Turbo-Jet

I would like to talk more about carburetor/timing settings to achieve maximum performance at WOT.

I made this post a while ago, a received some good responses http://forums.corvetteforum.com/zerothread?id=313950

I was curious about the exact differences between stumble/hesitation and bogging.

(Please correct me) from what I understand:

-A bog is too much air, too soon (air valve spring too loose, ?). This will occur beginning-middle of WOT, and the car feels like it falls on its face. Engine will pull slowly, then pick after 1-2 seconds, and perform normally.

-Hesitation and stumble are the same? Again this is caused by lack of fuel. If you go to WOT, and there is a slight delay versus input, then likely that accelerator pump shot is not enough, or primary jetting area is too lean.

What about a stumble at mid RPM?
What about stumble at high RPM?
What about a big bog just after shifting to 2nd? (1st gear range was fine)
What about a continuous WOT stuttering, especially at low-mid RPM?
What are symptoms of too-rich bog or hesitation? Is this possible, or just not very common?

On my car, I am trying to solve a high rpm stumble. As I accelerate hard from stop, the engine pulls strong until 4-4500 rpm, and then it misses badly; feels like engines cuts out briefly. This doesn't always happen. I am thinking it is staving for fuel. My float at 3/8" (as per Lars, and I have a new pump and filter). Seemed to improve low rpm throttle response, but is this too low to supply a 454 under full acceleration? What about the air valve? (Engine is stock, so is Q-Jet). My timing is set at 36deg at 2750 rpm.

My other problem is on my GSX-R 600. Sometimes, I have an off-idle to WOT stuttering problem, mainly in the 6k rpm area of 2nd gear. How do I tell if A/F is too rich or lean?

mrvette

Turbo, other than the usual rebuilding and cleanups....the Qjet is famous for twoother problems, one makes the other even worse....

first off, apparently with EPA, which unfortunately encompasses about all of Q jet production...has demanded the off idle transferr circuit be set super lean to avoid some sort of emissions crap....well that entire circuit depends on the exact size of the two brass capillary tubes in the top portiion of the carb...they need drilling out for more gas....how much...I don't know...
NOW of course Gm being cheap about it, made the carb bases out of potmetal so the shaft on the primary blades wears out to be egg shaped...allowing air in where it don't belong....making it even leaner....super sensitive to the slightest extra air...
so this information was told me by Brad Urban some 15 years or so ago...used to own the Carb Shop in Ontario Kalifornia....he did a carb for me...and explained what needed doing, since then of course it's common knowledge among super tuners...but most carb shops so not know about the off idle transferr circuit....
these two cases along with premature secondary air flapper opening are why Q jets are called Quadra Bog......

today, i'ts simpler to just get an Edelbrock or Carter AFB, and have over with it....

GENE

Phoenix79

I battled that problem for quite some time. I drove the vette till gas was depleted and filled it up with Mobil. End of problem. My problem was like yours...it didn't do it all the time and normally happened at 4-4.5K rpms. I had a new inline fuel filter ready to go in as changing the carb filter fixed the problem for 10 miles and then the major hesitations at 4-4.5K erupted once again. I would go to a different gas station after you deplete most of your gas and go from there. I drove the vette for 2 weeks feeling ashamed to drive past rice since I knew once I hit 75 I wouldn't be able to touch 80+. New gas and the problem ceased. Atleast for me.... I hope its as simple as that for you.

Matt Gruber

time to join the 21st century http://community.webtv.net/MATTGRU/AFtesting

73 LS-4

When you say it cuts out briefly, and misses. Does it recover after this and rev past 5k or is it just stuck at that point? Air valve problems normally result in an initial bog when opening the throttle if they are too loose, or a lack of top end power if they are too tight. Does your mechanical advance stop at 36 degrees, or will it go beyond this at a higher rpm then 2750 (maybe getting too much advance at high speed , unlikely but just throwing possibilities)?, what about fuel pressure?

:cheers:
Pat Kunz

Turbo-Jet

thanks for the ideas.
i installed proper bushings on the throttle shafts last year. as for drilling those tubes, i will save that for last

bad fuel? intersting, as all my testing was done onm same tank of gas. i will try another brand.

any more info on that sensor?

73-LS4: when it cuts out, it 'seems' to recover, but it is hard to tell because my natural reaction is to quickely left off the throttle, because it sounds pretty bad. i will try to stay at WOT. i am pretty sure the timing stops at ~2800, but I will recheck this too. what about if I use the brass stop bushing instead of the stock rubber bushing? my fuel pump is brand new, with new lines, so i hope it is safe to assume the pressure is adequate. with my current setup, it is hard to put a T in the pump-carb line.

thanks everyone. tonigh i am going to bring the float to 1/4", and then try new gas. (remembering not to make more than one change at a time...) fun stuff.

:cheers:

jdunne

Have you looked at the accelerator pump plunger seal.....the newer carterafb and edelbrocks use a plastic washer/seal instead of a leather seal......if it doesnt seal properly you dont get a full shot of fuel,hence a transition stumble...hope it helps.......john :chevy

Turbo-Jet

bingo! float to 1/4" and it pulls strong to redline :D

mrvette

If the problem returns shortly, and you have a mechanical stock type pump, you might try replacing that 3/8 input hose from line to pump, that curve is sometimes too tight for hose, and the extra suction will collapse it, starving engine for gas....can be temp related too....

GENE