High rpm miss

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aaronk785

aaronk785

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Hello all. Need some help with my friends car and its a ford but you guys are the best so I came here first. Buddy inherited 65 mustang with a 289 from his dad. Wanted more power so we rebuilt it. Bore was good so went with forged flat tops for 9.8 comp. Factory ported heads with 1.84 and 1.5 valves installed. Comp cams 282s solid cam. 230 at .050 and .528 lift with howards duel springs 135 seat 300 open. RPM intake 750 proform dp. 3400 stall and 4.11s. HEI distributor. Got it running took it out for a test drive everything seemed fine. Put on some easy miles then hammered it. Ran great. Would scream to 7000 no problem. Probably put on a couple hundred miles. He stopped by one day and said it was missing. took me for a ride and sure enough at 6300 rpm acted like it hit a rev limiter. Every time at the same rpm in every gear. Not fuel starvation because you can ease it up to 4500 and nail it and still 6300 misses. Running 6.5 fuel pressure at that rpm. Tried new plugs, no help. Pertronix in stock dizzy with matching coil with full 12v to coil. Exactly the same miss. Checked valve lash, all good. Spring look good but didn't check pressures but they all seem stout. Compression check good. I am stumped. Please any suggestions are greatly appreciated.
 
Have you taped a fuel pressure gauge to the windshield and made sure it stays at proper fuel pressure?

Springs crossed my mind; so none broken?

Acting like a rev limiter seems more like fuel or electrical, though, and I would first look at electrical with it missing at the same RPM in each gear.
  • Plug wires: Check for end-to-end resistance on each one, plus the coil spark wire
  • Coil could have gone bad; easiest to diagnose by R&R. What coil model?
  • Moisture and dirt under the distributor cap was common on 302's/289's, with the distributor up front and the fan blowing carp right on it.
 
Both ignitions are brand new. So are the wires and plugs. Its funny both ignitions miss at the same exact rpm. Both say they need a full 12v to the distributor which I have verified. I was thinking springs too. But ive broke springs before and was a totally different miss. Can an alternator short out at high rpm? Thanks for the reply. I will try what you suggested.
 
aaronk785 said:
Both ignitions are brand new. So are the wires and plugs. Its funny both ignitions miss at the same exact rpm. Both say they need a full 12v to the distributor which I have verified. I was thinking springs too. But ive broke springs before and was a totally different miss. Can an alternator short out at high rpm? Thanks for the reply. I will try what you suggested.
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To be sure, I'd rig up a voltmeter with the lead right at the coil+ feed out in the engine compartment, to make sure that the voltage stays up.... With a good alternator, the voltage on the battery + to - terminals with the engine at a fast idle speed ought to typically be in the 13.8 to 14.5 range. The voltage to the ignition box and coil + should be no less than 1 volt lower.

IIRC the Ford system back then used a resistance wire to the coil+ terminal, which serves the same purpose as a Mopar ballast; IIRC#2 that wire is in the harness inside the car.

The ignition system 'typically' will draw more current as it rev's up, so that will make sure the voltage is staying up... there might be some resistance in the wiring somewhere.... like that resistance wire mentioned above, or a poor ignition switch contact or firewall connection. Any resistance should show up as a low voltage to the coil+ with the engine running. (It probably will not show a low voltage with the ignition switch 'on' but not running with that Pertronix system.)

Agreed on the springs acting differently. As for an alternator shorting, unlikely it would just do that at a certain RPM range, but anything is possible; monitoring the voltage right at the ignition box would tell you that.

Regardless of the parts being new, 'infant mortality' failures do occur so you gotta check everything IMHO; troubleshooting by 'circumstantial evidence' will bite you in the a**! If the 2 boxes act the same way, then you can likely rule them out, but not something like the plug wires, despite being new. Look for resistance on each spark wire in the 3k to 10k range for most types.
 
do a cylinder leak down test my bet it slightly bent exhaust valves from over rev.
 
