High throttle miss

[blykins]

I would imagine your ignition is breaking up, or you're running out of gas (not enough flow, etc.)

[Texasdoc]

I plan on trying to GoPro the fuel pressure gauge if I can figure out how to mount it in the engine bay safely. How would I go about checking the ignition? New coil, plugs, wires. Mallory box seems to be firing fine. Timing light flashes when hooked up to each wire.

[blykins]

Does that box have a rev limiter? Is it completely cutting out or just slightly?

Can you post some pictures of some spark plugs?

[Texasdoc]

It does have a limiter - I checked it yesterday. It was set to 8500. I moved it back to 6250. I had raised it since the EFI ECU had its own limiter and I didn't want them fighting. I had forgotten to lower it when I removed the EFI. It is now at 6250. It is like a stumble. Not a complete shut down. Sounds like the engine is "burbling". It will go above the 4000, just little power and sounds like it is missing.

Here are some plugs. It is weird that when I replaced the plugs, I got them all at O'Reilly on the same day. Two boxes of four. They are all labeled as Autolite 3924s but one box had a black colored metal. The other box had a silver colored metal. ???

These are plugs 3-8 left to right. They have less than 100 miles on them. In person, they appear to be just a touch more white/gray than they look in the pictures.







Plugs 3, 4, 6



I replaced the intake gaskets after I last changed the plugs. I thought I was possibly getting some oil in the intake. I also removed the PCV system and am just running two breathers now.

[blykins]

Idle is a tad rich and one of your trunk hinge bolts is loose, but other than that your plugs look good on my phone. Little tannish/brown to the porcelain, strap looks good and I can see the heat marks on them.

How easy to take a look at fuel pressure?

[genolan]

I had a similar issue with a Holly Street Avenger 770 with Vacuum secondary and solved the problem by going to the lightest secondary spring, She was starving for gas at higher RPM's

[Texasdoc]

Thanks for noticing that. I tightened up that trunk hinge bolt - loose, just like you said.

This is a mechanical secondary carb. I have bigger jets that I can install, but will check the fuel pressure at WOT first.

I have a gauge on the fuel pressure. At idle, it is at 6.5. I'll have to figure out how to mount the GoPro to see what it is at WOT since it cannot be seen it from the cockpit.

[jhv48]

If ever a car needed to be analysed on a chassis dyno, this is it.

After all you've done, it has to be either spark disintegrating or fuel leaning out once the secondaries open up. Your primary jets are fine as evidenced by the fact that it cruises just fine and your plugs are good. Problems only seem to crop up when you go to wot. I'm ruling out a broken valve spring for now.

Before you throw more money at it, try bigger secondary jets. Go up about four sizes and see if there's any improvement. One of those existing jets also could have an obstruction in it. Easy to pull the bowl and see.

It's going to be something simple, you just haven't found it yet.

[Texasdoc]

Originally Posted by jhv48

If ever a car needed to be analysed on a chassis dyno, this is it.

After all you've done, it has to be either spark disintegrating or fuel leaning out once the secondaries open up. Problems only seem to crop up when you go to wot. I'm ruling out a broken valve spring for now.

It's going to be something simple, you just haven't found it yet.

I wish I had a dyno. The only one I know of is over in Dallas. I'm just about to give up and take it over there.

I didn't see anything in the jets when I took it apart. I'm going to check the fuel pressure with the GoPro. If that's fine, I'll disconnect the secondaries' linkage. If I can go WOT with just the primaries, it is the secondaries circuit. If it still stumbles, the primaries are involved or its the ignition failing at high rpm.

I've had the valve covers off. No obviously broken springs.

[blykins]

It actually wouldn't hurt to pull the valve covers real quick and look at the valve springs...

[blykins]

Well, its not always evident....could be an inner or damper spring broken, doesn't have to be a broken piece laying out in the open.

Not saying that's what's wrong, but it's something simple to rule out. The stumble and loss of power could be valve float from a spring giving up.

If you wanted to try something quick on the carb, you could jet up quite a bit and see if that affects it. On the dyno with these carbs, we normally just have to do a slight HSAB adjustment if anything at all.

[Texasdoc]

[ame="https://www.youtube.com/watch?v=FXqumjchbRE"]Youtube Link - fuel pressure gauge[/ame]

Here is a video of WOT. About 20 seconds in I go WOT. You can kinda hear the miss after the RPMs climb. It does ok for the first few seconds, then about 4500 rpm it has the stumble sound. It sounds worse in the cockpit. The fuel pressure bounces quite a bit but seems to stabilize around 5. You can see fuel thru the sight windows the whole time. Thoughts?

I have the MSD Mallory 110 gph fuel pump - External Link
Holley HP billet Fuel Pressure regulator - return style - External link

I think I'll try disconnecting the secondary linkage. If that doesn't work go up on the jets. I'm afraid it may not be the carb, but something mechanical as has been discussed. I haven't been able to install the WBO2 sensor yet and it may be a week before I can due to heat and work. I hate that it gets to be 90 degrees by 9 am.

[blykins]

It went from 6.5 psi down to 4-4.5 psi at WOT......

[Texasdoc]

Yes. And the fuel bowls are still full - at least the secondary bowl is. Is that enough to cause the stumble? Does it need 6.5 the whole time? I don't want to go thru and replace the entire fuel system (again) and have it still have the problem.

Is fuel pressure dropping from 6.5 to 4 enough to cause a problem if the bowl is still full?

[blykins]

Should not drop.....

But, jet it up 2 numbers all the way around and see how it handles it.

[fastd]

I have been dealing with a similar problem myself recently (subject is "WOT too Lean")

Please feel to disagree, but I am tuning my PVs only to WOT. I have 4.5s with a idle vacuum of about 11.5". If you agree, then increasing your PV to 6.5s is not the answer; but it could help.

IMO, a vacuum gauge and A/F gauge will help you more than taking it to a dyno unless you know the guys running the dyno are really interested in getting it right - it takes a lot of testing, tuning and patience to get it right on the dyno. You definitely have already demonstrated that you are willing to put in the time and money to get it right...but are they?

Given your situation, I wouldn't hesitate to jet the 2ndary main jet way up to see what happens. Really an a/f gauge can best tell you whats going on at all of the various stages; more efficient that anything else you can do. If you do jet way up, I would keep the 4.5 pv, got to WOT immediately to see if the engine feels good. Tuning for WOT is easier than tuning for both WOT and pre-WOT. If it does, then see how it feels pre-WOT and take it from there.

I have spent a lot time jetting the PVCR, the emulsion air bleeds, the IFR, the high and low air bleeds.

If it is indeed a carb issue, i.e. not ignition, etc. then there is a lot more you can do to tune specific parts of the curve than just main jets...

Curious, why did you take out the efi?

[fastd]

would be helpful to know what model # Quick Fuel you have; looks like 4150 style with sights in the bowls and no choke;

maybe this one?
Race-Q Series Carburetor 750cfm RQ-750 - Quick Fuel Technology

don't know where the model # is stamped on there but someone here should

[Gaz64]

I'd say it this one:
Street-Q Carburetor 750cfm - Quick Fuel Technology

You need to address the fuel pressure stability, I would step up the needle and seat one size, fit rear jet extensions and notched float to the secondary bowl.

Gary

[fastd]

Gaz64

That one doesn't have the clear sites on the fuel bowls.

[Gaz64]

That's interesting, maybe only one window per bowl on the other side like some Demon carbs?

The OPs has silver metering blocks, hence the carb I posted.

Hope Brent can shine some more info.

Gary