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74superTN
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Location: Port Saint Lucie, FL
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 Posted: Mon Jan 14, 2019 5:12 am Post subject: RPM fluctuates |
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So I'm helping my ex get her 77 bay back up to par because it hasn't been running correctly. A little history about the motor. It's a Type IV, Camper special and someone removed the Raby installed Mallory distributor. I tuned the carbs and timed it to the settings of the 009 dizzy. However, when it sits at idle and even when I'm driving, the RPM fluctuates up and down on the tach by a few 500 rpm or so. Any ideas? The last guy who touched it like I said removed the Mallory unilite but also replaced the bosch coil with some grey one. Any ideas? Dual Weber 40s, 1971cc camper special.
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alman72
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| Posted: Mon Jan 14, 2019 5:26 am Post subject: Re: RPM fluctuates |
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removed or stole?
as a friendly reminder to those who do not do their own work on their bus, always get your old parts back. many can be rebuilt, and are much better than a lot of what you can buy today. (Or they are still good parts, because the guy working on it just threw parts at it until it ran)
sorry OP, I have nothing to offer on the RPMs. I think I have read that the grey coil is not the best. |
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74superTN
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Location: Port Saint Lucie, FL
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| Posted: Mon Jan 14, 2019 5:46 am Post subject: Re: RPM fluctuates |
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| alman72 wrote: |
removed or stole?
as a friendly reminder to those who do not do their own work on their bus, always get your old parts back. many can be rebuilt, and are much better than a lot of what you can buy today. (Or they are still good parts, because the guy working on it just threw parts at it until it ran)
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Sadly, what it looks like happen is he broke the distributor cap for the Mallory somehow and didn't have a replacement easily available. So he just took it off and replaced it with an 009. He did put the Mallory back in a storage compartment but it needs a new cap.
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dstimm
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| Posted: Mon Jan 14, 2019 6:59 am Post subject: Re: RPM fluctuates |
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Could it be that your tach might not like your distributor?
This is instructions to my tach, and it mentions:
"If your tach fails to operate smoothly, the signal might of high sensitivity. Use the black/white wire instead of the blue/red wire".
https://www2.cip1.com/v/vspfiles/assets/images/eet4-1b32-00N.pdf
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Spike0180
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Location: Detroit, Michigan
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| Posted: Mon Jan 14, 2019 7:09 am Post subject: Re: RPM fluctuates |
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Thoughts on RPM Fluctuation:
1) Vacuum leak (wiggle/move your vacuum lines around while it idles. One might cause a severe effect to the engine when you wiggle it, showing a vacuum leak) You may try spraying some carb cleaner/break cleaner around the throttle shaft of the carbs too. This is a common place for vacuum leaks and could cause surging.
2) Something either loose or gummed up in the 009 distributor. Disassemble, clean, return to service (or better yet, get a cap for your Mallory unit)
3) Could be spark plug wires, check for cracks and make sure they are completely snugged up on everything.
I'm sure others will chime in with ideas
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Randy in Maine
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| Posted: Mon Jan 14, 2019 8:22 am Post subject: Re: RPM fluctuates |
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If it is a VDO tach, it may just need a $1 diode as they are very sensitive to "stray voltage".
"Will VDO tachs work with points ignition or Pertronix?
Yes, with diode in line on the signal lead. Instructions are as follows:
Purchase a diode #1N4005 from a local electronics store such as Radio Shack or Circuit City. Cut both ends of the diode so each is approximately 3/4” long. Crimp 1/4” female spade connector on the end of the diode with the silver band. Crimp butt-splice connector on the other end of the diode. Crimp the opposite end of butt-splice connector to the wire connected to the ignition signal source. Connect 1/4” female spade connector used in #3 above to terminal #4 on the back of the tachometer. Connect a ground (-) wire to terminal #3. Connect a switched 12-volt power wire to terminal #2. Set switches for appropriate amount of cylinders." |
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Wildthings
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| Posted: Mon Jan 14, 2019 9:06 am Post subject: Re: RPM fluctuates |
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| If it were actually fluctuating 500 rpms while in gear it would be slamming you back and forth in the seat. As others said, your problem is likely the tach not reading the signal correctly. |
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SGKent 
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| Posted: Mon Jan 14, 2019 9:17 am Post subject: Re: RPM fluctuates |
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| Wildthings wrote: |
| If it were actually fluctuating 500 rpms while in gear it would be slamming you back and forth in the seat. As others said, your problem is likely the tach not reading the signal correctly. |
yes, probably is the tach wandering rather than actual hunting. That said, any actual hunting at idle with carbs is likely to be a mixture and/or sync issue. Can also be a problem of weak distributor springs combined with too high an RPM.
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airschooled
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| Posted: Mon Jan 14, 2019 11:01 am Post subject: Re: RPM fluctuates |
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How does it DRIVE? Tachs can lie, and usually do on buses where the wire is 14-feet long and run haphazardly alongside other electrical wires.
