so I've got a 396 and I just installed a Lunati cam and a new edelbrock 750 and airgap intake. I'm having issues wit a bog/stutter at low RPM. It seems like the more timing I give it the happier it is, but I'm a pretty inexperienced tuner and don't know where to go from here. Heck I don't even know if timing is my problem.
I'm currently running 20* initial and the lightest springs out of my moroso HEI re-curve kit. It idles like a champ, but the thing stutters and bogs under 1700rpm during normal acceleration. Once the mechanical kicks in it feels good. It acts like it wants 25* initial, but that can't be right unless my timing light is waaaay off.
I had the medium springs in it and it bogged bad below 2k. without vac advance it was barely driveable. Changed to light springs, and switched the vac advance to manifold (40* at idle!). Runs alot better and idles is greatly improved but still acts like has some hesitation/stutter at low RPM and the timing is all over the place at 800rpm idle with the light springs.
so what do I do? did I get something wrong? The carb is out of the box other than setting the idle mix. Ignition is HEI and MSD 6a.
edit: oh yeah, I don't know if this is relevent or not, but I initially stabbed the dist at 12* advanced for break in. It wouldn't even make it around the block without backfiring or stalling when I tipped into the throttle.
you might need more torque converter and more gear.
but i'd start by fiddling with the carb and timing until you can get it to run better.
this is how you learn- change stuff and see what happens.
I just put together a SBC with 268 voodoo. It was doing close to the same.... I had way to much timing on it to begin with.
Long story short it seems to like the heavy springs, no vacuum advance and a lot of initial. Had to put the start retard on it. Cheap timing light so I don't know the exact total timing.
Also a MSD 6al with gap at .055" helped ALOT too. Plus went with a stronger step up spring in the Edelbrock carb.
Just trying to help. Many days of tuning on this one.
This 268 voodoo I worked on Never really pinged. It was crazy,, I could put it WAY up and it just ran sluggish and had kinda a miss from 1500 to 3500,, never a ping. Motor will burn those tires off now tho .
Your cam is not that big, so driveability should be pretty good.
Whats it do if you get on it a bit?
It sounds like it is REALLY lean
Since you had the intake and carb all off check for vacum leaks, and crossed plug wires. Check that it is firing on all cylinders as well. Could be a plugged orfice in that carb
when you first tip into it or pull away from a stoplight it acts like a car with a bad accel. pump. I switched the linkage to the biggest shot, but I realized that wasn't the whole problem. It hesitates at cruise when its under 2k. I only mashed it from a stop once and it spun the pegleg 315s without problem.
I did spray starting fluid all around the manifold and carb several times and everything seems fine, unless theres a leak in the valley or something. I've checked firing order.
Maybe I'll have to pull that carb back off and clean it real good. It is brand new though.
I don't want to lead you in the wrong direction,, just putting in my two-bits on this voodoo I build. I just had do a little at a time,,, It helps me to sleep on it and start fresh with a clear head in the morning.
understandable. I've left the garage every night this week shaking my head in frustration, and trying again the next day. Maybe I'll crank the initial up just to see how it acts. Just didn't sound right to me though. Like I said though, I'm pretty inexperienced with tuning.
was the cam degreed in-are the valves adjusted correctly-those carbs generally work pretty good-did it run ok before the swap
when a lot of stuff is changed at once, its hard to pinpoint sometimes
if the carb has a choke, you could try closing it a little just to see how it runs with less air
are you sure all the vac. ports are plugged on the carb-i have seen the rear one left out and drive people crazy
take a good look down the carb, is it giving a good squirt-what about float level-you can tune the heck out of those things, metering rods, springs, etc
the metering rods have 2 steps, thick and thin-at idle/high vacume, the rods are pulled down thru the main jets(thick part of the rod, making jet small) at low vacume, their is a spring in the barrel of the metering rod, that lets the rod rise and overcome the lower vacume, making the jet larger(power) diff size springs allow rods to rise and fall at diff vacumes, thus the tuning ability
of course, it may not even be a carb issue, and i have had to much coffee this morning
Set the base timing at 18* and shoot for 20* mechanical for 38* total all in no later then 3,000 rpms
Get an adjustable vacuum advance and adjust it so it adds 10* when it works.Now plug it into manifold vacuum.This will add 10* more timing to your idle timing so you will have 28* at idle.Soon as you give it gas,the mechanical takes over and the vacuum drops off. You should have very good throttle response after this and then you can move unto tuning the carb.
Very good advice above on timing etc ..... you need to ensure the timing & advance curve is close to above ......definitely use a vacuum advance and one limited to 10-12* is right on
You may have carb or valve train issues but at least get the timing set up correctly easy to do with a timing tape on balancer and timing light ...........too many people ignore this basic tuning step ......timing first then carb and then ?? FWIW
Just had a very similar problem. Other than checking for vacuum leaks and ignition also have a look on the fuel supply above idle. I don´t know how the Edelbrock Carb works, but on my Holley I opened up the idle feed restriction a bit and problem solved. In the lower RPMs had this stumble too with lean mixture, now she runs smooth there too.
I just installed the same cam in a 454 this week. This cam has great midrange power. I had a hesitation when I first ran the motor before my comp cam went flat that ended up being carb issues and a couple valves adjusted too tight. I would find a working carb and eliminate that then re-adjust the valves.
I think you will be very happy with your cam choice.:beers:
thanks for the advice guys. I don't have any car guy friends that could take it for a spin and help me diagnose it so you guys are all I've got
the damper installed fine, I installed the cam dot to dot. I set the valves using EOIC, 0 lash plus 1/2 turn before break in.
The stumble isn't nearly as bad as when I started, but you can still feel it lose all power when it shifts gears putting around the neighborhood. If I keep the shifts above 2k the power is fine.
