Hi Guys,

Happy Holidays!!!!!!!

Finally got my engine all back together. I welded up a set of rails in my shop, and mounted my '76 marine 351W and intermediate housing assembly to it. Wired it up, rebuilt the Holley 7163 (4160 marine version. vac secs, elect choke) with a renew kit,new plugs,wires, cap, rotor, points, and fired it up.

Ran good right off the bat. Worked on timing, tuning and was trying to set idle real low 500-600, just like the book says.

Once I get below 800 rpm, a surging lope developes. As I try to lower idle screw, the rpm starts surging and dropping then surging and dropping etc. goes from about 200-800.

I can hear a mighty hiss, or suck, coming from the carb primaries, not so much the secondaries.

This has a block that the carb sits on, that has passages from the primaries, around the secondaries, to a hose fitting, through the hose to a pcv valve, and into my starboard rocker cover.

If I pull the pcv up and out, there is a mighty suck back towards carb. when I pull this, the lope fades, and the rpms steady a little bit. If I stick my finger over the pcv, it gets suctioned into place. The lope drops away, the rpms stabilize, then in a few moments, the engine dies out.


Any ideas where to start looking?

Thanks, and happy holidays again.
Ken

 

Sounds like the wrong PCV valve and it's sucking too much air causing a lean condition.

 

With the engine off, the pcv valve will rattle if I shake it. If I have it out of the grommet and hold it, with engine running, lots of suction just whistles right through it. If I then stick my finger over it, I can hear it "clunk" sort of shut. Then engine stabilizes, but then eventually dies.

Ken

 

Your PCV valve is functioning normally. What's your base timing set at ? Sounds like you need to advance it then retry the idle setting. Also check for a vacuum leak in the carb spacer.

 

Hey Fellas... Merry Christmas

I claim to be several things, but a competant carb man, I am not. What is the best way to search out a vac leak?

Kenny

 

PS
Timing is 10 btc at 800 rpm. If I goose it, I can watch the mark advance. I have a marine prestolite dist that seems to be working aok.

 

Use spray carb cleaner, spray all around the intake. If there's a leak, you know when the carb cleaner gets sucked in. 10*BTC sounds good, maybe try 12 or 14 though and see if that helps.

 

OK baddad457,

I will give those a shot, very soon. As soon as the kids are done tearing open presents, out the door I go.

Thanks

 

I use an unlit propane torch to find vac leaks. Check the pcv valve as suggested, my 83 302 wouldn't idle right because the wrong pcv valve had been installed.
regards and Merry Christmas
rikard

 

I keep trying to work on it, but have many Xmas distractions. Got the kids settled down, but I have some family coming over later, for dinner. Wife is going nuts cleaning the house. My saving grace is that in front of my shop, I have a smoker going since 6:00 this morning (8 lb. brisket and a couple of venison roasts). Sorry honey, I gotta keep an eye on it. It's not my fault that I keep gettin cold, and have to duck in my shop to warm up (the 351W, that is, heh, heh, heh).

I haven't found any vac leaks yet, via baddad's method, but in the process, I believe that the sound that I hear is air getting pulled past the primary throttle plates. I do have a very strong vac under the carb, as I said, it just howls through the pcv.
I advanced the timing to 14 deg, and slowly closed down on the two idle needles, and slowly brought up the idle screw. The engine calmed down quite a bit, but the lope and surge begins to show up again when I get back down around 600 rpm.

I have been told that the stock marine cam that I have is probably a "wild" cam for high rpm cruising. Could that be affecting this low end problem?

Kenny

 

Yea, it could. Some cams just have a spot at which it's not going to idle under. Usually though the Marine cams are comparable to an "RV" type cam. 800 may be the least it'll idle. If you've bumped the timing up to 12-14* and lowered the idle mixture and speed screws, it well could be the minimum idle speed. Only retarding the timing would let it idle down. Is 800 too high for it to shift into gear without "banging" the drive too hard?

 

I have one of the old OMC electric shift drives. It uses electro magnets to slide the forward or reverse gears into the main gears. I have been told that it is absolutely critical to be at low rpms to shift this way. I have been told that it is so critical that some people add an ignition cutout at the shifter, so that if a shift is thrown, the engine "dies" momentarily. I have enough self control to avoid shifting too high. The two OMC books that I have call for 400-600 rpm in the tuneup section. So far I can get down to 600. I "think" that that would be OK, I just don't "know" that it is.

 

You don't happen to know the cam specs do you? An RV type grind usually will idle like a stocker in regard to rpms, that's the smaller RV type anyway. These are usually the 195-214* @.050 duration grinds. The bigger the duration numbers, the higher the minimum idle speed is.

 

I really am the direct opposite of a Pro when it comes to the carb and apparently the cam as well. I found in the shop manual that there could be 1 of 2 cams installed. I found this info for each. cam 1- lobe lift intake .278 lobe lift exhaust .283 valve lift intake .448 valve lift exhaust .456 cam 2- lobe lift intake .260 lobe lift exhaust .278 valve lift intake .418 valve lift exhaust .448 Nothing at all about duration.

Also, this doesn't mean that it is the original cam. I have evidence that the engine was rebuilt at some point (40 over aluminum pistons). I didn't see any markings on the cam, when I did a rebuild. A friend chucked it up on a metal lathe, and took some measurements with it turning. He said the cam was straight and the lobes were by far well within the max. allowable lift loss specs of .005 right from my Seloc Manual.

Oh boy, now what?! $%*#@
Ken

 

Both sound like the HO/Marine cams, it ought to idle at close to 600 rpms with either, if that's what's in there. The early 80's HO 5.0 used one of those cams from 82 to 85.