I have a 1969 L-68 (tri-power) Vette. I recently had the distributer rebuilt. After that was done, I noticed a popping sound in my exhaust. The car accelerates great, and the sound is not constant. It seems to be a bit worse when the car is first started (while cold), but does continue. I also notice a very very slight miss as I'm driving along at a constant rate of speed. I checked the timing and it is good. I also made sure the mixture is right. I was told that it might be a leak in the exhaust. I was also told it might be a plug misfiring occationally or a bad wire. I would think it would be more constant if either of those were true, though. I would also think it would tend to miss more when I accelerate, which it doesn't. What about the dwell? Might that be the issue? What does the dwell adjustment control? Can anyone help me figure this out? Thanks.
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Re: exhaust noise
Is it a single point or TI dist? What does the guy who rebuilt the distributor have to say? What parts were changed? What are the mfg/part number of the replaced parts? Was the dist. tested before it was returned to you?
What is the initial timing set at? What is OE spec? Did you set it correctly IAW the CSM? Is the spark advance map OE or modified? -
Re: exhaust noise
It is not a TI distributer. The guy who rebuilt it said he thinks it might be a plug or plug wire. I'm not sure he is right, though. I'm not sure what all was changed, but he told me the points were new or in good shape. I don't know the part numbers you asked about. I don't know if it was pre-tested. The timing is suppose to be set at 4, which it is. I don't know what IAW or CSM means. Nothing has been modified. Everything is stock (or at least suppose to be).Comment
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Re: exhaust noise
Anthony, I have the exact same car, and mine does the exact same thing. It seems to be a common occurance on the Tri-powers. If you search the DB you'll see a lot of instances of people asking the same question. I believe it is a over rich condition were unburn fuel is passed into the exhaust and dentonates. You'll probably notice it does it on decel or coasting not under load. High vacuum situations where the engine finds extra fuel to suck in that it doesn't need.
There were some recent fixes listed just yesterday on the site such as lowering the float level on the center carb some, which is very easy to do. leaning the mixture screws some, again easy. Making sure your power valve isn't blown, a little more effort.Comment
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Re: exhaust noise
Jamie,
Thanks for your response. I'll look through some of the other comments folks have left on this page. I did lean up the mixture a bit (as much as I think I can), but I hadn't considered lowering the float level on the center carb. I'll look into that. I'm thankful to hear that it is a rather common issue and perhaps not a serious one. I hope it is one I can resolve, though because it is really irritating. The explanation you gave is consistant with what I've heard. I just hope I can make an adjustment to fix it. I am going to also see if there is an exhaust leak from the manifold to the side pipe, which I've been told might be the issue with "post-ignition". I'm embarrassed to say I don't know what the dwell adjustment does. Is there a quick way to explain that? Might that be the issue? The reason I ask is, with the distributer having just been rebuilt, maybe the dwell wasn't set properly? Thanks again for the help you've already been.Comment
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Re: exhaust noise
Do you have the A.I.R. pump installed? I had a similar problem years ago that was caused by a bad diverter valve on the A.I.R. pump. It would almost always 'pop' on deceleration.
Dave T.Dave T.Comment
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Re: exhaust noise
Dave, that's a good thought but I don't have A.I.R. on my car. It was removed years ago. Thanks for the thought, though.Comment
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Re: exhaust noise
'Dwell' is the common name for the number of degrees that the distributor shaft can rotate between each close/open cycle of the ignition points. 30 degrees is considered the optimum number for most Chev engines.
In theory, greater dwell angle will result in more time for the coil to fully saturate the primary windings.Comment
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Re: exhaust noise
Michael,
Thanks for your specific answer to my question regarding the dwell. To follow up on that, what sort of impact does the dwell adjustment typically have on how an engine runs? If it is off (normally less than 28 or more than 32), might that create the issue I described in my initial question (popping sound in the exhaust with no impact on acceleration, but popping as the car is cruising at a constant rate of speed or at idle)?Comment
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Re: exhaust noise
Anthony, Dwell is exactly the number of degrees that the point remain (or dwell) closed. A distributor rotates 360 degrees. During that 360 the points have to open 8 times for an 8 cylinder engine. Divide 360 by 8 and you have 45 degrees of rotation for each cylinder.
During the dwell time the coil is receiving current through the low tension lead, as the closed points complete the circuit. The current is building up a magnetic field in the primary windings of the coil. When the points open the current is stopped and the magnetic field collapses and passes through the secondary windings of the coil. This process is magnetic induction (pass a current through a wire get a magnetic field, pass a magnetic field through a wire, get a current), and it creates a current in the secondary windings which is what comes out of the high tension coil lead to the distributor.
Ideal dwell is 31 degrees, leaving 14 degrees of rotation for the coil to discharge it's spark (collapse the magnetic field). If the dwell "angle" is too short the coil cannot build up a strong enough magnetic field and the discharge spark is weak. If the dwell time or "angle" is too long the coil does not have enough "Time" to discharge the full spark before the points close again. Points can be set with a feeler gauge, but a dwell meter is more acurate.Comment
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Re: exhaust noise
Jamie,
Great explanation. Thanks for the help. It sounds like it would be worth it (regarding the issue I'm having) to at least check the dwell. I've checked the float level and the mixture, and both are fine. The timing is also correct. I still also need to check for potential leaks in the exhaust. Another mechanic told me it might also be a problem with my side pipes themselves. I'm a bit skeptical about that, though, because it is happening on both sides, and this all started when I had my distributer rebuilt. I would think that if it were an exhaust leak or side pipe problem, the noise would be happening only on one side or the other, wherever the leak (or loose baffle, etc.) is. Maybe that's not true, though.Comment
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Re: exhaust noise
Keep in mind that adjusting the dwell also affects the initial timing, so that must be reset after any dwell adjustment. Also, I;m still uncertain as to the "rebuilt" description. What parts were changed, and was the curve checked on a distributor machine to verify it matched the original? Was the vacuum advance checked and verified? Many open questions on this issue.Bill Clupper #618Comment
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Re: exhaust noise
Bill,
Thanks for the tip. I'll make sure and recheck the timing after making any (if any are needed) adjustments on the dwell. As for all the unanswered questions regarding the distributer rebuild, it sounds like I need to go back to the mechanic that did that work and ask some of these questions. Thanks again.Comment
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Re: exhaust noise
Minor variations will hardly be noticeable, other than the ignition timing deviating from ideal setting.
In extreme cases of incorrect dwell, the engine may also misfire particularly at high RPM under load. Although it's certainly good to verify your existing setting as well as the timing, the symptoms you have described do not sound like dwell misadjustment to me. FWIW.
Just as curiosity, what spark plug heat range are you running?Comment
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Re: exhaust noise
Keep in mind that if ANY of the distributors internal parts, Mainshaft, Weights, or vacuum advance, the probability of thos parts meeting the original specs without reworking is virtually NIL, based on distributors I have rebuilt. At best they are servicable replacements that can be "adjusted" to meet specs in many cases, but seldom do they "hit the mark" as delivered.Bill Clupper #618Comment
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