Low RPM troubles

[Todd Hedges]

Anyone experiece problems with performance below 2500 to 3000 rpms?

My engine is very sluggish, and very slow to "rev-up" when below 3000 rpms. Runs like a truck motor.
Timing appears to be okay, if anything a little to advance.
36 degrees total timing at 3,000rpms.

What could the cause be for the slow reving?
Bad carb power valves?
Vac. leak?
Bad MSD 6AL?

I checked for arcing.

[Edley Rondinone]

How well is/are your fuel filters flowing? I had an intermittent "hesitation" due to a partially clogged fuel filter...

[Todd Hedges]

Good question.
I'll take a look.

[HighPlainsDrifter]

Todd,
You could check a couple things.
Check the positive and negative polarity on the coil,it makes a difference if the wires aren't correct.
Put a vaccuum gauge on the engine, if you have less than 10 inches at idle then your timing is retarded or the cam is retarded or too big for the street. If it is a hydraulic cam you can check or re adjust the rockers, if they are too tight the engine will lugg and not rev.
Check your plugs, see if they are light brown[normal] or black[rich]
Good Luck,
Perry.

[Clois Harlan]

Todd,

Is this a new problem or has it always been this way? What cam and manifold are you running?

Clois

[Todd Hedges]

Clois,
I can not really explain when this problem appeared.
THere were a host of other complications that I corrected, such as
bad plug wires, bad plugs, timing, etc.
Even after correcting those issues, the slow reving still exists,

I have a Ford SVO intake and a mild Isky cam.
(I'll look for the exact numbers on the cam.)


Thanks

[CJ428CJ]

Todd,

I'm running Isky hydraulic cam 351281. .565/.565 lift, 232/232 duration at .050, 108 LC. That doesn't seem to be too much cam for my application. It pulls strong from idle (1000 rpm) right up to about 5800 rpm.

If it were me, I think I'd probably check all of the following:

1. As suggested above, double check the valve adjustment.

2. Double check the timing both initial and total. My 428 likes tons of initial advance. I have it set for 20 degrees initial and 38 total, all in by 3000 if I recall correctly (light blue and silver spring and the largest diameter advance stop if you're using the MSD distributor). Once you have the correct springs and stop in, check the timing marks with a timing light. Make sure your timing advances as you rev the engine. If it doesn't the weights in the distributor may be hanging up.

3. If the above checks out, I'd move on to the carb. If you've messed around with the carb a lot already (changed jetting, adjusted the idle and idle mixture screws), try to get it back to factory specs to start with. To do that, put the original jets back in (if you've only changed them 1 or 2 sizes, you may be okay but if more than that, put the orignals back in for now). Take the carb off to make sure the butterflies aren't open too much. It's nearly impossible to properly adjust a carb if the butterflies are open too much. On a Demon, there's a grooved slot. The butterflies are supposed to be adjusted so that the visible part of the slot makes a square. If it's a rectangle, the butterflies are open too much. I assume Holley has something similar but I'm not sure. Now, put the carb back on but leave the turkey pan off until you're done adjusting the carb. Turn the idle mixture screws all the way in and then adjust them until they're about 1-1/2 turns out for starters. All 4 should be adjusted exactly the same. While you're adjusting the carb, advance the initial timing to about 25 degrees. Fire the engine up but if it doesn't idle, don't adjust the idle yet (if you do, count the number of turns and make sure you turn it back to that setting before proceeding). Make sure the engine is fully warmed up. After the car is heated and running, fire it up. If it starts to die, don't hit the accelerator, instead gently push the accelerator pump arm to give it a little gas. What happened? Did it improve or get worse. If it got better, the engine is looking for more fuel. If it got worse, it's getting too much. If it got better, turn the idle mixture screws out 1/8-1/4 turn (all four the same amount). If it got worse, turn them in 1/8-1/4. If the idle increases when you turn them in 1/8 -1/4 turn, turn them in another 1/8-1/4. Keep doing that until the idle stops increasing. At that point back them back out about 1/8 turn.

If adjusting the idle mixture screws has no affect on engine idle speed, you may have a blown power valve or one that's rated incorrectly. Put a vacuum gauge on and measure manifold vacuum. The power valve should be rated 1-2 inches lower than manifold vacuum at idle. I had to go from a 6.5 inches pv to a 5 inch with my cam. I had nothing but headaches until I made that change. Once you have the idle mixture screws adjusted properly, put the initial timing back to 20 degrees or so and then finally you can fine tune the idle with the idle butterfly screw.

That was kind of long winded. Hope it makes sense.

Chris

[Clois Harlan]

Todd,

How is the RPM Problem? Did you check your accelerator pump on your carb?

Clois

[Todd Hedges]

Wow, Chris.
Thanks for all the info.
I will work on this during the week and let you know how it works out.

Thanks for all the help.

[Edley Rondinone]

Double-check that you have the correct firing order...are all the spark plugs firing? Any bad plug wires? Maybe some kinks in the fuel line?