So I am experiencing a lean bog off idle and between gears. Am I correct in thinking the only real way to cure this is to play with the pump cams and/or shooters?

 

pump cams and shooters. one thing to remember is that up to 34 or something they all put out the same amount of fuel. just the bigger the sooner and the smaller the longer the squirt. i have a stock 305 and i went up two sizes. also timing will cause the same thing.

 

I think im gonna try to up to a .35 squirter to solve the same issue. my A/F goes to about 15 to 1 at tip in throttle and low 12's the rest of the time. I may have too much timing but the power #'s are there with no sign of knock so Im gonna try the carb first.

 

I am having a similar situation with mt Nighty Demon. I upped the squirter size, and that works great for when you floor the gas. However, it hasn't done anything for when I am cruising in that lean spot (and it shouldn't). My next two steps are going to be raising the primary float level and then mess with the IAB's. Let us know what works for you.

 

tip in is the accelerator pump circuit. cruise is jets i believe. and make sure the secondaries are opening under full throttle as well.

 

So too much timing can cause this?



I also run quite a bit of timing with the vac advance hooked to full manifold vacuum. I also don't notice any signs of detonation on the plugs but may just not be seeing it.



I guess the IAB's might affect the transition circuits slightly and it seems that my cruise is fine, just when I close and re-open the throttle do I have the lean bog. I don't see how the float level would affect it but it definitely sounds like its worth a try.
The acc pump squirts fuel anytime the butterflys are opening so it sounds like the pump circuit would be the main culprit.



Secondaries are opening; I was kicking around the idea of messing with the power valves but I think the PV would have nothing to do with it.




What about the primary throttle blades being closed too much???

 

No need to guess, they (IAB) will affect transitioning most times along with the IFR's.

The transition circuit as I have learned about it somewhat on my Demon is a mix of IFR and IAB and also from talking with a few very good carb tuners.

If you cannot seem to get rid of the "hole" between tip in and lo cruise chances are the transition is lean. This of course depends on if tip in is truly good and cruise on the main jet is good as well. If you play with the pedal in this area does it stay stumblin' or does it clean up?

Raising the float level will bring the main circuit in (activate) sooner and sometimes it is soon enough to cover up this hole between tip in and lo cruise. Can also crutch it by running some more primary jet. But honestly having the IAB and IFR adjustable is a better fix IMHO. I had to do this last summer to fix a lean transition and this winter modified my carb to be fully adjustable.

If your tip in is truly good as well as a steady cruise I would try:

Raise P float level to highest mark on bowl ( I did read this is a Demon right? )

If not much better and hi cruise on the jet is still good, stick a piece of wire in the IAB. Make sure it stays put, run it long under air cleaner base, wrap it around vent tube, anything to keep it in place. Grab a piece of MIG wire or whatever. .020 - .030. Not a permanent fix but this will tell you if bring the circuit in sooner cleans it up.

If still a hole persists try another piece of wire in the IFR's. Same idea, just want to see a change for better or worse before you determine how to make it permanent.

 

No


What is your timing set up.

Idle? w and w/o vacuum

total?

When does the vacuum can start to pull in? And when is it ALL in?

What do you have for vacuum @ hot idle fully warmed up?

It may. Easy enough to try.


How is the carb set up right now? Talkin' everything...

Floats?

Jets, Pump Cams, Squirters?

Idle Mix Screws?

Transfer Slots?

 

EDIT:

Just realized I misguided you with this bit of info. DO NOT put a piece of wire in the IFR is the transition is lean. This will only aggravate the situation (make it leaner). If making the IAB smaller does not cure the lean situation the IFR would need to be enlarged (this is where drilling and tapping the carb out for set screws like I did makes tuning fun). From my findings in my personal Demon tuning and speaking with carb guys about it, a typical step in tuning IFR's are generally in the .002 - .003" range. IAB's generally respond to differences of .005".

I had a lean transition and decreasing IAB's .005" and enlarging IFR's .0025" cured it and made for a very smooth transition.

 

Timing is (+/- 1-2*):
18 base
+ 16 mech advance
= 34-36 total

With vacuum advanced hooked up (full manifold vac) I'm idling at approx 38-40*.

Not sure when the vac advance kicks in or is fully in, the canister is kinda finicky but I do get that substantial advance at idle.



Vacuum pulls 11 +/- an inch at idle (1500 RPM). I'm shooting to bring that idle down to about 1100RPM.


Carb floats are just licking the slight glasses. 77 primaries with a 6.5 PV and 83 secondaries with the PV blocked.

Pump cams and squirters are stock for the Summit DP I have: Green primary cams and Pink secondary cams.
http://www.summitracing.com/parts/SUM-M08750MS/


Here's the condensed info: http://static.summitracing.com/globa...20m08750ms.pdf

Slots are almost fully closed on the primaries (!!!) and I have no clue where they are at on the secondaries but I would guess .025...

Idle mixture screws are a bit over 2 1/2 turns out each.




lol...don't worry, definitely wouldn't do that.

 

I see in your other thread that your idle has some large swings of a couple hundred RPM. I will say it does sound like vacuum leak as Damon clearly pointed out, be positive it does not have one... but a mismatched vacuum can can also cause an unstable idle.

First thing I would do is get a Mity Vac pump and figure out exactly where this can starts and stops. Using full manifold vacuum you need the can FULLY deployed at 2" less than your typical idle vacuum. So.... with 11" you want it ALL IN at 9". Too light of a mechanical advance spring can also cause an unstable idle. Try heavier springs just to see if it helps smooth the idle swings out.

Car off connect the vac pump to vacuum can and slowly pump the trigger while watching the vacuum can rod. Obviously cap and rotor need to be off. Watch closely and see at what vacuum reading on the gauge the rod begins to move... write it down! Now keep slowly pumping the pump and closely watch when the cans rod stops moving, note vacuum and write it down. This number needs to be 2" below your idle vacuum. Some adjustable cans can be adjusted to work, some cannot. Either way I would test what you have 1st.

Hmmm. Never worked with that model carb. but standard procedures should be able to get it in line. But what freaks me out is the .052" IFR and you are lean!!! What size are the IABs?

Again 2.5 turns out say lean transition but with a .052" IFR. ?? Does the carb have 4 idle mixture screws?

First thing I would do is get the vac pump to figure out what vacuum can you need or even if your current one is a good match.

Then take the carb off and baseline it. Set it up just as Summit sheet says. If 4 corner set the blades equal P & S side. If 2 corner just crack the backs from fully closed and set the P side to .020. See how it idles.

If still unstable swap in heavier mech adv springs and see what she does for a test.