I’m hoping someone can help with my Carb tuning problem.

Ok, I have just dropped in a bran new 350-4Bolt Main 290H.P G.M Crate motor. The motor has about 1,500 miles on it now. G.M. part # 12499529
http://www.lowcostengines.com/12499529.htm

I am using the recommended H.I.E. G.M. curved distributor, an Edelbrock PERFORMER, air gap (idle-500rpm) with a 600cfm Edelbrock carb with manual choke. I’m using a 700R4 with a 2200 stall. Running 2inch true duel exhaust with highflow cats.

The problem is when I floor it off a dead stop it will ether just stall or sputter. If it stalls it will just fart out the carb, if it goes at all it makes a sputter sound until the RPM’s come up. It has no low-end torch at all, but seems to pull ok at higher RPM.

The recommended timing for my motor is 10 B.T.D.C. I tried 2 different timing lights and both are giving me 10 B.T.D.C. So I bumped it up to 13 and it seem to help a little but not much. I put on the vacuum gage and adjusted the fuel / air until I got the max vacuum of 15 LBS. G.M says 15 LBS vacuum is correct for my motor due to the cam they use.

Edelbrock recommend I move the adjuster on the carb’s accelerator pump to the bottom hole in order to get more fuel into the carb. I did this and no noticeable change. They then recommended using different metering rods and springs, so I swapped in what they recommended and still not luck, (but it did help a little).

Any advise or ideas or suggestions would be greatly appreciated.

 

Nitro,
are you using 'ported vacuum', or manifold vacuum,
to the vac advance in the dist?

 

I have always just called it timed vacuum, not sure what the correct terminology is.

While standing in front of the car, the vacuum advance is connected to the port on the left side (passenger side). The port on the driver's side is capped off. The port on the driver’s side always supplies vacuum, but the one on the passenger side supplies vacuum at various RPMs, that’s why I call it timed vacuum. My H.E.I vacuum advance is connected to my timed vacuum.

Any Ideas?

 

tried changing the accelerator nozzles?

 

I tried that also, it was a #28, and I tried a new #28 still the same.

 

as in going up in size? I'm running a .43

 

Did you do anything else other than change the nozzel? Metering rods and springs? float ajustments? or jet replacement? I did not know they made different size nozzels, I assume the larger # means bigger nozzels, (more fuel) ?

 

On my 330 hp 350HO, I spent *** knows how much $$$ on springs, jets and other Edelbrock tuning parts. The only thing I could really do is move the bog throughout the power band. You need a real carb. I switched from the 600 cfm Edelbrock to a 650 Speed Demon and the difference was night and day. Due to the lean bog, I could only run anywhere from 13.8's to mid 14's at 104. With a 650 Speed Demon I bested a 12.88 and a 12.93 @ 108 in good weather.

Switch out that damn Edelbrock .

 

Ya, I may end up doing that. But the thing is I just deleted my L98 ( /87 5.7L T.P.I ) motor. I really don’t want to give up on the carb tuning just yet. And to be honest I’m still not 100% sure it is the carb, but at this time I highly expect it is.

This is the first time I ever used the “performer air gap” Edelbrock claims this manifold (#2601) is for idle to 5500 rpm. I also want to make sure that it is not the manifold causing this problem.

 

Definitely not the manifold, I promise you. I would say 90% certain carb, 10% vacuum leak/timing.

 

What is the torque converter stall speed?
The cam in that crate motor was the old L-82 L-46 350-350hp cam. It is 222@.050" .450"/.460" lift ground on 114LSA and is installed 1 degree retarded from split overlap. and will not like a very low stall stock type converter. Should have a 2200 rpm stall minimum to expect good throttle response. A 2800/3000 stall would be best.
if you have a typical stock converter the stall is about 1500rpm +or minus a few rpm. Way too low for this cam.
This cam was originally designed for a 11:1 compression ratio motor. (1969 L-46 350-350hp) It's a dud in a 8.5:1 cr motor like the 290hp crate motor.
Also should have a high rear gear ratio.
Try increasing the ignition timing to see how the response is.
Temperairly increase the idle ignition timing to 20degrees BTC and see how it responds from a dead stop.

