Mopar Muscle Association UK

OK sportsfans - another thread...

So; engine in and runs GOOD - pulls hard, ideal for racing haha. BUT not so good on the old 'keep cool' front. I have about 5-10 mins in traffic. This is all good documented stuff - there are lots of detailed article on how the flame front in the piston is different under no load and that a cruising/idling car will be MUCH happier with a good ol' slug of vacuum advance. But my problem is this - mine just wont have it. As soon as you connect vacuum (to either lower manifold port or upper chamber port - A+B on pic below) it will idle but touch the throttle and BANG ! straight back out the carb.

So, first thing is to get the ignition curve in. Having a good read around it seems that higher compression engines need respond better to a more aggressive advance curve than my old stock engine. This one seems to like 16+ initial (as opposed to 12ish for my old) which took the full advance to almost 40 - pinked.

So - Job one. Changed advance bushing to largest 'black' which limits mechanical advance to 18degrees total.
I then changed the spring rates to give me full advance at 3,000 rpm (see graph).
This now works - I have set car for 16initial and watching the revs she hits 34degrees total at 3,000 on the nose and stays there.
She runs nice and seems happy to start. Happy days.

Here is where the fun starts. As soon as I unblock the vac can and connect either to A or B on pic it goes nuts. BANG ! intake backfire as soon as you touch the throttle. I cant even get a reading off of it with the gun. So at this point I dont know if its dizzy prob, vac can prob, carb prob (although its worth noting the problem has followed from old engine to new, and the carb is different so it points to dizzy/can).

I can see the internal of the dizzy move when I suck on the pipe. I'm a bit lost to be honest...

Any bright ideas or do I need to speak nicely to a tuning shop (thunder road?? ) and just get them to set the thing up properly; carb and vac ? I need to get it sorted as it will make the car so much more usable when I am not panicking about traffic jams ! :-)

PS - All other aspects are good. Big ally rad, new thermostat, shroud and big block fan. Holds a rock steady 185deg with the slightest airflow - but obviously gets hot real quick once on idle only.

PPS - One other significant detail - I cant stop the overun on shutdown. At least half the time she will overrun when I turn it off usually engine in a cloud of smoke out the air cleaner. I've taken to shutting her off in D to stop it - but that aint curing the problem. I figure its worth mentioning as its all related to timing and heat I suspect...

This guy has same thing. But no solution
https://www.fordmuscleforums.com/all-fo ... nce-2.html

It sounds like the vacuum can is pulling in too much extra advance, to the point it's probably sparking on the next plug lead on the cap. You can normally adjust the vacuum can by poking a little allen key up the nipple, google will help you here. Running on? possibly the primary blades are open too far. Ajust the primaries until you have a square transfer slot and then leave it alone, adjust your tickover speed with the secondary screw. Even if that doesn't fix it, that's the correct way to set the carb up, much crispness of throttle will result.

Timing/vacuum/carb issues aside... were you thinking of fitting an electric fan... or two?
As you're not fussed about 'stock appearing' I'd have thought that would be a good solution to help with cooling when you hit traffic...

Blue wrote: Thu Aug 29, 19 1:20 pm It sounds like the vacuum can is pulling in too much extra advance, to the point it's probably sparking on the next plug lead on the cap. You can normally adjust the vacuum can by poking a little allen key up the nipple, google will help you here. Running on? possibly the primary blades are open too far. Ajust the primaries until you have a square transfer slot and then leave it alone, adjust your tickover speed with the secondary screw. Even if that doesn't fix it, that's the correct way to set the carb up, much crispness of throttle will result.

Hokey dokey. Will tweak and report back.
I agree ref the vac can - as I say I cant get a reading off it even. I dont think stock MSDs are adjustable although I assume I can fit an aftermarket adjustable. I'll poke an Allen key down it...


Rob - You are right; dont care stock appear and perhaps a big ol' relayed elec setup would help - BUT I think its masking the problem. The vac SHOULD work and produce much less heat as a result. I can wield a spanner but tuning I dont know enough about I'm afraid. To be honest I know a few mechanics who are lost once 'plug it in' doesnt work ! Needs a proper old school 'I know carbs and timing' shop I guess

You don't need no steekin' electric fans.....

