vacuum timing and carburetor setting

[Ed McDermott]

Hi

I need some insight into a problem in setting idle speed using the vacuum method.

Background:

1948 Dodge Custom fluid drive with a 1953 B-3-B model 306 truck engine.

1  no pointer on engine  for referencing timing marks on balancer.

2 pipe plug hole is plugged at bottom by broken bolt (guess).  No pressure escapes (or felt) when engine running

3 so trying to use vacuum method.

4 Engine ran reasonable  but could not set reliable idle below 800 rpm

5 rebuild carburetor as car ran rich and have no history on the car

6 Re-installed carb  and set out to time.  Warmed up engine.  vacuum advance was still connected.

Could not set idle below 1600 rpm.  turned idle mixture in but engine stalled out without getting much reduction in rpm before stall out.

Checked engine compression  (1-6) 120, 120, 120, 110, 115*, 120    * gauges may not have been tight enough in spark plug hole

checked vacuum at 1600 rpm and maxed vacuum with idle mixture and vacuum was steady at  19 in/hg with 1/2  in of jitter  pulsed the accelerator and vacuum went to 0 and returned to 19 in.

checked float setting reset float a little to insure full closure.

checked idle speed and was able to adjust lower to 1400 rpm min.

So I am looking for some guidance, as to what the problem in setting the idle rpm may be  as not sure what  to do next.

Thanks

Ed

 

 

[Jerry Roberts]

That pipe plug hole above number 6 piston is probably plugged with carbon .  Is your throttle linkage not backing off all the way ? Some throttle linkage  hung up a little bit ? If you disconnect the throttle linkage to try again , be sure that you have your throttle return spring hooked up so that your engine doesn't run wide open .  

[Ed McDermott]

the throttle linkage is connected to the spring and the throttle plate is fully closed with the rpm at 1400 rpm.

[kencombs]

5 minutes ago, Ed McDermott said:

the throttle linkage is connected to the spring and the throttle plate is fully closed with the rpm at 1400 rpm.

then there must bean air leak.  check gaskets and vaccum lines carefully.

Edited April 19, 2022 by kencombs

[rallyace]

Are you measuring manifold vacuum or carburetor vacuum? You want to be using manifold vacuum for timing. Start by disconnecting the vacuum advance and plugging the line. Adjust timing for max vacuum and then adjust the idle speed and mixture to get a smooth idle at about 600 RPM. You will very likely have to fine tune the timing as you work through this process.  This is what I did when I first started my rebuilt engine three years ago and I have not had to touch it since.

[allbizz49]

Check throttle shaft to throttle body clearance. It's a problem with these old dogs 

[Ed McDermott]

I am using manifold vacuum, but i had not disconnected the vacuum line (forgot).  Will do today as looking for a vacuum leak.

Also per Jerry Roberts, could the the pipe plug be carbon plugged.  The car arrived with no bolt in the pipe plug and I drove it short distances  and ran the engine over two years and not noticed any pressure escaping from the port.  So just wondering since I have no practical experience with a pipe plug.

Thanks everyone your suggestions help.

[kencombs]

43 minutes ago, Ed McDermott said:

I am using manifold vacuum, but i had not disconnected the vacuum line (forgot).  Will do today as looking for a vacuum leak.

Also per Jerry Roberts, could the the pipe plug be carbon plugged.  The car arrived with no bolt in the pipe plug and I drove it short distances  and ran the engine over two years and not noticed any pressure escaping from the port.  So just wondering since I have no practical experience with a pipe plug.

Thanks everyone your suggestions help.

I have 4 heads in the shop,  two not on engines, one on my old 218 and one on my 230 slated for the pickup.  ALL had that hole plugged with carbon.  And it takes a pretty got hit to knock it out.   I'd bet 99% of them have never had that plug removed, ever. 

[Jim Yergin]

You might want to look at this: 

 

[Ed McDermott]

So the solution in this post from Jim is that the vacuum advance diaphragm was bad.

Would this have been  noticed during testing  if the vacuum advance line was disconnected at the carb and the carb port plugged.  In my case there was no difference. I await your answer but it is an interesting thought as I mentioned I had a hard time with setting idle with the carb before being rebuilt.  Could only set it at 800 to 900 for steady run.

Thanks

Ed

[kencombs]

2 hours ago, Ed McDermott said:

So the solution in this post from Jim is that the vacuum advance diaphragm was bad.

Would this have been  noticed during testing  if the vacuum advance line was disconnected at the carb and the carb port plugged.  In my case there was no difference. I await your answer but it is an interesting thought as I mentioned I had a hard time with setting idle with the carb before being rebuilt.  Could only set it at 800 to 900 for steady run.

Thanks

Ed

Does that mean it just wouldn't slow down,or it would but got rough running?  Big difference in possible causes.

 

If it just won't drop below, it is getting air from somewhere, throttle blades or elsewhere.

[Ed McDermott]

thanks, will check more thoroughly.

Ed

[Ed McDermott]

Thanks to everyone who suggested an air leak.  And thanks to Ken Combs for persisting as I finally understood the issue.

Pulled the carb off and examined and found a linkage in backwards that gave a little resistance to the plate closing perfectly.  Cleaned up a few suspicions  areas , replaced the carb and was able to adjust to a smooth idle at 770 rpm.   Have not touched timing yet to complete the job.  It got a little cold in the garage so put off to tomorrow when its to be warmer.

Ed

[greg g]

Knowledge, assistance, patience, determination, perseverance are usually rewarded.  Good to hear you were successful.  Idle should be closer to 500 to 550 to do a good vacuum timing procedure.  

[Ed McDermott]

Thanks to everyone who suggested an air leak.  And thanks to Ken Combs for persisting as I finally understood the issue.

Pulled the carb off and examined and found a linkage in backwards that gave a little resistance to the plate closing perfectly.  Cleaned up a few suspicions  areas , replaced the carb and was able to adjust to a smooth idle at 770 rpm.   Have not touched timing yet to complete the job.  It got a little cold in the garage so put off to tomorrow when its to be warmer.

Ed

[Ed McDermott]

Hi

Thanks.  Managed to get rpm to 680 but won't run smoothly below that point.  Checking timing today to see if its correct or needs adjusting.  As I mentioned in the beginning I do not have a timing mark indicator and pipe plug is plugged and cant see what is plugging it.  car arrived and ran this way.  made up a timing pointer and installed to use as a reference as to where I am starting so that I can always get back to it.  If I can every find TDC the I will have a reference marker.

Ed

[greg g]

Pull the spark plug on number one, put your thumb over the hole with ign off bump the engine till you feel compression pressure against your thumb.  Then rotate by hand till the pressure is highest.  Then look for your timing marks.  A wire brush may be necessary.  Then you can highlight TDC with paint.  This mostly becomes moot when doing the vacuum procedure, but the marking will be helpful for future reference.  When I did mine via vacuum it runs at 6 btc.  But I have milled my head and block and am running dual intake so yours may be different.