Thanks for the offer Mike, but I don't think that would really have a big impact on a WOT problem. Much appreciated though!



Just thought of something - if I hook my GoPro up and post a video, would that help you guys better diagnose the issue?

prob not.

I can't answer your question but with the different versions of 914 fan I occasionally have to set timing without a visible mark for full advance.

All the fans are built the same even though the timing marks change.
That is to say, zero and full advance are always in the same location relative to the fins and one of the large ribs where the pulley bolts on.
Zero is about 4 1/2 fins past the large rib and 28 deg advance is just one fin past the large rib.
There are 4 large ribs and only one of them is anywhere near the correct timing marks.

Thanks Chris. Kev recommended I look into getting a quality adjustable timing light, which I agree with. I know that my "0" mark is correct, and aligns with TDC of #1, so getting an adjustable light would certainly eliminate any other concerns.

Next question... sorry, I know there are many....

John (FourBlades) had to pull the fuel tank out when the car was over with him to repair a pinhole leak. I wonder if some debris could have gotten in the lines since then and plugged the fuel filter. However, my FP gauge in the engine bay says 3 PSI, and is rock solid, whether I'm revving the car or not. Would it be possible for the fuel filter to be blocked, limiting fuel volume but my pressure reading still be good?

I'm also considering raising my floats on each carb up a little. Currently they're set to the Weber spec 32.5 mm. Would it hurt to bring them up a little?

No. If you have constant 3 psi, the flow rate is fine.
Unless the blockage is after the regulator.

The big number is where the float stops falling when the bowl empties.
Decreasing that dimension won't significantly change the carb operation.
If the float level goes that low in operation you have a fuel flow problem.
Decreasing the small dimension (10mm) will affect the fuel level in the float bowls.

You don't need the fancy timing light. Just set full advance at the last fin before the heavy rib. (3 1/2 fins from the zero timing mark)

Thanks for all of the replies. Since I've tried just about everything, I did some more reading and it appears that when a cam starts to really get worn, one of the symptoms is a lot of backfiring/popping out of the carbs. The last compression test I did put all of the cylinders around 110 PSI, which is further evidence that this engine just might be flat worn out. It wouldn't surprise me if it is, as I don't have any indication that this engine has ever been rebuilt.

At least, it makes sense to me.

Its about 2 inches away from the TDC mark on the fan.

rich

consider re-adjusting your valves to 0.004", it may help

Good call Tim, I might just try that.

Anyone know - Is there a way to measure valve lift on these engines using a dial indicator while the engine is still in the car?

Weird... went and drove it around the block after bumping the timing up about as far as it would go with the car still running. It still backfires at the same point, but I was able to "push" it past, and it then seemed to clear up and keep revving to 4,500.

Until it can rev past 6,000, it ain't fixed

Yeah... then I put it back on the ramps to go back to basics, mostly checking that I have the plug wires in the right order.

I guess you never had the heads off? I had a craked head once that would pop.

Reaffirmed that the SVDA was installed and timed correctly today. No difference.

I also adjusted the valve lash a little tighter than it was, which was probably closed to .008 than .006. REALLY quieted the engine down, but alas the backfire is still there, strong as ever.

Another thought I had - I guess it's possible this engine is a lot beefier than I might have ever anticipated and is just that thirsty on the top end. I've got new head-intake gaskets, manifold-carb gaskets and some smaller (read: richer) air correction jets on the way. I'm going to try one real rich mix for the top end before I give up on this engine, in case this is simply just an uber lean condition all along.

Still very strange though - each time I rev it up, sometimes the passenger carb is the one acting up, and then sometimes it's the driver's side now.

Did you rebuild the engine?

Could plugged or restrictive head vents cause this problem?

No.

Crap, I didn't want this to start getting spread between two different threads, so I'll try to keep correspondence here.

Just posted in the "SVDA" thread:

This is indeed an old motor. The numbers I have for compression indicate it *may* have some life left (110 PSI on all 4) but I'm not holding out hope. I have no idea if, or when it has been rebuilt, but certainly not by me. Everything I know makes me believe it is the original, with over 100,000 miles and probably not rebuilt - there were no rebuild tags or markings on the engine when I got the car.

Rich - I'll post a video sometime this weekend. I've got a GoPro, and as long as I keep the windows up, it should get pretty good sound.


Also, no head vents. This is a '73.

Hahaha

To late, you started them....


So, what are the carb specs again please.
Body, venturis, jets and e-tubes

Carb specs:

Weber 40 IDF (early models)
Idles: 50
Mains: 140
Primary venturis: 28mm
Accel pump jet: 50
Bowl exhaust vent: 55
Emulsion tube: F7 (or F11, didn't make much difference, but F7s improved transition)
Main A/C jet: 200

This is a 1.7 liter motor?

Have you check your linkage to make sure both carburetors have the same travel?
What you describing is what happens when one carburetor is opening more than the other. To check this measure the distance from the throttle shaft to where the ball joint is connected they should be the same.

I asked in the other thread, post a pic of the linkage, or a few

With those specs on a 1.7 you are not starved for fuel.

Ack! So many questions, which really makes me happy... I'm just having a hard time keeping up.

Yes, I agree that my jetting settings should be more than adequate for a 1.7, based off of Tomlinson's Weber Manual tables.... that is, assuming it actually is a 1.7 and not some rebuilt monster In my dreams, right?

The linkage is a Tangerine Racing cable setup, which I am really happy with. From every indication I have, it is synched... it's really hard NOT to have them synched with this setup actually. I need to re-synch again tomorrow since I adjusted the valves today, but before I did the valves today, they were synched both at idle and at 3,000 RPMs.

