1 (edited by Fishah 2018-04-23 10:28 AM)

Topic: Carb'd 350 runs like shit! Only on certain corners.....?

Hey guys!

   Calling out to the smartest gearheads I know, because our engine is making me unhappy. Sometimes. I know as much about old school engines as I do about women; nothing.

     Backstory:

Car is a 928 with a Gen 1 Small block. Holley Carb. 300ish-hp. Mechanical fuel pump. 9PSI at idle. 4-6PSI at top speed (per dyno run).

Last week, we were fighting an issue where the car would fall on it's face above 4000RPM. Dyno said off-the-charts lean. We swapped out for our spare carb. Problem solved...

     Lead up:

Took the car to HPDE here in Arizona yesterday. It was 95degrees out.

Typically at a Lemons race the car runs at 190 degrees. The track on Sunday was short, so no time to cool the engine. Also it was fking hot.... Engine started out around 210. As the day went on, and the car hot hotter our new issue got worse. Leveled off around 220. Never boiled over.

     Issue

On the slowest corner of the track, the RPM would get down to 2500 in 3rd gear. It was a quick switchback (left/right). As soon as I whipped the car back to the right, and floored it, the engine did almost nothing. It made a hell of a racket, but had ZERO power. It wasn't as if it was surging or falling over on it's face, it would just rev at the speed of molasses and the car went nowhere. Eventually as the revs climbed, the issue went away. Normal engine sound resumed, as did power.

I started using second gear on that corner, and the problem was solved. For a little while. Eventually it started happening in second gear also. This was as the car got hotter.

Eventually it started happening in a faster, third gear switchback earlier in the track also. 3000-3200ish RPM.

Note: There are two other places on the track where I was third gear, 3000RPM, that it did not have this issue. The difference? Those were constant corners, and not switchbacks. The issue only seemed to occur on the second part of a switchback corner.

     Questions:

Could this be a lack of fuel? Float levels not properly adjusted?
Could this be detonation? 87 octane in a hot engine?
Has anyone experienced this, or something similar? I'd love to hear ideas.

We have a LuckyDog race at Willow Springs in two weeks. The temps will be cooler.

Thanks everyone for reading!

#33 Ford Festiva - We Are Not Really From Iran (Retired)
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2 (edited by Guildenstern 2018-04-23 11:18 AM)

Re: Carb'd 350 runs like shit! Only on certain corners.....?

Could be float level too low, or you just may have found the limit of your fuel tank pickup.

You will get ping and detonation as the air/fuel mixture leans out due to running out of gas in the carb. If you have a mechanical fuel pump that can then create a low fuel pressure feedback loop where even if you can start pulling fuel from the tank again, you’re not able to pump it to the carb fast enough at first and that exacerbates the problem.

Also concider that your pump may be on its way out and can’t feed enough above 4000 sustained anymore.

Check also all fuel filters.

You’re starving gas somewhere along the line.

What you described is basically how my bike acts when I forget to turn the fuel selector on and it  starts to suck the lines dry.

Mistake By The Lake Racing (MBTL)
88 Thunderbird "THUNDERBIRDS ARE GO!", Ex Astris, Rubigo / Semper Fracti
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Re: Carb'd 350 runs like shit! Only on certain corners.....?

Since it only does it when hot, then might be vapor lock.  Make sure your fuel lines aren't running near your exhaust.

Put a fuel pressure gauge on it. Mount it outside on the hood or cowl so that you don't have fuel inside the cockpit.  Will tell you definitively if it is fuel related.

Re: Carb'd 350 runs like shit! Only on certain corners.....?

Also consider adding an electric fuel pump in the back of the car near the tank. This will keep pressurized fuel in the line feeding the mechanical pump making it work less hard. Use a larger metal line  between the two pumps and insulate it.

Re: Carb'd 350 runs like shit! Only on certain corners.....?

Since it ONLY does that in certain corners I would bet its the floats.  Talk to truck and off road guys, Holley bounce is a thing! 

As others have said it would be worth your while to put a fuel pressure gauge on it, and make sure to isolate the lines from heat.

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Re: Carb'd 350 runs like shit! Only on certain corners.....?

Does the carb vented it could be dumping fuel out of vents into motor. A quick left right could splash fuel out vents.

The fifth Element Porsche 924

Re: Carb'd 350 runs like shit! Only on certain corners.....?

