BarryK

this is the senario: ('65 L76 motor)

car ran pretty good including high rpm's except I knew distributor needed to get rebuilt as the timing curve was WAY off.
Over the winter as I was re-doing the engine and engine bay I had the carb sent out for rebuild.
I also replaced the distributor rotor and cap than ended up sending the distributor out for rebuild and re-curve. The distributor had hi-tension points and as they were only a few months old the guy working on the distributor kept them in. The guy working on my distributor (someone very, very well respected on the NCRS board and who has a Sun Distributor machine for proper re-curving) mentioned he wasn't too sure about the replacement cap and rotor I had just gotten from Paragon and put on and mentioned that if I noticed any high-speed break-up to replace them with different units that he recommended.
When reassembling parts back on the motor I replaced the spark plug wires with new Delco 508N wires as per recommendation from JohnZ and Duke.

Since getting the car running again I haven't driven it too much this season but when I have I've noticed it was breaking up at 4,500 - 5,000 rpms and higher.
I was told it may have been the float levels in the carb being too low and of course I remembers what the distributor guy said about the cap and rotor so today I went to fix the problem.

I re-set the float levels on the carb, and replaced the "new" distributor cap and rotor with the older units that I still had knowing I never had any problem with them and no high rpm break-up.
After setting the carb floats and replacing the rotor and cap I checked the timing and it's dead on at 12* initial, 36* total all in by 3,000 rpm and 52* total w/ vacuum advance.
I also re-checked my fuel mixture settings using a vacuum gauge and it's set based on the highest vacuum readings. I'm getting about 12 in/HG vacuum steady on my L76.

Took the car for a test drive and while it improved somewhat the problem is still there over 5,500 or 6,000 and higher rpm.
I don't like having my motor run that hard so I didn't stay up at that rpm range too long but now I'm wondering if instead of a "break-up" like points floating if instead it's maybe a high-rpm miss.

What is my best course of action to help diagnose and fix this issue.

To summarize:
1. the points are hi-tension and less than 1,000 miles on them
2. dwell is rock steady and timing is dead-on where it should be
3. distrib. cap and rotor are back to the "old" set I was running last year with NO problems like this
4. carb was rebuild and floats are set to the correct level and mixture is set right via a vacuum gauge
5. new spark plug wires (and new spark plugs)
6. new fuel filter

I'm getting a miss or a break-up of some type at high rpms.

Any help is greatly appreciated.
Thanks!

Grey Ghost

Have your tried a high octane fuel ?

BarryK

I run 93 octane which is what I've always used in the car without a problem before.
I'm not getting knocking or pinging (not that I can hear anyway over the sidepipes)

I happen to be going to the dragstrip this Sat so I can try putting in a few gallons of racing fuel to see if that will make a difference

kellsdad

Back when I ran engines with points type distributors, it seemed like the points had to be replaced frequently or I would experience a fall off above 5,000 RPM. Given all the things you've already done, if it were my car, I'd try a new set of points and condensor.

BarryK

Tom
yeah, I was thinking of that too. Just can't imagine that they went bad so quickly. There is less than 1,000 miles on this set

Vette-66

Don't think I can help directly but seems you want to break the problem into two areas, fuel, ignition.

You seem to have the ignition part pretty straight but you may want to check fuel/supply.

Does the miss happen with successive runs above 5000 or just once in a while when you run it up?

What carb are you running?
If your running vacuum secondaries, do they open FULL?
Do the throttle plates open FULL when the gas is floored?
Are your carb power valves functioning?
What jets are you running.
Have you looked at your plugs after a HARD run for signs of lean mixture?

Just suggestions of things you may want to check.

Chuck

BarryK

Chuck

the carb is the stock Holley 2818-1

1. I have no clue if my secondaries are opening FULLY or not when I'm at high rpms
2. same answer on the throttle plates
3. power valve is new from the recent rebuild
4. jetting is whatever the correct stock jetting is suppose to be as this is how the carb rebuilder sets them up
5. not yet, I'll pull one or two of the plugs tomorrow and shoot pics of them

thanks for the suggestions to look at

67vetteal

How old are your valve springs? Al W.

BarryK

Al

valve springs could cause a high rpm break-up or miss?
I didn't know that and never thought about the valvetrain as a possible culprit area.

