Technical Timing with Dual Quads - HELP!!!!

Progressive carburetors from the factory won't have idle circuits in the secondary carburetor.

 

You have to much advance in dist put a bushing on advance pin to limit it to say 12 degrees leave your timing at 25
where it run and starts good the 25 plus 12 advance gives you 37 total and drive it like you stoled it

 

Actually the top surface at the outer cir***ference of a piston and the distance it is from the head at TDC is what normally decides quench.

Then the thickness of the head gasket (which is also involved in quench) along with the size and shape of the combustion chamber and the design of the top of the piston will determine the compression ratio.

So essentially you can set an engines compression ratio as low as is feasible for your needs and still have proper quench. Basically they are two independent things. (Dare I say that ????)
Take a 472/500 Cadillac as an example. They were made with compression ratios from 8.25 to 10.25. Some had smog pumps and some didn't. They had basically two heads........76cc and 120 cc. They also had different piston shapes. No matter the head or the piston, they all had good quench. Today, people use aluminum intakes and eliminate the smog stuff...and use gas thats inferior to yesteryears offerings but still manage to elude detonation. Here are some pictures of some pistons for Cadillacs. Notice that the whole head of the piston doesn't get lowered, just somewhere toward the center. Basically whether you zero deck the piston or put it slightly below the deck and the compressed thickness of the head gasket determine quench. If the piston is not zero decked but slightly below the deck, it will have some effect on the compression ratio but not much.

The outer part of each of these pistons is what will determine quench, not the actual compression ratio.
If you notice the 8.2 piston above, when used with a 120 cc head you get 8.2.........but if you put a 76 cc head on it I think the comp ratio becomes an unusable 13 or 14 to 1. But the quench basically stays the same.

 

Just pissin in the wind here.........some food for thought so to speak and don't know if it has any merit.

First, I would do the piston stop thing with a degree wheel and find tdc between the two stop points. Then I would get the timing set to something reasonable. Once you have that set.......

On a recent episode of Engine Masters where they do the dyno testing, they compared two carb set ups. One was vacuum and the other was mechanical. They found that while the vacuum worked well (for street use)after some tuning, the secondary opening caused a drop in vacuum and subsequent inability to draw fuel as well thru the venturies. They hypothesized that the engine is an air pump and only capable of drawing a certain quanity of air into itself. If they provided more area (carb throttle size) then that amount of air could be drawn into the engine with less vacuum .......and vacuum is what controls the fuel being drawn into the engine.

With the mechanical linkage and the dual squirters that were on the non vacuum carbs, they made quite a bit more hp (I'm thinking 20 or so). Since the engine was incapable of drawing additional air, the additional fuel provided by the squirters had to be making the power difference.

If they put bigger jets in the vacuum carbs they might make up the power, but they would then run rich when crusing. Now thats a quick synopsis as best I can remember it. Does it apply to your problem....I don't know.......but its something to think about. If you want to watch it and see what they really said, its called "Pick a Carb Shootout" Season 7 Episode 16

Have you tried to see how it runs on just one carb?

 

Well 348 with 3x2s didn't have idle on the outer carburetors and the 409 2x4s didn't have idle in the front carburetor.
I think Pontiac didn't have idle in the outer carburetors of their 3x2s

 

No, I haven't tried to run it on one carb, at least not yet. Right now I'm trying to isolate systems and eliminate variables. I don't want to start changing a load of stuff all at once. I wanted to start with some real basics, first by checking compression. I have that. Then I cleaned the plugs. Now I'm on to timing, which may be my issue.

The carbs are brand new Edelbrock AVS2s, the ones jetted for dual quad applications. They came out of the box and onto the engine. They're not used, converted, or remanufactured. So when it comes to a potential issue with either timing or fuel delivery, I'm going to play my percentages and ***ume it's something I screwed up versus Edelbrock.

 

I wouldn't say you screwed something up, you just need to find your engines sweet spot.

 

What I meant was that you could simply put a solid homemade gasket under one carb without changing anything else other than disconnecting the linkage. That way, if the engine did run well, you would figure that the addition of the second carb has a role in your problem and that your ignition is functioning satisfactorily. Its a simple test thats easy to do and undo..........before you get into trying to adjust and/or tune the carbs themselves. It might not change anything, and that kind of tells you something also. There could be a vacuum leak or a carb could have a bad float......lots of little things other than maybe too much carburetion..........but if you test it with just one carburetor, that at least indicates that it runs well when you know for sure it doesn't have too much.........or if it still runs badly, that carb has a problem or something else is wrong. Just a process of elimination and doing the simple things first. Good luck with figuring it out.