Guys, thank you so much for all of your valuable opinions. I really appreciate the input.

I will go through everyone's suggestions and report back my findings.

Floats was my initial thought for the same reason  as Matt, but at this point it could be anything.

If it was a simple as a FP or clogged line, this issue shouldn't only happen on specific corners. It would be more consistent? And why worse when hotter?

I have a fp gauge under the hood, but adding one in driver view does sound nice to have.

---

Anyone think it's likely an event like this could cause long term engine damage?

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Re: Carb'd 350 runs like shit! Only on certain corners.....?

If it's partially clogged the hard turns when hot may be just enough strain to starve the carb.

And the hotter the car gets, the less lean your mixture has to be to start pinging. You may be right on the edge of some envelope corner that the turn just pushes you over the edge.

But simplest stuff first, you know it's a lateral G exacerbated issue so start with that stuff first. Tank and Bowl and work your way to the middle.

Mistake By The Lake Racing (MBTL)
88 Thunderbird "THUNDERBIRDS ARE GO!", Ex Astris, Rubigo / Semper Fracti
A&D: 2014 Sebrings at Sebring (NSF), 2014 NJMP2 Jurassic Park (SpeedyCop), 2012 Summit Point J30 (PiNuts)
2018 Route Sucky-Suck Rally Miata, 2019 World Tour Of Texas 64 Newport

Re: Carb'd 350 runs like shit! Only on certain corners.....?

What Holley carb? Some of them have side-hung floats.

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Re: Carb'd 350 runs like shit! Only on certain corners.....?

Is the stumble preceded by hard braking? You could be spilling fuel out of the rear bowl down into the main body. Does the rear bowl have a "vent whistle?"

Re: Carb'd 350 runs like shit! Only on certain corners.....?

What holley? and what size? What is the engine setup? 9psi is a tad high at idle. I'd think 7 would be best, under full load you'd expect it to drop to maybe 5. I'd probably use at least 89. We are running a 88 9.5:1 305 and use 93. " It made a hell of a racket" I'm guessing that would be detonation. I wouldn't want to do that too many laps. the spacer helps direct the air into the intake, but if you were lean you need to address your jetting, if you have a vaccum carb you might need to adjust the spring or the PV or both.

"get up and get your grandma outta here"

12 (edited by Fishah 2018-04-24 08:09 PM)

Re: Carb'd 350 runs like shit! Only on certain corners.....?

It's a Holley 600 I believe. Not sure if there is a more specific model number?  Engine was built by PO so I don't have specifics on the motor.

There is braking, but not hard braking. The braking would be about 3-5 seconds before this incident occurred.

The car was at the Dyno a week ago. A/F ratio was perfect throughout the RPM range. It has to be something related to the type.or corner and/or temps.
--

So we dug into it a bit today. No clear place for the exhaust to be heating the fuel lines. Maybe the header in one spot. I'll wrap the line in heat tape.

We checked the floats. The front bowl was overfull. The rear just right. We adjusted.

I also ordered a new fuel pump that has a built in pressure regulator so we'll be right in the range. I agree that our current punp was being erratic.

Also found some air in the water system so I fixed that.

Won't know more for two weeks until the race, but I think I've checked all of the obvious boxes. Will run a higher octane fuel also. Fingers crossed...

Thanks everyone!! I will report back.

#33 Ford Festiva - We Are Not Really From Iran (Retired)
#928 Porsche 928 - West German Pushrodders (Retired)
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Re: Carb'd 350 runs like shit! Only on certain corners.....?

You might want to grab a re-build kit and go thru it. If it has some dirt in it, that could cause issues. Good luck.

"get up and get your grandma outta here"

Re: Carb'd 350 runs like shit! Only on certain corners.....?

Does it look like this?This is the most common 600 CFM 4 BBL.
http://static.shop033.com/resources/E8/2024/picture/8A/15807370.jpg

This is a side hung float card.
If the front float was too high you could easily get over fueling/fuel spillage
though some corners.

In all candor, as much as I hate them, a Q-Jet Rochester is a better track carb for the 350.
While the Holley is sized correctly, it has large primaries, where as the Q-Jet has smaller
primaries leading to better torque delivery coming out of a corner.
The Q-Jet can deliver enough fuel/air to feed a 428 Super Cobra Jet Ford, and still be very driveable.
Additionally, the Q-Jet is much less sensitive to G-Force loading due to the float bowl design.

