rob81gsl

iTrader: (3)

i have an 81 gsl with a 4 port 13b and a 45 DCOE weber. i keep hearing this crazy noise that sounds like it is coming somewhere around the carby area of the engine bay. it only does it when i'm really getting on it at more that half throttle in fourth and all of fifth when i'm accelerating hard as well. the only way i can describe it is like a piston engine sounds when a rod is knocking really bad at high rpm's. The other thing that is stumping me is if it's making that noise if i breathe off the throttle pedal to less than like half throttle it will go away until i get back on it. let me know what you guys think, i have been stumped on this one myself for awhile now but i have an autocross this sunday and need to get it figured out soon.

Thanks in advance for any help

HeffBoost

Ouch!

Sounds like detonation under heavy load. Odd for an NA, but certainly not impossible. How well do you know your carb setup? Could you be running out of fuel? I don't know carbs enough to comment on the weber you mentioned.

Is it an old 4 port w/3mm seals?

Now that I read your description of the sound again I'm confused. A rod knock is a deep sound and detonation is more like pebbles being shaken in a tin can. W/o the word "rod" I would read it as detonation.

Maybe back your timing off a little and see if it affects it? Know anyone with a wideband you could borrow?

Cheers

rob81gsl

iTrader: (3)

it does sound like stones shaking in a can which really freaks me out. i'm getting PLENTY of fuel so i know thats not an issue. i believe its a 75 rx4 engine that i got from a old road race guy that had 0 miles on it when i got it. it now only has 2500 miles on it. and it gets even better, i just went out for a spin to check things out after i tinkered with my suspension it made the noise in third gear too. so i'm just worried about if it's gonna last or not. i need to get a timing light tomarrow, when i put the engine in a couple of monthes ago i just went by ear on the timing and just never messed with it so i'll have to give that a shot.

i'm open to anything right now so keep any ideas flyin at me

thanks

rob81gsl

iTrader: (3)

i honestly don't think it's detonation, so if anyone knows of anything else it might be it's getting to be chrunch time for me to get the finishing touches on the car for the autox this weekend

web777

Sounds like detonation. Is your timing OK?

Mschafe

I don't think its detonation. I have a Mikuni carb, and I had that exact sound under heavy acceleration. Kind of a banging or knocking sound coming from the carb, not the engine. I cleaned everything out good, re-did the timing and the noise went away.

SilverRocket

I'll jump on the detonation bandwagon, based on my experiences with said phenomenon... sounds like detonation under high load. When this happens, does the engine keep revving normally, or does it appear to slow down or hit a wall?

A couple points I can think of... first thing, stop eliciting the detonation, if you hear that sound, get off the gas. Change your driving style until you fix the problem. Second, have a look at your spark plugs; I've broken the porcelain on a couple spark plugs and had very similar problems. Third, check your timing; what kind of timing are you running?

You might need to go through your entire ignition system to see if anything's worn out.

EDIT: I see that you mentioned above that you set your timing without a timing light. You definitely should check it. It's possible that overly advanced timing has caused detonation and broken a trailing plug or two (I've done it a few times anyways). Definitely have a look at those plugs!

Lt. Dan

Sounds to me like what my car just went through... drain your oil, and check it for a copper tint.... I think you have a spun bearing.... Sounds just like a piston engine with rod knock...

Dan

rob81gsl

iTrader: (3)

i did have some detonation a couple of weeks ago for sure. i was coming home from the drag strip and i partially melted the tips off of both plugs on the front rotor. i was leaning out and low to mid rpms so i tweaked the weber. i have checked the plugs since then and they show that both rotors are running rich so i really don't think its fuel. I'm gonna have to check out exactly where my timing is set right now but i'm not sure where to set it, anyone have a good point where i should start it out at?? i'm still curious if it could be that weber though, it's been having more than its share of fits lately though. is it possible that it could be anything the weber moving or whatever under vaccum and higher rpms??? but whatever i do i need to get it fixed before sundays autox or else i'll be pissed big time if i have to miss it

thanks again for the help

rob81gsl

iTrader: (3)

ok i'm somewhat of a newbie when it comes to a timing like, i hooked it up and it just continuely flashed no matter what plug wire i put it on, except for the trailing coil wire, which did absouletly nothing. so i could use a little help with how to set the timing with the light

Rotarx7

iTrader: (3)

i had that same prob w my 7 like about 2 weeks ago. It ended up that my timing was too advanced and the float bowl was set too low, running out of fuel. After all that little tinkering she ran pulled like no other! good luck man

HeffBoost

Sounds like your trailings aren't firing. Something to look into but shouldn't be causing your problem.

I don't understand. That's what it's supposed to do. Then you point it at your main pulley and look for the marks.

I'm not sure of the stats for that old engine, but I would assume Ls would be 1-5*ATDC.

rob81gsl

iTrader: (3)

the problem was that it was flashing so fast that you couldn't even line it up with the marks on the main pulley, oh well i'll figure it out. hopefully thats just it, i'll keep messin with things and get er done.

thanks for all the help

HeffBoost

Hmm, maybe it's a problem with the light you're using. They say some don't work. Do you have access to another to try?

coldy13

Sounds like you timing is too advanced. Usually, detonaiton from not enough fuel will make it hit a wall while accelerating and it won't rev any higher. Detonation from too advanced timing will do exactly what you're saying, make a rattling noise under hard acceleration.

Rotor13B

Yep, I'm on the detonation band wagon too. I woulnd't drive it anywhere until it's fixed because if it is detontation, Instead of ten minutes under the light varifying your timing will turn into 1500 dollar rebuild.