Engine Backfires + CIS airbox Repair

[Noslo14z]

Recently my father and I had brought our 73.5 911T back to life. However we have had some issues with the motor and intake system (CIS airbox leak to be more exact). When we first got the motor running while the car was on jack stands it wouldn't rev over about 2200 rpm. As the car had been sitting for some time (8+ years), we decided that removal and inspection of the distributor would be a good idea. Upon inspection of it, we found various spots of corrosion on contacts as well as on the mechanical advance for the timing. We cleaned the corrosion off of the contacts and freed up the lobes for the advance to the best of our ability (they would come out and almost all the way back in upon releasing them). After this put it back into the car. Before we started the car, we had done a complete flush of the CIS as the plunger for the fuel dizzy wouldn't fall back down from a WOT position.

After this work had been done, we started the car and it purrs like it just left Meany West. So after we had finished rebuilding the brake master cyl, calipers & front hubs, we put her back down on the ground and took it for a drive the next day. For the first 15 or so minutes the car ran pretty well apart from a little bit of a jumpy clutch. But as it was driven a bit more the car started having issues in the 4500rpm+ range. It would back fire and not rev past that. We kept driving to see if it was just a minor hiccup, but it would get worse (not reving higher then 3700rpm). I have read many threads in regard to the CIS being extremely reliant on an air-tight airbox, and i suspect that our cracked one might be the culprit in this case. So my questions are these:

1. Is it worth trying to seal the cracks in the air-box if it spans a large section of it?
2. If so, what type of sealant/glue/epoxy would be recommended for this?
3. Are there other things in the CIS system that could be the reason for this?

[timmy2]

Looking at your garage, I noticed you have a different intake on the engine. What year of CIS are you running?

[Noslo14z]

It is the stock 1973.5 CIS.

[Nostril Cheese]

Originally Posted by Noslo14z

1. Is it worth trying to seal the cracks in the air-box if it spans a large section of it?
2. If so, what type of sealant/glue/epoxy would be recommended for this?
3. Are there other things in the CIS system that could be the reason for this?

1. You'll have to take the whole thing out in order to repair it. It isn't worth it. Get a new airbox and put a popoff valve in it.


3. You need to hook up the CIS gauges to the car and check what the system and control pressures are. You said the control piston is stuck. There may be other things stuck in the system as well.

[Noslo14z]

Nostril,

I agree that a new box is probably a good idea but the $250+ price tag for a new one isn't very appealing. If I fix it, the old one has to come out. If I get a new one, the old one has to come out. I would like to get a new one, but there are other things that need the money more than this. Unless I have to buy a new one i would prefer to keep the old one and just repair it.

[timmy2]

Slow set JBweld has been used on air boxes in the past.
Don't know how long it lasts.

[enzo1]

get a used one from a dismantler, think i paid a 100 and it had a pop=off already installed cracked air box pelican site:forums.pelicanparts.com - Google Search

[Sunroof]

I have a spare air box from my 1973.5T. I was told by my mechanic that I might have a cracked airbox, but that was not the case. I had a vacuum leak and I was running too lean! I replacedit with a new one however, and kept the older one. In fact I epoxyed the side wall anyways. Their is a pop-off valve alreadyinstalled on it. It has a few chips on it where the air filter cover fits on, but will work just fine. Drop me an email if your interested. It will be alot cheaper then a new one with a pop-off valve...!

Bob
1973.5T

[Sunroof]

Colin..
After reading about your issues did you do anything at all to the fuel tank? After letting it sit for eight years that fuel filter in the tank could be clogged. CIS always needs clean fuel. If your tank is low, under the tank and in the center is a large hex. nut that holds the screened filter that sits within the swirl pot. This is a very fine meshed filter that sits about three inches high within the swirl pot (about the size of a ignition distributor). It should be replaced or at least inspected. Once you drain the tank (syphon) you have easy access from under the car. You'll need to fabricate a bolt and several nuts to get that hex. fitting (19mm - 22mm?). Replacement filters are gasket not expensive. This first CIS tank will rust internally, especially with eight years of stagnant fuel and condensation build up. It seems that just about all 1973.5T owners have had some issue with the fuel tank, including me! Did you change the external fuel filter as well?

Bob
73.5T

[ossiblue]

Originally Posted by Sunroof

Colin..
After reading about your issues did you do anything at all to the fuel tank? After letting it sit for eight years that fuel filter in the tank could be clogged. CIS always needs clean fuel. If your tank is low, under the tank and in the center is a large hex. nut that holds the screened filter that sits within the swirl pot. This is a very fine meshed filter that sits about three inches high within the swirl pot (about the size of a ignition distributor). It should be replaced or at least inspected. Once you drain the tank (syphon) you have easy access from under the car. You'll need to fabricate a bolt and several nuts to get that hex. fitting (19mm - 22mm?). Replacement filters are gasket not expensive. This first CIS tank will rust internally, especially with eight years of stagnant fuel and condensation build up. It seems that just about all 1973.5T owners have had some issue with the fuel tank, including me! Did you change the external fuel filter as well?

