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Old 01-27-2007, 08:22 PM
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350 Chevy off Idle Issues

Hey guys, im new here. Heres my issue! Beware long post, sorry guys!

I have a 350 Chevy Class II from Jasper Engines. 350hp & 389 lb ft as dyno'd per them. Spec sheet is here: Jasper Chevy 350 Class II .
Im running a Holley 750 cfm double pumper, with Edlebrock Performer RPM intake.
When i crack open the thottle, i get a horrible bog untill i back off the throttle. Quick blasts to halfway, the tires will squeal and the car takes off. Sometimes will backfire through the carb when cracked to WOT. Vacuum at idle is between 6-8 in hg. Ive gone up from 28 shooter in the from to a 35, and in the back from 31 to a 37. All stock pump cams and accelerator pumps. Still have the hesitation, although its a little bit better.(I wont backfire through the carb anymore from what i can tell). Ignition is MSD HEI with MSD 6AL box. Bosch +4 plugs with MSD 8.5MM wires. Jets are now 73 in the front and 83 in the back. Car has more power it seems now once your off idle with these jets. Going to do a run and check plugs afterward to see color. I basically have 2 questions, 1) how do i clear up that off idle hesitation. Its even hesitates under free revv, with no engine load. Also, any ideas what initial and total timing should be. Jasper says initial between 8 and 12. Ive heard of people running as much as 20 degrees initial. Also, what RPM should this initial timing be. Again, im new to this and really dont want to blow this thing up from some stupid tuning mistake. Any help is apreciated. Thanks in advance!

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Last edited by 84Caprice57; 01-27-2007 at 08:32 PM.
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Old 01-27-2007, 08:40 PM
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I had a big block that did that.Sounds in the carb,a little big for ur motor.But should'nt be a problem.I adjusted my vacuum advance and it helped but did'nt get rid of it.I know this is'nt your problem but mines happened because of an electric fuel pump pulling too much fuel.Went through 4 carbs till I figured it out.Because it ran fine everyday for 4 months with that electric fuel pump.I dont know enough about carb but other people on this site should be able to help
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Old 01-27-2007, 08:49 PM
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350 Chevy off Idle Issues

You didnt say what your initial timing is,try and see if more initial timing cures your problem.Once you start hot rodding,all factory settings are useless.18-20 BTDC timing figures are not uncommon with a performance motor.
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Old 01-27-2007, 09:14 PM
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my inital timing was 12-13 in dr (about 750) 17-18 in park (950-1000) RPM. Ive recently read about using 20 degrees intial. So i just recently bumped to 20 degrees in dr, however i havent really ran the car. I think i gave it a quick off idle shot with same results as before. Ive also read about hookin up vacuum advance to FULL MANIFOLD vacuum for best off idle performance. I tested at idle with this setup (i didnt drive the vehicle), and i saw 40 degrees initial timing at 950 rpm. Wasnt sure if this was a good idea. Any ideas?
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Old 01-27-2007, 09:20 PM
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350 Chevy off Idle Issues

Try using no vacuum advance at idle,40 degrees is way too much.20 is tops at idle,40 is probably more than than too much with a small block.
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Old 01-27-2007, 09:25 PM
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I've actually ran with no vacuum advance the other day. (Forgot to hook it back up after playing with the carb) Same feeling off idle lol. This has been my problem ever since ive had this motor. Ive never really sat down and tuned it. I had a 650 double pumper from my other car i put on this one at first, that run just the same (less top end though). except that carb was 6 years old and never rebuilt. I ended up just going with a new carb to start from scratch instead of using used parts. Just an FYI for a lil more backround.
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Old 01-27-2007, 10:13 PM
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You need to shorten the distributor's mechanical advance travel limit.
This will allow more intial spark timing at idle (24degBTC) but the same 34-36deg at max advance. Shorten the mechanical advance travel from stock 20-24deg of travel to around 10-12deg. Swapping weights and springs in the distributor *will not achieve this alone*.
You have to limit the travel of the advance pin in the slot.
Some people weld up the end of the slot. Make sure you weld up the right end. (Requires dissassembly of distributor)
Get a adjustable vacuum advance canister (Crane). Limit the vacuum advance travel to about 15deg. Total timing (initial+mechanical+vacuum) at high vacuum part throttle cruise should not exceed 51deg combined.
Remove the carb and flip it over. Look at the throttle plates in the bores. Look at the idle transition slotd. Adjust the idle stops (both primary and secondary) so that only about .030" of the slot is visable below the throttle blade at idle.
replace the power valve with a 4.5" PV.
Reinstall the carb. Do not touch the idle speed screw. the throttles must be in this idle "sweet spot" or the motor will not idle right or have good throttle response.
Replace the spark plugs, they are carbon fouled. Gap at .035" replace the distributor rotor if it is suspect. (common problem)
Start the motor and warm it up. Set the timing for 24 initial and 34-36 total. When the motor is warmed up fully reset the idle mixture screws for best idle.
Be sure the PCV valve is hooked up to the carb and the oposing valve cover is vented. The carb is designed to include the PCV. if its eliminated the carb will not idle right..
From this point the idle speed screw should need little or no adjustment from you preset throttle opening position.
Use ported vacuum.
The cam in your Jasper motor has much more overlap than a stock cam. The extra overlap slows the fuel burn rate at idle (exhaust dilution simular to EGR) requireing more timing at idle. Should idle rock steady with a pronounced hot rod lope at 800-900rpm. If you have the right torque converter for this camshaft (2800 stall minimum) the idle will only drop slightly in gear. Should have a little more manifold vacuum at idle now too. Check for leaks.
For performance driving with this new aggressive timing curve I recomend a slightly cooler heat range ping resistant spark plug.
Try Champion RV8C plugs (.035" gap) for your Dart S/S heads. The carb is not too big. Should run fine with #70 primary #80 secondary jets and 4.5" power valve.
Check the distributor cap center post under the in cap coil.
replace with MSD PN 8412.

