engine idle

genolan · #1 (**permalink**) 12-06-2013, 06:09 PM

I just looked at a 2007 4300 miles Backdraft 408 stroker 520 hp and noticed the engine idle was around 1k rpm's even when warm (it was 30 deg air temp) and when shutting it off it had some dieseling when shutting off. We did a full compression test and all were 195. Is this normal if not what could be the cause. The car was running lumpy at idle even with new plugs put cleaned up at 2k rps. Mechanic said that it's a aggressive Cam and that's how they run. This was all part of a pre purchase checkout. We found a minor oil leak and the car needs some detailing.

madmaxx · #2 (**permalink**) 12-06-2013, 07:50 PM

Everything sounds perfect. Welcome to cobra world

MFE III · #3 (**permalink**) 12-06-2013, 08:31 PM

Most times, dieseling is caused by too high an idle.
You could probably drop the idle to 900 and it'll go away.
Wouldn't worry about it.
Buy it and enjoy it!

WardL · #4 (**permalink**) 12-06-2013, 10:20 PM

I was thinking about this today. My Cobra idles around 900 RPM when it is cold. It doesn't have a choke so it doesn't run very well when it is cold. By the time it warms up it idles at 1050-1100 which is too high. Why is there a difference between cold an warm idle? I have a Roush SRTW with a Bigs Stage 5 Holley carb.

vector1 · #5 (**permalink**) 12-07-2013, 01:49 AM

I'd fire the 'mechanic'.

tpatodd · #6 (**permalink**) 12-07-2013, 05:31 AM

So happy to read the responses here. When I bought mine (427SR) it idled ~1100 and I thought it was high. I turned it back to ~900. When I sent her to BDR's service they returned her at 1100 again.

I had some knocking when shutting her down so I have dialed it back to 900. Now it is seems perfect.

I am not sure if the 1100 is a suggested setting for my motor but I like the lower setting better.

genolan · #7 (**permalink**) 12-07-2013, 05:59 AM

Thanks all it sounds like it's expected. I guess these cars aren't production cars so I had better clean off my tools.

Ron61 · #8 (**permalink**) 12-07-2013, 06:25 AM

I had a pretty aggressive cam in mine and it idles at about 1100 RPM. Anything below that and it loads up. It really starts to smooth out at around 3500 RPM. I never have had it deisel even on some of our 118° Summer days here.

Ron

blykins · #9 (**permalink**) 12-07-2013, 06:40 AM

I set my solid roller engines to idle at around 1200 rpm, and they still shut off without running on.

Without knowing the engine's specs, anything any of us will say will be total speculation. However, a high idle with dieseling could be a sign of a vacuum leak.

genolan · #10 (**permalink**) 12-07-2013, 07:50 AM

Just a thought could it be stale gas the car has been sitting for 6 months? I was also thinking the Carburetor may be gummed up. It runs well once warmed up with no pinging and solid performance.

vector1 · #11 (**permalink**) 12-07-2013, 06:39 AM

I would find out why it experiences 'run on' before I bought it. The high idle for me is no problem, some guys like a low idle, mine idle 1000-1200 and don't run on. I would gamble a $5 it experiences pinging somewhere while driving in the present configuration. Then you got to wonder what else you are gonna find out if the present owner can't solve his 'run on' issue.

RET_COP · #12 (**permalink**) 12-07-2013, 02:42 PM

Set your idle and mixture settings on a HOT engine. This is the way it will idle HOT. Cold engines will not idle well until HOT. Keep you foot on the pedal at a higher rpm until HOT or just drive it away when it's warm until HOT.
Just having some meaningful fun but a carb with no choke will act like this.
As far a dieseling, bad fuel, not the right fuel, vacuum leak, wrong timing.
I usually set my idle at about 1000 rpms. Above 1200 rpms with a light spring in the dist. the advance starts to move so I stay below that.
My cam is in the high .500s with a 233/237 dur.

WardL · #13 (**permalink**) 12-08-2013, 09:09 AM

Thanks for the info RET COP. If the gasoline is properly stored, it won't go bad in 6 months. The primary issue for old fuel would be moisture or water.

RET_COP · #14 (**permalink**) 12-08-2013, 06:43 PM

Fuel from the gas pump is a crap shoot. You'd be surprised what comes out of some. During the very hot summer days I have had some minor dieseling issues. Not sure if it was some bad gas. When I add some GOOD racing fuel to the same tank of gas the dieseling is gone. SCR 10.75

195lbs. cranking compression is getting up there. What octane fuel are you using?
Another area to look at would be the heat range of your plugs.

Pman1961 · #15 (**permalink**) 12-08-2013, 10:02 PM

My car has an aggressive cam and idles 1150 +/- If I run my rpm's any lower the motor sounds like it is laboring or lazy. My distributor is locked out at 34 degrees. I can experience run on from time to time. I've got in a habit to have the brake on and turn the ignition off at the same time I'm releasing the clutch while in gear.