'70 hi RPM problem update

[adam schorr]

On Monday, 5 September 2005, at 5:12 p.m., I wrote "The all-stock 11:1 LT1 will not rev past 5300 rpm. It pulls strong until the fun is supposed to begin, then quits revving, breaks up, and backfires"

Suggestions posted were
1) Pull the plugs; examine condition; see if they are R43, or whatever. Go hotter.
2) Primary fuel bowl full could be fill of schellac due to non use
3) Other avenues

We pulled 4 plugs (two from each side of the block). They are R43-s and all looked fine. The gap is .035 Perhaps a whee bit more. With plugs in good condition, is a hotter plug swap still warranted?

My buddy thinks this is ignition related. Reminder from initial 9/5/05 post: The last work done by a mechanic was on the carb, and ignition.

Michael, Terry, Drew, Dave, or anyone, suggestions appreciated !

Thanks!
adam

[William C.]

Re: '70 hi RPM problem update

Change the rotor to a good high-performance rotor. and close the sparkplug gap to .030. your comment of .035 maybe a bit more in a high-compression engine is headed in the absolute wrong direction.

[Clem Z.]

Re: '70 hi RPM problem update

does it have TI or points and if it has points the spring is not strong enought,wedge some foam rubber against the moving point arm to add more tension.

[adam schorr]

Re: '70 hi RPM problem update

LT1 has TI Ignition.
Gap is set to factory service manual specs .035...Give or take one
one-hundreth, or thousandth. Think hardly enough to cause rev limit problem.

[Duke W.]

Re: '70 hi RPM problem update

The TI generates about double the per spark energy as the single point system, so it will fire a fouled plug that a single point won't, but AC heat range 3 is much colder than needed for street use. I used 3s for race track hot lapping (which were just right) and 5s for the street.

Plug fouling might be the issue. Try R45s and see what happens.'

Duke

[William C.]

Re: '70 hi RPM problem update

check the wires for proper resistance and use a hi-perf rotor.

[adam schorr]

Re: '70 hi RPM problem update

Duke, Bill,

The plugs are clean, fine. Look to be in excellent condition.

Duke, swap them anyway for 45's anyway?

The cap and rotor may were changed last month...To maybe NOT a hi-performance rotor as suggested.

[Duke W.]

Re: '70 hi RPM problem update

I don't think the plugs are the issue, but I recommend AC heat range "5" or equivalent for all SB street applications regardless of power rating or ignition system type. Since they run hotter they will tend to burn off deposits better, but they are not hot enough to overheat and cause preignition in any kind of sane street driving or even drag racing.

Bill's suggestion to check wire resistance is a good idea. Do you know the make/model model and age of the installed wires?

IIRC the OE type radio suppression wires should be no more than about 5K ohms per foot.

My experience with TIs and HEIs is that they tend to work perfectly or not work at all. Hi rev breakup is a common problem on single point distributors as things wear and get sloppy and even on new distributors on high revving mechanical lifter engines. The fix is a good "blueprint" overhaul including shimming up the end play to two to seven thou.

Another issue that can cause high rev breakup is a clogged fuel filter or other problem that prevents sufficient fuel delivery. As the engine's fuel flow demand increases with revs at WOT the mixture leans out until the engine misfires or just quits.

Duke

[adam schorr]

Re: '70 hi RPM problem update

Duke,

I will look into your wires question: Will consult my repair records, chapter meet judging sheets, and look at them to see if I can identify.

What did you mean by "IIRC" in your last post.

Thanks as always!
adam

[William C.]

Re: '70 hi RPM problem update

Adam, I did a tuneup a long time ago (about 73) for a friend who brought his own parts, 327/350hp. Used a "modern" design rotor and had exactly the problem you are reporting. Changed back to the original the problem disappeared. The factory design of the rotor changed to shorten the tip (lengthen the distance the spark must travel from tip to distributor cap contact) and that caused the problem noted, I don't know if it will fix your car on not, but an Accell rotor is a cheap investment after your aggrievaton.

[Mike S.]

Re: '70 hi RPM problem update

a colder spark will help high rpm miss fire. if that does not help.check valve spring pressure. good luck.mike

[Terry M.]

IIRC = If I Remember Correctly *NM*

[Duke W.]

or "if I recall correctly" *NM*

[Clem Z.]

Re: '70 hi RPM problem update

make sure the wires are all the way down into the cap,slip up the boot and push down on the wire into the cap. run the car hard till it misses,cut the engine quickly and coast off of the road,pull a plug,if dark it is ignition and if white it is fuel. you may have to do this several times to get a reading