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  View original topic: Aircooled 2.0 backfiring only under load
Westy Steve Tue Sep 16, 2008 7:38 pm

Alright, this is really kicking my ass. My Dad and I have been tirelessly working on his Vanagon. We've got the compression where it needs to be now. We think we've got all the vacuum leaks found. He currently has twin carbs on it and we went through the balancing routine (we think) successfully. It idles like a new car and when you rev it up it sounds really great. But when its under a load and you rev it up, it backfires.

Does anyone have any freaking clue what this could be? We're at our wits end with this.

Item #2:
Lastly, we're thinking of converting it over to a Weber Progressive even though we do believe we've got the twin carbs dialed in. Anyway, I have what i think is the correct intake manifold for it that was inside a parts vanagon. However, the parts vanagon was supposed to be fuel injected, so I'm wondering if it is some kind of intake manifold for a fuel injection system. Never seen what kind of manifold those have, or even if they have a manifold at all. Anyone got a photo of the manifold for a intake manifold? (i.e., like where the throttle body bolts on?) or does anyone have a picture of a intake manifold appropriate for the Weber Progressive?

Thanks for any help on either issue.

Steve

Lanval Tue Sep 16, 2008 10:02 pm

What year is that...? I'm assuming 77-79 baywindow, must be federal? Any reason not to go to a fuel injection setup on the 2.0?

Best,

Lanval

Randy in Maine Wed Sep 17, 2008 4:16 am

Sell the car before you go to a weber progressive. They be junk.

I am not sure why the FI was removed but you are paying the price now. When you buy carbs for this application, it is really a time and money saver to get them set up for your application or you can spend a fair amount of time gettting it correctly set up.

It sounds to me like you do not have the dual carbs dialed in correctly. I would suggest spending some time with an exhaust gas analyzer (like an Innovate LM1) to get the mixture correctly jetted and the carbs syncronized.

Before you do any of that though, make sure the ignition is 100% and set up correctly.

Tell us what sort of set up you have and how you are timed.

Westy Steve Wed Sep 17, 2008 5:53 am

Randy in Maine wrote: Sell the car before you go to a weber progressive. They be junk.

I am not sure why the FI was removed but you are paying the price now. When you buy carbs for this application, it is really a time and money saver to get them set up for your application or you can spend a fair amount of time gettting it correctly set up.

It sounds to me like you do not have the dual carbs dialed in correctly. I would suggest spending some time with an exhaust gas analyzer (like an Innovate LM1) to get the mixture correctly jetted and the carbs syncronized.

Before you do any of that though, make sure the ignition is 100% and set up correctly.

Tell us what sort of set up you have and how you are timed.

It is an early '82 Vanagon. My Avatar is *my* bus, but the sickly Vanagon is my Dads. Its currently set up on twin Webers with the PSDIT2 and PSDIT3 carbs. We set the valves correctly, and we're timed to 7 degrees BTDC. We're planning on timing it at 3000 or 3500 RPM when my Dad gets back from a trip later this week but I can tell you that we observed the advance and it seemed correct in the past. Its a mechanical advance distributor. Has an electric fuel pump.

Going back to Fuel injection at this point is not an option in the short run. Maybe in the long run, but parts availability in this area would keep this Vanagon off the road too long. Also, my personal bus is a 2.0 liter with a Weber Progressive and quite honestly, it runs very well and gives me good gas mileage (20+ mpg), so its difficult for us to consider going through the time and trouble of putting in a complete FI system (plus the down-time) for maybe 1 or 2 mpg. Florida is flat. We are so frickin' close to unlocking this mystery. My Dad's electronics will be fixed this weekend and interior is being worked on the following week. This bus is ready for the road except for solving this mystery.

Anyway, we did have a little bit of trouble balancing the carbs with the unisync, and we feel that the butterfly might be opening a tad early on the driver's side "master" carb to keep the unisync readings the same. Like I say...it idles great and revs up responsibly, but when you put it under a load, it stumbles and backfires.

Some of the issues we've considered is low fuel pressure, but there doesn't seem to be a time lag from when the engine stumbles. We will be checking the advance at High RPM, but it seems like the distributor is advancing based on past investigations and like I say...it revs up fine...which I think would not happen if it was not advancing right.

We're stumped

Steve

Randy in Maine Wed Sep 17, 2008 6:41 am

When a carb "barks" under load it is usually due to a running lean condition. Those carbs were initially set up for 1700-1800 engines and may need some jetting changes to get them to work well.

If it were me, I would send a PM off to "mharney" and ask his advice. He is the best out there IMO.

mightyart Wed Sep 17, 2008 7:33 am

If those are used Webers you are going to have trouble syncing them.
If they are used and not matched you are really going to have a time.
A weber progressive kit will cost about as much getting the FI together.
I've delt with alot of old and new Webers, I've also messed with a quite a few FI systems and I'd only consider a Weber in a Dune buggy or the like.
Never on the Air Cooled Vanagon.
To easy to drop a seat if the mix is off, easy to have the mix off on a worn Weber

mightyart Thu Sep 18, 2008 5:57 am

Westy Steve wrote:
Item #2:
Lastly, we're thinking of converting it over to a Weber Progressive even though we do believe we've got the twin carbs dialed in. Anyway, I have what i think is the correct intake manifold for it that was inside a parts vanagon. However, the parts vanagon was supposed to be fuel injected, so I'm wondering if it is some kind of intake manifold for a fuel injection system. Never seen what kind of manifold those have, or even if they have a manifold at all. Anyone got a photo of the manifold for a intake manifold? (i.e., like where the throttle body bolts on?) or does anyone have a picture of a intake manifold appropriate for the Weber Progressive?

Weber Progressive:


Fuel injection:

Westy Steve Thu Sep 18, 2008 11:19 am

mightyart wrote: If those are used Webers you are going to have trouble syncing them.
If they are used and not matched you are really going to have a time.
A weber progressive kit will cost about as much getting the FI together.
I've delt with alot of old and new Webers, I've also messed with a quite a few FI systems and I'd only consider a Weber in a Dune buggy or the like.
Never on the Air Cooled Vanagon.
To easy to drop a seat if the mix is off, easy to have the mix off on a worn Weber

Thanks for the info and the pictures. They clarified what kind of intake manifold we have. Dad's out of town, but when he gets back, we'll be exploring the theory that his mechanical advance distributor is sticking...but only under load due to torque. Ever hear of that?

If the problem is the carbs, I think we'll end up punting on the twin carbs, but we now have an intake manifold, and a used Weber Progressive. We're gonna try the progressive (at least in the short run), and if we ever spot a FI system locally, we'll consider robbing it....if for nothing else than the educational aspect.

Steve



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