#6 idle jet and idle speed

toddu · 09-30-2016, 04:13 PM

I thought I was down on cylinder 6. Can ran bad, plug horribly fouled. I removed carbs from that side, thorough cleaning, new plugs in 6 (twin plug).

I started it, but haven't driven it. Maybe runs smoother (now I'm paranoid so can't be certain). I can crack every idle jet holder with an almost instant stumble. On 6 I'm not sure I notice a difference. New plugs look fine (just idling though for a few minutes). Synchrometer shows it's pulling as much as other cylinders (compression ok??). Exhaust temp (headers) is in line with others (~300). Exhaust note sound off though. Again, maybe I'm being paranoid.

Thoughts??

Todd

911pcars · 09-30-2016, 04:30 PM

Check the O-ring on the idle jet holder. Engine (cylinder) will run roughly if it's compromised (torn/missing).

A cylinder balance test (short out each spark plug wire in sequence) will often reveal/confirm the weak cylinder.

Sherwood

Trackrash · Trackrash's Garage

Mine will do the same thing. I have bad guides and I can end up with an oil fouled plug.

If you are getting fuel fouling, I would check the floats and needle and seats.

toddu · 09-30-2016, 05:34 PM

I rebuilt the carbs over the winter. Car was running fantastic, sat about 2 weeks and just didn't seem right next time I drove it. Fuel in bowls was pretty dirty and discovered rain hats don't seal well.

Would anything else cause no perceivable change in idle loosening idle jet holder (assuming o-ring is ok, I'll check)? Air bleed adjustment?

Just pull the plug wires off 1 at a time? Exhaust being hot on that cylinder led me to believe it must be firing.

Todd

911pcars · 09-30-2016, 05:52 PM

Quote:

Originally Posted by toddu

I rebuilt the carbs over the winter. Car was running fantastic, sat about 2 weeks and just didn't seem right next time I drove it. Fuel in bowls was pretty dirty and discovered rain hats don't seal well.

Would anything else cause no perceivable change in idle loosening idle jet holder (assuming o-ring is ok, I'll check)? Air bleed adjustment?

Just pull the plug wires off 1 at a time? Exhaust being hot on that cylinder led me to believe it must be firing.

Todd

Assuming an engine produces 180 hp., one would assume each cylinder in a healthy engine contributes 30 hp. (6x30= 180).

An engine down on compression, or A/F mixture, or air volume will produce less; e.g. a holed piston will have zero compression and produce/contribute zero power. Equal power should occur at all engine speeds, from idle to max. rpm.

Shorting out a cylinder at a time (by momentarily disconnecting or grounding a spark plug wire) will cause that cylinder from contributing - the engine rpm will drop. Same effect by momentarily disconnecting the signal to the fuel injector.

Repeat this on all 6 cylinders. Engine rpm should drop equally. The cylinder that doesn't produce an equal RPM drop isn't contributing even though it's firing (trying). Pinpoint that cylinder to inspect (compression, leakdown test, carb and jet passages, injector, vacuum leaks, etc.).

toddu · 09-30-2016, 05:58 PM

Thanks Sherwood!

Todd

Gordo2 · Gordo2's Garage

Todd,

I assume you are talking about opening/closing your idle mixture adjustment screws (vice idle jet holders)? If it's idle mix screws, here's a few observations and tricks I've picked up from my efforts to optimize the PMO tune:

- If they ran well/right previously, but now seem off: clean the idle jets and idle jet passages with spray carb cleaner. This is such a fast/easy thing to do and it seems to address the primary issue that carbs experience. This also requires zero adjustments- so it keeps me from re-obsessing to find the "perfect" tune.

Sometimes it's heard to hear / notice a single cylender RPM drop as you adjust mixture screws.

- First keep in mind that unless the idle speed screw is backed out to allow full closure of the throttle plates/butterflies - I.e if the butterflies are slightly open, fuel will be drawn through your idle circuit, which can make it more difficult to notice changes made to the idle mixture screw (it's adding or reducing a small amount of fuel - but if the butterflies are cracked open - the mixture screws aren't the only contributor...).

- I like to wear earplugs when I adjust mixture: they seem to help isolate minor RPM drops as I turn the screws.

- I sometimes do a low and high RPM idle mixture screw adjustment, but I use my wideband to help. I tune low RPM with a normal lean drop method - then adjust Idle speed to ~2,500 RPM. At this RPM I've found the idle mix screws seem to have a more pronounced or noticeable effect on RPM drop. I've had great results doing a lean drop at this RPM, and making equal minor adjustments to hit an AFR range.

