Annoying Idle Surge

[rusty_rig]

Specs: rebuilt factory carb (17082204 E4ME), SBC 350, iron vortec heads, "baby hot cam" (215/223 LSA 108 473/473), flat top pistons, custom prom (L69 timing tables stepped down for vortec heads and target rich/lean set to .6 from .41), and custom MAP sensor voltage regulator to boost voltage signal

Car runs great except at idle... In park it is less noticeable but in gear it's honestly annoying. Basically, I'm getting a weird surge that shakes the vehicle for a few seconds then smooths out. No trouble codes, and dwell is varying between 30-35. I don't expect this thing to idle like a stock engine, but this feels almost like a misfire. Plugs are just a hair lean and introducing/removing air from the carb does not seem to improve things.

The only two things that come to mind is the knock sensor interpreting things wrong at idle or the volume of air/fuel in the idle circuit is simply insufficient for my specs.

 

[Clone TIE Pilot]

Check for vacuum leaks as they really effect idle quality. Do you have any main nozzle dripping? Even an incorrect flow PCV valve or crankcase air leaks can lean out the engine. Also l would check for exhaust manifold leaks because outside air leaking into the exhaust can throw off the O2 sensor.

I have not heard of a MAP sensor voltage regulator before.

 

[rusty_rig]

I have a slight nozzle drip currently, but I think it's because I changed the prom tune for less timing at idle (had to increase the idle screw to compensate). What's the best way to check for vacuum leaks? I can unplug the brake booster and plug the hose and it creates no change...

I'll see about posting some pictures of my map regulator setup - all it does is boost the input voltage to the MAP and bring the output signal into the proper idle range.

 

[Clone TIE Pilot]

Nozzle drip is generally caused by having the throttle blades too far open at idle. This can be caused by not enough base ignition advance, not enough idle air bypass, too much fuel pressure, vacuum leaks, or not enough fuel in the idle circuits


Sounds like you may need to add more base timing advance so you can close the throttle plates more. With Qjets, too much plate opening at idle causes nozzle dripping, too far closed at idle causes off idle bog.

 

[spongbob]

I have a slight nozzle drip currently, but I think it's because I changed the prom tune for less timing at idle (had to increase the idle screw to compensate). What's the best way to check for vacuum leaks? I can unplug the brake booster and plug the hose and it creates no change...

I'll see about posting some pictures of my map regulator setup - all it does is boost the input voltage to the MAP and bring the output signal into the proper idle range.

How long has the intake been bolted on to motor it's nessesary to re-tighten"them about 3 times over say the first week then again( 3rd) in the later same year.Maybe not the case in your motor but ! ....many/some times" the symptoms your mentioning, are from cam walk ... (bad)everything aside.. im an Advocate"of running the tappet's very near there top nil lash, somewhere around 20% of one turn ,untill at least some milliage is chocked up" if not forever..the lax lash seems to make some stuff more forgivable...just saying....thnx..need to check carb base gasket ? Or spray wd 40 onto that area all around where throtle gasket sandwitches? Fuel dripping don't seem right either : I might as well say it here..but the funky idel fluctuation is can be caused by small metal flakes in the oil deflecting the lifter valve stuff like break in oil: yes dump it twice and fast.. or oil over dew for a changes ( 6000) mile oil..even a good motor can manage to churn out tiny metal fragments some times you can see them" in the oil,certainly in an oil change...what oil do I want ? 5w20 Castrol "how many motors have I worked on ? Countless ...no not the result of the castrol" lol

 

[rusty_rig]

How long has the intake been bolted on to motor it's nessesary to re-tighten"them about 3 times over say the first week then again( 3rd) in the later same year.Maybe not the case in your motor but ! ....many/some times" the symptoms your mentioning, are from cam walk ... (bad)everything aside.. im an Advocate"of running the tappet's very near there top nil lash, somewhere around 20% of one turn ,untill at least some milliage is chocked up" if not forever..the lax lash seems to make some stuff more forgivable...just saying....thnx..need to check carb base gasket ? Or spray wd 40 onto that area all around where throtle gasket sandwitches? Fuel dripping don't seem right either : I might as well say it here..but the funky idel fluctuation is can be caused by small metal flakes in the oil deflecting the lifter valve stuff like break in oil: yes dump it twice and fast.. or oil over dew for a changes ( 6000) mile oil..even a good motor can manage to churn out tiny metal fragments some times you can see them" in the oil,certainly in an oil change...what oil do I want ? 5w20 Castrol "how many motors have I worked on ? Countless ...no not the result of the castrol" lol

 

[rusty_rig]

Timing light revealed something interesting - the timing at idle is fluctuating ±5 degrees. I'll check to make sure the intake bolts are torqued to spec, but what are some possible causes for the fluctuation?

 

[spongbob]

Timing light revealed something interesting - the timing at idle is fluctuating ±5 degrees. I'll check to make sure the intake bolts are torqued to spec, but what are some possible causes for the fluctuation?

I caint say whether the spark is following the fluctuation or the timing is following the fluctuation,maybe disconect the vacuume advance then recheck, is timing fluctuation still present? I can say a partial working vacume advance pod Is worse then none...try blocking the pvc to see if things are steadier ? ,dose closing the chock some with your hand seem to achieve improvement if so it's proubly something about the way its breathing...if this were my problem I would be trying some different things possibly trying a different carb , you may end up with an aftermarket carb to cope with the extra runner size .or at least some adjustment to the metering jets up front ( primary) your not going to hurt anything by trying 2 sizes larger on primary jets,while your in the oem carb check the pump nozzle check valve ( discharge) it supposed to controle dripping ?... if you have another distributor see how it behaves,,you going to have to narrow something down if you want that good solid pattern idle that just says I'm right...worse come to worse be certain your pistons are sealing equal that in mind watch the buildup of compression if 7 cylinders take 3 strokes to max out and 1 cylinder takes 5 strokes you might have a tight valve or a reason to check leakdown ect ect compression equality is very noticeable at idle speeds 700 or below even some of them computer carbs idled in neutral higher I don't know what exactly is happening but if your useing vortec heads it seems like you might use a tbi setup..no ? Is your motor a flat tappet or roller cam ....

 

[Clone TIE Pilot]

Timing light revealed something interesting - the timing at idle is fluctuating ±5 degrees. I'll check to make sure the intake bolts are torqued to spec, but what are some possible causes for the fluctuation?

Is the electronic spark advance connectef? To check base timing you must disable the electronic timing advance. CCC systems did away with mechanical and vacuum timing advance.

 

[rusty_rig]

Is the electronic spark advance connectef? To check base timing you must disable the electronic timing advance. CCC systems did away with mechanical and vacuum timing advance.

EST is doing it's thing - I burned another prom to give me 28 degrees of timing at idle and it's holding fairly steady. Vacuum gauge shows almost 15 in park and the dwell is varying between 30-35. I think what I'm dealing with is reversion in gear so the real question is how do I minimize it?? I'm thinking about raising my idle speed to where it smooths out, but I'll need to combat the engine dieseling... any ideas?