65 365 really rough idle

[Bob H.]

My car sat for some time but always had a mix of ethanol free and race gas in it. The rear float valve was leaking so I replaced both (very clean in the bowls) changed the power valve per specs and the accel pump

It is very rough at idle 800 and varies 200 rpm high and low. Cruising the highway at low speed it like riding a bucking horse. Give it a little gas or floor it and it runs like a 365 should.
Initial timing is set to 10 and vacuum advance brings it to about 25 degrees. Checked vacuum hoses, checked for vacuum leaks on the intake and carb using brake fluid and all ok. new points rotor and cap. choke ok
Do I just have a carb issue or could it be something else?

[Duke W.]

Re: 65 365 really rough idle

The 327/365 has relatively poor low speed tractability because of the extremely high overlap of the 30-30 cam. Normal idle behavior is 10" @ 900, and getting it going from a dead stop with minimal clutch slippage usually results in at least a slight stumble.

You should verify that the VAC is either an OE 236 16 or modern B28 replacement and starts at 4" and all in at 8". Setting initial timing on these engines is very difficult because the centrifugal starts at 700, and it is virtually impossible to get 30-30 cam engines to idle that low long enough to set. So use the total WOT method. The OE centrifugal is all in at 2350 engine revs, so set it with a dial back timing light at 38-39 between 2500 and 3000. Of course this assumes that the centrifugal has not been modified, so first test to determine the actual revs that centrifugal is all in then set total WOT advance a few hundred revs about this speed.

Thus set with a properly functioning VAC, total idle advance at 900 should be in the low thirties.

If you detect any detonation at 38-39 back it off in two degree increments until the detonation goes away.

Once the above is done, go through the idle speed/mixture setting procedure stating out with the idle mixture screws out 1.5 turns from the seat with a target final idle speed of 900 and it will have some "lope" varying +/- 50 to 100 revs from nominal, but it should be stable.

Further information is in my tuning seminar... Web search duke Williams san diego corvette tuning

Duke

[Bob H.]

Re: 65 365 really rough idle

Thanks. so it sounds like when getting 15 degress when connecting the VAC is an issue. When I first checked initial timing it appeared set after top dead center which I thought was my problem. Now I wonder if it was set there to compensate for the VAC adding the 15 degrees.. Like maybe someone set the timing with the VAC attached. Oops I'm confusing myself

[Patrick H.]

Re: 65 365 really rough idle

Further information is in my tuning seminar... Web search duke Williams san diego corvette tuning

Duke

Or, check on this site in the thread above:

https://www.forums.ncrs.org/attachme...1&d=1499167975

[Carl N.]

Re: 65 365 really rough idle

Might want to check valve lash and valve springs, also make sure a plug wire is not lose and coil wire is not shorting out

[Bob H.]

Re: 65 365 really rough idle

will report back. thanks

[Duke W.]

Re: 65 365 really rough idle

Thanks. so it sounds like when getting 15 degress when connecting the VAC is an issue. When I first checked initial timing it appeared set after top dead center which I thought was my problem. Now I wonder if it was set there to compensate for the VAC adding the 15 degrees.. Like maybe someone set the timing with the VAC attached. Oops I'm confusing myself

No, it sounds like your VAC is okay. Nominal max advance is 16, but 15 is within production tolerance. You should still verify the ID number and if functionally correct, check the start and max points, which should be about 4 and 8", respectively.

The VAC signal line should ALWAYS be disconnected and plugged when setting initial or total WOT advance, but there are still many that don't understand this, and setting initial at 10 with a 16 degree VAC connected means that actual initial is 6 deg. ATDC, which will result in poor performance, high fuel consumption, and likely overheating.

Duke

[Timothy B.]

Re: 65 365 really rough idle

How many turnes out are the primary A/F emulsion screws from seated. If the tune is good then it sounds like a lean surge, it's possible there is a slight vacuum leak in the throttle body to manifold or main body/throttle body that reveals itself during high vacuum conditions. It may be time to have the carburetor overhauled, surfaces trued and throttle body bushings installed.

Have you tried driving the car without the vacuum advance connected to see if retarded timing helps.

[Bob H.]

Re: 65 365 really rough idle

Tim. Disconnect the vacuum and it does run better. Is vacuum near zero at idle on the metering block port? A friend checked it and says it over 12 and that is the problem. If it is supposed to be near zero, what is causing it . is this a leak in the carb

[Duke W.]

Re: 65 365 really rough idle

Idle vacuum is near zero at WOT. At idle your engine should pull about 10" @ 900, and a 300 HP engine with the low overlap OE cam is 18-19" @ 500. Free rev both to about 2000 and they should be pull 20-22". Vacuum is an inverse function of load. The higher the vacuum the less the load and vice versa.

