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New engine with some stumble and some diesel....Too rich?

3K views 30 replies 6 participants last post by  0trbo4myCHEVUICK 
#1 ·
I've got some noticeable breaking up from 600-1100 rpms when coming off idle from a red light with light throttle. More gas and it clears through..
It is also dieseling for a couple seconds after shut down.

I am pretty certain this is a tuning issue but I am not sure where to start. It feels like it's loading up off idle or something. I feel like its running rich and maybe I need to adjust my choke..

I've got 9.5:1 compression, 270/278, .515/.530 cam, vortec heads, DUI HEI, QTF 750 elec. choke, vacuum secondary with annular boosters...
 
#17 ·
This says timing is too low, or carb is lean....but.... Carb may appear lean because of choke setting, cold weather air is denser....are you sure it isn't just the weather affecting it, especially if this is an open element air cleaner and not a factory warm air supplied snorkel air cleaner.

It has been colder than normal in Texas the past few days. Probably nothing has changed except weather conditions you are subjecting engine to this week.
 
#11 ·
Those cam specs you posted sound almost like its the same cam I am running in a 350 build. Are you running a lunati voodoo hydraulic roller cam by chance? 270/278 219/227 @ 50 515/530 lift. If so I have that cam in my 350 with dart 180 shp heads and a quick fuel 680 vacuum secondary carb but temporarily put my old 600 vacuum holley back on. You will need to run at least 16 degrees of initial timing with that cam or it will never run right.

I had a lot of problems with idling late last fall with my new carb and I had changed out some ignition stuff and marked my distributor and thought I was on the correct setting after putting things back together and fought for three days trying to get it to work right.

Finally I got my timing light out and was off by over 10 degrees. You can get a good timing light for about $35 or so and are as easy as heck to use. Also on your dieseling issue that happens a lot of time when you have to turn you idle up too much on the primary and expose too much of the transfer slot which will make it diesel when you shut it off

You might want to take a 3/32 allen wrench and open up your secondary butterflies by a 1/8 of turn and then close your primary about a 1/8 of a turn. Check and verify timing first before you adjust anything carb wise as it will make a difference.

I run 16 degrees initial timing with vacuum advance disconnected and have 15 degrees of mechanical timing all in by 3000 rpm and then run my vacuum advance on full manifold which I get 12 degrees from it so it comes out like this. 16 degrees initial plus 12 from vacuum advance gives me 28 at idle. Mechanical total is 31 degrees with 12 degrees vacuum advance for a total of 43 degrees while cruising and works really well for me on 89 octane but I have 9:1 compression.
http://www.corvette-restoration.com/resources/technical_papers/Timing101.pdf
 
#15 ·
I had a lot of problems with idling late last fall with my new carb and I had changed out some ignition stuff and marked my distributor and thought I was on the correct setting after putting things back together and fought for three days trying to get it to work right.

F
http://www.corvette-restoration.com/resources/technical_papers/Timing101.pdf
I wen't through the same thing, had to replace intake gaskets and wen't through hell getting the distributor to drop in the right spot. Turns out I had it a tooth advanced still when I finally took it to the shop for a timing check.
He told me what he set the timing at, but I forgot as I do most things.
 
#12 ·
#13 ·
Dieseling: are you running an electric fan on a switch? if so, you need to change your fan power to utilize a relay. running a fan with a live switch on full fan power can cause dieseling.

Carb: set transfer slot on main bbl's. take carb off, adjust idle screw to show correct amount of transfer slot. adjust base timing to control large adjustments in gross idle speed. use the idle screw for finer adjustments.

Base timing: be sure to remove your vacuum advance line from the can on the distributor and plug it (no vacuum leak). the engine will idle down. twist the distributor until the desired base timing is achieved. a dial-back timing light is nice for this, but you can do it with an incremented timing tab on the engine.

**are you sure that you're using the correct timing pointer? SBC's aren't all the same. some time TDC at 12 o'clock, other at 2 or 3 o'clock. **

lock down the distributor at the correct base timing and then rev the engine to roughly 3,000rpm to check the over all advance.

**FYI the timing note: '22* @ 3,000 rpm' probably indicates that you have a total mechanical advance of 22*. this means that you can expect to add 22* of timing centrifugally at 3,000 rpm. so, if you put the base timing @ 750 rpm, the advance weights aren't slung out and advancing the timing. so if you set your base timing at 14* @ 750rpm, ADD 22* to 14* and that's your total mechanical advance (36* total mechanical, which is probably close).

the engine will probably run 40-50* of timing advance at some point with the vacuum advance reattached. set your initial timing with the advance disconnected and plugged and you'll probably see a difference in idle quality.

i'd start there. honestly, i'd start with the timing first then check the carb after wards. timing checks are usually faster.
 
#14 ·
Thanks guys, I will buy a timing light and try to put some degree tape on..
The issue seems to have subsided for now, I am about to leave for work and it has been sitting for about 4 hours now so I will see how it acts cold.

Eric: Yes, I am running the voodoo 268, 60121 cam that you mentioned.
 
#16 ·
. A cam this size will lope at idle... might try 650 RPMs in park and 600 in gear and let it lope... this may cure the dieseling... by allowing cooler idling...


. ( May have to speed idle back up if have to take in for emissions testing )


. May also need a different size power valve if it's opening at idle...
.
 
#19 ·
Yes I think most of my problem is in my choke and has to do with the weather...

I turned my distributor a little to advance the timing and my low rpm stumble is completely gone now. I think Eric was right about this cam needing more initial to run properly....
Haven't had any more dieseling issue either although I still may try to turn my idle down a bit.. Although it sounds really good and full of bass at 750 rpms and any lower of an idle and my volt gauge starts reading low and oil pressure is down.

I really think the carb back firing when throttle slammed at idle in park has to do with the choke. I am going to go outside and mess with it now that it's been sitting all night.
 
#23 ·
I run a one inch 4 hole carb spacer but no exhaust heat crossover and I don't have egr on my intake and I run a 14x2 open air element and I can tell you it takes a little while before the carb gets warm. I can have it running for a few minutes and you can feel the heat from the engine and if you touch the carb its still ice cold. The carb for me needs a good 5 minutes of run time after my temperature gauge hits 180 for it to run fine. Carbs don't run well when cold and need some heat in order to run right but yet no too much.
 
#29 ·
Hard to tell in the poor quality cell phone pics but yeah, If you look closely you can see that I have a hose run from the heater core outlet port to the plenum under the carb and then into my heater core.
 

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#30 ·
A water line hooked up this way is not a crazy as some may think, in a cold climate it is one way to warm the carb up.

Dart a few years back used to offer a high rise dual plane intake called the "Kool Can" that had a water cavity under the carb, the idea for racing was to hook it up to a cooler of ice water and a small pump and use it to cool a hot intake charge down. Like an RPM Air-Gap on steroids :D
 
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