Need some help diagnosing what may be one or two unrelated issues on my D200. I know this is on the edge of being OT but both the truck and the drivetrain are technically HAMB-friendly and I know there are some carb guru's on here so I figured it was worth the ask. It's got a stock 318 with closed-chamber heads at ~9:1 compression, an Edelbrock LD4B intake, and I'm running an aftermarket small-cap vac-advance distributor firing an HEI and a 570 Holley "Street Avenger" although the issue I'm running into is pretty typical of any 4160-based carb I think. I'm fighting a hesitation off-idle when you punch it which is something I've worked through before with accelerator pump changes. If you roll into the throttle the truck is completely fine, but if you punch it (either from a dead stop or even while cruising 60mph+) it coughs and hesitates bad before taking off. The hesitation is worse when it's cold, so my inclination is not enough accelerator pump shot, but but before I go that route I wanted to make sure it isn't tied to my other problem. The truck is a stick-shift, so I've got it tuned to idle about 750 rpm. 10° initial advance, about another 12° at idle from the vacuum can since it's hooked up to manifold vacuum. I think the carb is on the edge of being too big because I had to completely close the rear idle screws and it's only got the fronts open ~3/4 turn (when I tried to adjust it as a 4-corner idle all the screws were open less than 1/2 turn all around). The truck has an aftermarket p/s setup which provides some significant drag at idle when maneuvering into a parking space for instance and it'll drop the idle by about 200 rpms, even more on a surface like hot sticky asphalt. I would liken this to the same issue you'd have going from park to drive except I can't tell since the truck is a stick. Is it possible my idle rpm drop and my stumble off idle are actually the same issue? I don't want to go chasing the squirter tuning only to find out later that it was some other issue causing both problems? I've never tuned a carb with p/s before and it's messing with my head since it's not a consistent load. Some reading has suggested that I'm relying too much on the vac advance for idle and that's why it's dropping out under the p/s load, but since I'm not giving it any throttle the vacuum stays about the same when the idle drops. Nonetheless is this an application where ported-vacuum would be better? I suspect both my idle stability issue, my hesitation, and the fact that my idle screws are so far in might all be related somehow? PS the primary transfer ports are not excessively open and the secondaries are completely closed at idle; no secondary transfer showing at all.
try ported vacuum and see what happens. Also see if you can modify the advance curve, to have more initial, with the same total advance.
Be sure the mechanical advance is at full return when at idle, you maybe fighting that along with the vacuum advance. It's easy enough to change the vacuum advance so I would start there and move it to ported. As for the idle screws, be sure the throttle blades are closed enough and you are actually using the idle circuits and are not partially into the run circuit.
I took it off and the transfer slots are showing about the right amount on the primaries and totally hidden on the secondaries, but it sure does seem to be getting extra fuel from somewhere based on the idle screws. The engine makes 21 inches of vacuum at idle, is there anywhere else it might be getting fuel outside the idle screws? I also bumped the powervalve up to a 10.5 based on the idle vacuum but that might have been a bit much?
Fuel can be drawn in through a bad power valve/ gasket , accelerator pump check valve . make sure your air bleeds aren't plugged . internal gasket problems are can other source.
I swapped to the ported vacuum source for the distributor and bumped the static timing up to 12°. Double checked the mechanical advance and it doesn't kick in at all until ~1200+ rpms (hard to say for sure trying to watch the tach and the light at the same time) and isn't all-in until pretty high rpm's. After switching the advance around the idle screws only required minor tweaking to get a good idle (I'm using a Fluke 88V for a tach so it's pretty sensitive). Didn't get to road test it, but it didn't really do much for my load at idle issue. I did find something that might well be important though. On a whim I popped off my PCV setup (aftermarket from Jegs) which was also added at the same time as the recent carb/intake swap and the idle didn't change much with the PCV disconnected but dropped almost 250 rpm's with the PCV port plugged. I know that there will be some additional idle air with a PCV setup, but 250 rpm's seems excessive? To see what would happen I tuned the idle with the PCV plugged and got it back up to 750 rpms and the load at idle issue was greatly improved. Less than 100 rpm loss with the PCV plugged. I want to road-test the truck with both the ported-advance w/PCV and w/o PCV and see how the stumble is. If it's PCV related maybe I can add an orifice to the PCV hose to reduce the bypass air from the PCV at idle?
