News:

Due to a technical issue, some recently uploaded pictures have been lost. We are investigating why this happened but the issue has been resolved so that future uploads should be safe.  You can also Modify your post (MORE...) and re-upload the pictures in your post.

Main Menu

Backfire help 56 SDV

Started by Bruce L. Wiley, January 05, 2005, 01:17:35 PM

Previous topic - Next topic

0 Members and 1 Guest are viewing this topic.

Bruce L. Wiley

I have new plugs, wires, points, condenser, cap, rotor, and coil. Plugs set at 35, points at @30 dwell, timing set at @2.5. The engine still has a slight miss and backfires. Any help!?!?!?

Johan 158

Hello Bruce,
Although Im not an expert on 56,I assume you made all your repairs, because off the miss fire. Check the following please,
compression and loading time of this compression, on all cilinders, loading time gives you an indication of the cam condition, it should be almost the same on all cilinders.
Also a small leak on an intake valve will cause the problems you have for sure.
Check your valve position (overlapping)  against the actual position of the piston/crank,your timing chain may have lenghtened or a cam lobe (intake) has flattened.Also a lean mixture due to a vacuum leak may cause miss firing some times back fire.Also have a look for
opening height and time of your valves.All this, if the simple solutions do not work,random order, if you wish. Good luck.

Bruce L. Wiley

Thank you. I will try that and let you know how it turns out.

Porter

 
"Vacuum Gauge Use

Connect vacuum gauge to base of carburetor to full vacuum source at idle. Then monitor your readings and compare readings to chart below.  
1-Steady gauge reading from 17-22 indicates normal engine in good condition.

2-A intermittent fluctuation at idle indicates a ignition miss or sticking valves.

3-Needle fluctuates as you increase engine speed indicates ignition miss, blown head gasket, leaking valve or weak valve springs.

4- Steady needle that drops at regular intervals indicates burnt valves or improper valve clearance, (the needle falls when bad valve is in operation).

5-A drifting needle goes high then drops goes high then drops in a steady movement indicates minor intake leak or improper carb adjustment.

6- A low reading but steady from 10-16 indicates late ignition or valve timing, low compression, stuck throttle valves, leaking carburetor or intake gaskets or a big cam.

7- Needle vibrates heavy at idle but becomes steadier as engine speed increases indicates worn valve guides.

8-A gradual drop in reading at idle indicates blocked exhaust system or excessive back pressure in exhaust system.
      Thats troubleshooting with one.  Now for fuel mixture adjustment with one.
 Disconnect vacuum advance at distributor and plug vacuum line, start motor with timing light and a RPM gauge attached and set idle and timing to correct settings. Connect vacuum gauge to full unported vacuum at base of carburetor, now shut motor off and turn both idle mixture screws all the way in but be sure that you only use light pressure as you can turn the idle mixture screw past being lightly seated by twisting hard and then you are damaging the idle mixture opening with the needle causing it to flare the opening trashing a good carburetor body so please only lightly seat the needles. now back each out 1 1/2 turns, restart motor and check idle and timing and adjust as necessary to correct settings, now back each needle out slowly a 1/4 turn each alternating between the 2 until you reach the highest vacuum reading, check and adjust idle speed and timing, now repeat step one of backing idle mixture screws out alternating between the 2 a 1/4 turn at a time until you have reached highest vacuum reading again. Continue the process adjusting timing/RPM/idle mixture screws until you have no changes occurring and have reached highest vacuum reading possible. You have now optimized your timing idle mixture adjustments. Reconnect your vacuum advance while disconnecting the RPM gauge, timing light, and vacuum gauge.
 Ok now for monitoring fuel consumption with a vacuum gauge.
 Connect the vacuum gauge to full unported vacuum and run vacuum line long enough for gauge to be in cab. Go for a drive monitoring the vacuum gauge. When you have high readings you are getting the most fuel economy possible, when you have low readings fuel consumption to efficient combustion ratio is lowered. By easing into your gas pedal rather than moving it rapidly you will notice vacuum readings stay higher which increases MPG "
 
 

Bruce L. Wiley

Porter, Thank you. That is EXCELLENT information. I will test it out this evening.

Porter

Not W.A.G. work, or replacing parts needlessly.

Good luck, let us know what you find.

Porter

Bruce L. Wiley

I cant get hold of a vacuum gauge until Friday. I did check the firing with the timing light. All plugs seem to fire correctly except No.2. It has an erratic light pattern. I changed the plug and checked the wire. It did not correct the problem.

Paul Flykt #18899

Great description Porter, I have printed it as a check list for my own use and am going to give my son a copy. Its an inexpensive tool that is easy to use.

