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

[ThePaintedMan]

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

Finally. That sounds logical and lucid to me! Sorry it took so long to get it through my thick skull (IMG:style_emoticons/default/headbang.gif)

I should have called Art a long time ago, but I knew he had sold the shop, so I wasn't sure if he was around to answer questions.

I might consider giving him a ring. In the meantime, I do have some extra of those diaphragms around from other cannibalized rebuild kits. I'll give them a shot when I get home. Next will be to try Tim's carbs. (IMG:style_emoticons/default/beerchug.gif)

[timothy_nd28]

Next will be to try Tim's carbs. (IMG:style_emoticons/default/beerchug.gif)

It's a ten minute job, with instant results (IMG:style_emoticons/default/popcorn[1].gif)

[ThePaintedMan]

Yeah, yeah Tim, I know (IMG:style_emoticons/default/smile.gif) This is more of a learning experience for me. Slapping your carbs on would be cheating! (IMG:style_emoticons/default/beerchug.gif)


So, I've pulled off one of the accelerator pump covers, praying that somehow I had installed the later style diaphragm designed for the adjustable lever rod, but I did not. As I mentioned, it's definitely the earlier style diaphragm, which, as Rich suggested, may come in a range of sizes. Getting the right one from the right supplier is probably a crapshoot. I do have two new diaphragms, which I'll install anyway, but below I've posted a picture of something interesting.




I've known that these style accelerator pumps have two different positions for awhile, ostensibly to provide more/less volume depending on application. The question is, which one is which? Do any of you have the fulcrum pin in the #2 position in the picture above? Tim's carbs have it in #1, which is the only reason I'm hesitant to try to move the pin on my carbs.

[ThePaintedMan]

Yeah, yeah Tim, I know (IMG:style_emoticons/default/smile.gif) This is more of a learning experience for me. Slapping your carbs on would be cheating! (IMG:style_emoticons/default/beerchug.gif)


So, I've pulled off one of the accelerator pump covers, praying that somehow I had installed the later style diaphragm designed for the adjustable lever rod, but I did not. As I mentioned, it's definitely the earlier style diaphragm, which, as Rich suggested, may come in a range of sizes. Getting the right one from the right supplier is probably a crapshoot. I do have two new diaphragms, which I'll install anyway, but below I've posted a picture of something interesting.




I've known that these style accelerator pumps have two different positions for awhile, ostensibly to provide more/less volume depending on application. The question is, which one is which? Do any of you have the fulcrum pin in the #2 position in the picture above? Tim's carbs have it in #1, which is the only reason I'm hesitant to try to move the pin on my carbs.

[ThePaintedMan]

The only other things I want to take a look at when I pull the carbs apart again are the check valves. Now, I know that the bowl "exhaust" valve is located at the bottom of the bowl (#13 in the diagram below) and can sometimes get stuck, or wear out as it is a one-way check valve, so I'm considering replacing it on both carbs. However, there seems to be some confusion on the Samba that the "pump demand valves" (#10 on the diagram below) are also check valves. I have pulled enough of these apart now, that I have never seen a ball valve in them or any indication that there is a way for them to clog. Can anyone verify this?

http://www.webercarbsdirect.com/v/vspfiles...ics/40IDF70.pdf

[jmill]

If I recall correctly there is a ball check in #10. That's why they call it a valve. They clog or freeze closed. Then all the fuel or most goes to only one barrel. If there wasn't a check valve you'd suck air not fuel into your pump diaphragm.

They clogged up on this guy http://www.914world.com/bbs2/index.php?sho...carb&st=100


post 208

[r_towle]

position 2 "would" make the lever depress the pump even farther....
Hmmmmm.

I will go out in a few minutes and take a alook.
I got mine working...so mine must be in the right spot...

I never saw that...but now that I have, I "must" try it.

Honestly, I was going to replace the whole thing with the adjustable setup, which I like better.

Rich

[ThePaintedMan]

position 2 "would" make the lever depress the pump even farther....
Hmmmmm.

I will go out in a few minutes and take a alook.
I got mine working...so mine must be in the right spot...

I never saw that...but now that I have, I "must" try it.

Honestly, I was going to replace the whole thing with the adjustable setup, which I like better.

Rich

John, I'll check mine this afternoon when I start pulling them apart. But I've never noticed/seen the ball valve in them. Thanks for the information.


Rich,
If you could take a look at that, it would be a great help for me. I do believe the longer lever length from changing the fulcrum point might help.

I do have one of the adjustable pump covers and all the hardware included, but I've never been able to track down another for the other carb. I'm sure WeberCarbsDirect or someone sells the whole assembly, but it's probably exorbitant. On top of that though, I think that the cam that rides on the throttle shaft is different as well, and there is no way I'm going to pull both throttle shafts again just to change the cams out. Major PITA.