I pulled the Jegs PCV out of the valve cover and found that it has such a large bypass opening that it was fluttering at idle. I also found by hooking my vacuum gauge to the brake booster port that I was losing almost 5 inches of vacuum with the Jegs PCV. I gutted my Jegs billet PCV so it's just an elbow now and I plumbed a stock 318 PCV in the hose under the carb. The stock PCV doesn't lose any vacuum on the gauge vs. a plugged port, and I'm now only dropping 40-50 RPM when I plug the PCV port vs. the stock PCV. Dead-idle power steering is much improved, however the hesitation/stumble is not any better both leaving a dead stop and also punching it while at a level cruising speed. Gotta be the accelerator pump now, right? What I'm confused about is whether to go up or down, or mess with the cam. I've found conflicting info in tuning guides. Based on the fact that the stumble is worse cold and gets better as the truck warms up (but still large & unacceptable) it should be a lean condition and I should go up a couple sizes on the squirter. However, I've also read that for heavy vehicles like trucks sometimes you have to drop the squirter size to lengthen the charge? My stumble is pronounced & immediate, and the truck recovers very quickly, so I still think it needs to go up a few sizes, but before I end up buying too may sizes can anyone comment on these opposing theories? I've got the red accel pump cam in (I think) the #1 position. I can drill out the squirter a size or two up for free (I have a good selection of orifice bits), but obviously that's irreversible.
I'd start with a #26 and work my way up.... Also change the front float bowl gasket with a new one... Chances are the pump shot is leaking right back into the fuel bowl...
The Holley Street Avenger 570 is a 4150-series carburetor, not 4160. Holley says to use port vacuum for the distributor. The carburetor actually is smaller than the 570 cfm rating. It also has two modes of tuning/w one being for economy. Holley recommends it for engines producing up to 300 hp. It is not too big for your 318. If you have not done so already, I suggest you go to Holley's website and read all of the information, including installation and tuning, for your particular carburetor.
Temporarily hook up your vacuum gauge with some extra hose so it's viewable in the cab while you're driving. Make sure it's connected to full manifold vacuum and watch how it responds under different driving conditions. Especially watch the vacuum reading when you encounter the stumble or flat-spot you're feeling. It may help you decide if it's related to the specific power valve you've installed.
Is there a scientific way to test the pump? I see a shot of gas as soon as the throttle is opened, but I've never considered that some of the intended volume is leaking back. I did change the carb to the blue non-stick gaskets the first time I had to open it up. I recall using genuine branded Holley ones, but it's been several years so I'm not 100%.
Before doing anything else, I'd make sure there's zero clearance between the accelerator pump arm and the throttle linkage arm. Any slack or play will cause the throttle to open before the pump begins it's first shot of gas. Then the next thing I look at is the diaphragm in the accelerator pump, as I've found they go bad and get hard very quickly with today's gasoline. I'd pull it out, and change it to an alcohol resistant type diaphragm that will stay pliable and not harden with the crap gas. I personally swap diaphragms before installing new carbs these days, just so I wont have to a few months down the road. Once you've done these two steps, then I'd look at timing, or other issues if it persists.
If it's leaking back, it's the little soft rubber seal under the accelerator pump diaphragm. So if you open it to look at the pump, pull that out and simply put a new one in. They're cheap and easy to change. But they rarely fail unless they've got some years on them.
My car sits for 5-6 months , I have opened up the carbs after sitting a few times & have noticed that accelerator pump diaphragms & power valves get " stiff" & don't function well until they've & soaked " a bit , I attribute this to the ethanol fuels . This could be what's affecting the OPs problem . A 31 is a pretty fair size shooter , generally found in "mild performance" carbs , I doubt that's the problem. If the problem only occurs during the instant the throttle is hit , I also doubt its a PV issue , as they take a millisecond or two to activate . Could possibly be an ignition timing issue ....when you tromp the throttle , any vacuum advance drops out , so that leaves static & mechanical timing .
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