 

If the engine sounds lazy maybe advancing the cam will be the answer. It will help with cranking compression.

 

Don't ***ume that the new carb's are ok. I got 2 brand new Edelbrock AFB's for my car and all 4 floats were set different, float height and drop needed to be set. Not saying that is your problem, but should probably be checked.

 

Folks, again, can't thank you all enough for the help. We're definitely on the right track now, or at least, seemingly much better than we were before.

@gimpyshotrods identified at least one of the issues; the timing mark was absolutely incorrect. I did my best to get the engine to TDC and I stuck a piston stop in the plug hole. Sure enough, about 30 degrees off. Zero mark is approximately in the 12:00 position.

I pulled the timing tab off and put some new timing tape on the balancer. Obviously this isn't an exact science, but it's probably within 2-3 degrees of being right.

I pulled my stop out, ****oned everything back together again, and lit it off. It definitely needed more timing. After checking with my vacuum gauge and seeing how it wanted to idle in gear, it seemed like best initial timing was about 12 degrees. With the vacuum advance disconnected, at 3000 rpm, it's getting about 33 degrees of total timing. That's almost exactly what the specification is in the manual for total timing (32 is spec).

For the hell of it, I hooked the vacuum advance back up to see what my timing would be with that on. When I had the car out last and it was running poorly with detonation in the upper RPMs, the vacuum advance was connected. Well, my timing went to almost 50 degrees at 3000 RPM. So I disconnected that and capped the port on the carb.

So far, it seems to be OK just sitting there running. When I get home today I'll take it for a drive and see how it runs.

Another couple observations/musings;
-Someone brought up fuel pressure. The fuel system here is fairly simple. A Carter 4070 fuel pump with 5/16 lines, feeding a fuel block on the firewall and distributing to both carbs. I have a pressure gauge mounted to one of the ports on the distribution block. Pressure is 5-5.5 psi with the engine off, but running it fluctuates between 4.5 and 2. This seems low. It also makes me wonder if it's creating some sort of lean condition under load where I'm putting a load of air in the engine opening 8 barrels, but there isn't enough fuel. Sort of a secondary issue to the obvious timing issue.
-I haven't checked/touched the floats yet on the carb
-I also thought briefly about the possibly that the stock converter might be contributing to the idle problem, as if it's stall is too low and the engine has to fight against a tight converter to then smooth out the idle. I don't really think this is an issue, but just thinking out loud.

 

can you post pictures of the carburetors?
both sides, front and back

 

Remember vacuum advance is really only working in cruise conditions, when you floor it vacuum drops and you are back to centrifical advance. You might want to try driving it with the vacuum hooked up and see how it behaves, it will most likely get better milage that way.

 

I’d connect the vacuum back up. If it’s using venturi it will still be 12 initial. You had no load at 3000 in neutral; 50* is normal for that.

 

After your timing is properly set you better check the float settings as mentioned. And several other adjustments, the are listed in the carb manual. You are also going to have to tune them, there is NO jetting or metering rod combination out of the box that is spot on for your particular engine combination. You have to see what the engine wants and likes. You also meed to get your fuel PSI in check, 5-5.5.

 

see post 8

 

As per your request, sir.

Here is a link to the carbs that I used:
https://www.jegs.com/i/Edelbrock/350/1903/10002/-1

https://www.jegs.com/i/Edelbrock/350/1904/10002/-1

They are in fact AVS2 carbs, I ground the rivets off that hold the ugly front tag on and took that off. They look much better without them.

Plumbing is as follows:
-Front carb has all front vacuum ports capped.
-Front carb has 1/4 NPT rear port with a 90 degree fitting to a hose barb that is plumbed to the vacuum canister. This provides vacuum for power brakes.
-Rear carb has large front port capped.
-Rear carb has front port manifold vacuum plumbed to transmission lockup
-Rear carb had the vacuum advance plumbed to the timed vacuum port. This is currently capped (blue).
-Rear carb large rear vacuum port is plumbed to a right angle tube that is connected to a PVC valve installed in the valley of the intake underneath the distributor.
-All vacuum lines are secured by safety wire in lieu of a clunky hose clamp

 

I took the car out last night to dinner. It actually ran the best it's run in quite some time. Aside from running out of gas about a half a mile from my house (in my defense the gauge was still reading 1/4), it was a good trip. Perhaps that was contributing to the low fuel pressure.

That said, it's still doing that half throttle "slurp" like it was doing before. Starts easy, idles great. Has good part throttle pick up from a stop. But half throttle acceleration is mediocre and I get that slurping sound from the engine. Pull my foot out of it, and it goes away. Which is one of the things it was doing before.

