[bhill86]

Title says it. Took my 77 T/A for a ride today and on my way back, around the corner from my house where I drive frequently, heading up hill, 25ish mph I stayed in 2nd gear and accelerated up the hill as I normally do and heard detonation. I was in a neighborhood at this point so I had limits to my speed but low rpm acceleration gave the same result the few times I could test it before I got home.

Last time I had a similar issue which, was this past summer, I discovered the mechanical advance mechanism in the aftermarket dizzy that came with the car 10 years ago darn near seized up and was stuck at full advance. I replaced it with a factory unit I happened to have and that seemed to fix the issue then.

I checked the timing tonight, 14° BTDC at around 775rpm (which is where I set it last time so it hasn’t moved nor did mech advance stick again) with vac can disconnected and open carb line plugged. Vac can adds 9-10°. I couldn’t check total tonight tonight cause if noise but it is set for another 20° or so for 34 total plus the can.

SCR is arguably 8.5-8.7ish. I say that because if anyone read a previous thread I posted I had engine work done by two different fellows in the past who each calculated my milled 6x4 heads at different cc’s. Being afraid of detonation in the past, and being inexperienced, I used to use 91-93 octane depending if I could get 93. Realizing that wasn’t necessary, I stuck with 89 up until recently when I filled my last tank with 87 thinking I could probably get away with it easily. My cam is a comp xe262h which I know builds DCR a little but is it enough to cause me detonation issues on 87 octane with 8.5:1 SCR?

Any ideas? Thanks in advance.

[tom s]

just for grins do a pumping compression test.You might have got a bad tank of gas and going uphill can load the engine more and cause it to detonate.Tom

[bhill86]

What do you mean my a pumping compression test? Just so I’m clear and don’t make any assumptions.

[tom s]

Pull all the spark plugs,pull the coil wire,wire open the carb throttle.Rent a compression tester from a auto parts store.Make sure battery is up full and check the pumping compression in each cyc.I think you make 3 rotations on each cyc.Report back.If it pumps more than 180 it could be a issue with less than 91 gas.Tom

[bhill86]

Alrighty. I have a tester but I probably won’t get time for this till Tuesday. As soon as I do, I’ll report back.

[Joe's Garage]

Both times it was bad fuel.

(I really hate to call that paint thinner crap they sell us gas....)

[77 TRASHCAN]

How many miles on engine?
How often is gas added to tank?
When was gas last added to tank?

There are different formulations for gas, typically a summer blend and a winter blend.
You may be caught in the middle of a weather change, it is close to Thanksgiving...

Go top it off with best octane pump gas you can find, hopefully with no corn whiskey in it.

It would still be a very good thing todo the cranking test! XE cams can bite you. Your compression ratio may be a little high for that cam, even though it’s been ok until now.....

[lust4speed]

Think we should remember that he had the same engine combo last week and last month -- and didn't have pinging. No use in dredging around with the existing build parameters for a change that just happened.

1. Tank of sub par fuel that was even worse than normal.
2. Timing variation but sounds like you have checked.
3. Vacuum leak causing a lean condition.
4. Junk in carb jets causing a lean condition.

Probably other items that I have left out.

[F ROCK]

this same thing happened to me a few years back.
Head gasket blew between cylinders #3 and#5.
Engine ran fine until I accelerated or went up a hill, then it would ping.

[steve25]

Was this drive you took yesterday done on the same fuel that was in the tank as your last drive, or did you add new fuel?

Summer blend fuel might still be getting dispensed in your area or at the gas station you go to even though the air temp has changed a lot !

Also your Carbs power power pistion that acctuates the metering Rods could be getting hung up on crap and your going lean at a certain throttle opening.

[bhill86]

Originally Posted by 77 TRASHCAN

How many miles on engine?
How often is gas added to tank?
When was gas last added to tank?

There are different formulations for gas, typically a summer blend and a winter blend.
You may be caught in the middle of a weather change, it is close to Thanksgiving...

Go top it off with best octane pump gas you can find, hopefully with no corn whiskey in it.

It would still be a very good thing todo the cranking test! XE cams can bite you. Your compression ratio may be a little high for that cam, even though it’s been ok until now.....

