I have a 75 with a yearone 350 engine and a th400. I recently swapped my old carb that I desperately tried to rebuild with a street avenger 670. Before I ran it, I went ahead and swapped the jets with 68 front and 73 secondary per Lars' instructions. Also using the tall yellow secondary spring that came in the carb kit.

The carb runs way better than the q-jet that I had on there, but I am getting a pretty good bog off the line. I have already made sure that the acc arm lever is sitting on the cam and not the spring. Timing is at 34* all in around 2500 rpm.

I logged a few runs with my LM-2 and here is a screen shot of one run to about 75-80mph (all runs are very similiar). RPM does not read right - it is erratic and too high, but it shows what I was doing at least:


The car idles at 750 rpm and the A/F is about 10.7 - too rich??
When I nail it, the A/F jumps way up there to 17-20
Then it drops down and levels out around 9.7 for the rest of the run - too rich again?

Where do I need to start?

 

Start by getting the rich AFR corrected (smaller jets, probably). Worry about the lean spike last. It's possible it's actually so rich at that point it is misfiring which shows as lean on an AFR meter. You may find the bog goes away once the rich issue is corrected.

 

Thanks, I will swap the 65/68 combo back in and re test. Will that also help the idle AF ratio? Or is that independent?

 

You are too rich, set idle for ~13 and try it again. If it comes down to 10-11 put in smaller jets. What does the steady cruise at 2500RPM look like, use a little smoothing too.

 

sorry, this stuff is basically new to me...so I'm not sure which to do first.

Use the idle mixture screws first to get idle afr to 13? Then if WOT comes up to only 10-11, then drop jet sizes?
Or go ahead and drop jet sizes and it will bring the idle afr down?

Is the idle circuit seperate or does it affect WOT?

Smoothing?

 

Jets do not change idle AFR. But idle richness has a small impact on WOT.

 

Idle circuit plays a big role in cruise, if you get you idle and cruise to around 13:1 to 14:1 and your WOT drops down to 10 then you put in smaller jets. There is a big learning curve with a wideband but once you understand what is going on it is well worth the time and effort

 

From Edelbrock carb manual.

Despite the enormous variety in engine designs, virtually all (spark-ignition 4-Cycle) engines have very similar A/F Ratio requirements. For fully warmed-up engines, the range of A/F is:
A/F RATIO CHARACTERISTICS
5 RICH BURN LIMIT: Combustion is weak/erratic.
6-9 EXTREMELY RICH: Black smoke and low power.
10-11 VERY RICH: Some supercharged engines run in this range at full power as a
means of controlling detonation.
12-13 RICH: Best power A/F: Un-supercharged WOT.
14-15 CHEMICALLY IDEAL: At 14.6 the A/F is at the theoretical ideal ratio with no
excess fuel or oxygen after combustion. Good A/F for part
throttle cruise and light to moderate acceleration.
16-17 LEAN: Best economy A/F ratio. Borderline for part throttle
drivability (worse than borderline if EGR is used).
18-19 VERY LEAN: Usual lean limit (Driveability).
20-25 LEAN BURN LIMIT: Varies with engine and system.

Even though engines will run anywhere between 5 and 25 A/F, the usual target values for an unsupercharged engine are a fairly narrow range (Figure 1). A/F is about 12.5 for the WOT and 14.0- 15.5 at part-throttle cruise. An intermediate value of about 13.5-14.0 is usually used for mid-range power (non-WOT acceleration).

 

You are close in your last statement reading literature from a site that is now moving towards modern V8's unfortunately in my experience with the Innovate forum and the designer of the LM-1 himself a generation 1 small or big block like it a little more rich. Nature of the beast.

I've been tuning with the LM-1 for some years now and both my 406ci and 427ci have lean misses at anything over 14:1. Now maybe a stock 195HP L48 you might be able to run it at cruise a little higher. This is from what I have seen

 

Thanks for the help guys. I didn't have time to make any changes yesterday but I did get a AFR reading at cruise.

50mph at 2000 rpm was 12.9-13 AFR (also much cooler last night than the earlier tests). I will lean out the idle screws today and see what I can get.

