Idle Fluctuations in Drive

[Mark H]

In park the engine idles very well all the way below 600rpm
I have set the idle at 780rpm resulting in fairly steady 15.5-16 HG and A/F around 14.6

However in drive the idle fluctuates between 440-610 rpm, but does not stall.  HG 10-12 but does falls to 8 and then comes back up.  Which is the issue  I would like help with.
A/F does lean a tad in drive from high 15s to high 16’s and the PP spring stays down.
 
Engine
Freshly rebuilt (600miles) Oldsmobile 461ci
Everything stock except camshaft including intake, heads, exhaust manifold and Qjet.
10.2:1 compression and 180 +/- cylinder pressure in all cylinders
Factory cam .220/.222 @.050 472/472 LSA u/k  changed to .224/.234 496/520 112LSA
TH400 with factory stall (2000ish) with 3.23 rear

Tuning includes 18 degrees initial and 36 total, running with manifold vacuum to vacuum advance

Rebuilt carb 7029251
Changes as follows:

Idle tubes drilled from .035 to .0375, DCR drilled to .055 pp spring is 9-7hg
Jets/rods changed from 70/49 to 72/49 with AU Secondary rods
No adjustments to any air bleeds or mixture holes, however idle bypass holes were plugged (experimental) but then both primary plates drilled to 0.09375 (3/32).  This helped in lowering the idle with the transfer slots only slightly exposed (square shape)
Mixture screws set at 4 turns out with minimal control other than turning them in to 1.5 turns out will cause a stall
Tip in at 2000rpm causes a 50-75 rpm decrease.
Spark plugs show a tendency towards slightly lean
A/F as follows: light cruise mid 13’s, cruising high 13’s WOT 10.8 to low 11’s

Generally the carb performs very well in all areas except idling in drive.

Any suggestions are welcome.

[77cruiser]

Is the idle set to best highest vac/rpm. 14.6 is cruise AFR idle needs to be richer.

[Mark H]

The idle has been set to best rpm/vac.  Thank you

[77cruiser]

In gear or neutral.

[Cliff Ruggles]

I see a couple of potential issues. 

You don't need a lot of initial timing with that engine combo and I'd switch it over to ported vacuum so it isn't adding so much. 

18 initial is pretty high for that little cam and 10.2 to 1 compression.  I'm surprised it isn't "bucking" the starter on a hot restart (high cylinder pressure).

Also verify that the timing is ROCK solid in and out of gear.  I cure a LOT of issues with Olds engines as nearly every single one that comes in here has some sort of advance curve "kit" in the distributor that's adding some timing at idle speed.

You can also run into issues using MVA as most vacuum advance cans have pretty strong springs in them and some of the timing may fall back when the trans is placed in gear and vacuum falls off a bit.......

[Mark H]

Hi Cliff, thank you for your feedback.  Over the last couple of years I have enjoyed reading current and older posts as well as your book which has helped me understand these carbs a bit better.

When I started the tuning process after the rebuild, like many others I couldn't get the idle below a 1000 and it would stall in drive.  I feel I have come a long way.

I was also surprised how much timing the motor liked, with a relatively mild cam upgrade.  The initial 18 degrees is where the timing landed after setting the total at 36.  The motor likes what it likes and I have had no issues.  The engine at idle also liked manifold over ported. 

However I will now turn my attention to checking for any changes in timing in and out of gear as well as comparing ported vs manifold in drive.

[Cliff Ruggles]

18 degrees initial timing is a BUTTLOAD at idle speed.

As a general rule of thumb anytime you find yourself having to crank initial timing past 14-15 degrees or so to make an engine happy you just missed the cam choice vs the CID and compression ratio.  In other words cranking compression is low and overlap is bleeding off too much compression at low RPM's.

The real fix is less overlap and or higher static compression.

Not enough fuel to the mixture screws will also require a LOT of timing at idle speed. 

I consider butt loads  of timing and tons of idle fuel "crutch" fixes for fundamental issues, but sometimes that's what needs to be done with these things.  Besides who wants your new "high performance" engine to idle as smooth as the 305 in a 1987 Chevy half ton truck?

I've made a very good living because of the choices folks made with the compression ratio and cam events and very quickly find their new engine build doesn't idle or run for chit with "normal" timing and fuel curves, so it's all good from where I'm sitting.......

[Mark H]

Hello again

I reviewed some of my notes from before adding bypass air and didn’t note the rpm fluctuations in drive.

So I temporarily blocked the throttle plate holes with aluminum pop rivets.  Air bypass holes in the main body are also blocked.

The idle in drive now only fluctuates between 480-510rpm.  Vacuum is 7-9mg and A/F is high 15’s although while cruising the A/F seems good at mid 13’s.

Total timing is 35.5 which results in 13.5 degrees at idle at 600rpm with VA plugged.  Adding VA from the manifold bumps the rpm to 750 with 23 degrees of timing and 15 mgs of vacuum.

Overall I’m happy with these numbers except the high A/F ratio at idle unless this is normal? and a lack of responsiveness from the mixture screws which I settled on 3 ¼ turns out.  Are these two issues connected and what steps could I take to improve? 

