Stuttering/surging

[ossieoz]

OK, i’ve put the 102hp engine into my Greenbrier and have had a few problems. Had a weird vacuum leak that is now solved :)

The only problem I have now is a stuttering/juddering/surging when I just cruise at low speed. I’ve near enough checked and changed everything and I’m pretty sure everything is balanced and correct. While accelerating it does seem to have a little miss but nothing consistent and it accelerates well... driving up hills go fine. On the motorway it goes well even at ‘high’ speeds.

I’ve tested another set of carbs, put in new spark plugs (AC44FF .035) and leads, dwell 33, timing set at 13 (even tested 16), checked distributor has correct weights/spring/cam and shaft not loose, double checked warm valve adjustments (1/3 turn in from zero lash), new coil, (tested with old coil again), idle speed set at just under 600 (even tried around 500). New condensor, new points... as said... I’ve swapped and checked everything but still have this quite severe surging/juddering at low to lowish speeds.

Am I missing out on some vital info on a 102hp engin... for example it ‘has to be 1 full turn’ from zero lash? Or any other info?

Or can i only assume that the heads have to come off so that i can test all valve springs are OK?

Or is there another possibility for my problem?

Thanks for any help/suggestions :)

Have a 1962 rampside with 110hp and 1964 monza cab also with 110hp that i’ve renovated and tuned that go very well, so i feel that I can quite a bit about setting up the dual carb Corvair engine.

The 102hp engine was bought on eBay last year already renovated, so I cannot or do not know if everything was done correctly.




1961 Greenbrier, 1962 Rampside and 1964 Monza cab.

[ossieoz]

P.S. have new universal joints installed


1961 Greenbrier, 1962 Rampside and 1964 Monza cab.

[Jerry Whitt]

An idea. Try advancing the spark more. Additional timing advance may give better performance. Let us know what happens

[64powerglide]

Float level set to high in the carbs??????

[ossieoz]

An idea. Try advancing the spark more. Additional timing advance may give better performance. Let us know what happens

So try 18 or 20?


1961 Greenbrier, 1962 Rampside and 1964 Monza cab.

[ossieoz]

Float level set to high in the carbs??????

Don’t think so... but will double check

I’ve tried two different sets if Wolf rebuilt/renovated carbs


1961 Greenbrier, 1962 Rampside and 1964 Monza cab.

[bbodie52]

It sounds like you've certainly thought this through and touched all the bases. It sounds like you are describing an unusual surging and uneven engine operation that only occurs at low engine speeds with minimal loading. Everything seems to smooth out and run better at higher speeds with more load on the engine! Trading carburetors has not appeared to help, as the problem remains even after the carburetors have been swapped. Uneven operation at low speeds would not seem to point to a fuel starvation problem, since high-speed operation draws more fuel, and the increasing demand seems to be fully satisfied! It also appears that you carefully checked for vacuum leaks throughout the system. So what comes into play only at low engine speeds under a light load?

Engine vibration may be more pronounced at low engine speeds, with a low frequency vibration possibly shaking engine components at a different rate at low speeds. Could increased vibration be introducing an intermittent ignition problem? Have you considered a possibly faulty connection in the wire lead between the ignition points and the negative terminal on the coil? This special wire is normally constantly flexing as the vacuum advance moves the breaker plate. Over the years the stranded wires inside the insulation can begin to break down. Clark's indicates that this wire is often frayed. Since the vacuum advance is more active at lower engine speeds, it would follow that the motion of the breaker plate is also more active at lower engine speeds. This might cause additional flexing of the ignition points wire lead, and if the connection is frayed the connection between the points and the ignition coil could develop an intermittent connection. An ohm meter might reveal a poor connection or a poor ground at the breaker plate that could be causing an intermittent ignition trigger at low speeds. The operation of the vacuum advance itself is also more critical at low engine speeds. At high RPM settings the centrifugal advance comes into play. But at low engine speeds it is the vacuum advance that is needed to introduce proper engine timing. I would recommend a careful examination of the distributor and the vacuum advance, and the connection to the ignition coil, to ensure that everything is properly grounded and that the connection between the points and the coil is solid and stable. It's only a theory, but you already seem to have addressed other major possibilities.

http://www.corvair.com/user-cgi/catalog ... IN&page=77

Part number C530: 60-69 DISTRIBUTOR POINT LEAD

Weight: 0 lbs 2 oz
Catalog Pages(s): 75,77(11«),77(19)
Price: $ 7.85

Part number C7825: HIGH PERFORMANCE POINT PLATE-62-69** ** THIS IS NOW A REBUILT UNIT **

NOTICE: Price includes core value: $20.00. You will be issued a refund of $20.00 when you return the used part.

Weight: 0 lbs 8 oz
Catalog Pages(s): 77(9)
Price: $ 54.20

[ossieoz]

An idea. Try advancing the spark more. Additional timing advance may give better performance. Let us know what happens

Tried this and the 102hp went like a ‘rocket’... probably advanced to at least 30/36 with some testing... is this possible to advance this much and still go well?

