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

[jmill]

That's a good linkage and wouldn't suspect that's your problem. Old worn out carbs aren't easy to troubleshoot. It would be better for your sanity to bite the bullet and purchase a new set.

We can make recommendations based on how the carb should perform but when other variables are in the mix (like worn out parts) it makes it very difficult to get it right.

Switching sides might narrow the problem down to the one carb. Actually telling what's wrong with an old broke down one will be hard to tell without holding it in my hands and seriously eyeballing it and maybe taking it apart. If they are old with worn out shafts I toss them over my shoulder and find better ones.

[type2man]

You might want to check the plug wires or cap for arcing at higher rpms. Those are old Webers you are running.

[messix]

check the dwell of the points, using gap only isn't always good enough.

and check to make sure that points and condenser wires are in good shape and connected tight.[ the connection under the cap]

and find or barrow a know good coil.

and ohm out your ignition wires with a volt ohm meter.

[rhodyguy]

george, i consider the carb to intake gaskets a ONE time use part. get new ones. they should be dirt cheap if you have a vw parts outlet nearby. when you tighten the carbs down, do it in an X pattern, like torquing down lug bolts, in a few stages. if you're using cut steel lock washers, don't. schnoor/belleville (cv bolt washers) washers work great. with the impression in the gasket it looks like your reefing down pretty hard. if you do one nut tight you may be cocking the carb and building in a leak.

[ThePaintedMan]

I agree with you Kevin, they are a one time use. Normally I wouldn't reuse them, but I'm stuck this weekend. Also, next time I won't crank down on them so much. I reinstalled the carbs last night and tried my best to check for leaks, but as far as I can tell, it must just be a leak from the inner portions of the throttle shaft. I have tightened them in the preferred "X" sequence as well (or as best you can with an even number of bolts).

I drove it to work today, as I'm leaving here for the DE. I did notice that once I got on the highway and it was warmed up, I could ease through 4,000 RPMs and it didn't do it. But whenever I was full throttle past 4,000 RPMS it would crack back through the throttle bodies like a SOB. (IMG:style_emoticons/default/confused24.gif)

[monkeyboy]

I would venture to guess that either the throttle shaft is leaking, or your accel pump needs to be looked at.

[ThePaintedMan]

I would venture to guess that either the throttle shaft is leaking, or your accel pump needs to be looked at.

That's a good thought. But I rebuilt both carbs a couple months ago with brand new diaphragms. I've also verified that both are pumping the same and sufficient quantities of fuel. So now I'm leaning toward a leaking throttle shaft. By now these carbs must be at least as old as the car - I got them off Ebay and they were previously installed on a 912. That tells me at least 30 years, probably more. (IMG:style_emoticons/default/wacko.gif)

[jmill]

With 55 idles, 140 mains and the F7 ET you shouldn't have any transition issues. You should actually be a bit fat. IMHO your mains are a bit too large. The 180 AC jets would enrichen the top end even more. If accel pump diaphragms are new and not wrinkled, check valves are good and fuel quantity squirt is the same on both carbs (actually measured) there's really only two things left. Float level is low or you have a decent sized vacuum leak.

One other thought, are the aux vents the same on both carbs? There's a few different ones.

[DBCooper]

That sounds like a lean condition alright. It would have to be a really bad throttle shaft to only show up over 4000 rpms, but it makes sense that it would show up when floored and a lot of vacuum in the engine. Try switching the carbs from side to side, find out for sure where you need to be looking. Easy enough to do the same thing with plugs and wires, while you're at it.

If you can't go over 4K you aren't going to have much fun at the DE. On the bright side when you're done with this that's going to be the best tuned 914 around.

EDIT: Do your Webers have the little screen fuel filter on the inlets? Is it clear? If you switch side-to-side take a peek while they're off.

[rhodyguy]

do your DE with a light foot. sounds like it runs rough enough that i'd be worried about breaking something. when you're back to investigating, close the bypass screws and see what you have for differing readings. should help to indicate the offending cyl. you may have to perform a leak down test. your carbs have the fittings for vacuum advance, yes? have fun this weekend.

k

[ThePaintedMan]

Well you guys were right on a number of accounts. Though I did have a blast finally getting the car on track and an excellent instructor in T.C. (Carr914) who was patient with me, not being able to rev past 4,000 RPMs was a major handicap. I had to short shift in a lot of areas where I would have left it in fourth or third. You can check my build thread in my signature for the videos that T.C. posted and in some occasions hear the backfiring (though by the afternoon I was shifting before 4,000 to avoid it).

Over the next week or so I'll start by pulling the carb off and do a better inspection of the float level and if that doesn't do it, I'll swap the carbs left to right.

At the track I did notice that throttle position definitely has an impact as well. At 4,000 RPMs if I were to ease into the throttle, I could mostly eliminate the pops. But whenever I hammered into it, it made it worse. Needless to say, T.C. was very understanding. Thanks for the continued help and input folks.

The good news is, it never really ran hot. Only once did I get the oil up to 220 and CHTs were below 350-360 most of the time.

Kevin, these didn't originally have the fittings for vacuum advance. But they've been drilled for them and all the ports are plugged.

[r_towle]

How far are you from TC or from URY?
Might help to have one of them (or both) over for a few beers to listen and poke around the engine bay with you...

rich

[ChrisFoley]

If you can partially mitigate the symptoms by engaging the pump jets, then one or more throats is going lean on the main fuel cirucut.

