Q-jet advice

[Dtret]

I could have put this in my restoration post but I wanted to keep it separate. 71 454. All the tech folks here you go. Builder dyno’d the engine again with the “ fixed carb. Garbage. New a friend that had an edelbrock sitting on the shelf in his garage had him bring it to him. Put on engine without any adjustments at all. Edelbrock on the left junk- jet on the right. 1st photo is hp and torque. Second photo is measured vacuum. Called the carb guy and he is positive nothing is wrong with his carb. Now keep in the mind this guy is old school with all the profanities and everything. I need ammo to go in and act like I know something. HELP. Thanks Dennis  photos got loaded backwards 

[MC1of80]

Obviously something wrong there. I know people running Q-jets on super stock cars running in the 8s. 

The Q-jet should have performed way better than that. We're the secondaries opening fully? The linkage giving full throttle? The choke fully off, letting the secondaries open? Upper secondary air flap opening fully?

Pop a Holley on it and leave all other carbs in the dust. I know, your looking for a stock look. Can't tell what's under the air cleaner. 

[Brew]

Best thing I ever did was get rid of the Q jet. 

[jft69z]

Damn, that's not even in the same universe. Tom covered all the obvious things to check, plus a viable solution. Something's very wrong with that carb setup/adjustment.

[1Bad454]

Yeah there is definitely something wrong with that carb. If the factory got 365hp this should at least be in that range.. 

[GatorDog72]

The secondaries are likely not opening at all. Its hard to find a good running q-jet, many of the rebuild kits are super cheap quality. If you don't care about keeping the car 100% original, just chuck the q-jet and get a holley. They're much less problematic. 

[Gush's monte]

Dennis, I had a similar problem with the LS5 rebuild on my 1970 Chevelle. When we ran it on the Dyno with the original q-jet the secondaries would not open. The engine builder believed the problem was due to the high vacuum the engine was making not opening the secondaries. They drilled two holes in the throttle plates which allowed it to open at throttle on the Dyno and made good numbers 410 hp and 530tq. My build was stock except a baby hr cam 211 duration cut at a 112 Lisa and compression ratio of 9.8. Tuning the same carburetor to run well on the street has been a challenge. It would ping under load at high rpm, like it was running lean. Changing jets did not help. Also the transition to open the secondaries never felt right (always seemed to open to late). Any adjustment to the secondary spring showed no change. I have since got another q-jet on line and it ran great right out of the box. No pinging with 78 jets. In my research I think some of the passages in the q-jet core still have blockage. When I rebuilt the carb I let it sit in solvent overnight an used small brushes to clean out any passages but there are some in the core of the carburetor that are blind which you can’t get a brush into.talking to my engine builder he thinks a sonic cleaning my get the blind passages cleaned out. May try it over the winter.

[Gush's monte]

Also, on my Monte I have a bigger cam but run a Holley 750. I find it much easier to tune, easier to notice the change in each adjustment. Either carburetor will work but takes time to get the adjustment just right. When the carbs are set up on the Dyno they are set just to make the best power, not real world driving or throttle response you my want.

[JG72]

I ran into the same issue trying to keep my (rebuilt) QJ on the car. Put it on the Dyno and it wont produce the power from the rebuilt engine ( bigger cam etc.) They slapped a Holley750 fuel feed they had in the shop and BAM! Horsepower went way up and motor loved it. Still can't figure out why. They ripped the QJ apart several times to try and make it work by drilling out secondary flaps etc. but to no avail.