Only fires on 2&4 at idle

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Booky
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Only fires on 2&4 at idle

Post by Booky » Sun May 27, 2012 7:43 pm

I have a 1776 with dual kadron carborators with a digital CB performance distributor. It fires on all cylinders at idle if you pull the spark plug wires on one and three at idle it does not change the RPM's at all. I took it to a dyno and it produced 89 horse power at the wheels. Can anyone assist in figuring out why at idle it does not fire? I spray carb cleaner all around the carb and intake and it did not change anything.
Thank you
Booky

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sideshow
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Re: Only fires on 2&4 at idle

Post by sideshow » Sun May 27, 2012 8:29 pm

The term you are looking for is "charge stealing" and is often found with dual single throat carburetors. It has to do with low speed wonky air-flow and is not considered an issue, just a feature.
Yeah some may call it overkill, but you can't have too much overkill.

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Piledriver
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Re: Only fires on 2&4 at idle

Post by Piledriver » Mon May 28, 2012 12:35 am

The factory dual carbs avoided the charge stealing issue by having a seperate idle air and fuel circuit feeding a large crossover pipe.
The engine basically would idle fully (and smoothly) on the "third" carb, built onto the (from distant memory) back of the 3/4 side carb, at least on the T4 versions.

You can waste a lot of time rejetting, if you make it fat enough at idle you MIGHT get all 4 to fire, but you will probably load up the leading pair of cylinders plugs and get really horrible gas mileage at best.

Just accept that it idles on 2 and runs great everywhere else ;-)

The possible cures are worse (or far more expensive) than the problem, which hurts nothing.
(IDFs, IDAs or ITBs/EFI etc)
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Re: Only fires on 2&4 at idle

Post by Bruce2 » Mon May 28, 2012 2:54 am

Piledriver wrote: Just accept that it idles on 2 and runs great everywhere else
Don't give up so easily.
I had a friend with Kads and her engine did that. Around here, we can't just ignore it, because we have to pass the tail pipe smog test.
The first thing I did was to increase the balance tube from ¼NPT fittings with 3/8" hose up to 3/8" NPT fittings with ½" hose. Then, because the manifolds have provisions for a fitting on both sides, I added a second balance tube.
Before I tested this, I took the manifolds and did some major surgery. With my long shaft carbide cutting bit in my die grinder, I cut out the wall separating the two runners. I did this all the way down to within about 3/4" from the bottom. This causes a massive increase in the plenum volume.
Once it was all on, the engine idled incredibly smoothly on ALL FOUR!
Then it passed our smog test.

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Piledriver
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Re: Only fires on 2&4 at idle

Post by Piledriver » Mon May 28, 2012 3:24 am

Cool. :mrgreen:
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Booky
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Re: Only fires on 2&4 at idle

Post by Booky » Sat Jun 09, 2012 7:36 am

Thank you for the info.

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Re: Only fires on 2&4 at idle

Post by Buggsy » Mon Jul 30, 2012 7:58 pm

Hi Booky,
Wondered if you tried this and if it worked for you also.

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Re: Only fires on 2&4 at idle

Post by Booky » Tue Jul 31, 2012 6:16 pm

Not yet I am concerned with cutting out the middle.

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david58
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Re: Only fires on 2&4 at idle

Post by david58 » Tue Jul 31, 2012 7:14 pm

Bruce2 wrote:
Piledriver wrote: Just accept that it idles on 2 and runs great everywhere else
Don't give up so easily.
I had a friend with Kads and her engine did that. Around here, we can't just ignore it, because we have to pass the tail pipe smog test.
The first thing I did was to increase the balance tube from ¼NPT fittings with 3/8" hose up to 3/8" NPT fittings with ½" hose. Then, because the manifolds have provisions for a fitting on both sides, I added a second balance tube.
Before I tested this, I took the manifolds and did some major surgery. With my long shaft carbide cutting bit in my die grinder, I cut out the wall separating the two runners. I did this all the way down to within about 3/4" from the bottom. This causes a massive increase in the plenum volume.
Once it was all on, the engine idled incredibly smoothly on ALL FOUR!
Then it passed our smog test.
Anymore details or pics?
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Re: Only fires on 2&4 at idle

Post by Bruce2 » Tue Jul 31, 2012 7:41 pm

I didn't take pics. The car's being driven.
What else do you need to know?

Booky, hack out the dividing wall! It works!

Booky
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Re: Only fires on 2&4 at idle

Post by Booky » Thu Aug 30, 2012 12:16 pm

I think I am ready to cut it out how wide did you make the cut?

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Re: Only fires on 2&4 at idle

Post by Bruce2 » Thu Aug 30, 2012 5:46 pm

I did it the full width of the cutter, plus some more. I'd guess at least 20mm. Go as wide as you can, but don't poke through to the outside world. Take it almost all the way to the bottom.

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rsw
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Re: Only fires on 2&4 at idle

Post by rsw » Sat Sep 01, 2012 12:56 pm

I seen this posted on another website when I was looking for CFM ratings.

"on a V8 with a single plane, each cylinder has full benifit of of all four barrels, on a VW with one ventura per cylinder, each cylinder only draws from one barrel.

So on a V8 with a 650 cfm, each cylinder can draw up to 650 CFM, because only one cylinder draws at a time.

running four 325 cfm venturas on a VW would be the same as running a single 325 cfm carb on a single plane intake".

So in reality what your doing is making like a single plane intake so each cylinder can share more of the cfm's?? Also would this mean you need to run smaller jets because of the easyer availability of gas/air mixture to each cylinder?

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Re: Only fires on 2&4 at idle

Post by Bruce2 » Sun Sep 02, 2012 1:29 am

I would say your post is way off topic.
So far we were only interested in solving the idle mixture problem of running two single bbl carbs. At idle, one cylinder steals the mixture from it's neighbour.

What you're talking about is the effects of a plenum at big rpms (maximum flows). This is not what we were discussing.
Personally, I never believed in plenum manifolds. If you take a V8 with a single plane manifold, then remove the intake and install an Isolated Runner manifold with four 2bbl Webers, you make way more hp. And have better driveability.

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rsw
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Re: Only fires on 2&4 at idle

Post by rsw » Sun Sep 02, 2012 7:24 am

Sorry if you felt my post was off topic, but what you guys are doing helps people like me understand the air flow of cfm's in the intake system. Yes you are working on the idle rmp range, but wouldn't what you are doing help with the uper rpm rang also? If you look at my post again, you can see see there was some questions there, not just comments.

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