What dial position due I need for straight stitch. All I can get is decorative stitches

There is a Stitch Width dial that needs to be set on zero. Make sure that no decorative stitch designs are selected.

UPDATE: It appears that your machine is computerized. Probably best to take it for service. Oiling will most likely damage your machine and do nothing toward resolving the issue.

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Please provide the make & model of your machine. If it is a mechanical machine, most likely the machine has seized so that the needle does not do the appropriate swing for zig-zag or decorative stitches. A good cleaning and oiling with fresh, good quality sewing machine oil will take care of a mechanical that is seized.

Will not zigzag or decorative stitches New to these newer plastic style...

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Difficult to diagnose. Try a brand new needle that is compatible with the thread and fabric.
All About Needles

If the machine is mechanical, it may need cleaning and oiling if the decorative stitch mechanism is sticking.

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According to the manual, you should not need to adjust the bobbin tension (and I located other sites which stated the bobbin tension cannot be adjusted on this machine). Apparently, all the tension adjustment is accomplished through the upper thread::
"EN Thread Tension Upper thread tension Basic thread tension setting: "4". (1) To increase the tension, turn the dial to the next number up. To reduce the tension, turn the dial to the next number down. A. Normal thread tension for straight stitch sewing. B. Thread tension too loose for straight stitch sewing. Turn dial to higher number. C. Thread tension too tight for straight stitch sewing. Turn dial to lower number. D. Normal thread tension for zig zag and decorative sewing. Correct thread tension is when a small amount of the upper thread appears on the bottom side of fabric. Lower thread tension The bobbin tension has been set correctly at the factory, so you do not need to adjust it. Please note: - Proper tension setting is important for strong seams. - There is no single tension setting appropriate for all stitch functions, thread or fabric. - A balanced tension (identical stitches both top and bottom) is usually only desirable for straight stitch construction sewing. - 90% of all sewing will be between "3" and "5". - For zig zag and decorative sewing stitch functions, thread tension should generally be less than for straight stitch sewing. - For all decorative sewing you will always obtain a nicer stitch and less fabric puckering when the upper thread appears on the bottom side of your fabric. 1 A B C 22 D"

Doing further research, I found this statement on a machine review site:
"After reading reviews online from where I've bought my drop-in bobbin machines, I think many of the negative reviews are due to the bobbin thread coming up without laying across the bobbin. It can cause the stitches to look very sloppy and no amount of tension adjusting can fix the stitches."

There are several machine review sites wherein 4423 owners stated their disappointment in the 4423's performance. Some said the machine failed almost immediately and others said the machines developed problems when sewing heavy fabric.

The "dial" in the back is for decorative stitches which require a "programmer" to be inserted with the stitch formation you want to form. For a straight stitch no cam is required in the back of the machine. The three dials on the front:

Upper Left: Stitch selector
Upper Right: Stitch lenght
Lower: Stitch width

Using the upper left dial select the straight stitch picture
Using the upper right, choose your stitch lenght
Using the lower, choose your needle position, left/center/right

it sounds like the decorative cam or gear is damaged.. it would be a good idea to have the machine checked

it sounds like something has happened to the cam gears that
do the decorative stitches, which means the sewing machine needs
to be checked..
but before you have the machine checked,, make sure all the
dial that are used for decorative stitches are at the right setting
one missed setting can cause a lot of problems

Why don't you download the manual which will tell you everything you need to know? get it here. http://sewingonline.co.uk/library/JONES-VX-950.html

First, make sure you select the straight stitch setting on the round dial. It is the one that has the dashed lines, looking like the stitch itself. Then, you may also want to be sure that the stitch WIDTH (dial with the picture that looks like a straight line gradually turning into a zig-zag) to the lowest setting, which is generally 0. With some machines, even after selecting a straight stitch setting, moving the stitch width can change needle position. Sometimes it will initiate the zig zag, other times it will move needle position left to right. Does that fix it?