Do not run it up in rpm beyond 3000rpm with the timing jacked up to 20 degrees BTC ,,
just evaluate the initial launch engine throttle response.

If, this makes the initial throttle response better, yu need to
modify the mechanical advance curve in the distributor.
remove the distributor and recurve the mechanical timing curve to allow this increased idle timing but limit maximum advance to 36deg at high rpm.
You have to limit the amount of mechanical advance travel.

Its about 24 deg now. Limiting the travel of the mechanical advance to 18 degrees will allow 18deg inital at idle and 36 deg total. the throttle response will be much better.

You can remove the cylinder heads and have .060" milled off the deck surface of each head and reinstall with new thin .015" steel shim style head gaskets to increase the compression ratio. 8.5:1 is very low for this cam. 9.5 to 10:1 would be much better. But doing this will get it up to 9.1:1 if your motor has head gaskets thicker than .015" the cr is even lower than 8.5:1 (composite gasket)
It would actually be 8.1:1. therefore yu could increase the engines cr 1 whole cr with the above mods. A real good step in the right direction. This cam needs compression.

Installing larger accelerator pump shooters in the carb will help. There are about 5 sizes to choose from. You can also drill out the one you have a bit at a time and see if that helps. .002" at a time.

A low carb float level will cause lame throttle response.
Edelbrock carbs need correct float adjustment. (Critical)
Run the car and shut it off. Pull the top off the carb and see how high the fuel sits in the fuel bowl. Should be about 1/2 full. If it is low adjust the float setting to allow correct higher fuel level. Low fuel pressure at the carb will cause this too. Should be 6-7 PSI.

Pulling the timing cover and advancing the cam in this motor about 7 degrees will help a lot. (cam timing offset bushing set)

Installing a new cam with much shorter intake valve timing duration would be like night and day. A much better match to a car with a stock converter and a low compression ratio. the cam is a mis-match.

A switch to Comp cam #12-208-2 265DEH or Crane #113502 Z-256-2 would bolt right in with new lifters an completely change the nature of this motor.
We're talking about a huge increase in low rpm torque.

Try removeing the spark plugs and re-gap them to .035"
Improper spark plug gap or carbon fouled plugs will really mess up the throttle response. No amount of tuning will make much difference until this is corrected. if they are carbon fouled replace them. Cleaning will not help.
Spark plug wire cap and rotor and coil must be in good condition or throttle response will suffer. Even new parts can be defective.

The cylinder heads on this crate motor are really lame.
You should consider a cylinder head swap to any of the available aftermarket heads. You could add 80 hp and 100ft/lbs torque by swapping to a good cylinder head.
Something with small combustion chambers and high flowing ports.
prime candidate for a Vortec head swap.

A "home ported" 305 head with new larger 1.94x 1.60 valves would really wake it up for low $$$.
expect 50++hp and 75ftlbs torque from this common effective low $$$ swap.
Your only problem then would be keeping tread on the rear tires.

 

Thanks for all the tips so fare

This motor seemed like a good deal at the time, actually cheaper then a base rebuild. Right or wrong I chose this motor because I thought that cam would be good for pulling my trailer as well. This is also why I chose a 2200 stall converter, as towing with too high of a stall will burn it out in no time.

For the time being I need to just get her running the best possible without pulling the motor apart.

I am running a “2200” stall converter, speed pro stage 2 shift kit with a speed pro hydraulic lock up converter, running with what I believe to be 3.27 rear end gear. However the converter stall does not seem at all noticeable, maybe I got ripped off on the stall.

