Post Top End Engine Rebuild Issues - 3.0 CIS - Backfire

[Johnathan]

Hey guys, I don't post too much over here since I can usually find what I need via. the search... That being said I'm stumped on my current issue...

Issue: Backfire/Popping thru exhaust at idle + low rpms

Background: Engine had a broken head stud + exhaust leak so I decided to jump in a do a top end rebuild myself. Replaced rings+rod bearings, rods balanced, heads rebuilt by a reputable shop, reground SC cams to 964. Replaced rusty heat exchangers with SSIs + a 2in banana muffler modified with 2 outs. And all the various other necessities..

Observations: drivers side exhaust out is hotter than the passenger side (~100degF via. infrared thermometer). The individual exhaust tubes seems to be cooler on the passenger side but only by about ~50degF. The popping seems to be coming from the passenger side. Cars drives well at speed, but pops when I come to a stop or am at partial throttle cruising thru the neighborhood.

Corrective Actions: Tightened all exhaust bolts, Adjusted my CIS mixture via adjustment screw.

Tests: Performed CIS pressure tests, checked out OK

Thoughts: Incorrect Cam timing on passenger side cylinders (Please don't let it be this one... I'm sure I set this dead on at 1.26mm). Exhaust leak. Incorrect mixtures... Bad luck???

Thanks for any help you can give guys...

[bruboy]

I have had this problem and it turned out to be gummed injectors.

At idle/part throttle the small a mount of fuel dribbles out rather than being atomised correctly by the injector. At decent throttle the extra fuel atomises better and masks the problem.

Its easy to fix, get the local Bosche agent to flush them through, they will also check the atomised spray pattern

[Johnathan]

Thanks Bruce, I'll check with my local shop to see if they have the equipment to test and clean the CIS injectors.

In hind sight I should have had the injectors cleaned when the motor was out of the car during the rebuild.

I'll keep you posted. Thanks agian

[efhughes3]

Ditto here on the sticky injectors. As a side note, I had them cleaned a few months before assembly, which is a no-no.

[cstreit]

Even a slight exhaust leak can cause bad exhaust popping. As you decclerate or idle, there is a vaccum created past the leak, sucks air in, pop pop pop

[DTW_JWA]

Originally Posted by bruboy

I have had this problem and it turned out to be gummed injectors.

At idle/part throttle the small a mount of fuel dribbles out rather than being atomised correctly by the injector. At decent throttle the extra fuel atomises better and masks the problem.

Its easy to fix, get the local Bosche agent to flush them through, they will also check the atomised spray pattern

Bruce, I just saw a 83 SC w/ 200K for $14000. The color is bad but the overall package seemed in decent shape. What do you think about the price?

[Johnathan]

Originally Posted by cstreit

Even a slight exhaust leak can cause bad exhaust popping. As you decclerate or idle, there is a vaccum created past the leak, sucks air in, pop pop pop

What is the best/easiest way to find exhaust leaks?

Before the rebuild, when I had the broken head stud I could hear a popping from the exhaust leak between the cylinder+head when the engine was under load at mid to full throttle.

[Johnathan]

Ok, Injectors had a bad spray pattern and soaking them in SeaFoam didn't help so new injectors went in...

Result: Same problem..

So I guess it's on to hunting down an exhaust leak. Any other suggestions, or how to find an exhaust leak?

[Johnathan]

Not my car, but it sounds just like this...

http://youtube.com/watch?v=AHuf3zgFzA0

[TibetanT]

Subscribing for information purposes.

Good luck with this one!

[TibetanT]

Okay, I guess I do have one question/suggestion.

Have you checked your idle speed and the fuel mixture? Are they at spec?

[Johnathan]

Yup, Idle speed is about 900, and I've moved the mixture adjustment all over the spectrum with the same results.

[Johnathan]

Ok, I'm still hunting this issue down (and I'm tired of throwing money at it). Turns out the #4 spark plug was fouled, and the plug wire had some corrosion on it at the distributor cap. So new cap, wires, and coil (also tested bad) all went in... Same problem.

I did buy a cheap leak down tester and compared cylinders I was getting about 3% on #4 (unless I was performing it wrong). I could hear a faint leak from the tail pipe, and a slightly louder, but faint leak through oil fill. Does this sound normal?

I'm at the point where I'm going to pull of the HE and see how the gasket and exhaust value look..

Would a damaged valve/valve seat cause a plug to foul?

Any other ideas?

The car runs great otherwise, and I have a HPDE coming up, could I do any further damage this slight backfire at idle?

[Eagledriver]

3 percent is great. You should do a couple of other cylinders for comparison. Hiss in oil fill is normal air past rings on leak down test. Hiss in exhaust is not normal and could mean a leaking exhuast valve. If that is the case it will slowly get worse and eventually burn the valve. No hurry to fix it though unless you don't want to buy a new valve. The fouled plug is probably from that cylinder misfiring and running rich as a result. Could you have hit the valve on a piston when you were doing the cam timing?

-Andy

[Cdnone1]

I swapped out my exhaust on my SC with a pre 74 system with a 2 in 1 out it started popping right away. But only when I lift off from heavy throttle.
I haven't been able to track down the leak, but all I did was the exhaust so it couldn't have been the injectors, wires etc.
A friend suggested that it might be running a little rich, but he also suggested the much shorted distance of the exhaust travel and might exasperate the situation.
Steve

[Johnathan]

Originally Posted by Eagledriver

Could you have hit the valve on a piston when you were doing the cam timing?

Unfortunatly, Yes they did make contact when I was performing my piston/valve clearance check. I stupidly did the quick check of screwing down the adjuster nut and rotating the engine (slowly by hand + wrench) and feeling for resistance. I thought felt a very slight resistance (felt like the cam was pushing down another valve), it did however move past that point very easly. But I did a double check and found there was clearance issue (0.2mm). I didn't think(secretly hoping) that this would do any damage or bend a valve. Am I wrong?

Assuming this is the problem, will I need to replace the valve guide as well (these were all just replaced in the rebuild)?

[Eagledriver]

It seems like it's very easy to bend the valves this way. I had a slight intake valve leak after hitting the intake valve when I set the timing. To be sure you should do your leakdown test on another cylinder to compare the exhaust hiss from it. You can also take off the exhaust and listen for the hiss at the port. If you spray some WD 40 on the back of the valve so it runs into the seam between the valve and the seat you will hear it bubble when the cylinder is pressurized. You will not need to replace the valve guide but you will need a new valve and have the new valve lapped in or the seat cut (your machine shop should be able to advise you on this).

One other thing to check before you go through all this is to look for a vacuum leak on the intake of that cylinder. Maybe swap the injector as well with another cylinder to eliminate that possibility.

-Andy

[justinp71]

I had a problem on a v-8 motor very similiar to this and I ended up crossing two of the spark plug wires. It was weird because the motor seemed to perform fine, it popped at idle and sometimes while driving... Might be something worth double checking.