Yup, that setup stays synced.

Hmmm

Did someone rebuild that motor at some point?
Starting to wonder about the valve timing....

I dunno. I cannot recall ever really trying to push the car hard enough in the first year that I owned it and got it back on the road to even cause these symptoms to appear. It could be that it's been like this all along, but ever since I was getting it ready for the DE, the backfiring has been the bane of my existence.

Rich, here's something interesting I noted today. The valve lash adjustment screws on the passenger side of the engine look pretty well adjusted in. As in, there are only a few threads left on them to allow me to continue to tighten them up. The drivers side seems to have closer to 4-5 threads showing on most of them. Not sure what that indicates, if it indicates anything.

May I suggest a slightly different way of adjusting your valves.
Pin the opposite valve wide open, then adjust the valve directly opposite that wide open valve.

Then you know you are on the bottom of the lobe.

If your ignition is correct, you have no manifold leaks, your linkage is setup, then it's back to the valve that may be opening to early for the intake stroke.

I have never built a motor with the valve gear off by one tooth, but this may be the result.

Try this.
Go for a run once hot....take it down the street and come back towards the house in a low gear, as its backfiring.......run it like that for about 100 feet then shut it off.
Do not let off the gas.
Kill the motor at the higher rpm,s on the main circuit, not on the idle circuit.

Then pull the plugs and rad them

Alternate would be to pull the oil pump and see if you have a stock cam gear, but even that could be put in wrong...

now that it is pulling good until the higher rpm i'm going with worn out/or broken valve springs.

I think I mentioned this before but have you tried swapping out the cap, rotor and wires. It seems to me like something is arcing and higher rpm. With the carb setup you have, you should have no problem revving unless there is a spark problem or even a distributor problem.

Check your tire pressure.

Clean your windshield.

Move your seat all the way back.

Do your back-up lights work?

Check your heaterbox valves.

Change your underwear.

Try more brand in your diet.

Don't forget the blinker fluid Paul!

How are your Muffler Bearings?

leak down test. i think i mentioned this earlier. you should reconsider my "best offer". round trip would be about $32. running it to 6k with a fi cam (and old everything else in the valve train) is pointless unless you just want to make noise.

k

So here's what I did today. New manifold to head and manifold to carburetor gaskets. No difference. Tried richer air correction jets. No difference.

Here's video of the carbs (sorry it's so crappy - my GoPro wasn't charged). It's hard to see, or hear but you'll notice that the backfires are coming from the rear cylinders on each bank. This is reaffirming my belief that that lobe on the cam is flat/going flat.


https://www.youtube.com/watch?v=C7NnZ0cHyvs





https://www.youtube.com/watch?v=9OkSH-2iCGs



Kev, I'm still considering a leakdown test tomorrow, if it's not raining like it is today. If it is a flat lobe though, it wouldn't show up on a leakdown, correct? A leakdown can only help diagnose bad rings or valves, correct?

don't know what to tell ya. what the hell are the wires running down the drain funnel for?

If the camshaft was getting smaller, it would not open the valve...

It would not open the valve all the way.

It would not backfire.


Adjust those intake valve like I told you, and take your time.

Rich

Yes, it would. I don't know why, but my dad's car did exactly the same thing when his cam went flat. It would idle well, but almost any throttle would cause it to fall on it's face and backfire. But that was a V8.

Adjusted the valves per Rich's instructions, and everything was still in spec.

I worked until it was dark so I could try one thing. I pulled the plug wires off of first the #3 cylinder and then the #4 cylinder. Neither one made a difference in the idle, so the car essentially seems to be running on two cylinders. I checked both plug wire ends, and both were emitting good spark, so I know the plugs are at least getting spark.

Neither mixture screw is making much of a difference at idle now either. I cleaned both carbs well before they were installed, and I am at least getting a good squirt out of the accelerator pump jets, so I'm at a loss as to why the passenger bank isn't doing its part to keeping the engine running. It appears to be getting fuel.

If you really think that the spark is there then I would try adding fuel when at idle through the carb. OK I can't tell you how dangerous that is and how much I would tell you to only consider it as a last resort. But you sound like you may be close to that point. If the idle changes then I think you know there is a fuel problem in the idle circuit. That might be faster than swapping carbs from side to side.

Does nothing to solve your problem but it may point you in a direction. Another option to check the valves is to scope them through the spark plug hole. I just got a decent USB scope from Amazon for about $40 bucks. It seems to be ok nothing great but it can also be used to inspect your cam lobes through the push tubes. It was called the Supereyes 7. It has a 7mm head and can take pictures and record video to your Laptop. Might make you feel better about the cam if you can look at it. Just a thought.

Status?

Thanks for checking on me Rich. I went ahead and replaced the carb gaskets as well as the idle mixture o-rings, even though both were new, just to rule out the last variables.

The backfire is still there and as long as the engine is completely in synch, it seems like it's isolated to the #3 cylinder.

However, I did another compression test as well as a leakdown this weekend. Compression still shows around 105-110 on all 4 cylinders and the leakdown was phenomenal - I didn't see really any loss at all on any cylinder.

The only thing I can think of at this point is a weak valve spring, as this is a high RPM/high load problem. What do you guys think?

On that note, is it safe/recommended to replace one, or several valve springs on a older, even "tired" engine? I'd like to give it a shot at least before I pull the engine and install my spare.

4,000 RPM is not High RPM/Load

I still think you have a Bad Flux Capacitor!