Correct fuel pressure for dual carbs? Options

[terrymason]

I'm having alittle trouble getting my dual carbs dialed in, the 1st is rich, and the second is lean. My fuel pump was putting out 3.5psi, but the guy I bought my carbs from said I should run 2.6psi instead. What do you guys think?

I've got 60 idle jets and 180 mains, which I thought was overkill, until he told me that I should run 2.6 instead of 3.5 psi.

[john rogers]

I ran 3.5 PSI with my four cylinder engines and about 4 PSI with the six cylinder. Before you do anything, get a flow meter to see if flow in each barrel is the same. Adjust the carbs so that they are all 4 the same flow. If one side is rich and the other is lean with the same jetting then it could be float levels as raising the float can make the mixture rich and visa-versa. Raising the fuel pressure is the same, makes it richer, etc.

[So.Cal.914]

3.0 lbs.

[lotus_65]

2.5 lbs

[SGB]

60s are mighty big. How big is the motor?

for mains, 125s are about right for my 2.o, but I run 50 idle jets, and 180 air jets.

[type11969]

The self-regulated pumps sold by VW shops for dual carb applications are set to 3.5psi.

Have you checked and set the float heights? That can make a big difference.

[rhodyguy]

any other symptoms? snapping/popping back thru the carbs as you open the throttle plates? back fire on decel? my first thought is you've got the droplinks misadjusted if the timing and valve adjments are correct. one droplink longer than the other creating a condition where one carb is 'ahead' of the other. the one "ahead' might be set correctly and the other carb is running lean. don't use the idle speed adj screws to try and correct this condition. it won't work. tearing your carbs apart is the last step when all else fails. get out the cb weber tech book and what ever syncro tool you have, go to the pre dial in settings (the cb book is pretty clear on how this is done), with 1 (leave the other alone for now) idle speed adj screw turn the idle up to about 2k and measure the flow on the front venturis on both carbs. if the flow indicated is equal, go to the troubleshooting guide in the book. the conditions and cures are very well discribed.

k

[terrymason]

I'm running 60 idle jets, 140 main jets, and 175 air correctors. This is a stock 1.8L with dual dell 40s that I've been fighting for a while.

All 4 barrels are flowing the same amount (around 7 on my unigauge meter).

type11969 - I just went through my dellorto book again, and didn't see any mention of setting the floats, and I have no idea how to go about that. Is it a simple process?

[terrymason]

any other symptoms? snapping/popping back thru the carbs as you open the throttle plates?

-Yep

back fire on decel?

- No, but my wbo2 gauge reads very lean on decel. My sensor is near the exhaust tip though, so it could be fresh air being sucked in.

I'm using the Dellorto superperformance book, although it sounds like the ideas are all the same.

The passenger (1) side carb seems fine, maybe alittle rich, and the driver side (2) seems lean, and backfires out the carbs. It sounds like you may be on to something here. I'll go back to my book.

[rhodyguy]

sometimes a bit more info is helpful. no mention of the type of pump, type of fuel presure regulator (stand alone or incorperated in the pump itself) and whether you're using a good fp gauge. how does the engine run when you're off the idle circuit? how the carbs behave at certain engine speeds is important. are the plugs for one carb drastically dif colors? are you measuring the flow at idle 850-900rpm?

k

[type11969]

If you have the dellorto superperformace tech book, it is definitely in there, I don't have the book in front of me though so I can't quote the page. I believe it is after the troubleshooting section for the DRLAs. There is a small diagram indicating the height needed at open/closed positions.

It is easy, potentially time consuming though to get dead on. All you have to do is pull the carb top, measure the float position at the extents of its travel against the bottom of the carb top, and set the position to be 5-6mm from the bottom of the carb top in the float highest position, and 12-13mm in the lowest. You change the highest position by bending the forklike tabs that engage the needle valve, and you change the lowest position by bending the tab that contacts the part of the carb top that retains the float (opposite side of the float from the forklike tabs). I pull off the carb top gasket to get an accurate reading. Be careful that the spring loaded pin in the needle valve is not compressed at all during the stroke of the float. This will close the valve prematurely. Make sure it is the forklike tabs that close the valve.

If one float is set to high and the other set too low, one carb will certainly be richer than the other due to the different fuel levels in the bowl (and may also cause fuel to dribble out of the bowl down the carb throat).

