View Full Version : dieseling problem
ChevyIIMan
07-30-2005, 11:26 AM
Ok after replacing the busted valvespring, after resetting the valves, resetting the timing, ect.... the car still diesel when you shut it off. Now the car seems to run fine, doesnt run hot. But will diesel every time you shut it off. I am at a loss as to why shes still running lean and need suggestions.
Compression on that cylinder is good so I assume I dont have a bad valve. What else can I check? Possibly a pushrod? I checked it on a straight edge and it looked ok. When setting the valve im going while running back off till they tick, tighten till they stop and then 3/4 turn. Possibly still a valve out of adjustment? Timing is @ 12 initial also. HELP guys im stumped.
MrQuick
07-30-2005, 11:43 PM
how high is your idle speed? check to see if your throttle blades are open past the transfer slots, also if your secondaries are open as well.
andrewb70
07-31-2005, 04:13 AM
Do you have an MSD box?
Andrew
ChevyIIMan
07-31-2005, 08:40 AM
how high is your idle speed? check to see if your throttle blades are open past the transfer slots, also if your secondaries are open as well.
1000 or so at idle, 800 or so in gear. Note nothing has changed on the car since before the valve spring broke but i have double checked all the settings.
ChevyIIMan
07-31-2005, 08:41 AM
Do you have an MSD box?
Andrew
Yes MSD Digital 6. I am going to try and back the valves off 1/2 turn or so to make sure im not hanging a valve open. but I wonder if I damaged the valve when the spring broke possibly? The cylinder has good pressure 210psi so I dont think anything is bent. I am going to pull and inspect all of the plugs and see what they look like.
andrewb70
07-31-2005, 06:16 PM
If you don't have a diode in the MSD wiring it will cause it to keep running even after you shut the key off.
Andrew
ChevyIIMan
07-31-2005, 07:36 PM
If you don't have a diode in the MSD wiring it will cause it to keep running even after you shut the key off.
Andrew
Well ive run this box for 2 years now and this is the first time its ever deisled. Once again the car never deisled until the valve spring broke.
LowBuckX
07-31-2005, 11:33 PM
I too have to agree that your throttle blades are open to far at Idle. Was my problem for a while also.
Maybe before/during your spring issue you needed a differant carb setting but now that everything is good you need to do more tunning.
Does your mixture screws have a big effect on your idle quality? If not
Adjust your throttle screw down till it is running very slow then play with the Idle mixture screws.
ProdigyCustoms
08-01-2005, 12:35 AM
Two things, if you throttle blades are to far open, you are idling off the jet instead of the idle circuit, which can cause this and a hard to adjust fat idle. Also, see if the fuel tank is holding pressure, make sure the tank is vented. If it is not vented properly, it will be under pressure it will push fuel after you trun it off causing it to diesel and make it hard "floaded" start.
ChevyIIMan
08-01-2005, 04:05 AM
Ill check the throtle blades tonight and see what they are set at. The tank is vented so no issues there.
Rubes
08-01-2005, 04:24 AM
I had a similar problem. it started all of a sudden too. Turned out there was a vaccuum leak in my brake booster.
ChevyIIMan
08-01-2005, 04:05 PM
Well I checked my carb settings, lowered my idle RPM to 900, backed the #7 valves off to 1/4 turn past clicking and still the same thing. So i pulled the plugs and found this.....
http://www.odiesgarage.com/Nova/images/G05Break/plugs
Check out 5 and 8 whats up with that?
zbugger
08-01-2005, 04:35 PM
#5 and #8 are more what your plugs should look like normally. Have you thought about why the valve spring broke? Have you looked at your timing chain at all? I'm just wondering if it hasn't stretched and is causing that.
yes 5/8 look good, the rest look like it is running too rich. Definetly put some new ones in. I don't think the timing chain being stretched would cause dieseling, especially if it runs fine. You need to take the carb off, and look at the bottom of it, there should barely be any of the transfer slots showing, they should look almost square. Adjust the idle screw to get them correct, that should get you the correct idle, also the idle mixture screws should be about 1 1/2 turns out. Also advance your timing to about 15 degrees and then turn down the idle, advancing the timing make ths idle go up, so then you can turn the idle down on the carb, this exposes less transfer slot. Do all that and let us know
ChevyIIMan
08-01-2005, 07:30 PM
The timing chain maybe has 10k miles on it if that. The spring had almost 40K with loads of drag passes.