I bought the Crane curve kit on Sat. My base trimming was 12 degrees B.T.D.C and total was 36 degrees B.T.D.C with the vacuum can disconnected. I changed the vacuum advance from the “timed” (ported ) side to the full manifold vacuum side (drivers side), and this seemed to help a lot. I then installed the two silver springs and also installed the timing limiter advance plate. I set the limiter plate to the 7th notch, my understanding is this will give me more advance off the line but limit my total timing at high RPM. If I am correct this should give me:
12 degrees initial
16 degrees crane adjusted vacuum advance
21 degrees centrifugal (curve= 600start- 2200 fully advanced)
49 degrees total with 28 degrees initial

However I still need to dial everything in, do some trial runs and make adjustments as needed. It started snowing so I had to stop for the day.

The fuel pressured is good, I’m using a carter mechanical fuel pump, and the float level looks good too. Edelbrock sent me some new metering rods and springs and this did help a little also. I adjusted the accelerator pump to the lower adjustment (“adds more fuel”). The fuel nuzzle (“ accelerator pump shooter”) is an Edelbrock # 28. I still need to try a different size nuzzle to see if this helps not sure of what size to order first.

 

By using full manifold vacuum to get additional needed idle ignition timing you run into a problem. As soon as you punch it, all that 16 deg of vacuum advance provided by manifold vacuum, instantly falls to 0 as manifold vacuum dissapears.
Right when you need it the most. You need more initial static advance like 18- 20deg initial. ( more the marrier, as much as the starter motor will tolerate) then limit the mechanical travel to 16-18deg centrifical distributor advance resulting in 36 total. All in by 2200 is too quick. the light springs are unstable at low speeds. More initial and a more stable curve maxing out at 3000rpm ia better overall. Then employ about14-15 degrees of vacuum advance all in during highest cruise vacuum reading. use ported vacuum to control it.
You'll find this method more stable, more responsive.

Useing full manifold vacuum and very light centrifical springs tens to casue the idle to hunt up and down with manifold vacuum and flair up when going from idle in drive to neutral
( cause the manifold vacuum rises causing the springs to flex)

That cam is about the worst for towing. made worse being in that low cr motor. Its way too big for towing. You'll be much much happier and richer by switching to one of the cams I suggested or something simular.
Not only will it tow much better , but the car will also be quicker in the quarter mile and get much better gas mileage. Especially with a modest 3.27 rear gear.

You can change this seemingly "good deal" into the real deal you were looking for with these mods.
You can sell of the Cam and lifters and heads as they are near new. And off set some or most of the cost of dialing it into a cool street/ tow package that performs.

 

Thank's again for the detailed information, but we got a foot of snow here today, so I can't play with my toy until the roads clear up.

I plan on trying what you suggested, hopefully by next Sat. I will let you know what kind of luck I have with it.

Just curious, what type of application would you say the cam I have would be best suited for? I don't understand why G.M. chose the "wrong" cam. What the hell where they thinking? A cam with the correct lift, duration etc. would not cost them any more to manufacture.

 

That cams origional application was for the 1969 Corvette L-46 350/350hp motor. it did make 350hp with the other matching parts of the combination. 11:1 compresion Camel back 2.02x 1.60 heads, decent stock dual plane manifold, corvette ram horn ex manifolds and 750cfm Qjet.
Came with a 4speed manual trans and mid 3xx gearing. 70 and 78 series bias ply tires of the era didn;t have much traction so they could afford to trade off some torque.

It was also used in the 1973-1974 Camaro/ corvette L-82 350 (245 net HP) with low cr smog heads 2.02/1.60's and 9:1 cr. But torque took a real hit from the cr loss. (these cars typically ran low 14's stock.

Since used on Mercruiser Marine 350 engines.
Gm has made millions of them and probabily is trying to use them up., not making new ones.
This is typical of GM as they also used the same "929" cam in virtually ever stock low perf 350 ever produced from 1967 to well into the 80's.
(195-202@.050 .395-.410" on 112LSA)

The cam used in GM's 350-330hp vortec headed crate motor would be a better match (212-221@.050 .430-.460" on 112LSA.)
Simular to the Comp 265DEH I recomended.

