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

[ThePaintedMan]

Well I cleaned and re-lubed my SVDA dizzy and went ahead and installed it with the Compufire setup. Though it didn't fix the issue, it does transition a little smoother and idles better.

Now the driver's side carb is acting up as well, which leads me further to believe that this is ignition related. I think that in the process of tuning the carbs and the transition to the warmer months, the plugs are probably fouled. I went ahead and ordered a set of new wires and NGKs since that was the cheapest/easiest/most logical next place to start.

Rich, that is interesting that you have documented different cams for the accelerator pumps. I'd like to add that information to my carb thread at some point.

John, I haven't really found a way to measure the jet volume with the carbs on the car, but now that I've rebuilt these things a few times, I plan to do that for every carb that I rebuild for others. I knew that the cams *could* be changed, but I would honestly recommend that people avoid it if at all possible. As you remember, it involves removing the butterfly plates, punching out that little roll pin and then driving the shaft out of the body. I think that process subjects the throttle shaft bores to a lot of trauma and getting the plates to align the exact same way they did is a nightmare. On the carbs I'm rebuilding for some of the guys here on the forum, I've let them know that unless there is a well-founded reason to remove the throttle shafts, they should always be left in. I think that Tomlinson recommends the same thing in his Weber book.


Anyhow, I'll let you guys know what happens. I'm hoping it's just the plugs/wires!

[yeahmag]

Weak or failing coil?

[ThePaintedMan]

Weak or failing coil?

Good call Aaron. That would be the next item to replace if this doesn't do the trick. At that point, with an all-new electrical system, I'll consider tearing into the freakin carbs again.

[stugray]

I think that process subjects the throttle shaft bores to a lot of trauma and getting the plates to align the exact same way they did is a nightmare.

I agree. Although the realignement of the plates is not that hard, just removing the original self locking screws that hold the plates in place can bend the shafts if you do not back them up properly. BTDT.

Stu

[ThePaintedMan]

I agree. Although the realignement of the plates is not that hard, just removing the original self locking screws that hold the plates in place can bend the shafts if you do not back them up properly. BTDT.

Stu

I forgot about that Stu. I've screwed them up before too.

The biggest problem with realigning the plates in my opinion, is like many "wear" components, once they've worn into their respective throttle bodies, they need to be reinstalled in the same one(s). Of course when I rebuilt my first carbs, I wasn't really paying attention to that. (IMG:style_emoticons/default/headbang.gif)

[ThePaintedMan]

Sorry to drag this back out, but I thought I would update on my progress, so if someone else ever has this issue it might help them shortcut and save all the time/money/hassle.

Purchased new plugs, wires and coil. Rather than try to isolate the issue, I was betting that replacing all of the ignition components at once would just get me back to better days faster. End result: nothing. Still a pretty good series of backfires at WOT at and above 4,000 RPM, now it's pretty much coming from both carbs.

So, as a last troubleshooting step and before I sink more money into it, I'll try to pull the carb tops off this weekend. I'm beginning to suspect that either all or at least one accelerator pump on each carb is not squirting sufficient volume. I'm back to having a large stumble when transitioning from idle to main circuits again as well, which leads me to believe they are the culprit. As of a few months ago, the car transitioned very well with little to no hesitation. It's possible some crap has begun to accumulate in the accel pump jets over time, I think.

The only other resort after that would be to take the carbs and manifolds off, install new gaskets and rebuild them both... again. (IMG:style_emoticons/default/headbang.gif)

[timothy_nd28]

Take that extra set of carbs you have laying around and slap them on as it. If you have the exact same problem, the problem lies elsewhere.

[yeahmag]

Have you verified with a timing gun that you aren't getting spark scatter at high RPMs?

[r_towle]

Weber pump part numbers

According to redline weber.

47407-207, adjustable shaft
47407-027 , early style.

http://www.webercarbsdirect.com/40_IDF_Rep..._Parts_s/56.htm

[ThePaintedMan]

Thanks Tim, I'm considering it.

Aaron, I timed it again last night, and although there is some scatter, I don't know what else could be done to fix it. I've tried 3 different distributors and installed the Compufire, along with new plugs, wires and coil. No difference.

