Another Carb question...

Posted by: Benforsp

Another Carb question... - 12/04/17 04:22 PM

Hey guys,

Have a BB with a moderately sized roller in it. It’s a 451 and am running a Holley 950 on it. I’ve tried a number of things but am having trouble with it back firing through the carb when I mash the throttle. Idles great revs great on the primaries but once I move to the secondaries no go. Backfired and nearly stalls the engine. I have the dizzy locked out at 37 degrees. Have tried it anywhere between 35-38 with the same result. I have 30 cc accelerator pumps. On the secondaries I’m using the brownish red pump cam with a #37 squirter. This carb ran great on a buddies BBC with similiar cubes and a big solid cam. Any help is appreciated and if you need more info please let me know. Thanks!
Posted by: Thumperdart

Re: Another Carb question... - 12/04/17 06:41 PM

For starters, make sure the secondary squirter is not plugged but those need help regardless......... thumbs
Posted by: Dave Hall

Re: Another Carb question... - 12/04/17 07:05 PM

Sounds like it's time for a Dom Dominator... grin
Posted by: Thumperdart

Re: Another Carb question... - 12/04/17 07:48 PM

I must say even healthy small blocks have picked as much as 5-6 mph and equal et's as long as they have gear and a good converter but that's when you know the 4150 was either not enuff or really jacked in terms of the tune............ beer
Posted by: Dragula

Re: Another Carb question... - 12/04/17 08:02 PM

ITS LEAN...under load.

I like #45 squirters in everything!
Posted by: DaveRS23

Re: Another Carb question... - 12/04/17 08:23 PM

iagree

I've also had good luck with big squirters at least in the primaries.
Posted by: RapidRobert

Re: Another Carb question... - 12/04/17 08:30 PM

Quote:
It’s a 451 and am running a Holley 950 on it. I’ve tried a number of things but am having trouble with it back firing through the carb when I mash the throttle. This carb ran great on a buddies BBC with similiar cubes and a big solid cam.
I'm assuming nothing was changed on the carb from him to you. at what RPM are you mashing it? vacuum leak? at the offending RPM is the carb being delivered enough fuel? A locked dist means no vac adv so no potential rotor phasing issue (near zero vac so a can loses its timing/rotor phasing shifts back CW). can you run it up to the offending RPM in your driveway & mash it & check the squirt? (fuel delivery needs would be less than a driveing mashing but it'd be a start). Keep us posted.
Posted by: Benforsp

Re: Another Carb question... - 12/05/17 03:51 AM

Originally Posted By Thumperdart
For starters, make sure the secondary squirter is not plugged but those need help regardless......... thumbs
[quote=Thumperdart]

Squirters are not clogged. I have gone from a 31 to a 37 and checked to make sure everything was clean.
Posted by: Benforsp

Re: Another Carb question... - 12/05/17 03:53 AM

Originally Posted By Dragula
ITS LEAN...under load.

I like #45 squirters in everything!


It has 31’s up front and 37’s in the back so you think 45’s in front and back?
Posted by: mopar dave

Re: Another Carb question... - 12/05/17 04:30 AM

45's are huge in a 950. Shouldn't need any more than 35's. I once had 45's in a 1100 dominator on a 511 and it didn't like it. Now have 33 front and rear.
Posted by: BradH

Re: Another Carb question... - 12/05/17 08:52 AM

What model # "950"?

What's the COMPLETE carb tune?

CR?

Cam specs?

Gear, tire, stall speed?

Launch RPM?
Posted by: Benforsp

Re: Another Carb question... - 12/05/17 10:08 AM

Originally Posted By BradH
What model # "950"?

What's the COMPLETE carb tune?

CR?

Cam specs?

Gear, tire, stall speed?

Launch RPM?


It's a 0-82951.

The engine has 440-1 heads, CR is 11.0 to 1. Pump Gas. Mostly a street car, 4.56 gear, 275/60/15 MT tires, 3800 stall.

