6 months ago, I tuned my 69 vert 350 Auto (no AC) with new points/condenser and a rebuilt carb. I set the dwell to 30 and the timing to 9 BTDC as per Chilton's recommendations. Idle was set to 800-850rpm in park, and 650rpm in drive with brake depressed.
Well the other day, my car started idling rough. Now it always has had that rumbling shaking that we all love. But this was different...more like an ceiling fan that is missing a blade... all out of whack. And it only happened after going 50-60 down the highway. And it only happens intermittently. And more importantly it only happens when I am at a light.
So, yesterday, I checked the dwell and it is 30.5. The timing however is 10 degrees BTDC with the car idling at 800. If I lower the speed screw down to 650rpm, the timing is 8 BTDC. Chilton states to check auto trans at 650 rpm. It doesn't say park or neutral or drive. It is out of whack, but I cannot figure out if I should go with my reading at 800 in park (10 BTDC) or 650 in park (8 BTDC). Or am I totally off the mark.
Just my thoughts, but maybe your vac advance is starting to come in at or just prior to 800 RPM? Are you checking your timing with your vac advance disconnected? Have to ask cause sometimes people forget about that, even myself. Also, is your vac advance hose connected to manifold vacuum or higher up (ported) on the carb? I run manifold myself, but lots of folks have mixed opinions about that, my car just seems to run better with it. I would start by making sure to disconnect the vac advance hose to the dizzy and plugging the source and the hose, then put the rpms down to about 600 or 650, just to make sure that no advance is starting to come in and time it to what you need it to be. Then after timing is set, adjust your rpms to where you want it to be for park and in gear, then plug your vac advance back in and see where it goes from there. Here is a good article on Vac advance http://www.hotrodders.com/forum/vacuum-advance-hooked-up-directly-manifold-bad-47495.html Here is one on this site https://firstgenfirebird.org/forums/ubbthreads.php?ubb=showflat&Number=230669 let us know how it all works out for you!
8 vs 10 on idle doesn't really matter with respect to making it shake like a leaf intermittently. I also believe factory/Chilton's timing specs are of no value with modern fuels and presumably an engine that has been rebuilt at some time in its history and has a different compression ratio than stock. Regardless of timing I think you're looking the wrong direction and changing timing is not going to solve the actual problem. Sounds like an intermitent misfire which would have me looking at plug wires and/or distributor cap/rotor contacts. Maybe even idle mixture or idle speed issue. Do you have a tach on the car? Can you tell if RPMs are simply dropping a litle bit too low when the shaking starts at a stoplight? Does shaking stop if you take it out of gear, allowing RPMS to raise up a little bit?
OK, I just took it out and had no issues with the shaking. But yesterday it was hit or miss (and more hit than miss)!
As for the vac hose, I did disconnect it at the distributor and plugged it for the test. The other end of the hose connects to a plastic thing (with other vacuum lines as well) on top of the front of the intake manifold (near upper radiator hose).
Shaking is only in gear, when I take it out, the RPMs go up and it stops shaking. Also, when i am moving along, it has no issues either. Car does have a tach which I confirmed yesterday with my dwell meter's tach function.
Oh, your running one of them thermostatic vaccum switches. That switch basically, opens up or provides vacuum based on the temp of the coolant in the manifold. I am really not familiar with that in depth and wouldnt know if that would cause problems if that switch was going bad. I would start looking at the items Crazecars suggested. You also mentioned you have a rebuilt carb, wondering if you might be having problems with the idle circuit or with something being stuck here and there.
I rebuilt it. I don't think it is the original carb (Rochester 2bbl) because the stamp says it comes from a 1968 firebird and mine is a 69. But I did the rebuild 6 months ago.
Next time it starts to shake rattle and roll, conect your timing light and check if the flash is steady. If so put it on wire number two, three,... just to determine if you're getting a steady spark at all cylinders. If you get one that has an intermittent pulse check the plug and wire on that cylinder.
Check your points. As they wear out the gap gets larger and affects the timing and dwell. If the points look burned it may be a bum condenser, there's a lot of that going around this time of year.
As crazycars says one or two degrees of timing and a hundred degrees of rpm wont give it "missing blade" symptoms, but a missfire will. A cross fire, one plug charge crossing over into another wire, will also cause grief, do you have old wires or a couple running parallel touching?
Next time it starts to shake rattle and roll, conect your timing light and check if the flash is steady. If so put it on wire number two, three,... just to determine if you're getting a steady spark at all cylinders. If you get one that has an intermittent pulse check the plug and wire on that cylinder.
Check your points. As they wear out the gap gets larger and affects the timing and dwell. If the points look burned it may be a bum condenser, there's a lot of that going around this time of year.
As crazycars says one or two degrees of timing and a hundred degrees of rpm wont give it "missing blade" symptoms, but a missfire will. A cross fire, one plug charge crossing over into another wire, will also cause grief, do you have old wires or a couple running parallel touching?
1. Will bring timing light with me next time to check..good idea.
2. Wouldn't the dwell seriously depart from 30 if it is seriously worn? Is 30.5 too far away from 30?
3. How do you manage cross fire of plug wires?
4. I will pull all the plugs and check gap, cleanliness and spark.
And if/where they do touch/cross over and touch, 90 degree crossovers will also prevent it. The parallel runs where hey touch or come too close together are the usual culprits, and usually it's only an issue between sequential firing cylinders.
Vacuum leak will also make it act like that?? When I bought my car it ran the same way. The seller said he rebuilt the carb but it was leaking from under the carb base plate and from the intake manifold gasket. What a relief after I sprayed all over and found the leaks. Good luck and let us know what you find.
1. works once in a while to find an intermittent miss; 2. The more the wear the more change. No, 30.5 is not a prob. Sometimes the cam follower can wear down as well which has the opposite effect as worn points, decreasing the gap and increasing dwell; 3. What Craze says; 4. Can't hurt
