SkylinesSuck

I have had some audible ping on my '75 350 (stock cam and stock HEI) for a while now. It happens most at part throttle, low rpms, but there still might be some at WOT in the higher rpm's (hard to say 100% because of noise). I want to say it's most prevelant when it is cooler out, but that could just be because I hav the a/c off and windows up and thus can hear it better. This isn't "OMG!" loud knocking, but midget hiding under my hood gently shaking a dixie cup with ping pong ***** inside it kinda loud. I can't hear it with the a/c on full blast, but on lower settings I can. I can definitely moduoate the amount of ping so with my right foot too FWIW.

In an attempt to fix this, I recently installed a new timing set (installed staright up), brand new Cardone dizzy wih stock weights and springs, tripple checked my timing (20* at idle w/out vacuum advance hooked up), and have even tried running without the vacuum advance, as well as the equivalent of about 95 octane gas. The later two thing help, but it was still there. Any input? Could I have adjusted something on the carb too lean somehow and cause this? The carb was rebuilt by "Sparky" a few years ago.

Just basically looking for some ideas on where to look and what to try. I am bringing the car back to the states in a couple of months so nothing more than minor tweaks and tricks will be tried now. Major fixes will have to get tried after it's boat ride.

oldcutlass

When you adjusted the carb, did you use a vacuum gauge? Did you adjust for the highest vacuum while keeping the 2 a/f mixture screws turned out as close to the same as possible? At what RPM did you set your timing? Back your timing down to 18.

Fun71

How is your EGR system?

SkylinesSuck

I had set the idle screws evenly using a vacuum gauge at one point at 800'ish rpm, but I've since monkeyed around with them and have probably gone leaner on them trying to remove some fuel smell at idle. Do the idle circuits have that much effect on part and full throttle driving? I will try pulling the base timing back a couple of degrees but since I tried disconnecting the the vacuum advance already, I wonder if 2 degrees will really help. Worth a shot though.

EGR is functioning properly as far as I know. The valve actuates properly when vacuum is applied, and it's hooked up to ported vacuum. I can apply vacuum with a hand pump at idle and see if the idle changes per the FSM test.

oldcutlass

The idle a/f mixture screws only work at idle, once the rpms go up out of the idle circuit they do nothing. The vacuum advance is over and above mechanical timing. If your total mechanical timing is too high it will still ping. If your fuel sucks it will also ping.

SkylinesSuck

Okay, I'll try backing it down a few degrees to see what happens. The trouble is I know it's "correct" now, so why would it happen? I know crap gas, but like I said I've tried the equivalent of "super" here which is like 95'ish octane.

Too lean can cause pinging, right? Just for arguments sake, how would one go about changing part throttle mixture?

oldcutlass

Rejetting the carb will richen the mixture. Pull your plugs and see what they look like.

M-14

Are you using any oil? Oil that gets into the combustion chamber drastically reduces the octane.

gs72

If you are running a stock air cleaner with a heat riser, make sure the flapper is open to ambient air and not to the exhaust manifold heat. Sometimes the thermostatic valve fails or the air bleed in the valve gets plugged and you pull your air charge off of the exh. Manifold on a warmed up engine. This will cause pinging and sometimes vapor lock conditions.

SkylinesSuck

Air cleaner/heat riser is functioning properly. I just verified this yesterday.

About oil consumption, very little. Not enough that I need to add any between oil changes, despite a small oil leak.

Fun71

I just thought this - what's the engine temp? Hotter engine temp will ping easier.
Another thought. Maybe you have carbon buildup in the cylinders. If so, Sea Foam or something similar may help.

SkylinesSuck

According to my rally gauge, temps are normal. The pinging is actually worse when it's cooler I think. I've heard of sending units not being very accurate though. I think I searched one out that was supposed to be though. Seafoam is worth a shot.

MDchanic

Remember, Japan uses the RON octane rating system, which gives numbers about 4-5 points above the AKI numbers used in the US.

This means that your Japanese 95 octane is the same as US 90 octane.

Theoretically, your 8.5:1 engine should run on 87 or 89 AKI octane, but if there's a lot of carbon in the cylinders, that could be out the window.

Ideally, use SeaFoam "everywhere" - that is, put at least a can in the oil, run the car briefly to distribute it, then spray some in the cylinders (try to get all over the piston crowns), and leave the car for at least a few days, then crank it over with the spark plugs out to avoid bending a rod, then pour a can slowly through the carburetor at a fast idle.
It WILL smoke a lot, so beware of your neighbors.

- Eric

olds 307 and 403

He is actually at 7.8 or 7.9 to 1. I have leaned out the secondaries on a 307 and it pinged bad. My current 350 pings on anything lately but 91 octane. It has 9.5 to 1 and a smallish 204/214 cam. I unhook the vacuum advance and it goes away. You are retarding part throttle timing 20 to 30 degrees, if you are unplugging and blocking the hose. I have found low compression Olds V8's can tolerate a lot of part throttle timing. As said, use sea foam or water and a spray bottle down the carb throat to remove carbon.

MDchanic

Agreed. I was referring to the nominal compression, but the actual compression is almost certainly in that range, if the combustion chambers are free of carbon.
If the CCs are all carboned up, it can be a point higher, maybe more, but more importantly, some of the edges / surfaces of the carbon deposits can get very hot and act as sources for preignition.

If using SeaFoam on piston crowns with the heads off, you can see it actually dissolve the carbon after it sits a while, and a non-aggressive brush can help a great deal, so letting it soak a while should help reduce the carbon (if any) a great deal.
For the sake of accuracy, though, I have never actually done an experiment where I used SeaFoam on an assembled engine, and then taken it apart to see how much carbon was removed.

- Eric

SkylinesSuck

Dialed it back to 18* BTC and enriched the idle circuits up about a half turn to achieve max vacuum. Might have helped a bit, but the ping is still there.

Also, about the gas, I was trying their "Hi-O-ku" (Hi Octane) which is 100 RON. That's why I was saying it was the equivelant to 95 octane (AKI). We used to get that on base for normal stateside prices which was nice, but now we get their crap 90 RON swill on base to increase their (AAFES) profit margins. If I want the good stuff, I have to go off base for it and that's like $7 a gallon even at today's super discounted prices.


I'm going to find my spare set of plugs and give the Seafoam a shot next week.

olds 307 and 403

If you can get Kleen-Flo products, their air intake cleaner and especially their combustion chamber cleaner works really well. I spray it down the carb till it stalls. Then I pull the plugs and spray it in the cylinders and let is sit over night.