Sorry - too much writing... More thoughts, but have to run.

Gordo

toddu · 10-01-2016, 08:05 AM

Hey Gordo, always appreciate the insight.

I was actually referring to cracking the idle jet holder while at idle. All cylinders had a drop in RPM (very rich). 6 was very discernable. If I adjust the idle mixture, I do start to hear some slight rumble in the muffler (turning adjustment in, which seems backwards).

I'll mess around with it tomorrow. No time today of course :-(.

Todd

Gordo2 · Gordo2's Garage

Quote:

Originally Posted by toddu

Hey Gordo, always appreciate the insight.

I was actually referring to cracking the idle jet holder while at idle. All cylinders had a drop in RPM (very rich). 6 was very discernable. If I adjust the idle mixture, I do start to hear some slight rumble in the muffler (turning adjustment in, which seems backwards).

I'll mess around with it tomorrow. No time today of course :-(.

Todd

Ok, but I'm not sure what you were trying to determine by cracking the idle jet holders open. Are you trying to confirm that you have fuel flowing through the idle circuit?

Wouldn't loosening the idle jet holder screw allow unmetered fuel (and some air) to flow to the idle mixture screw? If so, wouldn't the fuel still be somewhat metered by the idle mixture screw's needle (idle port closed if the mixture screw is screwed in / open if mixture screw is backed out)? I assume when you crack open the idle jet holder screw, the conical end of the idle jet backs out of it's seat, allowing fuel to flow past, vice through the idle jet.

It would seem that when you crack the idle jet holders open, the fuel being delivered at idle mixture port would still depend on how you have the idle mixture screws adjusted / set. However, cracking the idle jet holder screw open would introduce some air leakage...

All very confusing and difficult for me to sort out what doing this would tell me from a troubleshooting perspective.

If it were me, I would stick to the basics and become very familiar with troubleshooting and setup tips that Paul Abbott has on his Performance Oriented website. Its the best gouge I found for understanding the carbs.

Gordo

toddu · 10-06-2016, 08:53 AM

The troubling thing is that on all other cylinders, cracking the idle jet holder results in a change in RPM. On 6 it does nothing. I can completely remove the idle jet and nothing changes. I can put my finger over the air inlet holes at the top of the carb, nothing. I can turn the idle mixture screw in all the way and nothing. I put in new plugs on 6, but they were very fouled.

To me the car doesn't sound right (exhaust note) and I was guessing a cylinder down. I don't want to drive it. Maybe I'm chasing a rabbit hole. Very frustrating :-(. I have 4-6 carb off again. I can't imagine anything is blocked, but I'll check it again. Maybe I'll sway carbs side to side and see if anything changes.

Todd

911pcars · 10-06-2016, 09:46 AM

Cylinder compression OK on no. 6 cyl.?

If OK, could be the idle air and/or fuel passages in the carb body are clogged with debris. To clean, they are accessible via removing the lead plugs pressed into the factory-drilled passages. Review the cross-section drawings and service procedures in any Weber manual.

Paul at PerformanceOriented.com (1QuickS) might chime in with his expertise.

Sherwood

toddu · 10-06-2016, 11:30 AM

I assume the compression is ok. It's on my list of things to check. Engine has about 2k miles on a new build. Not sure it's an indication, but it pulls just as much as the others on the STE gauge.

Carbs are off now (again) for another cleaning. They are PMOs, so pretty easy to service. Seems to be an air blockage, just don't see where it would be based on what I've checked/cleaned.

Plan on switching them side-to-side when I put them back on and see if "problem" follows the carb.

Todd

911pcars · 10-06-2016, 11:36 AM

Once you remove the 4-5-6 carb, you could chase/confirm these passages are clear with a can of carb cleaner and straw nozzle to see if the idle air passage(s) are clear.

Sherwood

1QuickS · 10-06-2016, 10:16 PM

Get a Gunson Colortune to help with diagnosis. Very immediate & 100% definitive for idle mixture problem solving.

toddu · 10-07-2016, 11:53 AM

Compression is around 180, so don't think it's anything internal.

Going to clean carbs, swap them side to side, order new plugs (all very black) and a colortune.

Hopefully that will be it because I'm about ready to sell it.

Todd

Gordo2 · Gordo2's Garage

Todd,

Not sure what kind of ignition you are running, but are you sure you are getting spark to that cylinder?

Hang in there man - you will figure it out.

Good luck,

Gordo