350 dieseling

[Smoke6]

1st post... new to site. Here is the situation. I have a 1986 Chevrolet K5 Blazer. It originally had a 305ci with all the smog and emissions available. The 305 popped a piston top so instead of repairing the 305, I replaced it with a 350 which was remanufactured just before it was shipped to me. It has a MILD cam, Edelbrock Performer RPM intake, Edelbrock AVS Thunder Series 650 with electric choke, Flowtech headers, 2 1/2 inch true dual exhaust with Terminator mufflers and no EGR. The engine still has the factory fuel pump on it which apparently produces too much fuel pressure (as it used to feed a Q-jet). The problem I am having is that it runs quite rich and "diesels" about 95 percent of the time once it is warmed to operating temperature. I dont have a tachometer so I dont know exactly how fast the idle speed is, but it sounds to be in the 900-1000 rpm range at operating temp. At initial startup though it tends to run quickly when the carb is warming but after that the idle speed seems low enough. There is a defininte presence of rich exhaust but after adjusting the Idle Mixture screws all I accomplished was creating a hotter, rougher engine. I have moved the timing all around trying to get it to stop and am running out of fresh ideas on how to make it quit running on when I try to shut it off. Any help will be appreciated.
Smoke6

 

[crazy4offroad]

Are you sure it's running rich? Black smoke? On cruise or WOT only? Do you have a vacuum gauge? If so hook it up and tune the idle mixture screws till you get the best possible vac reading. Then turn them in (tighten) until you shave about 50 rpms off the idle. This is called "lean best" setting and will keep your plugs clean, but not so lean it will run hot or diesel. I know you said you didn't have a tach but you should get one. Also check your ignition timing. And if you have one of those nifty clear, cleanable/reusable fuel filters throw it in the trash, it don't filter anything. If you still have rich condition you may need to get a tuning kit and adjust primary jets, metering rods and different weight springs to get your cruise and step-up mixtures right. If rich on WOT you'll need to dial in your secondaries with smaller jets. Hope this helps.

 

[Jims86]

Run on is usually caused by a vac leak, and curb idle(throttle stop)screw open too much.
and as far as running too rich, check the float hight in the bowl...especially with an Edelbrock carb. I have found a few a little too high. Reset the idle screws at 2 turnes out from closed, and fine tune from there.
Make sure you check your timing in gear, with the vac advance unplugged and block the vac hose. Try for a 525 to 575 in drive.

 

[Smoke6]

Well... I dont have a vacuum gauge but i do have the "nifty clear filter" which to be honest has been a pain in the ass anyway. I will replace it. I will baseline the carburetor and reset the timing again. More to follow. How big of a job is it to replace the primaries if that should be required?

 

[crazy4offroad]

Well... I dont have a vacuum gauge but i do have the "nifty clear filter" which to be honest has been a pain in the ass anyway. I will replace it. I will baseline the carburetor and reset the timing again. More to follow. How big of a job is it to replace the primaries if that should be required?

It's a Carter style carb, it's easy. Maybe 10 screws to remove the top. After that you can set the float level, drop, change metering rods/springs, primaries and secondaries. I looked briefly on Edelbrock's website and couldn't find a manual that covered it all, gotta split here soon but you may be able to find one.

 

[Smoke6]

I have adjusted the Timing and reset the idle mixture screws to "Best" position about 1 7/8 turns out from closed. I have installed a fuel pressure regulator set to 6 psi as well. The dieseling has almost stopped now. I have noticed that it does not do it if the engine is not at operating temperature. After I have driven it for about 10 minutes... it either will or wont. When it does the idle speed is a bit faster than when it does not. Could it be a choke adjustment? I have the electric choke which is run to a 12 v accessory feed at the fuse panel (where it was tied in from the factory). The choke has about 3 screws which hold the rotary adjustment knob on the passenger side of the carb. What is the correct procedure for setting this. The reason I suspect the Choke is due to the fact that sometimes the idle at operating temp is sometimes 650 and sometimes is around 950 while in park. Could it be the throttle return springs? What else could it be?

 

[Swims350]

well since you set the timing, that should be ok, but what exactly is it set at? does it spark knock at all at wide open throttle? or under load? if so it needs backed off more.

The idle mixtures help some but did you set them with a vac. gauge to get the most vac. out of it? If not then they are not set "best".

The electric choke if it's holding the idle speed up, then yes it can very well be it.

Have you set the idle down? not mixture screws but idle speed?

Have you tried higher octane gas?

There's usually 3 things that cause it...

too high of idle,

too much timing advance

and too low octane fuel.

of course the idle can be numerous things, idle mix too out of whack, causing it to be high, advance too far making it high, vac. leak, etc.


Also another FYI you know if you shut it off in gear it won't diesel right? I had to teach my brother in law that trick.

 

[crazy4offroad]

I would get it up to operating temp, and while it's running loosen the 3 choke screws, and turn the housing counterclockwise (I think, lol) and you should see the choke plate open. Also look at the driver's side of the carb throttle linkage, down near the bottom, That's where the choke high idle cam is. when it is at operating temp it should not be set on that step-up cam. I think there is a link rod on one side or the other that has a bend in it that you can straighten out a little to also make a small adjustment. Check Edelbrock's website, I believe they have the manual for your carb and it includes proper adjustment steps.

 

[Smoke6]

Resolved!!

Thought I would take a minute to thank everyone for their help and update the Web with the steps which finally got the old truck running correctly.
Biggest problem was.... as predicted.... Vacuum leak. I had an intermittent loss of vacuum in one of the rigid plastic lines coming from the Evap canister heading to the front of the Carb. After correcting the low vacuum, I was able to get a fairly stable vac of around 16.
Once Vacuum was corrected, I moved to timing. While attempting to set my initial timing it became clear that I would not be able to use the existing Timing Tab on the front of the engine as it did not line up with the mark on the balancer. The best I can offer you on this one is, its somewhere between 14 and 18 deg. Essentially I just adjusted the timing until my vacuum improved more and the engine idle came up slightly. This yielded a new vac of around 18 or so.
I moved the to the idle mixture adjusment screws which were somewhere in the 1.75 turns out range. With the truck off I screwed them in all the way, counting the turs until they stopped to ensure the seats were clean. Then I backed them out to 1.75 and re-cranked. Using the Vac Gauge I adjusted each side in appx 1/8 turn at a time until the vac begin to deteriorate. This happened around 1.4 turns out on each side so I backed out the screws to about 1.5 turns on each and the vac came back up. After adjusting idle mixture I lowered the Idle speed to achieve about 800rpms at idle and about650-725 when in gear.
Furthermore, It became apparent to me while working through all of these issues I had a signifigant exhaust leak coming from the Header and Collector gaskets on the passenger side of engine. Thinking it was just a mild annoyance all along it never came to the front of my to do list until the truck was running better. So after getting the run on to quit.... I installed a set of REMFLEX header and collector gaskets and low and behold, not only do they work AMAZINGLY, the darn truck seems to run even smoother.
So, Long story short (too late), if others have a similar condition where the truck RUNS ON incessantly, CHECK VACUUM LINES. Thought mine were ok then I started wiggling them around. No matter what adjustment I made, it didnt matter due to the fluctuating vacuum. Just my .02!!
THANKS AGAIN.