TheSamba.com :: View topic - rpm changes when removing spark plug wire from distributer

Tue Jul 24, 2012 9:26 pm

hello

I would like to share u with this issue , from my knowledge , if we disable any spark plug from distributer side , rpm will dscrease and misfire will occure @ corresponding cylinder

I have the following , i try to remove the spark plug wire from distributer side one by one when the engine is running , i note and feel the drop in rpm when i remove spark plug wire # 2 and # 4 , the drop in rpm is clear and approx the same . When i did with #1 and 3 , the drop in rpm is not clear and not the same as # 2 and 4 , only i note a change in engine sound .
I also note the the drop in # 1 and 3 is approx the same even both not clear as #2 and 4

Spark plug is good and the wire is good and distributer cap is ok , valve is correctly adjusted , my car is working good @ idle and @ highway

Is there any problem in my car according to above scenario?

Thank u

Tue Jul 24, 2012 9:31 pm

umm doesnt sound like a problem, in each case, that cyl wont be firing so it doesnt sound like an issue. If it runs fine, dont mess with it. Thats when you start having issues.

Tue Jul 24, 2012 9:41 pm

is it normal ? What is the cause of this , just try to be proactive

Tue Jul 24, 2012 9:53 pm

cause of what? When you remove the wires the engine wont be running on all cyls, and will run baisically like shit. Whats the problem?

Tue Jul 24, 2012 10:04 pm

i know what happened when disabling any spark plug , i am asking about the scenario i mentioned above , the engine behave different in 1 and 3 than 2 and 4

Wed Jul 25, 2012 2:25 am

It is not a purely definitive test and any results should be taken lightly.

In other words......

The act of pulling wires will usually track down a cylinder already dead, but not be used to tune the engine.

If you suspect something move on to other diagnostic tools, or processes' to define.

Oscilloscope, compression tester, read the plugs, leak down testing, tear down and inspection, etc.

maybe the heads are burned up and cracked. Maybe the manifold is leaking.

It's just not a test a used as is to determine much. If you want to call it a cylinder balance give numbers (RPM) but still it doesn't tell much.

Wed Jul 25, 2012 2:54 am

DeathTrap wrote: It is not a purely definitive test and any results should be taken lightly.

In other words......

The act of pulling wires will usually track down a cylinder already dead, but not be used to tune the engine.
If you suspect something move on to other diagnostic tools, or processes' to define.

Oscilloscope, compression tester, read the plugs, leak down testing, tear down and inspection, etc.

maybe the heads are burned up and cracked. Maybe the manifold is leaking.

It's just not a test a used as is to determine much. If you want to call it a cylinder balance give numbers (RPM) but still it doesn't tell much.

X2 on that.. When you think you have A dead cylinder? pulling the plug wires one at a time tell you find one that don't change the way the motor runs, is A good way to figure out were the problem is :) But your looking for "NO change" If the motor runs bad when you pull the wire that's A good thing :wink:

Wed Jul 25, 2012 4:51 am

I'm not sure, but I thought I read on here that at idle, the 1 & 3 cylinders don't do as much work as 2 & 4.... maybe someone will verify that.

Wed Jul 25, 2012 8:44 am

is this a stock car, post a photo of the engine and year of car and motor. !

alfa2000 wrote: hello

I would like to share u with this issue , from my knowledge , if we disable any spark plug from distributor side , rpm will decrease and misfire will occure @ corresponding cylinder

I have the following , i try to remove the spark plug wire from distributor side one by one when the engine is running , i note and feel the drop in rpm when i remove spark plug wire # 2 and # 4 , the drop in rpm is clear and approx the same . When i did with #1 and 3 , the drop in rpm is not clear and not the same as # 2 and 4 , only i note a change in engine sound .
I also note the the drop in # 1 and 3 is approx the same even both not clear as #2 and 4
use a TACHO !
and at low rpm the balance on a vw is not perfect. for sure if the heat riser is dead or weak.

Spark plug is good and the wire is good and distributer cap is ok , valve is correctly adjusted , my car is working good @ idle and @ highway

Is there any problem in my car according to above scenario?

Thank u

if compression is ok , check it ,it only takes 15min or so, and valve lash is
at the VW spec, .006 (stock motor, stock push rods,etc)
and spark is correct and static timing is good.
then try this , raise idle with idle stop to 1200
and retest.

or fix the riser.

