In an article in this month’s Hot Rod Magazine (January 2019), a question from a reader is why their engine (Chevy 330 355 as I recall) is performing poorly after they installed new aluminum heads and a new Holley carburetor. No changes to the camshaft or block. In order to get the engine to idle & run well they now have to advance the timing to 30 degrees BTDC base timing. Hot Rod suggests this indicates a problem b/c the normal base timing for this engine is usually around 16 degrees BTDC. It’s the same distributor, but it had to be temporarily removed in the process of installing the new heads of course. Hot Rod says the problem may be the distributor installation is “off”. I can’t figure out how that could occur. It seems like all that’s required is for the rotor to be pointing more or less at the number one spark plug terminal at TDC compression stroke. Beyond that, it’s just adjust the timing according to the timing light as viewed on the balancer, right?
My question is: How could the distributor re-install being “off” result in the need to idle the engine at 30 degrees BTDC?
Simple, if you try to install the dist. with the engine at TDC and the rotor pointing at #1, the curvature of the dist gears will rotate it one tooth off. Straight cut gears would be noisy. The dist has to be installed so the rotor is pointed at #1 AFTER it is seated.
But can’t that be corrected by just rotating the top part of the distributor so that the timing is correct at the balancer? If it is physically impossible to rotate the distributor enough to get the timing at the balancer correct, then reinstalling it might be necessary. But in this case they can get the timing at 16 degrees ok, but the engine won’t idle or run well at 16 degrees.
30 btdc is 30 btdc. The distributor has nothing to do with why it runs better at 30 btdc than 18 btdc. It has something to do with the new head. Probably changed the displacement.
There wasn’t any change to displacement, as that’s a function of the block which wasn’t changed. But there was a slight change to compression caused by the new heads, increasing compression, but only slightly. Hot Rod said the increase in compression would more likely reduce the idle timing, not increase it.
Who knows if the carb they put on is the right size for that engine or if it is jetted right, has the best power valve for that engine or is even adjusted right. Who knows what intake manifold was used? The probably increased fuel and air flow quite a bit on the intake side, did they free up the exhaust. Did Hot Rod figure it out?
Most likely they installed the distributor one or two teeth too far behind (retarded timing)… So now the ideal timing is the number of degrees too far back plus the - number of degrees needed to be before TDC… This suggests being off by 45 degrees from TDC, anyone who doesn’t notice the distributor being off nearly 45 degrees retarded would be asleep at the wheel but… Lets say the rotor as it was installed was too far retarded at -45 degrees…so… +30 degrees gets you to near 15-16 degrees BTDC… Basically they are just making up the number of degrees off that they are from the - BTDC they were shooting for… This is off by A LOT…so how anyone didn’t notice is a mystery. Wouldn’t be the first time a reader of Hot Rod got in way over his or her mechanical ability.
I know it wasn’t mentioned but they could have the engine base timing off if they messed with the cam chain and “forgot” to mention that little tidbit. I find people tend to leave out info they don’t classify as “important”.
** Another source of issue can be that they have the distributor cap wired incorrectly…correct order but beginning too far off the mark… Which would produce the same issue and force you to rotate the distributor to catch everyone up so to speak. These are the things they should be checking…and probably solved by now…and never told us they fixed it… Sound familiar? lol
chevy dist has helical cam gear but tang on bottom for oil pump drive. you set motor to tdc and align dist as you drop it into block and if it does not drop down fully you slightly turn crankshaft to rotate oil pump tang. dist should than drop
Correct me if I’m wrong, but If the timing light is showing 30 btdc on the crank pulley, #1 cylinder is firing 30 btdc no matter how you slice it. If that is the case, the question should be why is it idling better 15 degrees off?.
I can’t imagine an engine running with the timing off 30* advanced or retarded. And 15* advanced would make starting impossible while 15* retarded would require holding the throttle open to start and continue to run and the exhaust manifold would quickly become glowing red. My best guess is that the timing light is connected to the #2 cylinder.