Blue wrote: Thu Aug 29, 19 5:01 pm You don't need no steekin' electric fans.....

My thoughts exactly. Well, I wouldnt mind, but I dont think it should *need* them... I want to get it running right.

So - the bad news is that the vac can is NOT adjustable - in fact there are forums everywhere with people moaning that their MSD vac can cannot be adjusted. Which seems really daft to me. There are a couple of people that claim they can modify mallory cans etc to fit. Either way, I need to pop the dizzy out and take a proper look, see i I can get something to fit.

I may try bringing timing wayyyy forward so I can at least get a reading and work out how far the vac is pulling it.
Its a puzzle...

Ref carb setup - will have to pull it and check.

OK. Next plan - as this is starting to annoy me.
Forms all over the place seem to be full of "No, MSD dont do an adjustable vac can. Not compatible with anything else. Rumours of various fits etc..".

So here is what I have found. The Vac can looks a dead ringer for an Accel31034 to me.
The body is the same, bolt holes the same, even the circular stamping on the wide bit.

The only difference I can see is there is a little dent to clear a bolt or something on the MSD - but it looks like something I could file into new vac can. Only question seems to be the neck length. Is it the same, or is the MSD a *tiny* bit longer? Cant tell really.

The only other option I have come up with is this little thing. Claims to be a vac reducer; I know I am getting WAYYYy too much, so I only need to adjust down. Whaddaya think ?

I think how well that valve thing works probably depends on how much vacuum the engine makes at tick over, if its not a lot anyway...
I'd change the vac can, that has to be the correct way to do it. I actually have a similar problem with the Barracuda that I haven't addressed yet, I currently have the vac disconnected and it runs fine and stays cool but I will have a play with it at some point.

if you have the initial advance set at 16*
and a total mechanical of 34* BTDC are you happy that it needs 34 * total

an efficient engine may not

I can run mine with total advance of 28* and see no appreciable difference other than noise and a slight roughness if i go higher. i'm not saying mine is efficient it just doesn't seem to like more than 28 you need just enough...more is not better and sticking with a rule of "thumb" max of 34 is best might not be the best for your motor.

the vacuum can will pull in a specific no. of degrees based on the level of vacuum
but only get to the total it can possibly add, if you can achieve the level of vacuum necessary so you need a can tuned to your max available vacuum.

the no of degrees it adds is a function of the can and lever/rod this is sometimes stamped on the can or its Rod, 8 would mean 8* at the distributor which equates to 16* timing that can potentially be added if you have enough vacuum.
and that is added on top of initial timing and mechanical timing appropriate to the RPM at the time based on level of vacuum

the vacuum level the can starts to work at, is a function of the spring inside that presses on the centre of the diaphragm. which may be adjustable via an allen bolt down the pipe, tighten bolt and compress spring, it is then harder to move diaphragm

you can in theory set up the vacuum can, to have no effect....becasue you never achieve the vacuum necessary to overcome the compressed spring

kinda pointless..but hey it is a starting point....

The vacuum can must be connected to the ported vacuum connector. the one above the throttle plates. it should see no vacuum at idle and add no timing at idle.
it should see and full vacuum with the throttle plate just passing the hole that leads to the tube for ported vacuum. but the level of that vaccum is dictated by RPM and engine load
On low rpm to high rpm acceleration this has some effect, the throttle sweeps past reasonably fast but it provides engine load based advance for that lower speed driving. you don't necessarily want mechanical advance here this is where vaccum advance has its forte providing load based advance.