Randall Ayers McGrew CLC # 17963

Me too, Paul!  I have a 56 to work on, and that was about the best treatise on How To I have seen lately!  Great job, Porter!  Keep up the good work.

Porter

The great thing about the message board is we have modern day Cadillac mechanics, old timers that specialize in the old Cadillacs and others that specialize in different years from the entire spectrum.

I got plenty of support last year from the experts when I rebuilt my 66 carb, replaced the waterpump, etc.

My specialty is autobody,paint and resto work but I like to troubleshoot, turn wrenches, tune ups , brake jobs, etc. Borne out of financial necessity years ago.

I chip in when I have something valuable to share, for most of us it is a gut wrenching, knuckle busting hobby of trials and tribulations, not too many millionaire collectors here farming out the work to a resto shop. Otherwise why would they be here ?

Porter

Bruce L. Wiley

I connected the VC to my carb. I have a reading that fluctuates between 13 and 16 on idle regularly at about 4 per second.. It smooths out some on increased RPM (not much).

Porter

He owns one, he follows the message board so his weekly 15 minutes of fame might get used up when he sees his subject name.

John is one of the expert mechanics here, and a "good ol boy" to boot.

Until then you might want to proceed with a full compression test, Mike Jones , one of the real mechanics here recommends a compression test at every tune up.

I have never done one on my 66 429 but it pulls a steady 20 plus vacuum reading, which I assume any healthy engine will do.

When you have an engine that doesnt run smooth after a thorough tune up and weak vacuum readings get out the trusty compression tester, it always served me well.

No sense in beating around the bush or not getting to the root of the problem.

Porter

Scott Hunt

According to your vaccum gauge reading you have a burned valve.

John Washburn

Bruce,

I have a few thoughts. If your vacuum reading is only 16 and you dont live at 4,000 feet or higher like we do in Colorado, you probably have a tired engine (vacuum readings drop approximately 1 per 1,000 feet)

The flucation of 3 is probably a burned or sticky valve.

The compression test will tell you a lot. Even better would be a
leak down test.

I would also make sure that the timing is set correctly, as this can affect the reading, especiall if it is late.

Good Luck

John Washburn
CLC 1067
Elizabeth, Colorado
Elevation 6,500 feet

Porter

John,

In addition to all the great advice here I never really picked up on the lower vacuum readings at higher altitude. I would imagine you have to retune and recalibrate all your cars, what are your methods for performing that operation ?

Another question, does that cold brewed Coors taste any better at 4,000 feet above sea level ?

Porter

Randall Ayers McGrew CLC # 17963

Porter!  I can answer that last one!  YES!  :)

John Washburn

Porter,

Who the hell drinks Coors, its like drinking water for gosh sakes.

Anyway, lots of stuff needs to be done to get an old car to run really well at altitude. Since Im at 6,500 I try to make sure they run well.

Timing needs to be increased. I generally try to go 1 degree advance per 1,000 feet, but it depends on the car. So a trusty vacuum gauge and a good ear help. If you go to far when you try to start the car it will let you know.

Also, usually try to reduce the primary jet size 10 to 15percent so that the car does not burn to rich. But again this is not an exact science. Took me three tries to get it correct on my 37 LaSalle. You can the kits to convert the carb but they can get really expensive for a Stromberg. Carter is a little cheaper and if you have a Rochester, not a problem.

The good news is that the cars run great on lower octane the highter the altitude.

John Washburn
CLC #1067
Drink Red Wine if you want to live to be a 100.



Jeff Maltby #4194

Porter. We stopped by the Golden Colorado Coors brewery for a few samples after we left the Sept 8 1974 Evil Knievel Snake River Jump in Idaho. Coors tastes much better at the plant then out of the bottle.

:<)

Porter

John,

Figured I could kill two birds with one stone, get some high altitude tuning tips and your take on Coors, since you are a local Colorado mountain man.

Cheers, what does one do when they transcend the high altitude to sea level , is the car still driveable or will higher octane gas help ?

We have some high altitude mountains here, you will need a good set of brakes on the way down from Mt. Washington, 6,288 feet above sea level and the temperature really drops when you get to the top.

Porter

John Washburn

Port - er,

Ive moved a few times in my life, and lived in Colorado twice (used to live in Golden, so had to take them out of towners to the Coors Brewery all the time). When I returned to the flat lands, like Seattle, Washingon, was too busy to pay a lot of attention to the cars. But since I take such good care of my cars (trust me on this and call me for the price of the Golden Gate Bridge, I can sell it to you cheap) they ran ok.

So to make a long story short, if the engine is in good shape and tuned properly it will run almost anywhere. But if the engine has issues then be careful if you take it up to the altitudes.

Hope all is well and that your team won...

John Washburn
CLC #1067
Elizabeth, Colorado