So with the vacuum advance disconnected, I know I'm not getting more than 32 degrees of total timing. In my head, it shouldn't be detonating at that. So is it a timing issue, or a fuel issue?

 

I use this to check the cam timing or the timing mark. Remove Valve cover and turn engine and watch valves on #1 when the exhaust is closing and the intake is opening and they are both open the same amount you are very close to TDC say within 5 degrees . Close enough to see if the cam is one tooth off or of the timing mark is off.

 

I used a similar method when checking initial timing, since that's the only way I can be sure I'm on the compression stroke and not the exhaust stroke.

 

thanks,
I don't see any glaring issues there.
very clean install.
electric choke on rear primary carburetor,
no choke on front carb with steel vacuum plug visible (barely)
vacuum lines look to be hooked up "correctly"
unused vacuum ports capped.

how does it run without the air cleaners?
or better how does it perform with better flowing air cleaners?
those look cool but how well do they actually flow?
(how much do they obstruct flow)

of course fine tuning takes time and is a coordination of all parts,
sounds like you are on the right path.
I would continue with the ignition timing
then work on the carbs, and repeat until you see no improvement.
(you are obviously doing that now)

I have run dual Edelbrocks on a half dozen cars and it's always like that.
on my current setup I am running in sync, non progressing linkage on a pair 600s
they required changes to rods, springs, main and secondary jets and squirters.
I had always tuned with typical equipment and ear and an eye on the plugs.
this time I am using an air/fuel gauge, it has been a real game changer.

 

So to stop detonation is using better gas not a option ?

 

Sounds like you are the right track as mentioned. The only thing I will add is, Edelbrock does not recommend using the large rear port for PCV. If you examine those ports from the underside of the carb they are configured different from the large front port. May have no effect, I'm not sure in your situation.

 

Thanks man, I spent a lot of time trying to keep it real clean. I figured if I'm going to put a Chevy where the 394 used to be, I need to take special care to make it look right. I'm sure it's going to take some trial and error to get the right combo. Maybe a call to Edelbrock might be a good move to see if they have any suggestions on metering rod/spring combo with this configuration.

Also a fair point. I've been feeding this 87 octane since the compression is so low I figured it would run on anything flammable. But it doesn't hurt to throw a tank of 93 in to see if it makes a difference.

 

Read the tuning manual, if you don't have one you can down load it off the web site and use the info to help tune your engine. A AFR gauge installed will be of great help to you if you are not a savvy, knowledgeable tuner of carbs.

 

I installed a wideband A/F gauge to help tune my dual W-1 Carters, it was a life saver and made short work of it. I also made up a throttle position gauge so I could see just how much throttle I was using and when. It was simple cardboard gauge taped to the dash with slot for a sliding pointer, a spring to keep tension on it, and a string tied to the throttle pedal. I marked zero throttle and full throttle, then divided the rest into equal parts. To my surprise, most all my driving was with 20% or less throttle, so I concentrated my tuning at the point. W-1s use mechanical metering rods, so I was able to pick a set of rods that worked for the amount of throttle I was using.

With the throttle position information, you will be able to give more details about how it acts when asking for help. A vacuum gauge mounted where you can see it also helps tuning.

You won't regret purchasing a wideband gauge.

 

Just food for thought here. When dialing in the injectors on the Fly, we had gas fouled a few plugs. Dried them out, cleaned them and they never were the same again. If yours got really wet, throw a couple new ones in place of those and see what happens.

I am also suspect of the air cleaners. As cool as they are, I agree that you should pop them off and go for a drive. It easy and cheap troubleshooting.

 

Just to wrap this thread up, the issue has been resolved. Every time I think I have a carburetor issue, it winds up being an ignition or timing problem, and this is no exception.

As folks correctly pointed out here, the timing tab was wrong to the balancer mark and was giving me an improper reference point. The engine was set to TDC, balancer remarked, and timing properly set. Mark Spencer at Spencer's Speed Shop also recurved the distributor for a bit more initial advance, all in at 36 degrees. He also reset the floats in the carb, which were off from the factory. Lastly, I had the linkage on upside down, which still worked, but was throwing the linkage ratio off.

It was like a new car after that, and I was able to put several hundred miles without issue on it before sending it off for paint.

I'd like to thank everybody for immense help, and especially, in correctly identifying the ultimate issue.

 

Best advice so far.

Twisting your head off and closing one eye while standing on one foot...in the dark with one had sttttreched to the back of the engine twisting a distrib. and holding a timing light with the other...it's enough to make a guy switch to SBF's !!!
6sally6

 

Post a picture of the linkage so we can see the difference.