Chevy Nova Forum banner

1 - 20 of 47 Posts

·
Registered
Joined
·
91 Posts
Discussion Starter #1 (Edited)
Hey guys first off thanks for all you your help in previous posts on input and influence in making some of my mods to car my car. Your advice has been well received. I just recently put a tunnel ram on my setup and need your suggestions in ironing out a few kinks. First off she runs like a champ, for the most part. Basically my issue is when I'm cruising along and put my foot down on the pedal, She dies almost as if shes not getting any fuel. If I'm cruising along and I feather it then slam my foot down she goes like stink. I've attached a video of what happens so you can see and hear what I'm referring too. I've got 450's with 30cc accel pumps, 2.5 power valves and 31 squirters. I just went out to the garage and gave it another go to make sure my facts are correct before posting this. I noticed that when I gave it full throttle the squirters appeared to have petered out making me think it could be an accelerator pump issue, perhaps 50cc is needed? Please adivse, kindly if possible. Thanks for all your help.

https://www.youtube.com/watch?v=HcdgTWeySbY
 

·
Registered
Joined
·
407 Posts
are your floats set right, or fuel level in the bowls? afr gauge might help. perhaps what you think is a stumble from fuel starvation is actually to much fuel. duel carbs tend to do that. I havnt played with holleys much but im about to learn cause i just puchased a tunnel ram set up with to holley 450s as well. picked it up with linkage and some sick air filters for 250, just need to find a good deal on a smaller ignition. oh another thing to check is your fuel pressure, i dont know what pump, regulator, or even exactly what your carbs like but you prolly do. or just keep ripping on it with that grin and she'l fix herself. :no::yes:
 

·
Registered
Joined
·
3,895 Posts
I'd bet money on either an accelerator pump issue (or lack there of) or your timing is too retarded (needs more initial).

First, check for vacuum leaks. If you start to cover the carbs with your hands (to choke it) and it idles up AT ALL, then you've got a vacuum leak to find.

Check the linkage for the accelerator pumps and make sure there's NO slack. Anytime the butterflies move, you should get fuel. If not, you'll get the stereo typical bog.

Proof that it's not getting enough fuel would be the fact that if you "Pump" the gas a few times, she responds favorably and "Goes like stink".
If everything is adjusted RIGHT and it still bogs, I would go up on the squirters a couple sizes well before I considered a 50cc pump!!

I would START with the timing. Advance it (initial timing) until you hear the starter "Labor" when you are starting it, then back it down a couple degrees. Then limit the total timing to achieve the max amount of timing it likes. Getting that right will yield a LOT more throttle response and better drivability that you would expect.:yes:

I'm not a big fan of using the vacuum advance of applications with sizable cams.:no: The lack of vacuum seems to make it problematic and harder to deal with....in my experience anyway. Some people have had better luck...To each their own.

Good luck!!

If all else fails, PM "BigAl". He's had quite a bit of experience with these types of applications as well.:yes:
 

·
Registered
Joined
·
91 Posts
Discussion Starter #4
are your floats set right, or fuel level in the bowls? afr gauge might help. perhaps what you think is a stumble from fuel starvation is actually to much fuel. duel carbs tend to do that. I havnt played with holleys much but im about to learn cause i just puchased a tunnel ram set up with to holley 450s as well. picked it up with linkage and some sick air filters for 250, just need to find a good deal on a smaller ignition. oh another thing to check is your fuel pressure, i dont know what pump, regulator, or even exactly what your carbs like but you prolly do. or just keep ripping on it with that grin and she'l fix herself. :no::yes:
Thanks for the reply. The floats are set right, thats what I thought initially myself. Thats crazy to think that its cause of too much fuel. The video doesnt show it but when I put it WOT I can actually see the squirters squirt then die off, thats why I was thinking not enough fuel hence the accelerator pump. I'm not smelling any excess fuel or seeing black smoke from the exhaust. The fuel pressure is at about 6 with a mechanical pump. I was debating switching it to an electric but I don't wanna throw money at it guessing. I'm thinking that it could be the power valves? right now I've got 2.5 in the carbs and 4.5 and 6.5 sitting on my bench. I should probably get my hands a vacuum gauge and see where I am at for vac since i've no clue at this current time. Your suggestion however of ripping on it with a grin is appealing though :) Thanks for the suggestions I'll give em a shot and keep you guys posted. PS I like your hood :)
 

·
Registered
Joined
·
91 Posts
Discussion Starter #5
I'd bet money on either an accelerator pump issue (or lack there of) or your timing is too retarded (needs more initial).

