First start after engine install problems
 

I have just reinstalled my engine after replacing broken head studs. When trying to start it almost starts but then backfires and dies. I assume it is my timing giving me problems.

I have checked and distributor is lined up with plug number one at the Z1 mark

Will it fire at all if I have distributor set 360 degrees off?

It is a 1978 911SC 3.0

Any suggestions appreciated

Thanks

check timing but first check pop up valve if you have one if it open.

Pop up valve ok

Plug wires going in a CCW direction?

Plug wires good

Power brake hose tight on air box.

Where is power brake hose?

On left side airbox .

Timing too advanced...?? Try retarding distributor till it runs then put a light on it to bring it into specs

I tried adjusting the timing both ways with no luck.

I think I have a leaking injector on cylinder 2 which is not allowing residual pressure to build up. Would this cause the backfiring and prevent it from starting?

is the dist 180 degrees out? you usually get a loud backfire when it is 180 out.
check the plug wires again. i have a bad habit of getting 4 and 5 mixed up

i just replied to your other post.

pull an injector, remove the plug on the back of the AFM, turn on the key and adjust the mixture CW until it starts to spray, then back CCW until it stops. or, try raising the sensor plate with your hand to richen the mixture while cranking.
make sure you have fuel getting to the fuel dist.

unplug the power to the WUR. thsi keeps the fuel pressure from going up. BTW waht is your cold CP?

Did you have problem before head studs repair.

It is very easy to have the distributor out 180. Z1 is #1 and #4 at TDC. One has the valves open. Pull one of the upper valve covers and find which side is trying to open the valves. That's the "wrong" one.

I did it too when installing a new distributor. :)

VFR 750- thats an old one.

No problem before dropping engine

I pulled the theft side valve cover and confirmed cylinder number 1 valve is loose at z1 and when the rotor is pointing to number one wire

If you think you have a leaky injector, that would be a problem. The engine would run poorly at best.

Only way to confirm is pull them out. If you put each one into a jar, you'd be able to see if one leaked more than the other. You will need to lift up the metering plate to get the pump to run (with the ignition in the run position)

Obviously this is a little dangerous, as you are pumping fuel over your engine. The safer alternative is to remove them entirely, and blow compressed air through them. I have an old fuel line connected to the injector, that I use to get a bit of fuel through the injector during this bench test. At 30-40 psi nothing should flow out. At roughly 50-55 psi the injector should spray a nice fan pattern.


If you don't have a compressor, you are stuck using the fuel pump to pressurize them.

I did this by lifting the metering plate. They all spray when I lift up and stop when I let it down.

If I were to unplug the AFM and turn key on should the injectors spray or just hold pressure?

Thanks

I find baby food jars are very usefull for this exercise .. just label them for each cylinder , then line them up on a bench and compare ..
the peas arent to bad either with a beer :)

this however sounds like a timing issue not a fuel issue ..

I hate to even bring this up but are you sure you timed your cams right ?

I am pretty sure☺️
I did check it a few times to make sure.

I am definitely losing fuel pressure as it doesn't build up like normal when I lift the sensor plate

i used the 16 plastic whater bottles for my 930.

if you unplug the connector to the AFM and turn the key on (pump running) nothing should come out of the injectors.
for your car to backfire like it is one or more injectors would have to be leaking a LOT. once it fired the first time that would clean out any gas in there then it should fire up. it may run bad. usually a miss or pop due to one cylinder getting more fuel,

pull the plug wires off and put them on one at a time. make sure you are putting them on in the direction of rotation of the dist. 1,6,2,4,3,5.

lift the sensor plate when starting. i think you are either too lean or its a plug wire issue.
if it is a too much fuel issue what about the CSV. you dont have it wired so it is on all the time?

What do you mean: "doesn't build up as usual"?

If you lift and they start flowing, do they flow continuously? If you let the plate go, do they drain down, or stop quickly?

If you raise the plate does it take a while for flow to get to the injectors?

i was going to add how long has it been sitting?
could the plunger be sticking inside the fuel head.
with the pump on, you should feel a lot of resistance as soon as you lift the plate. if there is any free movoement then the plunger is stuck. (this would mean all the injectors are dumping fuel when the pump is on).

i have to quote VFR

What do you mean: "doesn't build up as usual"?

