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

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Prior to mine changes, it had actually iron 76cc heads; one Erson video camer with 0.496/0.521-inch valve lift v 1.6:1-ratio rockers and 0.224/0.234 levels duration at 0.050-inch tappet lift; headers v 2.5-inch dual exhaust; and also a Holley dual-plane intake through a 600-cfm Holley carb. V the old setup, I would certainly pull 14 come 15 in-hg of vacuum in ~ idle. The ran nice well v 12 levels base timing and no vacuum advance. The vacuum breakthrough had resulted in a stumble off idle, which sound weird to me until somebody suggested taking the vacuum breakthrough hose off and also plugging it, then it ran favor a top.


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


With mechanically advance, that would be about 55 come 58 degrees full at 3,000 rpm. I"ve never had actually timing the high before, and I don"t know if it"s OK/normal or not. Ns don"t have any kind of weird symptoms or overheating concerns or anything. It runs at 180 to 190 degrees Fahrenheit at the cylinder head and also doesn"t have actually any significant driveability problems, various other than I have to tune the carb, but I feel this was much more important first. Any type of ideas ~ above this? Am ns crazy, or is there a major problem?Matt VendouernVia email


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No, this certainly is not normal. To run that much timing will normally detonate your motor into oblivion. Depending upon your cylinder heads, compression ratio, camshaft, and centrifugal breakthrough curve in the distributor, "normal" basic timing because that a street high-perf small-block Chevy is somewhere in the 12- to 18-degree prior to top dead center (BTDC) range, with full timing at full centrifugal advance about 34 to 36 levels BTDC, as review at the crank. But before you do anything (I recognize this sounds lame), the can"t pains to obtain a second opinion—try a different timing light and see if the results agree.


All right, therefore it"s not the time light. Take into consideration what might have changed/gone south throughout your cylinder-head swap. If the cam and also rocker ratio are the very same as before, idle vacuum must remain around the same at the very same base timing, or—due come a higher static compression ratio produced by the brand-new heads" smaller combustion chambers—be also slightly higher. This presume the valves and also the carb"s idle mixture and idle rate are properly adjusted. A misadjusted carburetor, excessively tight valve lash, and/or a vacuum leak could require way over-advancing the timing to crutch the underlying error so the automobile seems to operation a small smoother. Therefore, you need to acquire these items squared far at the normal base timing range.


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See all 6 photosA vacuum gauge is one invaluable diagnostic and tuning tool. If the vehicle won"t operation smoothly without overly advancing the base (initial) timing, examine for vacuum leaks. Girlfriend can also get pretty close on the carb"s idle fuel mixture mediate by turning the screws to obtain the highest possible idle vacuum reading.

A vacuum leak leans the end the engine, which can cause detonation and also burn valves. In ~ the most an easy level, if you put your hand or a rag over the carbohydrate at idle and the vehicle keeps running, you have actually a vacuum leak somewhere because that sure. Use a vacuum gauge for more granular diagnostics.


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


Are every the unused carburetor and intake vacuum harbor on your brand-new carb plugged off? Don"t overlook the large PCV hose nipple present on countless "universal" Holley carbs 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 examine valve circuits (either the devices themselves or the hoses feeding them)? in the interim plug the ports to those and also any other outside vacuum consumers consisting of the PCV valve and also see if vacuum improves. If vacuum go normalize, reconnect the hoses one in ~ a time until vacuum decreases—at the point, you"ve uncovered the negative hose or vacuum consumer.


If you changed the valves too tightly, they could not fully close or near late. Essentially, this is indistinguishable to an internal vacuum leak. Run the valve lash again if vacuum is still too low ~ the previous checks. The factory method is through the engine running at idle: through hydraulic lifters, back off the rocker-arm adjusting seed on the valve you"re checking till it start clattering, retighten until the noise just disappears, then walk 1/2 come 3/4 complete turns tighter. The running method causes an oily mess, therefore you deserve to also readjust the valves with the engine off making use of the EOIC (exhaust opening, input closing) adjustment method, as explained in Pit Stop, Dec. 2017 print edition (and top top the Web: Pit Stop, EOIC valve-adjustment method).


