rochester 2jet - any carb experts?

[ChevDog1]

My 1980 250 inline six had been swapped from the varijet carb to an old rochester 2G carb when I bought it. I have rebuilt the carb and fabricated a nice aluminum spacer ( http://67-72chevytrucks.com/vboard/s...=616795&page=7 ) for it, but still have some driveability issues. It is now tuned well enough to pass emissions testing, which I did last August.

The problem I have been having is with a flat spot, or bog, on acceleration. Steady state cruise, idle, and gentle acceleration are fine, but if I stab the throttle quickly, it noses over and I have to back off the throttle for it to recover.

Are there any carb tuning wizards on here that know what my problem may be?

[geezer#99]

Whenever I see a carb question, first thing I think is what's your initial timing set at?
That's because 90% of supposed carb problems are timing related.
So, what's your initial timing set at?

[ChevDog1]

Initial timing is at 10 degrees with the vacuum advance unplugged.

[geezer#99]

Bump it up to 14 and see if that helps.

[ChevDog1]

I had it at 12 before I took it in for emission testing and it failed. Had to back it down to get it to pass. It din't seem to change the bog either way.

I'm wondering if it isn't an accelerator pump issue. Some of the 2GV's used a check ball under the pump, and some did not. According to Mikes Carbs, he didn't think that mine was supposed to have one, but after all of the times I have torn this carb down chasing the problem, I'm wondering if I should try putting one in there. Not sure if I even have the check ball that came with the kit anymore. I'll have to check tonight.

[geezer#99]

Even 10 seems high to pass emmisions!
Are you sure your timing tab/balancer is accurate.

[ChevDog1]

The timing tab is as accurate as I can test for. Pulled the #1 plug and inserted a rod down to touch the top of the piston. Rolled engine over by hand and 0 degrees appears to be TDC.

[ChevDog1]

Thanks for the help guys! I started tonight by removing all of the vacuum lines I put back on it for emissions. Checked for vacuum leaks and found none. Dialed in a bunch more timing (kept adding until it started pinging, then backed it off a bit). It runs much better now. Still has a small bog on acceleration, but it's much better than before.

[jamyers]

If your ignition is good, I'd say the hesitation is the accelerator pump, and it wouldn't hurt to try a check ball.

[ChevDog1]

With the engine off and looking down the throat of the carb, the squirters appear to be working fine. They both give a real nice shot when you open the throttle.

[SkinnyG]

Experiment and fatten up the idle mixture screws to the highest vacuum reading (or fastest idle, if you don't have a vacuum gauge). This resolved a "flat spot" on medium acceleration on my Q-jet.

[jamyers]

Iirc, the one check all they all use is on the pump outlet side, so that when the pump retracts it can only pull from the intake. The "optional" second check all is on the intake side, so that with ANY pump movement, fuel can only move out the nozzles, for a more immediate and positive shot. Wouldn't hurt to try two check balls, or Doublecheck that the one check all is seating positively/cleanly.

[ChevDog1]

Ill try fattening up the idle first and see if that helps. If not, I'll see if i can find the checkball leftover from my rebuild and try adding it. When I rebuilt the carb (about 8 months ago!) I checked with Mike & Mikes carbs, and he was pretty sure that I didn't need one based on my description. I also used his video to determine if I needed one, and based on that it didn't. I have also read that if your 2GV has a fuel transfer slot in the side of the pump well, it is designed to not have a checkball at the bottom of the well. Mine has a transfer slot.

It is getting a really good shot at the squirters, and I'm guessing it wouldn't if it were pushing fuel back into the bowl.

Can my float level affect my carb in this way? I had to make an educated guess when I set it up, as this carb was never offered on my engine. Would it help to lower or raise the float?

[SkinnyG]

A higher float will richen the mixture globally. Lower float will be leaner globally.

[ChevDog1]

I will try a higher float setting and see if that helps at all.

I have also read about someone having a similar issue from running an inline filter in addition nto the one in the carb inlet. Something about restricting the fuel flow too much. Any of you heard of this?

[SkinnyG]

I doubt two filters is the problem. You aren't likely driving a fire-breathing dragon of a motor that can empty the float bowl at the touch of the pedal.

[ChevDog1]

Good Point! 150HP is hardly going to be taxing to even the fuel system on my riding mower!

[Oregoon]

Are you pulling your vacuum from the manifold or the carb, and if it is the carb, is it above or below the throttle plate?

[ChevDog1]

The carb only has manifold vacuum available. There are no vacuum ports above the throttle blades as far as I can tell. Why?

[jamyers]

Originally Posted by ChevDog1

...
It is getting a really good shot at the squirters, and I'm guessing it wouldn't if it were pushing fuel back into the bowl.

Can my float level affect my carb in this way? I had to make an educated guess when I set it up, as this carb was never offered on my engine. Would it help to lower or raise the float?

Yup, float level can have a huge effect on things. What were you using for your educated guess, and where did you set it?

Originally Posted by ChevDog1

The carb only has manifold vacuum available. There are no vacuum ports above the throttle blades as far as I can tell. Why?

What he's asking is whether you're using Manifold Vacuum or Ported Vacuum (which comes from just above the throttle plates, so you have no advance at idle but it all comes in and acts like straight manifold vacuum above idle.). I'm a believer in straight manifold vacuum, unless you're trying to meet some emission regs.

Which makes me wonder if you've looked at the ignition curve. Many engines can see real benefits in drive ability and throttle response by re-working the curve to bring in more ignition advance faster relative to rpm.

[Oregoon]

Originally Posted by ChevDog1

The carb only has manifold vacuum available. There are no vacuum ports above the throttle blades as far as I can tell. Why?

I had a bit of a flat-spot in my set up (I have a '74 250 with mighty monojet and '76 HEI), and pulling vacuum directly off the manifold eliminated it.

Check out the link and scroll down to post #3 for a very thorough explanation: http://www.chevelles.com/forums/showthread.php?t=94121

[ChevDog1]

I am running the distributor from full manifold vacuum. It is advancing from an idle.

I can't remember exactly what I used to set my float level, as it was about 8 months ago, but I remember that I was looking for applications with similar displacement and horsepower. After that, I think I lowered it once in an attempt to lean it out a bit. This may have been an attempt the wrong way. I'll try raising it today and see if it helps at all.

[ChevDog1]

I raised the float about 1/8" and it helped. The bog still isn't completely gone, but it is liveable at this point. It should be good enough to hold me over until I add TBI from a 4.3.

[Oregoon]

When you get around to doing that, please post up some pics of that swap! I'm interested in doing the same, especially with my truck being my daily-driver and not being subject to DEQ shenanigans.

[1974ChevyC30]

On my quadrajet, I had 3 things that I fixed that were causing a bog so check them all.

Float Level: When hot outside I wold get a bog from a stop due to flooding. Readjusted float to proper level.

Accelerator Pump: Seal was worn out from ethanol replaced and got even better

Of course, I use vacuum advance and my canister was toast so after I replaced that and did all of the above any and all bog was gone. These are just things to try from my experience.