Most tutorials on engine vacuum will say that only 10" at idle is too little and is an indication of a leak, like a burned valve. However this is not true for a high overlap SHP cams. The overlap itself is a "leak" at low speed and light load, so idle vacuum is low relative to conventional passenger car engines.

If you have an OE or OE equivalent carb the port for the VAC signal line should read full manifold vacuum.

It appears that you and your friend don't understand manifold vacuum behavior or the concept of a "spark advance map", particularly as it applies to the 365 HP engine. You need to study the tuning seminar in the link posted by Patrick and then reread post #2 before you do any more work on the car.

The vacuum characteristics of the 365 HP engine are well understood as is the optimum spark advance map, and the tuning seminar provides recommendations for all OE engine configurations. The first step is to verify that the spark advance map is optimized with the proper, functional VAC, centrifugal advance, and total WOT advance. Then perform the idle speed/mixture adjustment procedure that is outlined in the 1963 Corvette Shop Manual and '65 Supplement.

If this doesn't improve engine behavior then there may be another problem.

Duke

[Bob H.]

Re: 65 365 really rough idle

Hi Duke. Saying I don't understand the operation is actually an understatement. Since the carb hasn't been touched in 20 years, I sent it off today to be rebuilt.

When I got the car,

I blocked the ported output and checked initial timing which was set after TDC. Connecting the vacuum, then timing was at about 14 degrees. So to me it looked like someone set the initial timing to compensate.

Adjusting to

10 degrees initial and the dist connected to the ported vacuum off the metering valve, at idle I would have 25 degrees.

Disconnecting and blocking the port and it runs better.

After looking at your article and reading John Hinckley's article on manifold vs ported, isn't the ported at near zero at idle? If so, something was wrong in the carb.

So next I checked the vac number and it is a B3! What the hell is that? I will order a B28 and have it installed by the time I have the carb back

[Duke W.]

Re: 65 365 really rough idle

The OE 365 HP Holley is configured for full time vacuum advance. If you don't see manifold vacuum at idle on the VAC signal line then you might have an incorrect carb or someone plugged the VAC signal line. I've seen this before. "Ported" vacuum advance does not yield manifold vacuum on the VAC signal line at idle because the vacuum port is above the throttle plate in the carb. Ported vacuum advance was generally used on emission controlled engines, because with only initial timing, idle advance was well below optimum, and this increased EGT to help oxidize HC and CO in the upper exhaust system with injected air.

The 8" B28 (Airtex 1053V) is the correct VAC to use on the 365/375 HP engines assuming the cam is OE equivalent or something close. Install that and before hooking it up set total WOT advance at about 39 between 2500 and 3000, assuming the OE centrifugal is intact, which means it should be all in by 2350. Verify by testing before you set total WOT advance. Off hand I don't know specs for the B3, but I know it's not functionally correct for you engine.

Once the above is done go through the idle speed/mixture adjustment procedure starting with the emulsion screws 1.5 turn out from the seat.

I've recently set up three 365/375 HP engines as above (the 375 HP FI VAC signal line was plugged with a BB that I removed), and they have very good performance characteristics and reasonably good low speed tractability, better than I expected with such a high overlap cam.

Duke

[Timothy B.]

Re: 65 365 really rough idle

Bob,

One other think I thought about that could cause the lean surge, did you check that the vacuum advance control diaphragm holds vacuum. A leak here could cause the condition you describe. I suspect the incorrect vacuum advance control did not help your surge condition, it's possible the thing is seeking up/down causing a surge.

The vacuum advance connection on the 2818 Holley metering block is full manifold vacuum at idle, not ported like the exact same connection used on later Holley carburetors. Your mechanic may be used to seeing ported at this connection.

[Duke W.]

Re: 65 365 really rough idle

I checked the B3 on Lars' spec sheet... start @ 10-11", 23 crank deg. @ 19-21". Whether the installed B3 leaks or not is irrelevant. It's a boat anchor either way, just like a B1. In addition to having WAY TOO MUCH TOTAL ADVANCE, it doesn't even start until typical 365 HP 10" @ 900 idle vacuum, so it's not adding any advance at idle and then provides WAY TO MUCH total advance at light load. No wonder this engine doesn't run right, either at idle or normal light load driving.

Whoever installed it is a MORON!

I think at least half of the "engine doesn't run right" or "overheating" problems I get involved in are caused by a DEAD or functionally incorrect VAC. This simple, ten dollar part is very critical to proper engine operation, but most guys have no idea what it does, why it's important, and how to properly select a functionally correct one using the Two-Inch Rule; and there are only three, 8" (B28), 12" (B26), and 15" (B22) that are the best functional fit for any full time vacuum advance engine config. or one converted from ported to full time depending on transmission type - auto or manual. All the others are boat anchors.

Read my tuning seminar. A link is included in the last "sticky" thread by Richard Mozzetta.

Duke

[James W.]

Re: 65 365 really rough idle

Bob,




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