I have a 283 bored .060 over with a very mild cam. It had the original heads on it initially, which were not built with hardened valve seats 20 years ago, so they wore out. I replaced the heads with 601 casting heads from a 305 2 years ago. It ran well until I switched out the intake the first time. Discovered I had a cracked intake and have had to go back to my original Edelbrock Tarantula(yes, I know its not optimal). But ever since, I cannot get the motor to run smooth. I've tried to set valve lash more than once, replaces up wires, tracked vacuum leaks, adjusted timing and dwell...everything I can think of. I can get it running and rev it in park, but when I put the motor under load in gear, it runs VERY rough and I start getting a knock the more power I give it. Can the bump in compression have had a negative effect on the cam or piston rings? I am afraid I've either worn a flat spot on my cam, cracked a ring, or thrown a rod bearing. Any ideas? I'm at a loss. I live in an apt, so its not easy to just pull it apart and check her out. I know you "old guys"( I say that with the utmost respect) might have some insight here. I really appreciate it. Sent from my SM-G970U using The H.A.M.B. mobile app
"It ran well until I switched out the intake the first time" Can you switch it back? I know you said the other one has a crack but I would switch it back temporarily just to see if that Edelbrock Tarantula is the problem. ****s doing all this while living in an apartment, I've done it that way too. Sent from my SAMSUNG-SM-N920A using The H.A.M.B. mobile app
Check the timing again! [with a timing light] Did you disconnect and block-off the vacuum advance when you did this?
The Tarantula is what I originally had on it when it ran alright. I purchased a Performer from someone that was cracked, so I switched back to the Tarantula since its what I have on hand. I have a cast iron intake too, but the mating surface on the driver side of the intake does not match up well with the block. Sent from my SM-G970U using The H.A.M.B. mobile app
Yes, I did recheck the timing as well and blocked the vacuum. My timing is not what book calls for because of the single plane intake, but its where it should be for the equipment I have. Sent from my SM-G970U using The H.A.M.B. mobile app
OK, check the main jets in the carb [4 barrel ,I ***ume] What carb? It is running fine on the idle circuits , but after partial throttle it is leaning out [also causing detonation] The cast iron manifold is probably better, but how does it not match the block ???? [unless your block / heads are milled] If the bolt hole sline up, it should match [you leave the end gaskets out, and use a bead of silicone]
Is this a issue with the cast iron intake or the 305 heads? You very well may have a mis-match causing a vacuum leak on the bottom side of the intake.
I have a Qjet sitting on it and have to run fat even to idle the motorim 4 1/2 full turns out on each mixture screw. The cast iron intake was "reconditioned" and the mating surface got too much attention and it was taken down a bit too far(unevenly). Sent from my SM-G970U using The H.A.M.B. mobile app
Thats exactly where I found my leak. The bottom side of the gasket had oil on almost the length of the rail. I ***umed it was because of the "recondition" job. Sent from my SM-G970U using The H.A.M.B. mobile app
Oil along the bottom edge of the gasket is from "splash" in the lifter valley. If you have tracks up into the ports then maybe thats the area of leak. Also manifold gasket leaks rear their ugly head with coolant in the valley as well. What about the mating surface of the carb? [try and borrow another carb to rule that out] Is it a Q-jet or an old Rochester 4G or 4GC? [ The Q-Jet is spread bore , and the 4G isn't ]
It's a Qjet from a '76 C10. I had it completely rebuilt by an old machinist friend who went to the Rochester school in the '70s. I am a confident as can be in the carb. I do have an Edelbrock 600cfm from a mod-80s Blazer, but I'll need a different intake or adaptor to test it and to make a fuel line adaptor since the inlet is in the other side. Sent from my SM-G970U using The H.A.M.B. mobile app
The Edelbrock tarantula is a single plane intake with a mounting pad for a square bore carburetor. If you are currently running this with a Quadrajet mounted to it without any adapter plate, this likely could be the source of a vacuum leak or otherwise be causing issues. Your Edelbrock carb should bolt to the tarantula with no issues for a trial. Photos of your setup would also be helpful.
I am going to check my iron intake again. After doing some reading, I may have a couple of issues working against me. First, the 601 heads raised the compression slightly to 9.5 to 1. Second, i may be running lean on my fuel delivery, and running too low an octane fuel. Lastly, the single plane manifold really makes it difficult to tune correctly because the small bore just doesn't handle it well. I am going to try and make that iron intake work, releasing the valve lash and starting over on that as well because I believe I messed that adjustment up. Going to take a couple of days and work it over to see what i get next weekend. Sent from my SM-G970U using The H.A.M.B. mobile app
Likely you still have a vacuum leak. With it running at idle, remove the air cleaner and place a hand over the top of the carb, if the idle picks up and smooths out, you have a vacuum leak. You should invest in a vacuum gauge, even the cheap ones are a great diagnostic tool. How do the plugs look? They should be fairly uniform in color unless you have something messing up a single cylinder. Summit sells their own brand of intakes and they are pretty good, think of one of those and a gasket set. Make sure you have the right carb gaskets too. Have you tried 91 octane in it? With my 355 I'm close to 10:1 and I run 93 octane with a booster now and then.
You have a vacuum leak in the PCV system, I can see it in the photo. That hose coming from the oil breather is suppose to go to the air cleaner on one side and the other side should have a PCV valve going to the vacuum port under the carb. Try disconnecting the hose from the breather and plug it so the vacuum doesn't leak out and see if it runs better. If so, you need to get a PCV valve in that valve cover. The PCV has a small check valve that will close when vacuum gets low so the engine doesn't run too lean.. This is my 454. One hose goes to the PCV valve in the valve cover the other goes to the air cleaner on the clean air side of the filter so only filtered air goes into the engine.. Hope this helps..
So you have 2 PCV valves? I have the one going to the carb from the driver side, but the one on the p***enger side is just a breather tube to the filter. Sent from my SM-G970U using The H.A.M.B. mobile app
That breather in the image you are referring to has an integral PCV check valve in it. Thats why it goes to the carb. Sent from my SM-G970U using The H.A.M.B. mobile app
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