I Have a IR pyro heat gun meter, and can measure exhport temps.
the are pretty much equal. very very hot. over 500f. at idle
this varies by tune and idle actual speed, i run 900 RPM hot idle, (upper spec)
the carb runs better there. too.
better cooling, better fueling and my old gen dies at 805rpm(spec) so i avoid
the 800 end.

the dead cylinder syndrome is known.
the reasons are many and the truth is a ghost.
a: RISER BAD.
b: idle too low.
c: the firing order conundrum issues, 1,4,3,2 and the theory of , that when
2 fires, it depletes 1 which is next
or when 4 fires. it depletes 3 next, (same side depletion theory)
I don't believe it, yet (no data backing it up yet) (use your pyro)
but as a intake valve closes the vacuum was at peak and now drops
this dropping of vacuum ,makes the venturi action to very weaker.
at idle there is no real venturi, it's just a crack in the throttle plate.!
so if this velocity of air past that crack is too weak>?
mine has 5 idle nozzles, if not perfectly clean, mine idles and runs like crap.

and is very crude. and one can see if the pressure (less vacuum) happens
dynamically that the charge to the next cylinder may be a weak charge.
depleting the intake charge. (nozzles in carb die on the fly !)
Depletes or goes lean for a moment or at the wrong moment.

Raise the idle speed ,and retest.

the riser, if dead causes liquid raw fuel to pool at the carb to the head ports
this too has bad effects idle.
it goes lean and liquid gas don't burn so the jugs get air. not 13:1 AFR as it should.
they have this same issue with EFI, Throttle body injection systems, too.
they run 200F hot water end to end , intake , and water heated TB (throttle body) and all. this cures it , 99% , water otto not so unlike air otto engines.

on the bug we have this funky riser. it sucks, but can be made to work near perfect.
the sucks, is keeping it clean.

this is what i know about fueling.
MPI solves it. so does direct injection. but that is history......

i dont/cant do that snatch wire trick.
as the spark , jumps out and sends me to the parking lot.
flying. like Dark Knight.
sure does.! :roll:
there is a way, but requires paper clips under the spark boots
and you ground them, but MSD forbids shorting a CDI system for good reasons. RTM.
So, i use my pyro gun, and compression tester and then tune the carb.

cheers.

if you have misfire at idle, tell more.?

Wed Jul 25, 2012 10:24 am

thank u , specially for jalex and candymustang66 , my engine is a stock engine , solex dual carb , my attention go directly to cylinders contribution and the depletion theory , the misfire is clear @ run not @ idle on clynder 3 and 1 , misfie is clear @ idle on cylinder 2 and 4

as i mentioned i have no problem @ idle and @ run , i just disable spark plug one by one to check my engine , i will do compression test soon , but i want to understand the bahaviour of the engine

Wed Jul 25, 2012 10:31 am

TjdTaylor wrote: cause of what? When you remove the wires the engine wont be running on all cyls, and will run baisically like shit. Whats the problem? lol :)

Wed Jul 25, 2012 2:38 pm

your car never came with dual carbs. (natural heat too boot....)
why not put that minor (cough) fact in the first post?

the dual carbs are very sensitive to low idle
after all ,each side sucks 1/2 as much ! (compared to 4 to 1 stock)
so that idle setting, on both needs to both be set very carefully.
so they both draw the same.
and that they in fact draw at all , at idle.
and that the idle mix is right at idle on both, not easy to do , that.
can I assume it dont have a balance tube either.? custom set ups vary

the best setup for dual carbs is vacuum ports on each carb.
on motor cycles we use a classic mercury column
http://www.youtube.com/watch?v=CYgN2-96Ik0

or the safer differential vacuum meter. (tune it to zero done , electronic)_

with the column you see both sides pulling so you make them equal
then do so at cruise RPM (good systems have 2 settings. 1 for idle and 1 for off idle.)
but on bugs , they lack vacuum fittings and we yes the Snail deal
it needs to be very good to do idle flows.

i have 3 , types.
this is one i have that is for my 64 Spitfire, 45 years old tool, still works.

but nothing beats the 4 column, (using 2 for your dual)
the whole picture all the time, using it. zero fiddling back and forth bs.

im with other poster, Is there a problem, or just fishing for ideas
you welcome no matter...