High throttle miss
 

Chasing a high RPM/high throttle miss. Looking for thoughts - possible things I am missing. It idles fine at 13inHg. I can accelerate at normal speeds just fine. Cruise on the highway is fine too. However, if I start to get into it, it initially feels down on power. If I continue to apply more throttle, it starts to break up, missing pretty badly. I haven't gone WOT yet since I don't want to damage anything in case it is detonation instead of just a miss. Doesn't *seem* like an ignition problem but lots of forums say 95% of misses are ignition related, not carb related.

408W, AFR195 Renegade heads
Performer RPM Intake - 3/4 inch OPEN wooden spacer under carb.
New QuickFuel 750 carb (not sure exactly which one)
New MSD street fire wires
New Autolite 3924 plugs
Cam 230/236 - 112 lobe separation
MSD distributor - verified rotor phasing
Mallory Hi Fire 6AL

Recently reinstalled the intake to confirm no intake gasket leak. All cylinders appear to be firing at idle - headers are all hot.
Verified timing at 36 degrees, all in at 3000. Initial advance at 18. Black MSD bushing, one light silver one light blue spring. Timing appears stable while holding the throttle at 3000 rpm - no timing bounce.
No vacuum advance currently (make sure not over advancing)
No PCV currently - removed and the miss is a little better but not gone.
Verified TDC with piston stop.

Took the carb apart to see what was inside. It is one of their race carbs - no choke. 4.5 Power Valve. Main jets 74, secondary jets 82, secondary plate with PV plug.

Seems to me that the power valve is a little low. MSD says to use half the idle vacuum but I've read that this is not the case - just a possible starting place. What I think is happening is as I get deeper into the throttle, and move out of the transition circuit it goes lean (since the PV hasn't opened yet) causing a lean miss. Removing the PCV system helped since it is making it a touch richer (less unmetered air). Since I can cruise on the highway at 2500 RPMs without problems, seems the jets are fine. Idle circuit is fine. Just goes lean on heavy acceleration.

I have ordered a new PV 6.5 but haven't gotten to install it yet. Any other carb gurus have thoughts on what may be going on?

You need to verify what manifold vacuum this is occurring at with a vacuum gauge in the driver's view.
I would also have a fuel pressure gauge, or a fuel pressure warning switch to operate a lamp.

Was there anything abnormal like fuel hose debris inside the carb?

Have you checked the cap and rotor? MSD is known for failure is this area.

Gary

I'm planning on getting a longer hose to run the vacuum gauge into the driver's cab.

I have a fuel pressure gauge in the engine compartment by the carb. It is stable at 6 pounds, but can't see it when driving. Maybe I can rig up the GoPro. Pump is a Mallory Comp Red 110 gph unit with a Holley return style billet pressure regulator near the carb. It should have enough umph to keep the carb fed.

Nope, didn't find any foreign material inside the carb.

New MSD cap, New MSD adjustable rotor - phased so the rotor is in the center of the cap electrode when the plug fires at idle. The center electrode on the rotor touches the carbon button on the top of the cap. No corrosion anywhere since they are all new. All 8 plug wires get spark with the timing light test.

Thanks for the thoughts. I will get the vacuum gauge where I or a passenger can see it and also try to check the fuel pressure.

I would phase the rotor to fire closer to the leading edge.

Your fuel system sounds capable enough, what fuel filter are you using?

How about fitting a wideband O2 gauge to monitor A/F ratio?

I can adjust the phasing more, no problem.

I have a Holley Billet 100 micron pre-filter and 40 micron post-filter by the tank. This feeds up to the Holley billet 12-841 bypass regulator. One output on the regulator goes to the carb, the other output goes to the return line to the tank.

I actually have an WBO2 sensor and gauge on its way from Amazon to install. I have the bung already in the pipe from when I had the EFI system.

Oh, I also don't think it is the coil giving out at high RPM. New MSD 5527.

I have two 408's with 750's. I would go up two numbers on your main jets. Drop the timing from 36 to 32 or 34 at the most. Generally.

Both primaries and secondaries, or just the primaries? What jets do you have in the primaries and secondaries?

Both. 76 but I run PV's on both ends.

