The issue is after driving at highway speeds for 15 - 20 mins I start to hear a knock from the motor. The car will kick and buck like it is not getting fuel or spark and a loud knocking nose form the engine. It is worse under load. The engine will keep running and if I slow down or let it cool off it will come back to normal. I can drive around the city all day and not have an issue. I have replaced the fuel pump and rod as well as checked the fuel sending unit and all filters and lines and there seems to be no issue there. I have also tried a distributor coil and module and again no change. The problem seems to be worse after a short run on the highway and then trying to accelerate after getting off the highway. I was told it could be a carbon build up so going to try a seafoam cleaning to see if that helps. Any other suggestions?????????
81 corvette fuel or spark ?????
- Thread starter downhomeguy
- Start date
DHG...
My first guess would have been the coil.. But you covered that.
I'm thinking more of an electrical heat related issue, possible affecting the advance mechanism?!?
My second guess would have been ignition module too!
Check the wires to the module...they have to move during advance conditions..
I'm totally guessing here.
Good luck
Graham
My first guess would have been the coil.. But you covered that.
I'm thinking more of an electrical heat related issue, possible affecting the advance mechanism?!?
My second guess would have been ignition module too!
Check the wires to the module...they have to move during advance conditions..
I'm totally guessing here.
Good luck
Graham
Thanks Graham. Will check it out and hopefully post soon what solved the problem. Has been very frustrating, missing long weekend drives.
without actually hearing it, is the knock more like a higher pitch rattle when you accelerate ? if so I think the timing is too far advanced. Slow cruising you won't hear it much but on a harder pull it will rattle. But you say it kicks also, that could be a too lean a mixture on the carb too. Lean won't cause it to knock, advanced won't cause it to kick. sounds like a combo of possible problems
Another thing to check is the vacuum advance diaphragm on the distributor. If it has a hole in it, the breaker plate is not being advanced far enough to burn all the gas a higher speeds.
Probably why around town she runs fine and runs fine again when you slow from highway speeds. I would imagine if you lifted the hood at night you would see the exhaust manifolds glowing in the dark
as the gas is being burnt in the pipes and not in the engine.
Don't want to say how I know but I used to own a 1930 Model A with manual advance, Enough said!!!
Rob
Probably why around town she runs fine and runs fine again when you slow from highway speeds. I would imagine if you lifted the hood at night you would see the exhaust manifolds glowing in the dark
as the gas is being burnt in the pipes and not in the engine.
Don't want to say how I know but I used to own a 1930 Model A with manual advance, Enough said!!!
Rob
the spark plugs will tell a tale. when the engine is cool enough pull 'em and have look. do this after a city run and again after a highway run for a comparison. this is usually the first place to start to help point you in the right direction. is this one of those electronic q-jets ?
Have finally had the time and patience to figure out this problem. It turns out it was an issue with the camshaft. Have found two lobes on the shaft messed up and the valves were concaved and barely moving when the cam was turned. This was causing a pressure build up in the cylinder and giving me all the grief. Have the engine and tranny out so going to do a rebuild.
Way to go DG....
Sorry it was a bigger issue than you thought...but...
Nothing like a good tear down to get the juices flowing!
Graham
Sorry it was a bigger issue than you thought...but...
Nothing like a good tear down to get the juices flowing!
Graham
had you been using an additive to supply additional zddp in the engine oil ? if not, that is likely the source of the grief. i use delco e.o.s. in all my flat tappet engines, only about 8 bucks a bottle at gm dealers. an additive is a MUST. if the budget permits on the rebuild, go for a retro fit hyd roller cam.