Other things to check is air leaks. The rubber bits do get old and lose flexibility.

I found when I removed the CIS, the intakes moved a lot. Maybe the rubber sleeves are leaking? The injector sleeves also have an oring that get hard as a rock.

All the vacuum lines, air handling pipes and manifolds are potential leaks. The idle speed adjustment screw is really an air bypass.

If you sort out the fuel side, the next area is to check the air side of the equation.

Normally when you lift the sensor plate you can feel the pressure build quickly and it becomes almost difficult to hold up. That is not happening. I can easily hold up the plate with some but very little resistance.

Sounds like either the fuel manifold is binding, or your pump capacity is low.

Plugged filter? You'll get pressure, but not enough flow rate to sustain pressure.

I am loosing a lot of fuel. I put my fuel pressure tester on between the wur and the Fuel Distributor with the shut off on the wur side of the gauge. With the shut off closed it builds and hold pressure. With the shut off open it dumps fuel into my engine.

Would this eliminate the plunger being stuck?

Does this suggest cold start valve?

Just looked over your recent posts. Wow you do that rebuild fast! So that probably eliminates old fuel sitting around.

Still, your fuel distributor could be starved for fuel based on your observations.

Where is the fuel going?

If the plunger was stuck partially open, the fuel distributor wouldn't know why, It would just flow.

i think what you have been feeling is the control pressure rising as the WUR heats up which increases resistance on the sensor plate.....no?

do the fuel pressure test with the connector off the AFM. do not lift the sensor plate to do the test.

do the fuel pressure test again.
system pressure, valve closed should be about 4.8bar
control pressure:
cold- depends on outside temp but i dont think it should be below .8 with the engine off.
warm- connector plugged into WUR and heated up
with vacuum to WUR 3.2-3.6bar
no vacuum to WUR (IE engine off)- 2.7-3.1bar

if you can pull a vacuum onthe WUR while testing that is a big help. you should see about a .8bar difference with and without vac.

with a low contol pressure the resistance will be low on the sensor plate. the important thing is do you feel the resistance as soon as you lift the plate.

if you are lifting the plate a lot pull the injectors and put them in bottles.

you should just do what i said earlier and be done with the fuel side of this problem:
with the injectors in bottles:
let the WUR heat up and check warm control pressures (WCP).

check the plunger in the fuel head by lifting the sensor plate from 0 all the way to max.
(you can also check fuel volume for WOT this way.)


lift the sensor plate and hold it open about 3-5mm for 5 minutes or so so you can compare the injectors for fuel volume. this is for a good idle.

check each injector for spray pattern

check each one for leakage with pump on and sensor plate down. if they all leak the mixture is too rich. (if you are not sure about the fuel mixture adjust it CW until they start to spray then CCW until they stop).

once you do this you can eliminate the CIS.

The fuel seems to be running into the cylinders 1,2,3 it is running out the open exhaust port on cylinder 2

Will the wur heat up without the engine running?

I think you need to isolate some parts from the problem. From what you posted it could be injector related.

If it were me, I'd pull all the injectors and test them individually for opening pressure. Just to make sure that is not the problem, or a subtle addition to the problem (red herring).

Once you know the injectors are working properly and can only open when the desired pressure is reached, then you can eliminate them as being a problem.

Ok. I'm at work now but will check them tonight.

Thanks for all the feedback

If your CIS was working properly prior to rebuild, and you didn't do anything to it but remove and replace, then chances are the components are fine...

What happened to me on my first engine start attempt was the connection at the big plug at the front of the motor near the left side of the engine bay was loose and the frequency valve was not running... this will cause the car to not and or barely run.

When you lift the sensor plate, do you hear the frequency valve buzzing? It should be very distinct buzzing over the injectors squealing.

Regards,
Brian Petry
Springboro, OH

The cis was working well before engine drop however I may have damaged one of the injectors removing it from the injector sleeve.

I think the frequency valve is buzzing, I will double check that as well tonight.

Are you sure you have enought gas in tank. the rear same level as the front.

CIS troubleshooting..........
 

Is this happening with fuel pump running and air flow sensor sitting down? The fuel injectors should not be delivering any fuel to the cylinder/s during a FP test run until the FD plunger is lifted. Could you elaborate when the above condition occurs? Not sure what you have been doing for the test. Thanks.

Tony