Sounds like you have a little too much total timing. Try 16° initial; that will give you 34° total, which is probably about right for a 408.

This doesn't seem to be just an adjustment, as has been suggested by others.

How long has the Quick Fuel carb been on there?

Have you measured the plug lead resistances, including the coil?

Have the spark plugs been out for inspection/comparison?

Gary

New carb, coil, plugs, wires. All less than 100 miles. Had so much trouble with EFI - so I pulled it and replaced with new new carb & other hardware. Now trying to get carb working.

Ok, let's get your vacuum gauge and wideband O2 set up, then something might stand out.

Did you buy the Quick Fuel as a custom carb or a "generic" tune?

It's one of my custom Q-750's.

Try the 6.5 PV before you do anything else. Your logic up above is correct. If it fixes it, I'll reimburse you for the PV you bought.

Thanks, but I'm not worried about the cost of a PV. After all the money I put into the EFI, two EFI CPUs, 3 WBO2 sensors, EFI distributor, Ford module, high pressure fuel pump, etc - then the cost to rip it all out and go back to carb... the cost of a PV is nothing. I just want to get it working. I fought the EFI for almost two years, constantly at the computer working on the "tune". This carb already idles and cruises better than the EFI did.

Talking to Scott, he didn't remember my carb or what he put in it. He said he usually puts 6.5 PVs in manual shift cars and 4.5 PVs in automatics. This has the 4.5.

I'll change the PV first, set the timing at 34, hook up the vacuum gauge, and see what happens, then report back. It will be Wednesday before I can get back to it. Stuck in Chicago with a four hour plane delay.

Don't worry about messing with the timing. You don't want to change too many things at one time. I'd change the PV and take it for a drive.

The comment up above about 34° being right for a 408 is not necessarily true. It's not dependent on cubic inches at all, but total timing is mainly a function of the cylinder head and the compression ratio. One 408 with an Edelbrock head may want more timing than a 408 with a Trick Flow head, just because the combustion chambers are not made the same. It's ok to change timing later on to see if you can tell a different in your buttmeter, but don't change everything at once.

I'm confident that the PV change will make her run like a mashed cat. After that change, you can read the spark plugs to fine tune it.

How long does it take for plugs to show a change? Do I need to key it idle for so many minutes after reaching operating temp? If I drive it for x miles or y minutes, will that be enough?

I know that going WOT then turning off the motor immediately and coasting to a stop is the "right" way to read plugs. This really won't work for me.

If the plugs have done 100 miles already, they would show up an average of all conditions.

Lot's of idling would make the check almost null and void though.

Could be worth removal of all for inspection.

But as Brent has said, it sounds like a slightly late PV opening.

No carburetor will be exactly correct, most can be 90% straight up, and then some further tuning is necessary.

Ok, changed out the PV. Better, but still down on power and missing at >4000 rpm. Next is to install the WBO2 sensor to see where we are. I am going to re-do the exhaust header gasket and collector gasket before installing to make sure I don't have an exhaust leak that will make the sensor readings invalid.

One thing I noticed is that it is hard for the idle to come back down. I have it idling at 1000rpm. When I let off the throttle, it comes down to about 1400 then *slowly* drops back down to 1000 rpm (eventually). Pulling up on the gas pedal with my foot doesn't help. Checked the throttle cable and no sticking.

I noticed when checking my timing last that even though I have the black bushing in the MSD distributor (18 degrees), I am only getting 15 degrees advance. Don't know if this matters. It is uniform/consistent, revs up to max advance easily and stays there. I have the two light blue springs so it is supposed to start advancing at around 1300 and be all-in by 2500. I checked the cap for corrosion - nothing. Removed the springs and weights, everything looked fine. Sprayed a light coat of WD40 to help lubricate. No vacuum advance. No air leaks that I can find. Now I gotta research slow return to idle...

So you're cruising and you get it rolling, then go to WOT.....does it initially do fine and then all of a sudden starts missing when you get over the 4000 rpm mark?

Have you checked for a vacuum leak?

Yes. I start in first, quickly shift to second to load the engine, then mash it. No bog initially. Pulls hard until at WOT until about 4000, then starts to miss. Yes, checked for vacuum leak.