Asa":1tlh43z0 said:
Well, I'm dadgum tired of trying to adjust the accelerator pump by taking the top half off and fiddling with it only to find that I made it worse somehow. So, I picked up a spare carb that I'm going to mutilate so I don't have to remove the top to adjust it. Before I do this, any reason I should swap out this carb for the one I have on there now?
Click the pictures for big versions, long story short, 6554s and 75-1984, current carb is a 7115s and 75-2079
And yes, X, I do have a colortune on the way.
A spare is good. A person really ought to make one change at a time. If it really is gallin' ya that much, you could do a two 1/8" drill hole modified top to adjust the accelertor pump, and seal it with some gromets. One hole to preload the pump arm, then another to turn the screw. If that's what you have to do to maintain sanity, all good, your call. The screw does have about 10 turns, and it raises the 2.625" long metering rod rod up and down in the jet as per Motzing and The Frenchtown Flyers discussions.
You'll have to make surethe PCV inflow is set up properly/correctly as a closed system before you can even start. pcv set up,
it is 45% of the idle, the ignition if properly set up, 35%, and the jetting, just 20%, since you can raise or lower the metring rod to compensate.
The standard metering rod taper is 50 thou, that is, the standard taper from the small to the large step is about .040 to .050" long depending on the jet size it was designed for.
The ignition has to be basline mapped and re-check and controlled, with both with vacuum hooked and vacuum unhooked (cr_bobcats post #7 » Fri Aug 15, 2014 ) readings taken....this way.
viewtopic.php?f=1&t=72349&p=555788#p555788
cr_bobcat":1tlh43z0 said:
So I've finally gotten around to mapping the timing events of this new HEI style distributor. This morning I just measured with the vacuum can connected. Tomorrow morning I will measure with it disconnected and the hose plugged. I started at about 700 RPM as this was about as low as I could drop it and keep the engine running reliably.
RPM Timing
----------------
700 16
800 16
900 16
1000 16
1250 20
1500 25
1750 34
2000 39
2250 42
2500 45
2750 47
3000 50
I have an timing light with a built in tach and advance adjustment. Obviously all measurements here are approximate.
This seems a bit aggressive to me but obviously I don't know for sure what this profile should look like otherwise I wouldn't be here. I'm pretty sure my curve is off but that's based off of a gut feeling that I feel like there's more power to be had with my current setup. She just feels a little mushy. Am I right in my assumption here or are things looking ok? Do I need to take some more measurments with vacuum connected?
Pertinent info: idle currently set at 1000, Holley 1940 carb, stock exhaust, unknown cam (let's assume stock), C4, unknown rear-end (again, let's just assume stock)
I still plan to measure how much vacuum I have but haven't gotten that far yet. I've got a guage but I just haven't taken the time to do it. I should just do that tomorrow when I plan to disconnect the can and check to see how much mechanical advance I'm actually looking at.
My ancient $450 doller Sun Scope picks up all the ignition related faults, like crossfire and other faults, and air fuel ratio.
So your going to have to do it alone without that help.
Then adjust idle air fuel mixtures, and then find a way to define cruise and acceleration (wide open throttle) mixtures. Those last two are set by the metering rod and jet choices.
The baselines I gave you for jets are based on the jet to rod area from
http://www.blackhawkengr.com/Black%20Ha ... ustom.html.
the "General Jet & Rod Table"
http://www.blackhawkengr.com/General%20 ... Tables.pdf.
If you start with what was on the 7115s and 6554s with its basic jets, they will be very rich the way they came, the 75-1984 rod is way leaner as I guess its for a 240, and no doubt, will also have the 120-407 jet, the 107 thou one. With a 75-1984 rod and a 107 jet, it could work okay. TRY IT!.
It takes more time, but IMHO, the right way to go is lean, and then richen it off in steps by the main jet. So keep the 7115s metring rod, and adjust air fuel by 100 or 101 jet if you have to. Hopefully, what you have will work.
Recaping....from the graph....
Blue line is 10 up from the bottom, page 241. 75-2079, designed for a what
may actually be a 240 with 107 jet. Exactly it for your 7115s.
#75-2175 or 75-2176 (Red lines underneath), are the normal limits for 70's 300 with 107 jet. Rich.
The 75-1984 rod is
way leaner than your 75-2079, way too lean for a 98 jet, but might be okay with a 107 jet.
To repeat, the 75-2147, 101 and 75-1990, 101 jet, they will work well. Those metering rod/jets on 199-232 engines have a nice, lean cruise, a very rich idle, and a not very rich wide open throttle. They cope with PCV valves, they are tuned for them.
The rules are, if you can prove rich or lean by WOT plug readings, and idle by color tune, and mid range by loaded uphill 2nd gear 30-50 mph acceleration, you can then tailer the fuel curve.
So at the very most, you might want to look at either a 101 main jet later on if your mixtures with the PCV really are lean. Which I very much doubt.
Mike has a 75-1986, which, with your stock 98, richens the idle, and leans off the cruise, and keeps Wide Open throttle just the performance side of stoich.
See
http://www.hotrod.com/articles/ctrp-040 ... rburetors/
The only issue with the Co curve for exhaust gas checking listed is that engines can run best in the leaner than 14.7:1 ratio to 16.1:1 ratio cruise and idle, and with a nice fat spark, modern testing equiment removes the ablity to do that.
That's because of the EPA and Federal Emission standards. You can go that lean if your fuel is well atmoised, and you've got a good exhaust system, and your enghine runs cool enough. Between 1978 to 1981, all non feedback YFA 4.9's were more economical than feedback carb 80's 4.9's with strict 14.7:1 readings. With oxygenated blends, those trucks are running 15.1:1 actually.