Thread: Problems with 427

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I have a 60 over 427, in my 85 iroc. it is a 11.5 compression with a comp 294s 595 solild cam. The problem is i will hold the motor at 4000rpm with my hand on the throttlei will feel a thunk that feels like its coming from under the intake, Its not consistent. So iam hoping that the cam didnt kill a lobe. I need some help before i just sell the car lol
thank you mike

anyone have any idea what it might be

Any noise with the "thunk", like changes in exhaust or other? If you are holding it steady at 4000 does it "thunk" once or does it happen in some uneven pattern - like over 30 seconds how many "thunks" would you think? Just trying to understand what you are feeling, and maybe hearing.

Mike,
Do you have hydraulic lifters? If so, one of them could be failing.

Had a similar odd thunking sound in a SBC years ago. Finally pulled the intake, there was a 1/2" extension sitting in the valley! Every so often a lifter would hit it, and THUNK!!!

Just was RPope says.....sounds like something loose under intake. Hads oil pressure changed any?

Originally Posted by R Pope

Had a similar odd thunking sound in a SBC years ago. Finally pulled the intake, there was a 1/2" extension sitting in the valley! Every so often a lifter would hit it, and THUNK!!!

tore apart a 350 one and discovered a 12 inch screwdriver in the valley!

make sure your balancer isnt coming apart, seen that .what a mess.

soild lift cam, no difference in exhaust noise or engine rpm. Oil presure good. Iam going to adjust the valves tonight and put a fresh battry in it and see what happens then.

well, where to start. I found out what the thunk was. A rocker arm nut backed off and the rocker came off the pushrod. So that cleared that up. Another problem arose. I bought a new battary, and it will crank and start fine when cold, after it heats up it cranks slow and i have blow by coming out of the valve cover. If i let the car run it gets so hot that it will blow into the over flow. You can also hear it bubbling in the motor after you turn the car off.

On a side note, i bought this motor with a crack on the passenger side of the block. it will leak coolent out of it but the machine shop i go to said that i could use he barsleak block sealer and it will be fine. I just dont know if the crack got worse or what the deal is. I dont have the time or money to replace this motor. So it might go up for sale.

look for a block man . that sealer isnot perminent,i have seen them drilled and welded before.

Originally Posted by hucks 50

look for a block man . that sealer isnot perminent,i have seen them drilled and welded before.

Agree with huck. Back in the day my first 348 would start fine cold, but would not hardly turn over hot even with a new starter and new battery. Not long after I broke some rings under power and the smoke started pouring! The loose engine had severe carbon in the ring lands that expanded when hot in every cylinder, making it almost impossible to turn. Finally busted the oil rings, and that was that.
Sounds like you may have a similar problem, plus a cracked block. Time to look for a block, IMO.

I dont think it is a carbon problem, i dont even have one mile on it yet lol but with that crack why would i only get the blow by after its warm. I have no coolent in my oil or anything.

i installed a 160 deg thermo in mine and it helped alot ,that crack would scare me though.

well, come to find out the alt was not charging the battary. The wire that supplys power back to the battary was bad. So it was just running off the battary. Thats why when i ran it for awhile and tried to start it, it woud act like it didnt want to turn over. Also my 180 thermostat was not opening causing it to over heat. I took it out and replaced the wire and i was able to let it run with out over heating.