I'm thinking it's a lean condition causing it, but what do y'all think? The ignition and timing have been addressed today so it's obviously carburetor related.

I'm running 47k rods with 75 jets right now in my q-jet. If I ease in to it, it's ok. If I stomp on it, it stumbles then catches up. Whatcha think? Up the jets to 76 or 77 or something else needing to be addressed?

Timing is set to 34 degrees total centrifugal at 2500rpm.

Vortec heads, cam = 214/224, .442/.465, headers, sidepipes w/glasspacks (for those curious), Edelbrock Performer intake copy (professional products)

Ready set go!

 

This is a classic accelerator pump problem. You probably need a stiffer duration spring. This will force the pump shot through the nozzles faster. You could also drill the nozzles just a touch larger but I would try the spring first.

BigBlockk

Later.....

 

Where do I get a stiffer accelerator pump spring? The pump and spring in there are like only a year and a half old.

 

i have the exact same problem with my '77. i have messed with the accelerator pump, then i can get anything tuned even close to what i had beyond that. i have the edelbrock 750 cfm.

 

 

Did your carb sit "on the shelf" for a long period of time while you did your rebuild? I have the same problem and have heard that the accelerator pump will "dry out" if not used...just a thought.

 

that it is most likely the accelerator pump. The other problem could be that the secondaries are coming in too early. Are your secondaries vacuum or mechanical?

 

Mine is doing the same thing. I assume it's too lean because it didn't start doing it until I got my new exhaust system. My pump shot is very healthy, and it takes a good push to maually open my secondary air valves. I got an AF ratio meter to help sort this carb out, but now I have too much fuel pressure giving flooding problems and bad AF readings.

 

I'm sure Lars or olescarb could get you some or know where you could get them. I know olescarb likes #17074104.

BigBlockk

Later.....

 

Carbs unlimited has a set with 3 different sizes.

http://www.carburetion.com/quadrajet.asp

Not sure if this is what you need..I am not by any means a carb expert.

 

Nope. It never gets a break of more than a few days at a time.

Secondaries vacuum or mechanical? It's a q-jet... mechanical lower butterflies but the upper butterflies don't open until they're needed. I should probably get some smaller secondary metering rods, too.

 

Here's some info taken directly from Lar's tuning paper:

It is a very common speed trick to loosen the secondar windup spring to that secondaries will open very quickly. this is the single most common cause of a severe stumble or hesitation upon acceleration or transition into the secondaries.



Are you sure that your secondaries are not coming in too early?

 

The secondary windup spring hasn't been touched since lars last had it...

And I dare not tinker with the secondaries yet. Plus the secondaries wouldn't open just from hitting WOT from a stop. They don't open until at least 2500rpm or so.

 

Dont be so sure about that amigo.....


The primaries are TINY.


To the original question, also ensure your float level hasnt somehow changed.

 

Well I don't think the secondaries would open before 1000 rpm, that's for damn sure. Like lets say I'm at a stop sign then just floor the pedal. Stalls. If I'm on the highway and floor it, it doesn't do it.

Some more info for y'all. Prior to having the stock heads replaced with the vortecs it didn't have ANY problem. It ran flawlessly, with the exception of the chock pull-off being too rich. Ignition is where it should be now, the vacuum leak in the brake booster is fixed, and all of the vacuum lines seem to be fine. It's obviously not running rich because there's absolutely no stinky gas smell or black smoke ever.

Would it be worth it to try larger primary metering jets? I've a feeling my rod/jet combination is too lean for the intake/heads/cam/exhaust combination.

 

Could be. Lars had me go down a rod size after I put on my flow through exhaust system. I could read a lean condition on my spark plugs after I changed from from a stock exhaust to the flow throughs. How do the plugs look?

Did Lars vehicle test/tune your carb after rebuilding? If not, I would vehicle test/tune the secondary wind-up spring after you solve the primary problem.

 

I'm gonna pull a plug when I get back to my gf's house and take a look see. I'd guess lean.

Lars did test/tune my q-jet on his vette. I'm the only person that has taken the airhorn off and really tinkered much with it (the exception being the idle screws, choke pull off, and main idle screw) so I know the secondaries are just as he had them. I also know i need to go to a smaller secondary rod, that's kinda obvious.

Oh and I talked to a few carb shops and they're immediate diagnosis was the accelerator pump spring, also. So I'm thinking bigblockk is right, like he almost always is.

I'll call Larry at the Corvette shop on Monday, too, to see what he thinks. I might let him have a go at it if he can do it without making my wallet nearly disappear.

 

Just checked a plug. Definitely lean. Kinda white looking.

 

Well here's a little bump for all y'all. I took the vette in to the corvette shop to let them have a go at it.

Wasn't the accelerator pump spring, but the pump DIAPHRAM!!

Little background, I replaced the pump a year and a half ago because of a similar problem when I first got the carb. Got a brand new edelbrock pump and installed it. Still had the hesitation problem and the dyno shop cured it somehow, not sure what they did though.

So what was wrong with the diaphram? It was ripped at the top where the plunger pushes down on it. I brought them my original pump I had in it and it turned out to be an original GM one. >_< The GM pumps are skinnier in diameter. We got lucky and they found a diaphram that fit, in their parts assortment.

Also fixed the choke, apparently the rod was upside down. Not sure how that happened but it did. Also found out some of my well plugs are starting to leak. Easy JB weld fix, I'll wait till lars comes out to fix those. Power steering valve needs to probably be repaired or replaced. Doesn't bother me but it scares the crap out of anyone else. Apparently my dampener is spun, because he said he couldn't get an accurate timing mark. Seems odd though because I could get a timing mark with my light. He did it by ear/seat of pants and it isn't pinging so it should be ok. I'll try advancing it more, if there's no pinging tomorrow.

So results of the overcharge on labor for the little work they did? Runs pretty good. Idle is a little lower, but that's a simple twist of a screw. I prefer it to be between 600 and 700 in gear, so the alternator spins fast enough to hold a good charge. It doesn't stall or hesitate any more. Actually got the wheels to spin a bit from a stop. I just e-mailed lars a minute ago asking for recommendations on jets/rods since my head/intake change. I think I need to go up to 76 jets and see how those do for me. Until next time!