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Discussion Starter · #1 ·
New 355 with new Victor Jr heads, Victor Jr Intake, Comp Camps XE-294 HR,
Edelbrock 800 carb. MSD pro billet with 6AL ignition. Everything is brand new with only a mile or two from test driving it around the block. Timing is currently set at 18 initial and 38 total. Plugs look good, a tad on the rich side.
If you hit the throttle fast it completely stumbles or hesitates, you have to roll the throttle on gently and then as the rpm's pick up it runs perfect, It idles perfectly also. This occurs with the car sitting out of gear also, so i would rule out any converter/tranny issues.
Here is what I've tried so far: Checked for vacuum leaks, Checked timing over and over and over again trying different variations. Tried a Holley 750 dp, Tried an edelbrock 750. I've tried all three settings on the acc pump and i've tried some different step up springs.
The vacuum guage shows good vacuum and I've set the idle mixture screws using the vacuum guage.
I degreed the cam when I installed it also.
Any ideas on what else I should try? Thanks in advance.
 

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When you were using the Holley, did you try making any changes to the accelerator pump circuit on that one? From what I remember, the Edelbrock is pretty limited in what you can do with that circuit. Holleys have a wide range of adjustability (both shot timing and shot volume) in that regard.
 

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Discussion Starter · #3 ·
no i didnt make any changes to the holley as it was borrowed. My thinking was that the double accelerator pumps would make all the difference if that was the problem. Maybe a bad assumption though.
Is that what you're thinking though, that it needs a bigger shot off the get go?
 

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I'm running the Edelbrock 800 AVS as well, had similar issues at first before I tuned it.

Here's a thread I did a while back that may be helpful:

http://www.stevesnovasite.com/forums/showthread.php?t=201545

I had to do four things to correct the off idle hesitation:

1.) Install new squirters (comes stock with .035" you want the bigger .043" squirters)
2.) Move accelerator pump arm to bottom hole to allow furthest travel and longest pump shot
3.) Adjust float level to 7/16" or a little higher (3/8" may work) and float drop to 1"
4.) Set idle mixture to highest vacuum using vacuum gauge. Turn screws to achieve highest vacuum reading. This will get you very close and prevent over rich or lean conditions at idle.

You will also want to experiment with the step up springs. They are easy to swap out as they are located at the top of the carb near the choke horn. I found using the strongest ones you can without them bobbing up and down at idle in gear is the best way to tune those. You want them as strong as possible without always staying up so that as soon as you punch it, they move up out of the jets and let more fuel in. This affect part throttle hesitations more than off idle, but it might help you as well.

You're already running a lot of initial timing which is good. Probably cant go much more than that. Not sure what the duration is on that cam but if its really big you might be able to run 20 degrees initial and then limit your mechanical to 14-18 degrees so you're not going too high on your total.

MSD's biggest bushing is the black 18 degree bushing. So you could run 20 initial and 18 mechanical for 38 total. If you need larger bushings than that then you wouuld have to look at the FBO bushings that they make (10 and 14 degree bushings).

Doing those things above should help you out unless there is some other issue going on with your car. Sounds like it just wants more fuel just off idle when you punch it. When I swapped out the .035" shooters to the .043" shooters it made quite a bit of difference.

My car no longer has any hesitation off idle except when its first warming up since the engine is cold. Once its fully warmed up it runs great.

I have a 383 w/ 9.5 comp 234/238 dur @ .050" 112 LSA and run 14 initial and 18 mechanical for 32 total.

Good luck!
 

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What do you have for compression? It shouldn't buck the starter at 18-20 especially with that cam. I guess I may just be used to having a mini starter though. That cam should lower the dynamic compression a little. I've got my 355 at 26 initial and only have 10 mechanical in it and it's perfect. I also had a stumble. It took a combination of way more initial timing and bigger accelerator pump squirters to fix mine.
Forgot to mention you can get custom bushings for the MSD from a place called 4secondsflat. That's where I got mine. I bought a 10 and a 14 bushing so I could experiment.
 

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Do you know what your vacuum readings are in park and in gear at idle?

With some timing adjustments and carb tuning I was able to get up to 11" in gear (800 RPM) and around 16" in park (1000 RPM). This ended up being about 1.5 turns out for each idle mixture screw.

Edelbrock carbs don't work as well on low vacuum motors as Holleys do seems like from what I've seen. Works well on my engine though.
 

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Discussion Starter · #10 ·
Thanks guys, I have a feeling that maybe something a little different in the timing department like what Wad is saying might be the ticket. I will get some vacuum readings in gear tonight and post them. Right now while its in park i get 17-20 inches
 

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Discussion Starter · #11 ·
thanks for the replies

Thanks Cdahl, I've already done the floats, Acc pump adjustments and the IMS screws. But I will def try the bigger squirters.
The total duration on the cam is 294 242 and 248 at .050
 

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Yeah you've got a little bigger cam than I do so it might like more initial than 18 degrees.

The bigger squirters should definitely help you, but probably wont completely solve your issue. I bet if you bump up your initial timing, limit your total timing, and use the bigger squirters it will get you really close though!

Wadd2 just inspired me to order up a set of those FBO bushings from Don. He threw in a tuning guide for not much more as well. I'm going to try bumping up my initial timing as well this year.

I knew as soon as I jumped back on this forum after winter I'd start spending some money again haha!
 

