HEI Conversion, now no top end power

I've converted my V6 to HEI and have worked the intitial bugs out. All new cap, module, wires etc. Engine is totally stock and original otherwise.

Engine starts great and runs clean, nice idle and low end.

But..... Over 2500 RPM (estimate, tach not connected yet) it just runs out of power. The engine always pulled pretty stong up to 4K or so previous to the change.

I have not changed to the adjustable vacumn advance yet, but not sure if that's connected to this issue.

Any advise?

 

did you pull out any ballast resistor wires, and take power from a good battery source?
I'm just guessing, but you might be running out of power to feed the HEI at the upper RPMs

 

Ballast resistor bypassed.

 

make sure the vacume advance is plumbed to ported vacume. by this i mean the port should only have vacume above idle and nothing at idle. also make sure the advance diaphram on the distributor is working and also check the mechanical advance inside the distributor under the rotor. and make sure the base timing is set without and advance at all.

it can only be a timing issue. hei should make it run way better.

 

Sparky remembers correctly. I removed the vacuum advance can, locked down the vacuum advance mech in the dist.

It pulls great at idle and purrs at high rpm. It didn't really seem to affect my mpg much if at all. (at least compared to the stock dist.)

As others have said, it sounds as if the timing is 180* off. It can happen easily when you put the new dist back in.

The only other thing I can think of is if the mechanical advance is either sticking, not free or the springs are not correct (the curve as Patrick says)

 

I agree, an old post of Sparky's helped me identify that problem with my 4.3. Idles great, no power. As Sparky said if you have dual exhaust one tailpipe will be cold.

 

Very interesting suggestion regarding the 180' out.

I had a bad backfire situation on the right side, which we diagnosed as a bad valve spring. We found a cold cylinder there as well. Replaced all the valve springs and now the backfire is gone.

I wonder if I had both problems? Will run it tomorrow and check the right side temps!

 

Good news to report, but as usual not exactly what I expected.

The distributor was out of sequence or just 1 tooth off. We rotated it a tooth and immediately the heat returned to the right side cylinders...except for 1 (#2).

So a little more diagnosis found a bad spark plug! The plugs were new within a couple of months so I never even considered that possibility. 1 new plug and it runs better than new.

My guess is the backfiring was a combination of the out of sequence ignition and the bad plug. The valve spring was probably not a big issue, although it did stop backfiring after the springs were changed, before any of the other fixes. So who knows.

Now lots of joy in mudville.

Thanks for all the suggestions, I also found guidance in a very detailed Odd Fire HEI guide written by Rich Motts at www.jeepster.org/swchapter/hei-2.html

 

I forgot about that article. I know you solved the problem but I was interested in this little tidbit from Motts' write up.

"Gap the plugs a little wider, .040 is fine. A wider gap will give a better idle, too wide will kill top end performance." which makes sense.

I keep mine at about .050 or so.