Pcm 351, any carb tweaks or tips?

[matt k]

I rebuilt my 1987 comp's holley 4160 over the winter, put it back on last weekend, and am hoping to do some final tuning this weekend. It's got a blue acc pump cam on it, i believe a 5 or 5.5" power valve in it, and i might play around with the vac secondary spring a bit. While i'm messing around with it, does anybody have any hot setup tips, tricks, or ideas?

[cadunkle]

Run with a vacuum gauge at 21-25 MPH and go with a power valve rate 1"-2" lower than that reading. Jet down (1-2 sizes at a time) until you get a slight lean miss or stumble at steady light cruise 21-25 MPH, then go up two sizes. Go with the lightest vacuum secondary spring that does not cause a stumble and doesn't result in open secondaries at light 21-25 MPH cruise. Not worth tuning secondary jetting on a metering plate setup if you're not cruising on your secondaries... unless you have a bunch of Holley metering plates laying around.

Also note more timing will require less jetting. Avoid detonation though, General good starting point is 10* initial, you may be able to ru as much as 14*-16* but don't push your luck. If in doubt 10* should be good and likely will not detonate.

[CornRickey]

Lot of good stuff there

[cadunkle]

Note, power valve 1"-2" LOWER than your light cruise reading. When vacuum drops below the power valve rating, it opens and you're on the enrichment circuit dumping in lots of extra fuel. You want this during acceleration or when running flat out or at higher speeds, but not at cruise or riding speed. You only want to be cruising on the main jets. Sorry for the error! So power valve lower than your cruise vacuum.

Also, go with whatever the most modest accelerator pump cam you can that doesn't cause a stumble when accelerating or getting up on plane. Same idea with accelerator pump discharge nozzles... Smallest that won't result in a stumble. Saves fuel and keeps plugs from fouling when these parts are not oversized.

[matt k]

Awesome, that is some great information, thanks a lot.

I thought I was a little closer to set than I am. The boat runs great under no load, on the trailer. But in the water, under load, i'm getting a stumble/backfire when i open go from idle to WOT. Acc pump is shooting perfectly, so I don't think that's the problem. Have another question though: my drivers-right-side idle mixture screw works as expected. My drivers-left-side screw doesn't seem to do anything, even if i close it completely. Do I have a clogged metering block, or is that normal? If I bump the float too high, i get fuel leaking from the venturis on both sides, but i'm not sure that means I don't have a clog.

Step 1 this weekend is to put some fresh gas in. Tuning on last season's gas (even with stabilizer in it) is a waste of time. Step 2 is going to be to pull the metering block and make sure the left side isn't clogged or partly clogged. Step 3 is going to be throwing in a new power valve, because backfiring might have torn my old one, and I have another lying around. After that, I'm not quite sure where to go.

[cadunkle]

But in the water, under load, i'm getting a stumble/backfire when i open go from idle to WOT. Acc pump is shooting perfectly, so I don't think that's the problem.

Put fresh gas in it, if it still lean stumbles a larger discharge nozzle is required. If that helps, but result it it accelerated, then stumbles/backfires, then accelerates, you may need to try some other pump cams or go to a 50cc pump. That would be odd on a stock engine though. So first try going up a few discharge nozzle sizes. Also make sure you don't have any vacuum leaks that could be leaning it out.



Have another question though: my drivers-right-side idle mixture screw works as expected. My drivers-left-side screw doesn't seem to do anything, even if i close it completely. Do I have a clogged metering block, or is that normal? If I bump the float too high, i get fuel leaking from the venturis on both sides, but i'm not sure that means I don't have a clog.

First make sure your throttle blades are not too far open at idle. You want just a tiny square of the transition slot exposed, like .060" - ish. Too much and it'll pull enough fuel form there to negate any change of the idle mixture screws. If this helps you may have to open the seocndaries a hair more at idle to get desired RPM, typically a screw on underside of base plate on a 4160.

Failing that, clean out the metering block and main body. Once soaked/cleaned spray carb cleaner through all passages, then blow out with air.

[matt k]

Thanks again. I'll give that a shot this weekend when I get a chance to work on it.

[jet]

Or you can use my carb mod that I did. I unhooked mine, the back two that is. lol. Dont need it and dont need it trying to open when weighted down. I did this before the BIG prop change but just left it undone.

[matt k]

you just disabled the secondaries, forcing them closed all the time? Interesting. I'll keep that option in mind.

This morning I pumped out all the old gas, pulled the secondary actuator off and checked the diaphragm and cleaned the vacuum passage, pulled the metering block and blew a bunch of carb cleaner through the idle fuel passages, and replaced the fuel filter. After lunch I need to go get some fresh gas and see if any of that helped.

[jet]

Yep, the only reason you would need a secondary would be if your racing or wanting to get up to top speed the quickest you can-racing! It wouldnt help you (wasting gas) if the came on while weighted down, thats usually a smooth run up to speed and a 2-barrel and a good prop is all you need to get up to speed quick! but I know there are guys (including my friends) that love to full throttle across the lake. Theres NOTHING fun about full throttle when you have 6 people in the boat. You pound your boat across the waves just bouncing around scared for your life so the driver can have a lil fun. NOT fun for everyone else. Its equal to doing power turns. lol

[matt k]

Ha, yeah, I'll admit I like to take the occasional blast across the bay when i'm out by myself, but with 3 people in the back seat, my boat porpoises pretty badly over maybe 35mph, so I tend to keep it down when we're loaded up.

On the plus side, it seems like I've got it pretty well sorted out now, nice smooth acceleration from idle to max speed od somewhere around 45 (don't have my gps, and I haven't calibrated my speedos yet this year).

I have a slight surge with the throttle just cracked open right around 1000rpm, that goes away by 1200 (and isn't present at all with the throttle wider open). I may dick around with it a bit to clean that up, but only if I get bored ;)


Thanks for all your help everybody, it feels awesome to get back out on the water!

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