Last edited by F-BIRD'88; 01-27-2007 at 11:09 PM.
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Old 01-27-2007, 10:45 PM
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Vacuum is way to low for your engine specs. Find the cause of that and it will probably help your other issues. Look for vacuum leaks or very retarded timing to start with
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Old 01-28-2007, 12:17 AM
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Thanks for all the info. Ive just recently changed the cap, rotor, coil, wires, plugs, all this past week. Also have always used the low resistance bushing in the cap as well. I have someone who is going to set the timing curve for me. Its some thing they mentioned as well. My Vacuum advance is adjustable it seems direct from MSD, and from what I checked max vacuum advance never passed 50 degrees. The 4.5 power valve is something i was planning on doing this weds, as the speed shop was all out this past week when i went there. Im guessing thats going to fix my part throttle issues ive had when i bog then lurch. Again, thanks for all the advice. As for the accelerator pump nozzles, 37 front and back as of now, does this sound too big/small for my setup? Speed shop says 37 in general is way too big, however most holley tuning guides ive read, state if your bogging keep going up until stumble is gone. Does this statement sound correct or am i wasting my time with the nozzles? Also, have checked for vacuum leakes MANY times, all accessories disconnected vacuum does change. As for manifold leaks, ive checked those, and again, found nothing.
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Old 01-28-2007, 05:25 PM
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Any one have any advise on the accelerator pump question?
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Old 01-28-2007, 08:35 PM
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Quote:
Originally Posted by 84Caprice57
my inital timing was 12-13 in dr (about 750) 17-18 in park (950-1000) RPM. Ive recently read about using 20 degrees intial. So i just recently bumped to 20 degrees in dr, however i havent really ran the car. I think i gave it a quick off idle shot with same results as before. Ive also read about hookin up vacuum advance to FULL MANIFOLD vacuum for best off idle performance. I tested at idle with this setup (i didnt drive the vehicle), and i saw 40 degrees initial timing at 950 rpm. Wasnt sure if this was a good idea. Any ideas?

Wait a minute......

1) You have 7 inches of vacuum at 950 rpm and when you plug the vacuum can on it ADVANCES 20* ????? You definitely have a vac can problem. Keep it disconnected until you get it replaced with the proper can for your tuning.

2) Jasper, the engine builder, gave you specs for timing. Why doesn't that work??? Have you asked them??? They are solid reputable knowledgeable people. Didn't they give you a distributor curve recommendation?

3) They recommend an air valve carburetor and you are running a double pumper. And the boosters are not ideal, either. You can play jetting games for days, but you are chasing your tail.

I recommend that you set the engine up as THEY recommend, first.
You trusted them with your money, now trust their judgement as to what carb and stuff to run.

Just common sense.
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Old 01-28-2007, 08:41 PM
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This is true. With the vacuum can attached, the advance is at 20 degrees where i set it. Even with it disconnected, its not effecting the advance untill it comes to part throttle. Ill have to send them another email as to what type of timing curve to run. When it came to the carb i didnt think a double pumper vs vacuum secondaries would be a big deal. Ive never favored vacuum secondary carbs, thats why ive always stuck with a double pumper. Anyway, will have to give contacting them again another shot.
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Old 01-28-2007, 08:50 PM
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Quote:
Originally Posted by 84Caprice57
This is true. With the vacuum can attached, the advance is at 20 degrees where i set it. Even with it disconnected, its not effecting the advance untill it comes to part throttle. Ill have to send them another email as to what type of timing curve to run. When it came to the carb i didnt think a double pumper vs vacuum secondaries would be a big deal. Ive never favored vacuum secondary carbs, thats why ive always stuck with a double pumper. Anyway, will have to give contacting them again another shot.
That is NOT what you said previously. You are feeding us conflicting information.

Their web site says air valve carb. You are overcarbing when you hit the throttle and trying to crutch it with bigger shooters and bigger accelerator pump. IF you are willing to change a lot of stuff and suffer until you can get it tuned decent enough to drive.....go for it.
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Old 01-28-2007, 08:58 PM
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Also on the Jasper website you linked, their cam lift specs do not compute.
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Old 01-28-2007, 09:39 PM
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Sorry about the mismatched info, I was probably in a rush not paying attention to what I put down. As for the Jasper link, thats what they got on their website, so i have no additional info for you. Any thoughts on putting a throttle body fuel injecting system on it? Ive been thinking of this for a long time as well, but haven't been ready to spend the money. Would my 650 double pumper be better suited for this motor? Or am I wasting my time with either of them? Any thoughts?
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