First, check for vacuum leaks. If you start to cover the carbs with your hands (to choke it) and it idles up AT ALL, then you've got a vacuum leak to find.

Check the linkage for the accelerator pumps and make sure there's NO slack. Anytime the butterflies move, you should get fuel. If not, you'll get the stereo typical bog.

Proof that it's not getting enough fuel would be the fact that if you "Pump" the gas a few times, she responds favorably and "Goes like stink".
If everything is adjusted RIGHT and it still bogs, I would go up on the squirters a couple sizes well before I considered a 50cc pump!!

I would START with the timing. Advance it (initial timing) until you hear the starter "Labor" when you are starting it, then back it down a couple degrees. Then limit the total timing to achieve the max amount of timing it likes. Getting that right will yield a LOT more throttle response and better drivability that you would expect.:yes:

I'm not a big fan of using the vacuum advance of applications with sizable cams.:no: The lack of vacuum seems to make it problematic and harder to deal with....in my experience anyway. Some people have had better luck...To each their own.

Good luck!!

If all else fails, PM "BigAl". He's had quite a bit of experience with these types of applications as well.:yes:
In regards to going with larger squirters, I've got .031 right now is there a suggested stair step to climb at. I saw a post on another forum the poster seemed to have the same issues as me, he said he went to a 50cc pump and .041 squirters and saw a huge positive difference. He has a mild 350 same carbs and intake. .031 to .041 seems like quite the jump, any thoughts? My timing I believe is at about mid to low 30's. I'll also try your suggestion of reaching out to BigAl. Thanks for your help. I'll post the progess
 

·
Registered
Joined
·
1,286 Posts
The problem with the mechanical 450's is there's no pump shot for the secondaries, are these mechanical or vacume secondaries? Holley made some 450 vacume secondary carbs calibrated for ford applications but not too common. Disconnect the secondaries and give it a try to confirm it's a lean bog situation first. Pump cam shape and squirter size will have an affect on volume and duration of shot, also make sure you take any preload out of the pump arm adjustment-set it to no slack and leave it. What timing curve does it have now? Vacume advance? cam?
 

·
Registered
Joined
·
91 Posts
Discussion Starter #7
The problem with the mechanical 450's is there's no pump shot for the secondaries, are these mechanical or vacume secondaries? Holley made some 450 vacume secondary carbs calibrated for ford applications but not too common. Disconnect the secondaries and give it a try to confirm it's a lean bog situation first. Pump cam shape and squirter size will have an affect on volume and duration of shot, also make sure you take any preload out of the pump arm adjustment-set it to no slack and leave it. What timing curve does it have now? Vacume advance? cam?
They are mechanical secondaries. Not sure on the timing curve and I do have a vacuum advance. I currently have 2.5 power valves just picked up a vacuum gauge and took some readings, 15 at idle. Meaning I would need to have 7.5 or 7 PV's. Will that change effect my current issue?
 

·
Registered
Joined
·
3,895 Posts
They are mechanical secondaries. Not sure on the timing curve and I do have a vacuum advance. I currently have 2.5 power valves just picked up a vacuum gauge and took some readings, 15 at idle. Meaning I would need to have 7.5 or 7 PV's. Will that change effect my current issue?
Fix the timing issues before you do anything else to the carb!!! Otherwise, everything you do on the carb side will have to get REDONE once the timing changes are made!!

Yes, there is NO DOUBT that the power valves you have now are WAY off.......But you don't know what size you'll need until the timing issues are addressed because that will ABSOLUTELY effect your vacuum readings.:yes:
 

·
Registered
Joined
·
3,895 Posts
If you want to keep the vacuum advance, then you're in for a steeper learning curve on the tuning.