 

Today I also changed the acc. pump nozzel to a #33, and moved the acc. pump ajuster arm back up to the top ajustment. (more fuel).

The combination of switching to manifold vacuum and using the distributor advance plate seemed to help the most. The distributor locking plate gives my motor more timing where it seems to want it (off the line, out of the hole). It also locks the timing out from going over 50* total with an advance setting of over 16* vacuum. I always run premium fuel so this may be one of the reasons I am not getting any timing noise, she seems to really like lots of timing.

When I spoke to the Edelbrock rep. he told me to see page # 12 of my Edelbrock owners manual, section “long duration camshaft” and sure enough it said to:, “Connect it directly to manifold vacuum”. That section also had a recommendation to use a mechanical curve to give me plenty of initial spark advance. Bottom line hear is if I had of just spent 5 min, reading the manual that came with my new carb I could have saved many hours of frustration and some money too.

I still have some more tweaking to do, but now I know what my motor is looking for. Full throttle runs out of the hole are no longer a disapointment. For a low compression motor (8.5.1) this thing is really starting to work well.

 

any more progress?

 

I had these problems with my eddy 600 as well, turned out to be a combination of low fuel volume (i was using a stock 350 pump) and a crack in the line for the vac advance. After i replaced the pump with a 6V Holley 110 GPH and fixed the vac line it ran like a champ again... then i moved up to a bigger carb (eddy 750) and it runs even better!

 

I would add one more thing to your list of to-do's: A really high powered coil. Something from Accel or MSD or Proform, in the neighbourhood of 50,000 volts. It will likely give you that final satisfaction you desire when coming out of the hole.

 

Out of the hole (of the line) full throttle runs are fine now as long as I Hold the brake a bit before I punch it right to the floor. Manually shifting the automatic 700-R4 also helps a lot. I have upped the accelerator pump nozzle to a #44, the adjuster is still at the top notch. This motor seems to want as much fuel and timing as it can get to get her moving well.

New related problem. 1st gear all good, 2nd gear puts down rubber, 3rd gear ok up to about 3800 RPM. At around 3800RPM while under full throttle I am getting a sputter. (would this be from the secondaries?)

Another problem (not sure if it’s related or not) is while under light acceleration the carb sounds like it’s sucking air, but as soon as you give her some gas that sound goes away. Light acceleration to me doesn’t seem good at all; I really have to crush the pedal into the floorboards to get it moving.

Any suggestions?

The Speed Demon carb is on my parts list but right now I am Edel_broke so I need to try every possible salution of getting this Edelbrock to work right. So much for Edelbrocks claim of “These carbs work great right off the shelf” WHAT A CROCK OF **** THAT IS!

 

If it was the secondaries, it would do it in 1st and 2nd as well. Sounds like your fuel delivery system isn't keeping up with the fuel demand.

Sounds like you still have some part throttle tuning to do.

 

Many, many "carb problems" are actually ignition problems. I hope I'm not wasting my breath here, thus I have recommended a 50,000 volt coil. Just do it, it is not expensive and will eliminate ANY ignition problems you may have--short of a physical component literally disassembling itself.

 

Bump

 

No, you are not wasting your breath. Your suggestions are much appreciated.

My understanding of the coil may be wrong. My understanding is that a stock H.I.E. coil puts out around 35,000 Volts. I have read in many different places (books, internet, etc.) that a stock coil puts out more than enough volts for a mild engine. I have be lead to believe that buying an expensive coil is just a waste of money. However it is highly possible that I have been steered in the wrong direction in regard to my coil knowledge and what is considered a mild engine.

If you could give me any links or detailed information on H.I.E coils it would be appreciated.

I once had a Crane coil and I hated it because, if I was not extremely careful while adjusting the trimming I would get one hell of a ZAP . Is there a newer designee that avoids electrocuting you if you touch the distributor?

