I have actually a 355 small-block Chevy in my 1971 Nova and also just swapped top top a set of used aluminum heads. Ns was told the heads have 180cc entry runners, 64cc chambers, 65cc exhaust ports, and 2.02/1.60-inch valves. I also put a new 750-cfm Holley vacuum-secondary carbohydrate on it.

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Prior to my changes, it had iron 76cc heads; one Erson electronic came with 0.496/0.521-inch valve lift with 1.6:1-ratio rockers and 0.224/0.234 levels duration in ~ 0.050-inch tappet lift; headers v 2.5-inch double exhaust; and a Holley dual-plane intake v a 600-cfm Holley carb. With the old setup, I would pull 14 come 15 in-hg the vacuum in ~ idle. That ran pretty well v 12 degrees base timing and no vacuum advance. The vacuum advancement had caused a stumble turn off idle, which sounded weird to me till somebody suggested taking the vacuum advance hose off and also plugging it, then it ran favor a top.


But after i swapped the heads and carb top top the thing, it currently doesn"t desire to run almost everywhere under 30 levels base timing. I actually uncovered that out since I gained frustrated with it sputtering and running choose crap in ~ 15 and 20 degrees base timing, so ns turned the distributor as much as it would go, and it runs well.


With mechanically advance, that would certainly be around 55 come 58 degrees full at 3,000 rpm. I"ve never had timing that high before, and also I don"t recognize if it"s OK/normal or not. I don"t have any type of weird symptoms or overheating worries or anything. It runs at 180 come 190 levels Fahrenheit at the cylinder head and doesn"t have any significant driveability problems, other than I need to tune the carb, however I felt this was more important first. Any type of ideas on this? Am i crazy, or is over there a major problem?Matt VendouernVia email


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No, this certainly is not normal. To run that lot timing will normally detonate her motor right into oblivion. Depending upon your cylinder heads, compression ratio, camshaft, and centrifugal advance curve in the distributor, "normal" base timing for a street high-perf small-block Chevy is what in the 12- come 18-degree before top dead center (BTDC) range, with total timing at complete centrifugal advance around 34 come 36 degrees BTDC, as read at the crank. But before you perform anything (I understand this sound lame), it can"t pains to obtain a 2nd opinion—try a various timing light and see if the outcomes agree.


All right, for this reason it"s no the timing light. Take into consideration what might have changed/gone south during your cylinder-head swap. If the cam and also rocker ratio are the exact same as before, idle vacuum need to remain around the very same at the same base timing, or—due come a greater static compression ratio created by the brand-new heads" smaller burning chambers—be even slightly higher. This suspect the valves and also the carb"s idle mixture and idle speed are effectively adjusted. A misadjusted carburetor, excessively chop valve lash, and/or a vacuum leak could require means over-advancing the timing to crutch the underlying error so the automobile seems to run a tiny smoother. Therefore, you need to acquire these item squared far at the typical base time range.


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See all 6 photosA vacuum gauge is one invaluable diagnostic and also tuning tool. If the vehicle won"t run smoothly there is no overly progressing the basic (initial) timing, inspect for vacuum leaks. You can additionally get quite close ~ above the carb"s idle fuel mixture mediate by transforming the screws to attain the greatest idle vacuum reading.

A vacuum leak leans the end the engine, i beg your pardon can reason detonation and likewise burn valves. In ~ the most basic level, if you placed your hand or a rag over the carb at idle and also the vehicle keeps running, you have actually a vacuum leak somewhere because that sure. Use a vacuum gauge for an ext granular diagnostics.


The most most likely place because that vacuum leaks room at the entry manifold gaskets or under the carbohydrate baseplate. Spray a soap/water equipment or parts cleaner in ~ the "joints" and see if the idle speeds up; if that does, you"ve discovered the leak. Occasionally you"ll have an intake manifold gasket the leaks the end the bottom right into the engine valley—that"s really tough to spot v these rapid tests. Usually, you deserve to only discover such a leak through pulling the intake and also looking because that blow-by indications on the input gasket itself.


Are all the unused carburetor and also intake vacuum harbor on your brand-new carb plugged off? Don"t overlook the huge PCV hose nipple current on plenty of "universal" Holley carbohydrate at the behind of the baseplate under the secondary float bowl.


Are there any type of vacuum leaks in the automatically trans vacuum modulator or the power-brake vacuum booster inspect valve circuits (either the gadgets themselves or the hoses feeding them)? in the interim plug the ports to those and also any other external vacuum consumers including the PCV valve and also see if vacuum improves. If vacuum does normalize, reconnect the hoses one in ~ a time until vacuum decreases—at the point, you"ve discovered the negative hose or vacuum consumer.


