Avoneer
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| posted on 14/1/04 at 09:32 PM |
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Hi Stu,
Just read this in Des Hammils book:
"If the engine being tuned does not have sufficient idle speed advance, it will be impossible to get good clean performance below 2700 to
3500rpm without resorting to very large idle jets and lots of turns out on the idle screws."
I set my timing to 8 degrees BTDC as per the Haynes manual for a 2ltr Pinto with single carb.
Doh!
Maybe my twin carbs and fast cam have changed that somewhat - maybe I should set it more in the region of 12 degrees (estimate). Will have a go
tommorow!
Looking promising for a change (and money saving).
Pat...
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However a large number of electrons were terribly inconvenienced.
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Stu16v
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| posted on 14/1/04 at 10:13 PM |
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As per the 'timing' thread, using a timing light is a waste of time for setting up. What Des is saying is spot on, I dont know if he
explains the various reasons why. If he doesnt, here goes....
When you bolt a pair of twin carbs onto your engine, the first thing that is usually affected is the vacuum advance-that is, it is usually left
disconnected. Idle/low rpm is exactly where the vacuum advance is applied the most, so already your engine is now running quite a bit more retarded
when compared to the orignal spec. Ive no idea exactly how much the Sierra vac can advance the timing, but it could possibly be in the region of 8 to
10 deg!
Trouble is, you cant just advance the dizzy the same amount to compensate, because at higher revs, when the vac advance mechanism stops working, the
timing would then be 8 to 10 deg too far advanced, and holed pistons would result 
Compounding the problem even more is the issue of 'lumpier' cams. At low rpm, when the engine is 'off cam' the gases flowing
through the engine are not as as efficient as a standard cam, and what tends to happen is exhaust gases mix with the fresh inlet charge in the
cylinder. This diluted mix burns slower, and needs more advance from the ignition timing. But at high rpm, the situation is reversed, so the cylinder
fills better, and needs less advance than a standard engine!
To summerize, a standard dizzy in an engine with twin carbs and a 'fast' cam is far from ideal, in fact the complete opposite to what the
engine probably requires. Thats why a timing light is good for diagnosing misfires, and nothing else on a modded engine.
If there was some way of reconnecting the vac advance, that would go a long way to helping.......
Dont just build it.....make it!
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Northy
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| posted on 14/1/04 at 10:20 PM |
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Pat, you might want to have a word with Col, he has his Vacuum advance connected 
Graham
Website under construction. Help greatfully received as I don't really know what I'm doing! 
"If a man says something in the woods and there are no women there, is he still wrong?"
Built 2L 8 Valve Vx Powered Avon
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Avoneer
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| posted on 14/1/04 at 11:14 PM |
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Cheers Stu.
Got my vacuum advance connected from the dizzy to one of the carbs. Will have a go at using the timing light tommorow to set it nearer 12 and see what
happens!
Pat...
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However a large number of electrons were terribly inconvenienced.
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Northy
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| posted on 15/1/04 at 08:06 AM |
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Pat don't you need it connecting to atleast two of the cabs to stop pulsing. And you really need to use one of those anti pulse thingys. The
pulsing is also made worse by a lumpy cam
Graham
Website under construction. Help greatfully received as I don't really know what I'm doing!
"If a man says something in the woods and there are no women there, is he still wrong?"
Built 2L 8 Valve Vx Powered Avon
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SteveO
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| posted on 15/1/04 at 10:32 AM |
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Hi guys,
I've just passed my MOT pre SVA and as you know i had sme problems. with the webers i had to get an amazing 72year old guy who was amazing with
my 40s and he talked and i listened. first like you, one set was running too rich because unknowingly the metal piece on the 2nd carb by the side was
not allowing full closure on that carb when the 1st carb is fully closed, so we took it off totally as it wasn't going to link a third carb
anyway.
Then screw down all 4 idle screws, get the two spring mounted screws for the rev(don't know the name) just touching the horizontal plates, if
the engine stalls turn the idle screw half a turn. Then to balance and tune remove the plug leads from 4,3 and then 1,2 listen and compare the sound;
adjust rev screws until they sound the same. do the same with plug leads 1,3 and 2,4 but here adjust the idle screws, do the same with 1,2 and 3,4.