I don't know how old the valve springs are. I bought the car just over 2 years ago supposably about 500 miles after it completed a body-off restoration including a motor rebuild but based on many things I found I have no false impressions about the quality of much of the work that had been performed during this restoration by the PO and I've had to go and re-do a lot of it myself. Point being is that the motor was supposed to have been rebuilt but I can't vouch on it or what would have been done if it was really rebuilt so the valve springs are an unknown for me.

Grey Ghost

Since you are going to the drags. Try emptying the fuel tank; as much as possible. Then put in the high octane only. Mixing as little as possible with the 93 octane.

Another option is to put it on a good dyno. You can make several test and tune runs to find out if it is fuel starvation, timing, etc...this may save you time, money and the aggravation of swap till you drop on parts. Some of the dyno techs can diagnose a problem from the start.

At the NASCAR races, you can hear them revving them up to the 9 grand range to check timing.

67-427ci

Hey Barry,
Have you given any thought to your coil? I had a similar problem last year with a miss and (at times) a backfire at RPM's over 4,000 (using heavy or wide-open-throttle). Below that RPM (and using normal throttle) it ran strong, idled well, etc. I went the route of rebuilding my carb's, replacing the fuel pump, reworked the distributor, replace spark plugs & wires, etc. After a few weeks of pulling my hair out someone said to change the coil. I did and it was an immediate fix.
Roy

BarryK

Roy
the coil crossed my mind but wasn't sure if it would be the cause or not.
Based on your mentioning it I'll have to start considering that also.
Thanks!

Formula Outlaw

Barry, I put 110 octane in my L76 a few months ago and the car absolutely HATED it. The higher the octane the slower the burn time. Basically, all the fuel was not being ignited before it was shoved out the exhaust valve. Car was definitely down on power.

I don't think that the 93 octane you normally run would be the problem.

You might think about installing the Breakerless SE single wire ignition system, an electronic ignition conversion kit that fits in the stock distributor so it looks stock. It really made a big difference in my car. Just a thought.

I wish you could drive mine. You'd swear there was an electric motor under the hood now it's so smooth and spools up so fast.

Do miss the sound of those solids though....

But the performance is not to be believed....

BarryK

Formula

I know a lot of people love the electronic conversion kits but I'm not convinced by them. I DID look at them right after I got my car but after posting about it and getting replies from JohnZ, Duke, and others plus searching the archives on the subject I'm pretty convinced to stay with my points. I believe based on what i've learned that a correctly and properly set up points ignition runs fine and the electronic conversion don't give any real benefit or performance advange.
i'm not saying this to start a discussion on this subject and in NO means to argue against the conversion kits - only that for ME I'm not convinced that there is any real advantage to switching - as long as the point distributor is set up correctly which at this point I'm actually pretty sure mine is.
I'm open to anything but I'm not so sure that this issue is from my distributor at all.
The coil is a definite possibility.

A good friend whose advice I trust and respect as much as JohnZ's emailed me and he has me considering a possibility that I would never have considered so i'm going to try his idea and if it solves my problem I'll post back saying what it was.

in either case I'm still open to any other ideas to look into incase this possible cure doesn't solve my problem

Ironcross

The only way to get past 5500 is to install a solid lifter camshaft. Hydraulics quit pulling right around where yours stops. In otherwords they are maxed out. If it`s a solid lifter cam, then I too would be concerned.

BarryK

Wally

yep, solid lifters and cam in my car.
Stock L76 w/ 30-30 cam.

in the past I've had the car up to past 7500 rpm going strong and still pulling hard before I got too scared for the motor and backed off her.
Never had this high rpm break-up or miss before this season.

Ironcross

Then I would check the valve train Barry because a L76 is a good engine.

BarryK

Wally, that makes two of you now indicating towards my valvetrain.
I didn't want that kind of trouble!

it's always something.

If it turns out it's in the valvetrain that it will have to wait until the fall. i'm not about to tear that apart now.
Guess I'll be learning about a new area of the car I've not gotten involved with yet.......

Ironcross

It`s easy to pull the overhead covers and check for a simple thing as a broken spring or bent pushrod. Easy to repair if so, and if not then your looking at a possible overlooked ignition problem. The last I really doubt as you installed everything new.

BarryK

yeah, no problem taking the covers off but i've never fooled around or even really studied the valvetrain too much when I've had them open.
Unless it's something REAL obvious than I'm not going to know too much even what to look at. Time I learned though