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Engine air fuel issues

tsaarts

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Hi.

My setup is big block 400, Edelbrock E-street heads, air gap intake, Comp Cams 218 duration flat tappet cam ja Street Demon 780 carb. Compression with stock bottom end and 75cc heads should be between 8...9. Timing at idle is 18 degrees and thermostat is 160 degree. It is just street car. It will not see any racing which means driveability is priority.

My issue is that I bought AFR gauge to temporary monitor my air fuel ratio. But seeing air fuel made me thinking of some questions.
1. My idle in park is 850 rpm (afr 13.2). In gear it drops to about 650 rpm and AFR goes to about 14.2. At these low rpm-s idle is a little bit lopey and AFR is fluctuating +/- 1 point. (These afr numbers before were about middle numbers). Is this fluctuating normal? - For my logic it seems to be okey because idle is a little bit unstable with this cam.

2. Another question is that after driving a bit and then stopping engine temperature goes a little bit higher, to about 180 degrees. With this temperature idle goes about one point richer and stays this way. It does not stall or anything but should i go with higher temperature thermostat and is this thing normal? If i go to 180 or 195 thermostat should is see also 20 degree higher engine temperature and same mixture offsets?

3. I have been tinkering with one another stock spec engine before in my life and as i remember with stocker cam i had very stable idle and none of these strange things stated above. Is these things to be expected with mildly modified engine?
 

Challenger RTA

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While reading through,It comes to mind to back the timing off 1-2 degrees. try it I could be wrong nothing to loose!would need more info. everything working Timmig chain,vacuum advance,distributor? Were are you getting you vacuum from manifold or carb?It sounds like a timing issue.
As RPM increases, more ignition timing is needed
For higher octane fuels, more timing is needed due to slower flame speed
For large combustion chambers, more ignition timing is needed
For forced induction, less timing is needed because of faster flame speed
For emission controls, less timing is used to reduce smog compounds
For richer fuel mixtures, more timing is needed due to slower flame speed
 
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Chryco Psycho

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At 650 rpm the power valve may be opening from lack of vacuum so changing the power valve to a lower rating may help .
160* Tstat is too cold you need 180* for the engine to run properly , the engine runs richer at 180 because the fuel is mixing with air better .
 

rikkitik

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I would agree that a 180 t-stat would be a better choice.
Since your idle afr actually goes 1 point lean at idle in gear, 13.2 up to 14.2 , I doubt there is any issue with the power valve. Particularly with a relatively mild cam. A bit of fluctuation in afr at idle is not uncommon.
The leaning of the mixture in gear, could easily be attributed to the fuel being burned due to the slightly increased load.
13.2 - 14.5 at idle is rarely an issue, and actually pretty clean. It wouldn't be a bad idea to cross check/correlate your afr with a vacuum guage (if you have access to one) to gain further information.
 
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Dodgeboy

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I've got a 383 (Comp cams solid cam, 260 duration, 21° timing & 6" vacuum @ idle,with a holley 850 carb. I use a vacuum gauge to set idle afr, I find this is easier to find the engines happy spot then to shoot for a certian afr. Having said that, my idle afr is 14.20 in neutral, it bounces from 13.8-14.5. I will use the afr gauge to tune everything else. I have a 170° thermostate but she's usually running around 180°-185°.
I have had the afr gauge for 2 years now and it's never given me a rock solid reading at idle, it normally bounces around a bit (less then 1 point), i don't know if thats normal or not.
 

tsaarts

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I would agree that a 180 t-stat would be a better choice.
Since your idle afr actually goes 1 point lean at idle in gear, 13.2 up to 14.2 , I doubt there is any issue with the power valve. Particularly with a relatively mild cam. A bit of fluctuation in afr at idle is not uncommon.
The leaning of the mixture in gear, could easily be attributed to the fuel being burned due to the slightly increased load.
13.2 - 14.5 at idle is rarely an issue, and actually pretty clean. It wouldn't be a bad idea to cross check/correlate your afr with a vacuum guage (if you have access to one) to gain further information.
I had 195 thermostat lying around and changed that. Now the temperatures while city driving and idling are between 195 and 210. I m planning to order 180 thermostat. By the way with these higher temperatures it seemed to me that engine is running little better.
I also used vacuum gauge. Idle in park i had 900 rpm and 13 inch of vacuum. When in gear it dropped to 700 rpm and 9 inch of vacuum. Now i have a question. If ja want to swap to power brake is this vacuum enought?

Also saw that when tuning carb idle with bonnet open and after a little driving everything seems to go a little bit leaner. I have read that it is caused of fuels today boiling because of high temperatures under the hood And one solution is to use electrif fuel pump with return or just use some special fuel wilter with return line. It does not cause any stalling or anything but just with AFR gauge all these kinds of changes are seen.

Overall i think by drivability standpoint idle is okey. with steady state city cruising speeds air fuel ratio was about 12 - a little rich. Carb manual states that it should be about 13.5 and according to manual i should start chaning metering rods.
 
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Chryco Psycho

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Generally a running engine will not boil fuel especially if the heat crossover is blocked in the intake , it is running leaner because the fuel is evaporating better with the higher engine temp & you can adjust the mix leaner as the engine is using the fuel more efficiently .
You should be OK with power brakes as you don't use the brakes hard at idle & should have built up higher vacuum in the booster before applying the brakes .
 

rikkitik

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Good that you're looking at your vacuum a bit. AFR alone, at idle, can be tricky and misleading. When watching AFR though, don't drive yourself crazy worrying about fluctuations/subtle shifts in mixture readings as the engine operates. As CP mentioned, the engine is just using fuel more efficiently. And as you yourself mentioned it's actually running fine, just something you noticed. Try to watch for "trends" rather than "blips".
Pay attention to how it actually runs and responds, correlate that with your AFR, vacuum, and "feel". And, continue to ask questions. 👍
I would try to lean out that 12:1 cruise though.
 
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tsaarts

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Thank you for the input. I checked few things before i move on to cruise tuning. First my vacuum gauge is old and real vacuum was 3 inches more which means in idle 900rpm vacuum was 16" and in gear 700 rpm it is 12". I looked over all vacuum lines. For example PCV valve was stuck closed, but it didn't changed much everything. Idle screws are 1 3/4 turns out.

People using street demon 750 report that often carb is very rich out of the box - same as mine. I ordered tuning kit with some rods and jets - the one for 625 demon because it seams that this content is more suitable for me.

I have on more question. When i was testing then with moderate acceleration afr is too rich even high 10s. But when rpms goes up to about 2000 it gets too lean - about 15. Is this caused by secondaries open too early? What should be guidelines to cure that? Haven't been tested this yet by myself.
 

Chryco Psycho

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Well to be honest I never even knew this carb existed , I have seen one or worked on one but it looks like the AFB carb . Which is about the worst carb design created .
So I have no clue how the secondary air door is controlled ,if it is by a spring like the AVS you can change the spring tension , if it is weighted like the AFB all you can do is drill holes in the counter weights .
Having said that I doubt the air door is opening too early , you can force the throttle plates open but not the air door , so at low RPM the engine cannot use the fuel from the accelerator pump but higher up it is not enough , it seems there is virtually no adjustabilty in the accel pump circuit which is one of my major complaints about this style carb along with no adjustability for air bleeds .
So how to help ... it seems you need less accel pump fuel & lighter springs under the rods so the power valve / extra fuel as vacuum drops .
 
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