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 Idle vs Main Fuel Jets? Hot climate base? 
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Joined: Sun Dec 21, 2008 10:50 am
Posts: 96
Location: Atlanta
Post Idle vs Main Fuel Jets? Hot climate base?
Hello again,
I have been trying to figure out a low RPM (1000-1800) lack of power and I am still on dead end streets. I have 55 Idle jets, and will try the 60s just to see how things change. I was looking at the carb diagram on the manual, and can't understand how the fuel flow is different from the Idle Jets from the Main jets. From the diagram, it looks like both jets feed the same line. Anyone with knowledge on this carb design can help me understand the fuel / air flow right out of these jets?
Thanks,
JC

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Jerome
CRJ Driver


Last edited by boeing7873 on Fri Nov 24, 2017 12:46 pm, edited 1 time in total.



Fri Nov 03, 2017 11:53 pm
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Joined: Wed Apr 14, 2004 9:58 pm
Posts: 1401
Location: Idyllwild, CA
Post Re: Please can someone explain Idle vs Main Fuel Jets?
In most carbs, the "idle" ports are around the side of the bore just below the throttle plates; engine vacuum helps to pull fuel through them. The "Main" fuel ports on the other hand are above the throttle plates in the low vacuum portion of the carb venturi. Both of these control the amount of fuel by the size of an orifice (the "jets") and these are what you can change to get more fuel flow (larger orifice) or less fuel flow from a smaller orifice.

The Pinz carbs have a HUGE accelerator pump flow (compared to many carburetors) and this large fuel flow is necessary for good off-idle performance. You can tell if this is the issue by trying to get some power out of the engine without touching the throttle; obviously there is not much power available, but do you get any power with the throttle just cracked open (and held that way), cracked a bit more, and so on. The accelerator pump responds mostly to a relatively rapid opening of the throttle, what is needed under normal driving when starting from stopped. The linkage to the accelerator pump is such that the pump moves the most for the smallest throttle opening. The pump lever has three holes, the linkage needs to be in the innermost hole to get the maximum pump rod motion compared to the throttle motion. Sometimes the pump valves get stuck open or closed which affects how much fuel is injected; sometimes the pump piston goes bad (older ones are a leather cup, newer ones are synthetic), sometimes the pump flow is blocked by crap in the carb passageways. If it is an "off idle" lack of power (when you are just getting into the pedal and throttle) then chances are good that it is an accelerator pump issue.

It is also possible that your spark is weak, and introducing more fuel/air into the cylinders can prohibit a spark, but this shows up with great idle but poor higher rev performance, spitting, missing, etc. and doesn't sound like what you are describing. The 4500 rpm boxes are notorious for failing and cutting spark power at all revs; you can easily test this by unplugging the 6 pin connector from the box under the dash and making a jumper to connect the middle of the top row three pins to the middle of the bottom row three pins on the harness end of the connector. This jumper is what carries current to the coil, and what the rpm box "chokes off" (originally intended to do so only above 4500 rpm, but aging boxes can fail and do so at all speeds).

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Jim M.
712W and 710M


Sat Nov 04, 2017 5:34 pm
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Joined: Sun Dec 21, 2008 10:50 am
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Location: Atlanta
Post Re: Please can someone explain Idle vs Main Fuel Jets?
Hello Jim,
I have given up on my ignition (pertronix) since weird things happen to me and cant explain, I just spent $700 on the SSI electronics...

First, the engine does not run well at 7 deg advance, it does much better at about 12 or 14. So have to check my TDC again and make sure my pulley is well marked. Also, I just started to notice that at low RPMs, the electronic Tach meter does some sputtering in synch with the engine. If it was a mixture issue, why would the needle of this electronic instrument jump like crazy when the engine starts missing? I am seriously suspicious of that ignition now.

Having said that, back to the carb setting and jets... my understanding is that the idle fuel ports are below the butterfly plates, and those are metered by the idle screws. and like you described, the main jets are up in the carb venturi. What I am confused with is why are the four jets in the upper side of the NDIX carb apparently feeding the same fuel circuit based on the manual diagram. Or are they totally different circuits?

