My velle has been running again about for two weeks. Only problem I have is it bogs when giving it slight throttle while at cruising speed. Example, say you are going 40, but want to go 45. Well, I give it a little gas, and it bogs hard, then suddenly picks up, but picks up WAY too much. It kicks down and normally throws me into my seat with 3x the loudness. Not good for just driving around town. Im thinking this is a tuning issue with the carb. It has no issues pulling away from stopsigns, nicely, just this off-idle like stumble from cruising speed (say, cruising speed around 1800rpm). Carb is a holley 670 street avenger with stock jetting on a 350 (10.4:1 compression, vortec heads, .480 lift 280 duration cam).

I would try bigger squiters.

larger pump sqirters or it could be something as simple as to much vacume advance.

Actually, if it bogs, then runs like a raped ape, it's getting too much fuel. But the issue might be your secondary opening rate.

To check fuel, open up the gap on your front squirter, this will automatically lessen the volume and bring it in later. IF that helps, then put the setting back to where it was, and drop 2-3 sizes.

Havent tuned a vacuum seconady Holleyt in so long, I wouldn't know where to begin on that, other than trying tighter and looser seetings.

But again, lean is when the engine kinda ' softly gasps',too rich in the shot is always Bu----WAAAAH style. So it's getting too much.

Tightne up the secondary and see if it helps. Open up the primary pump gap, and see if it helps.Don't do both at once though, change one variable at a time so you know which direction you should be going. You set three things at once, you'll never know which helped.

As always, floats must be right on before you start fiddling. My first step 'on a buddies car' is to check the float settings before anything else is done.

As Gene aludes to it sounds like your sec's are opening too soon. Get a spring kit and put a heaver spring in the diaphram housing. The key here is you said It has no issues pulling away from stopsignswhich eliminates the accl pump and squirters. And this it bogs hard, then suddenly picks up, but picks up WAY too muchis indicative of the sec's opening too soon. You should not feel vac sec's open like you would mechanical's.

Dennis, I **think** he has one of those adjustible ones on there, so it's wicked easy, kinda like a Q Jet.

How do I adjust the secondaries on the carb? (total noobie)

Yes, the Avenger series carb has a quick change lid on the daiphram can. 2 screws removes the lid, swap the spring and go. Holley has a kit with color coded springs, it's real easy.

There are a couple extra springs in the kit not mentioned. They match in color but are shorter. I think they slip in just before the same color listed below like the yellow...

Color------Load------opening-rpm---full open
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
White----Lightest
*Yellow--Lighter------1620--------5680
Yellow---Light--------1635--------5750
Purple----Med light----1915--------6950
Plain-----Med----------2240--------8160
Brown----Med heavy----2710--------8750
Black----Heavy--------2720------not full open

*short spring
This is for a 350 engine, the more cubes the lower the opening rpm and the lower the full open rpm is reached.

This is published in several Holley books...

So if I put in the "purple" spring, it would probably fix the problem? Or then it wouldnt fully open?

Yes and no, you're on the right track though. If you currently have a lighter spring than purple then you should show some improvment by delaying when the sec's open. Don't worry about the sec's opening all the way on a vacuum sec carb. Consider it an "on demand" carb only giving your engine what it asks for. It's much harder to over carb with vac sec's.

I doubt it's a secondary thing. Even with the lightest spring the secondairies won't open with only slight throttle. Just not enough signal. Really seems like an accelerator pump adjustemnt. Something to try is run the idle mixture richer. If the problem gets better then I would say not enough fuel. If it get's worse too much fuel. Reset idle mixture, then adjust pump arm.

Also check timming.

Steve

Upon inspecting the box the carb came in, I found a bag full of goodies that include, guess what, secondary springs. I pulled it apart and found a silver spring in it. The box included a yellow and black spring. Upon testing the yellow and silver springs between my finger, I did indeed confirm that it is a silver spring, not a white one, as the yellow was softer than the yellow. I put the black in, but havnt run the car yet. We'll see what happens. I also noticed the primary and secondaries are coated with a brown dust, it looks like carbon, but its brown. I found it odd.

You do have fresh gas in it Justin? Anyway, it may not be the spring, but try it and see. I leanred a bunch about these carbs jsut by changing things around and seeing what happens. Again, check float level on a hot motor to make sure. You should see the gas just at the bottom of the sight plug.

I thought the problem was at part throttle at cruize speed, if it was bog at instant wide open then for sure I'd go with the secondary spring being wrong but I still think probably a lack of fuel or to much timing advance. At wide open the vacume advance pulls out. Just for the heck of it disconnect the vacume advance and take her for a drive to see if that has a positive effect. Heck, disconnect the secondaries at the diaphram link and take it for a run, this will eliminate the secondaries and isolate the problem to the primary side if not related to the secondary opening. Just some ideas I would try.

The vac advance isnt plugged in. Its capped off. Timing is about 11 initial, 32 full.

Am I reading a transmission kicking down in the middle of this? Where is it setup to downshift? 50% throttle? With the revs up a bit I've seen a light spring start opening the sec's without flooring it. You just need some air flowing through the pri throttle bores to create some vacuum in the venturi in the side of the bore.

If you are not getting the proper squirt you should have a stumble and maybe a lean back fire if you just take your foot off the brake and nail the gas. If the car launches ok, leave the accl pump alone. Make sure the car is warmed up well before making this test. As for testing the vac sec's you might want to hold the car in 2nd and be going just fast enough that the trans doesn't downshift when you step on it. Without a downshift in the middle it will be a lot easier to figure this out.

The way my trans is built if I leave it in 2nd it WONT downshift, so it should be easy to test.

Another thing I just thought of. Under slight gas a high pitched whistling noise comes out of no where, goes away with more throttle.

