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74' Coupe de Ville
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Discussion Starter #1
472cid + 4mv Quadra, 1974.

Cleaned and idle mixture adjusted good too.

Park/neutral or in drive -> full throttle and it stumbles and strikes on exhaust when I lift pedal up.

On road, it stumbles second or two like it shuts off, then car accelerates but futututu and poor performance. Works ok with other pedal positions.

Secondaries spring works and I adjusted like YouTube video told me. If I snap those on finger, they pop open and close fast.

Metering rods are stock dg.


Vacuum hoses are good condition. Acceleration pump squirts good amount of fuel.


Power piston is not adjustable on carb in car, is something missing?


What would be my problem?

HEI distributor and mechanical weights won't stuck. I'm not sure for vacuum clock but it should not do things like that.
 

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1980 FBC
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3,163 Posts
Check ignition pickup wires in distributor. Give them a gentle tug where they go into the pickup.

Those break from vacuum signal flexing. Right before they let go they become intermittent based on engine load.

Otherwise I'd look for something weak in the ignition system.

Full throttle can require more firing voltage than idle.
 

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74' Coupe de Ville
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Discussion Starter #3
I changed old dist cap and rotor back. I don't know was that half a problem or loose connections BUT now it stands full throttle, maybe very tiny lag but not fatata anymore. Backfiring is still there, I think that is less too.


Idle revs okay too but pedal lift and couple massive backfire comes.
 

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1977 Coupe (blue), 1977 Coupe (yellow), 1977 Coupe (green)
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Use a Mighty Vac and see if the vacuum advance canister holds vacuum and advances the timing. The HEI can also be worn. Check the distributor shaft end play. Look for wear that will allow the reluctor to rise out of the field of the pick up. I have seen the body bosses worn to the point the HEI is useless. The centrifugal advance mechanism has a pin which fits in a slot. The pin is supposed to have a nylon bushing which rides in the slot. When it is worn out, it allows more advance and the lower limit (starting point) is compromised. The pin will actually wear into the plate adding more advance and then extend the advance more.
If these things are all good, look to the crank free play. More than five to eight degrees free play suggests your timing chain should be replaced.
 

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1980 FBC
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Slight backfiring on deceleration closed throttle can be the A.I.R. pump check valve. It gets manifold vacuum and closes air injection off at very high vacuum.

Edit to add. Diverter valve.
 

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74' Coupe de Ville
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29 Posts
Discussion Starter #6
Mighty vac? Do you mean normal house hoover? :D

Vacuum advance vacuum hose comes under distributor on block. Is that right place? Should I use ported or continuously vacuum?

Crank free play, I try rotate balancer on hands clockwise/anticlickwise and check if it gives free play?
 

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1977 Coupe (blue), 1977 Coupe (yellow), 1977 Coupe (green)
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Mighty Vac or Vacuum tester is a hand held vacuum pump which will reveal if your vacuum motors are working properly. Will show how much vacuum it holds or the rate of leakage. The vacuum line to the canister should come from the manifold. Not sure about '74, but it may go to a TPS that restricts vacuum until the engine is at operating temperature. The proper Service Manual will have that information and would be a worthwhile investment for you.
Crank free play: The distance the crank will move until the timing chain catches against the cam timing sprocket. This resistance is noticeable. You can move the assembly by hand or use a strap.
 

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2002.5 F55 CORSA STS, 2014 Explorer XLT FWD
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Find the HP paperback 8 X 11 book on Rochester Carburetors by Doug Roe.

Here's a good vacuum pump for carb work.

Depending on emissions level and tuning the vacuum advance canister can be connected to either ported or manifold vacuum. You need the diagram for YOUR engine/car/model/year. Is it on a sticker on the underside of the hood at the driver's side ?

VDV - Vacuum Delay Valve
VCV - Vacuum Check Valve
.......... and several more variations on the theme were used.
 

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74' Coupe de Ville
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29 Posts
Discussion Starter #9
Okay,thanks. That kind of tools arent popular in my country, maybe you dont even find those.

If i release pedal up at normal steady speed it wont backfire.
 

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74' Coupe de Ville
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Discussion Starter #11
Yes i have :) rockauto ships with fedex in 3-4days. Ebay and other usps deliveres can take month...
 

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1977 Coupe (blue), 1977 Coupe (yellow), 1977 Coupe (green)
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If your Vacuum advance canister is defective, you can get one from Crane, available at most part stores. They are adjustable, so you will have to set it to your engine specifications.
 

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74' Coupe de Ville
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Discussion Starter #13
I have been driven about 150miles with new spark plugs. Those are like new, shiny white. What causes that?

Idle is little bit rich,I smell it and backfiring is on exhaust no carb. So it can't be lean?
 

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74' Coupe de Ville
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Discussion Starter #15
If i try look mixture by vacuum gauge at cruise speed, what would be good hg reading? Holleys advice video says "higher reading, accurate jetting". Does vacuum drop with too lean or too rich only?
 

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1977 Coupe (blue), 1977 Coupe (yellow), 1977 Coupe (green)
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If the white color you are referring to is like a crystal glaze appearance (hard to describe) (light metal grey), it could be what is called high speed glazing. Usually caused by high speed shut down from WOT. Often seen in racing. Shouldn't affect your plugs performance.
 

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74' Coupe de Ville
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Discussion Starter #17
But what it does to engine? I dont wanna hole in my piston :(

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In picture they look gray/tan. Not so much in natural, more whiter.
 

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70 Deville 77 Fleet 78 Seville 92 Deville 03 Deville
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I have seen plenty of white spark plugs that look scary but the vehicle is running fine. They will get some color eventually. I believe if your carb was set to factory specs, as listed in FSM or Chiltons, it would work perfectly. I'm not so sure a carb will really cause backfires. I have only seen it when the floats aren't working and the engine is blubbering badly. It's very easy to get backfires with crossed ignition wires and that Cadillac firing order throws me for a loop every time. I know autolite plugs are probably ok but it is awful easy to point a finger at them. I guess that's where I would be looking.
 

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74' Coupe de Ville
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Discussion Starter #19
Yeah i know but delcos werent at same place with other parts when i ordered rockauto.

Wires are okay.


Maybe nothing to worry about color.
 

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1977 Coupe (blue), 1977 Coupe (yellow), 1977 Coupe (green)
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No, that tan coloration is what you get with today's fuel. You will see a moderate change when you go from the Winter blend to the Summer fuel blend.

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Yeah i know but delcos werent at same place with other parts when i ordered rockauto.
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The Autolites are Ok, but the AC's will endure much longer. Last forever in these engines. You may have to check the gap every couple years. Don't assume the gap is set right out of the package with any plug. Always ensure the gap is set properly before installing.
 
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