Don"t skipping the distributor installation. In ~ the ideal base timing, is the rotor appropriately aligned v the cap"s No. 1 terminal on that cylinder"s firing stroke? If not, you may have actually reinstalled the distributor slightly "off."


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See all 6 photosRotor phasing on distributors without a vacuum canister: according to MSD, the rotor tip need to line up through the cap terminal or it is in on the top edge of the terminal when the spark occurs.MSD
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See every 6 photosRotor phasing v a vacuum can: ~ above a distributor that transforms clockwise prefer a Chevy does, MSD says the rotor tip need to be just to the ideal or just after the terminal as soon as the spark occurs v the vacuum disconnected and also plugged. With the development connected and the engine in ~ a steady speed off-idle, the rotor tip should be just to the left or before the terminal.MSD

Although it more than likely isn"t the reason of your immediate problem, in ~ this suggest it sure can"t hurt to check the distributor rotor phasing under to run conditions. Rotor Phasing is characterized as the alignment between the rotor tip and also the distributor lid terminal once the spark occurs. Not correct alignment may reason the spark to run to the following closest terminal or another ground bring about a misfire and also power loss. Street-style distributors with vacuum can be ~ may have actually less-than-ideal rotor phasing (even through the vacuum advance plugged). MSD offers an flexible rotor to correct this difficulty that fits MSD distributors and also many other distributors that use similar caps and also rotors.


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See all 6 photosCheck rotor phasing through drilling a large hole in the distributor lid in line with the No. 1 terminal. Run the engine in ~ a secure speed and observe the rotor guideline with a timing light. Keep in mind the relationship between the rotor tip and also terminal as soon as the spark jumps. Noting the rotor guideline with white correction liquid makes this easier to spot.Norm Brandes
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See all 6 photosMSD"s two-piece flexible race rotor (PN 84211, left, compared to traditional MSD one-piece rotor, right) have the right to be supplied on many MSD "points-style" small-cap distributors, and many various other Delco-style small-cap distributors. It offers up come 20 degrees of advancement or retard and is straightforward to collection up with accurate 1-degree detent adjustments.Norm Brandes

Except for possibly the rotor phasing under running conditions, the coming before things to look at room the most most likely culprits due to the fact that they autumn into the "things you"ve readjusted or likely have actually disturbed" category. But if none of the above helps, you"ll have to dig deeper, looking at various other things seemingly not directly related to her top-end swap.


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


On a factory-style balancer whereby the outer inertia ring is insulated indigenous the inner ring by a rubber strip, the outer ring occasionally "walks" in relation to the within ring. One proviso this might be happening is if the time keeps wandering—it won"t stay placed where you set it at. (If friend still are using ignition points, the time will differ slightly as the points wear.) A delaminating balancer must be instantly replaced. If using factory balancers in a warm rod, an old-school trick is painting a white or yellow vertical line down the side of the balancer aligned with the TDC mark on the balancer edge. If the white heat starts to relocate so it no much longer lines up through the TDC mark, the balancer is delaminating.

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If the carb is correctly adjusted, there space no vacuum leaks, the distributor is properly installed, and also TDC alignment is correct, that pipeline the camshaft and also timing chain. Remove the front cover and inspect the time set. Carry out the dots ~ above the upper and also lower sprockets heat up at TDC No. 1 cylinder firing place (No. 1 piston at TDC and both intake and also exhaust valves closed)? If it"s a multikeyway chain, is the reduced sprocket top top the suitable keyway? Is the timing collection worn or damaged (chain slack or damaged sprocket teeth)? If every this checks out, that pipeline the video camer itself. You"ll have actually to inspect its opening and closing points through a degree wheel versus the timing card supplied with the cam. Exactly a slightly progressed or retarded cam with a multikey sprocket or offset video camer bushing, but likely the camer would have to be ground method out the spec (very rare) come replicate her weird over-advanced scenario.