Just got back from a ride in my '72 U code Supreme and while I was out, I hit the gas from a dead stop. She lit em up fine, but then around 30 mph while still on the carpet, she started backfiring bad. Could it be because I opened the spark plug gap .005"? She's fine if I'm up around 60 and hit it..pulls all the way with no issues. Just when I'm on it from a dead stop. I took it out last week when it was cooler out and it was fine, so maybe it's the 95 deg heat today? Or maybe bad gas? I filled up at the same station I always use with 93 premium. Checked the plugs and they're fine. Thoughts?
Setup:
Holley 770 Street Avenger
Edelbrock Performer intake

 

Too lean of fuel mix usually causes this. Check your accelerator pump.

 

It just started this today and the carb is only two yrs old, I drive it at least once a week-10 days.

 

It is NOT being caused by a change of 0.005 in plug gap.

 

If the car is fine off the line but breaks up when you reach about 30 MPH, the problem is likely running out of fuel in the float bowl. The right way to diagnose this is to tee a fuel pressure gauge into the line at the carb inlet and look at what the fuel pressure is doing when the car starts breaking up. This sounds like a fuel starvation issue.

What carb, what fuel pump?

 

If there is a lot of wheel spin off the line and through first gear, the fuel pump may not be able to keep up and the carb float bowl is running dry. My 1970 Supreme had this issue, but only when there was a LOT of wheel spin off the line, then at near redline in 1st gear it would have issues. It all depended upon how much wheelspin there was off the line.

 

He stated his carb is a Holley 770 Street Avenger. I owned both a 1967 442 and a 1972 442 (both in the 1970s) and I never had an issue with fuel delivery during burnouts for any extended period of time/length. Of course, I ran only Rochester Q-Jets on both of those cars - they'll dump as much fuel as you give them. Holleys, well welcome to Holleys, IMO. I agree it sounds like fuel delivery (hesitation) - somewhere. Most likely the carburetor. It appeared to me, the majority of my friends who ran Holley's spent more time working on Holley's than they did driving their cars. My statements should not stop you from enjoying your Holley, Dave.

 

Well, either it was bad gas or partial vapor lock. After letting it sit for a couple hours, I poured some lucas in the tank..drove it for about 10 minutes. Hit my favorite stretch of road and hammered it from a dead stop...she walked sideways and pulled all the way to 100 before I backed off. Tried it a second time just for good measure and same thing..no issues. Hammered it from 60 and she pulled all the way to 110 before I whoaed it down and she was still asking for more. I think I'm 👍

 

I do not know why some people think Holley carbs are so great for everyday drivers. A well set up Q-jet will out perform a Holley all day long and get better mileage.

 

As was put to me when I was younger, Holleys do great at idle, and WFO.

 

I like the simple Holley 3310. They work extremely well and have a manual choke.

 

Holley carbs are for people too lazy or too ignorant to work with a Q-Jet

The only way a Holley will make more power over a properly tuned Q-Jet is if the engine requires more airflow than a Q-Jet can provide. And considering a Q-Jet is capable of at least 750cfm, that’s a pretty stout street engine.

 

Based on what info?

 

Not quite. There's no question that Holley carbs will make more top-end power. They are a bare-bones carb designed for wide open throttle performance. Minimal obstruction in the venturies and fuel flow paths, simple metering systems, easy to adjust. That's exactly what you want at the drag strip. It is NOT what you want on the street where you care about idle quality, part throttle performance, and accelerator tip in from a wide variety of starting conditions. The Qjet is arguably the most sophisticated 4bbl ever developed. It has numerous transition circuits and provisions to cover the wide variety of operating conditions that you experience on the street. This complexity is why they require more skill and patience to tune properly.

The reality is that 99% of the people have no clue as to what is the best setup for how they use their car. They read magazines, watch You Tube, and copy that with no understanding that the setup that produces peak power at W.O.T. on the dyno on Engine Masters is likely not the combo that is the best for their street-driven car with mismatched gearing and crappy shortie headers. Couple that with the fact that most people have no clue about carb jetting or advance curve tuning, and all they care about is that a shiny brand new carb out of the box runs better than the crusty, leaking OEM carb that they took off. It gets them to cruise night and back, which is all they care about.