To make it simple, switch to a mechanical advance distributor!!
Then bump the timing a couple degrees at a time until the starter labors a bit once it's warmed up.
Then back it down a couple degrees and limit the curve to get the amount of total timing you need (as much as possible before you get any signs of detonation [pinging]).

The process can be done on a dyno in just a few minutes. In your driveway and on the street, it will take a bit longer.

Either way, the dividends will be HUGE!!!

More performance and drivability is given up with timing issues than with ANYTHING else!!! :yes:IMO

Once the timing is set RIGHT......the carbs will be a cake walk.:cool:
 

·
Registered
Joined
·
375 Posts
Who put the 3.5 PVs in there? With 15" at idle use 8.5 -10.5 PVs. What pump cams do you have? Try the blue cams. Have you set the pump arms correctly? Can you see the springs on the pump arms compressing? Sometimes these need to be shimmed to stiffen them up. Go up .005 at a time with the shooters. If you get to .040 it's time for the 50cc pumps and hollow screws. (Around $100 at Jegs.) Don't try disconnecting the secondaries, the pump shot will be too much without them. (That only works with double pumpers or vacuum secondary carbs.) I converted mine to vacuum secondaries using spare parts, now I use .026 shooters. Advice? Shop the swap meets for a pair of 600 VS carbs. (Around $100 with 2 rebuild kits.) Like everyone else said get the timing up there first. That's where you start.
 

·
Registered
Joined
·
91 Posts
Discussion Starter #11 (Edited)
Ok so I made some changes before I read the posts (my email didnt notify me that there had been any new ones). I changed the jets to .040 saw no difference put in the 6.5 PV and saw no difference. Came in read the post, took a reading of the timing myself (didnt leave chance to my buddies reading shouted over the running motor). Turns out timing was at 26 degrees not 36 he must have been reading it backwards. So I bumped it up to 30 31 degrees. I now have no more bog when I snap the throttle. Took it out for a spin she ran much much better. However there is still a little hesitation when I put the throttle down when moving forward. Again if I feather it, then mash it I smell the pleasent aroma of burning rubber :D Granted its a tunnel ram not an rpm intake so is that to be expected with this setup or can I do some further tuning to minimize if not eliminate that lag? Perhaps when I take another read of the vaccum and insert the proper PVs that would address the remaining lag issue? Again with the timing at 30 or 31 degrees 6.5 PVs and .040 squirters can it be improved? Thanks again for everyones help, very much appreciated :yes:
 

·
Registered
Joined
·
1,286 Posts
try 25 squirters now that your timing has been corrected. Powervalve should be selected based on what vacume under load at cruize speed you want it to open at. Take a vacume reading at cruise speed and watch the guage drop when going up hills-this is when it should open the enrichment circuit to give you more power under load.
 

·
Premium Member
Joined
·
3,532 Posts
Add more timing still, try to get your total around 36-38* and see how it reacts. My motor likes 41* of timing so don't be scared to try 36-38*, sbc love timing!!!:yes:
 

·
Registered
Joined
·
91 Posts
Discussion Starter #14
Add more timing still, try to get your total around 36-38* and see how it reacts. My motor likes 41* of timing so don't be scared to try 36-38*, sbc love timing!!!:yes:
Cool, Ill give it a try. What kind of signs indicate too much timing? I did notice that at 31 the starter seemed to be working more/more resistance at the initial turn over. Does that make any sense?
 

·
Registered
Joined
·
121 Posts
After playing with timing, if it still acts up, i'd look at installing larger jets, sounds like thats your problem if your needing to pump on the peddle to make it go, I'd bet the main metering is lean on ecceleration.

When you go with that much plenum volume (TR) the carb/carbs will need to be jetted up in most cases.

You NEED to get that timing up to at least 36-37* total.
 

·
Registered
Joined
·
91 Posts
Discussion Starter #16 (Edited)
After playing with timing, if it still acts up, i'd look at installing larger jets, sounds like thats your problem if your needing to pump on the peddle to make it go, I'd bet the main metering is lean on ecceleration.

When you go with that much plenum volume (TR) the carb/carbs will need to be jetted up in most cases.

