351M tuning help
Thread Starter
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Tuned
Joined: Aug 2014
Posts: 292
Likes: 6
From: Florida
351M tuning help
I bought a vacuum gauge to assist in my trouble shooting. Now I'm even more confused and lost. The vacuum gauge bounces between 15 and 20, at best. I've tried the gauge on two different ports and with the vacuum advance attached and disconnected(and plugged). I've recently put new plug wires on, no change. With the timing light, I've noticed cylinder one will "blink" at what I think is a correct rate, then the light stops for 2-3 seconds but the engine doesn't sound different. I'm starting to think its the coil. Is there anyway to test the coil or diagnosis it? I will try to attach a video of the vacuum gauge, but who knows.
https://youtu.be/h4q1jghmDfQ
edit: I couldn't post the video directly so I tried to link an YouTube video. It is not a link, but you can copy and paste to YT and watch it there
https://youtu.be/h4q1jghmDfQ
edit: I couldn't post the video directly so I tried to link an YouTube video. It is not a link, but you can copy and paste to YT and watch it there
Post Fiend
Joined: Apr 2002
Posts: 19,311
Likes: 97
From: Waterloo, Iowa
It sounds a little bit like the timing is off. Vague idle, not crisp. Have you checked to see that the outer crankshaft damper weight hasn't slipped? This is a common defect, then when somebody tries to time it with a light, well it's not going to be anywhere close to accurate.
As a practical matter try (slowly) moving the distributor clockwise or counterclockwise as required to achieve the highest, steadiest needle vacuum indication without regard to the timing marks. See if that helps.
See if you can get a high, steady needle, hopefully around 18" to 20". If you live at altitude maybe only 15" or 16" is possible, but rock steady needle is what you want to see with a stock motor, the higher the better. Vacuum gauge is always connected to a source of steady manifold vacuum, meaning a port below the throttle plates. Some carburetors don't have a constant manifold port. If this test smooths everything out, you need a new or rebuilt crankshaft damper.
As a practical matter try (slowly) moving the distributor clockwise or counterclockwise as required to achieve the highest, steadiest needle vacuum indication without regard to the timing marks. See if that helps.
See if you can get a high, steady needle, hopefully around 18" to 20". If you live at altitude maybe only 15" or 16" is possible, but rock steady needle is what you want to see with a stock motor, the higher the better. Vacuum gauge is always connected to a source of steady manifold vacuum, meaning a port below the throttle plates. Some carburetors don't have a constant manifold port. If this test smooths everything out, you need a new or rebuilt crankshaft damper.
Post Fiend
Joined: Apr 2002
Posts: 19,311
Likes: 97
From: Waterloo, Iowa
I looked over some earlier posts you made. Keep in mind a tune-up has a cylinder compression test as the first step. A vacuum gauge also measures compression, at least indirectly, very well but it's nice to know for sure what we're working with from the start. A bouncing needle on a vacuum gauge generally means something that affects one cylinder, maybe a sticking lifter or burned valve, or a broken valve spring. A generally low reading means something that affects all cylinders equally. Study the vacuum gauge charts carefully, there are probably 5 or 6 that are real good, commonly available to download online. There could be more than one thing going on here. You're getting closer to diagnosing the problem. The gauge doesn't lie, there is a defect somewhere in thar.
Thread Starter
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Tuned
Joined: Aug 2014
Posts: 292
Likes: 6
From: Florida
Update. I made sure the vacuum gauge is attached to ported vacuum. I then disconnected the vacuum advance from the distributor and plugged the line. I started the engine and rotated the distributor while watching the vacuum gauge. The overall amount improved to about 16-20 but the needle was still rapidly bouncing, so it was hard to tell. At that point the timing mark was somewhere between 18-20 degrees. I played with the throttle and was getting a lot of backfires in the exhaust. What are the chances of the timing chain skipping and does that sound like my problem? I'm helping a buddy fix his truck tomorrow so I may not get anymore testing/diagnostics done for a few days.
Post Fiend
Joined: Apr 2002
Posts: 19,311
Likes: 97
From: Waterloo, Iowa
Vacuum gauge should be connected to manifold vacuum. I can only recommend at this distance to start from scratch to zero in on the fault. Good ole process of elimination. A cylinder compression test, and also verify true TDC with a piston stop. Make sure the ignition system is not misfiring, wires crossed, fouled out plug etc. The key thing you want to see with a vacuum gauge reading is a steady needle, at least with a stock engine, and the higher the better.
A bouncing or erratic needle means a valve is sticky or burned, sticky lifter, plug misfiring, broken valve spring maybe. Did the fuel tank have old varnished gasoline maybe?
