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. http://www.yenko.net/ubbthreads/images/graemlins/naughty.gif

[VintageMusclecar]

Joseph;

I just spent a little time scouring through the parts book comparing the published specs between the 2 carbs.

I don't normally like to publicly offer tuning advice like this anymore, but for starters you might try just bumping up your primary jetting by 2 sizes to see if that improves the situation or not. Leave the power piston and primary metering rods alone until you see how the jet change works.

If you need parts, let me know.

Eric

[PeteLeathersac]

Before you dig into the carb, are you positive you don't have a vacuum leak anywhere?.
Besides checking lines, fittings, advance system components plus power brake and modulator parts if applicable, I seem to recall some Oldsies had a ported point on top of the intake that required a sealed bolt intstalled if unused?.

Always tough replicating problems in the shop that only show up driving under load at certain revs but if a manual inspection of the components show nothing amiss, sometimes intake leaks can be found at stand-still by hooking up a remote tach and trying different constant revs while keeping a close eye on the tach for variations as you mist methal-hydrate out of a spritzer bottle onto suspect vacuum areas...be careful of course and keep a fire extinguisher at hand!.

May be best if equipped to isolate brake boost and modulator system parts out of the equation and doing a drive first just in case...then onto the other things?.
Good luck!.

~ Pete
http://www.yenko.net/ubbthreads/images/graemlins/beers.gif

[JOZW30]

You guys are awesome! Thanks so much for the advice!!

I will re-check my vacuum to be sure before messing with anything else.

I re-built the carb before finishing the motor. It's really clean.

Car has the correct manual disc brake set-up mandatory with the 4-speed W30 package.

Eric, I will double check the obvious and make sure my basic timing is good. If I still have this issue, I'll try your advice above. After that I'll contact you privately for some "professional" help.

I really think this car can run alot better than my limited exp./skills can manage. But I love learning too.

Thanks again!

[SmallHurst]

Also, check to see if the vacuum advance is working properly. It could be that you have a leak in the canister and it does not show itself until the advance is needed. Pull a vacuum on the canister and see if it advances the weights like it should. Personally, when I am at the track, the advance is unhooked so that it has no possibility of coming into play. (Not like it would have a chance under a full throttle blast!)

[JOZW30]

Just one more general tuning question, I have the info for the initial timing for the stock motor set-up. But if you have have "upgraded" internals for performance (cam, higher compression psitons), do you still start at the factory initial setting?

How much advance can you go with? I've always been told to back off once you start to feel it pinging. Is this true?

It's hard to tell with the cam, I usually just advance until the motor smooths out and start from there. How much is too much initial timing?

[SmallHurst]

What I have done is what has been told for me to do. Personally, I do not worry about initial timing. When I set up the timing, I am looking for my total timing and in by a certain time. For instance, on the Olds BB, you have enough torque to bring the full timing in by 2500 rpm. My setup likes to go 36 degrees all in by 2500. When I am running with that much advance, the gas had better be good. I have tried to go farther with the timing, but it seems that my track time decreases. Brought the timing back to 36 and the track time came back. http://www.yenko.net/ubbthreads/images/graemlins/rolleyes.gif

[JOZW30]

Thanks for all the help guys. Seems I've got my tuning woes surrounded.

The two size up jet change realy helped the lean surge (it's almost completely gone).

When I went to really dial the timing in, I discovered that I am only getting 11 degrees of mechanical advance. It is there at about 1900 RPM, and doesn't go any further. It's not the original distributor (generic re-manned 455). I do have it though.

I'm assuming this is not good. My question is, what is my best solution? Can someone recommend a distributor guru? How much does a dialed-in distributor (I'd prefer to leave this to an expert)cost? Is it possible to buy them "off the shelf", or is it only possible to get your original re-done?

Last question, is it OK to drive the car this way? I mean, am I hurting anything or just giving up performance?

Thanks again!

[VintageMusclecar]

Joseph;

Check your PM's

Eric