Stumbling Tunnel Ram on hard acceleration [Archive] - Chevelle Tech

: Stumbling Tunnel Ram on hard acceleration


HopkinsChevelle
Jan 4th, 07, 5:42 PM
I have been driving my chevelle around a bit lately and now that its pretty much broken in I have been putting my foot in it a bit.

Today I tried to do a hard lauch from a stop. It just stumbled like crazy. Ive played with the springs and bushings a bit and did what my engine builder recomended. I believe its a black bushing and two light silver springs. It definatly runs better but not what i want.

I would assume that the problem is dialing in the timing. Any recomendations

Engine combo

402 block .030 over
XE 274-280 at 050 236-242 568 578 lift 110 lobe seperation.
290 closed chambered heads 2.19 intake valve, stock exhaust valve
mild porting
10-1 comp ratio
MSD box, MSD pro billet distributor, MSD 8.5 wires
Edelbrock tunnel ram, two 450 holley mechanical sec
Keith Black hyper pistons, nitrous gap
piston and rods ballanced
stock rods with ARP rod bolts
summit headers
3 inch pipes with dyno max race bullet mufflers

I think i have the timing set about 14 degrees advanced. any ideas appreciated!

Jason Snyder
Jan 4th, 07, 6:22 PM
ITS THE 450 CARBS!!!!!!!!!!!!!!!!!!!!!!!!!!! those are the worst carbs to tune !!
The reason is = when you nail it ,the secondaries open and DON'T get any fuel squirt!! IN OTHER WORDS MASSIVE LEAN BOG!! Think about it? if you tried to run a carb without a primary fuel squirt(discharge nozzles) it would almost die when you gave it gas ,right? YES IT WOULD.

The only cure for this is to install huge primary discharge nozzles!! like size 55-65 (you may have to drill the stock ones out). Another trick is to install a power valve that is real close to your vacuum reading(just under though).

Sooner or later you will want holley 1850s , they are much easier to tune! THEY are vacuum secndaries.

ratuned
Jan 4th, 07, 6:57 PM
i would try tuning what you have a little more before i dumped the carbs. i would check your vacuum. i will guess you will have 12-13" at idle. a good start might be 8.5 power valves. i believe that the primary squirters are 31's. maybe bump them up to 36's. if its still not there try 40's. make sure you use the right screws. best of luck mike

mac762
Jan 4th, 07, 7:02 PM
Maybe you could install 50cc acelerator pumps. Or do those carbs already have those?

HopkinsChevelle
Jan 4th, 07, 7:17 PM
Thanks for the replies guys, I had the carbs worked over by a local carb guy that knows his stuff. I told him exactly what i had going on in the engine and he said he would set them up. Maybe I should have asked, may have to run down there tomorrow. I dont know what size primaries or power valves i have.

Also, i went with mechanical secondaries because i was told it would be easier to set up. Was I lead astray there?

Jason Snyder
Jan 4th, 07, 7:31 PM
i would try tuning what you have a little more before i dumped the carbs. i would check your vacuum. i will guess you will have 12-13" at idle. a good start might be 8.5 power valves. i believe that the primary squirters are 31's. maybe bump them up to 36's. if its still not there try 40's. make sure you use the right screws. best of luck mike

Iam not saying dump the carbs, i am just giving you advice ! because i went thru this same problem!! I learned all the tricks to making a tunnel ram streetable.
Those carbs come with 8.5 powervalves stock...

Jason Snyder
Jan 4th, 07, 7:39 PM
Thanks for the replies guys, I had the carbs worked over by a local carb guy that knows his stuff. I told him exactly what i had going on in the engine and he said he would set them up. Maybe I should have asked, may have to run down there tomorrow. I dont know what size primaries or power valves i have.

Also, i went with mechanical secondaries because i was told it would be easier to set up. Was I lead astray there?
Real quick ,WHAT DOES SET THE CARBS UP MEAN? you can't do much to these carbs(maybe jets/pv/discharge nozzles). REALLY you can't tune carbs off the engine (you can only guesstamate) !!!! MAYBE his guesstamate is off a little.You need to go back to him and explain the lean bog condition, then he can adjust from there.

Tom Mobley
Jan 4th, 07, 8:01 PM
What Jason said. those carbs are mechanical secondary with no sec pump. It's a recipe for massive lean-out followed by massive bog. I've never understood why Holley even sells those things. There's a number of ways to cover up the problem by creating more problems. By the time you a huge pump shot out of it it will start fouling the plugs.

