Heres the deal, I have a pre '86 305 in my truck. When your driving down the road at a constant rpm, it will begin to detonate. Or at least what sounds to me me like detonation. It’s a loud pinging sound. If you change rpm by either leaving off the gas, or adding more gas it goes away for a few seconds and then comes back. It seems to happen more when the engine is hot.
After looking at the plugs they look great. There is only about 4000 miles on them. They look like they might be just the slightest bit lean, but pretty much normal. The carb and fuel filter was changed at the same time.
So I thought maybe there was a few loose rockers, but thats not the case either.
So anyone have any ideas where to go? I do have another motor for the truck, but I’m just not ready to drop it in yet…
Normal operating temps. Not overheating or anything like that. It seems to get worse as the outside temp goes up. I was thinking about a colder thermostat for summer, but not sure if that is the real cure. Since the plugs don’t look like they have been runnnig hot…
how do you have your vacuum advance hooked up? you should have it on a timed vacuum port which only pulls vacuum off idle. the other ones pull vacuum all the time. the difference is one is above the throttle blades the other is below.
then how much timing initial and total are you running? do you have a recurve kit in the distributer and last how much timing does the vacuum advance make? 15? 20?
Running an edlebroke 1406 carb. THe vacum advance does run from the timed port. Total timing is 36 degrees I believe, but to be honest I’m not sure. It is set at whatever the chiltons manual tells you to set it at. No recurve, and not sure about the amount of advance.
After reading through the carb manual I starting to thing I need to retune the carb. It seems like it is leaning out during what Edlebrock calls cruise mode. But on the other hand, I had a simialar issue before changing the carb, with the old quadrajunk. But then I know it was leaning out, so it may have been the same problem.
And then you have neonracer, that has the 1403, that is jetted leaner from the factory, and he has had no problems that I know of.
Not sure how much this matters, but, if I run the truck to the point that the motor reaches normal operating temp. it will run on after shutting it off. But if I let it sit and idle for 4-5 minutes, it shuts off normally.
This almost sounds like a rich condition during off idle running, doesn’t it?
how do you know it’s 36 total? what is the initial set at? in cruise mode you would get surging if it was lean, not pinging or detonating. a couple things just to try would be bump total timing back 2 degrees and try it.or unhook the advance and run it and see what happens. the next couple things are just ideas for ya if you wanted to try it. 89 octane, recurve kit in dist. i always use the lightest springs to get timing in the earliest. also change the vacuum advance canister to one with less advance in it. if the one you have is 20 you could try a 15 or something.
Well I removed and cap the vacuum hose from the vacuum advance. the noise seems to be gone. the engine seems a little sluggish, but the noise is gone. So I guess I would need to find out what canister is in there, and maybe back up the initial timing also. sound right?
try some better octane, if thats not it check the timing, and there is a little trick to check the timing chain, take off the dist cap and grab the lower crank pulley and rotate it back and forth there should not be much play in the time the cap moves to the time you switch direction of the crank…if there is alot of play then you need a timing chain, and this is kinda old school but i have seen those trucks have alot of ignition timing dwell, so check the play in the dist shaft…
LOL