Low RPM/off Idle stumble

Bosshawg600

Bosshawg600

PDP.com FireFighter
Joined
Aug 3, 2006
Messages
3,651
So the truck runs pretty good now (for not having seen the road yet). but for some reason there is an off idle stumble, probably from 850rpm to 1K rpm, after that it just wants to skyrocket, controlled though.

probably one of the weird things about it is that when trying to start, if I just give it a little skinny pedal, it instantly dies out. Usually this helps start up, but not on this new motor.

im thinking this might be the high timing/big injectors/low compression piston combo. Im running somewhere around 25-28* timing. Should i back it off a bit to help start up and get rid of the stumble, or is this just a new motor quark that will work itself out???
 
PDP 4k's for now. Seems as though when I push the throttle just a bit, it sounds and feels like its flooding it out, then kicking back in and taking off like normal.
 
Im never 100% positive on anything.

Pump lift was 7.3mm on a 160 pump, but there is variables in there that might have changed it slightly (shaft moving, bad math, guage not 100% accurate, things like that) so thats why I used an "ish" number. With whats in the motor, 25* is the sweet spot.

My fuel pressure is a constant 45psi, read off an Areomotive regulator.

Truck runs fine other than that, idles fine, drives fine, all that stuff.

Im just thinking it might be the HUGE Competition injectors combined with the low compression pistons somewhat flooding it out. Ill see how it does when the motor is run for a while and at operating temp.
 
my truck is a mostly stock 94 12v 160 pump, other than ddp stg 2, after the sticks it had the exact same simptoms as you speak of, fergot 16.5 timing
 
Well, just got back from trying to start it. Every time I try and give it a little throttle to get it going....instantly dies. and it doesnt take any throttle at all to do it, just a little bump and it instantly dies out.

Timing hasnt slipped, all fuel lines are bled, I am once again at a loss with what the heck is going on.

Im thinking of taking the Scheid injectors out and puting the stock ones in...maybe the injectors are out of ballence or just not working right?????
 
Ok, went out and got it stated again.

Takes FOREVER to start! But one of the strangest things is....it doesnt Black smoke AT ALL. Reving it up...nothing, driving it down the road...maybe just a little grey smoke.

It does haze a LOT and just continuly bellows white. No once its started, everything runs fine, all pressures are good, no oil consumption nothing weird.

Im just gonna run the numbers again and figure out were exactly the timing is. And If all else fails, swap injectors.
 
Yep. i had at least 10K miles on them before i pulled the motor. So those are fine.

But I will say, I did notice the RPM's at idle are at 600. they were 900 before the motor was torn down.....
 
Ok, Im gonna go out and try to check the idle and Gov. springs.

But Im gonna post something i know im gonna get hammered on for!

By me saying the truck is impossible to start...we auctually have to start it and keep it running on ether for about 30sec to a minute before it will auctually start to run on its own. All we do it just give it a little squart accross the front of the turbo so it sucks in the fumes and not a ton of liquid going inside. But it take quite a bit of time before we can give it less and less ether and it will finally run on fuel.

This whole time when it is running on ether, if you as much as touch the throttle...instantly killes it. So I will sit there with my hand on the throttle and slowly try and rev it. 3 to 4 times it will instantly kill the motor, but after anout 45 seconds of trying this it will spit and sputter but finally rev a hair while bumpuing the throttle. A few revs later and it will finally rev and run on its own.

Something is Definatly not right!!!!!!!!
 
Bring your idle up, if that doesn't work,
Then back the timing off, and see if it runs better.
I had my truck at higher timing and it required ether everytime to start it, just a shot though. Bring the timing back down and it fires right up.
 
Just got back. i think it might be the idle. I gave it a small shot of ether, gave it about 1/4 throttle and it fired up pretty good.

Now I got to find out why its so sluggish and has almost no low end power. Had to use 1st gear evertime just to get in moving....never had to do that before......but at least it runs. Thinking of taking it out for a nice long road trip in a bit (well just from the shop to the house).

might just play with the AFC and see if that takes care of the sluggishness (its slid all the way back and the housing is all the way back to) so basically Im running as little fuel as possible out of the pump...minus the Lazer cut DV's!

But i think it smokes just a bit much at idle.......

idlesmoke.jpg


idlesmoke2.jpg
 
Ok, Im in desperation mode!

I thought that it might have been my competition injectors....well it isnt. I swap out my big stick for a set of stockers and it still does the same thing!

Brought the idle up a bit....makes it harder to turn over.

Checked my math on the timing and it is sitting at 22.5*. Im gonna try and set it again some time and see what that doe and what the differences are. I know I did the timing right, and now i know my math was pretty good. but this is the last thing I can think of.

Guys I REALY need some help here! I gotta get the truck up and running by TS or the entire TS trip is gonna SUCK with out the truck!!!!!!!

Smokem? ZStroken?? ED guys??? I need some pointers!!!! Ive done everything suggested to no avail. Im ever will to pay VERY WELL if someone can get my truck running!!!!!!!!
 
When I put the 4k in my pump and rebuilt engine it wouldn't idle on its own for about 30 seconds in early winter, but with more miles it starts easier and runs on its own with temps in 30s. Are you letting it warm up any before trying to drive? Mine would spit and sputter for a minute or so even at 60+ degrees outside. The more I drive the better it starts and runs. It might be partly the new engine, I know mine has gotten better with time. Adjusting the idle up helped mine the most about starting with no throttle. My timing was only at 18.5 and it took about 5-8 seconds of cranking to start. I would drop the timing back to 18-19 and see if that helps.
 
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