Fluctuating RPM's
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Elder User
Joined: Dec 2015
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I redid the test correctly. Here are the results:
Idle: 575 RPM (± 20 fluctuation) vacuum of 0
Revved engine to 1000 RPM (± 0 fluctuation though it flickered to 1010, but I think it was the tachs fault) vacuum of -5
Revved to 2500 RPM (fluctuation ±10) vacuum of -20
When the engine is revved to 1250 (I did not check at higher speeds) the advance seems to differ by one switching back and forth.
Idle: 575 RPM (± 20 fluctuation) vacuum of 0
Revved engine to 1000 RPM (± 0 fluctuation though it flickered to 1010, but I think it was the tachs fault) vacuum of -5
Revved to 2500 RPM (fluctuation ±10) vacuum of -20
When the engine is revved to 1250 (I did not check at higher speeds) the advance seems to differ by one switching back and forth.
FTE Legend
Joined: Jul 2010
Posts: 32,875
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From: Northeast, OK
If you are only seeing a fluctuation of 20 RPM on a digital tach at idle you are chasing ghosts. Can you hear any RPM change? I doubt it, and I doubt there is a problem. These aren't fuel-injected vehicles. Some authorities have said it is amazing that engines will even idle, much less do so smoothly.
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Elder User
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I meant to add this to my previous post.
I added a clamp around the hose where it connects to the carb and that dropped the fluctuation from 75 or so to around 20. I think it may have just been a vacuum leak because that hose was pretty loose on there. Would that have caused the fluctuation?
I added a clamp around the hose where it connects to the carb and that dropped the fluctuation from 75 or so to around 20. I think it may have just been a vacuum leak because that hose was pretty loose on there. Would that have caused the fluctuation?
Lead Driver
Joined: Apr 2007
Posts: 5,958
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From: Houston
Gary, I actually am running full vacuum on my dist. I tried moving it to ported vacuum and the truck just had a roughness in the idle. rpm didn't move on the tach it was steady but just a roughness in the truck. Switched it back to full vacuum and now with the A/C off you cant even tell the truck is running it is that smooth running.
I think some engines will like manifold and some will like ported. I know my emission sticker says it should be ported. But doesn't like ported. My '78 Mercury is set to ported and was when it was running idled great but that could have been due to the fact I have a primitive map sensor that works in conjunction with a yellow DS2 box to advance the timing more than the vacuum and mechanical advance can during high vacuum settings.
I think some engines will like manifold and some will like ported. I know my emission sticker says it should be ported. But doesn't like ported. My '78 Mercury is set to ported and was when it was running idled great but that could have been due to the fact I have a primitive map sensor that works in conjunction with a yellow DS2 box to advance the timing more than the vacuum and mechanical advance can during high vacuum settings.
FTE Legend
Joined: Jul 2010
Posts: 32,875
Likes: 48
From: Northeast, OK
I've not encountered an engine that doesn't work well with ported vacuum. However, they will require the idle stop screw to be turned up as the idle will be much slower without the vacuum advance. On the other hand, I can see that an engine will "like" an extra few degrees of timing at idle, and that could cause it to feel like it is running more smoothly.
The problem I have with ported vacuum is that usually the advance is set to be in the middle of its advance range with the vacuum the engine produces at idle. That means that any fluctuation, like the A/C compressor coming on or going off or the choke coming off or dropping the transmission in gear, will change the engine's RPM. And that RPM change changes the vacuum, which changes the advance, which changes the RPM, which..... Basically you are in a feed-back or for/next loop and any change has ripple effects.
However, if you use a vacuum advance that isn't in the middle of its adjustment range at the vacuum the engine generates at idle you can get away from those problems. But, that takes investigation and tuning, and most people don't know how to do that.
The problem I have with ported vacuum is that usually the advance is set to be in the middle of its advance range with the vacuum the engine produces at idle. That means that any fluctuation, like the A/C compressor coming on or going off or the choke coming off or dropping the transmission in gear, will change the engine's RPM. And that RPM change changes the vacuum, which changes the advance, which changes the RPM, which..... Basically you are in a feed-back or for/next loop and any change has ripple effects.
However, if you use a vacuum advance that isn't in the middle of its adjustment range at the vacuum the engine generates at idle you can get away from those problems. But, that takes investigation and tuning, and most people don't know how to do that.
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