I have a 355 small-block Chevy in my 1971 Nova and also just swapped on a collection of provided aluminum heads. I was said the heads have actually 180cc intake runners, 64cc chambers, 65cc exhaust ports, and 2.02/1.60-inch valves. I additionally 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 cam with 0.496/0.521-inch valve lift v 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 also a Holley dual-plane intake v a 600-cfm Holley carb. Through the old setup, I would certainly pull 14 to 15 in-hg of vacuum in ~ idle. It ran nice well v 12 degrees base timing and no vacuum advance. The vacuum advancement had resulted in a stumble turn off idle, which sound weird come me until somebody said taking the vacuum advancement hose off and also plugging it, climate it ran prefer a top.


But after ns swapped the heads and carb ~ above the thing, it now doesn"t desire to run all over under 30 levels base timing. Ns actually discovered that out since I gained frustrated through it sputtering and also running prefer crap at 15 and also 20 levels base timing, so i turned the distributor as much as it would go, and it operation well.


With mechanical advance, that would be around 55 to 58 degrees complete at 3,000 rpm. I"ve never had timing the high before, and I don"t understand if it"s OK/normal or not. I don"t have any kind of weird symptom or overheating problems or anything. It runs at 180 come 190 levels Fahrenheit at the cylinder head and also doesn"t have actually any major driveability problems, various other than I need to tune the carb, however I feel this was much more important first. Any ideas ~ above this? Am ns 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 typically detonate your motor into oblivion. Relying on your cylinder heads, compression ratio, camshaft, and centrifugal advancement curve in the distributor, "normal" basic timing for a street high-perf small-block Chevy is what in the 12- to 18-degree before top dead facility (BTDC) range, with total timing at full centrifugal advance around 34 come 36 levels BTDC, as read at the crank. But before you execute anything (I recognize this sounds lame), the can"t hurt to obtain a 2nd opinion—try a different timing light and see if the results agree.


All right, for this reason it"s no the time light. Consider what could have changed/gone south throughout your cylinder-head swap. If the cam and rocker proportion are the exact same as before, idle vacuum need to remain about the very same at the very same base timing, or—due come a greater static compression ratio generated by the brand-new heads" smaller burning chambers—be even slightly higher. This assumes the valves and the carb"s idle mixture and idle rate are effectively adjusted. A misadjusted carburetor, excessively chop valve lash, and/or a vacuum leak can require way over-advancing the time to crutch the underlying fault so the automobile seems to run a small smoother. Therefore, you require to gain these items squared far at the common base time range.


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See all 6 photosA vacuum gauge is an invaluable diagnostic and tuning tool. If the vehicle won"t run smoothly without overly progressing the basic (initial) timing, inspect for vacuum leaks. Girlfriend can additionally get pretty close top top the carb"s idle fuel mixture convey by turning the screws to obtain the greatest idle vacuum reading.

A vacuum leak leans the end the engine, which can cause detonation and also burn valves. At the most an easy level, if you placed your hand or a rag end the carb at idle and also the automobile keeps running, you have a vacuum leak somewhere for sure. Usage a vacuum gauge for an ext granular diagnostics.


The most most likely place because that vacuum leaks are at the entry manifold gaskets or under the carb baseplate. Spray a soap/water equipment or components cleaner in ~ the "joints" and see if the idle speeds up; if the does, you"ve uncovered the leak. Sometimes you"ll have actually an input manifold gasket that leaks the end the bottom into the engine valley—that"s really hard to spot v these rapid tests. Usually, you have the right to only uncover such a leak by pulling the intake and looking because that blow-by signs on the intake gasket itself.


Are every the unused carburetor and also intake vacuum harbor on your brand-new carb plugged off? Don"t skipping the big PCV hose nipple current on many "universal" Holley carbs at the behind of the baseplate under the secondary float bowl.


Are there any kind of vacuum leaks in the automatic trans vacuum modulator or the power-brake vacuum booster inspect valve circuits (either the devices themselves or the hoses feeding them)? temporary plug the ports to those and also any other external vacuum consumers consisting of the PCV valve and see if vacuum improves. If vacuum walk normalize, reconnect the hoses one at a time until vacuum decreases—at the point, you"ve uncovered the poor hose or vacuum consumer.


