I just added 1.52 ratio roller tip rockers from comp cams to my motor and it seems to be running lean i think. Here are my symptoms. It stumbles a little somtimes at about 2500 to 3000 rpm while moderately accellerating and it will stumble a bit at about 5400-5600rpm under full throttle. In both cases is is just a quick stumble that is just noticable and the tack just bumps then it is fine. I also got a large gunshot sounding backfire once today. I have 1.5 stage richer metering rods in both cruise and power...should i try these??? And/Or could change the setting on the arm of the accelerator pump.(edelbrock 600, 350ci, XE256H cam, headers...)

Thanks guys
Smokin

 

Have you read Lar's paper on Q-jet tuning on the FAQ? I know you don't have that carb, but a lot of general carb tuning principles he writes about would apply. I learned this tip from it: to verify lean condition, disconnect the accelerator pump and flick the throttle. You should get a slight hesitation. Mine was almost stalling, so I knew I was WAY lean. I used his charts to pick out a combination jet/rod that would work for WOT. Now I'm working on getting the cruise and idle dialed in.

 

OK....i disconnected the accel pump and blipped the throttle to near WOT. It hesitated slightly as the secondairies started to open. Does this mean im Lean. Im just trying to make sure i understand what you are sayin. If it is properly tuned it should not hesitate??????(is that not the point in the accelerator pump, to remove the hesitation as the secondary circuit comes on???)

 

Well at least with a Q-jet, when you remove the accelerator pump linkage, that's when you should have a slight hesitation when blipping the throttle, so It sounds like your jetted and rodded right, at least if it was a Qjet. The theory is that if there is no change, or better response with the accelerator pump disconnected then it would be richly jetted.
I think the point of the accelerator pump is to provide a squirt of fuel to compensate for the sudden increase of airflow when the flaps open (primary and secondary in some cars) as you mash the throttle. The Qjet won't open the secondary airvalve until the engine creates enough airflow which in turn lifts the secondary metering rods up, providing more secondary metering area. At least I think that's what I derived from Lar's paper. If you haven't yet, check it out. It sure helped with my Qjet. I was getting a bog (secondary air valve spring had NO tension on it), poor throttle response ( jetted too lean) and intermittently opening secondaries (air horn tightened too much by Bubba and distorted. resulting in the flaps binding). I'm not sure how any of that would apply to your carb, but it might give you some ideas.
hopefully someone with Edelbrock experience with chine in.


[Modified by isosceles, 11:31 PM 6/24/2003]

 

where is the FAQ i cant seem to find it???

 

The FAQ is http://www.corvettefaq.com they have a great wealth of tips and instructions on it. I think the tuning papers are under engine, I can't quite rember.

Isosceles
I'm having some trouble figuring out which rods and jets to get. I'm trying to follow the charts, could you explain which is a 10%change.

 

Smokin -
If your carb is the Edelbrock version of the Carter AFB, the tuning tips I cover in the FAQ section do not apply - they apply to Q-Jet tuning only. The AFB is a completely different animal...

 

Also.. if you're getting a backfire... the likely cause (assuming correct timimg, your wires are not cross-firing etc) is for a valve to be open when it's not supposed to be. The spark ignites the fuel and the expanding flame finds a way past the open valve either to the carb (intake valve) or to the exhaust (exhaust valve). Since you recently messed with the valve train, take a second look at your set up procedure for the rockers. One or more rockers may be too tight or something may be binding. For example, a bent valve can hang open.

 

When the tach dances or bumps as you put it, its usually an indication of some kind of ignition problem.

 

that could be...i will check that out...and as for the rocker set up i put them all at 1/2 turn past zero preload. That should be good???? it may have just stuck once or somthin cause it never did it before that or since so who knows why it did it. Cars are like women...so unpredictable. and then they give you the "IF YOU DONT KNOW WHATS WRONG WHY ARE YOU TRYIN TO SAY SORRY AND FIX IT" crap. I HATE that. hahah later guys. Keep postin your ideas. Thanks a lot

smokin


[Modified by Smokin80, 2:47 PM 6/25/2003]

 

BTW, let me make it clear that when I mentioned that bouncing tach needle indicates ignition problems, that only pertains to electrical tachs, which is what I assume you have in yur car.

 

sure is!!!

 

Of course it depends on what you have in it already. Lars' paper recommends changing one thing at a time, first being the metering area at WOT. Since all the rods have a .026 metering area at WOT, it makes it a little easier to figure out. But whenever you change the primary jet size, the cruise/metering size will be affected if you don't also change the rods.
Here's what I did, following Lars' paper:
Determined the need for rejetting by disconnecting the accelerator pump, flicking the throttle and almost seeing the car stall. Also, covering the airhorn at idle with my hand actually increased the idle speed, as covered in Tech tip 3 of that paper.
1. Made sure float level was correctly set. That's the first step covered in the "procedure'section of Lars' "How to tune a Q-jet (basic)" paper.
2. Took carb apart following tech procedure #1 off paper
3. determined that I had .071 primary jets and ..40 rods.
4. go to the chart labeled "figure 2" and look up your WOT metering area based on your jet size, but use the .026 rod to determine this. Remember that all rods are .026 at the WOT metering area. So for my .071 jets it is .00342826
5. Next, you'll need to consult another file on the FAQ labeled 'jet change chart', and Excel spreadsheet. In the second column there are a bunch of metering area numbers. Find the one closest to yours at WOT. Mine was .0034286, so mine matched up best on row 50.
6.Then scroll across until you get to the green shaded area on that same row, stopping at the column with the number closest to 10.000, but not over it.
7. Scroll up to the top of that column and write down the metering area. That number for me was 0.003739281. This is the my new WOT metering area I will try to get with my new jet/rod combo.
8. Go back to the chart labeled "figure 2" in the Tuning paper and look at the WOT metering areas for all the different sized jets. Remember to only look at the areas with the .026 rods. The jets I selected were .074 because with the .026 rod tip (remember all the rod tips are this diameter) the metering area is .00376991. From the spread sheet, I was shooting for .003739281 and this is as close as I can get (pretty close!)
9. Now you need to figure out which rods to use so that your cruise/idle metering area doesn't change. If you use the same rods you have now, and you increase the size of your jets, those same skinny rods might give too much metering area at cruise. Go back to the 'figure 2' chart and look at your cruise metering area for your old jet/rod combo. For my 71/40 set up, it was .00270256. This time you will use the actual rod size you have in that table, not the .026. The part of the rod that affects cruise metering area is .040 in diameter. (again, the tip is .026, that's the part that affects WOT).
10. So now, you need to go to the "figure 2" table and look in the section with your new jet size and pick out the rod that will give you close to the same metering area. I needed .074 jets to increase my WOT area by 10%. I need rods to give a cruise metering are of close to .00270256. Looking down the list I see that .045 rods will give me the closest to that at .0271041.

If you still have issues with the cruise/idle metering area, follow the instructions in the paper to test that. Lars will have you lock out the secondaries and make some test runs. An easy way to figure out rod change sizes is to multiply your metering area by 1.1 for leaner (thicker rods mean less metering area) or by .9 for richer ( skinnier rods mean more metering area).
I hope my example helped. It worked for me. I am still messing with the cruise area a little. I ended up going with my new jets and back to my old rods, but I think that makes my cruise a little rich. I still need to test that.
Follow the rest of Lar's procedures in the paper. That guys KNOWS Qjets!


:yesnod: