Strange timing issue

[Some Car Guy]

So just for kick, I went from 66s to 67s and the stumble is pretty much gone once warmed up. However the “response” feel seems to be a tad less although smoother. I’m thinking this might be because of it being a bit too lean and then the mains come in? Don’t know. I was thinking of bumping up the timing so I get about 35° full mechanical advance. But that would put me at about 16° initial, which I’m not sure if that is bad or not. Any thoughts? Also… For those of you who are running stock 383 HP engines with manifolds, is your jetting roughly the same?

Thanks,
Mikey

I doubt you’d have a problem with 16, just try and see. As long as the total doesn’t get over what the engine will tolerate on a given fuel, you’ll be fine. I found my factory assembled 70 383 hp didn’t make any audible pings at 38, but wasn’t smooth. It was choppy feeling. I pulled it back a little bit at a time and settled on 34 total. Felt free and fast with that, no more choppy feeling. I suspect that might’ve been some sort of “silent” knock I was feeling.

I don’t remember the jetting.

 

[Some Car Guy]

One of the 'problems'.....with this sort of problem......is that folks do not make a big enough change. If you suspect leanness, go up 2-4 jet sizes, not one. Making a big change will point you in the right direction & THEN you fine tune from there.....

Try this simple 5 min test: engine idling [ in gear if auto ] slowly adv the dist to see if idle rpm increases/gets smoother. Keep going until you have the highest rpm. Lock the dist & test drive. I am betting the stumble will be gone. Keep rpm below 3000 because you are only testing for tip in response. Report back.

You really have to be careful with timing. I’ve explained this to you on here before, not sure why you are going down the road of engine damage yet again.

Typically it will be all in by like 2200. What you are laying out there could cause 25 degrees(or more????) at idle, followed by 16-26 or who the hell knows how much advance. So most distributors will have him at 40 to maybe 50 plus degrees, like you cause with that one guy that heard the bad noises with 52 degrees.

It’s just not something to be trifled with for any length of time. The two things that absolutely kill gasoline engines are over revving and pinging.

To attempt such, you really need to recurve the distributor to have basically no advance and then use the initial as you stated.

 

[Geoff 2]

SCG,
Re-read what I said....which was to limit rpm to 3000 to prevent pinging. Your nonsense in post #22 shows you have little experience in engine tuning.
25* at idle [ or more ] at idle. What ignorance you display. How about 50* at idle?? [ See below ]. Ever heard of locked dists that give 35* [ or more at idle?????????? ]
Once the reqd idle timing is determined, the dist is THEN re-curved.
GM cars in the 1960s used vac adv connected to man vacuum. That added timing at idle. There were cars that had 10.75:1 CR & very mild cams.
When my GTO left the showroom, it idled at 26*. 6 init + 20* added by vac adv. Chevs were generally about 24*.

Educate yourself by reading the links below which are about the IMPROVEMENTS gained from more idle timing.

 

[Some Car Guy]

Yet again, Geoff displays a lack of understanding and a willingness to cause damage to an engine. Pretty easy to see what I wrote and how timing advances into a dangerous range quickly. Thinking that an engine with no load on it at idle is the same as one pulling a car down the road is demonstrating his inability to comprehend the issue.

SMH

 

[Dave6T4]

My car used to have a stumble/surge right at cruising speed. 440 with Edelbrock 750 cfm., MP electronic distributor. I had overall timing set at 32 degrees, v.a. disconnected @ 3000 rpm. At cruise, with v.a. connected, I figured I had too much total advance. I was able to tune the surge out by playing with the advance stop in the distributor canister.

 

[MikeyM]

Thanks guys.

Geoff - copy, will do. I think I have a little more room to go as far as mechanical advance but it will likely end up about 16* initial. Any issues with that? I think the previous owner had an advance limiting plate installed because the timing is at 14° initial currently and I cannot get it to go past 31/32, mechanically, at 3000. Have to run some errands so I will get It warmed up and try it when I get back.

Car guy, thank you. I recall you posting up that information before and that is what I used to help initially time the engine to where it is now. It’s just that having no real history on this motor, I’m just trying to be very careful as I proceed forward. Apples and oranges here but my VW bus with a type for air cooled engine calls for an initial timing of 7°, it is also set at 16° with the mechanical advanced limited. Total advance on that is 32° but on those air colds, you don’t want to go any higher. I know the same applies here, I guess it’s just a matter of not overdoing it in combination with the correct jetting.

Thanks again. Really appreciate all the input.
Mikey.

 

[MikeyM]

So here's a little update. After checking and readjusting timing, it settled in at 16* mechanical (VA disco'd) and at that setting I could not get it to go past 32* total mechanical advance when going up to 2500+ RPM. The engine is much more responsive, pinging, idles better and stronger and the RPMs don't drop as much when put into gear. I think I'm noticing a bit less throttle pressure to maintain the same speed at cruise (makes sense) and the stumble is almost gone and at this point it's improved from a true stumble to a barely present hesitation. In fact, once the car warms up, I can barely notice it but I don't think anyone else would be able to if they didn't know. NOW, once warm, there is no "stumble" but I can feel it a bit during transition. Not that I do this often but if I mash my foot to the floor from a dead stop the tires break free with no stumble.

I changed from the orange cam to the yellow cam (cuz I lost my blue cam) and that seemed to help a tad. The idle mixture screws are about 3/4 - 4/5 of a turn out, adjusted in gear with a vacuum gauge and vacuum is between 13-14 in gear, 15ish + in neutral. Power valve is a 6.5. I spent some time really dialing in the idle mixture in gear, repeatedly going from side to side and I think that's a big part of the drivability. I can live with how it is, although it's not perfect, because it's completely drivable and has good manners, once completely warm. Until then you just have to be easy on the throttle.

So... now my power valve question. I have read that PVs add 6-8 jet sizes more once opened based on the vacuum. But add to what? The fuel bowl? The discharge into the venturis? I also read that the correct "non Holley" way to select a PV is to go 2 numbers less than vacuum at cruise. If that is the case, my cruise vac is about 15-16" around 2500 RPM so I guess that would land me at 12.5. But can anyone explain why or if I would need to do that at all? Would it help off-idle at all? Is it supposed to? Or is it supposed to add fuel under heavy load at higher RPMs?

OH.. almost forgot. I'm seeing all kinds of differing graphs regarding the Holley accelerator pump cams.. One says blue comes in the fastest, another says yellow, another says brown. Can someone tell me which pump cam(s) come in sooner than orange? From what I can find, blue should be perfect.

Sorry for all the questions but I feel I'm almost there.

Thanks for the continued input.

Mikey