VCCA Home Forums TECHNICAL - 1945 AND NEWER 1965-1970 68 timing\dwell settings

Could some one please tell me what the timing and dwell settings are for a 68 impala 307 ? I see the timing marks, on the engine, have an A then a O then a B. Thanks, Joe.

Timing needs to advance 1 degree for each 1000 feet above sea level. Set it with the vacuum line to the advance removed and plugged. 2 degrees before TDC(each tic mark = 2 degrees, so 1 mark ccw of the 0); 28-32 degrees dwell (dwell numbers are inverse to gap; 28 is wide and 32 is closer; adjust towards the middle. Rough idle might be caused by too high a dwell. Missing at high rpm might be too low dwell. Changing dwell also changes timing. Points need to be clean and aligned.)

This is what the book calls for, but there are many factors that determine actual advance needed. Knock point under load and back one or two degrees is good. Changes in elevation of a couple thousand feet can make it run rough. So can changing brands of gas.

Richard, Thank you for the info. Its really helpful! Never thought about the elevation factor. I"ll be in the garage today and trying to get this timed. Many Thanks,
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Joe's 37

Joe,
Put the timing light away.With engine at temp,set the dwell to 30 degrees.Leave the vaccum advance plugged in as well.Only need a tach and vac guage.Best way to time a stock non hi-po small block of this era TODAY,is to simply advance the timing until idle won't increase any longer and eng just starts to miss,then back timing off a little and go for a ride.Goal is to run as much advance as poss on TODAY'S GAS.If it pings/spark knocks,just retard the timing a pinch more and take her around the block again.When you reach most advance without ping lock down the dist.Then set idle speed and use the vaccum guage to adjust idle mixture screws. Good luck,Jim

Ok Guy's, here's what I did today. Replaced the points, condenser , rotor and dist. cap. and plugs. Put four of eight plug wires on. Boy, those front four wires will have to wait for another day. Then warmed her up. Set the timing to 8 degs. above TDC. Set the dwell 45 degs., that's where it smoothed out. It still stumbles a bit, before kicking into over drive, but runs a whole lot better now. Any ideas on where to re-rout those four front plug wires? Thanks, Joe.

The dwell needs to be set first right at ~30 degrees. Then adjust the timing and the carb. Are you sure you were reading the right scale with 45 degrees? Sounds more like the six cylinder scale when the eight cylinder scale is at 30... If you recheck your timing, it will likely be way off if you adjusted the dwell after adjusting the timing.

Hi Richard, Going back out to re-due this today. Been years since I've used my meters, so i've gotten a little rusty at it. Many thanks, Joe.

Just remember that the dwell is a fixed setting. Actually, with new points, you can set it a little wide (28-29 degrees), as there will be some wear on the fiber pusher. You should check it again after a few hundred miles and set it at 30. Then check your timing again, because it will change with the dwell. Good luck on it!

Richard, I reset the dwell to 30, It stalled on me a coupla times, but got to stay running until I could reset the timing to 8degs. Seems to run alright at low Idle but, on high rpm it misses a lot. Could it be the mixture adjustment screws need tweaking??Thanks, Joe.

Idle mixture screws will not change the way it runs above idle. Have you checked the ignition system for causing your running problem at higher speeds?

Hi dens41, I was in the process of replacing the four front plug wires, you know, the ones that run through the front engine mounts! Then got called away for the rest of the day. I swear those wires are the old originals. Hopefully, this will take care of the problem. Thanks for the info, Joe.

Just a follow up here, I replaced the four front spark plug wires today, on the 68, and that took care of the problem of the engine missing. Reset the dwell/timing and the low idle/fast idle smoothed right out. "Thanks to all for your info" Joe.

Glad to se you found the problem. Thr rear ones will seem like a piece of cake.

dens41, Would you know the fast idle and low idle RPM'S on the 68? Thanks, Joe.

Do you have a manual or auto trans? Air conditioning? Idle solenoid? Is the decal still there with emissions info?

Manual = 700 rpm Auto = 600 in drive

Hi Rich, It's a auto. trans, no air, no Idle solenoid. Thanks, Joe.

