Newbie with 355 issues. Please no flames
Hello all I've been lurking on the site for quite a while now and have gotten tons of tips and had many problems solved by reading other posts but I haven't been able to pin point this one so I finally registered to post.
Here's a little back ground info. I rebuilt a 350 to and put in my cj-7 a few years back with help and guidance from a mentor/friend of mine. It was my first complete engine build so as I look back I wish some components were researched a little more but hindsight is 20/20.
4 bolt main 350 from a 1979 K20
bored .030 over (355 now)
stock heads, valve springs, rockers, push rods, and lifters
Do NOT have flat top pistons
Edelbrock performer intake with performer 1405 600cfm carb.
brand new HEI ignition with stock spec ac delco plugs
comp cams xe274h cam (230/236)
timing set to 14 btdc (still learing about total timing so I dunno what its at)
runs very rich
has a bad high rpm studder when "winding it out"
I've read many problems of people running rich with the 600 edelbrocks so thats a concern but not first priority.
This studder is driving me crazy. It seems like I can run 1st gear up to about 4500 or 5k with plenty of pull but when I go to 2nd it seems to studder very bad until I let off the throttle completely and ease back into it. Im no where near a pro at this but I can't figure it out. Is that valve float, due to stock springs with the aggressive cam? Is the cam too big? Carb too small? I'm lost.
Sorry If I used any wrong terminology or sound like just a plain dufus as I'm still learning.
Any and all help is greatly appreciated.
The problem is obvious. You can`t run stock valve springs on a big cam. The springs must match the cam. Why is because right off idle, the aggressive lobes of the high performance cam causes the springs that are too weak to create valve bounce. Through the RPM band power is way down from where it should be due to this. You said you didn`t have flat top pistons, which do you have? Dish top? Dome top? If the pistons are dished and with stock heads your compression ratio with a stock replacement head gasket is around
7.9:1. That cam has to have at least 9.5:1. You can`t stick a big cam in a stock engine and expect it to work correctly. The high RPM stutter can be caused by many things, but I`m willing to bet it`s caused by valve float as the stock springs can no longer stabilize the valves on a cam that big at upper RPMs. The easiest fix would be to swap cams to something alot smaller, a edelbrock performer plus cam would work and it would live with stock valve springs. But every cam should use it`s matching springs.
Lastly the rich idle is caused by the base timing being too low. Now the carbs throttle blades are so far open to get it to idle with the big cam the butterflies are off the idle circuit slots which is causing the carb to draw from
the boosters at idle, causing what I call a "runny nose"
The timing should be 12 degrees before top dead center with vacuum advance disconnected and plugged. Afterwards, reconnect the vac advance to a full time manifold souce, this is a source that pulls vacuum full time, including at idle. Recheck timing, it should be 18 to 24 degrees before top dead center. When you connect the vac advance to a manifold souce the first thing it`s going to do is idle up, then you lower the idle speed on the carb and it places it back on the idle slots.
DV is dead nuts on, but I would like to add a bit. Since I'm nver one to leave something alone, I thought I would throw my 2 cents in. If I were doing this, I would think back on what pistons I put in there. Also, shoot out the casting numbers of the heads so we can all see what the chamber size is. They could range anywhere from 58cc to 76cc.
If you remember having dished pistons you could opt for a decent set of heads with 58cc chambers...or at the very least, 64cc. If cost is a prohibitive factor, you could always grab a set of 305 HO heads like the 416s to bump up that compression. Yes, they will require some mods to be able to you use the cam you have.
I would also get a dist curve kit and use one silver spring and one blue spring to get mechanical advance all in by roughly 3000 rpm. If you can find someone who knows how, or you can research a way to do it yourself, you could limit the mech advance to about a 14* or 16* swing. @ at 16* swing, you could set your initial timing at 20* , use a 10* vac can to make that 30* and you'll have a WOT timing at about 36* above 3000 rpm.
That will give you a nice snappy little SBC. Now, if cost is a huge factor and performance is on the back burner, stab a stock-like cam in there, or the one that DV recommended, and call it a day.
Replace the cam.Another problem that can happen is "VALVE FLOAT".This can accure at high rpm.High rev's on stock springs and the spring can't control the valve when opening and closing.Worst that can happen is to break a spring and drop a valve into the cyclinder.Good luck.
I'm very late in my response so I apologize for that first and foremost.
Still haven't fixed the issues. It's not my daily driver and my wallet says to wait a bit before purchasing anything not a necessity.
The pistons I have are standard pistons so I would assume they are somewhat dished.
Where would I find the casting numbers on the heads?
What would be a better fix? New heads? New valvetrain? new heads and new valve train? New cam? :smash:
Another problem I'm having is it seems to be pulling the choke by itself. I've googled it every way to sunday and can't figure that one out. I should have just put a cummins 4bt diesel in in the first place.
The casting number is under the valve cover .
Whoa. You said you used the stock lifters?!?! You should never ever use a flat tappet cam with used lifters. The old lifters are married to the old lobes of the old cam. I think it is safe to say that you probably have irreversibly damaged your cam by reusing the lifters.
But back to DoubleVision's point: I think the easiest thing to do is replace the valve springs. With the correct ones. You can do this with the correct tool without removing the heads. Here are the springs.
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