Capt. Delinquent Racing
RUST-TITE XR4Ti - '21 ARSE-FREEZE-APALOOZA  I Got Screwed
The One & Only Taurus V8 SHO #31(now moved on to another OG Delinquent)
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Re: Carb'd 350 runs like shit! Only on certain corners.....?

I might think about a q-jet for this app also. My experience is limited to what they do on Cad 472 motors, but in that (incredibly thirsty) app they work fantastic, I was recommended to stick with one by a Cad guru of decades experience. They're fiddlly when worn out but completely rebuilt ones are cheap and readily available on the shelves at O'Reilly.

The secondary benefit (see what I did there) is the awesome roar of the huge secondaries on the Q-jet smile

I really think the corner being a variable is pointing at a float issue, if it delivers all the power you want on a dyno, but falls on its face under lateral G, "sloshing" is the word that comes to mind...

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Re: Carb'd 350 runs like shit! Only on certain corners.....?

DelinquentRacer wrote:

Does it look like this?This is the most common 600 CFM 4 BBL.
http://static.shop033.com/resources/E8/2024/picture/8A/15807370.jpg

This is a side hung float card.
If the front float was too high you could easily get over fueling/fuel spillage
though some corners.

In all candor, as much as I hate them, a Q-Jet Rochester is a better track carb for the 350.
While the Holley is sized correctly, it has large primaries, where as the Q-Jet has smaller
primaries leading to better torque delivery coming out of a corner.
The Q-Jet can deliver enough fuel/air to feed a 428 Super Cobra Jet Ford, and still be very driveable.
Additionally, the Q-Jet is much less sensitive to G-Force loading due to the float bowl design.

We are using the factory quadrobog on our 305, it has been slightly modified for our setup but works very well. Plus it gives us 50 points for other things.

"get up and get your grandma outta here"

Re: Carb'd 350 runs like shit! Only on certain corners.....?

DelinquentRacer wrote:

Does it look like this?This is the most common 600 CFM 4 BBL.
http://static.shop033.com/resources/E8/2024/picture/8A/15807370.jpg

This is a side hung float card.
If the front float was too high you could easily get over fueling/fuel spillage
though some corners.

In all candor, as much as I hate them, a Q-Jet Rochester is a better track carb for the 350.
While the Holley is sized correctly, it has large primaries, where as the Q-Jet has smaller
primaries leading to better torque delivery coming out of a corner.
The Q-Jet can deliver enough fuel/air to feed a 428 Super Cobra Jet Ford, and still be very driveable.
Additionally, the Q-Jet is much less sensitive to G-Force loading due to the float bowl design.

Yep, that's the one.

Our current Holley is brand new; less than a week old. Hopefully shouldn't require a rebuild.

That Rochester definitely does look more suited to what I'd be doing. A new carb isn't in the budget for this next race, but I will keep that in mind as a solution if this continues. Fortunately Willow Springs doesn't have a lot in the way of quick switchbacks.

#33 Ford Festiva - We Are Not Really From Iran (Retired)
#928 Porsche 928 - West German Pushrodders (Retired)
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18 (edited by ross2004 2018-04-25 12:52 PM)

Re: Carb'd 350 runs like shit! Only on certain corners.....?

This was the best thing I ever did to my carb'd engine, hands down. Although if I were to do it again, I'd get the Holley Sniper instead.
https://image.ibb.co/isPy3c/52130002_L_80df0c21_5e3a_4392_98fb_e4c68d33c916.jpg

Re: Carb'd 350 runs like shit! Only on certain corners.....?

DelinquentRacer wrote:

Does it look like this?This is the most common 600 CFM 4 BBL.
http://static.shop033.com/resources/E8/2024/picture/8A/15807370.jpg

This is a side hung float card.
If the front float was too high you could easily get over fueling/fuel spillage
though some corners.

In all candor, as much as I hate them, a Q-Jet Rochester is a better track carb for the 350.
While the Holley is sized correctly, it has large primaries, where as the Q-Jet has smaller
primaries leading to better torque delivery coming out of a corner.
The Q-Jet can deliver enough fuel/air to feed a 428 Super Cobra Jet Ford, and still be very driveable.
Additionally, the Q-Jet is much less sensitive to G-Force loading due to the float bowl design.

Why does everyone hate the Quadra-Jet, yet always have like 5 things they do better than other carbs?