Buy a pair of 1850s, save yourself the headaches. You can get secondary diaphragm housing lids with a tube so the secondaries are equalized by running a vacuum hose between them. Also, you can scrounge around for or just buy a pair of 650 double pumpers.

BillK
Jan 4th, 07, 8:45 PM
Hoppy, (name ??)

Just my opinion, but 14 degrees initial timing, if you do not have vacuum advance, is probably a big part of your problem. Unless you have a vveerryy fast advance curve that is getting a bunch of timing in at say 1200 rpm, I dont think it will cut it. Just for grins, grab the distributor and advance it about 10 more degrees and see what happens. You may not want to run it too hard that way because you will probably have to much total timing, but it will tell you if it is the problem. If advancing it wakes it up, get a vacuum advance distributor.

69-CHVL
Jan 4th, 07, 9:24 PM
We'll, I just got done bringing back a t-bucket to life over the winter. It has a SBC 350, tunnel ran w/2 Holley 450 vac secs STOCK

This thing will rip your head head off! No stumbles, hesitations, no nothing!

Bill got it right, go after your timing. That cam IMO is gonna want at least 18* to start, probably more. Heck, I run 20* intial. Watch that your total doesn't go over 38* though.

Advance your timing alot before doing anything else.

Mike Feudo
Jan 4th, 07, 10:31 PM
It's cold outside that is part of the problem. When I ran one on a bracket car we had real problems if it was a long race and it got into the cold part of the night. I would find a set of 450 vacuum carbs if that is all the airflow you want. Like others has said that 450 mech. carb is not the greatest thing ever invented.

HopkinsChevelle
Jan 4th, 07, 10:49 PM
Mike i wondered about the cold weather myself but really its acted the same under acceleration for awhile now. Im just now trying to diagnose the problem. Id like to be able to do a burn out and not kill the car.

Bill and 69-CHVL, Ill try playing with the timing a bit tomorrow and see what i get. I thought about advancing it some more. Ive done a few searches and it seems they (tunnel rams) like alot of initial timing.

I really have more money in those carbs than i should. I bought them off ebay then had them both gone through. Id hate to have to get rid of them if i dont have to. The distributor is a Pro Billet MSD non vacume, id hate to have to swap it out too. I thought i was buying good parts when i put it together. Maybe i didnt think it out well enough.

540Hotrod
Jan 5th, 07, 12:31 AM
You're on the right track...you need LOTS of timing. I ran one on the street on a 396/375 with an L-88 cam. I modded dist to cruise around with 42* just barely off idle. Worked fine...but gas was better then.

Try locking out the dist and setting it at 36* or so and see how low speed response works. Be careful though...hypers don't like detonation and I see you have a M-21 so you don't have a lot of first gear anyway.

For sure carbs need tweeking to either get a a secondary pump shot..they used to make kits to do it..or take them back to carb guy and see if he will convert them to vacuum? I ran 660 center squirters and they worked fine with a little work.


JIM

1968 hot rod
Jan 5th, 07, 1:40 AM
I have dealt with this problem with a pair of #1850 vac secondary carbs and a Edelbrock Street Ram also the warmer the manifold got the better the car drives.
The things that cured the lean part throttle and full throttle misfire were as follows.
I locked the timing for full advance 36 in this case (automatic car a stickshift may require a timing curve)
At part throttle the car would hunt and hesitate also had a "flat spot" when pulling from a stop at light throttle.
This is a 383sbc with a cam of similar specs as yours.
I pulled the carbs and verified proper transition slot positioning.
I then opened up the idle fuel bleeds to .029 and installed smaller idle air bleeds .065 and used .032 squirters with red dp accell pump cams.
This was an instant cure for the part throttle light accel problem .
Don't forget these are 600 vacuums, a 450 mech may have differant size bleeds.

To deal with the full throttle lean backfire.
These 600's had secondary metering plates instead of metering bodys.
I had to open the transition slot idle feeds.
These are the small holes in the top of the plates.
Buy yourself a Holley tech book with lots of pictures to identify the fuel passages.
I prob had the carbs off and apart 6 times,make SMALL changes and keep notes on what ya did.
If you get lost pm me.

71rat408
Jan 5th, 07, 3:34 PM
We'll, I just got done bringing back a t-bucket to life over the winter. It has a SBC 350, tunnel ran w/2 Holley 450 vac secs STOCK

This thing will rip your head head off! No stumbles, hesitations, no nothing!

