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Rough idle 1936 Dodge d2


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This is a lot like asking a doctor to diagnose your belly ache from 1000 miles away. There is no one thing that can cause a rough idle on a 1936 Dodge or any other car. There are a thousand things.

If you went to a doctor he would ask some questions and do some test before he made his diagnosis.

So, do a compression test. If compression is good, and uniform on all cylinders, next I would look at the ignition system. Possibly you have a defective plug wire or spark plug, or maybe you got 2 wires mixed up. It happens, especially #3 and #4.

If ignition checks out perfect next thing is carburetor although they seldom cause a rough idle.

Could be other things like a vacuum leak at the manifold/block or manifold/carb joint. I could go on all night but am not going to bother without more information.

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When was it rebuilt? What did they do? Did it ever run smooth since you got it back? How long since it ran, or how long since it ran right?
The engine was rebuilt prior to 2000. The engine was dropped in 2006 prior to finished ground up restoration. I've driven approx 250 miles since then.
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At 250 miles (depending on the completeness of the rebuilt), a rough idle (depending on the current idle RPM) may be normal.

What is the RPM specification for engine idle for your engine, and at what idle RPM (measured by a known tachometer) are you running?

Another member asked if the engine runs rough only at idle?

It may simply be that the working components of the engine need to get to know each other before they will work well with each other.

Jon.

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At 250 miles (depending on the completeness of the rebuilt), a rough idle (depending on the current idle RPM) may be normal.

What is the RPM specification for engine idle for your engine, and at what idle RPM (measured by a known tachometer) are you running?

Another member asked if the engine runs rough only at idle?

It may simply be that the working components of the engine need to get to know each other before they will work well with each other.

Jon.

Thanks Jon. I'll check the rpm. The engine was a complete rebuild.
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If it sat around for 15 years there is a good chance of sticking valves or rings. Do a compression test and find out. Also add some Marvel Mystery Oil, Redex or other upper cylinder lube to the gas and maybe the oil.

Thank you. Will do.

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Your compression ratio is 6.7:1. This should give you a compression pressure of around 100 PSI.

As long as all cylinders are within 10% of each other and reasonably close to that figure you should be good to go.

Also, your engine may run better and have better compression after you put a few break in miles on it.

If one cylinder has 0 compression you just found the sticky valve. In this case come back and we will tell you how to fix it in a few minutes.

Edited by Rusty_OToole (see edit history)
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OK you passed the compression test. All cylinders have some and they are within spec.

Sooty plugs are a clue. Could be the carb is off, or maybe just a lot of running with the choke on and no chance to blow out the carbon on the hiway.

My next step would be to check the ignition and see that all plugs are firing like they should at the correct time. 3 and 4 are the easiest to mix up. Last step would be to adjust or rebuild the carb.

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I would suggest that you look at the ignition timing. There is a thread "Questions on ignition timing - 36 Dodge" by 1936 D2 that would be helpful in this regard as it appears setting at the recommended 4 degrees ATDC may not be the best setting.

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Thanks Rusty and Stude17. Read the questions on ignition timing . Cleaned the spark plugs,checked gap reinstalled and did a vacuum check. Interesting when I hooked up the tester just before the vacuum advance on the distributor the rough idle smoothed out. The initial vacuum was 16 mmHg so I turned the distributor to get 18mmHg. Ran smooth. Thinking there maybe something wrong with my connection to the vacuum advance or vacuum leak somewhere.

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Sounds like your vacuum advance unit is faulty ( leaking diaphragm) and will need to be replaced or repaired. To test if this is the case you could blank off the vacuum line and run on the centrifugal advance only which is probably what has been happening anyway.

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To test the vacuum advance you can disconnect the line at the distributor and suck on it. You should see the points plate move (with the dist cap off). Plug the end of the line with your tongue and the points plate should stay, if it goes back there is a leak.

An air leak or vacuum leak can mess up the idle especially if it is intermittent.

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Well Rusty, I decided it was easier to remove the distributor and test as you described There were a couple screws loose on the vacuum unit; tightened those and tested. It's working properly. I had set the point gap while on the car earlier but I think sometimes that can't always be accurate; so bench checked and reset the points; they were a bit closed. Put the unit back in the car, reset the timing at 4 ATDC and checked the vacuum which ended up at 20mmHg. Car is running a lot smoother. Still a slight bit of roughness or miss. I wonder if maybe caused by valve lash. I was thinking #1 piston that was about 7 lbs than the rest of the cylinders.I recall recently rebuilding the carb.

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You don't need to remove the distributor, you can do the test on the car. If you have a vacuum pump it is easier.

Is the gauge steady? If so you don't have a sticky valve. See #5 in this diagnosis chart

http://www.secondchancegarage.com/public/186.cfm

I wonder if your motor doesn't just need some breaking miles and the usual touch up to head bolts, valve adjustment, timing and carb adjustment after 100 miles?

Edited by Rusty_OToole (see edit history)
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I realize I didn't have to remove the distributor but actually it was easier for me. The vacuum was not sporadic. Steady a 20mmHg. I was thinking of valve adjustment. Never had doe it while engine was in the car.

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I realize I didn't have to remove the distributor but actually it was easier for me. The vacuum was not sporadic. Steady a 20mmHg. I was thinking of valve adjustment. Never had doe it while engine was in the car.

Noticed a little oil burning out of the oil blow by tube.

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I've checked everything that I've posted as far as vacuum , timing, points, plugs, compression. Everything is fine except for #1hole is 95 lbs while the rest are 102. Was thinking valve adjustment since hasn't been done since the rebuild and the engine only has about 250 miles on it.

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