Rich, I knew about the difference in diaphragms. I'm positive I have the correct ones (early style) installed.

[904svo]

Remove the Compfire and try it with stock ignition.

[rhodyguy]

(IMG:style_emoticons/default/agree.gif)

[ThePaintedMan]

It was doing it with the stock ignition, as I have stated previously in the thread. I tried several different distributors, all with points. Then installed the Compufire. Neither way made a difference.

[carr914]

Maybe it's time for one of these

http://www.msdignition.com/info.aspx?taxid=8&taxid2=425

[yeahmag]

The MSD won't help if it's not an ignition problem...

George, you don't have access to an LM-1 or some other wide band O2 do you? That would tell all...

[toolguy]

Have you tried a new distributor CAP. . arcing inside, maybe cracked. .
The most common carburetor problem is bad ignition. .

[r_towle]

Rich, I knew about the difference in diaphragms. I'm positive I have the correct ones (early style) installed.

I keep mentioning it and here is why.
There are MORE than two of these parts.

The key difference I have seen is the middle cam follower length is the longest on the old style (correct version) pump setup.

If you put in the shorter one (its not much shorter) you starve out at the top end.
Put one in that is too long (there is a long one that fits the old carbs) and you weap fuel all the time.

I have no idea how to know for sure which one is right except to replace them.
I got into this and ended up with about 5 different pumps before I had one that was correct for my carbs.

Every retailer swore what they sent was the right one, new part number, superceded part number blah blah blah.

I ended up asking a guy to send me one of each....
Tried them till one fit.
The outer rubber is the same, its the length of the middle portion that changes on every single one of them.


I also read earlier today that the air correction jets get clogged...but that would not account for a lean backfire like you are having.

Did you try the other carbs you have laying around?
I would go there at this point to get you focused on the issue....carbs or ignition.
rich

[ThePaintedMan]

Cap and rotor are brand new as well (on both types of distributors I tried.)


Rich,
I *think* I know what you mean now. Are you suggesting the CAM on the throttle shaft is not what differs from carb to carb, but instead the "button" length of the diaphragm can be different based on the kit purchased? As in, perhaps I'm getting a sufficient *shot* of fuel from initial throttle tip-in, but as it moves to WOT, the accel pump circuit actually closes, and it goes way lean on the very top end?

[r_towle]

Cap and rotor are brand new as well (on both types of distributors I tried.)


Rich,
I *think* I know what you mean now. Are you suggesting the CAM on the throttle shaft is not what differs from carb to carb, but instead the arm length extending from the accel pump housing does? As in, perhaps I'm getting a sufficient *shot* of fuel from initial throttle tip-in, but as it moves to WOT, the accel pump circuit actually closes, and it goes way lean on the very top end?

Yes sir....you got it.

Like I said...btdt...

YOu know who knows all this is Art Thraen...who is now retired and sold aircooled engineering to blackline racing.

So, here is BLR,s phone number...they still do the work and Art is helping in the transition, but not for long.

801 747-3342
Rich

[r_towle]

We just wanted to fill everyone in on what has been keeping us so busy over the past year and a half. We have been in negotiations with Art Thraen to purchase all of Air Cooled Engineering's equipment and assets. We reached an agreement and are now the owners and operators of Blackline Racing LLC. Justin has been working at ACE since 2008 and will continue to perform all of the engine and trans building, machining and service for Blackline Racing. Dave Hogarth has worked at ACE since 2005 and will still be performing all of the top notch carburetor work that ACE was known for. Colton will be leaving his current employment and running the business side of things to get work moving through better. We are currently working on a website that we hope to be up and functioning inside 3 months and we do have a facebook page if you would like to follow us on their.
This has been a dream of ours since we first started working together at our local VW shop 7 years ago. That dream only intensified when we saw what we could do with both of us working together on our Bonneville projects. We consider ourselves amazingly lucky to have this chance to make our obsessive hobby into our own business. We truly appreciate all the support everybody has given us; here in the forums, buying our lifters and inserts and of course out at the Salt.
We are very excited to start this new chapter in our story and are looking forward to what this means to the future of our Bonneville racing.

Thank you everyone,
Justin and Colton McAllister
Blackline Racing
801-747-3342