Bob
73.5T

^^Just saw this thread and these were my thoughts as I was reading your original post.^^

Your symptoms strike me more of a fuel starvation than a massive air leak. If you did not r&r the tank, then you likely have a clogging problem. Before you tear out/replace the airbox, do some checks on the fuel supply. Do you have a fuel pressure gauge set? A must for CIS engines. Even if you don't, you can check the fuel flow rate (do a search for how) to determine if you are getting the volume necessary to your injectors. I'd suggest you go this route first, given the history of the car and the symptoms you describe.

[Sunroof]

L.J's right on. Clean fuel is everything and a car that has sat for eight years will probably have issues with fuel varnishing. If you elect to remove the tank and have it cleaned (radiator shop will boil it out) be careful about internal coatings. That swirl pot is made of bakelite similar to the distributor. Not many coatings will stayed adhered to it as I experienced with "Red-Kote". As a result I got stuck dead on the road several times due to the coating clogging the filter after flakes of the red-kote came off the swirl pot. So much for lessons learned!! So at a minimum get that old gas out and flush with fresh gas and a clean internal filter and go downstream from there (fuel pump, fuel filter, fuel accumulator, WUR, fuel distributor, injectors, etc)...............................I highly suggest that once you get everything running correctly add a can of Seafoam or Chevron Techron to help clean the system particulary the injectors. Do this back to back or evertime you fuel up.

Something else to check is the Airboot on the back and top of the airbox. If a crack delvelops in the airboot you are not going very far. The rubber could be dry rotted. Remove it (two clamps), inspect it and maybe clean up the top of the sensor plate while your at it.

Keep us posted on the progress. Their must be an accumulative 1,000 years of CIS experience on this board so keep tapping into it.

Bob
73.5T in Sepia

[E Sully]

Just to get things right, lets see what we are working with. Your garage shows: Mid Year CIS fuel injection system upgrade, 2.7 P/C kit.
As Bob wrote, make sure the fuel tank is clean, and filters are changed. Full tune up should be done, oil and gas filters, filter screen in the bottom of tank, distributor cap, rotor, points, rocker clearance checked. Original fuel filter is NLA and later style needs to be adapted. Updating A 1973.5 911T Fuel Filter

Were the P&C's done when you owned the car, or by the first owner? Any information on the build, parts changed would help. Original heads with cast in injectors, or later intake runners with injector ports? What WUR is installed, does it have the Throttle Position Switch on the throttle body? Later cis eliminated the TPS and used a wur with vacuum connection. Were the cams changed? Original or later ignition distributor, fuel distributor. Pictures and or numbers on the parts will help. It's got to come off for repairs or replacement either way, might as well make sure the parts are correct.

Tank screen location. Pelican Parts - Product Information: 911-201-141-00-M100


Throttle Position Valve


WUR, original was 0 438 140 001, later types 008, 129.


Ignition Distributor, original was 0231 169 008, later 2.7 type 0 231 184 00x can be substituted.


Fuel Distributor

[timmy2]

Ed,
In the third post he claimed to have the stock 73.5 CIS on the engine.

[Sunroof]

The stock 1973.5 tank has only two external fittings versus three in the photo. The internal filter location is correct in the photo but on the early tank has a big hex nut that has to be removed to access the filter housing. No matter what the concensus is to attack the fuel system, which after eight years needs to be freshened anyways. I know I would start at that point after being idle for eight years!

Bob

[Noslo14z]

Firstly thank you all for the advice on this issue.

In regards to the whole clean fuel idea:

Before we had started the car originally, we did a couple of flushes of the fuel system. We put something like 4 gallons of fuel into the tank and we just kept the fuel running through the system until the fuel looked almost as good as the fuel we put into the car.

Also when we turn the ignition on, and we hear the fuel pump start whirling, it does change sound sometimes, as if the pickup is sucking air. Would this be yet another symptom of the dirty fuel tank? Also, I've dealt with these type of fuel drains on the bottom of tanks before and at times when they come out, they bugger up the surrounding material, making fitment of it almost impossible when trying to re-install. Anyone experienced this of the 911s?

[E Sully]

Noslo14z,
You won't hurt the tanks threads removing it. Fuel Tank Screen Allen Size?
The gas may be cleaner, but the change in pitch is probably due to the the filter and tank screen being clogged with debris.

Dennis,
I just want to verify it since it has had a change to 2.7 P&Cs. It may be original, but it seems to me that if changing to 2.7, some change should have been made to match intake to new capacity. Original 73.5 heads had the most restrictive ports and mild cams. I don't know when changes were made, but he is not the original owner, and it is possible for replacement parts may have been used at some time. Considering it has running issues, I just want to verify the setup before trying to get it running right.

Bob,
The tank is definitely original to the car. The filter screen is not in place since I used a plug to seal the opening when repainting. The third fitting on the right with a cork is capped in normal use. I have done a lot of research on the cis tank. Almost all 73.5 are like mine. Here is one from the Classified Forum with the cap installed.


There is one unique one that I came across that may be a different style. The owner had it is the 73 S he is restoring, and believed it to be a '73 cis tank due to swirl pot and full size spare.


This is a later cis from a 1975. Space saver, no third capped fitting.

