Hello all,
Tonight I'll be buttoning up the car for the first DE this weekend (if it passes tech this time), and I've got one last thing I'd like to track down before hitting the track. A few times I've ventured into the 4,000 RPM range and I get a lot of backfiring, primarily from the passenger side carb, I think. Otherwise it runs great at all of the other RPMs. Right now it's got a crappy 009 distributor with points, but seems to work well enough so I've decided to stay with it to keep the headaches down.

Would it be logical to assume that the high RPM misfire is because of spark scatter with the points setup? I ask because I have a Pertronix unit ready to go on a spare distributor that I'd like to try, but with only a few days until the DE, I'm not going to go poking and prodding unless there is a good chance this could fix the issue. Otherwise I'll just short-shift


Whatcha think?

You've only ventured to 4000 rpm "a few times"? You'll spend a lot of time above that at the DE, so how does it behave past 4000? Get better, or worse? Two things, point float (or a small gap, or bad points), a weak coil or condenser on the ignition side, or plugged primary jets or a low float on the carburetor side. I'd bet on the carburetor since you say it's from one side, and it's the easiest to check/fix. so do that first. And you should put that Pertronix in already, it's doing no good in the box. It will clean up any points problem. Do that, see where you are. Get crackin, then go slay some V8's!!!

EDIT. I'm assuming you've adjusted the valves and they're good, carbs are well synced and no carb/manifold/head gasket leaks?

Yeah... what can I say.... I'm a wuss. I'm always afraid I'm going to blow something up, so I rarely venture beyond 4,000. Guess it's time to take the skirt off

Everytime I've gone up that high and I hear those symptoms I've backed off, or shifted so I'm not sure what happens beyond it. Once I get the string alignment done, I'll go piss off the neighbors tonight to see if I can replicate it.

The low float makes sense, but I've set both to spec a few months ago when rebuilding them and they haven't come off since. It is possible I screwed one, or even both up though.

I'll check my points gap and/or install the Pertronix if I have time tonight. If not, maybe tomorrow after work. If that doesn't do it, I'll play with the carbs, but everything should be all set now that I've got the jetting figured out. Valves were just set as well and the carbs are synched, but those are a couple things I should check before this weekend.

DB, what is your name by the way? I've never caught it...

Well, my ex-wife seemed pretty sure it was Dumb Bastard. That's not right, from my personal point of view, but it kind of stuck, so DB it is.

What fuel pump do you have? Could also be you don't have adequate fuel pressure (volume). You'll notice it after a hard pull. Float level drops because pump can't keep up and you run lean.

Carb plumbed farthest from the pump will lean out first.

I'd look at stepping up on the main jet size if you have good fuel volume and it's not ignition related.

I have had issue with rev-limiting rotors over 4K, spitting and sputtering, not sure if your using one..........

As far as I know, I've got plenty of fuel volume and pressure. This is a carter rotary style and I've got a quality inline regulator and gauge set to 3.5 PSI. No way to tell what's happening with the pressure when I'm driving, but when it's revved in the driveway I don't see any FP fluctuations. Even if that were the case, the passenger side carb is the one closer to the T (which is the one that I suspect is giving me issues). Would make much more sense for this to be a jetting/fuel issue than ignition if it's only one carb giving me trouble.

Think I might actually start with the mains. I've got 135s in there now and I have a set of 140s that I could try. Its possible my synch is off just enough at the higher revs and that I'm already borderline lean, so that at those RPMs it goes lean enough to start poppin.

No rev-limiting rotor on this distributor. Like I said, it might be easier to start with the jets since I know my way around carbs much more than ignition stuff. Eventually I'll probably look for a L-Jet dizzy and throw points on it just to cut down on the spark scatter if nothing else.

Thanks for the help and ideas!

First off, you ARE a wuss.....at a de, redline is where you play.

Get a timing light and see what it says when you rev it to above 3500.

You may need to set the timing differently

Might be an ignition issue but if not and your looking for enrichment just towards the top end you can drop your AC jet size down below 2.00.