You NEED to get that timing up to at least 36-37* total.
K, I'll give it a shot tomorrow and post the results. On the chance that the issue is still present what would the suggested jet size be? Currently I've got 65s i think, I never really looked too close for some reason 65 comes to mind. I picked up the carbs at a swap meet assuming they were stock (everything else on the carb appeared to be) the stock jets are 58 I believe.
 

·
Registered
Joined
·
1,286 Posts
What cam and compression ratio does this engine have? 59 main jet front is stock I think for the 450. I wouldn't stray more than a few jet sizes from stock. What is your initial timing set at? You'll probably want around 18ish at idle with the vacume advance disconnecter and 36-38 total at 2500-3000 rpm with the vacume advance disconnected. Then you'll want to limit the amount of vacume advance to what the engine wants-it may be only 8-10 degrees. The best way to set it up is to tune the curve with the vacume advance disconnected then once your mechanical curve is figured out then start adding some vacume advance in small amounts untill there's a loss in response at highway cruize speed and back it down some. The timing curve needs to be dialed in or at least close before any work gets done on the carb. When you mentioned you had 31 degrees-how did you check it? at idle? with the vacume advance connected? <that could be your bog right there.
 

·
Registered
Joined
·
3,895 Posts
What cam and compression ratio does this engine have? 59 main jet front is stock I think for the 450. I wouldn't stray more than a few jet sizes from stock. What is your initial timing set at? You'll probably want around 18ish at idle with the vacume advance disconnecter and 36-38 total at 2500-3000 rpm with the vacume advance disconnected. Then you'll want to limit the amount of vacume advance to what the engine wants-it may be only 8-10 degrees. The best way to set it up is to tune the curve with the vacume advance disconnected then once your mechanical curve is figured out then start adding some vacume advance in small amounts untill there's a loss in response at highway cruize speed and back it down some. The timing curve needs to be dialed in or at least close before any work gets done on the carb. When you mentioned you had 31 degrees-how did you check it? at idle? with the vacume advance connected? <that could be your bog right there.

:yes::yes:Thank You!!!:yes::yes:
 

·
Registered
Joined
·
3,895 Posts
K, I'll give it a shot tomorrow and post the results. On the chance that the issue is still present what would the suggested jet size be? Currently I've got 65s i think, I never really looked too close for some reason 65 comes to mind. I picked up the carbs at a swap meet assuming they were stock (everything else on the carb appeared to be) the stock jets are 58 I believe.

Don't worry about even touching anything on the carbs (except MAYBE the idle speed adjustment on the linkage) until the timing is set RIGHT!!!
Otherwise, all you'll be doing is beating your head against the wall with aggravation. :rolleyes:

Here's some good reading material....

http://www.stevesnovasite.com/forums/showthread.php?t=67503

http://www.stevesnovasite.com/forums/showthread.php?t=11689

http://www.stevesnovasite.com/forums/showthread.php?t=43076
 

·
Registered
Joined
·
91 Posts
Discussion Starter #20
What cam and compression ratio does this engine have? 59 main jet front is stock I think for the 450. I wouldn't stray more than a few jet sizes from stock. What is your initial timing set at? You'll probably want around 18ish at idle with the vacume advance disconnecter and 36-38 total at 2500-3000 rpm with the vacume advance disconnected. Then you'll want to limit the amount of vacume advance to what the engine wants-it may be only 8-10 degrees. The best way to set it up is to tune the curve with the vacume advance disconnected then once your mechanical curve is figured out then start adding some vacume advance in small amounts untill there's a loss in response at highway cruize speed and back it down some. The timing curve needs to be dialed in or at least close before any work gets done on the carb. When you mentioned you had 31 degrees-how did you check it? at idle? with the vacume advance connected? <that could be your bog right there.
Taking your advice mixed with a previous comment about the PV's I disconnected the vacuum advance and checked the vacuum pressure still holding strong at 13 to 14 at idle. So my PV's currently 6.5 should be sufficient? Then I hooked up the timing light and saw that I was still at 31 degrees at idle with no vacuum advance in the mix. Took it out for a test run and its the same story, dies at WOT runs good only if throttle is feathered first. Is my vacuum advance not working correctly adding to the list of possible issues?
 
1 - 20 of 47 Posts
Top