Ignition question: Ronnie: chime in please

[Rockfish39]

Im almost embarrased to ask this, but it has been on my mind for some time.

409 with a 340HP intake and a correct 7025124 4GC carb. Initial advance is set to 12 adv. Starts up and idles just fine, now here is the problem. The vacumme advance can in the distributor is tied to the only vacumme port on the carb, which is manifold vacumme. The 7025124 does not have a venturi vac port on it.
The instant you take that carb off idle, BOGG!, will even stall out sometimes :mad:

Now, I suspect that there are two primary contributors to this.
1. insufficient pump shop from the accelerator pump on that 4G carb
2. the momentary, but immediate loss of vacumme advance the instant you take it off idle.

I had an argument a while back with Dave Ray who insisted that factory 2*4 409 ignitions are setup in this same way. Though, the only way that I ever got any performance out of my 2*4 setup was to connect the distributor can to a venturi (spark advance) port on my rear AFB. This way, works great. smooth idle, and launches when I mash the pedal.

Back to the 4GC problem.

Now, one "work around" was to not tie the distributor to a manifold vacumme source and ramp up the inital timing to 18. Wont stall out, but wont launch either.


So my question is: Setup in a totally stock configuration, what is wrong??? Surely it didnt bogg like that when it was factory new.

:dunno Rock

[Ronnie Russell]

Lou, First, let's assume this is not a Roch. 4GC that you modified for E85. I remember you were experimenting with that in the past. Your knowledge of the 4GC is far superior to mine so I would start with basics. I would substitue AFB to determine if the problem was fuel delivery related. I know that sounds like a lot of work, but it would determine if the carb has a major problem inside. Would be happy to ship you one for the test. Try it , then ship it back. It sounds like the problem is simply too little initial shot or too much initial shot. Too much would show black smoke which you would have probably noticed, so it sounds like a simple accelerator pump defect, but I know you have already checked that. The 340 should run great, even with only 10 dg. intital. There is a remote chance it could be ignition realted, but to know for sure it would take a known healty carb to rule out fuel delivery problem. Point dwell being way off could also cause that problem,,, but I know your tuning ability, so I have to keep going back to the 4 GC having a problem. Wished I could be of more help, good luck , hope you find the problem.

[models916]

Are you setting the initial timing with the vacuum disconected? Should be. I set mine with a timing tape at 36 all in at 2500. Takes the distributor out of any problems. You can go on to solving the problems.

[Rockfish39]

Hey Ronnie and Models!
Thanks for your input... Just a little more background information. Not running E-85, not yet anyways... Not available in Florida at the present time.

Still using 93 octane gasoline.

I set my initial timing at 12 adv, with the vacumme can disconnected, of course.

When I start pulling up the RRRs I clearly can see the mechanical advance moving my timing up accordingly. So far so good... The counter weights and springs are doing their job.

Total curve is limited to 32deg (via a stop inside the distributor), all in by 2600 rpm
So that box is checked.

OK, so now the engine is idling at 800, vacumme can is disconnected, ignition timing is set to 12 deg adv. Ready for the next step

Connect the vac can and ignition advance jumps straight up into the mid 20s and the idle runs right up to about 1200. :eek: OK, I kinda expected that too because by tying manifold vacumme to the distributor, the can pulls full vacumme advance at idle.

The trouble begins the INSTANT you go off idle. With ignition advance now hovering around the mid 20's, touching the throttle plates drops manifold vacumme and goodbye vacumme advanced ignition timing. Naturally, it takes time (in milli-seconds) for the distributor to spin up and for the mechanical advance to overcome the sudden loss of ignition timing, but by then it's too late BOGG CITY.

So, how is this orginal setup supposed to work without bogging ????

[jr.W]

Rockfish my thinking is it would be too little pump shot also. I had that problem
big time with a dp Holley going into the back half of the carb when I was younger.


jr.W

[fatride]

Rockfish, I had the same exact setup, timing wise as you are describing and had a bogg big enough that I thought the bumper would hit the ground! No bogg after tuning the carb. Right now I have the timing set at 21 dgs initial with 38 dgs all in at 2800 rpm. I have an adjustable vacuum advance unit hooked to manifold vacuum that will not pull advance until manifold vacuum hits 15". At cruise with over 20" of vacuum, engine runs with 48/50 dgs timing.

[Ronnie Russell]

Lou, Since your timing is all in at 2,600, you obviously have weaker dist. springs installed ( that is a good thing ) . If you were to set initial at 16 or 18 and leave vacuum control disconnected and plugged, the engine should run great and accelerate fine without a vacuum control. Mecanical advance range is 22 to 24degrees . Stock situation-- 10 dg + 22 to 24 = 32 to 34 dg. Add v.c. = 48 to 50. By using 18 initial + 22 to 24 mech. = 40 to 42 total without v.c. Car will run great that way IF there is not another problem. That is not the set-up you want but my point is I think you have other problems other than vacuum control. A big vacuum leak in the system would be a suggestion, but I know you have already searched for that and it would be sorta obvious. Sorry, the only thing I can come up with is the lack of initial shot of fuel.

[LongIsland63SS409]

Rockfish,

I have the same setup on my car, 340 HP 4GC.

I have the same bog as you describe off idle but not as bad.

I have not had a chance to diagnose the problem but I believe it is the carb pump circuit.

Let us know what you find out.

Mike

[models916]

36-38 seems to be the total timing for these engines. With better heads you could turn it down a little, but the stock GM iron stuff we run needs a lot of lead. Mine runs hot and flat at less than 36.

[63z11]

I too had the same problem with the 340 motor, ENGINE rather I just pulled from my car. The problem a least in my case was the acc. pump. Bigger problem was only 1 or 2 people sell the correct 1 for an 09 or corvette carb. The one on mine was only a year or so old and already didnt pump.:bang

[LongIsland63SS409]

63Z11,

"Bigger problem was only 1 or 2 people sell the correct 1 for an 09 or corvette carb"

Are they still available?

Mike

[Rockfish39]

Right now I have the timing set at 21 dgs initial with 38 dgs all in at 2800 rpm. I have an adjustable vacuum advance unit hooked to manifold vacuum that will not pull advance until manifold vacuum hits 15". At cruise with over 20" of vacuum, engine runs with 48/50 dgs timing.


The next time that I set this up (again) Im going to trying tuning to these specifications. The inital seems a bit aggressive to me, but as long as I have no hot start issues with that much advance, it would sure beat the heck of that terrible bogg.


Thanks for all the imput!!!!

Rock :cool:

[Phil Reed]

LOU..................what's with the FLA address???????????????:dunno:dunno:dunno Have you been thrown out of Virginia or just go voluntarily????

Can you still talk inot your watch!!????:rofl:rofl:rofl

[Rockfish39]

I think Tommy Nolen has an inside track with the Governors Office or something...

:D