I still cant figure out what is going on with my car. While cruising at 2800 rpms (about 45 mph). I am getting a slight pulse or hesitation every so often. Not really consistent everytime. I just installed a new MSD distributor (replaced MSD HEI one) and a new Blaster Coil and upgraded from Performer to Performer RPM. I am not sure what is causing this. I switched from the heavy springs, to the light blue springs and then to the lightest springs in the distributor and it is still doing it. I pull roughly 15-17 inches of vacuum at idle and have a 6.5 powervalve. I do notice a slight whistling coming from the carb but off idle it goes away and it did this before my new manifold swap. Could it be a vacuum leak or maybe a bigger powervalve or something? I can tune a lot but this one has me stumped. I dont know what else to try.

Thanks, Erik

Back out your idle mixture screws one turn and see if it still does it. This assumes your carb goes richer if you back out the screws, some have reverse idle mix screws.
David

David has you on the right track. Also try going up a couple of sizes on the primaries and see what difference that makes. Sounds like you have a lean surge. My car actually runs its best track times jetted lean and it has a slight surge cruising @ 2000-2500.

I was thinking lean too. You were describing two problems before -- pinging and hesitation, the spring change suggestion was to help with the pinging.

Have you changed jets or power valves at all? I found that if the primary jets are on the lean side (not necessarily bad by itself) and the power valve is late (rated too low for the vacuum) there will be a hesitation when tipping in the throttle.

Making the idle mix screws richer is a good test technique -- if the stumble goes away then you would be confirming that the carb is going lean. For a permanent fix I would 1st make sure the right PV is installed, then maybe a step richer on the primary jets.

Hang in there . . .

I am running the car with the vacuum advance plugged. There seems to be toomuch valvetrain noise when the vacuum advnce is connected. And this MSD distributor is not adjustable on the vacuum advance. I will have to turn all timing down to get it to work alright. I just haven't messed with it yet. But last night I intsalled the lighter springs and took a test ride and it still hesitated very slightly. And it does it every so often, not consistent.

How will adjusting the idle mixture screws help my cruising at 2800 rpms? I thought those were just for idling. The power valve should be correct, it is a 6.5 powervalve and I will have to check my vacuum again but at idle it was 16-17 inches. I hope I can get this new car figured out. I have the Goodguys coming up and want to be ready for it, alot of cruisins here in Ohio as well.

Thanks, Erik

Part throttle lean surge. Normally it is caused by too much vacuum advance and too lean a low speed. Are you sure you don't have a vacuum leak somewhere? The whistling sound would sure indicate one.

I was thinking vacuum leak as well. But the car ran fine with the old manifold and the carb made the sound then. It must be inside the carb itself. It sounds like it is coming through the venturis or inside the carb. I did bump the timing up from 34 degrees to 38 degrees. Maybe this motor only likes 34 degrees or something lower like that. My last car liked 38 degrees.

i still dont understand how turning the idle mixture screws would affect cruising, they are just for idle. The guy at MSD said that it sounds like a lean surge to him as well. He suggested turning the timing down alittle or jetting up by one to two sizes. I will mess around tonight somemore and see what i come up with.

Thanks, Erik

the idle "circuit" influces drivability well in to the 2000+ rpm range. The reducing the size of the air bleed for the idle circuit will richen the top end of the idle circuit as the engine transitions into the main circuit.
Get an LM1 from innovatemotorsports. smile.gif

The idle mix screws affect the part throttle fuel curve too -- they aren't intended to use for adjusting part throttle mix but they do have an affect and may be helpful for tuning.

The idle screws will make a difference during cruse, it's an easy way to slightly richen the cruse mixture, although not by much. The idle mix will be too rich but you can put up with that for a test.

Making you idle air jet smaller will affect upper cruse ranges, if your air bleeds can be changed that will be the area you want to look at for a permanent fix.

Another quick fix to richen cruse is to raise the float level a little, primary side only.

If you haven't tried it, start the engine and turn in your idle speed screw to the rpm where you are having the problems and see if it does it just sitting in your driveway. If it does, then try turning the mixture screws richer at that speed setting or tip in the choke a little to richen the mixture and see if the engine smoothes out.
This will point the way to which way you need to go, rich or lean.
If your engine is running on the idle circuit, you need to tune that circuit, not the main jets. The main jets are working when you see fuel coming out of the booster nozzles.
David

Well, last night I turned the timing down to 35 degrees and took the car for a ride. It seemed to be running fine and no surging, but I can feel a lack of power as compared to the 38 degrees of timing. The seat of the pants difference was noticeable. I am still trying to figure the whistling sound coming from the carb. I know it is coming from the primary butterflys, when you crack them open alittle it goes away and when I turn them back to close them all the way it goes away. But it is loud and sounds like a mini blower or something. This motor generates 17inches of vacuum at idle, and you can definetly hear it sucking air through the butterflys and making the whistling sound. At idle should the butterflys be completely closed or is that up to the idle screw and where it is set at? I am slowly learning with this car, it completely different then my other one.

well remember at cruise, your probably not getting the extra 3 degrees of advance.. it probably ends up pulling them above 3000. If it is all in by 3000, you might want to slow it down a little.

But, the back butterflies should definately be used to help trim the drivability. Don't open them more than the front, but they should certainly be cracked. It helps keep the front transition slot small.

On a holley with a ported vacuum source (above the venturi), there is often a "whistle" as you begin to pull vacuum through the circuit. this usually goes away as you open the throttle blades further.Also, remember that when you turn down the total timing by adjusting the distributor you also lower the initial timing. This "richens" the motor slightly and may be enough to make the surge go away. Advance the intial/total timing and it leans out again. I would go up two jets on the primary and reset the timing where it made the most power.

Doc