Dual Dellorto carb setup, the carbs tend to snap under load. After tinkering for a while with airflow and linkage and not resolving, I am going to go to basics and check fuel pressure. How do I measure this? Is there an inline guage I can buy?

confirm valve adj and timing first thomas. what pre-install setup proceedure did you use? air bypass screws partially open or closed? you'll need a good gauge installed prior to the carbs. what type of pump? facet, self regulated rotary? with the linkage disconnected, is the flow = side to side at idle? both sides snapping back up thru the carbs? at what rpm? specifics...

k

I am still using D-jet so my gage is different. I got a gage from McMaster Carr and some hose. For FI, a 60 PSI gage is best for measuring in mid range (29PSI). My gage is also filled with fluid so it reacts smoother. You can get gages in all sizes and ranges and from cheap to expensive.

Pump is rotary cbperformance. I'm letting her cool down right now to do a valve adjustment. Carbs were prebalanced by the rebuilder, and I am measuring the same air flow on each carb, at least at idle. Snapping happens under load, or when, after when having to slow down, you give it some gas to speed back up. I just cleaned the main jet stack, clean as a whitle they were.

BTW, fuel filter is clear and shows 1/2 full with fuel. Should I blead it air out of the fuel lines?

you have a return setup still, right?
you HAVE to add a regulator! carbs will want .5lb-3lb pressure depending on application. your probably pushing 7lb with that pump.

paul

and you've got a quality pump, so get a quality regulator too.

oh yea, and a gauge. mine's pretty lame compared to others trick setups... but it works.

Click to view attachment

the cb pump he's running does not have a return line feature. internally regulated at 3.5#. unless the pump is defective i doubt it is the problem. could be a vaccume leak, linkage is out of adjustment (if the flow is the same when attached that shouldn't be it unless when the shafts are off the idle speed screws the linkage falls to a natural position). if just one side is making the racket it can indicate which side is wrong...that's why i asked at what specific rpm it's occuring. could be at the transition circuit. if thomas looks down a throat, checks against his paper work and supplies the vent size that should be of some help eliminating potential culprits. i'd turn the engine speed up with the one idle screw and see what the side to side flow rate is.

Ok, I just finished adjusting the valves. Nothing was seriously out of adjustment.

Next, I went to Sears, and bought a cheap made-in-china fuel pressure guage. I'll be returning it, it's crap, but.... I am reading 3 to 3.25 psi before the carb Tee, in the engine bay. The guage is not that fine to read more precisely. It's a very new CB pump, by the way.

So I measure my flow again, it looks the same. I drive it, same crap, snaps under acceleration (right carb), even bogs a bit. Then, at idle, i thought I heard a snap...reved it up a little more by hand...let it idle again....snap (right carb)!.

So it's snaping occacionally even at no load at idle.

Your lean going into the transition.

About where is you adjustment screw on the threaded shaft for your accelerator pump?

Is it about in the middle of the threads? More towards the end? What????


Clayton
Clayton

More towards the end. About 1/4" of thread showing before the bolt. Oh wait a minute... the right side has less thread showing, hardly any... and that is the one that snaps. I guess they should be exactly the same, or is there a reason that they are not?

only if you want them to squirt the same amount

(YES)

I'm on it... but, is there a general rule as to how far they should be threaded in at?

You can get a small vial that can be lowered down into the throat and used to measure the enrichment jet flow quantity. Ideally, they should all be equal.

I've seen carbs set up where the enrichment jet squirted with each bounce and bump of the engine, resulting in a erratic mixture.

Try about 1/3rd from top of threads.

If no worky......

Then 1/2 from top of threads... Somewhere in there is TYPICALLY what works.

C

Well, I tried both of those settings, and several in between. I drove it at each setting and I still seem to get that snap under load. I am starting to get really frustrated, so I am coming to the board for help. Help!

Questions:

1. Is it the more you thread the nut down the threaded shaft, the more squirt your accelerator pump will provide, or the other way around?

2. What does it mean when, under load, you hear a sound from the carbs that kinda sounds like gargling?

Thomas,
My Dells had a pretty good vacuum leak at the base of the intake. Jake said that it was pretty common since the intakes are poorly cast. I had to sand mine with a belt sander to get the bottom nice and flat then seal it with some sealer to the intake side of the phenolic.
As a side note, I had my carbs rebuilt by Art Thrane and they still needed to be synched. I think the synch is kind of specific to the motor isn't it?

I actually watched the accelerator squirt into each car. One carb sprays the whole time as the throttle is opened, but the other carb cuts out at about 1/2 travel. Its like the acc. pump gets stuck or something. Should I unscrew the 4 screws that hold it on?

Get ready to catch some fuel when you undo those four. Perhaps a small tear/leak in the acc pump diaphragm...

All sorts of funky crap collects in those acc pump chambers since they are the lowest portion of your carb, it is possible that one side is clogged up pretty good. The acc pump circuit drillings can get clogged also. If there is crud in the acc pump circuit, there can be build up on the seat for the little steel ball that keeps pressure in the circuit therefore bleeding off pressure and only allowing a partial squirt. On the side that has a lesser squirt, if you pump that side multiple times, does the squirt get better? If so, my guess is that those little balls aren't seating.

Last thing, are your pump jets all the same (measured with a jet gauge)?

I believe the spec is .2ccs per two full strokes. I use a trimmed down baby medicine dispenser found at CVS to measure (and WD-40 instead of gas if the carb is out of the car).

Well, I took the carbs off, and made sure the accelerator pumps were working, and adjusted them so the accelerator pump would be functional through the range of the carb's operation. Reinstalled the carbs, and put a this sealer on all four carb manifold gaskets, top and bottom. Get it running and balanced again, and same thing: the right side carb almost always backfires under acceleration. Occasionally is will backfire under idle. I am very frustrated. To recap, I have done:

Valve adjustment
Timing adjustment
Fuel pump pressure measured
Main and idle jet removal and cleaning
Balanced with synchrometer
Varied idle mixture from 2 to 3 turns

What have I not done? Time to send these away I think.

Did you clean out the acc pump circuits?
Did you verify that the acc pump check balls are seating properly?
Did you verify that the acc jets are the same size from side to side?
Did you verify that the acc jets are not partially clogged?
Did you measure the pump volume and try to equalize it from side to side?

You may consider getting new pump diaphrams and springs for both sides to rule them out.

-Chris

Sounds like you are too lean. Are "idle" jets perfectly clean or maybe too small?
Edit: is there an air screw that you have turned out too much letting in too much air? And if your selling them, how much?
I just thought of something. Have you checked compression? Maybe you have a broken valve spring or something. Sounds like the intake charge is coming right back through the carbs...