hey guys, i was looking into this book i got on engine tuning and such and it was mentioning a Tach-Dwell that they used to set the fuel/air mixture and other components, i thought people used a vacuum gauge for this?

am i wrong? or can you use both. if i wanted to adjust things like my mixture, idle speed which should i buy?

 

get things straight ...
the dwell is the relation to how long the contact breaker is closed and open. if it's closed for a long time it can 'load' the coil longer for better spark, however if too long the open period is too short. the instant it opens it sparks.
has nothign to doo with A/F mixture. you want to set it to manufacturers specs. there's no point tuning there. or simply get a pertronix so you don't have to adjust points anymore

if you change dwell and you get a better vacuum then this is solely for the reason that you actually change the initial timing. if the points close longer they will open later (spark retarded), if you adjust so it's closed for short period it wil advance the spark.
this is one of the reasons you have to constantly qadjust the points. they wear and ignition quality and timing suffers.

so what tools do you need:
a) if you want to keep your points some people simply use the cardboard of a matchbox to adjust the points. but a dwell meter is far more accurate and measures the lot dynamically. i'd say you get a pertronix or similar module for the price of a dwell meter. so i'd rather get that. I still have one from the past and i come across more than one vehicle to adjust

b) you absolutely need a timing gun. and it should be one where you can set the advance (adjustable dial, or digital).

c) a vacuum gauge. get one of those that don't dampen. some gauges will have some kind of damping so if the pressure changes rapidly the needle in gauge is steady. that's actually something you do not want. you want to see if it 'misbehaves'. however with a big cam this can be misleading (tghis will cause some jumping as well)

d) a tach, but if you get a good digital timing gun, this will have one in there already

 

The dwell - tach is an ignition tool, not a fuel/air tool.

 

sweet thanks guys.its seems the book i was reading used the tach-dwell tool as a tachometer, im not sure if this is possible but i quote "adjusting the fuel and air mixture using a tach-dwell meter"

im sorry as this seems like a simple question but:

what can i adjust with a vacuum gauge? i was going to buy a tach anyway so thats out of the way. im assuming i can adjust the mixture using the tach correct?

 

quote "adjusting the fuel and air mixture using a tach-dwell meter"
basically there's a
- dwell-meter: only used to adjust points. nothing else
- timing gun: to adjust ignition timing, nothing else
- tach: only to read rpm
- vacuum gauge: used to read manifold vacuum

sometimes you get a few of those copmbined in one. so it's not unusual if you have a dwell meter, that the same machinery would also be capable of reading rpm (tach).
heck there's even multimeters made for mechanics who can gauge both.

back to the subject.
to adjust the idle mixture screws on a carb (and only those) there's typically the following way:

set them to lets say 2 turns out (all of them depending how many the carb has) at the same time. then open them up by half turn, then try closign by half turn,. if closing them by half turn causes the engine to run better, keep closing (always both at the same time) until the engine runs best
don't close them any further when it's starting to run worse again. you want the leanest (most closed) position on the carb that makes the engine run best.
example: start with two turns out (each). close all by half turn (runs better), close all another half turn (runs crap). go back a half turn and only close quarter turn (doesn't seem to make much of a difference). leave it there

well and this knowledge of hearing and reading the engine running good you can actually measure
a) using the tach. adjust idle mixture screws so you can have the highest idle in the leanest position.
simple example: 2 turns out 850rpm, close by half turn 850rpm, close another half turn 750. go back half turn to where it had 850 wand close quarter turn, result 830. go back to where it was 850 and close only 1/8 turn: 860. leave it there.
usually not that black&white but that's the general idea. inbetween you might have to adjust the curb idle as well

b) using a vacuum gauge. if you hook the vac gauge to manifold then the more vacuum the engine generates under the throttle oplate the better the signal for the carbn and the better the engine will run. higher rpm will go togerther with hiogher manifold vacuum.
So you could simply attach the gauge, go two turns out and start going back in until you get the highest vacuum with the least mixture screw opening again

maybe that makes sense now?

 

Many people use the "tach" feature of the tach dwell unit to set the fuel mixture..The leanest mixture will give you the highest rpm you can reach on the tach by turning the mixture screws..It's pretty much the same as getting the highest vacuum reading using a vacuum meter because vacuum increases with rpm..

 

thanks guys this helped alot.