Have a 307, when i got it i just dropped it in an elco, planning for a changeout later. Just opened the oil cap, clean as could be, so i put it in, got it running, carries 60 psi pressure (def. not stock) and runs great. Recently, thanks to some posts on the small motors, i decided to keep it and work it over if necessary. Figured heads, pulled valve cover (clean as could be, springs looked almost new) and shock-416 H.O. heads-what i would have done. Now the question: next thing I figured was the cam, but lots of work to change it (a/c, p/s, etc.) and with the oil pressure and heads I'm wondering. Yesterday I hooked up a timing light tach, was jumping all over the place-like from 825-840-855. Idle is fairly smooth, but not totally "clean" like a stocker (duals on it). I'm going to put a 4 bbl. on it, but can't decide what to do about a cam. What's your thought???
The cam should be chosen to work with the rest of the car. We have no idea what the rest of the car is...so we could only offer blind guesses.
If it runs good now you can just drive it. The tach jump could a distributor being worn, does the timing move too? With the longer stroke using a 283 bore it will have more torque and a bigger cam can kill some of that if it’s important to your driving style.
Drive it then decide. I do not understand your timing light numbers and I have been a mechanic for over 50 years. A good starting place is 12 degrees advance. Unhook the vac advance hose to get an accurate number at idle. I have a god to cam none original that I like for small bore motors in a daily driver. if you decide to change the cam let me know and I will get you the part number. That said drive it then decide.
No genius with words--the question is Would you change the cam or leave as is? Its a playcar driver only. Timing mark did not move, vac advance plugged. Initial timing now at 12-14 degrees.
The tach is showing us that the idle is not perfectly steady, which is typical with a larger than stock cam, but also if there are other issues with the ignition or fuel systems, or valve problems, or whatever. Like he said, drive the car and see how you like it. If it feels sluggish off the line, it might want a smaller cam. If it feels like it runs out of breath early, it might need a larger cam. We can only guess.
Check for vacuum leaks, then if all good check ignition system. I had an off topic AMC that did that same thing. The idle was up and down. I swapped out the pick up and stator wheel (?) inside the distributor and it cured that nonsense. Keep us posted.
I have a tach --its rock solid at around 800 rpm and reads fine all the way up. The timing light tach was the one that jumped around -i'm thinking it might have had some type of electrical interference. I have an HEI in it, no problems with it that i know of and the motor doesn't have a miss anywhere. My thought here is that the motor was overhauled, better heads, high volume oil pump, and the question was did they put in another cam with a little more lift than stock? If so, I'm a happy camper and I'll just make the carb changeout. It feels strong for a 2 bbl 307 motor, seems like it wants more fuel-reason for the carb.
So it was the tach on the timing light jumping around? I assume a digital tach? Maybe a lesser expensive unit? That might explain the RPM swing? Am I using too many question marks?
Drive it. If you like it,leave it. If you don't,change it. Makes no sense to plan changes until you have some seat time in the vehicle.
Naw, hell, I use question marks on everything-especially in my old age. Right, its a timing light digital unit, but its not a cheapie-Innova Pro digital, now about $125 or so.
I just knew someone would say that--correct, that's the perfect foolproof way, and of course the way I was trying to avoid. You know what i was looking for: ----- "its definitely got an upgraded cam in it, just go with the carb".
I've wondered about that. Chevy never put a 4 bbl on one (that i am aware) from the factory, nor had any hipo parts for one. Just seems to me that if a 283 in its base stock form could handle one, a 307 should be able to also. Maybe the old 283 had better base stock heads. My thinking is this thing was overhauled, and had good parts put in it then surely they would have looked at the cam. I've been wrong before--many times.
Well, just thinking. The 307 has a larger bore than the 305. And you have some better 305 heads. You have a shorter stroke than the 305. Only “I wonder” is how far the piston in in the holes, type of pistons, and what the CR is. I think your heads are like 56/58 ccs. With the say “high performance “ era they were used on a 305c it would have had a dish with 4 valve reliefs. That gave the in the 9.2 :1 CR. So assuming the similar with a 307 piston (I’ve no idea) I wonder why it need to idle so high? I’d think 600-650 would be about right, unless as you think, it’s due to the cam to “keep it running”. Anyways, always tough to upgrade anything if you don’t know all of what you have. If I wanted the 4 barrel with the CI of 307 and racing wasn’t a thought, I’d lean to the smaller Edelbrock in the 500? CFM range.
Good thinking. The idle number is basically my deal for my a/c. S. Texas is already hot, around 89 today. It'll idle slower, but i just keep it there-800. Being cheap, i'm going to use an old Carter performance carb, afb style, rated at 625cf. I'm using stuff i have, Edlebrock c3b manifold with the Carter, and the HEI on it now. --and you're right on, I don't have a clue about the guts of the thing.
You are asking a question about a cam that can't really be answered without some investigation (which you don't want to do). I simply told you what you said you wanted to hear.
If that is the original cam, it's not a matter of if, but when the cam will get eaten- they were a problem (soft) straight from the manufacturer and assembly plant!
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