Thread Starter
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Tuned
Joined: Aug 2014
Posts: 292
Likes: 6
From: Florida
The vacuum gauge is attached below the throttle blades, which means manifold vacuum. 🤦*♂️ My mistake. My engine is not completely stock. I have a holley 600, victor jr 4 barrel intake and long tube headers. I'm pretty sure everything else is stock. I forgot to mention I also ohm'd the ignition coil, it passed. I think the change in blinking speed is from the timing light's "pickup." I recently changed the plugs to E3 diamond fire, which are suppose to never foul out. But I will check them when verifying TDC. Thank you so much for your help
Thread Starter
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Tuned
Joined: Aug 2014
Posts: 292
Likes: 6
From: Florida
I was just looking at that chart, too funny. It looks like I'm leaning towards loose valve guide ... GREAT! I might swing by the parts store and rent a compression tester and verify TDC tomorrow. It'll probably be after dark on the east coast.
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Post Fiend
Joined: Apr 2002
Posts: 19,311
Likes: 97
From: Waterloo, Iowa
Yeah I saw that too. Loose valve guide sort of fits doesn't it? The gauge charts are usually really accurate too, it is amazing to me how many different faults and diagnostics can be done with those things. There's usually one or two different or extra faults on every chart, check them all out. Let us know what you find, it will help somebody else out. Make sure it really is what you think it "might" be, before tearing into it at least.
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Post Fiend
Joined: Apr 2002
Posts: 19,311
Likes: 97
From: Waterloo, Iowa
I forgot to mention I also ohm'd the ignition coil, it passed. I think the change in blinking speed is from the timing light's "pickup." I recently changed the plugs to E3 diamond fire, which are suppose to never foul out. But I will check them when verifying TDC. Thank you so much for your help
Crossed plug wires would for sure. Any chance the plug wires are in the wrong sequence firing order? Always check for easy stuff first, a simple loose plug boot can look just like something expensive.
Thread Starter
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Tuned
Joined: Aug 2014
Posts: 292
Likes: 6
From: Florida
Time for an update! I know it's long overdue, but life got in the way and I was helping my buddy try and get his Camaro into the Low 11s. So bought a new coil and it fixed the misfire! Woohoo! Still does not run quite right, it stumbles when I give it throttle. I ignored the timing gun and used the vacuum gauge to adjust the distributor. With the highest "steady" vacuum the timing mark was reading 30 degrees BTDC. So now I'm pretty sure the timing marks on the balancer have spun. I used a hand-me-down compression gauge(pretty sure it's not accurate) and all cylinders were around 125 to 130 on the gauge except for cylinder number 6. It peaked at 75. That's the same side as the ticking noise I'm hearing, that I'm assuming is a valve noise. Before I pull the valve cover off what do you guys suggest I check or do? What should I check and look for once the valve cover is off?
Post Fiend
Joined: Apr 2002
Posts: 19,311
Likes: 97
From: Waterloo, Iowa
Did you have the vacuum advance port disconnected and plugged? A fast idle will pull in quite a bit of vacuum advance. Best to use a piston stop to check the true TDC.
Lead Driver
Joined: Jul 2017
Posts: 9,580
Likes: 1,162
From: San Jose, CA
Another thing I thought about was a too-lean mixture. Is that listed on the chart anywhere?
Reason I'm going there is that you listed headers, a small 4bbl carb and a high-rise, high-rpm intake manifold (Victor Jr) and if you put the carb on right out of the box it might have jets a few sizes too small for your breathing.
A cam would certainly have changed that, but even with the stock cam those other somewhat mismatched parts could be the culprit.
And did you check for vacuum leaks at some point during this testing?
Paul
Reason I'm going there is that you listed headers, a small 4bbl carb and a high-rise, high-rpm intake manifold (Victor Jr) and if you put the carb on right out of the box it might have jets a few sizes too small for your breathing.
A cam would certainly have changed that, but even with the stock cam those other somewhat mismatched parts could be the culprit.
And did you check for vacuum leaks at some point during this testing?
Paul
Thread Starter
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Tuned
Joined: Aug 2014
Posts: 292
Likes: 6
From: Florida
Now that you ask, I dont remember disconnecting it. 🤦*♂️ The idle sound correct though. I'll have to get a tach later.
Thread Starter
|
Tuned
Joined: Aug 2014
Posts: 292
Likes: 6
From: Florida
Another thing I thought about was a too-lean mixture. Is that listed on the chart anywhere?
Reason I'm going there is that you listed headers, a small 4bbl carb and a high-rise, high-rpm intake manifold (Victor Jr) and if you put the carb on right out of the box it might have jets a few sizes too small for your breathing.
A cam would certainly have changed that, but even with the stock cam those other somewhat mismatched parts could be the culprit.
And did you check for vacuum leaks at some point during this testing?
Paul
Reason I'm going there is that you listed headers, a small 4bbl carb and a high-rise, high-rpm intake manifold (Victor Jr) and if you put the carb on right out of the box it might have jets a few sizes too small for your breathing.
A cam would certainly have changed that, but even with the stock cam those other somewhat mismatched parts could be the culprit.
And did you check for vacuum leaks at some point during this testing?
Paul