Well I threw everything I had at it tonight. Did it in steps - first bumped up to 140 mains from 135. Then F7 emulsion jets, which I had been meaning to install for awhile to try to improve the transition from idle to main.

Then I sucked it up and installed the Compufire. I tested in between with some bursts around the block and no difference. However, I did get it up to 4,800 once when it didn't backfire as much.

Ran out of time as I have to drop it off at the shop tomorrow before work to get it aligned. But when I get it back I'll play with the synch again. It's the only thing I can think of that would cause one bank to act up and not the other.

I may try those A/C jets though. I've been told 180s can really improve the top end over the stock 200s.

O.k. , just my 2 cents worth... lean everywhere, increase the size of the main jets. Lean on top , decrease the size of the air correction jets. If you really think it's one side over the other and they are synch'd , ( open as well as idle ) then make sure you don't have any vacuum leaks. Carb base gasket, intake manifold gskt ? Extra air in means lean on top ! Spray a little carb cleaner around the carb base at idle and see if anything changes. If it does, fix the leak. Good Luck !

Heck, I rev my street engine past 4K routinely - before upshifting, and once in a while in 5th as well.
I take it right to redline on a regular basis in the lower gears.
On the track you should be revving to 52-5500 rpm before every upshift.

If I didn't rev my little 1.7 past 4K, I wouldn't make it out of the driveway.

Have fun and good luck at the Track

Guess I've been babying her too much. I think it's my lack of funds and ability to rebuild an engine that causes me to be so careful with it. But if you guys say go for it, I don't need to be told twice

The car is at the shop getting aligned now. When I get it home, hopefully tonight, I'll play around with the synch some more and triple check for leaks. Chris, I have a quick question for you regarding your cable linkage. I'll send you a PM.

back firing on accel or decel? exhaust only, not out of the top of a venturi? what about stability at 4k cruise mode?

Acceleration and cruising, but for whatever reason its right at 4,000 RPM and up. Definitely coming from the top of the carb, which is what I've always called "backfiring" as opposed to what many call backfiring - burbles from the exhaust (which is actually fuel-overrun).

Hi guys,
Running out of time, but at least I think this is a simple problem. "Old dog" had it right. Vacuum leak at the base of the carb. Gaskets are all new, but check the pictures below. Any of you familiar with Weber IDFs know if this passage is supposed to be open? Are the gaskets supposed to cover up these holes? I've noticed it before, but figured they were supposed to be open to the manifold. I can't quite tell if they are compressed all the way, or if there is really a vacuum in this location. You can see the indentations in the gasket below.

Leaving for the DE tomorrow, so I'm hoping to get it fixed tonight. If the leak isn't coming from here, I can't figure out where it would be coming from. Nothing else stood out to me.

On that note, can anyone think of a quick fit that would get me through the weekend? I think Permatext grey is gasoline resistant.

Gasket is correct. Hole is correct. A vacuum leak would show up at idle and not 4K. I'd also take a close look at your linkage. If it's wonky you'll get pops. Show us a picture of your linkage.

John,
Its pretty dark here, so pics would be difficult. But it's a Tangerine Racing cable setup that has worked great so far. Carbs were still synched, both at idle and at 3,000 RPM after checking with my snail gauge.

I though that port was correct on the bottom, but could never tell if a PO had messed with them. The reason I'm suspecting a leak between intake and carb is via the old carb cleaner trick. Sprayed the base of the carb with cleaner and the idle dropped noticably. I'm going to look at other areas of the carb and inspect for potential leaks, possibly the manifold vacuum tubes (these are an older set of carbs that actually have two brass tubes protruding from the base of the carb for manifold vac.) It's possible that the throttle shaft is leaking as well, since these are very, very old. One carb has already been re-bushed, but I can't remember which one.

easy test

Switch carbs side to side and see if the popping follows the carb.

Still might want to consider checking the timing at full advance...
the 009 is not curved properly for our motors and it may affect your performance at full advance..
It may also not be delivering full advance.

rich

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.

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

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.

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.

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.