mechanical advance is basic RPM based advance basic and a bit rubbish like a throttle position sensor is rubbish and a MAP or MAF is better
vacuum advance is clever it is engine load based advance for 2 purposes its a basic sensor style input that is emulated by MAP or MAF on FI cars
Its used to pile in the advance on pull away or acceleration from idle based on vacuum at the carb, which indicative of engine loading. not RPM
It pulls in a huge quantity of advance at High RPM cruise to deal with massive lean, low mass of air/fuel mix per cylinder. i.e vacuum disrupted running at high engine speed and nearly closed throttle

on cruise this has maximum effect. the throttle plate is held in the "just past the hole" position with the full "suck" of 8 cylinders turning over at say 2500 ++ rpm with the throttle blocking off much of the normal flow through the carb...you are running mainly on the idle circuit with the vacuum stretching the mixture to a thin and horribly lean state hence a huge bunch more advance is nice to set it off.... its helping to light what is essentially a wet log with wet matches in a room with little oxygen

if you connect this pipe to the inlet or carb below the throttle plates it will see idle vacuum and just mess with your initial timing.
its a bodge to get some cars to start and idle with a dizzy that has way too much mechanical advance, that needs to be run with a very small initial advance to compensate. don't go there its a mugs game.

anyway i have rambled

vacuum gauge plugged into the ported vacuum
and friend to read it while you drive??

lift off the throttle and cruise at various RPMs and see what max reading you get.
if you see some at idle then is the throttle held open too wide on the stop screw? are you idling on the mains?

idle vacuum is pretty pointless here as the vacuum can, should not see idle vacuum when plugged into ported vacuum port

do you have a miti-vac type brake bleeder?

if so you can use it on your vacuum can pipe to see what vacuum you need to activate it fully by watching the rod as you pump the air out of the can.

so

34 degrees total? too much in too early? with potentially 16 -25 extra degrees of vacuum advance on top for a split second or 2 as you press the throttle gently, to accelerate away.
16+25 is 41 might be ok..... 34+ 25 = 59 oooooooooooooooooooo000 oooo oooo might be ok at 4000 rpm with the throttle nearly closed and a weak thin mixture, not too good at 1300 rpm with the accelerator pumps in full-on squirting mode that might make a big flame shoot out of the carb. do you need heavier advance springs to keep the mechanical advance off while you transition through the area where vacuum is higher?
that is, as far as i know how it is supposed to work.

Hence the "all in as fast as possible" advance curve is perfect for racing engines used as such, with no vacuum advance.... but you are not doing that. you want to drive it and have load-based ignition timing, designed for the street. maybe heavier springs? maybe not 34* total

34* Is touted as the best total mechanical figure for lots of engines but it really depends....you need just enough...maybe that's too much for yours

mine likes 28 dunno why just does...and it was a joy to find this out....

or
34 degrees total in at just the right time, yay.... but vacuum advance plugged into NONE ported vacuum so is adding 16-25 extra degrees on top of your 16 initial at idle and as you press the throttle gently to accelerate way 16+25 = 41 at 800 -1000 rpm...... slightly smaller oooooo etc

its needs to be on the ported vacuum

more experimentation needed


dave

Holy moomoo Dave - that's an epic one even for you ! Thanks !! Food for thought in there for sure - but as always, I have some questions...

Firstly, I have ordered the adjustable vac advance - my basic plan was to retro fit to the MSD, wind it in so it does nothing, then slowly introduce it turn by turn, see if I can at least get a reading off the damn thing. Thats planA.
PlanA pt2 is to perform Blues 'check throttle plates' - I'll pop the carb when I do the vac.

Now for the questions -
My driver here isnt to make car drive better - it runs really good - its to get the cooling under control. My understanding is that this is achieved by using the vac advance to pull it right forward at idle, allowing the slow burn flame time to ignite fully. The heat under the bonnet is caused my mechanical advance being too late ideally for idle and buring fuel gitting the inside of the exhaust; hence everything gets so hot so quick. (again, I am fairly convinced my cooling system is good; overkill if anything). Your post suggests that at idle (the very conditions where I am making the heat) the timing will be purely mechanical ?

The other limitation I have is the bushing and the advance control in the MSD. I can change the curve to about 6 different options, but the limitation is the advance stop bushing. The largest one has a 'range' of 18 degrees (which is the smallest range) - so if I bring it in from 34 down to (say) 30 it will bring my mechanical initial all the way down to 12 - too low. (although it would be interesting to see if this allowed the vac advance to get a reading).

