This 1965 L76 motor pulls fine up to about 5500 RPM, then breaks up. What should I look for?
Joe
Joe
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Re: Motor Breaks Up After 5500 RPMI
If secondaries aren't opening, then I'd suspect you'd have a hard time getting past 4500 RPM. Typically, the description of an engine 'breaking up' (running so 'ratty' RPM refuses to climb) correlates to an improper air/fuel mixture (typically too lean). You can STONE the problem instead of knee jerking with various 'try this' and 'try that' by renting time on a chassis dyno where engine spark is recorded/displayed and tail pipe exhaust gas is recorded/monitored. That'll NAIL the generic source of the problem....
Had this same symptom with a BB tri-power setup about a year ago. All the King's horses and all the king's men (change this, change that, try this) had absolutely no effect. Engine refused to climb above 5100-5300 RPM.
I rented a dyno and did the 'trick' above. Nailed it! Today's alcohol impregnated gasoline reduces the boiling point of fule in the carb bowls. As the car neared max RPM (also max BTU generation under hood) we observed mini geysers of fuel erupting from the vent stacks of each carb nearly at random!
To test the hypothesis, we soaked rags with water, set them in the refigerator, let 'em cool down then wrapped the tri-power fule lines and immediately re-ran the test. She climbed effortlessly to red line! Did it twice. But, after that she started acting 'squirrely' again and we found the shop towels were warm....
Suggests the BB tri-power system was VERY near its thermal dissipation limits when originally designed and today's environmentally friendly fuel pushed it over the edge! Don't think gas guys go out and test their mixtures on 30-50 year old carb equipped cars.... Virtually all of what's built today is some form of FI with the gas under reasonable pressure. CAN tell you I've never seen this 'funny' on a properly built and tuned 4-Bbl carb though. -
Re: Motor Breaks Up After 5500 RPMI
My hunch is either the fuel filter or distributor. If the engine just quits - like someone turning off the ignition - your fuel filter could be clogged and it's just running out of fuel. If it's a heavy misfire that keeps the engine from revving any further, the distributor is a likely culprit. You can check the fuel filter by removing it and blowing through it. It should offer little resistance.
To check the distributor, free rev the engine to over 5500 with a dwell meter attached. If the dwell drops more
than two degrees the distributor needs work. All engines with redlines over 5500 should
use the 28-32 oz. breaker arm tension points, though Chevrolet did not specify them for
SHP/FI small blocks. (I don't think they even became available until about 1966.) This
might be a quick fix, but...
considering age, you should pull the distributor and overhaul/blueprint it. Check the
shaft bushings for wear and have them replaced if the shaft is loose, (Several
specialists can do this job.) If the shaft bushings are snug, regrease the upper bushing
well and the tach drive gear. Install a new breaker plate that fits snug and wobble free,
shim up the end play to two to seven thou, and install the 28-32 oz.points. When you
reinstall the drive gear make sure the dimple is pointing the same direction as the rotor
tip or you won't be able to achieve correct initial timing due to interference of the
vacuum can with the manifold. If you need a new vacuum can use the NAPA/Ehclin
VC1810, which has the correct specs for SHP/FI small blocks.
My '63 L-76 would rarely rev to the redline in the first few years until I figured out how to
make the distributor work properly, but the flaky distributor, which acted like a rev
limiter as low as 4500 probably kept the engine from throwing the #7 rod and holing the
block. When the engine was down for overhaul at 115K, a maganflux inspection
showed a significant crack across the bolt head seat.
DukeComment
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bad valve springs
simple, your valve springs are weak.Comment
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Motor Breaks Up After 5500 RPMI
John:
Valve springs are brand new Z28 type, which are considerably stiffer than the original style. Engine is newly rebuilt with about 100 miles on it. Valve springs were shimmed as needed to achieve spec height. I would have guessed valve float too, if I didn't have the above information.
JoeComment
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Motor Breaks Up After 5500 RPMI
Jack:
I just got the carb back from Jerry Luck who did a super duper restoratoin job on it. Prior to this rebuild, the engine had a bad stumble/pop thru the carb, and 5 of the 8 plugs showed white insulators. The stumble/pop is gone, and now it feels like a rev limiter @5000-5500RPM. Possibly still too lean, but not as much as before?? If so, I don't know why that should be. Jets and power valve are stock, as per Jerry. Since I live at sea level, I can't see any reason why rejetting should be needed. Jerry reported that there was some warpage in the metering block(s), which was (were) machined true. One of the many questions I asked before I sent the carb to him, was whether he loads the test engine and checks secondary opening @WOT--he said he does. He tests all his jobs on an engine prior to shipping, and he swears that if any problem, it won't be the carb.