Also, nothing like a good Google search.
https://www.speedwaymotors.com/Mallory-271-Cap-Dis...gLH__D_BwE
Robbie
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74superTN
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| Posted: Mon Jan 14, 2019 11:13 am Post subject: Re: RPM fluctuates |
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It ran fine but seemed to be sitting high in the RPMs at 55 to 60 mph. It was around 5000 RPM during that time...Does that seem high?
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airschooled
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| Posted: Mon Jan 14, 2019 11:22 am Post subject: Re: RPM fluctuates |
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| 74superTN wrote: |
| It ran fine but seemed to be sitting high in the RPMs at 55 to 60 mph. It was around 5000 RPM during that time...Does that seem high? |
That seems right, for 3rd gear. 
The fix is a diode installed in the tach signal wire. IN4001 or up (IN4002, IN4003, etc.) will work fine, with the grey stripe on the diode facing the tach.
I didn't know where to mount the diode, so I put one in the dash and it smoothed out the tach needle and made it correct. I put another in the engine compartment and it cut out a slight buzz on my radio. 
Ignore the tach for now and enjoy driving it until you get the Mallory cap ordered. An engine of that pedigree deserves to be run as-designed.
Robbie
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timvw7476
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| Posted: Mon Jan 14, 2019 12:25 pm Post subject: Re: RPM fluctuates |
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The intake-to-head gaskets do eventually tear apart, leading to horrible
throttle response. Maybe time for fresh ones. |
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SGKent
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| Posted: Mon Jan 14, 2019 1:42 pm Post subject: Re: RPM fluctuates |
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| 74superTN wrote: |
It ran fine but seemed to be sitting high in the RPMs at 55 to 60 mph. It was around 5000 RPM during that time...Does that seem high? |
Pay attention to how you phrase things. Are you saying that the engine is turning 5000 RPM at 60 mph in 4th gear, or are you saying that the tachometer that has been installed is indicating that?
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74superTN
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Location: Port Saint Lucie, FL
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| Posted: Mon Jan 14, 2019 1:54 pm Post subject: Re: RPM fluctuates |
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| SGKent wrote: |
| 74superTN wrote: |
It ran fine but seemed to be sitting high in the RPMs at 55 to 60 mph. It was around 5000 RPM during that time...Does that seem high? |
Pay attention to how you phrase things. Are you saying that the engine is turning 5000 RPM at 60 mph in 4th gear, or are you saying that the tachometer that has been installed is indicating that? |
The tach on the dash is reading close to 5000rpm in 4th gear while cruising at 60mph. This is a new issue that has all of a sudden appeared since the new coil and distributor were installed by the random mechanic.
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orwell84
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| Posted: Mon Jan 14, 2019 2:58 pm Post subject: Re: RPM fluctuates |
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| 74superTN wrote: |
| SGKent wrote: |
| 74superTN wrote: |
It ran fine but seemed to be sitting high in the RPMs at 55 to 60 mph. It was around 5000 RPM during that time...Does that seem high? |
Pay attention to how you phrase things. Are you saying that the engine is turning 5000 RPM at 60 mph in 4th gear, or are you saying that the tachometer that has been installed is indicating that? |
The tach on the dash is reading close to 5000rpm in 4th gear while cruising at 60mph. This is a new issue that has all of a sudden appeared since the new coil and distributor were installed by the random mechanic. |
I would really suspect your tach and doubt you are spinning your engine at 5k. |
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aeromech
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| Posted: Mon Jan 14, 2019 3:05 pm Post subject: Re: RPM fluctuates |
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If you need a diode, I just made up 10 today. Send me a PM
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airschooled
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| Posted: Mon Jan 14, 2019 9:41 pm Post subject: Re: RPM fluctuates |
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| orwell84 wrote: |
| I would really suspect your tach and doubt you are spinning your engine at 5k. |
Problem is, a Raby engine at 5k sounds just like every other engine at 2k. 
Robbie
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a1steaksauce
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| Posted: Mon Jan 14, 2019 10:16 pm Post subject: Re: RPM fluctuates |
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| aeromech wrote: |
| If you need a diode, I just made up 10 today. Send me a PM |
Sign me up too. PM sent. |
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74superTN
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Location: Port Saint Lucie, FL
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| Posted: Tue Jan 15, 2019 4:12 am Post subject: Re: RPM fluctuates |
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| asiab3 wrote: |
| orwell84 wrote: |
| I would really suspect your tach and doubt you are spinning your engine at 5k. |
Problem is, a Raby engine at 5k sounds just like every other engine at 2k.
Robbie |
Seriously. That engine has always sounded so damn good....The ex kept it in the divorce 
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sjbartnik
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| Posted: Sat Jan 19, 2019 3:30 pm Post subject: Re: RPM fluctuates |
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Hi. The engine will always spin at x rpm at x road speed in x gear. Unless you change something mechanical like changing the gears in the transmission or changing tire/wheel sizes in a big way.
Nothing electrical can make the engine run at a different rpm at a certain speed. The engine and transmission are physically locked together mechanically by the clutch.
Therefore, this seems like a tach issue.
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