detonation problem

[86c20]

ok so my 86 with 350 is a pia. i have to replace the plugs every 6 months or about 5k. they will faul out then run like crap. i have run good plugs cheap plugs and e3 autolight. the e3 lasted about 15k then the detonation came back and one was missing the ground on it. joy joy. so new plugs and full tune up and 1000 miles and detonation agian. tonight i took the dizzy and turned it up and down gets better gets worse but still there and i cant turn it any more. also i checked all plugs still in one paece for now. there are no timing marks or pointer on the motor. have timed by ear by vacc and with a gm tech that put alot of them in have removed most of the emis stuff also. motor is all stock any help???

[rich weyand]

Sounds like you need a hotter plug. What are you running?

Also, Autolite 24s would be much less prone to fouling.

[mattybilt79]

What carb are you running? Sounds like the timing is advanced too far. Not going to be easy to set without timing marks obviously. Engine will run way advanced but think of what pistons look like if plugs are totally burned off? Find the procedure for finding tdc on number 1 cyl its on this site I can't remember all the steps and at least get a base setting try to mark your timing cover or get an aftermarket pointer put on and go from there way cheaper than melting pistons!

[rich weyand]

If you start getting detonation after 1000 or 2000 miles on new plugs, and not before, then timing is probably not the issue, or it would have been the issue all along. Plug fouling can also cause detonation by having garp on the plugs stay hot/glowing and light off the mixture prematurely. That's why I want to know what plugs you are running. Sounds like they're too cold to me.

[86c20]

refresh me on plugs. thought colder plugs stoped detonation? as they stay colder. stand by as i get the plug box. they are champ of some kind. wanted a better plug then the cheap crap i norm run lol

[86c20]

25 rv17yc is what is in there now.

[rich weyand]

Too hot can cause detonation, because the plug tip gets so hot it's like a glow-plug, and stays hot between ignition hits.

Too cold can cause detonation, because they foul and the fouling debris continues to burn between ignition hits.

In your case, you're too hot. Those Champion plugs cross-reference to an ACDelco R46TS, or an Autolite 26 or 27. You ought to be running more like an ACDelco R45TS or an Autolite 24, which is at least one full step colder than what you have in there, maybe two.

Try the Autolite 24s. They are more foul-resistant than the ACDelcos.

Note that "hotter" plugs have nothing to do with high performance. They are the same plug with less contact surface to conduct heat from the insulator through the threads to the head. That is, "hotter" has to do with temperature, not performance. You want plugs to run hot enough not to foul -- hot enough to burn off oil -- but no hotter.

[86c20]

ok i will try the autolite 24. at this point i will try anything. this is just an on going problem with this truck for 8 years now. in mass and on marthas vineyard the gas we get.... well it sucks. that simple to. and at 4 bucks a gal for reg.

[86c20]

rich i have to ask. what do you think of the fancy plugs? platinum and the like. seems to me all the same stuff but want to ask

[rich weyand]

Originally Posted by 86c20

rich i have to ask. what do you think of the fancy plugs? platinum and the like. seems to me all the same stuff but want to ask

Not worth the money and can actually run worse in these older engines.

Go with Autolite 24 or ACDelco R45TS.

[86c20]

ok we have an update. new plugs autolite 25 and the detonation has gone down alot but still have some that is enough to worry about. i am thinking that it is time to time it right. get a pointer tape and have at it. i did turn the dizzy tord the fire wall and she ran better but cant turn it anymore. turn the other way she stalls out.

[rich weyand]

Like I said, Autolite 24s.

[86c20]

aahh crap. and thats what i get for not writing it down. dammit. will get the right ones in the am. have to go there any way. good thing i get a great deal.

[dfrank]

I run the acdelco r43ts in my 383 stroker that doesn't have any problems and that is even colder than what rich is mentioning. The acdelo r45ts is the factory spec plug i believe. An msd ignition would probably fix your problem also, but thats around $800 for that.

[rich weyand]

ACDelco R45TS is factory spec, which cross-references to Autolite 25.

Given the severity of your detonation problem, that is why I backed off one to Autolite 24. That, and the fact that a mechanic for the dirt-track racers around here said all they ever used in the SBCs was the Autolite 24, because they are the least prone to fouling, which lets you run a colder plug, which gets you away from any detonation problems.

General rule: run the coldest plug you can that does not foul. You want the plug just hot enough to keep itself clean, and no more.

[86c20]

so the 25 is stock?? and still have problems. has to be the crap gas we get here. when i was in school in the real world never had any problems with the truck. i like msd but this is my beat up dd that has more rust and holes then the titanic lol.

well i will get the 24s and let you guys know how i do. this truck is great to me but some things on it are a pain to get right.

[rich weyand]

Yeah, but when you get it right, it is so sweet.

Yeah, the 25s cross-reference to the stock plug, but a lot of people run colder (R43TS). That race car mechanic was insistent that the 25s were too hot for the SBCs and the 24 was the way to go. Since the Autolite are so resistant to fouling, he's probably right. GM probably specs a little hot to keep people from having fouling issues.

Note that "hot" in a plug is purely a matter of the operating temperature of the body of the plug. It has nothing to do with how hot the spark is, or how hot (i.e. high performance) the plug is, just what temperature it runs at. You need it just hot enough not to foul. No more.

[86c20]

UPDATE.
well got the 24's in still pings not anywhere as bad as it did before so i am goingthe right way. i have turned the timing down as far as the firewall lets me still pings but she starts and runs smother. i think next is timing marks tape and light to see where it really is. never been right.
want to say thanks to rich for starting me in the right point.

[rich weyand]

I don't recall: Did you check the MAP sensor? If the computer doesn't know the throttle position, the timing coming out of the computer could be way off advanced from what you think it is.

[86c20]

lol rich wrong detonation problem i was just reading that one. no computer on mine at all.

[rich weyand]

Ah. OK. No way to know by year, because so many of these trucks have been modified.

Hmm. Have you tried running premium in it? Gas is getting crappier and crappier, because there are so many regulations about additives and ethanol and such, and most cars have knock sensors, so the consumer doesn't notice.

The gas here is pretty good. I haven't had to go to premium.

Yet.

[86c20]

thought of that too but at 4.15 a gal can do it every week. but that is whats in it now.

[rich weyand]

Premium, and it's knocking?

Have you checked that the vac advance is working, and not sticking advanced?

Even without timing marks, you should be able to mark the wheel and see that when you disconnect the vacuum, the mark moves compared to vacuum advance connected.

[Corts60]

Since you don't have timing marks, grab yourself a 9/16" wrench and hit the road to find a hill. Loosen the distributor where it is tight but you can move it by hand. Do a couple passes up the hill, retarding the timing a bit every run, until the pinging goes away. If the pinging persists and the engine starts to run like garbage because of the retarded timing, then you have another issue.

This is obviously a backyard way of troubleshooting, but until you get your timing marks fixed, this might be your best bet.

[86c20]

i have turned the timing down as far as i can do to the fire wall. runs well there. lots of power and smother.

i am thinking the advance. i swapped the dizz out years ago do to the timing box going out so i put a reg dizzy in. now i have a qjet carb what side do you run the vacc to for the dizzy? left or right? keep in mind i dont give a crap about the emisions at all. just want her to run good. also i am taking the air pump offas the tubes are rusted out and just leaking exhuast. need a plug ig you know what size it is?? it is just one big line on each bank.

can you give me a brake down of how to check the advance? just want the right steps so i can report back with good info. thanks guys this has really helped me alot to get going the right way