Dual 1bbl Carbs Occaisional Sputter On Rough Roads - Fuel Slosh?

[Kizbo]

Forgive me if this has already been discussed before, I'm sure it's likely been brought up...as my search revealed other posts that have touched the tip of the topic anyways, but really hasn't been talked about in detail from what I can tell or currently find. I'm sure I'm not the first person to encounter this issue...I know that even though I run a set of carbs that are a little less common (dual Solex 34 pdsit), there are guys out there that run dual weber 34s and kadrons that are similar in some respects. Anyways, I thought I could at least bring up my experiences in an effort to further "perfect" my ride and lend some help to future searchers on the topic.

Before the story, here are my engine specs;

Engine - 1641
Cam - engle 110
Carburation - dual Solex 34 pdsit carbs (28 vent(s), 145 mains, 130 air, & 50 idles) - Carbs balanced and manifold is connected by tube
Heads - Stock dual port, stock valves (no porting that I'm aware of)
Ignition/Distributor - 009 static timed at 8 deg & flame thrower coil
Fuel - Stock fuel pump

Yesterday marked the 2nd journey out and about riding in my sand rail since putting it back together. My 1st journey, although enjoyable and for the most part successful, revealed some tuning issues and some sputtering on rougher terrain. Let me just interject here and say that where I ride is for the most part gravel, dirt and sand roads...and when the season is right, a Mississippi river sand bar. With that said, most of the terrain that I ride isn't as horrible as say a rocky mountain trail, but it has its share of potholes. One road in particular is unavoidable and is horrible! (and is where the majority of said sputter is mostly taking place)....it's nothing but reclaimed asphalt (huge chunks I might add) used to make a gravel road - Bump city!

After the 1st outing, I thought I might need to check the fuel pressure as I had read that Solex carbs don't like much above 2 psi. As I figured, the fuel pressure was up past 4 psi on the stock pump. So I installed a fuel pressure regulator and gauge and then regulated it down to 2 psi (as far as I could get it). This seemed to make a big difference on its own to the quality of the ride. It definitely felt as though it remedied any potential loading up that was previously going on.

So, when out on the 2nd outing this weekend, everything was running well (maybe except for the slightest hesitation at lower rpm - which is probably due to the cam). However, when I get to the road-from-hell, it wants to sputter like crazy unless I take it at a snails pace. I would like to hopefully remedy this or at least make it a little bit better if anyone had any ideas to share.

So here's my question(s);

1) Is this sputter more than likely a flooding condition (fuel slosh in the bowl), or is it possibly a form of cavitation?

2) Is there anything that has/can be done to help this (i.e. bowl vent extensions, lowering float)?

I know that traditionally a centermount is the way to go for offroad, but I'm hoping that I could at least clean-up this phenomenon a bit if possible. I have a couple other stockish engines I'll put a center mount on in the future, but for now I'd like to make it work...plus, I'd like to work towards licensing this baby for the road this winter, and I know that the current set-up will seemingly work for that nicely.

Any thoughts or suggestions are always great!...If not for my benefit anyways, it could help the guy doing the search on it tomorrow. Thanks.

[I Ride Sand]

[dustymojave]

Yup! Bowl vent extensions.

Center pivot" for offroad is purely a Holley 4 bbl carb deal. Has NO bearing on other carbs used on VWs, (and a Holley 4bbl is extremely rare on a VW).
_________________
Richard
Offroading VW based cars since 1965
Tech Inspection 1963 - 2012 SCCA/SCORE/HDRA/MORE/MDR +
Retired from building Bajas, Fiberglass Buggies and Rails in the Mojave Desert. Also Sprints & Midgets, Dry Lakes, Road Race cars. All types New and Vintage
SoCalBajas Member
Kicked Cancer's A$$...1st and 2nd round...Fight ain't over yet.

[Kizbo]

[quote="dustymojave"]Yup! Bowl vent extensions.