I've been fighting with my dells for the past few weeks too, so I feel your pain. I think I solved my biggest issue by realizing that the needle valve were getting closed before I thought because I was compressing that spring loaded pin. The symptom was a high rpm leanout like I was running out of fuel. The way the floats were set, basically I was.


-Chris

[terrymason]

sometimes a bit more info is helpful. no mention of the type of pump, type of fuel presure regulator (stand alone or incorperated in the pump itself) and whether you're using a good fp gauge. how does the engine run when you're off the idle circuit? how the carbs behave at certain engine speeds is important. are the plugs for one carb drastically dif colors? are you measuring the flow at idle 850-900rpm?

k

I don't know what brand pump, but the regulator and 2 inline fuel gauges are sceptre brand that I got from pep boys. I've stopped using the fuel regulator (cranked it up to 5, it's max), and both pressure gauges (one before each carb) read a steady 3.5 out of the pump.

Having said that, I'm getting a new pump and jets (may not need the jets) from CB performance as soon as the west coast wakes up.

I have only measured fuel pressure at idle, something for me to do when I go home.

Car seems to idle fine, but when I goose the gas, I get a backfire out of the 2nd carb.

I've only pulled the plugs on the 2nd carb (backfiring one), and they look ok. I'll pull all the plugs tonight.


Thanks everyone, it sounds like I've got a good list of things to try when I get home. specifically:

1. measure the flow around 2K rpm - verify that it's 3.5
2. lengthen the linkage on my lean carb to verify that their isn't a situation where one is leading the other.
3. pull and compare all plugs

Once I get my new pump in the mail, I'll throw that it. If that doesn't fix things, I'll open my carb up and adjust the float level.

[rhodyguy]

the only reason i suggest turning the idle up is to steady the needle on your syncro tool. the pulsing at reg idle can make the neelde flucutate a bit. i prefer a steady reading. if the front 2 throats are flowing the same, move on to the next step. there's lots to the equation...potencial vac leaks at the intake to heads all the way to the base plates for the linkage. you'll like the self regulated rotary pump from cb. start with a set of 50s for the idle jets. cb should have them. also, ask if they have the little o-rings for the idle jet holders. now would be a good time to replace them when you do the jet change. get 8 of them, i think they are the same as the ones for the idle/air mixture needle screws. piece of mind.

k

ps they're open right now

[TravisNeff]

does your pump rattle a bunch? That would be an indicator of a facet fuel pump (not the best). Looks like a square box with a fuel fitting on either side.

[type11969]

Check to make sure your idles are clear, also verify that your pump jets are flowing and that the volumes are correct. Either of these can cause backfiring/hesitation.

Spend the time to set your floats anyway, the more you can get your carbs dialed in to each other, the better they will run!

-Chris

[terrymason]

I talked to Aaron from CB performance, who was awesome. He asked some really good questions, and I think has put me on the right track. A couple of things that came from it.

1. covering the barrel of the first carb will kill the engine - covering the barrel of the second carb does nothing.
2. no amount of adjustments seem to do anything to the second carb.

It really sounds like I have a air leak in my second carb. When I go home tonight, I'm going to spray carb cleaner around and see if I can find it. That would pretty much explain alot of what I'm seeing.

[rhodyguy]

"no ammount of adjustments"...are you refering to the idle air mixture needle screws? if so, plugged jet or issues with the circuit. you're getting close. closing off both throats (one at a time) on the suspect carb has no effect?

k

[type11969]

Plugged idle jet or circuit

[terrymason]

"no ammount of adjustments"...are you refering to the idle air mixture needle screw? if so, plugged jet or issues with the circuit. you're getting close. closing off both throats (one at a time) on the suspect carb has no effect?

k

The mixture screws seem to have no effect on the 2nd carb (the speed control does). On the first carb, closing just the first throat (at idle) will kill the engine. On the second carb, it has no effect. I haven't tried closing both throats, as I wasn't thinking to test that, I just happened to notice it.

Luckily, I get off work at 4 today. I'm pretty excited check this out!

[terrymason]

does your pump rattle a bunch? That would be an indicator of a facet fuel pump (not the best). Looks like a square box with a fuel fitting on either side.

It is pretty loud. You can hear it at a stop light with the engin on. I'm going to order this:
http://cbperformance.com/catalog.asp?ProductID=577
unless anyone has a better one.