As mentioned earlier my idle system has 0.0375 tubes and .055 DCRs with no changes to the size of the stock mixture holes. My primary side is 72/49 with 9-7 pp spring which is staying seated at idle in drive.

[Cliff Ruggles]

Lack of response from the mixture screws may simply be the throttle angle is too high exposing too much transfer slot or a little nozzle drip.

13.5 is actually pretty rich at cruise and you are rich at full throttle as well.  I'd put the .070" main jets back in it and see if you can get the APT screw out of the baseplate.  I make one here in the lathe with a spring on it.  This will provide full control of the part throttle A/F using .070" main jets and 49B primary rods.....

[Mark H]

Hi Cliff

Thank you for your quick response.

I don't believe the transfer slot is open too far but I will confirm next time the carb is off likely when I change the jets back to 70's and I will also check for nozzle drip.

I read some other posts on removing the APT cover/screw.  There seems to be some issues  from us DIYers in breaking the screw and or damaging the baseplate when drilling the broken screw out.  Although the turnability advantage is intriguing, as my part throttle is not an issue I think I will leave that alone for the time being.

I do have a 0.038 drill bit. Do you think enlarging the tubes from .0375 or even going to .039 would have any change on richening up the A/F at idle(currently sitting in the high 15's)?

[quadrajam]

It wouldnt hurt anything to go ahead and open up the idle mixture screw holes.

[Mark H]

Hello

An update and a few questions.

I have a slight nozzle drip and exposed transfer slot beyond the desired square shape.  It takes ¾ of a turn of the idle screw to eliminate the drip, however this drops the rpm in gear and the motor stalls.

My idle system is .0375 and .055 dcrs with air bypass blocked  and I am using 71/49B combo with a 7-9 hg pp spring.

My AFR is showing low 12’s WOT, low 13’s during light cruise and high 13’s during cruise.
My idle out of gear is 720rpm 13.0AFR and 15.5 HG
In gear idle is 480-510 rpm 13.5-13.7AFR and 7-9HG

I have excellent idle screw responsiveness an can see changes in AFR with every full turn.  The screws are currently set at 2 ¾ out.

I’m very happy overall with the responsiveness, idle in gear and all around performance.

However,

I realize that I need to lower the throttle blade angles to eliminate the drip and likely need some bypass air to accomplish this.
Stock configuration of the carb had approximately 0.057 bypass holes and I realize by blocking them and going to .09375 in the throttle plates  was too much air and possibly caused the wider rpm fluctuation of over 100rpm in gear.

Questions
1.   If I leave the nozzle drip as is what is the downside?

2.   If I start adding bypass air I would like to start slow with drilling a single hole in only one throttle plate or do I need holes in each plate to balance our the air?

I would prefer to start slow with one 3/64 hole(0.046) then 3/64 x 2 if needed or a combo of 3/64 and say 1/16, trying to keep below the stock .057.

 Thank you

[Kenth]

No need to drill the throttle blades for bypass air.
With your engine/cam i would open the original .059" bypass in throttle plate to .095"-.100".
I would open the idle tubes from .036" to .039"-.040", downchannels from .051" to .055" and idle needle holes from .085" to .095"-.100", keep the original main jetting.

Set initial timing to 10-12° and use the ported source for vacuum advance (you may try full manifold vacuum later).
Also, set the idle mixture screws for best vacuum/rpm in gear at 650 rpm´s, under 500 rpm´s are way too low.

This should make a strong steady idle in gear and a smooth transition to low speed driving.

HTH

[Cliff Ruggles]

NEVER drill the throttle plates for idle bypass air.  The Q-jets idle bypass air system is EXCELLENT and a very precisely controlled vacuum leak.  Using it you can fine tune the position of the throttle plates for the correct amount of transfer slot coverage, or exposure, depending on your engines needs.

You are still rich at cruise and may be adding a bit too much timing with the VA as well.

Didn't see where you tried to remove the APT screw and install an adjustable one.  It is an excellent feature and provides the tuner with full control of A/F without taking the carb apart and changing parts......

[Mark H]

Hi Kenth

My first attempt at dealing with the idle issue was using recipe #2.  38 tubes, 55 dcrs, and .09375 mixture holes with ported VA and stock jets/rods.  It did not run well.  I ended up starting over and going back to stock tubes and a new baseplate for stock mixture holes and now I am at .0375 tubes and 55dcrs.

Timing is set at 12 initial and 35 all in and it really likes manifold vacuum at idle.

I know i can get the idle higher to 600 with bypass air while eliminating the nozzle drip.  The question is how much bypass air.

Hi Cliff

I understand that it is preferred to use the carb's bypass air over drilling the plates.  As mentioned the bypass holes are plugged and before I drill them out I would like to get a rough idea how much bypass is needed and plan to get to that number slowly with holes in the throttle plates.  Once I'm happy with a number I will replace the plates and drill out the stock bypass holes to the size that's best.

I'm not comfortable at this stage trying to access the APT screw.  The 71/49B gets me close to the stoichiometric ratio.  I have read elsewhere that due to ethanol in today's fuels that the 14.7 number is closer to 14.1 and I am very close to that number and my spark plugs have very good color.    I would be interested in you thoughts on 14.1 and some input into my two previous questions will help me move forward.

Thank you to both of you.