Idle problem

[Cliff Ruggles]

Impossible to read plugs on this new fuel.

Always tune for best results.  If it needs more fuel the engine will tell you.  Start with idle, set the mixture screws for best idle quality and highest vacuum a the leanest settings.

Test heavy part throttle next, and dial in the jet size which provides the best power at heavy part throttle, climbing up steep grades, passing w/o the secondaries, etc. 

Then tune the part throttle for best off idle at light throttle openings, steady cruising, etc.

Last tune full throttle with the secondary metering rods.....Cliff

[dlphil10]

Good to know, thank you for confirming spark plug reading is difficult. The plugs look white, but more gas makes the idle unstable and causes it to be a little shaky. I know I need to get my advance curve coming in faster, but I think it still needs more gas in part throttle.

If I ease into the gas it'll do good, but it bogs with part throttle stabs. Heavy part throttle seems good (~5-7inhg). I'm thinking this would be helped by changing the power piston since this lines up with the orange spring that I have. If my drive idle vac is 13.5in hg. Would it be ok to use the stiffest spring? I have the long green, orange, and small light blue spring that is shorter than the green. I belive it came wth the plain spring, but it was lost.

[Cliff Ruggles]

Yes, you can use any of the springs for fine tuning they will all be down with that much vacuum at idle.....Cliff

[dlphil10]

Awesome. I know there are two light blue springs in the kit, is there a length difference between the weak one and strong one? The one I have is shorter than the green spring.

Would the power piston spring be the first thing to change for part throttle tuning, or should I be look into fattening up the idle circuit to free up more of my transition slots?

[Cliff Ruggles]

The power piston will be down for most light throttle/normal driving.  You can increase fuel to the main system by adding idle fuel with larger idle tubes/DCR's.  This can help with transition issues and being too lean right off idle as well.  Just be very conservative when tuning in that area as it does increase fuel to the mixture screws and exposed transfer slots at idle and you can put too much fuel there and loose control of idle A/F with the mixture screws......Cliff

[dlphil10]

Light throttle seems to be good. Plus, when the car is warm and in park, the idle screws turned all the way in don't stall the motor. They definitely choke it down though. So I don't think the idle circuit needs any more fuel correct?

[Cliff Ruggles]

Correct, if you can slow the engine you have control of the idle fuel.  There is usually enough transfer slot exposed so that fully seating the mixture screws may not kill the engine clear out.  That fact varies from one Q-jet to another due to how much bypass air is used, how the idle system is set up, and the length and location of the transfer slots.  If you take a look at a few dozen baseplates from different Q-jets you'll quickly notice that the specific location and length of the transfer slots can vary considerable from one to another. 

This is why I always recommend to be very careful and conservative with idle system modifications when setting up your Q-jet for a non-stock application.  I would also avoid swapping parts around from one unit to another for the same reason.  The parts may look the same but there are minor differences in them you may hot see that will effect the end result, sometimes negatively.........Cliff

[dlphil10]

Well that is good news. This baseplate looks like it matches woth the bowl. No passages are blocked and the transfer slots are long. There's even one horizontal slot as well. Maybe I could drill out the idle air bypass a bit and regain more of my transition slots? I believe they are only 0.040" right now. This may help with my part throttle response too correct?

[Cliff Ruggles]

If you aren't seeing fuel coming from the nozzles at idle (nozzle drip) then it will have enough idle bypass air.  This assumes that you also have control of the idle fuel from rich to lean with the mixture screws.......Cliff

[dlphil10]

I switched to the green power piston spring and that seemed to help power at part throttle. It also really seems to transition through the pedal amd the other carb circuits pretty nicely. No bog when the secondaries open. Really the only thing is a slight hesitation sometimes off idle amd a minor flat spot off idle. Power piston gives healthy squirts on the outer hole of the lever. I'm thinking I may need a bit more timing off idle honestly.

The only thing that makes me concerned is the tiny black speckles on the porcelin. They aren't heavy or shiny, amd there isn't a lot of them. Plugs are white after idling still, but 1.5 turns out is where the vacuum stops increasing. I ran them out to 6 to see if I could get the plugs to change, but they didnt. It seems to have the fuel it wants so I don't know if the specks have been on the plugs when I got them or what.

[dlphil10]

I can back out the mixture screws to 6 turns and it doesn't really bog. It may lower the vacuum a quarter to half an inch after 1.5turns out but it doesnt drop anymore. Could that be an issue? It really seems to have a better response at 1.5turns out though.

[Cliff Ruggles]

The idle mixture screws are set for the best idle quality at the leanest settings, then move on to something else.

This is done by backing them out far enough so it idles great, then slowly turning them in until you hear a change in the engine speed or very slight drop on a vacuum gauge or hand held tach.

Move them back about 1/4 to 1/2 turn or so until the engine smooths back out.  Repeat with the other side and balance them if you are running an open gasket under the carb or single plane intake.

So basically turning them in until you hear the engine slow is finding the "lean drop", then back out till it idles the best and you have the best A/F ratio at idle for your engine.  Of course make sure you do this with the engine fully warmed up and fully heat soaked.

If you are unable to establish good idle characteristics in this manner and can continue to back out the mixture screws with zero effect (doesn't go rich), then the idle system is marginal for what you are doing and may need a tad more fuel made available to the mixture screws via larger idle tubes and/or opening up the DCR's slightly.

Keep in mind when doing this that you need to first check for "nozzle drip" to make sure ALL the fuel at idle is coming from the idle system.  Also be aware that increasing fuel to the mixture screws for better idle control also puts more fuel to the transfer slots.  If you are having issues or lean symptoms right off idle during transition, and/or minimal fuel at part throttle or limited control with the APT system, adding more idle fuel will help that scenario as well.......Cliff

[dlphil10]

Thanks for that explanation, that helped.
I opened up my DCRS to 0.052". Then I had bad nozzle drip so I opened the idle bypass from 0.040" to 0.080". I also noticed the screws holding the baseplate and gasket had worked a little loose. It's better, but has nozzle drip at 750 rpm idle. At 700rpm the edge of the nozzle looks wet, but there no visible drip.

Sonce I just barely got the drip to stop at 700rpm should I add more idle bypass air and open up the idle circuit since it's lean? The idle tubes are 0.037, DCRs are 0.052, mixture screws are around .1. The idle vacuum is 14ing. It seems like that should be enough for it..

[Cliff Ruggles]

Sounds like you are getting close.

A good way to decide whether to add bypass air vs idle fuel is to remove a small manifold vacuum hose and re-adjust the idle speed. 

Then look for nozzle drip and tweak the mixture screws to make sure you have full control from rich to lean. 

If you do not additional idle fuel is needed.  After you add some idle fuel then do the same test to make sure no nozzle drip and full control with the mixture screws from rich to lean......Cliff

[dlphil10]

I remember the original motor would bog when I turned out the mixture screws on the 2 barrel. I seem to only get this one to bog down with turning the screws in. Turning them out does make it flubby amd erratic at around 3 turns out so it makes a difference. Is that enough or should I expect a clear bog when riches the mixture?