my buddy has a 400 sbc, 4bbl, small cam, accel dizzy. the problem is when you set the timing as it should be its dead, light stumbling at low speeds. set the timing where it "kicks" hard when starting it runs great. have changed the dizzy, carb, no help. whats up? ideas?
Are the advance mechanisms working?? Esp. the centrifugal advance. Sounds like a distributor that has it's timing locked-in. Cosmo P.S. Vacuum advance connected to manifold vacuum??
It could be that your harmonic balancer is "twisted" such that the degree marks on it's cir***ference are off by a few degrees. This happened to me. After several unsuccessful attempts to use a timing light to get the timing right, I finally got it right using trial & error at my "top-secret high-performance automotive test facility" (three telephone poles on a desolate road).
Another Accel?? That could be the problem. Does it have vacuum advance?? You probably need vacuum advance for the application. Try a stock HEI, if it'll fit. Just FWIW, vacuum advance is needed to work on the street, and having it doesn't take away from performance, either. Cosmo
we tried a stock dizzy, it did better but the same type problem, dead at the proper setting. turn the dizzy t'll it kicked back hard an it ran perfect.
Sounds to me like the balancer might have slipped too. I've had more than one do that, they slip on the rubber bushing inside, especially if they have gotten greasy or oily. Try timing it by ear, trial and error. I like the three telephone pole test strip, sounds like what I use.
we did "it kicks back like hell when it runs right and sounds right. turn it back so it starts o.k. it runs like ****.
Sounds like a combination of a few things. First find true TDC using a pistons stop and mark it on the balancer. Check all the plug wires to make sure you don't have any that crossed and all that basic stuff. You want to make sure that there is no little simple problems before you really get into parts swapping and tweaking. I am ***uming you have a stockish 400 with stock open chamber heads, and the factory compression ratios which were around 8:1 actual. Now you need wither timing tape on the balancer or a dial back timing light. With the big bore, low compression, and stock open chamber heads that 400's came with, you will need a bunch of advance for it to run real good. Probally something like 42 degrees total. The big bore means the flame front has to travel farther, low compresion and the poor open chamber design makes the flame front kinda lazy so that it moves slower. So you need more timing to get the burn just completing at around tdc - this gives you max cylider pressure, and cylinder pressure is power. Most 4" bore small blocks with decent compression are happy around 36-38 degrees with closed chamber heads and flattops, and the new style vortec heads like only about 32-34 degrees it seems when running flattops. Set the timing to where it runs good but is hard to start (too much initial timing it sounds like). Make sure the va***n advance line is disconnected. Read from the timing tape or dial back light what the initial timing is (bet its something like 20-25 degrees BTDC). For discussion, lets say its 20 degrees. Take the timing light and bring the engine rpm up to about 3000 and hold it there. See where the total timing is when the engine is running good. Say its 40 degrees. Take away the inital timing to give you the amount of mechanical advance. Now back off the inital timing to where it starts ok without kicking back and read the initial timing. We will keep that as a base. Lets say its 14 degrees. Now since you want 40 degrees total timing because thats where it runs good, but it will only start well at 14 degrees, you will need 26 degrees mechanical advance. Depending on the distributor, you usually need to take a small rat tail file and extend the mechanical advance slots so that they mechanism can extend farther to give more advance. If you are getting too much advance, you have to braze the advance slots shorter and file them to where you want. While you are in there, you might as well see if there is an advance kit availble for your distributor and change the factory heavy springs for some lighter ones. Some heavier weights will help also. These will speed up the curve. You will have to test it to get it perfect but a a few rules of thumb are: lighter car = faster curve steeper gears = faster curve manual transmission = faster curve heavy car = slower curve highway gears = slower curve automatic = slower curve When you are done you should be able to easily start the car and still have it run like crazy.
What racerrick said, plus; If you find the dampener is spun, definitely replace it because the 400 crank uses an externally balanced dampener and flywheel and it'll be out of balance with a spun dampener.
you have Vacuum at the advance connection I guess? The simplest thing it can be is this: slack in the timing chain.....how old is the engine [in miles]?...its a common thing for a Chev to have stretched so much that it can no longere be timed at the book setting......Also if you are referring to setting the timing "by the book" on a 400 that would be incorrect anyway.....if modified with cam and etc that would be slow as hell -Weren't they set at TDC originally? Also 400's were built at GM with their camshaft set 3 degrees retarded for tourque so Im saying the original timing spec is now BUNK as a result of your upgrades.
Good one...I forgot about that. If the balancer has spun on any engine, swap it. I saw what happens when the ring comes off at 5000rpm at my friends engine shop...smashed the hell out of the car.
You are setting the timing with the advance unhooked, right? What kind of compression does this engine have?
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