Still a miss at 4,000 RPM..., Backfiring at WOT, even with all new ignition components Options

[ThePaintedMan]

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?

[rhodyguy]

prob not.

[ChrisFoley]

Can anyone verify that the marks on the Bus fans are way different than ours? I advanced my timing by a good ten degrees or so on my way home and the car ran much better.

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.

[ThePaintedMan]

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?

[ChrisFoley]

...
Would it be possible for the fuel filter to be blocked, limiting fuel volume but my pressure reading still be good?

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

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?

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)

[ThePaintedMan]

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.

[r_towle]

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)

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

rich

[timothy_nd28]

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

[ThePaintedMan]

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?

[ThePaintedMan]

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. (IMG:style_emoticons/default/confused24.gif)

[carr914]

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

[ThePaintedMan]

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.

[vw505]

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

[ThePaintedMan]

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.

[r_towle]

Did you rebuild the engine?

[timothy_nd28]

Could plugged or restrictive head vents cause this problem?

[r_towle]

Could plugged or restrictive head vents cause this problem?

No.

[ThePaintedMan]

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.

[r_towle]

Hahaha

To late, you started them....


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

[ThePaintedMan]

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