Frankie the Fink

Hate to impose on my Corvette pals again but I'm stuck on the Chevelle. I am still getting a slight, but noticeable, surge around 2500 RPM (about 65mph at cruise). I've pulled the mechanical timing back with advance limiter tabs to about 18*, which, on top of the 16* initial gives me 34* add 10* on that for the Vac can and that makes 44* all in at 2500 RPM. This reduced the surge quite a bit but I don't know if dropping advance lower makes sense at this point. The car has a nice 700RPM idle with 17* of vacuum.

The car has a 4777-7 Holley with mechanical secondaries, stock jets should be (F) 67 (R) 73 and PV 6.5. Squirters are .028. There is 1200 miles on the rebuilt motor. Haven't examined the plugs after the last 30 mile run today but I'm really thinking about bumping the front jets up two sizes and see what happens.

Thoughts ? Keep backing off the timing...or bump up the jets ? And, yes I am a Holley novice but learning

Nowhere Man

Is the dizzy advance curve set right with the correct springs and weights

Frankie the Fink

Isn't that sort of the question I'm asking ?

The stock weights are in the Pertronix billet distributor; the medium springs are in their too. I put two limiter tabs in to pull the timing back from 24* mechanical to 18* mechanical which seemed to help quite a bit. The car idles and accelerates really well...its simply this lingering surge (trailer hitching if you will) at cruise. At this point I can't see 34* (44* with vac advance) total being too much timing causing a lean surge so my dilemma is whether to start dicking around with the carb or not.

Frankie the Fink

Its not an OEM engine ... it's a 'built' 396 making about 425-450HP...

BarryK

usually surging is from too much vacuum advance. You said you reduced the mechanical timing back but I'd actually try putting the mechanical timing back to where it was and instead reduce the amount of vacuum advance.
A quick and easy way to test that before making all the changes would be to disconnect the vac adv (don't forget to plug the line) and see if the surging goes away without any vac adv at all. If so you probably found the answer than you can reconnect the vac advance but limit it to only come in at a limited amount

Mossy66

The jet sizes for a '66 427 L72 carb are 70 front and 76 rear. (780 cfm carb) I have no idea if the that engine is similar to yours though. Also the L72 has vacuum secondaries, so I don't know how that compares to a mechanical secondary carb.(how far open are the secondaries at 2500 rpm)

I would try larger front jets before retarding the timing.

Also, is the surge at steady throttle at 2500, or are you accelerating from 2500? The reason I ask is that normally, the power valve is supposed to be about half of the idle vacuum. You're a little lower, but not by much.


Gerry

Fawndeuce

Since it's a built motor and not stock (variables), and since everything seems to be working right Frank, I'd get it on a dyno for some testing/tuning.
If you knew for sure that the jetting was right for the app etc., then I'd go for an adjustable vac can to dial out the surge.



Paul

Frankie the Fink

I think its under jetted....however I will do a short run tomorrow with vac advance plugged and then ****** a plug out from each bank. If I'm still surging I think I'll swap in larger jets...its about a 10 minute change from what I've read and seen. The car is running well its just this last ****ling problem I need to work out. If all else fails I guess I can look for a local dyno tuning outfit locally...hope it doesn't go that far tho.

I did add an adjustable Accel vac can to the Pertronix dizzy so I can dink with that some more as needed... Holley says a car with idle vacuum over 13" will do fine with the stock 6.5 valve...I do have a spare 10.5 though but will screw with that last of all.

DansYellow66

It does sound like too much timing. My car also suffered from that. Have you simply plugged the vacuum line and driven the car to veriy that the surging/missing stops? I think I had to take all but 8 deg vacumm advance out of mine to cure it - but that 8 degrees still helps it's running temperature and idle quality.

Bluestripe67

Frank, I think the days of reading plugs with todays unleaded gas are gone. Maybe I'm wrong, but! If you are experiencing a lean surge, have you seen an increase in engine temp or any new (if any) discoloration on the headers, assuming you have headers. Dennis

MikeM

Sounds a tad lean to me. My brain is reeling with your timing information.

Frankie the Fink

Car has headers not discolored at all....temp is rock solid at about 180* cruising and stops at 190* at stop lights with A/C on and moves right back down on take off. My plan is to cruise tomorrow with vac adv plugged...if it still surges go up two jet sizes on the carb; if it no longer surges then back off advance.

Mossy66

It looks like your timing is conservative enough.
I am by no means a carb expert, but some of the research I've done seems to indicate that at 2500 rpms, you might still be in the idle mixture area of the carb. The main jets take more effect higher up in the rpm range. Maybe try fattening up the idle mixture before you try anything else? ( hey, it's cheap and quick!)


Gerry

Mossy66

Here is some reading material for you!
http://www.bob2000.com/carb.htm

Good luck!

Gerry

AZDoug

Lean surge is certainly a possibility.

As far as the ten minute jet change. Maybe, if the float bowl gaskets are brand new.

Get a small cup to drain the fuel out of the float bowl into, when you remove one of the lower bowl screws.

Have a set of new metering block gaskets handy if that decides it wants to come off also.

BTW, don't forget to hook up your fuel line again before starting the car, an acquaintance forget that part,and his car spent the next 4 months in intensive care getting the fire damage taken care of.

Doug

JohnZ

If it has #67's in the primary, go up two sizes to #69's and see how that works. You won't know until you pull the float bowl off what's really in there, but go up two sizes from whatever you find. Some Holley blue (reusable) bowl gaskets will simplify future changes.

Frankie the Fink

Well ... just to follow up. I had already made up my mind to take JohnZ's suggestion...
So I went up two sizes on the jets...from #67 to #69...helped quite a bit but the car still surged some at cruise.

Then my Italian temper kicked in yesterday and I went 'medieval' on the Chevelle.
So much for changing one thing at a time.

1) Changed from timed (ported) vacuum on the Holley to full manifold vacuum
2) Changed power valve from 6.5 to 10.5
3) Put initial timing (w/o vac advance) back to 12* @ 700ROM from 16*
4) Put advance limiter tabs in the dizzy for 20* max mech advance
5) Hooked vac advance back up (I THINK -- not positive) that it's cranking in 10*
6) Readjusted idle mixture/speed

Result: Car is running the best it ever has, cool running, smooth idle. No surge that I can detect at any speed...pulls out strong. On very, very rare full throttle deceleration events I'll get a backfire (just a slight 'pop' really). I can live with that for now.

Plugs burning nicely and no overheating so I'm done tweaking for a while unless the mileage is in the cr@pper...

JohnZ

I'd hang onto that 6.5 power valve - you're probably going to want to put it back in.

Frankie the Fink

I've got it John...and it can go back in 5 minutes. Holley gives conflicting advice...their tech videos say if your car pulls 13+" of vacuum at idle the 6.5 is fine. However, their FAQ guide says to run at cruise with a vacuum gauge in the car and put in a valve 2" below the minimum cruising vacuum....which would be up pretty high in my Chevelle.

Then there is always the advice to put in one 1/2 the size of your idle vacuum -- that would be 8.5 for me. So, what's a fella to do ?

AZDoug

Hook up a vac gauge that goes inside the car, then drive around while someone else keeps an eye on the gauge.

You may be OK with an 8.5, but JohnZ is right, that 10.5 is so gonna suck gas. The 6.5 was put in for a reason.

Doug