C2 low speed cruise "jerkiness"

[Randy S.]

Hello all.

Looking for some guidance on what might be wrong with my 66 300 HP. When I cruise thru the neighborhood, under 20 mph and under 2000 rpm, the car won't cruise smoothly and has some "jerkiness" to it. Almost like I am stepping on the gas in short rapid bursts. I don't see a difference in rpm but I can feel the uneven speed.

I suspected ignition so here is some data. Initial timing (vac line plugged)11* BTDC. I previously bumped initial timing to achieve 36* WOT. Spec is 6* BTDC.

Idle vacuum is 17.5" both with a "Tee" in the vacuum line and without. With the vacuum can connected and at idle the total timing is 28*.

The spec for my distributor is 16* vacuum advance at 12" so it appears to me I am getting full vac advance ( 28-11= 17)at idle. IIRC Duke suggests full vac advance 1-2" below idle vacuum. I am using a Napa VC680 vacuum can which is spec'd at 16* advance at 16" vac. The above measurements were taken without restraining the centrifugal weights in the distributor so maybe they are worthless. Spec on 111153 distributor is 0* centrifugal advance at 900 rpm so in theory the centrifugal weights have not yet begun to advance.
I do not have a Mity Vac so I cannot spec the can directly.

I am assuming that at low cruise speeds my engine would develop 17.5" vacuum.

I tried setting the initial timing back to 6* BTDC but no significant results

I would appreciate any feedback

Randy

[Duke W.]

Re: C2 low speed cruise "jerkiness"

First, the VC680 (ID B1) vacuum advance control is worth its weight in scrap metal. I do not recommend it for ANY Corvette engine application. Many require 18" or more to pull to the limit.

Some moron put the VC680 in parts books as a substitution for the OE VACs years ago, and that mistake continues to haunt! If you can gather up enough of them, they could be melted down and cast into something useful - like anvils or boat anchors.

The VAC I recommend for 300 HP w/MT is VC1802(B22), 16*@15", and VC1765(B22) or VC1808(B26), both 16@12", w/PG since converter load reduces idle speed and vacuum in Drive relative to a MT idling in neutral.

You can cross reference these part numbers to Standard, BWD, or Niehoff on their Web cross reference catalogs. They should all have the same ID. I don't trust the Delco catalog or cross reference, and unless you find a 20 year old NOS Delco VAC, which is unlikely, they won't have the original five digit stamped ID because Delco buys their VACs from Standard Ignition Products, which now owns all the above other brand names, so all the parts are the same, just different "brands" for different marketing channels.

Always verify that you have the correct VAC ID number and check that it meets specs before installing.

Also, according to the specs I have your max centrifugal is 30@5100 (check against your CSM and AMA specs and also check on the car - maybe someone put "light" springs in it sometime in the past 40 years), so I suggest backing the initial down to 6-8, which will bring total idle timing down to 22-24, which is more appropriate for a low overlap cam. High overlap cams, only, need total idle timing in the high 20s to low 30s.

Many manual tranmission cars are subject to this kind of jerkiness at low speed due to driveline lash. An improper or malfunctioning clutch hub damper can be a cause - or excess ring and pinon lash, worn U-joints, excess transmission countershaft endplay that increases gear backlash - anything that increases driveline lash. So a thorough check of driveline lash is in order.

It can be exacerbated by a lean off idle condition, so an easy thing to try is richening the idle mixture a bit, which will richen the off-idle slightly. When setting idle mixture you usually find a range - say 1/4-1/2 turn where idle speed and vacuum are maximum, so set yours at the rich end of this range.

Dirty carburetors can also go out of calibration due to deposit buildup in the tiny idle/off-idle passages or dirt in the air bleeds, so if all else fails, remove and disassemble the carb and give everthing a good cleaning and inspection.

This was sometimes a problem on my SWC, and no-doubt the CR trans, 3.08 axle, and high overlap cam can make can make it a handful to drive at "creep speed" being as how idle speed (900) in first gear is about 10 MPH. You tweak it as best you can then learn to drive around it when necessary. It's a resonant harmonic and one way to kill it is adding some damping, which can be done by dragging the brakes a little.

Duke

[Randy S.]

Re: C2 low speed cruise "jerkiness"

Duke,

I ordered the VC1802 today and will install after confirming spec

I have conflicting data for max centrifugal advance. My distributor is 1111153 300 HP M/T. The AMA specs say 30* @ 5100 (same as L79) but the column is labeled 396 325 HP L 35, an obvious error. Assume both header and data is incorrect

My 66 shop manual spec section says 26* @ 4100 . I assume I can map this centrifugal advance by disconnecting VAC can and check advance @ rpm with my dial back.

Thank you for the very detailed response.

Randy

[Duke W.]

Re: C2 low speed cruise "jerkiness"

The specs I have are from Corvette News Vol. 9 No. 2.

Yes, you can easily check the actual centrifugal curve with a dial back light. If 30@5100 is correct you will have to free rev the engine to 5100+ to verify.

Another way to check check total without having to rev so high is to get a spring kit and check total with light springs, and if light springs don't cause low rev detonation, leave them in.

Duke

[Wayne M.]

Delco Specs for 1111153

From 1964-68 Supplement DR-324S-2.

Centrifugal degrees and RPM (1/2 of engine deg and rpm):

500rpm = 0-3deg
750 = 6.5-8.5
2550 = 14-16 (this corresponds with your 5100rpm at 30 deg crank)
Vacuum can: 1115355: 5-7 inches Hg to start advance

Max adv. @ 11.2-12.7" Hg (for max. 8 distributor advance degrees)

Hope this helps

[Robert Willis]

Re: C2 low speed cruise "jerkiness"

Maybe, but, in the seventies I had a Chevy car do the same thing. Someone had just tuned it up and the new plugs were duds. The car I fixed idled ok and was good on the road but at about 15 mph is was a bucking bronc.

[Randy S.]

Re: Delco Specs for 1111153

Wayne and Duke,

Is it reasonable to expect both the 300HP(1111153) and 350HP have the same centrifugal advance 30 deg at 5100 rpm?

I never questioned this until Duke brought it up today and I noticd my shop manual spec differed from the AMA spec

Randy

[Duke W.]

Re: Delco Specs for 1111153

According to the CN specs both the '66 300 and 350 HP engines have the same centrifugal curves, but different VACs, which I expect is the only difference between the two single point distributors.

Duke

[David H.]

Re: C2 low speed cruise "jerkiness"

Mine did this also until I replaced the AC43s with AC45s