High end miss??????

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dwgibbs

dwgibbs

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I have a 318 cast date of 1974... My son and I recently installed a wieland intake a new holley 600cfm electric choke carb, flowtech headers, and XL 268 Comp cam and new lifters, Its an automatic, stock 7.25 rear.... It runs and sounds awesome at idle and low speed however wen we take her on up to 75mph and up, the engine starts missing... new plugs/cap/rotor... it is stock ignition... any ideas on what/were to start looking?? the bottom end is all stock as is tranny... thanks in advance for the help
 
since the engine will be revving about 2,000+ rpm's for prolonged periods of time @ 75mph, your float bowls in the carb could be running dry if the floats are not adjusted right. or the advance curve may be out of sync...somebody else will chime in soon and set ya on the right track the folks here are great!
 
check fuel....plenty.......ignition?? have considered installing a new MSD but from all I have read, the stock ignition is more than adequate, unless of course i am reading old material
 
stock ignition will be fine as long as everything is running properly. I had a top end miss, jets were too big. plugs were black. What color are your plugs? Timing at full advance?
 
have new plugs in now....wen i changed them they were all Black and fouled.... carb was dumping way to much fuel.... have since changed carb with the new 600 Holley... timing is at full advance..or it was, havent checked it lately though....has new timing chain and gears...
 
I had the same issue on an old derby car a few years ago. When I put the new intake on, I ended up having a leak on the back of the intake to the block surface. I pulled the intake off, sealanted the **** out of it and put it back together. Problem was solved for me. I would check that area, cost you nothing and may be as simple as that.
 
springs are stock... runs great and burns the tires from a dead stop..... the miss is at high speed only and in 3rd gear.... around 2500 rpm and up...... but not in lower gears and higher rpms 4-5k does fine......???? checked intake and no leaks
 
Check your rubber fuel lines at the tank outlet and fuel pump inlet. I had a problem (slant six) where the engine would start missing at ~55 mph, like starved for fuel. A fuel lines was too soft and collapsing (damn ethanol). If you replace, use the newer "fuel injection" type. Not cheap at ~$5/ft but not affected by current fuels.

Another miss I had (2.4 L) occurred when accelerating, in 3rd gear going ~45 mph. It was a spark miss and went away after new plugs. I had even cleaned and re-gapped the old plugs, but no luck. When it occurred again a year later, I lowered the gap from the factory (0.055" recall) to 0.050", which totally fixed the problem. That was a tip from a Neon turbo site. If it starts missing when you open the throttle, you may have the same issue. It is harder for a spark to jump the gap at higher cylinder pressure (throttle more open, rpms low). Sparks can jump many feet in a vacuum.
 
You should be running spring pressures recommended by the cam manufacturer.
A couple of things...

Is carb jetted stock?
if it is...it's too lean, and needs to be jetted like the manual choke version eddy with, the electric choke version is jetted LEAN and will cause a surge with full advance cruise rpm.
check the psi and hoses like others mentioned as well.

The timing...
whats it set at? and do you have the vac advance hooked up?
if you do...unplug it from the canister and plug the hose for the next test drive.
Also check out the coil, test it.

I would just cover all these and more but in an order of 1st the simple/pointing toward kinds of issues, visual then testing and tuning..
 
Thanks Ya'll
the cam manufacture (Comp) said i didnt need to change the springs. (talked with same Comp Tech many times and this was one of my ? )... the Holley is jetted stock... will re-jet them last if needed... fuel lines appear good... the vaccum advance is not hooked up, nor plugged.... it is now (plugged)....i will also get the coil tested.. I didnt have any of these issues before i installed manifold/carb/cam.... the car ran great....... thanks again for pointing me in a direction....
 
dwgibbs said:
I didnt have any of these issues before i installed manifold/carb/cam.... the car ran great....... thanks again for pointing me in a direction....
Click to expand...

Did you check the intake manifold gasket? Get in there near the distributor with a mirror, it was tough to see.
 
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