Car dropped 250 RPM and is now stuttering
Can anyone advise me on where I should start in order to track down this issue that just popped up.
I was driving around today and things were going great. I stopped for gas and when I started the car back up I noticed that the idle revs we sitting at 500-450 instead of the 750 they are normally at. On the drive home I was experiencing some stutter (like surging but opposite, I was losing power not gaining it) under light load, specifically noticeable between 1500-2000 light throttle.
I'm not quite sure where to start. I'm letting it cool down now but it seems like all the plugs are connected still, which was my first thought.
That is a pretty radical rpm change.And it seems to be in the lowspeed circuit. And it came on like a switch, immediately on start-up, not a minute and a half later.
There are really only three things that can cause such a huge jump;
1) a jumped timing chain.
2) a change (uncontrolled) in timing
3) A fluctuating vacuum leak.
The cam chain tension is easiest to test. Then the Timing. And last is fuel.
Of these, and in conjunction with the 250 drop in idle speed,yet it runs at 500rpm; I would like to rule out a vacuum,or even a fuel, problem.Another reason, to look elsewhere, is because it was like a switch.
I am kindof leaning towards the timing, but first;
I do not think this problem has anything to do with the voltage supply. I have seen MSDs run just fine at 10.5 volts. But never the less, put a timing lite on it and see what the strobe says as the rpm is slowly varied. If the timing is not jumping around. Then check the chain tension.
Timing
>52* at 2200 is usually too much timing. My guess is one or both of your springs is/are not working correctly. 42* would be a better number. So, IMO, I would slow the mechanical advance down to; all-in no earlier than 2800.And I would check out the springs and the entire advance mechanism, for binding or sticking.I might also consider slowing the vcan down some, but the engine would tell me yes or no on that, after the mechanical has been slowed down a tad.
If your engine is getting 52* at 2200, imagine what it is getting at 1500 to 1700; my guess is waaay too much, And it's current behavior is it's way of protesting.
>And finally it
could be a combination of the timing changing AND the engine flooding.
This with an automatic only;
if the boosters are dribbling, then the engine will go rich, and the rpm will drop. With the rpm drop, the vacuum will drop. If it so happens that the vcan is overly sensitive, then the timing will drop out.And the rpm will further drop. Then if the boosters stop dribbling at the new lower rpm. the AFR cleans up, the rpm begins to climb, the Vcan maxes out, and the rpm rises. This can go on forever, as the TC sucks it all up.
The timing check is easier, so I listed it first.
A wildcard is if your metering rods are not staying down. You cannot run the engine with the covers removed, but you can loosen the cover screws and rotate them far enough to see the pistons and then lock the screws down again. Now you can run the engine up and watch them.This is a highly unlikely situation.
Another wildcard is a problem in the brake booster. Or the PCV. These are easy checks, just pinch the hoses closed.
And one more, an intake leak in the valley. But this is usually accompanied by blue smoke. This is another easy test, just seal the engine, remove the pcv from the port, and check there for a vacuum. With the pcv out, there should NOT be any vacuum there,lol.
And another; the PCV hose should be connected to the carb at the port that the manufacturer has designated for it, usually at the front.And the Vcan hose should be connected to the spark-port.And the booster; to the vacuum port on the manifold.
BTW,
your flyweight springs are not retracting the timing cam properly; you should never have to return the rotor back to the base timing by hand. While I don't think this is your problem,I do think this needs to be fixed.
Kindof another wildcard but, If you have been using secondary cracking as an idle air bypass, and if the adjustment has gone away, that could explain the idle dump. but this will have no effect anywhere else except at idle.