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After reading, I noticed the OP says he has the XE294HR. I believe we have the same cam. In fact I'm sure we do. Mine is the comp. cams XR294HR and it has .242/.248 at .050 duration. I have 10.5:1 or so compression and AFR 195 heads. My question is are you sure your vacuum readings are right? The most I can get out of mine for vacuum is 7 or 8 in park! Run's like a scalded dog though, lol. Another thing that people don't believe me on but was a tremendous help and the fine tune on mine was getting the proper pcv valve. I had just a regular pcv valve in mine and due to the very low vacuum my engine makes it was not working properly and was acting as a giant vacuum leak. I put one in for a 427 corvette motor and it runs like a different engine. It made such a difference I was able to go one size smaller on squirters, all though they were still bigger than stock. Also changed idle, everything. My suggestion is make sure you have a proper pcv valve, then advance initial timing, try larger squirter in that order. Keep in mind I'm using a Holley style carb. though which is a much different animal than an Eddy
 

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Interesting point about the PCV. Never thought about that.

Seems like there are a lot of threads out there where guys are running pretty big cams without enough initial timing and it causes a lot of tuning issues. I've been trying to figure out what the best initial timing setting would be for my own car and it seems like its a lot higher than I originally thought.

Here is a neat chart that gives some guidelines on what initial timing to use based on cam duration (scroll down to the bottom):

http://4secondsflat.com/Carburetor Sizing and Series Charts.htm

With you guys having duration of greater than 240 @ .050", going off of that chart, it recommends 22-26 degrees of initial timing. OP is running 18 and having issues, while Wadd2 is running 26 and his engine is running awesome. Seems to me like bumping up the initial to at least 22 would probably make a big improvement.

You wont be able to do that without those additional bushings from FBO (or you could try to make some yourself I guess). You could try the following timing setups:

22 initial + 14 mech = 36 total
24 initial + 14 mech = 38 total
26 initial + 10 mech = 36 total (setup Wadd2 is using successfully)
28 initial + 10 mech = 38 total

Those bushings are kind of expensive relative to what they are, but then again if that solves your tuning issue it'd be totally worth it IMO.
 

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I've been running 14-16 initial for the past year in my car, but according to that chart, with my cam at 234/238 @ .050" I should be running 18-22 initial. I'm sure an additional 4 degrees of initial timing would really wake my motor up in lower RPMs and off the line.

I ordered up a set of those bushings from FBO today, should get here early next week. I'll let you know how they work out and if my engine runs any better with the additional initial timing.
 

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Interesting point about the PCV. Never thought about that.

Seems like there are a lot of threads out there where guys are running pretty big cams without enough initial timing and it causes a lot of tuning issues. I've been trying to figure out what the best initial timing setting would be for my own car and it seems like its a lot higher than I originally thought.

Here is a neat chart that gives some guidelines on what initial timing to use based on cam duration (scroll down to the bottom):

http://4secondsflat.com/Carburetor Sizing and Series Charts.htm


With you guys having duration of greater than 240 @ .050", going off of that chart, it recommends 22-26 degrees of initial timing. OP is running 18 and having issues, while Wadd2 is running 26 and his engine is running awesome. Seems to me like bumping up the initial to at least 22 would probably make a big improvement.

You wont be able to do that without those additional bushings from FBO (or you could try to make some yourself I guess). You could try the following timing setups:

22 initial + 14 mech = 36 total
24 initial + 14 mech = 38 total
26 initial + 10 mech = 36 total (setup Wadd2 is using successfully)
28 initial + 10 mech = 38 total

Those bushings are kind of expensive relative to what they are, but then again if that solves your tuning issue it'd be totally worth it IMO.

WOW!
Great information here!

thanks for sharing this info!

Mark
 

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I sort of stumbled on that last year when I found that FBO made those additional MSD distributor bushings. They have a pretty cool website with a lot of neat info on there.

I'm still learning and have a lot less experience than a lot of guys on here, but it seems like that chart is a good starting point for initial timing if you're not going to run your timing locked out.

The Demon carbs website also has a carb sizing / initial timing type chart. It seems a little less aggressive than the FBO one, but between those two you couud get a rough idea of how to setup your timing curves.

Here's the link to the Demon chart:

http://www.demoncarbs.com/Tech/DemonSelectionGuide.asp

There are three tabs, Mild, Performance, and Radical referring to your camshaft profile. I think somewhere between the Demon chart and the FBO chart is a good starting point for determining your initial timing, outside of obvious factors like how your car runs and your particular setup, etc.
 

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Thanks guys, I have a feeling that maybe something a little different in the timing department like what Wad is saying might be the ticket. I will get some vacuum readings in gear tonight and post them. Right now while its in park i get 17-20 inches
This seems like a lot of vacuum for that cam. I would expect much lower. :confused:

Anyway, while more timing might make everything a bit better, 18° initial should be enough to prevent any massive off-idle stumble when you open the throttle hard, if you had the proper amount of fuel. I'd work on increasing the size of the pump shot. But it wouldn't hurt to play around with timing as well, you might find that both items need to be further optimized for best results with your particular setup. :)
 

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Discussion Starter · #20 ·
Thanks again guys

Tons of good information guys. Wadd2, what exactly are you considering the proper PCV valve? I'm currently using a check valve type breather.
I'm going to try ordering the bigger squirters and def play around with the timing some more.
I'll take a pic or video of the vacuum guage so you more experienced guys can tell me what exactly I'm looking at.
 
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