Are they just diffrent and therefore BAD, or is there something I missed while working on my friends because I just followed instructions?

Mistake By The Lake Racing (MBTL)
88 Thunderbird "THUNDERBIRDS ARE GO!", Ex Astris, Rubigo / Semper Fracti
A&D: 2014 Sebrings at Sebring (NSF), 2014 NJMP2 Jurassic Park (SpeedyCop), 2012 Summit Point J30 (PiNuts)
2018 Route Sucky-Suck Rally Miata, 2019 World Tour Of Texas 64 Newport

Re: Carb'd 350 runs like shit! Only on certain corners.....?

Back in the day when I played with Holley carbs I converted my 4bbl to center hung floats right after I purchased it as I knew of the problem with the side hung floats under hard cornering. This was on a 390 cfm Holley on a Ford 2.8L V6.

Re: Carb'd 350 runs like shit! Only on certain corners.....?

jimbbski wrote:

Back in the day when I played with Holley carbs I converted my 4bbl to center hung floats right after I purchased it as I knew of the problem with the side hung floats under hard cornering. This was on a 390 cfm Holley on a Ford 2.8L V6.

Come.visit AZ sometime Jim! You can stay in my guest room and modify my carbeaurator smile

#33 Ford Festiva - We Are Not Really From Iran (Retired)
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Re: Carb'd 350 runs like shit! Only on certain corners.....?

We run the same Holley on a 302.  Checking the float level is easy. Remove the two large flathead plugs, one on the side of each bowl. With the motor running, fuel should be just to the bottom of the hole. Loosen the flathead screw and turn the nut underneath the screw on top of the bowl to raise or lower the float. Next, verify that no one has modified the linkage to the secondary to be mechanically opened. It is a common redneck thing to put a screw or something in it to force the secondary open at WOT. This does not work because the secondary does not have an accelerator pump. Secondary on that model is vacuum operated only and will never open without a load. Third, I recommend an electric fuel pump, located below the tank, with a pressure regulator set to 4.5psi. The mechanical pump can cavitate and cause vapor lock.
We had all these problems, with similar symptoms to what you describe. The engine and fuel system are now extremely reliable.

Team Fairlylame
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Re: Carb'd 350 runs like shit! Only on certain corners.....?

ross2004 wrote:

This was the best thing I ever did to my carb'd engine, hands down. Although if I were to do it again, I'd get the Holley Sniper instead.
https://image.ibb.co/isPy3c/52130002_L_80df0c21_5e3a_4392_98fb_e4c68d33c916.jpg


If you want to do something like this on the cheap, A junkyard TBI system can be put together for next to nothing. Either use the stock ECU from the donor and get a eprom burner or run it on speeduino.

Re: Carb'd 350 runs like shit! Only on certain corners.....?

Fishah wrote:
jimbbski wrote:

Back in the day when I played with Holley carbs I converted my 4bbl to center hung floats right after I purchased it as I knew of the problem with the side hung floats under hard cornering. This was on a 390 cfm Holley on a Ford 2.8L V6.

Come.visit AZ sometime Jim! You can stay in my guest room and modify my carbeaurator smile


Don't temp me or I may just do that! 
Now that I have my own H&N restraint (NecksGen) and a Lemons License I will be looking for an arrive and drive or two this year.

Currently working with the Bad Decisions team and their Dustbuster and '48 Plymouth.

Re: Carb'd 350 runs like shit! Only on certain corners.....?

jimbbski wrote:
Fishah wrote:
jimbbski wrote:

Back in the day when I played with Holley carbs I converted my 4bbl to center hung floats right after I purchased it as I knew of the problem with the side hung floats under hard cornering. This was on a 390 cfm Holley on a Ford 2.8L V6.

Come.visit AZ sometime Jim! You can stay in my guest room and modify my carbeaurator smile


Don't temp me or I may just do that! 
Now that I have my own H&N restraint (NecksGen) and a Lemons License I will be looking for an arrive and drive or two this year.

Currently working with the Bad Decisions team and their Dustbuster and '48 Plymouth.

Good to know! Our only remaining race in 2018 is the 24hr @ MSR. I believe I have a full roster of local guys for that race, but we'll see. Would be happy to have you as a pilot, so I will definitely reach out if the opportunity arises.

#33 Ford Festiva - We Are Not Really From Iran (Retired)
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