dwell and ignition timing

[benzboy]

hey all. I cant seem to get my car running properly.

a little about the car
1973 911 E 2.4 mfi.
I did a top end rebuild. heads redone with all new valves seats seals ect. throttles tested and in great condition mfi pump bench tested and set properly. the only upgrade I done was going from a stock 84mm piston to a 85 mm je piston.

ok the problem
I got the car to idle very nice. but when I give it gas, I get backfiring and sometimes flames through the stacks. I never had a car with points before. I replaced the points and think I got it to the correct gap. .012inches or .30 mm. I read hundred of threads about checking dwell. some say its not important some say check it . I ordered a dwell meter. should be here I a few days. whats the best way to hook it up? I do need the car running to check dwell right?

im going through the CMA. so far air filter and plugs are good. next step is dwell and ignition timing.

a few notes. I did notice my plugs are very black and sutty. which means im running rich. would that also have anything to do with possibly the points and dwell not being set??
also with at idle I timed the car at 5 degrees atdc. when I set the idle with vacuum hose off at 2000rpms, im at about 22btdc. that seems hight to me. I didn't check it at 6000 rpms, just because of the popping and sometimes flmaes shooting through the stacks.
sorry for such a long post. im sure this has been covered before. but this is driving me crazy. been working on this for a month or so.

thanks
tony

[Flat6pac]

The vacuum hose is an idle only, disconnect it and go for idle at 900@ 5degrees BTDC.
Once set, you need to go up in the RPMs to see if the dist. is advancing correctly to 35 degrees btdc. If you have an advance problem you will hit a RPM wall at about 3kto 3.5k rpm
Dwell is properly set points but a scope reading..you need that reading correct.
The only guy I know that sets up MFI, correctly, is 40 miles away in Delaware.

Bruce

[tobluforu]

You say that the pump is setup, what do you mean? As you are probably well aware of there are many little things that need to be just right, look up CMA. Once you have the right plugs, timing is set, dwell set (not needed if you have cdi), no air leaks, valves are set, etc, then you can move on to the mfi. Have you adjusted the mfi pump, is correlation pretty close-rods equal length, pump rod the right length, etc?

[benzboy]

my Porsche mechanic set the pump for me so I assume its pretty close. I didn't adjust the pump. since that's the last step of the CMA. the rod pump is right on at 114.2 mm. I think I have the t.b's set right with my synchro.

I still think it could be the points. since I never done them before and never gapped them before either. that's why I want to check my dwell.

I do get a little popping coming through the intake stacks when revving the motor. not sure if that could be air leaks. I have everything tighten down pretty well with new gaskets.

[Jdub]

How is your distributor? Any sticking, bad springs, or otherwise? Sorry not to be of more help here - all my dwell time has been under late '60's MB coupes.

[Bill Verburg]

Originally Posted by benzboy

hey all. I cant seem to get my car running properly.

a little about the car
1973 911 E 2.4 mfi.
I did a top end rebuild. heads redone with all new valves seats seals ect. throttles tested and in great condition mfi pump bench tested and set properly. the only upgrade I done was going from a stock 84mm piston to a 85 mm je piston.

ok the problem
I got the car to idle very nice. but when I give it gas, I get backfiring and sometimes flames through the stacks. I never had a car with points before. I replaced the points and think I got it to the correct gap. .012inches or .30 mm. I read hundred of threads about checking dwell. some say its not important some say check it . I ordered a dwell meter. should be here I a few days. whats the best way to hook it up? I do need the car running to check dwell right?

im going through the CMA. so far air filter and plugs are good. next step is dwell and ignition timing.

a few notes. I did notice my plugs are very black and sutty. which means im running rich. would that also have anything to do with possibly the points and dwell not being set??
also with at idle I timed the car at 5 degrees atdc. when I set the idle with vacuum hose off at 2000rpms, im at about 22btdc. that seems hight to me. I didn't check it at 6000 rpms, just because of the popping and sometimes flmaes shooting through the stacks.
sorry for such a long post. im sure this has been covered before. but this is driving me crazy. been working on this for a month or so.

thanks
tony

dwell is just a more accurate way to set the point gap, set the gap on the wide side of spec which will have the dwell on the low side.

It sounds to me as if there is a timing issue, but w/ mfi it could be mfi timing, ignition timing or cam timing you need to be sure that all are correct

[benzboy]

im positive the cam and mfi timing is right on. would the points not being set properly make it pop and sometimes flames shoot out the stacks?

[tobluforu]

Points are easy, car will run pretty damn good if points are close, even a cc works at times.

[Bill Verburg]

Originally Posted by benzboy

im positive the cam and mfi timing is right on. would the points not being set properly make it pop and sometimes flames shoot out the stacks?

it's unlikely, as point gap closes up from wear the timing retards a little bit, but since you have the gap at least in the ball park, whatever ignition timing you have is set by rotating the distributor, are you sure you put the distributor back in phase? IE that the engine is at tdc compression stroke(there are 2 tdc's, you have to be at the end of compression where both valves are closed, not at the end of exhaust where they are open) w/ the distributor rotor pointing at the notch in the distributor(also should be pointing to the back of the blower, and that the ign wires have all been installed correctly, 1, 6, 2, 4, 3, 5, be sure that the cap is seated in the notch.

be sure that you are using the correct pulley mark 5ATDC is just to the left of TDC

you can also do a static test of timing before starting the engine by using a test light/probe across the point set just to be sure

[wwest]

Will the engine actually start/run with the timing 180 degrees off?