Took the car for 1st road test yesterday with the restored carb. When I pull the plugs, I'll post what I find.
Here's some food for thought. I am using an old 202 coil that I picked up in Carlisle for 10 bucks--fresh coat of paint and it's good to go. The motor fires up very quickly. Can this coil be the cause of the problem? How, exactly can I test for a weak coil? It would be great if this is the problem--I'll just drop in a new Accel unit.
JoeComment
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Re: Motor Breaks Up After 5500 RPMI
Trivia - all '67-'72 small-block V-8 applications (283/302/307/327/350, hydraulic or solid lifters) used the same valve springs; 80# @ 1.70" closed, 200# @ 1.25" open. The howling 7000-rpm 302's and LT-1's used the same valve springs as the grocery-getters; quite a tribute to the Chevy valvetrain designers.Comment
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Motor Breaks Up After 5500 RPMI
Duke:
Could it be points bounce at only 5000-5500 RPM? I am not certain, but I think that are Blue Streaks. The fuel filter is brand new, as is practically everything on this newly restored car. Fuel line, fuel tank also new. Fuel pump was reused, and it is the replacement for the SHP pump (forget now what number it is--looks like the "correct" unit, except for the missing "AC" logo).
I rebuilt the distributor, and installed all new bushings and mailshaft, which were all wiped out. Dimple is where it should be (in line with rotor), and end play was set to lower limit of spec. Wobble is almost nonexistant. For more info, please read my replies to John38706, and Jack Humphrey, both on this thread.
You might remember, we had many long discussions about my 30-30 camshaft, and carburetor issues--just to jog your memory. Lash is set per your method, which gives me a nice, lumpy idle with around 10 in-hg.
JoeComment
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Motor Breaks Up After 5500 RPMI
John:
That is amazing. I trust that the grocery getters were using overdesigned springs, and not vice versa.
The shop that set up my heads (Heyad Racing Heads--Cranbury N.J.)showed me the difference between the old and new springs. The old tested @60 pounds, and the new tested @110 pounds (don't know what height, but they were the same, of course for comparison's sake), which is most likely "valve closed". Could the old springs have been so tired? I was so concerned, that I asked Tom, the owner, if he should pin the valve spring studs. He told me it was not necessary. Is it probable that some studs have begun to pull out of the heads. This would certainly cause valve float.
JoeComment
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Correction
John:
I meant to say rocker studs, not valve spring studs, but I think you knew that.
JoeComment
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Re: Correction
the plugs will tell you if it is lack of spark or fuel. lack of spark the plugs will be dark gray and lack of fuel the plugs will be white. run her thru the gears till it misses several times and quickly pull the plugs without drivng back to your home or idling. take a plug wrench with you and do this in a spot were you can coast to a stop and plug the plugs.Comment
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PS
make sure all the plug wire are all the way down into the distrbutor cap towers and coil. pull back the boots,insert the wires and then put back the boots. i have see where not doing it this way the boots will keep the wire from bottoming out in the cap towers.Comment
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Re: Motor Breaks Up After 5500 RPMI
The Standard Ignition "Blue Streak" points are available in both standard 10-23 oz and 28-32 oz. breaker arm tension. What it the Standard Ignition part number of your installed points?
DukeComment
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Re: Motor Breaks Up After 5500 RPMI
To add to what John said all pre-'67 engines going back to the dawn of the Duntov cam used the same valve springs including 327s with the 30-30 cam. The early valve springs are essentially the same, but have slightly different specs.
When Duntov designed his cam "off budget" he knew he could not change anything else in the valve train, so he stretched out duration with about the same lift, which reduced valve acceleration, thus allowing the standard valve springs to control the valve at up to 6500 RPM plus a little more for insurance.
All subequent Chevrolet high performance mechanical and hydraulic cams followed the same philosophy, so the only difference in valvetrain is the cam itself and the lifters in the case of mechanical lifter cams. Everthing else is the same as a two-barrel 283.
DukeComment
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make sure the heat riser valve is not frozen shut
as i chased a miss in a corvette and found this problem years ago.Comment
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