78, L82, spark knocks after it is warmed up, i have re set the timing and if anything it seems worse.. Any sugestions ?? Thanks

 

Did you check the advance mechanism in the distributor for sticking? Did this happen after filling with gas and if so have you tried another fill up?

 

What are your timing settings? initial, mechanical all in minus vacuum and all together mechanical, initial and vacuum at 3000 rpm. How soon do you have the mechanical fully advanced (at what RPM)? In what condition does it detonate. WOT, all the time or light throttle at cruise?
Are you using a digital or dial timing light or standard light with timing tape or marks on balancer?

 

The advance isn't sticking, I had a shop set the timing,all I was told that it was set back to 8 degrees from 10 and that the factory setting is 12.. It does it after it is warmed up ,under load and also light throttle when cruising..

 

if the shop set the timing how do know the advance mechanism inside the distributor isnt sticking. A stock 78 L82 has relatively low compression so should have no issue with pump gas. Another possibility is the EGR valve is stuck open.

 

Is there a way to tell if the EGR valve it stuck open ?

 

this video, even though a newer vehicle, may help you could also eliminate it with this

http://www.summitracing.com/parts/mr...make/chevrolet

 

'78 L82 engines were built to run knock free on 87 octane gas with conservative stock OEM timing.

If you've already got 4 degrees less advance than stock, it's likely not timing or it's not really knocking (detonation) that you're hearing.

 

Ok, thanks a lot..

 

If you have a shop doing your maintenance then be prepared for a lot of frustration. It will be difficult at best and likely nearly impossible for us to help you diagnose the problem without you knowing the specifics. I'm not sure how to help unless your doing the timing yourself and have some knowledge of what changes have taken place. Do you have the ability to do th timing yourself?

 

I don't have the tools to do a lot of the work myself..

 

Ok, I have an idea.
If you are experiencing detonation at light throttle and not at WOT then I would suspect that the vacuum advance is the culprit. If you did not have detonation before taking it to the shop these guys may have actually advanced your initial timing and not retarded it.
We can retard your timing very simply by unplugging the vacuum advance can. This will not fix your problem it will only be used to help diagnose it and to see if your are in fact having detonation problems.
I am assuming that your mechanical experience is limited. If I am wrong let me know and we can move along more quickly. I don't want to insult you just trying to help if I can.

In this picture you see grey cap with wires coming out, that is the distributor cap. It is mounted on the distributor. Coming out the front of the distributor you'll see a silver conical can with a large hex shape on the end. This is your vacuum advance can. Yours may look slightly different but it should be close to this.

It also has a black hose connected to it that runs to the carburetor. To access this area you may need to take off your air cleaner assembly.

[IMG][/IMG]

I want you to disconnect the black hose from the silver can and plug the hose with a bolt or golf tee or something that will make a seal and not fall out. Don't worry about the can and it's hole, it will be fine.
Now put your air cleaner assembly back on. Go take the car for a drive. It will not run as well and may seem gutless or have a hard time idling but don't worry about that.
What you are listening for is the absence of the rattling detonation sound that you have been hearing. If it is gone then your timing is too advanced. If not then it may be something else your hearing.
Then try stomping the throttle wide open briefly, I'm not saying go fast this can be done in first gear or from a stop. If you hear the detonation start let off the throttle immediately.
Like I said If I'm being too simplistic let me know.

 

Thanks , I will try that.. It might be a few days before I have a chance to get it out and test drive it.. But I will let you know as soon as I do.. Also, I won't be insulted at all.. Thanks for taking the time to help..

 

Do you have a way to do this test without going too far from home. Like in your neiborhood? The reason I ask is if you run with your timing seriously retarded the engine will run hotter and the exhaust manifold will get pretty hot as well.
So you don't want to run for any longer than necessary in this condition. If you can't stay within a few minutes of your place then locate the items I pointed out and make the changes once you get somewhere where you can test it, and after testing it change it back to the normal condition to drive home.

 

Thanks I have plenty of places to drive and stop in my neighjborhood.. I will try that as soon as I can and let you know.. I havn't had this car very long , it is a numbers matching car snd the motor has headers ,,42000 miles on it .. I don't know if the motor is stock other than headers..

 

I did get a chance to drive it this afternoon, and I didn't hear any knocking at all.. I didn't idle to well ,other than that it seemed to run fine.. Can it be left this way and just adjust the idle some ??

 

If you did that you would have lousy gas mileage, and poor performance off idle and all around.

The guys who did your timing may have inadvertantly advanced your timing instead of setting it correctly. Based on this assumption your can either take it back to them and see if they will fix it correctly or you can tune it by ear as I will explain. This procedure is far from ideal or accurate but it can get your detonation to stop. Best plan is to get it fixed or learn how to fix it yourself. Get some books on tuning chevy 350's or get on line and do a lot of searches to find out how.

So to fix this for now what we can do is loosen the distributor hold down bolt and rotate the distributor clockwise. You want to do this about 1/8 inch at a time. Use the connection that the vacuum hose connects to on the vacuum can to gauge the distance you are rotating the distributor.
So loosen the nut which is down below the vac advance can where the distributor goes into the block just enough to rotate the distributor.
This will take a 9/16" end wrench. At least on mine it is. Could be 5/8". Once you got it loosened a little bit get a ruler and set it up so you can measure the distance your going to rotate it using the connection of the vacuum can to do so. Move it by pressing on the vac can rotating it towards the firewall of the car or in a clockwise direction.
Tighten the hold down bolt and test drive. During these drives you will have the vacuum hose connected to the vacuum can unlike your previous drive.
Listen for the detonation. If still there repeat the procedure again. Move it 1/8 inch, and remember to tighten the hold down bolt each time very important!
Repeat this procedure until the detonation is no longer heard. You will then have a roughly timed engine that at least will not destroy itself.
When all done you may then adjust the idle to something that works well.
As you go along you'll notice that your idle speed will get lower. This is correct, as you retard the timing idle speed reduces as you advance the timing idle speed increases.
So note your idle speed now when the engine is fully warmed up. Write it down. If you loose track or accidentally bump the distributor and don't know where it's at you can get back to where it started by turning it in the correct direction to get back the same idle you had before.

In this picture my finger is over the distributor hold down bolt. That hole next to it is where the distributor goes into the block.

[IMG][/IMG]

 

I guess you never checked the EGR?

 

Melwiff makes a good point. don't exclude the EGR as a cluprit. I tend to go after what was done last as the cause, but that is not always the case.
If you didn't have detonation before you took it in but did after you got it back, then very likely related to what they changed or managed to mess up for you. EGR is really easy to take off two bolts. Make sure not all carboned up and vacuum diaphram works that's about it.

 

i think the timing was the problem, after two drives/adjustments I can just barely hear it now.. I ran out of time ,,will try again tomorow if time allows..