[yeahmag]

Can you remind me if this happens at steady throttle state? If it's just the accelerator pumps not working correctly you should be able to slowly accelerate and the problem not be apparent.

[ThePaintedMan]

Can you remind me if this happens at steady throttle state? If it's just the accelerator pumps not working correctly you should be able to slowly accelerate and the problem not be apparent.

No, Aaron, steady state is fine. Initially I thought that it was only happening at 4,000 RPMs or above, but what I discovered is that I can replicate it when I hammer the throttle above 3,000 RPMs. If I use partial throttle, I can actually get above 4,000 RPMs but as soon as I apply additional throttle, it backfires again.

To boot, there is a significant stumble now as well transitioning from no throttle to partial throttle. I had this problem before with small main jets, but solved it with larger 135 mains. It has slowly crept back though, and even 140 and 145 mains don't seem to help.

My goal is to pull the carb tops off, check the condition of the valves in the fuel bowls, clean/ream the accelerator pump jets and also probably move the pin down on the accelerator pump arm fulcrum.

[ThePaintedMan]

Okay, this is hard to explain... but I think I found the problem. When I had to pull the throttle shafts when I first rebuilt the carbs, I didn't know a whole lot about the ins and outs of these carbs.

The throttle shafts are drilled with an offset hole along the X axis for the accelerator pump cam. I believe I mistakenly swapped the throttle shafts, and the left one became the right and vice-versa. In other words, there appears to be a "left" and a "right" as you cannot get the accelerator pump cam holes to line up with the holes in the throttle shaft correctly if installed wrong. I assumed the PO had buggered them up and they simply wouldn't align so that I could get the roll pin to drive straight through them. Instead, it was probably my fault (IMG:style_emoticons/default/headbang.gif)

This is an oversight that's easy to do, and I'm pretty sure it's whats causing the accelerator pump squirts to be mis-timed.

Will report back tomorrow if I can get them buttoned up and installed.

[carr914]

(IMG:style_emoticons/default/unsure.gif) (IMG:style_emoticons/default/wacko.gif) (IMG:style_emoticons/default/drunk.gif)

[jmill]

That would do it.

[r_towle]

Hope that does it.

[ThePaintedMan]

...And it didn't do it (IMG:style_emoticons/default/smile.gif)


I dinked around with the carbs all Friday night and finally decided to go ahead and eliminate variables. I installed Tim's carbs, which were in much better shape as far as the throttle shafts and butterflies were concerned. They are the same, earlier type 40's with the cam driven accelerator pump. Just got it all bolted back together and took it around the block. Still a lean backfire at WOT, which can be replicated in the driveway or under load. (IMG:style_emoticons/default/headbang.gif)

The only thing I have not tried, was to move the fulcrum pin on the accel pump down. This should provide more volume, especially at WOT.... at least, I think....

[carr914]

George, try changing out the Fuel Pump. When you couldn't get the car started here at my house, I was thinking it was starving for fuel, like it was in constant vapor lock

[rhodyguy]

is the fuel pump elevation = or below the tank outlet?

[timothy_nd28]

You happen to have another set of carbs laying around? Third time's the charm (IMG:style_emoticons/default/idea.gif)

[ThePaintedMan]

T.C. and Kev, the fuel pump is brand new and it is mounted on the back firewall over by the heater tube inlet for the long (which is slightly below the fuel line level). It also has a quality fuel filter inline before the pump. There is also a quality regulator on top, mounted to the trunk firewall set at 3 PSI and it splits into the two feedlines for the carbs just after that.

Tim, those carbs are awesome man. I rejetted them with the jets (50 idles and 140 mains) that I had in my carbs, and slapped them on. The throttle shafts are in great shape. I think you've found a customer (IMG:style_emoticons/default/smile.gif) I'll PM you about buying them from you. I'll probably just put these other ones on Ebay (listing their flaws of course) and be done with them.


Additional note though - when I visited Len in Tampa before stopping by T.C.'s, he mentioned that on his Triumph he had dealt with the timing marks being off on the harmonic balancer. Obviously we don't have to deal with that, but I'm starting to suspect that my timing marks on my fan are way wrong. 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. No real stumble anymore, and it revs much better. Still a backfire at WOT, but it doesn't seem as noticeable. Should I keep going and add some more timing advance to see if it continues to improve?

[Mblizzard]

George I had to go to 75s on the idle jets before all of my problems went away. I have not gone through all of the information here so you may have already done this. I have a set of 70s that I can send you if you want to try them out for a cost of a stamp.

Again you may have done that already so I make not be offering any help here.