It takes a long time but when we put it on the analyzer this guy got it spot on Lamda under 1%, HC <800 and CO2 was way under. (I used the Dave
Andrews jetting for 7500 power band, road rally kent cam with vac advance on one of the inlets.)
hope this is useful.
steve
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Stu16v
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| posted on 15/1/04 at 07:11 PM |
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quote:
Pat don't you need it connecting to atleast two of the cabs to stop pulsing. And you really need to use one of those anti pulse thingys. The
pulsing is also made worse by a lumpy cam
Spot on Northy, ideally it wants to be connected to all four (not a big prob if you have managed one, just do it another three times, with three T
pieces). If that proves to difficult, an anti pulse thingy will help.....
Dont just build it.....make it!
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Northy
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| posted on 15/1/04 at 07:18 PM |
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I was going to connect to all four
Wow, I knew something
Graham
Website under construction. Help greatfully received as I don't really know what I'm doing!
"If a man says something in the woods and there are no women there, is he still wrong?"
Built 2L 8 Valve Vx Powered Avon
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Avoneer
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| posted on 15/1/04 at 11:13 PM |
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I'm only connected to one of the carb outlets, the other three are plugged and I can idle smoothly so there!
Re-set my timing to between 12 and 14 and gues bloody what - I have some tweakability with the idle mixture screws!
Whoopee!
Borrowing a gas analyser soon as well - what should I be looking at?
Cheers everyone so far.....
Pat...
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However a large number of electrons were terribly inconvenienced.
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Stu16v
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| posted on 16/1/04 at 06:22 PM |
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I would personally still try and get the others connected too. Easy way to check if it advancing fully is to let engine idle with vac connected and
note where timing is with timing light. Then pull off pipe,block hole at carb end and suck the pipe yourself until you are blue in the face. If you
can get more advance by sucking, you probably want the other three connecting...
CO% and especially Hydrocarbons can give you good indications of what is going on with the engine. CO% is the mixture strength, and with the engine in
a good state of tune it should run sweetly at around 2-2.5% at idle.
The Hydrocarbon reading is an indication of unburnt fuel. High readings can be caused by a number of things. Excessively high readings are usually
caused by misfires, very weak mixtures, very rich mixtures (flooding carbs), low compression, incorrect ignition/cam timing and to a certain extent
high lift cams. This reading needs to be as low as possible. A standard engine in good health/state of tune shouldnt exceed much more than 300ppm,
however your mileage will vary.
[Edited on 16/1/04 by Stu16v]
Dont just build it.....make it!
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Avoneer
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| posted on 17/1/04 at 11:36 PM |
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Hi,
Managed to get myself another pair of plain and simple 40's, not emission type like my current ones, and they are already jetted and set up for
a 2ltr.
Hopefully, will make things even easier!
Pat...
No trees were killed in the sending of this message.
However a large number of electrons were terribly inconvenienced.
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Stu16v
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| posted on 19/1/04 at 09:48 PM |
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quote:
Originally posted by Dave Ashurst
Stu16v,
Sorry to be thick, but how does that work?
I'd like to fit a vacuum advance dizzy but I'm having trouble with the logic of connecting vacuum advance to MORE THAN ONE side draught
tube manifold - so that it works properly.
Also what's an "anti pulse thingy"?
do you make it or buy it (and where from) ?
thanks and regards
Dave
With the vac advance connected to one cylinder, it will pick up the strong pulses of the single inlet tract, enough to constantly vary the timing.
I'm not entirely convinced it is a huge problem, as I doubt the vac advance would be able to react quickly enough to the pulses in the first
place, thereby givng itself a nateral damping in the first place.
But by connecting two opposing cylinders, or ideally all four, you might get a less strong peak vacuum signal, as you rightly say, the pressure will
try and equalise out over the other three cylinders. However, as the vacuum doesnt completely disappear in any of the inlet tracts, you will get a
higher average signal when compared to a single tract, coupled with the fact that the butterflys will need to be closed off further (the idle
speed will increase with the vac connected). A bit like single phase/three phase electricity really....
The minus point is that you may find the carbs will want setting up again around the progression stage if they are running sweetly presently, because
as mentioned above, the butterflys will need to be shut down further to keep the same idle speed and potentially this will take them away from the
progression drillings, possibly creating a flat spot. There again , it might just cure one too....
A trip around a scrappy will source you an 'anti pulse thingy' If you have a look on Mk3 Escorts, VW Polo's and stuff like that,
you sometimes find them in the multitude of vacuum pipes around the back of the carb. Careful though, some are one way valves. Easy way to check is to
see if you can blow through BOTH ways of the valve, one way should give a significant restriction but not block the flow completely, which helps damp
out the pulses.
Or fit a programmable electronic ignition with a throttle position sensor....
HTH Stu.
Dont just build it.....make it!
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