However the jolting is still there :(

I think there is some kind of miss fire as when I let the car coast with out putting my foot on the gas the jolting has a rhythm and progressively gets worse


1961 Greenbrier, 1962 Rampside and 1964 Monza cab.

[ossieoz]

It sounds like you've certainly thought this through and touched all the bases. It sounds like you are describing an unusual surging and uneven engine operation that only occurs at low engine speeds with minimal loading. Everything seems to smooth out and run better at higher speeds with more load on the engine! Trading carburetors has not appeared to help, as the problem remains even after the carburetors have been swapped. Uneven operation at low speeds would not seem to point to a fuel starvation problem, since high-speed operation draws more fuel, and the increasing demand seems to be fully satisfied! It also appears that you carefully checked for vacuum leaks throughout the system. So what comes into play only at low engine speeds under a light load?

Engine vibration may be more pronounced at low engine speeds, with a low frequency vibration possibly shaking engine components at a different rate at low speeds. Could increased vibration be introducing an intermittent ignition problem? Have you considered a possibly faulty connection in the wire lead between the ignition points and the negative terminal on the coil? This special wire is normally constantly flexing as the vacuum advance moves the breaker plate. Over the years the stranded wires inside the insulation can begin to break down. Clark's indicates that this wire is often frayed. Since the vacuum advance is more active at lower engine speeds, it would follow that the motion of the breaker plate is also more active at lower engine speeds. This might cause additional flexing of the ignition points wire lead, and if the connection is frayed the connection between the points and the ignition coil could develop an intermittent connection. An ohm meter might reveal a poor connection or a poor ground at the breaker plate that could be causing an intermittent ignition trigger at low speeds. The operation of the vacuum advance itself is also more critical at low engine speeds. At high RPM settings the centrifugal advance comes into play. But at low engine speeds it is the vacuum advance that is needed to introduce proper engine timing. I would recommend a careful examination of the distributor and the vacuum advance, and the connection to the ignition coil, to ensure that everything is properly grounded and that the connection between the points and the coil is solid and stable. It's only a theory, but you already seem to have addressed other major possibilities.

http://www.corvair.com/user-cgi/catalog ... IN&page=77

Distributor Wire.jpg

Part number C530: 60-69 DISTRIBUTOR POINT LEAD

Weight: 0 lbs 2 oz
Catalog Pages(s): 75,77(11«),77(19)
Price: $ 7.85

Part number C7825: HIGH PERFORMANCE POINT PLATE-62-69** ** THIS IS NOW A REBUILT UNIT **

NOTICE: Price includes core value: $20.00. You will be issued a refund of $20.00 when you return the used part.

Weight: 0 lbs 8 oz
Catalog Pages(s): 77(9)
Price: $ 54.20

Thanks for the tips but i have brand new distributor wire and have tested both old and brand new vacuum advances (bought from Clarks). Points plate is a renovated one. But I’ll try swapping to another one (yet again).

I do have 51 jets in my carbs and thought that might be causing the problem... but switched to the recommended 49’s for the 102hp and it didn’t help with the judder. All that happened was the 102 had less power.


1961 Greenbrier, 1962 Rampside and 1964 Monza cab.

[bbodie52]

As you seem to have just proven, the car needs more timing advance. But not just static initial advance, it needs timing advance at slow engine speeds (provided by vacuum advance) and then it transitions to more advance at high engine speeds (as the vacuum advance disengages due to low levels of vacuum at the carburetor vertical spark port at open throttle) with higher RPM — as the centrifugal advance takes over.

"ossieoz" Fri Jun 08, 2018 1:56 pm

Jerry Whitt wrote:
An idea. Try advancing the spark more. Additional timing advance may give better performance. Let us know what happens

Tried this and the 102hp went like a ‘rocket’... probably advanced to at least 30/36 with some testing... is this possible to advance this much and still go well?

However the jolting is still there :(

If the vacuum advance canister is faulty, the diaphragm may not be opening consistently and smoothly in response to carburetor spark port vacuum. This could be caused by a leaky vacuum hose, or a faulty vacuum advance diaphragm mechanism that could be intermittently advancing the timing and then dropping out. If this is happening you may still feel a jerky surge of power as the vacuum advance kicks in and then drops out cyclically even as you dialed in more static initial timing. Try disconnecting and plugging the vacuum advance hose, and see if the jolting disappears. If it does, check the hose for leaks or try replacing the vacuum advance unit.

Just to confirm, your vacuum advance is connected to the spark port (vertical) and not the choke vacuum break port (horizontal)?

[Jerry Whitt]

As the advance in timing did help, an immediate thought comes to the Harmonic Balancer. Many of the older balancers have a rubber ring that deteriorates. This allows the timing mark to actually move.

If this were to be the case, could cause "jolting" or other vibrations.

[64powerglide]

Is your dwell tach needle bouncing at idle or is it steady?? If it's moving does it go up then down or drop then go back up?

[terribleted]

Hmmm 102 should not have a balancer on it. What about improperly clocked distributor?

[64powerglide]

102 HP will lose high speed power if the timing is set over 18 advanced.

[ossieoz]

As the advance in timing did help, an immediate thought comes to the Harmonic Balancer. Many of the older balancers have a rubber ring that deteriorates. This allows the timing mark to actually move.

If this were to be the case, could cause "jolting" or other vibrations.

I have no harmonic balancer on the 102hp and I believe none is required (?).


1961 Greenbrier, 1962 Rampside and 1964 Monza cab.

[ossieoz]

Is your dwell tach needle bouncing at idle or is it steady?? If it's moving does it go up then down or drop then go back up?

I have a digital Actron dwell meter... fluctuates a little at idle from 31.5 to 31.8

At 1500 rpm it goes down slightly to 32.1/32.2


1961 Greenbrier, 1962 Rampside and 1964 Monza cab.