About 4500 miles on the engine and I drive around 1000-1500 miles a year during the summer so I get gas more than a couple times a season.

Gas was added about a quarter tank ago. Maybe 2 weeks ago? I can’t remember off hand.

[bhill86]

Originally Posted by lust4speed

Think we should remember that he had the same engine combo last week and last month -- and didn't have pinging. No use in dredging around with the existing build parameters for a change that just happened.

1. Tank of sub par fuel that was even worse than normal.
2. Timing variation but sounds like you have checked.
3. Vacuum leak causing a lean condition.
4. Junk in carb jets causing a lean condition.

Probably other items that I have left out.

Vacuum leak could be. I think I’m due for throttle shaft bushings in my carb and maybe a good once over. It’s been probably almost ten years since it’s been rebuilt and I don’t remember what jets and rods are in it. Which I’d like to know. I figured it might be time to have a look and clean it out soon.

[bhill86]

Originally Posted by F ROCK

this same thing happened to me a few years back.
Head gasket blew between cylinders #3 and#5.
Engine ran fine until I accelerated or went up a hill, then it would ping.

That’s interesting. I wouldn’t have guessed that. Hope that’s not my case.

[bhill86]

Originally Posted by steve25

Was this drive you took yesterday done on the same fuel that was in the tank as your last drive, or did you add new fuel?

Summer blend fuel might still be getting dispensed in your area or at the gas station you go to even though the air temp has changed a lot !

Also your Carbs power power pistion that acctuates the metering Rods could be getting hung up on crap and your going lean at a certain throttle opening.

Recent drive was same tank of recent 87 octane. I probably have less than 80 miles on this tank I’d guess.

[n20ta2]

That octane is to low.. I would run the highest octane available to you, bet that clears it up.

[ta man]

Have the temperatures dropped outside?

[bhill86]

Originally Posted by n20ta2

That octane is to low.. I would run the highest octane available to you, bet that clears it up.

I’m not opposed to an easy fix like that if that’s required. Just didn’t want to put a bandage on another issue if it was something else.

[bhill86]

Originally Posted by ta man

Have the temperatures dropped outside?

Yes they have. The day in question was up in the low 60s but we’ve been ranging in the 40s and 50s for highs lately.

[nUcLeArEnVoY]

What was your engine temp? I mean, going uphill at cruise speeds provides a nasty combination of max vac advance, potentially high centrifugal advance depending on your advance springs, and of course high engine load, so if anything would bring about pinging, it's that. Maybe up that fuel octane a bit? I only run REC-90 in my '79 400, but I'm pretty sure it's lower comp than your's, since I'm still using stock vacuum advance which adds 25 degrees at 11" on top of an 18 degree initial timing as per stock spec (but then again, my advance springs are EXTREMELY stiff/heavy, so I'm getting very little centrifugal). But even with all that advance, I don't get pinging... but I'm also in South Florida where hills are those things you see on Cuban girl's chests.

As has been mentioned, run a quick compression test with a good tester using schrader valves. I would think you may have to up that octane a bit. I say go back to the 89 - clearly it was your happy medium. Vac leak could also be possible, but I would think you would have noticed your idle raise or drop depending on your mixture. If the only difference between this newly onset pinging and no pinging before is that you tried a lower octane fuel, then I think that may be where you should start.

[Schurkey]

Originally Posted by tom s

Pull all the spark plugs,pull the coil wire,

GROUND the distributor end of the coil wire, otherwise coil voltage goes sky-high and the coil windings (and their insulation) are in danger.

Originally Posted by tom s

wire open the carb throttle.

I just turn the fast idle cam to the fastest-idle step. The engine isn't even turning 200 rpm, it doesn't need the throttle wide-open to supply enough air.

Originally Posted by tom s

Make sure battery is up full

Very important. Listen to the cranking speed--the last cylinder should be sounding just like the first cylinder.

Originally Posted by tom s

check the pumping compression in each cyc.I think you make 3 rotations on each cyc.Report back

I crank until the needle quits going up. Typically 5--6 pumps.

And don't forget to verify the accuracy of your compression tester now and then. If the gauge isn't accurate or repeatable, you need a new gauge.