[messix]

Well you guys were right on a number of accounts. Though I did have a blast finally getting the car on track and an excellent instructor in T.C. (Carr914) who was patient with me, not being able to rev past 4,000 RPMs was a major handicap. I had to short shift in a lot of areas where I would have left it in fourth or third. You can check my build thread in my signature for the videos that T.C. posted and in some occasions hear the backfiring (though by the afternoon I was shifting before 4,000 to avoid it).

Over the next week or so I'll start by pulling the carb off and do a better inspection of the float level and if that doesn't do it, I'll swap the carbs left to right.

At the track I did notice that throttle position definitely has an impact as well. At 4,000 RPMs if I were to ease into the throttle, I could mostly eliminate the pops. But whenever I hammered into it, it made it worse. Needless to say, T.C. was very understanding. Thanks for the continued help and input folks.

The good news is, it never really ran hot. Only once did I get the oil up to 220 and CHTs were below 350-360 most of the time.

Kevin, these didn't originally have the fittings for vacuum advance. But they've been drilled for them and all the ports are plugged.

not being able to fire a dense charge in the combustion chamber is a primary sign of a weak ignition system..... try a new coil and check the wires!!!!!!!!

[jmill]

If you can partially mitigate the symptoms by engaging the pump jets, then one or more throats is going lean on the main fuel cirucut.

It actually sounds like the opposite Chris. He's going lean when he stomps on it like the pump jets aren't supplying enough fuel. Check your pump adjustment nuts. Might be too loose on the carb that pops.

[ThePaintedMan]

Again, thanks for the ongoing advice gents. I hammered it again tonight while on the highway, this time at 3,000 RPM. When stomping on it, immediately to full throttle, I get a lot of lean backfiring. Again, only from the passenger carb. To me, this doesn't necessarily indicate an ignition issue (though I suppose it could be). The driver side seems to do just fine.

Here is what I've done so far to recap:
Changed distributors (still a crappy 009, but that only seems to be a problem on the low end with the flat spot)
Installed a Pertronix/Compufire
Installed F7 emulsion tube to richen things up throughout the rev range
Installed 145 mains on the offending carb, even though the other is still running fine on 135s

Still sounds like some kind of vacuum leak to me, but what do I know. (IMG:style_emoticons/default/biggrin.gif) Also, given the fact that this problem persists even after the initial accel. pump jet shot, (i.e. WOT) I don't think it's accel jet related. This weekend I'll pull the plugs and do a compression test to get a better feel for what's going on. Possibly a sticky valve, I dunno. Again, I'll play with the float as well and even swap sides with the carbs. But for now I'm just going to park it and be happy I made it through the weekend without blowing it up (IMG:style_emoticons/default/piratenanner.gif)

John, these are the older style Webers without the ability to adjust the adjust the pump engagement. It's fixed.

[messix]

Again, thanks for the ongoing advice gents. I hammered it again tonight while on the highway, this time at 3,000 RPM. When stomping on it, immediately to full throttle, I get a lot of lean backfiring. Again, only from the passenger carb. To me, this doesn't necessarily indicate an ignition issue (though I suppose it could be). The driver side seems to do just fine.

Here is what I've done so far to recap:
Changed distributors (still a crappy 009, but that only seems to be a problem on the low end with the flat spot)
Installed a Pertronix/Compufire
Installed F7 emulsion tube to richen things up throughout the rev range
Installed 145 mains on the offending carb, even though the other is still running fine on 135s

Still sounds like some kind of vacuum leak to me, but what do I know. (IMG:style_emoticons/default/biggrin.gif) Also, given the fact that this problem persists even after the initial accel. pump jet shot, (i.e. WOT) I don't think it's accel jet related. This weekend I'll pull the plugs and do a compression test to get a better feel for what's going on. Possibly a sticky valve, I dunno. Again, I'll play with the float as well and even swap sides with the carbs. But for now I'm just going to park it and be happy I made it through the weekend without blowing it up (IMG:style_emoticons/default/piratenanner.gif)

John, these are the older style Webers without the ability to adjust the adjust the pump engagement. It's fixed.

cracked plug? have you swapped plugs side for side?

[rhodyguy]

when it's dark outside start the car and mist the the spark plug wires with a spray bottle and check for spark scatter. check the flanges on both ends of the intakes with a straight edge. when you run the comp test do a leak down as well. are you using phenolic (fi) intake gaskets? just guessing...weak valve sealing, spent valve guide? "misfiring during acceleration", 8 choises. #8 is interesting re jetting.

[r_towle]

question.

Can you simulate this with no load?
So, can you make it backfire above 3000 rpms while your standing next to the car?

Rich

[ThePaintedMan]

when it's dark outside start the car and mist the the spark plug wires with a spray bottle and check for spark scatter. check the flanges on both ends of the intakes with a straight edge. when you run the comp test do a leak down as well. are you using phenolic (fi) intake gaskets? just guessing...weak valve sealing, spent valve guide? "misfiring during acceleration", 8 choises. #8 is interesting re jetting.

I can check the plug wires for arcing, if that is what you're suggesting Kevin. However, they're brand new. I straight edged both sides of the intakes before installing them, but if I end up pulling them off, I'll do that again. Yes, phenolic gaskets between the intake and head.

question.

Can you simulate this with no load?
So, can you make it backfire above 3000 rpms while your standing next to the car?

Rich

Rich, yes, if I remember, it does this without load. If it only happens under load, that's usually electrical, correct? Unloaded would point to fuel delivery?