Just so you don't overlook another possibility, you really should make sure you don't have a vacuum leak. I have seen this result in strange behavior since the carb can't get the right information. Recently, my 350 had some hesitation and the majority of the problem was the plug at the electric choke housing had fallen out - simple fix. All these other ideas are great once the basics are covered.

To find a leak take some carb cleaner and spray around the intake gasket and carb while the engine is running - if the speed changes find it and fix it. Don't forget the booster is a possibility.

Thats the thing, there is only one vac line on the whole car (one to the trans). I re-torqued the intake bolts, they are fine.

Still a good idea, it can be very subtle and not easy to find. Try it, you may have a leaky hose or a carb gasket leak and don't realize it. It could even be at the trans modulator.

I just tried the new spring, made no difference. Also i tried to leave it in 2nd, it still bogs.

the stiffer spring made no difference, so now you have isolated the problem to the primary side of the carb, a vacume leak, or a timing issue-but it sounds like the timing numbers should be on with no vacume advance.

Tried richening up the idle?

Steve

. Ok back to the front side then, adjust the pump arm so you have more clearance at the bottom of it by tightening up the screw.( this is that screw with a spring in the center the bottom of whioch actually rides on the pump arm going into the pump housing.) Shorten the spring part by tightening up on the screw. This test will tell you if you have too much pump shot. if this cleans it up a bit, put the setting back to where it was ( count turns when you tighten it), and get a squirter that's 2-3 sizes smaller up there. You might not be able to see the numbers on the squirter without removing it.

BE DARN CAREFUL NOT TO DROP ANYTHING DOWN THE BORE OF THE CARB WHEN REMOVING THE SQUIRTER.

Yes you can fish it out, but...

Rather then change the squiter, you can also change the pump cam.

Steve

Originally posted by wanarace:
Rather then change the squiter, you can also change the pump cam.

Steve I'm certainly no "carb guru", but I did sleep at a Holiday Inn Express last night :D

If the pump cam is currently in the #1 position try the #2 position. This will delay the pump shot by a few hundred RPMs. If the initial pump shot is the problem this may delay it enough to cure the bog. If this doesn't work then you may try another pump cam with a less agreesive profile as was suggested. My engine idles around 900 RPM in gear and I foot brake the car to ~1,600 RPM. I use the #2 pump position so that I'm not already way into the pump shot (read poor transition) when I launch the car.

As far as checking for a vacuum leak there is a simple test. With the engine idling cup your hands over the top of the carb. If there is a vaccum leak it will continue to run, if not the engine will stall.

The Holley web site tech info has some pretty useful tuning tips including these on accelerator pump tuning:

http://www.holley.com/HiOctn/TechServ/TechInfo/TI-221.html

That is some good info. :D

That Holley website is excellent for basic carb information. After reading about 15 minutes worth of information I fixed an off-idle hesitation problem last night quickly. My plan was to check the accel pump adjustment, then try cam location changes to narrow it down. When I found the pump was so far off (on a new carb) the real problem was more obvious. I moved the adjustment over 1/8" to get the setting they describe and what a change! Crisp off-idle response without any cam change.

I imagine that being that short on the adjustment caused the shot to be delayed and not enough volume creating a hesitation. My assumption (wrong) was that a new carb would be correctly adjusted from the factory - never hurts to check.

I had a similar problem one time, and it was my flex fan blowing my mixture from the squirters. I cured it by using a 50cc pump and tubed squirtters. If you have rubber line in the system like back at the tank check it!! Also make sure you have fuel line and not vacumm line etc. graemlins/sad.gif

Moloko,

I have the exact same trouble I'm still sorting out. I changed pump cams to a lesser shot, that helped a lot but didn't solve it completely. Next I'll be dropping down several shooter sizes as I know Ive got a big on now.

Keep us posted when you solve this, I'll do the same - but I think you just need to lean the shot out.

Did I read that you didn't have your timing advance connected? That could be part of it at lower rpms, too.

Well, after playing with my carb a bit more I noticed the throttle shaft has a LOT of play in it. Way too much wiggling going on, and I think it's affecting the carb. Seems the shaft will move some as it's opening the throttle blades before the pump shot even has a chance of getting into the carb.

I started scratching my head after really bumping up the pump shot and still not getting anywhere. When I actually watched the carb in action, half the time the pump only moved part way, which is how I noticed the throttle shaft moving around. Also, gas was coming out the shaft onto the intake manifold. Looks like it's time for a new carb for me.

well I just bought a vac guage today. Is there any way this can help me? A few guys said this will point me towards any problems I had, but never told me how use it.

I've also gotten more info about the bogging. At any low rpms (like when cruising around 30mph) the car runs like crap, it sounds like its misfiring, hesitating, etc, until you give it a little more throttle. Then it takes off like a rocket. The throttle is like an on/off switch.

You know, it really sounds like you're timing is messing you up too. Is your vacuum advance hooked up to the timed port on the carb? Is your distributor sticking? An easy test is to advance your timing more, then see if that helps.

Back to the carb - my 3310 is pretty dated, and it's been apart many times. One of the biggest problems was with vacuum leaks INSIDE the carb. One time when I started having trouble with it I took the carb off the car and re-tightened all the screws - it solved the problem somehow.

I'm in the process of getting a new Holley from BIGS (www.bigsperformance.com) - and for not much more than a new Holley, color matched to my car. I'll let you know how that works out, but so far they've been pretty responsive.

XTreme, a loose throttle shaft causes a vacuum leak also because air will pass the loose bushings and cause it to act like the butterflies are slightly open.

Eduardo, that's why I decided to get a new one. It was really pretty loose - I'm surprised I didn't notice it before.