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Old 03-19-2017, 11:18 PM   #1
PotteryPal
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Arrow 289 Vacuum Gauge Reading

I was checking the tuning on my 67 289 with a vacuum gauge and noted aside of it being on the low side of the normal range (i.e. 18-20in mg) that under acceleration the vacuum increases sharply. In my experience this isnt normal. Shouldn't it be decreasing under acceleration?

I also have the problem that I can only turn the dizzy so far clockwise before the vacuum advance nipple hits the coolant hose. Any ideas the fix this issue as well?

https://www.dropbox.com/s/mru1kp6qy8...%20PM.mov?dl=0
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Old 03-21-2017, 12:58 AM   #2
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Thats close to normal. (totally depends on cam) sounds like it running well so there is not likely a vac leak

at 850 rpm I get 14" (this is idle) hot engine (cold engine makes way less vac)

under light acceleration vac will drop BUT on say level ground in 2nd gear higher rpm will give higher vac readings. So vac will drop as you accelerate to 30mph then as you back off to maintain 30mph vac may read 18ingh for example in the same conditions vac may read 21inhg at 40mph in 2nd gear.

In the video you can see vac drop as the engine accelerates and then come up quickly as it holds rpm then revs down.

No load quick rpm changes will give large vac spikes like you saw.

under load at WOT in any gear vac will be zero

The more you lug the engine the quicker vac will drop off. half throttle accel in 5th gear at 40mph is going to cause vac to drop into the 2-3" range

half throttle accel in 1st at 10mph could up around 12"+

Last edited by Gun Jam; 03-21-2017 at 01:00 AM.
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Old 03-21-2017, 06:08 AM   #3
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Default Check Your Timing !!

Quote:
Originally Posted by PotteryPal View Post
I was checking the tuning on my 67 289 with a vacuum gauge and noted aside of it being on the low side of the normal range (i.e. 18-20in mg) that under acceleration the vacuum increases sharply. In my experience this isnt normal. Shouldn't it be decreasing under acceleration?

I also have the problem that I can only turn the dizzy so far clockwise before the vacuum advance nipple hits the coolant hose. Any ideas the fix this issue as well?

https://www.dropbox.com/s/mru1kp6qy8...%20PM.mov?dl=0
Check your ignition timing with a good (strobe) timing light -- with vacuum hose to the distributor disconnected and plugged.
The conditions you describe (low vacuum; vacuum pot interference) indicate a distributor that is a tooth off.
If you are still running a "points" distributor, ditch it for an electronic type -- in the long run, they'll give more power, increase fuel mileage, and hold a tune better and longer.
At your altitude (Houston), your stock cam should give a steady 19-20 inches at 800 rpms.
Bosch Double-Iridium plugs gapped at .040 for a long-life tuneup.
Good Luck !!
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Old 03-21-2017, 12:29 PM   #4
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Originally Posted by 95 MERC View Post
Check your ignition timing with a good (strobe) timing light -- with vacuum hose to the distributor disconnected and plugged.
The conditions you describe (low vacuum; vacuum pot interference) indicate a distributor that is a tooth off.
If you are still running a "points" distributor, ditch it for an electronic type -- in the long run, they'll give more power, increase fuel mileage, and hold a tune better and longer.
At your altitude (Houston), your stock cam should give a steady 19-20 inches at 800 rpms.
Bosch Double-Iridium plugs gapped at .040 for a long-life tuneup.
Good Luck !!

I was thinkning about efi and electronic dizzy. There are a lot of options out there. Is MSD all the rage? Or are there other I should consider, i.e. Holley dual sync, Petronix? If I change the dizzy now can I go efi later or should I do it all at onece in a complete kit?
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Old 03-21-2017, 12:34 PM   #5
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Thats close to normal. (totally depends on cam) sounds like it running well so there is not likely a vac leak

at 850 rpm I get 14" (this is idle) hot engine (cold engine makes way less vac)

under light acceleration vac will drop BUT on say level ground in 2nd gear higher rpm will give higher vac readings. So vac will drop as you accelerate to 30mph then as you back off to maintain 30mph vac may read 18ingh for example in the same conditions vac may read 21inhg at 40mph in 2nd gear.

In the video you can see vac drop as the engine accelerates and then come up quickly as it holds rpm then revs down.

No load quick rpm changes will give large vac spikes like you saw.

under load at WOT in any gear vac will be zero

The more you lug the engine the quicker vac will drop off. half throttle accel in 5th gear at 40mph is going to cause vac to drop into the 2-3" range

half throttle accel in 1st at 10mph could up around 12"+
Thanks for the info. I re-routed the fuel line to make some more room. As well I checked the timing. It is running 6deg BTDC initial timing and under vacuum ~2200rpm 35deg BTDC. It is running well. Just thirsty. I sill check my new plugs in a couple weeks to see what they are telling me.
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Old 03-21-2017, 06:37 PM   #6
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6 deg initial might be a bit low but within normal range. 35 all in at 2200 is fine assuming you have the vac adv disconnected.

If you've got 6 initial + 29 adv for a total of 35 + 10 for vac at 45 over 10ingh then you are good.

if you've got 6 initial + 19 for a total of 25 all in at 2200 + 10 vac for 35 above 10ingh your timing is seriously low and will cause reduced fuel mileage, and significant power loss.

With vac disconnected your total timing should be 32 to 35. at 2200 to 3000
These engines dont run a tooth off. you might get it to back fire or stumble along if you use some throttle but they wont "run"

They dont slip teeth but once then shut down right away

However the distributor gear can slip on the shaft in rare cases and retard timing

I dont think any of that is your issue.
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Old 03-21-2017, 06:46 PM   #7
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6 deg initial might be a bit low but within normal range. 35 all in at 2200 is fine assuming you have the vac adv disconnected.

If you've got 6 initial + 29 adv for a total of 35 + 10 for vac at 45 over 10ingh then you are good.

if you've got 6 initial + 19 for a total of 25 all in at 2200 + 10 vac for 35 above 10ingh your timing is seriously low and will cause reduced fuel mileage, and significant power loss.

With vac disconnected your total timing should be 32 to 35. at 2200 to 3000
These engines dont run a tooth off. you might get it to back fire or stumble along if you use some throttle but they wont "run"

They dont slip teeth but once then shut down right away

However the distributor gear can slip on the shaft in rare cases and retard timing

I dont think any of that is your issue.
I'll check it again this weekend. Thanks for the different scenarios..
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Old 03-21-2017, 07:31 PM   #8
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I got eager and went out and checked it with the vacuum advance connected. I got this:

6 initial + total of ~42 - 45 @2200rpm. It was hard to get an exact measurement at high rpm because my scale stops at 30 btdc. I made an extra mark for 40 btdc.
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Old 03-21-2017, 08:33 PM   #9
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good!

is there an actual problem? if not I think you are good to go
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