I will check the Idle and carb, first im going to do a leak check on the intake. I assume if it was just an Idle issue the car wouldent read lean while on the road.
ChevyIIMan
08-02-2005, 04:24 PM
yes 5/8 look good, the rest look like it is running too rich. Definetly put some new ones in. I don't think the timing chain being stretched would cause dieseling, especially if it runs fine. You need to take the carb off, and look at the bottom of it, there should barely be any of the transfer slots showing, they should look almost square. Adjust the idle screw to get them correct, that should get you the correct idle, also the idle mixture screws should be about 1 1/2 turns out. Also advance your timing to about 15 degrees and then turn down the idle, advancing the timing make ths idle go up, so then you can turn the idle down on the carb, this exposes less transfer slot. Do all that and let us know
Done.... the dieseling seems to be gone but still a lean to almost off the scale a.f reading. I am going to put some more miles on it tomorrow and see how the plugs look.
well it might be time to play wiht the jetting, glad that got rid of the dieseling. I just had a similar problem, and doing what i said pretty much got rid of it also. Altough I would of liked to had my idle at 950(manual car) than the 875-900 now so my alternator would charge just a little more at idle. What kind of carb is it, and what jets are in it? Also AF guages can lead you wrong, if your plugs are that black, i don't see how it could be lean, unless you were running way to rich before at idle, definetly put some miles on it and check the plugs and let us know, also when it is lean? at idle or crusing
ChevyIIMan
08-02-2005, 06:44 PM
well it might be time to play wiht the jetting, glad that got rid of the dieseling. I just had a similar problem, and doing what i said pretty much got rid of it also. Altough I would of liked to had my idle at 950(manual car) than the 875-900 now so my alternator would charge just a little more at idle. What kind of carb is it, and what jets are in it? Also AF guages can lead you wrong, if your plugs are that black, i don't see how it could be lean, unless you were running way to rich before at idle, definetly put some miles on it and check the plugs and let us know, also when it is lean? at idle or crusing
When you fire it up it will read rich until the O2 sensor heats up ( a few seconds) then it will begin to slowly creep to lean. and stay there Idle , cruise, WOT you name it. The plugs will tell if the 02 sensor is bad after a few more miles.
It has a 750 DP with 69's in front and an adjust a jet in the rear which I have no idea where its set but that would only effect WOT right? Is it possible that a weak exhaust spring could have been giving me a false Rich reading? then as it got worse the reading went lean?
Is there any way to test the O2 sensor? Pull a wire or two off the plugs on that side, should givef a rich reading with dead plugs right?
No idea on testing the o2 or whether a weak exhaust spring would throw it off. However i think 69's are pretty lean for a 750dp. I don't know off the top of my head but i think stock they are 72's or something. You are correct about the secondaries, that would be only if your foot was in the pedal more than about half way. Forget about everything you had before, you need to start new and start reading those plugs, BTW the AF guage is only usefull at part throttle and idle, and even then they aren't that reliable. I think you miht of been either
1. idling rich before and lean when crusing
2.you were too lean in the primaries, but when you went to WOT the jetting in the rear was too much and would run too rich making the plugs black. I would find out what was stock in the front of that carb and put it back to that, then find a way to adjust the rear jets to whatever stock would be. Go from there, I bet you are too lean in the primaries with those 69's
Rubes
08-02-2005, 07:45 PM
... should givef a rich reading with dead plugs right?
I may be all wet here, but Ill take a stab at that. I would have thought so too, but the O2 sensor is measuring the amount of Oxygen in the exhaust and comparing it to the amount of oxygen in the free air (not comparing air molecules to fuel molecules). If you never fire the plug, all the O2 that went in comes right back out again, and should indiacate LEAN, since O2 should be used up in the combustion process.
I dont know, makes sence to me, and yet it doesent :pat:
Rubes
i checked the stock jetting for you it is;
71 primary
80 secondary
so you are a little lean in the primary, What are the specs on the motor? i have a feeling the carb is too big for the engine, and someone tried to de-jet it to run better, but the carb is still flowing X amount of air and needs the gas to match. Does it bog at all? I would put the 71's in the primay back in.
