Ok. I have read mos all of the carb tunning threads in this forum. While I have gathered some valuable info I still need help with my particular application. I have a 1980 with a 350 that has a comp 280h cam and a 650 cfm mechanincal secondary carb built by AED. My most recent problem is cruising rich and an off idle stumble. I do have most of the necessary components to properly tune this thing, just looking for some guidance.

 

Off idle stumbles are real comon once you switch to a higher perfomance cam.. Best thing to do is work on the idle circuit first, which will probably solve your stumble as well...


First take a vacuum reading at idle, when warmed up and then install the proper PV size.. I believe they all come with a 6.5 from the factory so that may be good depending on your vaccum reading...
While you have the carb off, flip it over and open up the seconday throttle plates to about .015.. This will raise your idle but allow you to turn down your curb idle making sure your transfer slots are not exposed, which causes a very rich idle.. This may require a little tweeking but .015 is a good starting point..
What is your timmining set at and how is the curve? A good starting point is to have 16-18 initial with 36-38 total and all in by 3000.. This will require you to limit the total advance to 20 degrees, which can be done with heavier springs in the HEI. Bigger cams like more timing.. I had mine set at 20 initial but it still wanted more so I actually locked mine out at 36 and now my stumble is totally gone and the off ifle response is fantastic!! BG also recommends 20 initially on cams with over 240 of dur at .050..

Once you make these changes, drive it and see how it responds and then go from there... Dont make to many changes at once in case you go in the wrong direction with one setting, which means you will have to backtrack to find it.. In other words, dont start changing jets just yet.. The above changes are all I needed to make a huge difference in idle quality and cure my off idle stumble... I originally thought my jets were to big but after making adjustments and fixing the idle circiut, I actually believe it wants bigger jets now.

 

Not total, centrifugal ( or mechanical ).
Except this,

 

My cam duration is 230 @ .050. My initial timing is set at 20 deg with 36 total all in before 3,000. My idle mixture is spot on running between 14.2 to 14.8 AFR. I have 11 in hg of vacuum at idle. All of this in gear. I had read that my problem could be a lean surge and it appears that it still is. Current PV is 8.5 to try to help with this. I have put on a larger squiter nozzle but still get a lean spike when giving a short burts of throttle (not opening the secodary side). How much of a role do the pump cams play here? Also my AFR during cruise is around 13.8 or so. Anyone know what is optimal for a carbed car?

 

This is good general advise, but the power valve has nothing to do with running rich at cruise. Out of the box the 650 DP should be set up almost perfect for your combo. Do you know if it has been changed from the factory setting? When you say it runs rich - what do you mean? Black smoke? Smell? Sluggish acceleration?

 

What intake manifold are you running? If it's an Edelbrock Torker II or a similar single plane your off idle stumble can be directly related to that. I went through this many years ago with a Torker II on my 355 with an aggressive cam and even after playing with the tune it was only finally eliminated by switching to a performance dual plane. Edelbrock told me that this was pretty common with the Torker II.

Also, check your primary accelerator pump adjustment. When I bought a modded Holley DP carb I picked up a stumble off idle and found it was a poorly adjusted accelerator pump. You'd think they'd adjust them correctly from the shop but apparently that's not always the case. Good luck.

 

Factory settings have been changed a little along the way. Running rich as in black spark plugs, and an AFR reading of 11.0, and poor gas mileage.

I am running a Wieand stealth ram. Dual plane.
And I did say off idle stumble but really it has a stumble with any sudden acceleration at all RPM's that does not open the secondaries.

 

OK. I have a good idle and my cruise circuit is no longer rich. However WOT is rich and I still have the sudden acceleration stuble (lean spike). I plan to reduce the rear jets a couple more #'s to reduce the WOT rich condition. But this stumble thing has me stumped. I does it from a stop or from a 2000 rpm roll. The WB spikes to 16:1 AFR with a sudden shot of the throttle. Any ideas?

 

Did you open up your secondary throttle plates to .015-.020?

 

I beleive this thing is finally worked out. I went to the holley site did some reading/research on the web. I was hesitant to open my secondary blades because of the fact that I am only running 357 cu in. I found the best results with accelerator pump cam and squirter changes. No more stumble or lean spikes. Took it to the track yesterday with good results also.