams and ported vacuum. The good part was after the inspection you could just hook the hose back up and reset the timing.

As always, just my opinion, but it works for me.

Tim

I remember that, Tim, and that's exactly what I did.....put the smog crap on, got the car cerified, and took it off again.....LOL. Skirk, you might consider ditching your points and condenser and switching to a Pertronix conversion. No more points, no more dwell hassles, just install it, set it, and forget it.

Here's a suggestion...put in an HEI, or a Pertronix II or a Crane Cams Distributor Kit and you won't hae to worry about points or dwell anymore. May cost a few more bucs, but, in my opinion, well worth it.

seem to have the points set correctly now, runs smoothly, idle is about 950 rpm. when i shut the key off sometimes the engine wants to keep running, any thoughts on this issue?

The throttle could be open a bit to much causing the higher idle and dumping enough fuel down upon shutoff.

Try backing off the idle speed screw on the carb and see fi it still idles smooth at around 700-800rpm

950 or even 800 is NOT idle rpm for an inline six. Unless you have a big cam, there should be no reason that it would not idle at 500-600 rpm. It should idle well at 400, but oil pressure will drop very low and most factory specs call for around 600.
If it is idling high because of poor carb adjustments, it could have a vacuum signal from a ported vacuum connection at 950 rpm, if the throttle blades are open too far.
It is running when you turn the key off because the throttle blades are open too far and then you are getting pre-ignition from hot carbon deposits, allowing it to diesel.

I would first make sure you do not have a vacuum leak by spraying the intake gaskets and carb base with carb cleaner.
You said you checked the dwell with a dwell meter. You didn't say you had checked the timing with a timing light yet. Check the timing.
Next, you need to adjust the idle down to where it belongs.