 

So what are you using for a fuel pump?


Also, the upgrade to the Accel coil helped my 355 alot.. it started quicker and seemed to all around run better.

 

My fuel pump is a Carter. I have used the same type of fuel pump on a different motor, with higher compresion and 100 more hp with no problems. The fuel psi is really good, all the lines and the pick-up unit are all new also.

I am running Talor wires. The disrtibutor is new also, it is new from the G.M. performance cataloge and has (had) a macheicl curve. GM peformance reomends my motor run a 10* B.T.D.C. I am running 13*, I have added lighter springs to the stock weights, I have also tried using a machanical advance plate on the vacuum can. As per Edelbrock, I am us the vacuum on the drivers side of the carb (manifold vacuum). For my motor manifold vacuum really works much, much better.


Purple82TA just looked at the link, she sounds nice. I would be carfull that you don't cut your arm off with that flex fan and no fan gard though.
I run Sunoco Gold (highest octain avalible), my timing is curved to give me as much addvance as possible at the low end without killing it on the top end. I believe the timing is pritty much maxed out. I have tried more timing than what I am running, but if I give it any more it starts running a bit hot. The motor has a 4 core rad with 3 tornato sucker fans, and an oversized trany cooler in front. The A/C system has been completly deleted.

Starting is not a problem, the motor has one of those "CRC" high speed starters, and new flex plate.

 

Hmm.. i'm still using the passenger side for the vac advance on mine and i'm pretty sure i've got a little more than 290 HP....

I'm actually planning on taking the vac advance out on mine and just locking it in at full advance.. lol

 

A powerful coil will provide a strong enough spark to actually allow you to widen your plug gaps considerably. Most people use around a .035 gap but a good coil will allow you to widen that gap to .045 or .050, thereby giving you both greater power and mileage. A more complete burn of the available fuel.

A stock ignition module may not be able to provide timely spark at the higher rpm ranges, as well. It is possible that this is part of your problem in third gear.

It sounds like you have a GM Performance distributor, which will likely cover all the issues I've raised here. They come with a good coil and module from GM.

 

My motor has very little vacuum at idle, it's like 14 @ 900 rpm, the manifold vacuum works best with mine. I was also thinking on welding the weights open on an old distributor that I have kicking around to see how that works.

I looked at your video, your car sounds nice. That fan looks a little scary to me though, have you thought about using an electric fan?

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A powerful coil will provide a strong enough spark to actually allow you to widen your plug gaps considerably. Most people use around a .035 gap but a good coil will allow you to widen that gap to .045 or .050, thereby giving you both greater power and mileage. A more complete burn of the available fuel.

A stock ignition module may not be able to provide timely spark at the higher rpm ranges, as well. It is possible that this is part of your problem in third gear.

It sounds like you have a GM Performance distributor, which will likely cover all the issues I've raised here. They come with a good coil and module from GM.

Thanks, I did not know that I could increase the spark plug gap if I used the 50,000 volt coil. My distributor is one of those G.M. performance ones #9344086 I think. Do you know if there is any way to check and see if it really is the 50,000 volt one? I put my hand on it a lot and I never got a shock yet, but maybe that was only the Crane coils.

 

You'll have to go by what GM advertises as the specs for this distributor. I do know that if it is the Performance Parts distributor it is a step up from the stock rteplacement part.

 

Ok I just scored some newly worked 1967 camel hump heads, 64 chamber, with the 1. 94 intake and 1.50 exhaust.

I am going to use a Victor Reinz 5746 gasket (with a compressed thickness of, “ 0.26”) the heads have been shaved . ten thousand off.
Can anyone take a stab at what my compression ratio will be? My motor is described above.

I also just rebuilt one of my 750 Rochester’s with a modified divorced choke, (now works as manual choke). So I'm going to try the Rochester on a Performer and see if it's any better than my Air Gap and 600 eldelbrock.