Thank u all. Here is what ive done today. Ran coil off of a relay has full 12v with car running. Lashed valves. Unplugged tach. Put old plug wires back on ran perfect with these just wanted new. Put old plugs back in. Changed carb back to 600 holley. Voltage good at high rpm. No rev limiter. Never had a cd box. Went back to hei distributor. Nothing helped every time any gear 6300 and will not rev a bit higher goes into total chaos at about 6350 and just falls on its face. Im thinking it has to be springs. Just weird, one day 7000 rpm next day 6300 max. This thing sings and sounds beautiful to 6300. I'll keep at it. Thank you for all the help I'll try some more suggestions.
 
what is the spring rate? sounds like valve springs have gone south. cheap springs do not have a long life. need a spring tester and check pressure on heads.
 
I'll keep on the electrical try more things. The spring rate is 300. They are new howards springs only about 200 miles on them. I've had real good luck with howards products. Dual with damper.
 
Probably ignition.

If you wanna check for spring bounce... pull the rockers and look at the rockers wear pattern on the stem, if is a straight line parallel with the head then its fine... if it looks like an x or you find some angled while others parallel... it's the springs.
Stock **** might rotate but not with heavier springs they wont.
 
Went out and pulled some rockers. Sure enough one with butterfly pattern, and several with wear pattern off by 45 to 90 degrees. Wow springs didn't even last 200 miles on a cam with slow ramps should be easy on springs. Guess I know what I have to doo now. Must of got a bad batch. Thanks for all the advise.
 
aaronk785 said:
Went out and pulled some rockers. Sure enough one with butterfly pattern, and several with wear pattern off by 45 to 90 degrees. Wow springs didn't even last 200 miles on a cam with slow ramps should be easy on springs. Guess I know what I have to doo now. Must of got a bad batch. Thanks for all the advise.
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when you get it apart, look at the spring seats on the heads for digging in of the springs.
That will double verify the springs are bad or installed at the wrong height. Good seat pressure is very important as well as open, some people try and get away with the lightest spring they can be it fitment issues, less Machining of guides, trying to get the last quarter horsepower out of their engine... But when a valve hits a piston at high rpm it destroys an entire motor in most cases ...so it's just not worth it.imo

Solid or hydraulic cam?
Seat psi should have been 130-145 seat depending....figuring a 7 psi loss after break in which is typical of springs.. which is why you need to figure that in if you're looking to run then on the low end of spec'd pressure.
110lbs turns into 90 something real quick after some heat cycling.
 
aaronk785 said:
Went out and pulled some rockers. Sure enough one with butterfly pattern, and several with wear pattern off by 45 to 90 degrees. Wow springs didn't even last 200 miles on a cam with slow ramps should be easy on springs. Guess I know what I have to doo now. Must of got a bad batch. Thanks for all the advise.
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What rockers?
 
aaronk785 said:
Summit racing aluminum roller rockers.
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Bingo! Mopars aren't the only engines that can have poor rocker geometry. I'm guessing there was nothing wrong with the springs, and valvetrain instability weakened them prematurely.
That motor will likely need a pushrod that is at least .100" longer than stock, and can be quite a bit more than that depending on the combination. How wide is the wipe pattern on the tip?
 
Push rods are .125 longer than stock. Thats as short as I could go and not have the rocker body rub the stud shoulder. Wanted shorter to get my sweep centered. Right now it just a bit to the outside. I would say the sweep is .060 to .070 wide.
 
Installed height is recommended 1.750 with 135 seat pressure. That about 20 over the 115 they recommend. After lash cam is .506 lift. coil bind height is 1.062. So lots of room. I'll do what it takes to make it right. Learning all the time.
 
aaronk785 said:
Push rods are .125 longer than stock. Thats as short as I could go and not have the rocker body rub the stud shoulder. Wanted shorter to get my sweep centered. Right now it just a bit to the outside. I would say the sweep is .060 to .070 wide.
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Centered is not good geometry, minimum sweep is, no matter what is says in some performance book or magazine. Centered is a nice bonus once minimum sweep is achieved.
It sounds like there is a longer than stock valve in the heads, but that might not be the case. Regardless, the sweep should only be about .025" with that amount of lift. You are almost three times that much, and the valvetrain is unstable. The reason the rocker runs toward the outside of the stem isn't because the pushrod is too long, it's because the rocker is too long. It's not an anomaly to have this problem, but rather standard fare when using bolt on valvetrain parts. They design the rocker for use with stock length pushrods so guys can bolt them on without having to buy new pushrods or figure out pushrod length. The problem is, the geometry is different for the roller rocker, and it causes problems like your buddy is having.
This is what I have to do with GM heads, and Ford is no different.
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20181128_092951.jpg