Ok, awesome! Sounds like a potential fix exist. So now let me ask...is this just as simple as attaching a fuel line for some length or is there a better way or art to making it work? Thanks

[dustymojave]

Been a little while since I looked down the throat of those carbs. Refresh our memories with a pic or 2.
_________________
Richard
Offroading VW based cars since 1965
Tech Inspection 1963 - 2012 SCCA/SCORE/HDRA/MORE/MDR +
Retired from building Bajas, Fiberglass Buggies and Rails in the Mojave Desert. Also Sprints & Midgets, Dry Lakes, Road Race cars. All types New and Vintage
SoCalBajas Member
Kicked Cancer's A$$...1st and 2nd round...Fight ain't over yet.

[Kizbo]

Ok so here's a quick look down into the carbs. I was in a hurry and forgot to get the dimension of the brass vent, but nevertheless here's a view of it. This is the dirtiest these carbs / buggy has been to this point - we've had a very unusually wet & muddy summer around here and just haven't had time to washer and clean her up.

I will say this too.... that little brass circle/plug with the hole in it just above the outlet pipe (vent jet maybe? - unsure of the terminology), I have noticed that it stays a little moist with fuel. Not sure if that maters or if it's significant by any means.

[pullstart]

Is there enough of the vent sticking out to even get a fuel line on there? If so, I bet one of those small spring clamps for lawn tractor fuel line might do the trick holding it in place...

[Vanapplebomb]

PDSIT carbs with 4in of 1/4in fuel line. I replaced the brass pieces. Pulled them out, drilled out the 6mm hole in the top casting to .250in, pressed in a new 1/4in brass tube with a flared end, and slipped the fuel hose over the flaired end.

_________________
1800 Type 4 Berrien 295

http://www.thesamba.com/vw/forum/viewtopic.php?t=487021

[Kizbo]

This is some good info, and thanks for the pics. Yeah, those darn vent tubes really don't have much extension into the carb barrel at all do they...looks like a modification is due. I compared the length of the vent tubes from these carbs with a 34 pict, and unless my perception is somewhat jaded (with respect to the top of float bowl to top of vent) it would appear that the 34 pict extends much further. I'm surprised that fuel would slosh that much to come out of the vent tube....but I guess much like when running with a coke in hand, it doesn't take much for it to spill all over you either.

I'll be sure to post any pics of modifications to the vents.

34 PICT

Now the 34 PDSIT

[dustymojave]

The replacement brass tube needs to be a tight interference fit in the carb top.

It must not be a slip-in fit. If it is, it will slip out and fall down the throat, probably jamming the throttle wide open and then sucking it into a cylinder and doing damage there. It's amazing how much damage small brass pieces can do to aluminum pistons and heads in a moment.
_________________
Richard
Offroading VW based cars since 1965
Tech Inspection 1963 - 2012 SCCA/SCORE/HDRA/MORE/MDR +
Retired from building Bajas, Fiberglass Buggies and Rails in the Mojave Desert. Also Sprints & Midgets, Dry Lakes, Road Race cars. All types New and Vintage
SoCalBajas Member
Kicked Cancer's A$$...1st and 2nd round...Fight ain't over yet.

[andk5591]

I dont know those carbs, but doesnt a 145 main seem pretty damn big? I know on Kads, would run a 132 or 135 on that engine.
_________________
D-Dubya Manx clone - 63 Short pan,1914.
Rosie 65 bug - My mostly stock daily driver.
Woodie 69 VW woodie (Hot VWs 7/12).
"John's car" 64 VW woodie - The first ever
Maxine 61 Cal-look bug - Cindy's daily driver.
Max - 73 standard Beetle hearse project - For sale
66 bug project - Real patina & Suby conversion
There's more, but not keeping them...

[Vanapplebomb]

Richard brings up a good point. You do need to make sure it is a tight fit. After pressing mine in, I also peened over the tube in the float bowl just for an extra measure of safety. Just took a flat chisel and gave the inside edge of the tube a couple good pops. To get the new tube back off you would probably have to drill it out. Stucker than stuck.