Bill got it right, go after your timing. That cam IMO is gonna want at least 18* to start, probably more. Heck, I run 20* intial. Watch that your total doesn't go over 38* though.

Advance your timing alot before doing anything else.

I think the key here is you had vacuum sec carbs. The 450 mechanical secondarys don't not have a secondary accelerator pump.

I ran the 450 mech. on my 72 and had similar probems to the original poster. After to talking with The Carb Shop, I decided to use the 1850 Holleys. I added secondary metering blocks (had to mount the carbs sideways), drilled holes in the primary throttle blades (idle screws weren't responsive), and went to .031 shooters (off idle stumble), I also tied the secondary diaphragms together.
No more problems, the car took of without any hint of stumbling. This is the way the car was when I ran the 11.81. Produces about 6" of vacuum at idle.
I also drove this car quite a bit around town, and had good street manners.

69-CHVL
Jan 5th, 07, 4:15 PM
Yea, I missed that his carbs were mechanical...or sort of.

I stil would like to see some more timing though.

Jason Snyder
Jan 5th, 07, 4:49 PM
We'll, I just got done bringing back a t-bucket to life over the winter. It has a SBC 350, tunnel ran w/2 Holley 450 vac secs STOCK

This thing will rip your head head off! No stumbles, hesitations, no nothing!

Bill got it right, go after your timing. That cam IMO is gonna want at least 18* to start, probably more. Heck, I run 20* intial. Watch that your total doesn't go over 38* though.

Advance your timing alot before doing anything else.
Hey would you mind sharing the list number to those carbs your using?I never knew they made 450 vac secondary carbs,I WANT SOME!!

69-CHVL
Jan 5th, 07, 5:07 PM
I'll look at it next time over there. They look like 1850's, had a metering plate, and the vac sec housings were connected together w/a vacuum hose to keep the secondaries opening evenly I guess. They were old, and the throttle shaft still have a touch of slop in 'em. I think the #'s were 45xx.

Now, this is on a T-bucket which weighs what, ~2000lbs? But the throttle response is incredible.

ratuned
Jan 5th, 07, 5:22 PM
we ran those mechanical 450's on a 68 firebird. it had a 400 sbc10.5-1 comp with afr 195 heads and a 294s comp cam. carbs were stock and with a 10" converter and 3.73 gears it was responsive. maybe we just got lucky. i would most definately agree that 2 600 vs holleys would be a better choice though imho. mike

HopkinsChevelle
Jan 5th, 07, 6:27 PM
Wouldnt two 600's be to much for my engine? I had two vacume secondary holleys but i was told mechanical would be easier to use.

Rich-L79
Jan 5th, 07, 6:40 PM
Wouldnt two 600's be to much for my engine? I had two vacume secondary holleys but i was told mechanical would be easier to use.

The great thing about vacuum secondary carbs is that if they are set up right, the secondaries won't open unless they are needed and therefore you should always have good velocity through the carbs. To some extent, if the carbs are too much for the engine it simply won't ever open the secondaries. With mechanical secondaries you are openning all the barrels at once thus providing too much venturi volume when the engine can't even make use of it. As an experiment you could try gradually dipping into the throttle at first instead of punching it wide open from a low RPM point.

Jason Snyder
Jan 5th, 07, 7:42 PM
Wouldnt two 600's be to much for my engine? I had two vacume secondary holleys but i was told mechanical would be easier to use.No two 600s will be fine !! I ran a 355 with10.7 compression ,old fuelie heads and a hydraulic cam 236@.050 with.510 lift and 106 lsa .. the intake was an old tri.y with two 1850 holleys ,this was in a 65 nova and ran 11.90 all day long,and driven to the track!!
Basically tunnel rams can run larger carbs because they atomize much better! Its kinda like having a 7 inch spacer in a single plane intake ! EVERY INCH YOU RAISE THE CARB ,THE LEANER THE FUEL MIXTURE!!

struggler
Jan 8th, 07, 5:40 AM
FWIW I used 2x 450 mech.sec. carbs on an Edel. Street Ram on my 355 for a while. I had 4000 rpm of stall, carbs were stock except for 31 squirters. Air speed didn't do it justice until about 5000rpm, however it idled fine and never flatspotted, was just a bit flat at WOT until the air speed picked up a bit. If I was to do it again I would use a pair of 1850's, just to slow the sec. opening a bit.

The 450's should be bearable, just up the squirter size until your good.