High Speed Misfire

[nmoreilly]

HI,

This is my first post

I have a 68 L-89 with stick 3x2 and TI. The problem is that around 60-65 MPH (constant load/speed) a miss has developed (intermittent missing/bucking etc). On acceleration (normal/hard) there is no problem and above 65 there is no problem. The carbs were rebuilt recently at a shop in northern NJ. I have had a shop doing this and other work on the car but I get the sense that I am getting screwed . At this point I am going to start taking the time to lookin into the problem myself starting with ignition. Do you guys have any immediate thoughts as to where to start first if anyone has had similar problems.

Also, the car has a vibration that is between 1800-2300 RPM with/without load. As the car warms up the vibration disipates substantially but is till noticeable. I have had the following items checked by the above shop (at least they claim to have checked and charged accordingly)Harmonic balancer, Fan/Fan clutch, Flywheel resurfaced. Could the other problem be related or is this more likely to be separate and internal to the motor?

Also, if anyone has had any good luck with any shops in Northern NJ for engine rebuilds I would appreicate the information. I have heard Nostalgi Motors in Boonton has a good rep

Many thanks to all in advance to reply,

Noel

:confused :confused

 

[Jack]

Welcome aboard Noel!

Welcome to CACC NOEL. You'll find CACC has many friendly folk wishing to help/learn! This is a VERY friendly place. Although it's near impossible to diagnose online ... I'll take a stab anyway. Seems you have misfire at steady 60-65mph. And it seems you have no misfire at launch ... & none above 6500rpm. At first blush it seems the ignition is ok ... I'd start looking at the carb(s) themselves ... or possibly an air leak at intake manifold/carb base(s). Just hang in there & someone else'll come along with a solution. BTW, we like owners' vette pics here!!!
JACK:gap

 

[Ken]

Welcome To The Corvette Action Center nmoreilly!

Can't help ya, just wanted to welcome ya.

 

[tangee-vette]

Welcome Noel.......When I purchased my Vette it also high a high speed miss. Somewhat as you described. Turned out to be plug wires. Hope this helps ya . Rick

 

[nmoreilly]

Thanks guys for your words of welcome and help

Noel

 

[AKRAY4PLAY]

Hello Noel,
I have run into several things that cause a miss at mid engine rpm, around 3000. I believe that Rick is on the right track. Usually speaking, USUALLY, when an engine misfires intermittenly, it is electrical. Try changing your spark plugs and then wires. Plugs are cheap. I normally run Autolite 26's in my small blocks. They seem to hold up to high compression and high ignition voltages better than anything else, plus they are wallet friendly!!! The three duces are sweet when dialed right, but I have no idea how help you there, sorry man. As for the vibration, it could be caused by some misfiring cylinders, or worse, the tranny. My '90 Chevy sport truck did the same thing your Vette is. Changed plugs and no more problems. If that does not help the situation, get a can of starting fluid or WD-40 and follow Jack's idea. You may even want to look at vacume leaks first, since it is cheaper to diagnose. Sorry I could not be of more help, good luck, Brian

 

[Ken]

Hey Noel, I didn't catch the "vibration" thing the first time around.

Mine does the same thing beginning at around 1800-1900 rpm on up, peaking at around 3200-3300 rpm. I haven't gone too far into it yet, other than to pinpoint that it IS where the vibration seems to be emanating -- from the engine. :eyerole

I am now leaning towards it being the bracketry on the front. I'll see once I start digging into it, hopefully soon.

Good luck!

_ken

 

[rpounds]

I would suspect plug wires, cap, rotor. Even if this is not the problem, the 100 bucks or so spent is well worth it.

If this does not cure the high speed miss, get a compression check. You may have a valve that is slightly sticky or a bit tight. A sticky valve would also explain why the engine runs a little rough before it warms up. Another thing to check would be the condition of the valve springs, although the indication would be that this is not the culprit, because the engine smooths out at higher RPMs, I would not rule it out as a problem.

I have a hard time believing that it has to do with carburation. Carburetor problems usually expose themselves in a much different manner. However, because you are running 3x2s, the center carb may have some problems. Remote, but certainly not out of the question.

Hope some of this points you in the right direction. Sounds like a very nice car.

Ron

 

[Bullitt]

T.I.

Welcome Dude! If I'm not mistaken, your distributor has a module in it that is hooked up to a seperate transistor igntion box from GM. The module or box could be going out, causing an intermittent problem in the power band. These distributors are expensive to replace and difficult to find. I think Corvette&Car Parts magazine did a series of articles on multiple carburetion a year ago. I only got one of the issues, but I'll try to find it to give you an idea of the issues you might want to backorder. I've always ran one carb, but a couple look cool. It's getting harder to find the old-timers who really know what they're doing, when it comes to carbs.--Bullitt

 

[redmist]

I have nothing but praise for OLSEN ENGINES right over the Jersey border in Nyack NY. The owner's name is Charlie. Money is not this guys driving force in life.
He worked with me on my engine, doing only the peices I wanted him to do, ie machine work. Gave me great suggestions and help me debug some problems I had when I put the engine back together and in the car.
He builds engines from 4 cam ferraris to SBCs primarily for racers.
845-353-0500
The racing season is almost over so you should get even more attention.

 

[nmoreilly]

Guys,

Again, many thanks for all the responses. Ken good luck with your vibraition hunt...one thing I am going tolook at is a possible bent or misaligned pulley which might cause it. I am hoping that it is not internal balance problem

Noel

 

[nmoreilly]

To Redmist,

Thanks for the tip with Olsen. Who did you use to get your engine in/out. I have bit of a problem in that the car is strored remote from where I live and would have to be stored where (or close to) the work is being done

 

[lone73]

engine miss at cruise

I had these EXACT same symptoms. I changed out everything I could think of on my '73 L-82. I even tried an HEI dist. with a new hot wire to it. New wires, plugs, several different carbs, resealed the intake, plugged off all vac ports to eliminate a possible leak, new valve springs and seals (while I was at it). I thought maybe i had a very weak spring that was allowing valve float or something. I finally decided after messing with it for months that it must have some ground down lobes on the cam. I was about to go looking for a dial indicator when I decided to just throw in the towel, as well as a zz4 crate motor. I figured I'd just get it rebuilt later and save some aggravation. It's now sealed in the baggie & crate that the crate mtr came in.
good luck with it
lone73