Engine mystery

[rwgrimes]

Hello,

I have a 74 Targa that is bone stock original.

Started acting up today after taking it in for an alignment (saw another post with same scenario, these cars do not seem to like auto shops!).

Engine idles smooth at around 950 but the moment I accelerate, it begins to stumble and misfire (backfire on intake side). Car will accelerate slowly and once it passes 3500-4000 takes off and runs like it is suppose to. Fuel consumption seemed very high also (1/4 tank used for 35 miles of travel).

Going to look at ignition and vacuum pieces tomorrow but was hoping this might be a "seen it before condition" that the experts could help point me in the right direction on.


Thanks in advance!

[DRACO A5OG]

After you check the above ( spark plugs for symptoms ), check your fuel pressure, HB has a real cheap but affective tester $20.

It maybe your Fuel Pressure Regulator or like part in your SC

Yeah, our baby's feel mistreated at shops, like day care :-P

[kodioneill]

If it started happening right after your alignment I'd check for crushed wires or fuel lines under the car. Check the fuel pump area in front of the l/r tire.

[Bobboloo]

First and easiest to check would be the dizzy. The vacuum line on your dizzy is just a retard for idle. It retards the idle by 5 degrees. Disconnect and plug it. Put a timing light on your dizzy and check to see if it advances smoothly as you push the throttle rod. It should be 0 degrees at idle and should smoothly advance as use accelerate the motor in RPMs. If you see the timing suddenly jump or hang up when changing RPM's then the dizzy needs attention. Max advance should be about 35 degree on the crank pulley and should be marked. If the dizzy works properly then your problem is probably air/fuel related.

First off. How long has it been since the fuel filter has been changed? If it's been a while then change it and while your at it check the flow from the fuel pump. When you check the flow examine the fuel for contamination.

If the car is bone stock then it has the early version of CIS where enrichment is controlled by a valve on the throttle body. (later versions of CIS used vacuum for enrichment). So vacuum can be ruled out for any enrichment issue on your car unless it has been modified.

The symptom you describe sounds like the car is going lean when the throttle opens. (Common symptom when the WUR is getting out of spec or fuel delivery is below spec.) That you have unusually high fuel consumption is counter to that.

At this point (having eliminated spark as the issue) the best thing to do would be to check for vacuum leaks and check fuel pressure. (WUR)

Vacuum leaks most commonly occur with these cars when the airbox gets cracked. Does your car have a pop off valve? If not then it should have one because back fires will blow the box. If your car doesn't have a pop off then visually check the box for cracks around the plenum for the intake runners. If it does have a pop off then check the pop off valve for cracks at the base where it is glued.

That your car idles smoothly at 950rpm makes me think that you probably don't have a crack in your air box but it doesn't hurt to check since you say the car was backfiring.

Now for fuel pressure. The only way to check this is with a CIS gauge. If you don't have one or feel comfortable going this deep then you should probably take it to a shop that's good with Bosch CIS injection. If you want to proceed let me know and I can provide you with the specs.

[rwgrimes]

Hi Bobby and others,

Ordered a CIS fuel pressure gauge today and should have it this week-end.

Air box has a pop-off valve installed. Found a charred spot on the distributor cap down by where the clamp holds to distributor by points wire connection so will be replacing.

Filters and plugs are about 2 years old and have about 3000 miles on them, will pull plugs this week-end for signs.

Looking for timing light and dwell meter that are tucked away in the attic somewhere.

Underside of car looks undisturbed.

Thanks for the responses so far, I'll let you know what I find.

Rob

[rwgrimes]

Finally received CIS pressure gauge.

Fuel pressure readings:

System Pressure 52 psi - below low spec limit but previous owner said system pressure always ran low with car.

Control pressure 22 psi, air temp was around 68 °

Control pressure climbs smoothly to ~49 Psi in about 2-3 minutes as engine heats up.
Partial throttle pressure is ~52 psi.

Distributor timing looked correct.

Thinking the CDI needs to be looked at next, would this be a waste of time or is System Pressure an issue being that low?

Thanks for the help.

[Bobboloo]

Your car is running lean. That's why it's stumbling and your getting backfires. First off you need the correct system pressure which should be close to 5 bar. Your at about 4 bar and thats not enough. Your control pressure warm is to high which makes the car lean. This is probably due to low system pressure.

So to set the system pressure you need to remove the nut on the right side of the fuel distributor that has the tiny shims. They are very small and easy to lose so be careful when you remove them. They are behind the spring that puts pressure on the needle valve. What you need to do is add more shims. More than likely you don't have any shims available so you could take a small thin washer cut it off a piece and inserted behind the shims. A little bit goes a long way so you may need to remove one of your shims to get to the right pressure. Once you've established the right system pressure then we can move on to control pressure. 4.8 bar is the correct system pressure and that equals 70 bar. After you get the correct system pressure remeasure your control pressure and it should be lower

[T77911S]

check the fuel enrichment circuit

[prebordao]

Check here for an easy to make additional shims
How to make pressure regulator shims...

[rwgrimes]

Thank you all,

I'll see if I can round up some shims otherwise I'll try the Tuna Helper!