No. Remains unresolved at this point. They offered a number of things for the car owner to check, the distributor being “off” is one of them. They didn’t explain what they meant by “off” though. Which is the topic of this thread, what did they mean by “off”?
As one of their possible explanations, Hot Rod indeed suggested there might be a timing light problem.
This was another of their theories. Vacuum leaks at the carb also.
I may be mistaken about the engine displacement number. But the only parts the owner replaced were the heads and the distributor, so the displacement, whatever it was, couldn’t have changed.
That sounds correct to me. Whatever the timing light is showing on the balancer is what the engine timing is, irrespective of whether the distributor shaft was installed at the “correct” orientation or not. If the shaft isn’t oriented quite right, the person with the timing light will just rotate the distributor cap/base to a different position that they would have if the distributor shaft had been installed differently. This will only work if the distributor cap/base can be physically rotated to that position of course. The distributor cap/base will interfere with the engine parts in certain orientations. But since the owner was able to achieve both 15 and 30 degree btdc by rotating the cap/base, the shaft orientation must not have been off much.
I got to thinking, maybe by ‘off’ Hot Rod means some of the distributor high voltage wires are connected to the wrong spark plugs. That could certainly cause this. But you’d think however you set the timing the engine would still run pretty poorly.
The distributor is not installed properly… They wired the cap incorrectly… The timing light is not on the #1 cylinder wire… They messed with cam timing… They forgot the woodruff key in the balancer? There are plenty of places this person could have screwed up. All these things are quickly and easily sussed out however.
We can only guess which avenue they took to get to confusion land here. The mere fact they are confused about any of this speaks to their mechanical ability so… If any one of us were onsite we would have the answer fairly quickly.
@Honda_Blackbird got it right. From what info is provided, I would say as the engine is reportedly running fine, it comes to woodruff key, or damaged timing mark plate or they are reading the timing wrong. But like he says, asking the right questions would give the best results.
Yeah but they weren’t supposed to have had the harmonic damper off the engine. I was just kicking around things that would produce the symptoms seen here. Besides…how do you know I got it right? Aren’t we discussing someone who wrote in to Hot Rod Mag ? LOL… I dunno
I think you may be going a little overboard on your criticism of the person with the car problem there HB. I certainly have run into technical problems repairing cars which perplexed me to no end. The owner of that car is simply asking for ideas why it only runs well w/the timing set at 30 deg. Marlan Davis is the technical editor at Hot Rod who wrote the reply, and it would be hard to find anybody anywhere that’s more knowledgeable about technical stuff w/ older cars than he is.
The topic of this thread btw isn’t why it runs better at 30 deg. The topic here is: how could a faulty distributor installation cause that? So far the only theory that makes sense to me is
plugs wired to cap incorrectly, but then why would it run well at any timing setting?
Problems w/the timing light, triggered from the wrong plug, balancer problem are not related to the distributor installation. At this point I think the conclusion is that there is no explanation how a faulty distributor installation could cause this symptom.
New theory: Is there a distributor installation problem that could cause the firing to occur in between the number one wire and the next wire, so the only way to get the spark to the number one spark plug is to over-advance the timing? Wrong points, defective points, point gap way too large or way too small? Still wouldn’t explain why it runs well at 30 degrees though. But it might explain why it runs poorly at 16 degrees.
Why would the cap being wired incorrectly run normally at any timing setting? Easily, lets say you have the firing order correct on the cap…but you labelled the number 1 cylinder location one unit off… The cap would have the correct order…but every wire is one place off. You would need to rotate the cap almost exactly the amount they are doing in this instance…
You think I was being overly critical? I dunno, perhaps. I don’t have any doubts if I (or us) were onsite the problem would be solved in short order however. There aren’t many areas to look into on this one…so the solution is not a difficult one.
Who knows…we will never know the solution here, there is no back n forth with the OP so all this is a theoretical exercise… so this thread could go on Ad Nauseum. But the cap could be the source of this issue…and if the order is correct but with the wrong starting point it could explain why it is so confusing to the person in this situation. They probably looked into the firing order and the cap would pass that inspection…if they didn’t take into account they started the sequence in the wrong location…