 

Same experience as Motorhead here. My 454 will not run smooth at no load if it is any leaner than 13:1. The more load I put on it, the leaner it will run. Moderate acceleration it is fine at 15:1 (I still richen it to 12.5:1 at WOT, of course). It also doesn't like to idle leaner than 13:1. At 14:1 I can feel it miss @idle and it will hesitate when taking off.

BTW, I have EFI so it s easy for me to test all kinds of different AFRs. A bit more difficult with a carb as a change in one area will usually impact other areas as well.

 

Real quick question before I test...

I have an adjustable vac advance canister on there until I get the correct one for my vacuum. should I leave it disconnected while I test? I figure WOT will stay the same, but cruise and idle might change?

 

Vac advance needs to be hooked up, unless you plan to always drive with it not hooked up.

 

I plan to always use one, but I don't know exactly what this "adjustable" can is doing. Only thing I know is that it pulls ~18*. Just not sure at what vacuum it starts at.
I guess it will be consistant right now and then when I get the correct one, it should only take a slight tweak of the idle A/F ratio...if any.

 

I think you may need to do your test over a longer period of time/rpm range. From 75-80mph is only a split couple of seconds over a very short rpms range. Maybe find a longer stretch of road and do it in 3rd gear from about 2,000rpms till redline. This should give you a bigger picture of how it is doing in your entire usable rpm range.

 

I recommend a book called "Super Tuning and Modifying Holley Carburetors" written by Dave Emanuel. You don't even have to buy it, just go down to your local public library and borrow it. If they don't have it, they can get it. Once you really understand how a carburetor works, the tuning process becomes much, much easier.


Scott

 

Sorry. I meant from a stop to 75-80 mph.

I had some time to mess around with the AFR. Unfortunately, I left my SD card at my office so I couldn't log any runs. I will get it tomorrow.

But I did get my idle and cruise up in the 13.5 range with a quarter turn of the mixture screws. Still a small bog off the line, but it has improved.

 

Just out of curiousity, set your idle mixture screws using a vacuum gauge, and tell us what stoich you end up with. Record where the screws are right now, so you can go back if you need to.

I'm not saying any of these things is your problem, but it is a good idea to get these things squared away first to eliminate them as variables.

1.) Set float level.
2.) Set the accellerator pump up. Here's how you do it: With the engine off, take one hand and pull the throttle linkage to WFO. Take your second hand and push down on the lever on the bottom of the pump diaphram. Take your third hand and measure, with a feeler guage, if there is any clearance between that lever and the arm that rides the cam. Should be at least .015 to make sure it doesn't bind. Now let the throttle close, and make sure it isn't on the fast idle cam. from here, the slightest movement of the throttle should start to activate the accellerator pump circuit.
3.) If you haven't recently double checked your timing, do that, just to make sure. Again, eliminating variables here.

OK, there's gonna be more tasks that we will bark out at you, but that's about all I have for you for right now, get these three things cleared up and get back to us. If you have any problems performing them, or if you already have done them, let us know.

Just fyi, the idle circuit is pretty much completely separate from the main circuit. Not technically, but effectively. The idle circuit pulls fuel from the main fuel well, just like the main circuit, but from there, it passes thru the idle feed restriction, which act as a sort of a jet for the idle system. From there it goes to the transfer slot. On the way to the transfer slot, it branches off and goes past the idle mixture screw, and then to the idle discharge port. So, the idle is metered by the mixture screw, the transfer slot is metered by the idle feed restriction, and the demand for fuel is dictated by manifold vacuum.
As the throttle blades are opened, manifold vacuum drops, lessening the metering signal to the idle discharge port, and the transfer slot takes over. Once sufficient airflow is achieved in the venturi, the main system takes over, which gets it's metering signal from the venturi effect of air rushing thru the restriction of the venturi and the booster. Once you are on the main circuit, the idle circuit isn't really getting any metering signal, so it really isn't providing much fuel, if any, IF the system is operating properly.

Scott

 

Are there any markings on the can? If you can find out who made it, you can probably download directions for how to set it up the way you want it. Actually, they are probably all made by the same company, so they are probably all exzactly the same.


Scott

 

is the accelerator pump adjustable on that carb? you might try that too if it's still bogging. i know that helped with the last carbed mustang i had. i had to shorten the stroke to get a smaller shot of fuel.