Think about it. How many people think the optimum trans is a Powerglide because that's what all the drag racers use?

 

I've been watching the responses to this thread, and haven't replied because it got way off topic, but since the can of worms was opened, I'll throw out my .02.

I won't debate the fact that the Q Jet is a better carb, however, my Holley works well for me. Other that the hiccup for which I started this thread, it behaves very well. Very responsive, no issues with driveability whether I'm in slow traffic or cruising at 70 on the highway, and idles very well. Maybe it's my application, or maybe it's my tuning...I dunno, but, again, I have no complaints with mine.

 

Based on the number of people who think a universal” fit Holley will be ideal for any engine. Look at the different part numbers for the same engine in different cars with different options, how can any “universal” carb be correct for every engine? And before anyone says you need to tune it for the application, how is that any different than than setting up a Q-Jet for the engine it’s bolted on?!

The very things that make a Q-Jet “too complicated” are the features that make them great. I’m not a Q-Jet genius, but I know enough about them to make them work. Like so many other things with our cars, the engineers who designed them knew far more than we do, worked far longer, with more resources and testing than any of us could ever hope. Instead of trying to out-engineer the people who designed and built the cars, tweak what is already there to suit your needs.


I have had multiple Holley carbs on my car over the years. I have had a 750 vac secondary (3310) a 750, 850, and 950hp. None drove anywhere near as well as the Q-jet, none ran any quicker at the track, and none got anywhere near the fuel economy of the Q-Jet. If the car doesn’t drive as well, does t run as fast, and costs more to enjoy it, why bother??

I’m convinced most people who bitch about Q-Jets think that $25 rebuild kit and a couple cans of carb cleaner from Advance Auto is all that needed. They completely ignore the sloppy throttle bushings,
carboned up air bleeds, and mismatched parts and label the carb as “junk”.

A properly calibrated rebuild kit, with the parts to repair 50 years of wear and tear is cheaper than buying a Holley that will require lots of tuning to dial in.

I don’t hate Holleys. If I was building a 1000hp engine, or a engine with forced induction, something that needed more airflow than a Q-Jet could provide, I’d use a Holley. I seriously doubt you can fix a problem, or improve 90% of street engines by bolting on a aftermarket carb (of any make) instead of fixing a Q-Jet.

I have broad shoulders, I’m ready for the comments.

 

The Powerglide WAS the ultimate trans, until the 2 speed turbo 400 was developed, or aftermarket gearsets with different ratios for the 400 was designed 😁

I couldn’t agree with you more, the Holley is a great primitive bare bones carb. I question if Holley will make more top end power, all things being equal. If the engine requires a specific amount of airflow, and both carbs are capable of meeting those needs, and the tune is correct, does the engine really care what carb is bolted to it?

 

This is the best response to the “which is the better carb?” debate!!




Nothing beats the wail of a Q-Jet at WOT!!!

 

I won’t say what I believe others ‘think’ when they’re working on carburetors, but I find working on a Q-Jet to be quite simple - especially when there is and has always been a plethora of excellent documented information for these carbs. Remove throttle linkage & a half dozen bolts it’s in your hands & on the bench. The idle passages are excellently engineered. You can upgrade needles/jets, seats, & rebuild almost every detailed piece of the carb. Choke adjustments are clearly established in literature as well as fast idle cam adjustments. I don’t believe it’s mysterious or complicated I believe it’s a matter of following prescribed instructions. Not so bad really.

 

x2

 

I had three cars with Q jets, I rebuilt each one of them including a CCC Qjet in my 88 Caprice (it needed to come apart for a TPS replacement anyway). I found them all to be rock solid dependable (except for the TPS). The choke mechanisms always worked and they always started easily no matter the air temp. I seldom had to adjust the mixture screws.

 

The same can be said for most carbs. Holley and their variants probably have the most options available for tuning. Most carbs are not plug and play (while they may run the engine right out of the box) and can benefit from tuning to what the engine needs.