ChevyIIMan
08-02-2005, 08:02 PM
Yeah 71's were stock in front and I will swap those out. Unsure how I got 69's in there but they have been there for years from what I recall. 71's will go back in and well see how she does. When I tok the front bowl off today and saw the 69's I knew something wasnt right.
No bogging, I bought the carb new and jetted it a while back. Specs on the motor is...
355ci
GM L31 Vortecs
Comp 274 xe cam
RPM air gap manifold
Orginally ran a TH-400 w/ 3600 stall
Now has a 200-4r w/ a 1800 stall
Thank the tranny change could have anything to do with it? I swapped trannys maybe 1600 miles ago.
well that is borderline for a 750, I wouldnt' rule it out, but it might be too big, however it is probably fine. you might put the 71's in and find out they are too big, . I don' tthink the tranny would do anything if anythhing it has better gearing
LowBuckX
08-02-2005, 10:20 PM
You went from a 3600RPM stall to a 1800rpm stall? How well did the motor take that? 3600 sounds too big 1800 sounds to small.... sorry off topic
ChevyIIMan
08-03-2005, 04:03 AM
The motor was doing fine with the intital swap, 1800 may be a hair low for the cam. But it loved the 3600 stall... Ill eventually goto a 2500 or so.
ChevyIIMan
08-03-2005, 08:37 AM
Although since im not drag racing it like I used to I have comtemplated seling my 750 off and buying a Edelbrock 650cfm Thunder Series AVS carb instead. The 750 DP was great for drag racing but maybe not so much for a cruiser/road car now.
ChevyIIMan
08-03-2005, 04:57 PM
Well ran the new plugs. While I was attempting to set the idle mixture I check the plug they all had a light black coating on them from the idling. I took it out on the highway for maybe 5-10 miles or so and came home shut it off without any idling and they are all WHITE as can be.
See the pictures here http://www.odiesgarage.com/Nova/images/G05Break/newplugs/ , the black dusting on them after idling covered the entire plug, once they were run the black was either totally gone or gone from the tips of the porcelain. Just white porcelain, the picture may be deceiving but they are bright white. And you can see the discoloration in the ground strap also.
Oh and shutting it off today she deisled again. Now I checked the plugs shortly before heading out for my trip and they were all a nice tan color, even on the dark side a bit but she did run rich. Why all the sudden does EVERY cylinder run super hot? What in the world changed in the course of 1400 miles?
paul67
08-04-2005, 04:43 AM
Would the timing do that, maybe its advancing to much on open throtle see if it pings under heavy load, it might be a vac pipe does 1 go to the carb . just a guess
paul67
ChevyIIMan
08-04-2005, 05:02 AM
No pinging under load that I can tell. Timing is at 12 initial and 28 iirc total. Ive checked all the vac ports on the carb, none of them leak and they are all sealed up, the carb gasket doesnt leak, the flange is flat also. Im going to check both my fuel filters and make sure they are not clogged, check my fuel lines for bends or kinks that may have happen on the trip. Either way its not getting enough fuel or too much air. Now I just gotta figure out which one and why.
paul67
08-04-2005, 07:37 AM
Are you running elect or mech fuel pumps, on when I had a elect fuel pump it would run ok but would run lean found that the pump gears were worn got flow but very little pressure, found this because the fuel filters were only showing 1/2 full changed to mech pump no probs.
paul67
ChevyIIMan
08-04-2005, 08:53 AM
Electric Pump but the fuel pressure guage at the carb shows fine pressure.
paul67
08-04-2005, 09:06 AM
what pressure are you running
ChevyIIMan
08-04-2005, 09:13 AM
what pressure are you running
5~5 1/2 psi if I remember correctly.
paul67
08-04-2005, 09:14 AM
Also the presurre when the engine at open throttle
No pinging under load that I can tell. Timing is at 12 initial and 28 iirc total. Ive checked all the vac ports on the carb, none of them leak and they are all sealed up, the carb gasket doesnt leak, the flange is flat also. Im going to check both my fuel filters and make sure they are not clogged, check my fuel lines for bends or kinks that may have happen on the trip. Either way its not getting enough fuel or too much air. Now I just gotta figure out which one and why.
