The Dodge Challenger Message Board

Dave, i have read through the procedures you wrote out for Setting Ign Timing. I have a 360 with a fairly big dur cam (about 290) but i can get the car to idle reasonably happily in gear at about 650-750 RPM, so its not that big a cam. I have found that without my vacuum advance connected, i am still getting advance figures of around 45+ degrees at only around 2500rpm. I have put the initial on about 18 degrees as you said. My vacuum at idle (manifold vacuum) is about 10.5-11 " HG, so its not too bad. I need that much initial timing in order to be able to reduce the carb idle speed, otherwise i get horrible stumbling on part throttle/coast due to the transfer port not working correctly (its a T Quad).

The dizzy is just the std MP electronic, with chrome ECU. Should i have that much advance at such a low rpm with no vac advance connected? I may need to recurve the centrifugal weights, but not totally confidant in doing that, also no idea where i can get springs. It drives well with not a hint of pinging with no vac advance connected. And that is with a 2.76 gear set, in top gear going up a hill at 3/4 throttle. I have not tried connecting the vac advance to manifold vacuum. The resultant increase in revs from this might allow me to reduce the initial timing and therefore will bring down the total to what it should be - 32-34 degrees? Thanks.

I just hooked it up again and ran it and this is what i found: If i take vac advance off and block that carby timing port, and with initial timing set at 18deg, then i get well over 40 degrees at under 2500 rpm. If i work backwards and set the advance to what you recommended which is about 34 degrees at 3000rpm, the initial at idle (out of gear) is at around 0 deg. Now if i connect the dizzy vac advance to manifold vacuum ( it has about 10-11 In HG) as was suggested, i get a huge initial timing boost at idle and so have to back off the dizzy. But doing this seems to upset the way the engine runs, it idles erratically and the 'all in' timing is still around 40 deg. The carby kerb (curb?) idle then has to be increased, so i am back to the problem of too much transfer port being exposed and runs badly at part throttle

Do you think that 10-11 InHg is too much manifold vac to be able to use for vac advance?

Help ! Any suggestions? Thanks

Steve, I think Dave' s at the Euronats so here's what I suggest. First thing is to either get a new vacum cansiter that allows an adjustment so you can get the vacum advance you need at a much higher vacum. It sounds as if the vacum advance is 'coming, in' too early. The engine produces it's highest vacum during de-cel followed by light throttle cruising. You need to shorten the distance in the advance plate of the dizzy you have now to limit how much mechanical advance you need.. This will limit the total mechanical advance along with your initial you should have 34-36 @ 2500-3000,,the springs only control when this mechanical advance happens it doesnt limit anything. Stiffer springs= late advance weak springs= advance happens very early. You need a vacum canister that is either adjustable and can provide the amount of degrees you want at a certain vacum level. What is the number stamped on the arm of the vac cansiter? Example is a 7.5 which would mean 15 degrees of advance for this unit,,you can use a 3/32 allen wrench to adjust the cansiter to bring it in later. Take a piece of paper and draw a line like a graph, then with a dial back timing light you can plot your curve and 'see' what the advance curve is for your setup. A mighty vac hooked to the canister with the engine running can provide you with how much vac. adv. the canister is providing at what vac in inches the engine is making and when this vacum advance happens and at what inches in mercury.. Typically big cams with a lot of overlap and intake duation run better with no vac adavance, because the vacum is erratic and pulls the spark over to the next terminal and this causes misfiring, chugging, runs badly ect.

Thanks Eddie, i appreciate the suggestions. I'd be interested to see what Dave says too when he gets back. I think you may be right about the Vac adv coming in too early. I will have a look at the reading on the cannister (cansiter -spelling?) arm tomorrow. I didn't even realise that was there. Also not sure if it is the adjustable type, but will have a good look. I also think it might be a good idea to run around for a while with no vac adv, like you say. This way, i will be able to put more idealist initial timing in and still be closer to the 32-34 degrees total. Dave said he only has about 8 In Hg, perhaps you do as well. Like i said, i think i may have bit too much manifold vacuum (10-11 "HG) to connect the dizzy adv up to that. I don't have the separate Mighty Vac vacuum motor either, but just tonight i did win a Digial Engine Multimeter which has a digital Tacho on it, so hopefully i will have accurate tacho readings when tuning.

BTW, how do i put a limiter on the centrifigal adv plate in the dizzy?

Thanks.

Weld the slots shorter on the mechanical advance so that the weights will only advance the distributor 16 degrees.

That will then give you 18 initial and 34 total.

Then fit springs (you can get distributor spring kits) to get a nice even advance (check the timing in 500rpm steps from 1000rpm) right up to 3000rpm max.

If you don't have an adjustable vac can weld the slot on that up to. You can live with only 10-15 degrees vac advance. Or do without any vac advance at all. because of your high initial timing you don't really need it.

You should have no vac on the source for the distributor at idle (if you are using the factory timing port on the carb) so if you have any vacuum there you need to close the throttle more.

Thanks Dave. I have now disconnected the vac advance completely as it still has too much (by your theory) just on centrifugal. Will putting in a spring kit reduce the total? I don't think it will? Regarding the the Vac advance, Eddie said to look at the number stamped on the vac advance arm. He said a 7.5 would mean it gives 15 degrees advance total. Well mine has R11 stamped - does that mean it gives 22 deg? That is heaps?!. I am still getting over 40 deg with no vac advance (with 16-18 initial). Could the harmonic balancer have slipped a touch? That would account for these readings. If the car does not ping under load or heavy throttle with even 40+ degrees, at 2500rpm, is that still ok?...because you said whatever max advance you can get away with, is good for it. Still not sure! Thanks, Steve

No mate. You need to weld (or epoxy) the slots the weights ride on so they are shorter to reduce the timing. You don't want more than 34-36 total depending on how good your fuel is. The springs only effect the rate of timing increase.

If you are not getting any pinking at 40 degrees i am VERY surprised.

ok, i understand now -the centrifugal weights slots, not the vac advance.

Yes, i had vac advance connected and about 44 degrees at 2500 which was all in. Going up a reasonably steep hill, with 2.76 rear end, in top gear at 3/4 plus throttle and no pinging at all!

That's why i suspect the timing marks might be out due to the damper slipping a touch. I think it can happen.

ta.

That can happen if the damper is failing.

You will know for sure if it suddenly exits the car through the hood or inner fender.

I arrived in Sydney a week ago after driving from Perth (3000miles). I drove all the way with no vac advance. It has about 14-16 initial and at only 1500 rpm is at 30 degrees, at 2500 its at around 36-37. The car went like a dream although fuel usage was higher than i expected at 17-18.5 mpg sitting at between 65 and 80mph.

But when i get back i will be pulling the dizzy and recurving it. I found out a great way to get a good starting point for the mech advance. And that's by using a school geometry protractor fixed to the top of the shaft, so that a degrees readout can be obtained. It is quite accurate, but obviously can not take into account some variables.