Attempted to tune SU Carburetors but still backfiring on intake

[jalexquijano]

THe compression test showed the following results:

 

cylinder 6 --------------------- 175 psi

 

cylinder 5 --------------------- 180 psi

 

cylinder 4---------------------- 180 psi

 

cylinder 3 ---------------------- 185 psi

 

cylinder 2 ---------------------- 180 psi

 

cylinder 1 ---------------------- 180 psi

 

Any way i do notice that when i remove the spark plug terminal from cylinder number 4 there is no change in the idle while with the other cylinder there is a noticeable drop of rpm. What could be the problem with cylinder 4? Anyone? Spark plug is still getting black. Car is now backfiring at the exhaust every once in a while.

[Mark Maras]

Good compression. I doubt that you have much, if any, spark to #4. Check the dist. cap, rotor, #4 wire & the connections. Try swapping #4  plug & wire with # 3 or #5 & see if the problem moves to another cylinder. 

[Chickenman]

Delete post.

Edited November 4, 2015 by Chickenman

[cbuczesk]

Put your timing light on plug wire #4 to see if you have spark.

 

Chuck

[jalexquijano]

Put your timing light on plug wire #4 to see if you have spark.

 

Chuck

 

Ill check on that! The distributor cap and rotor are Brand new! Im using  a Pertronix 1761 ignitor and 3.0ohms distributor bypassing the stock ballast resistor! THe NGK blue cables are Brand new also! The Airtex 8012S electric pump is pumping great without overflooding the float valves of the SU Carbs.

[jalexquijano]

Good compression. I doubt that you have much, if any, spark to #4. Check the dist. cap, rotor, #4 wire & the connections. Try swapping #4  plug & wire with # 3 or #5 & see if the problem moves to another cylinder. 

Drove the car last night, arrived to a location, turned the engine off and after 15 minutes started the car again but when reaching to the first intersection the car stalled and died. I will check again on spark plug number 4 to see if its fouling. With the compression results i have posted here do you think one of the valves could be warped or burned?

[jalexquijano]

Good compression. I doubt that you have much, if any, spark to #4. Check the dist. cap, rotor, #4 wire & the connections. Try swapping #4  plug & wire with # 3 or #5 & see if the problem moves to another cylinder. 

 

Mark,

 

Just woke up this morning and took a picture of the last 3 cylinders. As you can see Cylinder N°4 is the darkest one! I did close the rear carburetor by 1/8 of a turn to lean it a Little bit more:

 

 

I guess the car stalled because plug 4 is completely fouled as you can see in the separate picture. The cap, rotor and plug cables are new. Questions are:

 

1. Could this valve be to tight and needs to be adjusted?

 

2. Why if i remove the spark plug wire from cylinder 4 i cannot notice any change in idle while with the othe 5 plugs it is noticeable? Even with a Brand new plug?

 

Your advice if greatly appreciated! I will be taking the car for valve readjustment this saturday!

Edited November 5, 2015 by jalexquijano

[Mark Maras]

 A Z engine will run on 5 cylinders. #4 not firing didn't cause the stall. At this point, not enough info to diagnose that.

Compression ratio figures, I don't remember if this engine is new or old but either engine would run fine with these #s. If the engine is new, the #s should be very close if not the same.

 However, the valves MUST be adjusted to specs before ANY troubleshooting including a compression test. If a mechanic has been doing this in the past, IMO, YOU should pull the valve cover & check the clearances. It's the only way to be sure & it's not difficult.

 #4-compression is good, & it is fed by the same fuel supply as #5 & #6. The only reason the cylinder is not firing, IMO, is little to no spark at the plug. The coil supplies elec. to all the plugs through the distributer. The pathway to the plugs is very simple. As the dist. turns, the rotor distributes the elec. to the contacts inside the cap. From there it goes into the plug wires & into the plugs. Fact, there are 5 cylinders firing. The problem has to be located somewhere from the dist. cap to the plug. A quick check with an ohm meter will tell you if have a bad plug wire. Compare it to the others. Other options already mentioned, timing light on #4 plug wire or just trade #4 wire with another one to see if the problem moves with the wire. You've replaced the plug, if the wire is good the only thing left is the cap. Rare, but I recently had one go bad on one cyl. Ohm meter check, inside to outside & double check the connections at the cap & plug.

[Jarvo2]

Might want to run a vacuum gauge on the engine to see what else is going on.  A compression test is only one part of the diagnostics.

 

[Mark Maras]

You're right. Old school mechanics would use a vacuum gauge first. It still may be the best diagnostic tool for the money.

[Jarvo2]

I've got one of these mounted on my balance tube & it really helps with tuning.

 

http://www.summitracing.com/parts/sum-g3125/overview/

[siteunseen]

I've got one of these mounted on my balance tube & it really helps with tuning.

http://www.summitracing.com/parts/sum-g3125/overview/

FANTASTIC! I wanted to mount the big I have somehow but others talked me out of it. Now you have helped me again, I didn't know they made those.

Which port did you use? I've plugged all of mine except for the brake booster. And I've already tapped them to NPT threads.

Thank you, I love gauges!