If you readjusted the valves also tightly, they can not totally close or near late. Essentially, this is indistinguishable to an internal vacuum leak. Operation the valve lash again if vacuum is still as well low after the vault checks. The factory technique is through the engine running at idle: v hydraulic lifters, ago off the rocker-arm adjusting nut on the valve you"re checking till it beginning clattering, retighten till the noise simply disappears, then walk 1/2 to 3/4 full turns tighter. The running technique causes an oily mess, so you can also adjust the valves through the engine off using the EOIC (exhaust opening, intake closing) convey method, as described in Pit Stop, Dec. 2017 publish edition (and on the Web: Pit Stop, EOIC valve-adjustment method).


Don"t skip the distributor installation. In ~ the proper base timing, is the rotor appropriately aligned through the cap"s No. 1 terminal on that cylinder"s shoot stroke? If not, girlfriend may have reinstalled the distributor contempt "off."


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See all 6 photosRotor phasing on distributors there is no a vacuum canister: according to MSD, the rotor tip need to line up v the cap terminal or it is in on the leading edge that the terminal when the spark occurs.MSD
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See every 6 photosRotor phasing through a vacuum can: top top a distributor that transforms clockwise choose a Chevy does, MSD says the rotor tip must be simply to the best or just after the terminal when the spark occurs with the vacuum disconnected and plugged. With the advance connected and also the engine at a steady speed off-idle, the rotor tip must be just to the left or prior to the terminal.MSD

Although it more than likely isn"t the cause of your instant problem, in ~ this suggest it certain can"t ache to inspect the distributor rotor phasing under to run conditions. Rotor Phasing is characterized as the alignment in between the rotor tip and also the distributor cap terminal when the spark occurs. Not correct alignment may reason the spark to jump to the following closest terminal or an additional ground resulting in a misfire and also power loss. Street-style distributors with vacuum cans may have less-than-ideal rotor phasing (even v the vacuum breakthrough plugged). MSD offers an flexible rotor to exactly this problem that fits MSD distributors as well as many other distributors the use similar caps and also rotors.


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See all 6 photosCheck rotor phasing by drilling a huge hole in the distributor lid in line v the No. 1 terminal. Run the engine at a secure speed and observe the rotor pointer with a timing light. Note the relationship between the rotor tip and also terminal when the spark jumps. Marking the rotor reminder with white correction fluid makes this much easier to spot.Norm Brandes
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See every 6 photosMSD"s two-piece flexible race rotor (PN 84211, left, compared to standard MSD one-piece rotor, right) have the right to be supplied on many MSD "points-style" small-cap distributors, and many other Delco-style small-cap distributors. It supplies up to 20 degrees of advance or retard and also is straightforward to set up with specific 1-degree detent adjustments.Norm Brandes

Except for maybe the rotor phasing under to run conditions, the coming before things come look at space the most most likely culprits because they autumn into the "things you"ve readjusted or likely have actually disturbed" category. Yet if nobody of the above helps, you"ll have to dig deeper, looking at other things look at not directly related to her top-end swap.


Does the height dead center (TDC) mark on the harmonic balancer line up with the "0" mark on the timing tab in ~ TDC? over the years, there have actually been several various factory time tab locations and even various "0" mark locations on the factory balancers. The time tab for the little 6-inch-od balancer is also different 보다 one because that a large 8-inch balancer due to the fact that the larger the balancer, the better the distance between each time mark. To verify suitable TDC alignment, you require to find true TDC for the No. 1 cylinder. TDC stops the screw right into the spark-plug hole are obtainable from numerous outfits, consisting of Summit Racing.


On a factory-style balancer where the external inertia ring is insulated indigenous the inner ring through a rubber strip, the external ring periodically "walks" in relationship to the within ring. One reservation this may be keep going is if the time keeps wandering—it won"t stay placed where you set it at. (If girlfriend still room using ignition points, the time will differ slightly together the points wear.) A delaminating balancer need to be instantly replaced. If using manufacturing facility balancers in a warm rod, one old-school trick is paint a white or yellow upright line under the side of the balancer aligned with the TDC note on the balancer edge. If the white line starts to move so it no much longer lines up v the TDC mark, the balancer is delaminating.

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If the carb is properly adjusted, there are no vacuum leaks, the distributor is effectively installed, and TDC alignment is correct, that leaves the camshaft and timing chain. Eliminate the front cover and inspect the timing set. Execute the dots ~ above the upper and also lower sprockets line up at TDC No. 1 cylinder firing place (No. 1 piston at TDC and also both intake and also exhaust valves closed)? If it"s a multikeyway chain, is the lower sprocket ~ above the appropriate keyway? Is the timing collection worn or damaged (chain slack or damaged sprocket teeth)? If all this checks out, that leaves the camer itself. You"ll have to examine its opening and closing points with a level wheel versus the time card gave with the cam. Correct a slightly advanced or retarded camer with a multikey sprocket or offset electronic came bushing, yet likely the camer would have to be ground way out that spec (very rare) to replicate her weird over-advanced scenario.