GrandSportC3

Since I swapped out the regular exhaust for the side pipes with maxflow mufflers, I'm getting a slight hesitation when accelerating from lower RPM's. It almost seems like the engine is getting too much fuel. If I accelerate harder with the secondaries kicking in, there is no hesitation, just at low RPM. I'm a little confused as I would almost expect the opposite. I went from a fairly restrictive system to a free flowing system. I'm thinking in changing the primary jets to a smaller size or am I missing something? Did any of you who went from restrictive to free flowing exhaust have simar issues and what was the solution? BTW, my carb is a heavily modified Holley 750..
Details on carb in this thread..

https://www.corvetteforum.com/forums...lley-carb.html

REELAV8R

You are slightly lean on the primaries I believe. You'll soon know if you go leaner and it gets worse.

GrandSportC3

I don't think that I'm running lean but I could be wrong.. It's almost like the engine is receiving too much fuel but maybe it's too little?? I'll try go with bigger jets first and if this won't work, I'll try smaller ones. Too bad that I don't have a oxygen sensor and a gauge for a/f tuning.. I used to have a wideband on my race car with a exact read out of the A/F ratio at any time.

Big2Bird

When you reduce back pressure, you increase air flow. Increased air flow requires richer jets. Chugging/surging/hesitation is caused by a lean condition.

JimLentz

GrandSportC3

Thanks. Will try bigger jets and wilm see what will happen..

jim2527

If you had a choke I'd say close it a tick...if it gets better your running lean. But you don't have a choke so try taping/blocking off part of the air cleaner.

GrandSportC3

A carb with choke tower would kill my air flow as I'm running the drop base and thin filter.. hood clearance with the Team G intake is a problem..

Kubs

Did you figure out what the issue was? How much higher did you go in jet size?

I know this is not apples to apples but I will be changing my exhaust soon due to a muffler rotting out on my '71. They appear to just be factory replacement mufflers the previous owner put on, but I want to change them to a cheap Summit street/strip chambered muffler for a little more sound (previously couldn't hear the car over the cooling fan it was that quiet). I am not spending money on a whole new exhaust now since I want sidepipes eventually. Engine is the stock 350270hp with the original Q-Jet, which I just rebuilt, and stock 2" pipe. Do you think I will have to re-jet the carb as well, or will the Q-jet be ok since they are kind of a variable CFM carb, to a point?

GrandSportC3

Haven't had a chance to play with the jets yet.. I've been way too busy.. Hopefully I will get to play with the jetting this week.. Drove the Vette to work today.. PERFECT WEATHER!! I think that your setup should not require any re-jetting.. I don't even know how re-jetting on a q-jet works.. Never worked on those.. I'm a Holley guy..

7t9l82

Buy a vacuum Guage they are cheap and easy to use. Adjust your mixture screws to the highest vacuum at idle. Try that before changing your jets.

jim2527

Tape off your air cleaner with duct tape a little at time and keep adding. If it gets better your running lean.

GrandSportC3

What does the idle mix have to do with accelerating when on part throttle? Does the idle mix also affect the mixture when not at idle?

Kubs

Yeah this is my first QJet too. I have only worked on 1 other which was a Holley. I am an EFI guy...

I did find that the '71 carb has number 74 jets in it stock which seem on the bigger side to me, so I think you're right. I'll probably be OK.

7t9l82

It may be too lean at idle to transition seemlessly. You may need a step up on your squirter. When I went to an air gap manifold I had to go up 2 sizes on the primary squirter and one on the secondaries.people describe things differently so sometimes its tough to diagnose .

GrandSportC3

The easiest change is to just try different size primary jets and if that won't work, I'll check for other potential sources of the issue..

7t9l82

Pretty easy to change a squirter.

GrandSportC3

never did that.. Would have to look up how to change it..

GrandSportC3

UPDATE... Finally, after 3 months, I got a chance to take off the fuel bowl on the primary side to check the jets. It had very small 70 jets in there.. They are definitely too little.. Not sure how much bigger jets I should get.. I have some jets from my previous cars but they are all in the 90 to 99 range... Don't have any jets that small.. I also wonder if there is a way to identify the accelerator pump. It has

34R-2178B stamped on it with a 8 below.

I think that I might try 74 jets but I'm not sure if that will be too much of a step up??
Hole size on size 70 is .0735 while it is .0800 on the 74's

The squirter is a 37.. I believe that this should be sufficient..

GrandSportC3

Realized that.. only one bolt holds it in place..