Cruise hesitation

[FL113Dave]

Hello,
I have a 1968 Monza with a 140 4spd. Anyhow, I've been struggling with what I think is carburation issues for a while. The ignition is solid with a Petronix unit from Clarks. The car idles great and full throttle is better than I think it should be. When I'm at steady throttle cruise speeds the car surges like it is lean. Like I said, full throttle is great, this car will lay rubber with 14" tires no problem. I have plugged the PCV from the balance tube, the balance tube to the carb pads have new hoses, the choke pull offs are good as is the vacuum advance. I'm at a loss on where to look next, These aren't complex machines... Any help or direction is much appreciated.

[Jerry Whitt]

With the pcv plugged, you are likely to develop oil leaks from many places. As the pistons compress the air fuel mix, and the cylinder fires Some blow by gets by the rings and then pressurizes the complete crank case and other places.


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[66vairguy]

A lean condition is possible, but the PCV allows so little air into the vacuum balance tube that plugging it should make little difference at cruise. Also it is not a good solution, unless you just did it briefly to see if it made a difference.

You could have a carburetor problem, but another possibility is you've got too much ignition timing advance under light load while cruising. Disconnect the vacuum hose from the distributor vacuum advance canister and plug the hose. Take it for a cruise. This should have NO effect when accelerating on power. If that solves the issue your vacuum advance canister may be allowing too much advance or your basic timing is too advanced. With no vacuum advance your mileage will drop a little, but it won't harm the engine. Too much timing advance is never a good thing for an engine in the long run.

You didn't mention which Petronix unit and coil you are using. Keep in mind a Petronix unit is more tolerant of a worn out distributor, but a badly worn distributor will still cause problems.

[corvairsince70]

I had the same issue with my '68 140 PG. If the primary carburetors you are using are in fact '68, they are not your friend due to being smog carbs. GM intentionally leaned them for emission purposes. I replaced my '68 smog primaries with '66 primaries and the problem vanished. Smooth as glass now, no hesitation.

[FL113Dave]

Jerry,
I am also an ASE Master Tech :-). The PCV is only plugged at the balance tube. It is still open to the air filter si I dont see any damage done there. I will hook it back up because it was only a trial.

I'll try the vac advance to see if that helps.

Also, I have '66 carbs on the car

Thanks for the suggestions

Dave

[66vairguy]

The 66 carbs are the same as 65 - EXCEPT they had a revised idle circuit that can be a problem getting the idle to hold steady, other than that they are fine UNLESS they are the Calif. emissions control carburetors.

BTW - the "SMOG" carburetors work fine, when they work! They are very difficult to repair if any corrosion is present. Even the non-"SMOG" 66 carburetors with the idle emulsion circuit cannot be repaired once the inner casting tunnels corrode - BTDT!! This is why 65 carburetors are getting scarce.

NOTE: if you take the carburetors apart - check for a missing P.E.S valve (looks like the check valve for the accelerator pump). The 140's had a lean spot just as the secondaries open so the primaries were modified to correct that with the addition of the Power Enrichment System (P.E.S.)

If you don't have Bob Helt's carburetor book, then buy it!!

[66vairguy]

What distributor are you using??? Is it the original 68 unit?

[FL113Dave]

The distributor is a rebuilt 66 140 distributor from Clark's years ago. The carbs are a mismatch of ones that I have working. I tried Grant's carbs but they all sucked. I have 54 primary jets and 55 secondaries - trying to get rid of the lean condition. Still the same as before. I plugged the PCV as an attempt to further richen it up. I'm ure I'm missing something basic but I can't find it.

[FL113Dave]

Can you believe it has over 250,000 miles on her?

262K to be exact...

[64powerglide]

Could you have the float level to high & are getting a bit of overflow at a steady speed?

[FL113Dave]

Kalamazoo? I grew up in Ann Arbor. I'll check - Good suggection

[66vairguy]

The distributor is a rebuilt 66 140 distributor from Clark's years ago. The carbs are a mismatch of ones that I have working. I tried Grant's carbs but they all sucked. I have 54 primary jets and 55 secondaries - trying to get rid of the lean condition. Still the same as before. I plugged the PCV as an attempt to further richen it up. I'm ure I'm missing something basic but I can't find it.

O.K. definitely try disconnecting the distributor advance. The correct 66 140HP manual trans vac. adv. maximum is about 4 degree's less than the more common 110HP unit. Also note the 140HP manual trans. distributor maximum advance is 18 + static (36) and it's all in by about 2,800RPM (around 60MPH). In the past some have had to "limit" the vacumm advance by putting a hole and pin in the canister arm.

I run 51 jets in the primaries on my 140 with 65 carbs. That's what was used in the 65's and the 66 leaner carb's got 49's. I know folks push the bigger jets for a number of reasons, but GM never had an issue with these jet sizes. My cars run fine and the plugs are right on.

I was also tempted to run bigger jets, but after taking apart a few engines with "big jets" in the carbs I found the piston rings jammed full of carbon and the pistons showed damage from carbon debris - and these weren't high mileage engines either. That little extra power and slightly cooler engine temps just aren't worth the carbon build up issues - just me!

Good luck and let us know what you find.

[64powerglide]

Just down I-94 a bit. GO BLUE!!!!!!!!!