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.

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.

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.

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

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.

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

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.

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!!!!!!!!

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.

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. 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

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?

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.

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

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.




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?

A cheat for the manifold to head gaskets is to either use a non-hardening sealant or some axle grease. I've had mine trued up on a mill and still used the sealer. Hylomar Blue is what I've been using for a few years now.

No.
You can't get the carbs onto the main circuit without any load on the engine.

Which may bring you back to ignition.
you are certain you have the firing order correct?
1432, clockwise.

Dont take this the wrong way....most of us have chased our tails and found it was that simple.
The car will run if you flip the wires...just not to well.

Wires also tend to arc alot at higher RPM's

Dark at night, run the motor...get it up to 3500 rpms.
Simple test is to touch everything bare handed.
touch each wire, run your hands down it from one end to the other.
Touch all around the distributor cap.

One shock will sting, but its harmless.

If you get frustrated, flip the carbs from one side to the other.
If it follows the carb, you know where to look for the problem.

Float levels, air bypass screws, cracked covers...its all possible to let in air leaks.

If it was me, I would touch the whole ignition system.
If nothing happens, flip the carbs, see if it follows you.
If it stays on the same side of the car, change both plugs and wires.
If is still stays, pop the valve cover off and rest the valves LOOSER on that side of the car.

If you are getting a rapid fire machine gun like backfire at the higher rpm,s its problably ignition related.

If you are getting random (no beat) back fires at the higher rpms, it may just be a vacuum leak you missed.



Rich

I wouldn't go exposing your self to 40k of volts! it's not that I will kill you but that it can if you have the right circumstances kinda like Russian roulette!

I would run it up in rpm and grab a wire off the cap with some insulated pliers and see which wire has less affect on how the engine runs.

have you switched to electronic ignition yet?

I could be a bad cap or rotor... even if they are new.


when was you last valve adjustment?

Played around some more last night. I can still somewhat replicate the backfiring even when there is no load on the engine (i.e. in the driveway). It doesn't backfire as hard or as repetitively, but revving open throttle at 3,500-4,000 does produce some backfiring.

I did not witness any arcing last night, either through touch or visually. Went ahead and cleaned the cap and rotor, with no difference. I did not try the "pull the plug" test, but I can try that tonight. But to answer your question Rich, it is indeed much more of a repetitive "machine-gun" like backfiring. Which is the only thing making me think it's ignition - otherwise, all signs point to fuel delivery.

My other thought is to ask Tim (one of the guys I'm rebuilding a set of carbs for) is if he'll let me rebuild them and then swap them out with these carbs just to try to rule them out.

Yes, it has the Pertronix installed now. It didn't make a difference unfortunately.

While I had it running, I went ahead and put the car up on ramps so it could cool down so that I can re-check the valves tonight as well.

Checked the valves, everything was within spec.

Rechecked the wires tonight when it was much darker, and I could actually see an occasional arc from the coil wire and #3. It appears that where the wires clip into the holder on the tin has cause some very slight damage to the casing. I could produce some momentary arcing from that spot on each wire, which sucks because they were brand new 2 months ago. Took the wires out and taped them with some e-tape just to see if that fixed the problem. Went around the block and hammered it. No dice.

I'm going to order new carb to manifold gaskets, FI manifold to head gaskets, new wires and plugs. I'll try that next week and see what happens. If that doesn't do it, I'll see if Timothy will let me install the carbs I'm rebuilding for him as a last resort to see if it's really the carbs on this car giving me headaches.

Use some light rtv gasket between the phenolic spacer and the head, and between the spacer and the manifold.

Rich

So, you just replaced the wires, not too long ago.
Still have the old wires kicking around?

Rich

My SWAG is still the accelerator pumps. I'm not real familiar with the real old ones but did your rebuild kit come with new cams? The roller wears the cam out and you won't get the right volume. It's been quite a few years but I recall they came with different sized diaphragms and cams. Did you install the larger diaphragm and cam?

Just bombing around I see this fairly complete kit has three different sized diaphragms. No cams though.