Hi Everyone : I have a 289 in a 1967 bronco that I put in. the guy I bought it from said it ran good. When I tried to start it it would not start come to find out he put a bigger cam in an he also put a new timing gear set in it but he is a chevy guy an he timed it to a chevy after I played with it for a while I put he timing right I it started but # 5 an 7 dont really get as hot as the others . I did a compresion test an # 5 an 7 are around 80 ish psi the others are 120's. Does anyone know why it is doing that ? Thank for your help Rick

 

If it had a cam change you may want too back off the rockers a half a turn/full turn just to be sure the valves are sealing, Then redo the compression test then do a bleed down to see if it might have a burnt valve cracked head etc, bad exhaust you'll hear it in the tail pipe intake in the intake,cracked head/gasket radiator,bad rings in the breather but take the rockers completely off to do a leak down.

What did the plugs look like?

 

agree, re-adjust all the valves.

 

do not assume the worst... first as recommended re-adjust the valves.... second read the spark plugs on those cylinders.... see what they tell you.... compression goes somewhere... either past the rings or past the valves... or into the adjoining cylinder (blown head gasket) but you will hear that one because it will sound like a dead miss....

If you have a cracked head or block three things will occur... one or all.... coolant will weep into the oil and it will foam and turn into a milky mess... oil will seep into the coolant and you will have chunky little blobs in the rad or the coolant will turn a burned brown color.... and the engine will run hot (boil over hot) as the compression gasses will heat the oil and coolant....

most likely you will find either a bad valve seat, or misplacement of ring gaps, or cylinders were not honed/bored properly, or were too large and the engine required a rebore or bigger rebore....

first attempt to adjust the problem from the valve train....

second look at spark plugs, read color and condition of threads... as in the threads are no good in the heads and the seal is bad there (seen that one) and also make sure the compression bung was sealed as well on those cylinders... (headers can make getting that thing in the plug hole and seated properly really fun)

then if the problem does not go away.... you have to tear things down...

 

All good advise. One thing I would check for is rocker studs pulled. Place a straight edge across top of studs to see if they have pulled any. A higher lift cam would put more pressure on them and they are press fit in stock configuration. Even carbon buildup on the valves can keep them from closing correctly.

 

gfw is the man here... that is also likely a problem... if your heads have pressed in rocker studs... and you have a bigger than stock cam... then it is very possible that the increased valve spring pressure created by the heavier springs needed to stabilize the valves with the bigger cam have cause a couple pressed in studs to back out of their holes slightly creating a out of time valve that causes a miss sound...

use the steel straight edge from a combination square across your rocker detent nuts to toubleshoot and see if the stud ends are all level... if not the springs are pushing them out of their pockets... I am assuming you do not have screw in studs and guide plates with poly locks.... If you do... leave the vc off and run the engine... back of the poly locks on the cylinders with the missed in them and see what happens when you adjust the locks down... pushing the rocket into the valve spring... or backing them off....

Lastly:

Another thought here.... this engine have a flat tapped cam... u know if you do not use the additive in the oil it will self destruct...

I pulled my favorite rig-job rebuild on a junk 70s for 302 for a 75 p/u by pulling the motor down to the crank in the block... no... I did not replace bearings.... or rod bolts...
Ran a dingle ball hone down the bores.... re-ringed the valley of death pistons... and put that dog back together... and installed some 289 heads (58cc) chamber on that puppy... they had screw in stuff... I also put in a .480 .480 cam.. Those motors run like that... really well... actually the dish piston and small chamber is better than a pop-up and a larger chamber... pretty awesome for junk...

That trunk was rough... it was one of those heaps with the really tall front springs that eats the front tires to death in 25k miles unless you flip them around on the rim at 15k miles so by 50 they are cupped and shot.... and it was rusty.... but ran like hell... in a good way...

well a buddy of mine had a sixer one of those trucks and the engine in it after long term abuse with a 4bbl and offy intake and efi exhaust mans developed a rod knock....

I gave him the parts and motor to convert... and we converted his truck with my parts to a v8... and I told that dude not to change the oil without adding the additive...well he changed it without the additive....

Three months later... it had no power.... when I pulled the engine down the carnage was unreal.... the cam was gone and the lifters where everywhere.... it was a shocker... should have took pictures... it was that bad...

So put that additive in there if you don't have a roller! And... you may need to make sure that something like this is not happening to you...

 

One other thought, new cam new firing order. I know later model 302's had a different firing order than 1967.

I do realize this is an older thread, would be interested in finding out what happened.

Paul