I've got a set of spare valvecovers I'll cut up so I can adjust the valves running this weekend and reset the timing where it "should" be. If that doesn't work I'll slap the old qjet back on it to see how it acts. I've never done a cam install before so theres a chance I just fudged up something stupid.
1/2 turn is to much.Back off a 1/4 turn and it might work better.I find with perf cams only use 1/4 turn after 0 and they work better.But thats only my 0.2 worth
Don't forget the small stuff! Had the same problem! Changed things i did not need toFor me it was a simple vacum leak from the carb.so check all your gaskets#1 One small leak of any sort can cause NIGHTMARES
Did you install the cam with a three keyway timing set? Did you use the correct dot? May be retarded and this is why you are needing more timing.
Just a thought.
thanks for all the advice guys. I did use the three posistion timing set, but I'm positive I installed it straight up.
I'm going to re-set the valves running and warm, and double check the damper to make sure my TDC really is TDC. I'll see if I can round up a timing tape too just incase my dial-back light is screwing with me, and reset the timing where it "should" be. I'll re-torque the intake too just too be sure. If all that doesn't work I'll slap the old qjet on it and see if that solves it.
Rob, If I can't get this thing in line this weekend I'd love a second opinion.
I recommend my 'Quick & Dirty' method for verifying that the cam was installed correctly. The reason being, IF the cam is installed even slightly retarded, one of the obvious indications is that the engine takes more timing to run right, and there is a lock of bottom-end power and response.
You have this, so it is worth checking it out.
You only have to remove a valve cover.
Turn the engine up to TDC, and look at #1 cylinder's valves.
If BOTH valves are closed, you need to turn the engine one more turn so that both valves are off the seat at TDC.
Check the heights of the top of the retainers to the spring seats. The intake valve should be .030" to .060" CLOSER to the head. If the exhaust valve is closer, the cam is retarded and you have to take off the front cover and install the cam correctly.
The most common problem is that you have lined up the keyway with the dot on the keyway. The correct mark is on a tooth a couple of teeth counter-clockwise from the keyway. The cam does not have to be taken out to fix this problem.
well I set the valves running, verified the cam wasn't retarded, re-torqued the intake, reset the base time and recurved the dist so the timing was solid at idle, and still acts up. I was going to swap the old qjet on it, but my manifold isn't compatible.
at this point I think it has got to be carb or a vacuum leak in the valley. I'm going to see if I can steal the edelbrock carb off my dads vette and install it to see if anything changes. If it doesn't then I guess I'll pull the intake and re-seal it.
although after reading this thread I'm wondering if I just mis-diagnosed the problems because of my inexperience. His symptoms are exactly the same as mine, and he is also running a 750 edelbrock carb. Maybe I'm just a carb tweak off what the car wants?
Thanks for the help thus far guys. This has been a pretty steep learning curve for me. :clonk:
I agree that it is a carb issue :yes:. I have almost the exact setup you do (same cam and carb on a low compression 454). I had a similar problem and set my timing as mr 4 speed suggested 18* with the vaccum adv. unplugged, plug it back in and shoot for 38* by 3000rpms.
The 750 worked great on the motor with a stock cam, when I went to the lunati it all went south! I ended up going richer on the primary jets, a lighter step up spring and the closest hole to the carb on the accel pump. Now she runs awesome! :thumbsup:. Throttle response, idle everything. Took a lot of tuning to get there. Edelbrock has a kit (1480) for your carb that you can get at Autozone to tune your carb and your manual has a chart in 4% increments richer/leaner so you don't have to guess.
awesome, hopefully mine will be running like a champ soon. I ordered my kit since no-one local stocks it. My carb is a 1411 so I ordered the recomended 1489 tuning kit. Can't wait to get it dialed in.
Did I miss something? Everyone is talking timing and carb.....Did anyone ever suggest setting the IDLE MIXTURE with a vacuum guage? What about the choke, is it partially "on?"
What is the manifold vacuum at idle?
If you have a vacuum leak....the engine will run weird regardless of what you do. I've had a few goes at vacuum leaks.....
Live long enough, you'll see most every kind of issue
A vacuum guage is not only a great tool to help you diagnose a problem, it can also help others here help you out as well. It is so worth the money it's not even funny. The less guessing, the better...
I'll hitch-hike on what Astro said...
(carefully) look into the carb when you try to rev the engine. If you do not see fuel squirting in from the accelerator pump, move the mechanism to the middle hole. My 68 RatVette likes the hole closest to the carb body like Astro said.
I also had to jet the carb UP 2 primary steps and 3 in the secondary. My tired Brand H 3310 couldn't hold a candle to the 800 AVS Eddy then. In all fairness, the 750 Brand H was tired and I'd not converted the secondaries from metering plate to metering block configuration.
The Eddy carbs come with a good tuning guide, copies are also online. As most everyone says, you are probably lean.....
1. Get a vacuum gauge on her....I get 14" vacuum with an RPM intake, 800 Eddy Carb, and an Erson cam with .545/.545" lift, dur 235/235*@.050" and 111 LSA, the old HiFloII grind, have 781 heads, big valves added. Set the idle speed low then fiddle with the mixing screws to get the highest idle. If you cannot get 12" you probably have a vacuum leak...
2. Check that you are getting a good shot from the accelerator pump, adjust if necessary
3. Hang on!:yes:
This is an older thread, you may not receive a response, and could be reviving an old thread. Please consider creating a new thread.
Related Threads
?
?
?
?
?
Team Chevelle
5.1M posts
115.6K members
Since 1998
A forum community dedicated to Chevrolet Chevelle owners and enthusiasts. Come join the discussion about restorations, builds, performance, modifications, classifieds, troubleshooting, maintenance, and more!