If you adjusted the valves too tightly, they could not fully close or nearby late. Essentially, this is tantamount to an internal vacuum leak. Run the valve lash again if vacuum is still too low after the ahead checks. The factory an approach is with the engine running at idle: through hydraulic lifters, ago off the rocker-arm adjusting nut on the valve you"re checking until it starts clattering, retighten till the noise simply disappears, then walk 1/2 come 3/4 complete turns tighter. The running technique causes one oily mess, therefore you have the right to also readjust the valves through the engine off making use of the EOIC (exhaust opening, entry closing) mediate method, as defined 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 appropriate base timing, is the rotor effectively aligned through the cap"s No. 1 terminal on that cylinder"s firing stroke? If not, friend may have actually reinstalled the distributor contempt "off."


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See all 6 photosRotor phasing on distributors there is no a vacuum canister: follow to MSD, the rotor tip should line up through the cap terminal or be on the leading edge that the terminal once the spark occurs.MSD
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See every 6 photosRotor phasing v a vacuum can: on a distributor that transforms clockwise prefer a Chevy does, MSD says the rotor tip must be just to the appropriate or just after the terminal once the spark occurs v the vacuum disconnected and plugged. With the breakthrough connected and also the engine in ~ a steady speed off-idle, the rotor tip should be simply to the left or prior to the terminal.MSD

Although it more than likely isn"t the reason of your instant problem, at this allude it certain can"t hurt to examine the distributor rotor phasing under to run conditions. Rotor Phasing is defined as the alignment in between the rotor tip and the distributor lid terminal once the spark occurs. Not correct alignment may cause the spark to jump to the next closest terminal or another ground leading to a misfire and power loss. Street-style distributors with vacuum can be ~ may have actually less-than-ideal rotor phasing (even with the vacuum advance plugged). MSD provides an flexible rotor to exactly this difficulty that fits MSD distributors and many other distributors the use similar caps and rotors.


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See all 6 photosCheck rotor phasing by drilling a large hole in the distributor cap in line with the No. 1 terminal. Run the engine at a secure speed and observe the rotor tip with a timing light. Keep in mind the relationship between the rotor tip and also terminal when the spark jumps. Noting the rotor guideline with white correction liquid makes this less complicated to spot.Norm Brandes
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See all 6 photosMSD"s two-piece flexible race rotor (PN 84211, left, contrasted to standard MSD one-piece rotor, right) can be provided on most MSD "points-style" small-cap distributors, and also many other Delco-style small-cap distributors. It uses up to 20 degrees of advance or retard and is simple to set up with precise 1-degree detent adjustments.Norm Brandes

Except for maybe the rotor phasing under running conditions, the coming before things come look at room the most most likely culprits due to the fact that they loss into the "things you"ve readjusted or likely have disturbed" category. However if none of the over helps, you"ll need to dig deeper, spring at other things look at not directly related to your top-end swap.


Does the height dead facility (TDC) mark on the harmonic balancer line up v the "0" mark on the time tab at TDC? end the years, there have actually been several various factory timing tab locations and also even different "0" mark places on the manufacturing facility balancers. The timing tab because that the little 6-inch-od balancer is additionally different than one for a big 8-inch balancer due to the fact that the larger the balancer, the greater the distance between each time mark. Come verify appropriate TDC alignment, you require to discover true TDC for the No. 1 cylinder. TDC stops the screw into the spark-plug hole are easily accessible from numerous outfits, consisting of Summit Racing.


On a factory-style balancer whereby the outer inertia ring is insulated indigenous the inside ring by a rubber strip, the external ring sometimes "walks" in relation to the within ring. One reservation this may be continue is if the timing keeps wandering—it won"t stay put where you set it at. (If you still space using ignition points, the timing will differ slightly as the clues wear.) A delaminating balancer should be automatically replaced. If using manufacturing facility balancers in a warm rod, an old-school cheat is painting a white or yellow upright line under the next of the balancer aligned through the TDC note on the balancer edge. If the white line starts to move so it no much longer lines up through the TDC mark, the balancer is delaminating.

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If the carbohydrate is properly adjusted, there space no vacuum leaks, the distributor is appropriately installed, and also TDC alignment is correct, that leaves the camshaft and timing chain. Eliminate the front cover and inspect the time set. Do the dots on the upper and lower sprockets line up at TDC No. 1 cylinder firing position (No. 1 piston in ~ TDC and also both intake and exhaust valves closed)? If it"s a multikeyway chain, is the reduced sprocket on the suitable keyway? Is the timing collection worn or damaged (chain slack or damaged sprocket teeth)? If all this checks out, that pipeline the video camer itself. You"ll have to inspect its opening and closing points with a degree wheel versus the time card offered with the cam. Exactly a slightly advanced or retarded camer with a multikey sprocket or offset electronic came bushing, but likely the camer would have to be ground method out the spec (very rare) come replicate your weird over-advanced scenario.