Sure sounds like.....

[wwest]

Originally Posted by benzboy

hey all. I cant seem to get my car running properly.

a little about the car
1973 911 E 2.4 mfi.
I did a top end rebuild. heads redone with all new valves seats seals ect. throttles tested and in great condition mfi pump bench tested and set properly. the only upgrade I done was going from a stock 84mm piston to a 85 mm je piston.

ok the problem
I got the car to idle very nice. but when I give it gas, I get backfiring and sometimes flames through the stacks. I never had a car with points before. I replaced the points and think I got it to the correct gap. .012inches or .30 mm. I read hundred of threads about checking dwell. some say its not important some say check it . I ordered a dwell meter. should be here I a few days. whats the best way to hook it up? I do need the car running to check dwell right?

im going through the CMA. so far air filter and plugs are good. next step is dwell and ignition timing.

a few notes. I did notice my plugs are very black and sutty. which means im running rich. would that also have anything to do with possibly the points and dwell not being set??
also with at idle I timed the car at 5 degrees atdc. when I set the idle with vacuum hose off at 2000rpms, im at about 22btdc. that seems hight to me. I didn't check it at 6000 rpms, just because of the popping and sometimes flmaes shooting through the stacks.
sorry for such a long post. im sure this has been covered before. but this is driving me crazy. been working on this for a month or so.

thanks
tony

You can check dwell with an analog volt/ohm meter, R1 range and adjust to 70% needle deflection.

[wwest]

In the past I have seen wrong distributor rotors (to short) do this.

[benzboy]

im sure I put the distributor back in on tdc of cylinder 1. but to check. if I pull valve cover off when at tdc on 1, the rockerers should be able to move just a little bit, right? im sure all wires are connected properly.

[Bill Verburg]

Originally Posted by benzboy

im sure I put the distributor back in on tdc of cylinder 1. but to check. if I pull valve cover off when at tdc on 1, the rockerers should be able to move just a little bit, right? im sure all wires are connected properly.

w/ an E there is a certain amount of overlap at tdc where both valves are still slightly open, therefore no wiggle, but both should be very close to closed w/ #1 cyl at tdc
if you are backfiring through the intakes there is a timing issue

it could be
ign timing
cam timing
mfi timing
firing order error

double check everything

[Bill Verburg]

the wrong tdc is when cylinder #4 is at tdc on compression

#1 is the furthest back in the car on the left side
#4 is the furthest back in the car on the right side

an easy way to find tdc is to put a pressure/vacuum gage on #1 then rotate the crank by hand,

the crank rotates clockwise, when you get pressure on the gauge folowed by vacuum that is tdc compression

at tdc compression for #1 the rotor needs to point at the #1 wire in the distributor

[bcgreen]

Originally Posted by wwest

You can check dwell with an analog volt/ohm meter, R1 range and adjust to 70% needle deflection.

Oh, I haven't heard of that yet. How do you check the dwell with the analog volt/ohm meter? How do you hook up the wires? Is it just as accurate as a dwell meter?

[dicklague]

once you get the engine running OK, I highly recommend using an Air/Fuel ratio instrument to to adjust the MFI.

There are some good threads here on MFI as you may know.

I fiddled with my MFI for years with quite a bit of professional help and when I finally installed a Daytona-Sensors Wego IV permanently I finally got the mixture really dialed and got ride of the flat spots.

You are already in the loop on CMA so get all that right and then fine tune the MFI.

What I like about the Wego IV is that I get 2 hours of data and it records AFR, RPM, MAP and charts it out, and you can also put it in an excel spread sheet. Then you know what is really happening with the mixture over time and not must what the meter says at that moment.

You can convert CO readings from the CMA to AFR easily.....just Google for a chart.

One other thing I found very valuable is taking out the warm up regulator stack and putting in 6mm screw for manual adjustment. That gives you wide range mixture control once you have dialed in low speed and high speed.

One more tip is to install a Pertronix ignition to replace the points. Best $110 I have spent on my 911. You get the points and dwell right and 5000 miles later the rubbing block wears away and you can feel the engine running a bit fuzzy. I even had the all but close on me and the 911 would not run. With Pertronix you never adjust......it just works no mechanical rubbing. Just an air gap.

[Bill Verburg]

I have to apologize for post #15 I had second thoughts(often a bad idea) and changed my initial correct response to a wrong one

at TDC compression, both valves are on their seats and yes the rockers will wiggle by the amount of lash you have set

Here's a valve event diagram for S cams that I put together, E will be similar but w/ less lift and duration


TDC for each cylinder occurs twice in 720° of crank rotation the first is at TDC compression which I have arbitrarily labeled at 0°, the second is at TDC exhaust which is at 360° where both valves are off the their seats, it is this overlap that allows properly designed headers to extract additional power at specific rpm due to exhaust resonances

[wwest]

Originally Posted by bcgreen

Oh, I haven't heard of that yet. How do you check the dwell with the analog volt/ohm meter? How do you hook up the wires? Is it just as accurate as a dwell meter?

Just measure "resistance" between ground and the (disconnected) distributor output from the points with the distributor spinning at a goodly speed....

Just as accurate as any analog dwell meter.

[Peter Zimmermann]

Just to be absolutely sure, the MFI pump is timed 40 degrees AFTER CYL #4, not cyl #1 - correct?