hey, you were supposed to advance the timing to 15, your timing is too low, you need to advance you timing, that raises your idle, and lets you turn down the idle on the carb reducing the transfer slot, and lets less air in. Okay grasshopper? also 5 1/2 PSI is too liitle for a holley they like 7psi, however thats probably not your problem, oh yeah also check the springs and weights under your cap like paul mentioned make sure they aren't sticking
ChevyIIMan
08-04-2005, 09:28 AM
I did advance it to 15, turned the idel down ect... no diffrence everything stayed the same. Tonight I will pull the distributor and check those weights, its a pain to get to on my distributor honestly (Mallory Magnetic Breakerless Distributor). I can try and bump the pressure up a bit, paul not sure of the WOT pressure dont have a guage to try on the road. Can I just rev it up in neutral and see if it drops?
just bump the pressure up, revving it in neutral will do nothign. Also make sure your floats are low
ChevyIIMan
08-04-2005, 10:26 AM
just bump the pressure up, revving it in neutral will do nothign. Also make sure your floats are low
Make sure they are low? or arnt? Ive got them set so with the engine running if I slightly rock the car fuel runs out.
ChevyIIMan
08-04-2005, 03:08 PM
Fuel system checked out ok, I pushed the PSI up to 6.5 and still the same.
Charley Lillard
08-04-2005, 03:44 PM
Time to grasp at straws... Have you changed brands of gasoline ? Good gas ? Gas tank or cap vented ?
ChevyIIMan
08-04-2005, 04:03 PM
Well I was on vacation durring that time I had 5 diffrent brands in it and itleast 2 diff brands while it was showing lean. It has a frsh tank of the local gas I normally use in it now, and the fuel cell is vented.
Also I changed the exhaust from dual 2.5 to single 3in pipe but had that on the car for close to 1000 miles before these problems started.
well i didn't think it was causing anything, just that holleys like 7PSI, did you check the weights on the dist? what vacuum does the car pull at idle and what powervalve do you have?
ChevyIIMan
08-04-2005, 07:40 PM
well i didn't think it was causing anything, just that holleys like 7PSI, did you check the weights on the dist? what vacuum does the car pull at idle and what powervalve do you have?
Stock powervalve and the car is pulling 5-6 @ idle, bounces a tab but I assume thats from the cam
what 5-6 at idle???? that is less than my cam? Is that in park or drive? you probably need a 3.5 or 4.5 Powervalve, stock is probably 8.5/ or 6.5, which is probably opening and enrichening the circuit
ChevyIIMan
08-06-2005, 08:59 AM
what 5-6 at idle???? that is less than my cam? Is that in park or drive? you probably need a 3.5 or 4.5 Powervalve, stock is probably 8.5/ or 6.5, which is probably opening and enrichening the circuit
Thats in park. If the Power Valve was opening and enriching the circut would the car run rich and not lean??!?! Once again same cam, same carb for years. No problems till this. Now I took thsi reading from the port on the front lower of the carb where my transmission used to hook up to, is that correct?
yeah thats correct, I'm all out of ideas
paul67
08-06-2005, 11:10 AM
you say that you used to connect the trans to carb, then maybe its pulling to much vac when warm, i had this on a range rover ticked over when cold then increased when it got warm because it drew more vac and altered the timing. just a thought
paul67
paul67
08-06-2005, 11:34 AM
Ok, over here in the uk we call this preignition , popped to see and old school car chap and he says this caused , if the plug are white the carb fuel need richening up and the timing retarded as the cylinders are hot causing it to over run , classic car that used to do this on tv was the detective columbo's as it still ran when was walking away from it .
paul67
ChevyIIMan
08-06-2005, 12:49 PM
yeah thats correct, I'm all out of ideas
yeah last thing I can do is pull the carb fully apart and clean all the passages out, make sure it not leaning itself out, then just try jetting it up.. Other then that I too am out of ideas.
ChevyIIMan
08-19-2005, 06:26 PM
Well I completely stripped downt he carb had it hot tanked and put it back toghter.... same issues. Next im going to try a buddys carb. On the vac readings that was a problem with the guage the car pulled 13with a diff vac guage
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