The studs are relocated so the roller doesn't come close to rolling off the edge of the valve when the correct length pushrod is used.
The only other options are to use a stock rocker, a rocker with an offset trunnion (Crower has some good ones), or get a shaft mount system. All but the stock rocker scenario is going to have some additional cost, some worse than others.

I'd put a length checking pushrod in the motor and keep lengthening it until you get the narrowest sweep on the valve tip. Then note how far off center it is, and that is how much too long the rockers fulcrum length is for that head. If it's at least .100" from the edge of the tip, run it with the longer pushrods. If not, then you will have to decide what your next move will be.
One other option you can try is a roller rocker for a small block Chevy application. They are shorter than a Ford, and may allow you to have the right length pushrod, and stay reasonably centered on the valve tip.
Let us know what you find, because this is good info for the small block Magnum guys running the stud rockers, too.
 
aaronk785 said:
Installed height is recommended 1.750 with 135 seat pressure. That about 20 over the 115 they recommend. After lash cam is .506 lift. coil bind height is 1.062. So lots of room. I'll do what it takes to make it right. Learning all the time.
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You could use a shorter valve and installed height with that amount of lift. Or, use a spring will a taller coil bind height. You have .182" coil bind clearance at full lift, which is about .100" too much. Less coil bind clearance stabilizes the spring somewhat, but it won't stop the instability from incorrect rocker geometry.
 
aaronk785 said:
Summit racing aluminum roller rockers.
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Just checked Summits website. You can get a single rocker for a small block Chevy for $20 if you can't borrow one from someone local to check things out. That way you aren't buying a whole set without knowing whether they work or not. If they do work, then your buddy with have a spare in case he ever breaks one.
 
B3RE. Thank you so much for explaining this to me. I must be reading the wrong material. This was tech from a reputable cam maker. Said to change push rod length to get sweep centered. That's the difference between me a garage builder and a pro like yourself. OK. Went out and put in a .150 longer push rod. Sweep is down to just under .040.and right at about .100 from the edge. This is the original set I ordered but they didn't center the sweep. So I went shorter. 7.1 to 6.950. Am I close enough or not? OH! and I think their small block chevy valves. Thanks again for sharing your knowledge. I'll try the sbc rocker to see if that helps if what I have wont work.
 
aaronk785 said:
B3RE. Thank you so much for explaining this to me. I must be reading the wrong material. This was tech from a reputable cam maker. Said to change push rod length to get sweep centered. That's the difference between me a garage builder and a pro like yourself. OK. Went out and put in a .150 longer push rod. Sweep is down to just under .040.and right at about .100 from the edge. This is the original set I ordered but they didn't center the sweep. So I went shorter. 7.1 to 6.950. Am I close enough or not? OH! and I think their small block chevy valves. Thanks again for sharing your knowledge. I'll try the sbc rocker to see if that helps if what I have wont work.
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I know, Comp has had this info wrong for a very long time, and doesn't seem to want to correct it. Don't beat yourself up, there are a lot of "pro's" who don't do it right, and bad info in print doesn't help anyone, except maybe the one selling the rockers.

That's still off a fair amount, all things considered. I'd see if you can get it down to .030" or less, and use a SBC rocker if necessary. If you keep lengthening the pushrod, it will get to a breakover point where the sweep will start getting wider again. Making the sweep as narrow as possible, will make the valvetrain a whole lot more stable.

Edit: I misread your post. I thought you went from a +.125" to a +.150" pushrod, and it dropped to .040" sweep. I see now that you went an additional +.150" over the +.125", so effectively +.275". You will have to fine tune with an adjustable pushrod for the narrowest sweep, which may be longer or possibly SHORTER than where you are now. If it needs to be a little shorter, the the existing rockers will work, although it might be a bit nicer with the SBC rockers.
 
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