Good Morning,

My car has developed a slight miss at idle and cruising at certain RPMs (up to around 1800). You can see the engine slightly vibrate but it is more noticeable at the tail end where you can hear a "pah, or a puff" sound out the exhaust.

I have reset/verified the following:

points gap .020 (new points and condenser)
dwell set to 31dg
Timing is ~10-11dr (without vacuum advance)
Vacuum off carb/manifold is 20hg and very steady
Idle is 775-800rpm

I have also replaced the cap and rotor.

Here is where it gets interesting,

At idle with manifold vacuum connected to the distributor, the car misses making the sounds described above. When I plug the vacuum line, the misses goes away.

With the vacuum line plugged, I set my timing light to 36dg and rev the engine up and the timing (mechanical & advance) comes all in around 1800-1900 RPM. This seems pretty early based on some article I read by Lars (should be around 2500).

It seems to me that some worn springs would cause this problem causing more advance at idle than needed.

After I checked the springs, albeit never seeing new ones, the spring doesn't even constrict any more - they just stay constantly expanded. Once I saw some new ones on the Internet, I can see where the springs are completely worthless.

My guess is that the worn springs, with the vacuum attached, is causing more advance than needed at low rpms (800-1800). This would also cause the total timing to come in early.

I ordered a new spring/weight kit and it comes with three different colors (black, silver, and looks like gold).

I figured the difference is the tension but what I don't know is what should go on a very, very mild 350 with low horsepower?

Also, I have never troubleshot a distributor issue so if you see something wrong in my diagnostics, please let me know!

Jim

 

I think you have hit the nail on the head.

 

As for what springs, I believe Lars' paper calls for springs of 2 different colors. You may want to reference that paper you were reviewing, see if it tells which ones. In general though, put in weaker springs until you hear pinging under acceleration and then go a spring stronger to make it go away.

 

Full advance is coming in too early as you noted...as Mike and Jeff said you are taking the right approach to the problem. I don't know that a mild 350 is vastly different from other Chevy small blocks as far as timing so Lar's advice should still apply for the timing curve...

The vacuum advance diaphragm could be weak too. I would get the mechanical advance perfect and THEN bring in the vacuum and if things go whacky you know that's an area to look into. The fact that the miss disappears when you plug the vacuum line bothers me..you may have more than one issue.

 

With the vacuum plugged, the misses go away at idle but do show up as I increase the RPM. From about 1200-1800 it starts again.

 

Thanks Jeff, I did re-read it and saw the part you were referring to. I didn't quite make out what he was talking about until I saw how the kits are sold.

jim

 

Hi Jim,
I didn't have the paper handy when I posted my reply, otherwise I would have looked it up and added it to my post for you. But, glad you found the data you needed.

Jeff

 

I've never read the referenced paper but with your engine tune, you may find that full mechanical @2500, expecially with the vacuum advance may still be too much advance.

If you are still getting the "trailer hitching" and "bucking" at very low speeds, you know it's too much. Even if you have no spark knock.

 

"With the vacuum plugged, the misses go away at idle but do show up as I increase the RPM. From about 1200-1800 it starts again"
You may also want to check for worn dist bushings. Put the points on high cam and move the shaft up top side to side. See if the points open and close with just side to side motion. If there is enough movement, say enough to close the points even when they are on high cam,you have most likely found the problem that is making your engine break down at higher rpm. Good Luck.

 

An incorrect advance curve, whether coming in too early or too late, wont' cause a miss at any rpm. You have other issues.

Obviously, if your springs are defective and giving you no advance curve at all, you need to fix this problem. You can get a nice curve that comes "all in" at about 2500-3000 rpm by using the 2 gold springs in the Mr. Gasket kit 928G.

But this won't solve your miss problem. You should be looking for vacuum leaks and other ignition problems such as plug wires, plugs, and cap. I'd start by pulling plug wires loose at idle to see which cylinder is giving you the miss after you verify that you have eliminated vacuum leaks as the source of the problem.

Lars

 

Perhaps a recurve of the distributor is the cards. If you need a contact I can provide a source. The person who did mine was extremely reasonable and has a supply of stock weights, springs and autocams if necessary.

 

I think he is maybe talking about bucking/jerking instead of an actual misfire. Maybe the OP should clarify.

The timing curve he posted will cause that problem.

 

You didn't say if the timing advances when you connect the vacuum hose back up to the vacuum advance can. You only said that the miss goes away when you plug the hose. It sounds like the vacuum advance diaphragm inside the can is blown, causing a vacuum leak when you connect the hose. Hook up your timing light again, and see if the timing advances when you connect the hose--it should advance anywhere from 12 to 16 degrees with the hose connected if it is working and you have a can rated for the proper amount of vacuum your engine develops at idle.

 

IMO the gold springs will bring in the curve fully (with "37" weights) between 2500-3000R. Even still, this might be too quick to avoid some part throttle spark knock if you are running vacuum advance as well.

 

Thanks Everyone,

i swapped out the old springs with one black and one silver and the problem remains identical.

I'll replace the diaphragm next and check plugs/wires. The plugs/wires only have 500 miles, if that, on them. However, you never know!

And to clarify...yes, this is more of a bucking/jerking when driving. While parked, the exhaust sounds more of a hollow "pah" sound and the engine jerks a little. I'm assuming this is translating to the jerking feeling when cruising.

I also noticed, while parked, the "pah" noises are more prevalent after holding her at 1800-2000RPMs and then dropping her to idle.

I don't think there is a vacuum leak, this is based on the vacuum meter on the manifold port of the carb. It is solid, unwavering at 20-21hg.

Jim

 

Jim,

Try two gold (stiffest springs) and see what happens. IMO the black and silver springs are useless for a street application that uses vacuum advance as well.

Nick

 

Ok,

I have replaced the vacuum diaphragm and the problem actually got worse.

With everything "normal" up, at 800RPM my total advance is around 40 degrees which I know is waaaay too much at idle. The "universal" can adjustment's don't seem to make much of a difference. I'll try the "gold springs" and retest.

Problem is definitely too much vacuum at idle, I just have to figure out what is causing it!

jim

 

Which VAC are you using? With 20-21 inches engine vacuum a B-1 or B22 VAC will work well. Your idle timing should read about 26* w. the VAC connected to a full time source and initial timing @ 10*.

 

26* is about right on with the initial set at 10*. I don't know which can is most correct for your application. However, if you have 40* idle, something is amiss with the mechanical advance mechanism. You will need to start with the gold springs, as I am absolutely sure that will prevent any mechanical advance below 1200R. Then you can ascertain how much base timing you have and how much advance the vacuum canister is adding.

If all seems normal after doing the above, then disconnect and plug the line to the vac and use a dialback light to see how much mechanical advance you're getting and how fast it comes in. I know others will disagree here, but in my opinion even the stiff springs will allow the vacuum advance to come in too soon for use with pump gas. You can experiment with cutting coils out of the gold Accel/ Mr Gasket springs to elevate the curve in the RPM range, or better yet find some OE springs from an old distributor that are made from larger wire diameter and therefore have a higher spring rate.

 

Great! The two gold springs has made the engine run great at idle and at all RPMs without any Vacuum Advance.