Worth doing though. The extra length and flared end helps keep the fuel hose nice and secure.
_________________
1800 Type 4 Berrien 295

http://www.thesamba.com/vw/forum/viewtopic.php?t=487021

[Kizbo]

[Dark Earth]

[Vanapplebomb]

Factory in '72 had 137.5s on 26mm vents, so 145 on a 28mm vent is probably very close.
_________________
1800 Type 4 Berrien 295

http://www.thesamba.com/vw/forum/viewtopic.php?t=487021

[Kizbo]

[Vanapplebomb]

Phenolic spacers insulate the carb so they don't get too hot. Not sure what that has to do with your fuel pulling issue though.
_________________
1800 Type 4 Berrien 295

http://www.thesamba.com/vw/forum/viewtopic.php?t=487021

[dustymojave]

Fuel puddling on the butterfly is what the idle shut-off solenoid on the 34 PICT-3 carb (golden canister at lower left of pic) is for. This puddling wastes fuel, can result in a fire, and can cause dieseling on shutdown.

I don't know if one could be adapted to the carbs you have though.
_________________
Richard
Offroading VW based cars since 1965
Tech Inspection 1963 - 2012 SCCA/SCORE/HDRA/MORE/MDR +
Retired from building Bajas, Fiberglass Buggies and Rails in the Mojave Desert. Also Sprints & Midgets, Dry Lakes, Road Race cars. All types New and Vintage
SoCalBajas Member
Kicked Cancer's A$$...1st and 2nd round...Fight ain't over yet.

[Vanapplebomb]

...except that is for the air/fuel bypass below the throttle plate...but you are right about it cutting fuel to prevent run on. Still not sure what is up with fuel poolling on the throttle plate...

The only PDSIT carb to use the air bypass solenoid was the drivers side carb on dual carbed busses...and some dual carbed Opels. It cut air/fuel to the central idle circuit.

The type 3 PDSIT carbs and bus PDSIT carbs all had a pilot jet cut off on each carb to cut fuel supply to the progression drilling a in the throttle body...I still doubt this is your issue though. It shouldn't be pooling on top the throttle plate even though some drilling a are above. Vacuum on the lowest drilling should keep anything from comming out the upper drillings...You could replace the pilot jet with one of the pilot cut off solenoids to cut fuel at shut down and see if that is the issue. Most were 55s, but there were some 52.5s as well. They are probably close enough for this application. I don't think you will notice much of a difference at idle.

How much fuel is pooling?
_________________
1800 Type 4 Berrien 295

http://www.thesamba.com/vw/forum/viewtopic.php?t=487021

[Kizbo]

You may be absolutely right in thinking that the puddling is not directly related to the carb's heat. My original thought was that it "might" be related due to a couple of internet searches that described the Weber 34s having a similar problem that was resolved with a phenolic spacer....that and some information that claimed that the PDSIT carbs came from the factory (mostly) with a spacer to reduce a boiling effect out of the vent, which I guess wouldn't be an issue when I do the venting upgrade providing that it's actually happening in the first place.

As far as how much it's puddling... it's really not too much, although I do notice a little bit more in the drivers side carb as opposed to the passanger side. It's hard to describe the volume of fuel without a picture, but maybe it's somewhere around level with the top of the throttle plate shaft. I only noticed this when I had the air cleaners off to sync the carbs....during shut-down, when the engine was hot (as well as 95 deg outdoors), I noticed a number of drips out of what the Germans call the "austrittsarm" (not labled below) that dumps the fuel from the main jet into the venturi. I call it the "outlet arm" as I'm unsure of the terminology.

What I thought "might" be happening was; since this outlet arm is at a lower level than the vent tube (as shown below), maybe the heat was causing any residual fuel in the arm/tube to boil and expand and as a result drip out of there before it could puke out of the vent tube? This however may not be the case at all and I (admittedly) probably lack a little understanding on some of the finer points of this carburetor. Thanks for the continued info.