[Sunroof]

If the pump is making noise it might be cavitating as it starves for fuel. Yep, something before the pump. I would not hesitate to change out or at least inspect that mesh filter. It can be cleaned and reused unless something has cemented itself to the screen. Flushing with the filter in-pace is not the cure! I will send a photo later today of a 1973.5 tank cut open exposing the swirl pot, fuel lines and baffle. You will see that the filter sits inside the pot which is not very big. This is where the debris gets trapped and gets sucked onto the filter. Removing the filter and then flushing is the best way before the filter is replaced. Drain the tank, remove the filter, flush the tank with gas (not more then a half gallon or so) and see what comes out. Rust? Clumps of dirt? Sludge? You might then want to remove the tank and get it cleaned out. Replacement tanks are at $900 - $1200 even though you can use a newer CIS tank. The only change is the fuel capacity versus a tire saver spare. The 73.5 tank is unique in that it can hold a full size spare tire but gives up on fuel capacity (something like a hadful of liters) Thats about it. Guts on the CIS tanks are the same.

You stated that you ran gallons of fuel through the system. Where did you let the gas discharge?

Bob

[Sunroof]

Okay, here are some photos of a 1973.5T CIS tank opened up. The swirl pot had its cover removed to expose the hole where the screen filter fits. Based on the angle and pot location, particulate would settle near the pot and get drawn in to the filter. The amount of particulate within the pot itself after sitting for eight years could be significant, especially internal rusting.

When I had my tank boiled at a radiator shop with a caustic solution it was recommended I use an internal coating called, "Red-Kote". Little did I know that Red Kote does not adhere for long on bakelite (swirlpot) and flakes of the coating came off slowly from the pot and impacted the filter, suddenly shutting me down on the road. So take a valuable lesson from me on this experience. Several tank refurbishment shops will slice the tank in half, sand blast and hand coat and reseal and coat the outside as if you had a new tank. The price less than a replacement tank. This is the ONLY way to go as far as I am concerned.

Keep at it.

Bob
1973.5T

[Noslo14z]

Bob,
Your thoughts match direction of where we are headed (based on excellent advice of you and other pelicanites). Thanks for the great photos of the 73.5 fuel tank cut-away. We can vaguely remember looking inside a 76 911 fuel tank of my late grandfathers, however we cannot remember what it looked like specifically (it was clean and free of corrosion and deposits from old fuel, so we moved on).

The fuel that we ran through flushing the system (back on the 73.5) was captured and recycled into the lawnmower. To answer where we let the fuel out of the system, we hose-clamped tubing where fuel-in-line enters engine bay (driver-side) before accumulator & filter so as to not run the old fuel through the fuel dizzy and the rest and any of the CIS system.

E Sully,
Thank you for the great pictures and comments on the fuel tank(s) and other components.

2.7 P&C done under first owner (we are 2nd owners). Done by Ralph at Meaney West (His comments on the invoice where: Make 2.7 CIS). Heads have cast in injector ports, with the small diameter steel intake runners. The invoice shows new replacement of 6 intake and 6 exhaust valves (wonder if reasoning for all valves being replaced was because the head valve ports where enlarged?), 6 injectors, 12 valve adjusters and nuts, inter. shaft bearing, main bearing, 6 rails, 2 chains, 1 set inter-shaft GS, 2 chain gears, 12 rockers and their shafts, 12 valve guides, and wrist pin bearings, oil pump, oil tank.

To confirm, when my father purchased the car many years ago, the car had already had all motor modifications done to the car and was running excellent with less than 20k after the 2.7 upgrade.

Part Numbers as asked for:
Ignition Distributor: 0 231 169 008 JFUD
Throttle Position Valve mounted on Throttle Body
WUR: 0 438 140 001
Fuel Distributor: EP/LKM 6/3 0438 100 004
Injectors: 150/0437502 & 180/???
Spark Plugs: W225T30 RO 245

Current plan of action:
Note: Engine starts, idles, and runs to around 3,500 rpm fine, by around 4,000rpm power goes down like low fuel pressure (or low fuel flow).
1) Will probably first remove fuel tank sender unit /cover to provide visual inspection of the inside of the tank.
2) Inspecting and cleaning in-tank mesh screen. Reason we didn't do this previously is because of comments related to potential damage to the fuel tank removing the allen plug in the bottom of the tank. Any thoughts/comments/tricks to not damage tank would be appreciated (have no problem fabricating 22mm tool for removal)?

[E Sully]

I would say you have a completly original CIS from those part numbers. It looks like the car had a nice overhaul with the 2.7 P&Cs.
It sounds like the initial problem would be the airbox leaks, maybe at the rubber boots to the runners also. All rubber fuel and vacuum lines should be checked and replaced if cracked or brittle.
Gas may be a problem, but the best way to check is remove the filter screen. It is fairly easy once the tank is drained, and you will not hurt the tank. Just a word of advice, buy a CIS fuel pressure test set. System and control pressures need to be adjusted properly.
That covers fuel and air, finally there is spark. You need a good clean spark at the right time. It sounds like the advance weights might be sticking a little. If you have pitting on the points, rotor, or cap then replace them. The points must also have the correct gap.