Tuning help

[JOZW30]

Looking for some tuning advice. My 72 W30 runs pretty good, but I've noticed that the car seems to "surge" slightly while at cruising speeds (40+ MPH).

It doesn't seem to be missing. The engine feels smooth, just feels like the car is lurching very slightly. If I accelerate even slightly, it goes away.

It's a 455 with an NOS 1970 W31 cam. The car accelerates smoothly, and there is no pings or knocks. Plugs are dry, & look good.

Am thinking maybe a fuel delivery issue? Could it still be a timing issue?

Any thoughts?

Thanks guys.

[TDW]

It sounds like a lean surge. I would go one size up on the main jets and see if there is an improvement. That is assuming the timing was set with a light, and you are pretty sure it is correct.

[JOZW30]

Thanks Tony, I think the timing is close. I probably need to mess with it some more.

Does anyone publish a "tuning for idoits" manaul? I'm not an expert. It gets confusing. I bought a good digital timing light & have a vacuum gage.

Is there a how-to that spells out the steps 1-2-3?

Do you set the timing to factory specs even when you've used non-stock internals (cam & higher comp. pisotns)?

I think you are right though, it sure feels like it's not getting enough gas.

I'll start with the timing.

[WILMASBOYL78]

Check with Eric Jackson for any carb issues...Vintage Musclecar on this site...you won't have to go far..he's in Ohio, too.


wilma

[Keith Tedford]

If the jets and metering rods haven't been changed from new, then you probably are having lean surge. The 780 Holley on our COPO Chevelle had #68 primary jets and there was noticeable lean surge at 60 mph. No power either. Getting the jetting and timing right made a BIG difference. GM was dealing with emissions by leaning out the carb and retarding the timing with a very slow advance curve it seems. Jim Hand is a Pontiac guy but he has a lot of Q'jet jetting information that would be a good ball park for your engine. Do a Google search for him.

[Casey Marks]

The factory W-31 cam is VERY low on vacuum. So much so, that there were no primary metering rods in their QJets from the factory .... just jets. My bet is on the power piston spring for the primary metering rods ......

That cam might have 6" of vacuum on a good day. If the power piston spring is for a higher vacuum cam, the spring will hold the rods out of the jets, giving a false transition from idle to primary. ie - the engine doesn't have enough vacuum to pull the rods into the jets. The power piston spring has to overtake the primary metering signal to pull the rods out of the jets. That is your idle-to-primary transfer circuit in the carb.

[VintageMusclecar]

I'd say Casey pretty much nailed it. QJets and low vacuum cams are an "interesting" mix to say the least. What carb do you have on the engine? (ID number)

Eric

[JOZW30]

Carb number is 7042251. The correct carb should be 7042953 for the stick cars (am looking for one). The only published difference I can find is that the primary metering rod should be a "47B" vs. a "50B.

Do you think this is enough of a difference assuming my timing is OK?

I was thinking about finding a 71 W30 auto or stick carb, since this is probably closer the where I'm at now.

Should I just start with the primary rods? Assuming I can find some?

The car's a blast to drive other than this one issue.

[JOZW30]

Oh, & last time I messed with the timing, I was getting fairly steady 10 or 12" (can;t remember now)of vacuum on the gage while figiting with the idle screws.

Figured it was just how the W31 cam is in the 455 instead of the 350? I would have panicked seeing 6" to be honest.

[SmallHurst]

The W-31 cam was used in the '68 H/O 455 non-air cars. With the addition of the 100 extra cubes, it mellows the cam enough to have power brakes. I do follow what Casey is saying with the carb. What revs are you running when the surging occurs? Just trying to think if the carb may need an overhaul and get some gum cleaned out?

Mr. Hand is also a memeber of this board.