My accelerator pumps work great. Still, no power out of idle, have to pump...
Thank you for the comments.
Best,
Jeronimo


PS: Bumber suggested this reading:


http://www.aircooled.net/vw-carburetors-jetting-102/

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Jerome
CRJ Driver


Sat Nov 04, 2017 10:02 pm
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Joined: Wed Apr 14, 2004 9:58 pm
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Location: Idyllwild, CA
Post Re: Please can someone explain Idle vs Main Fuel Jets?
Quote:
First, the engine does not run well at 7 deg advance, it does much better at about 12 or 14.
Don't worry about it, mine runs best with a much higher advance, too. Remember that the spec engine is in military trim to make it the most bulletproof in all conditions with all fuels, 50 years ago, and not necessarily what we have or expect today.
Quote:
...started to notice that at low RPMs, the electronic Tach meter does some sputtering in synch with the engine. If it was a mixture issue, why would the needle of this electronic instrument jump like crazy when the engine starts missing?
Remember that the tach is getting pulses from the ignition and trying to convert the timing between them into an rpm that it expects to display with the needle. If the engine is "sputtering", meaning that it slows way down without firing, then catches, the rpm is suddenly much higher and this rapid and sudden change may be affecting the way the tach responds. Have you put a timing light on each plug just to see if they are getting a consistent spark regardless of the "sputtering", or do you seem to be missing some spark? The tach also expects pulses of a certain strength. You should expect to get 16-18 volts at the positive terminal of the coil (with the negative meter lead on good chassis ground and engine idling); any less than close to this range and I would suspect the 4500 rpm box. Of course most of this issue should go away with the SSI.

I haven't studied the carb manual and flow diagram, so I can't answer your question about fuel circuits. But as link says, make sure that everything else is right before messing with the carbs. Unless you have blocked passageways or stuck/leaky valves in the carbs (usually due to crap getting in there), air leaks from warped or loose carb sections, or too much/too little fuel from the pump, a well running Pinz will be happy with almost any jetting (within reason). I live at 5500 feet elevation and routinely go from sea level up as high as 11,000 feet without changing jets; however, since most of my driving is in the 2,000 to 7,000 feet range we did drop the main jets from 140s down to 130s to run just a bit leaner in the thinner air, but it wasn't a huge difference.

Check everything: carb bowl fuel level, choke (enrichment, actually) circuits are turned off with a warm engine, voltage at the coil, valve clearances, fouling of plugs, etc. and rule all of these problems out. If you still think it is carb related, send them to Jim L. and he will get them working right. I thought I knew alot about carbs, but he is the master.

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Jim M.
712W and 710M


Sun Nov 05, 2017 11:26 am
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Joined: Sun Dec 21, 2008 10:50 am
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Post How many holes in the carb base? Difference for hot climate.
Thank you for the reply. Good comments!
So enough of doubting about the ignition. Got the PinzSSI. No changes. Engine spits and backfires when cold, then gets a little better but not by much. I put the ignition at manual (should be 7 degrees, right?) and runs just like before when I used to put the pertronix at 7. It used to run much better at about 10. So I will advance the SSI and see what happens.

But still have the same problem, no power on transition from idle to about 1800 rpm. If I pump it, then runs ok, as the accelerator pumps are working fine.

Question: I notice my carb base has a total of 5 bores, one for the idle screw, the other four right out of the electro cutout valve. The manual indicates that this would be a hot climate base, as the regular should have a total of six bores. How many do yours have?? I am running 55 jets, the manual indicates 60 jets for the hot climate version.

Thanks!

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Jerome
CRJ Driver


Fri Nov 24, 2017 11:44 am
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Joined: Sun Dec 21, 2008 10:50 am
Posts: 96
Location: Atlanta
Post Re: Idle vs Main Fuel Jets? Hot climate base?
So today I replaced the 55 idle jets with 60 jets, o-ring types... and what a difference!!!
My high jets are 135s and I am thinking about getting 140s.
I think I may have totally solved the problem now.
It still needs about 2.25 turns of idle screws... less and will have a minor hesitation.
But again, could it be that the base I am running is for hot climate when I should have a standard one?
JC

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Jerome
CRJ Driver


Fri Nov 24, 2017 11:33 pm
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