Really interesting article here that explains what I was trying to say about the 'hot idle'; having re-read it after your post I am reading it differently. It doesnt say IDLE requires loads of vac - it says 'light load'... but that doesnt explain the hot idle of course

https://www.hotrod.com/articles/set-ign ... rformance/

With a minimum of air and fuel entering each cylinder, this means the mixture is not tightly packed. Here’s where things get tricky: You may have always thought of the combustion process as an explosion—the spark goes off and, boom, combustion occurs like a bomb. That’s not what happens. The reality is the spark plug fires and it takes a generous period of time for the combustion gases to burn completely across the top of the piston—much like a prairie fire across a large valley. The more densely packed the grass is, the faster it burns, while sparse areas burn more slowly.

We can apply this prairie-fire analogy to the combustion space. At WOT, the air and fuel are tightly packed and burn quickly, so we don’t need as much timing. At 2,800 rpm at WOT, 32 to 34 degrees of timing could be just about perfect for a typical pump-gas street engine. However, at very light throttle (14 to 16 inches of manifold vacuum), the air and fuel are far less densely packed in the cylinder. To make the most power possible at part throttle, we need to start the combustion process much sooner—perhaps as much as 40 to 44 degrees BTDC, depending on the engine’s individual demands.

This much timing is only needed when the engine is under very light load. Since manifold vacuum is a great indicator of load, early engine designers used a small vacuum canister attached to the distributor to advance the timing under high manifold vacuum to create a load-based timing curve independent of mechanical advance.

ok

1)using vacuum advance to fix you problem is not what i think you need to do
2) at idle all advanced mechanisms should be idle i.e nothing but initial advance no mechanical no vacuum.
3) the timing and fueling for idling needs to be corrected

vacuum advance is for after that

so i think you overheat due to over rich idle settings and not enough initial advance
you need a bit more idle rpm. get it all set then dial that back a bit

suggest
1) gunson colour tune, tune to light blue flame front on cylinder 1 and say cylinder no 3 or 4 or 5 or 6 i.e 1 cylinder at the end of the motor and one middle one

2) then use the kick back method to find initial timing i.e find out what it wants. 1-2 degrees back from where it tried to break your starter motor. you will hear it labour to start don't go further or you will get to starter destruction.
3) limit total to 34 or less
4) use the gunson, to tune to blue flame again with the idle mixture screw

or use lambda sensor to tune to 12.5-13.5 at idle slightly rich.... not 14.7, the ideal, becasue it will 15.7 or 16.7 on a hot day and only 14.7 at exactly the same temperature pressure..... and well you get the gist.... you want leeway and slightly rich is better than lean.

keep in mind with a race cam you may need to idle at 800 or 1000 rpm i.e 200 + over what the book says for standard

if you can't get a good tune at idle
wrong sized carb
wrong jets
wrong rods
running with the throttle so wide open its idling on the main jets/slits/ slides/orifices/venturis whatever you carb has i.e its not idling its running under no load...

wrong whatever
you may need to drill throttle plates

you need to be idling on the idle circuit
you need enough initial advance for a clean burn
you need enough rpm to create enough vacuum to get good distribution at idle all plugs firing, might still be lopey but all plugs firing something off

then worry about the rest.

your cooling system obviously works you can drive about and not over heat
the engine is not under load when idling if it gets hot its something to do with what's going on when idling
nothing is going on when idling other than igniting small quantity of fuel, way less than when driving so heat should be less
so its either the fuel or the timing or both

the fact you can drive around indicates good at all places in rpm range where there is some rpm.

Dave

Never seen this - Oh wow !
http://www.gunson.co.uk/tech-torque/Col ... on-chamber

Anyone happen to know thread size for a 440 ? (its 14mm)

1950s old skool stuff matey

works a treat

do not drive with it in

30 quid

will last a lifetime
avoids hours of bergering about
i think you are a mornings work away from total success

use for idle mix and accelerator pump actuation.

its too hard to use when driving at 70

i have one that fits hemi 6 and i use the same plugs as you



Dave

Blimey I'd forgotten all about Colortune, amazed you can still buy it. Last used one to set up the twin 40's on my Anglia in the 70s.....