Also no emissions sticker to be found!

Jim, Since I don't have a manual for this car yet, could you please tell me what the vacuum is to be set at? and where a bouts the idle mixture screws should be?(how many turns out)? The reason I'm asking is because, I have the dwell at 30 deg. and the timing at 2 deg. BTC. It runs ok at Idle, but when I put it into gear it wants to stall. I'm going to get a new vacuum meter today. I can always use one! Thanks, Joe.

Hi Joe,
Use the vacuum port lower rear at carb base where the air cleaner housing plugs in,as your best source for the vac guage.Should be able to easily pull a min of 18 inches vacuum at idle when warm.
Set the idle mixture screws at 1 1/2 turns out to start.Let her warm up before you adjust anything.Should end up at approx 2 turns out.After each small turn of each idle mix screw,give her a few seconds to tell you if she likes it.
Have a tach hooked up as well.This will also help with the carb adjustments.When warm that 307/'glide no a/c combo probably likes to idle in park at 600-650 rpm.Should drop 50-75 rpm in drive.Good luck! Jim

Many Thanks Jim!! Couldn't get back at it today, will try agin tomorrow, Joe.

Adjust for max vacuum. 18 is good for sea level, but vacuum is another thing affected by elevation. We are happy with 15-16 inches here at ~6000 feet. You might adjust the timing for higher vacuum also. A couple more degrees advance might help. The idle speed is supposed to be set at 600 rpm with the tranny in drive. Adjust the final speed after making all other adjustments. Having a good parking brake and/or good sized rocks blocking the tires is important for safety reasons... Someone inside with their foot on the brake will also do.

OK, I went out and checked the vac. today, on the 68, and got it to about 17. Then re-checked the dwell because the engine was starting to run rough, (engine was warmed up at this time), and it was jumping around between 30-35. Before I could check the timing it stalled!! The car would not re-start. Pulled #1 plug and wire, held it to the manifold and had the wife turn it over a few times-NO SPARK. Is the coil shot now or what?? Any Ideas will be appreciated. Thanks, Joe.

With no spark when hot, pour cool water on coil and see if that restores spark. If not then check condenser. If coil comes to life when cooled then replacing coil is best option. I have had the coil on my '28 get overheated (internally shorted) several times. Only when lugging or very slow speeds in very hot weather. Low engine speed heats the coil more than almost anything else. The problem is not uncommon. Have had it happen with coils made in China and Mexico.

Thanks for the advice Chipper. After it cooled down, I replaced the points and condenser, and it still didn't start. Next is the coil. How does one check the condenser?? What else could it be besides these two things? Maybe the timing gears? Thanks, Joe.

It could be coil, distributor cap, rotor, ignition switch, coil wire. Even could be new condenser (test by determining the gap the spark jumps) or new points. Don't trust any part you replace until you are sure it works. There are plenty of occasions where a new (just out of the box) part does not work. Yes most work but, not all (particularly if made in China).

Boy, I just replaced all of this except the coil. All of it was made in Mexico!! Joe.

Mexican made is better than China but not as good as Made in USA.

Hey Chipper, that was it !! I put in a new set of points and condenser and it fired right up. I must have got a bad set before. Agin, thanks TO ALL for the help, Joe.

Glad to have helped. Learned over the years that the problem with spark is rarely the coil. Points, then condenser lead the list.

OK, here I go agin. Everything was running fine up to now. Now it surges and misses when I sit at the light or just start out from one. Out on the highway,it seems to run ok. This is on a 68 chevy Impala 307 2 barrel. Any help on this will be appreciated. Thanks, Joe.

Based on my experience with 60s and 70s Chevy carburetors, the Rochester two-barrel has several annoying problems unless clean and exactly adjusted. Hesitation, inconsistent idle, surging are three of them. They also typically get lower gas mileage than the four-barrel.

Hey Chipper, thanks for the info. I'll have to get some carb. cleaner, and try to adj. the idle mixture screws. What would you suggest, turn an a half or two on those, or when it sounds right? Thanks, Joe.