I haven't really driven the car much. I am just trying to get the carb to respond right then I can fine tune it, but I feel like I am too far off.
Posted by: Thumperdart

Re: Another Carb question... - 12/05/17 11:05 AM

Originally Posted By Benforsp
Originally Posted By BradH
What model # "950"?

What's the COMPLETE carb tune?

CR?

Cam specs?

Gear, tire, stall speed?

Launch RPM?


It's a 0-82951.

The engine has 440-1 heads, CR is 11.0 to 1. Pump Gas. Mostly a street car, 4.56 gear, 275/60/15 MT tires, 3800 stall.

I haven't really driven the car much. I am just trying to get the carb to respond right then I can fine tune it, but I feel like I am too far off.


I did one for a BB Chevy and does not have these issues so call me and I will shoot you the tune and compare to yours if u'd like....... thumbs
Posted by: BradH

Re: Another Carb question... - 12/05/17 04:15 PM

You still haven't given any details of the cam or the carb's current tune. That doesn't make it any easier to try and help.

The 0-82951 is the "Street HP", right? Check the actual venturi diameter and see if it's 1.38" or 1.56" I've seen conflicting specs, depending upon when the carb was made I guess. The tune would likely vary quite a bit depending upon which size it is.

I have no first-hand experience w/ the "Street HP" stuff. I'd hope that they're "flexible" enough to be tuned to work w/ your combination.
Posted by: Benforsp

Re: Another Carb question... - 12/05/17 06:59 PM

Originally Posted By BradH
You still haven't given any details of the cam or the carb's current tune. That doesn't make it any easier to try and help.

The 0-82951 is the "Street HP", right? Check the actual venturi diameter and see if it's 1.38" or 1.56" I've seen conflicting specs, depending upon when the carb was made I guess. The tune would likely vary quite a bit depending upon which size it is.

I have no first-hand experience w/ the "Street HP" stuff. I'd hope that they're "flexible" enough to be tuned to work w/ your combination.


Hey BradH,
This is from the Holley site. It should be all the same with the exception of the larger rear squirter, accelerator pump change and 2.5 PV's.

I'm not with the car at this moment to know any of the air bleed sizes. What other information is needed?

Barrels
4
Booster
Down Leg
Brand
Holley
CFM
950
Choke
None
Emission Code
3
Finish
Shiny
Fuel
Gasoline
Fuel Inlet
Dual
Material
Zinc
Model
4150
Primary Main Jet
84
Primary Power Valve
6.5
Primary Pump Nozzle Size
31
Product Type
Carburetor
Secondaries
Mechanical
Secondary Main Jet
84
Secondary Pump Nozzle Size
31
Supercharged Application
No
Throttle Bore
1.750 inch
Venturii Size
1.563 inch
Part Number
0-82951
Posted by: Benforsp

Re: Another Carb question... - 12/05/17 07:07 PM

Originally Posted By BradH
You still haven't given any details of the cam or the carb's current tune. That doesn't make it any easier to try and help.

The 0-82951 is the "Street HP", right? Check the actual venturi diameter and see if it's 1.38" or 1.56" I've seen conflicting specs, depending upon when the carb was made I guess. The tune would likely vary quite a bit depending upon which size it is.

I have no first-hand experience w/ the "Street HP" stuff. I'd hope that they're "flexible" enough to be tuned to work w/ your combination.


http://www.compcams.com/Company/CC/cam-specs/Details.aspx?csid=739&sb=1

Installed straight up with 1.6 ratio rockers to bring the lift up.
Cam Detail....
Posted by: BradH

Re: Another Carb question... - 12/06/17 10:28 AM

Hmmm...
-> 440-ish cubes with "big" heads that work better on 500+"
-> moderate CR
-> 260+ at .050" street roller
-> limited stall speed
-> street-tuned(?) carb w/ 1.56 venturi & downleg boosters
...

Starting to look to me like a mismatch of parts, of which the carb is likely one of the "What's wrong with this picture?" items, along w/ the converter being too tight. The carb may be workable, but I'll take a s.w.a.g. that the way it's set up right now is... uh... "wrong".

FWIW, the backfire upon WOT acceleration (IIRC what you posted originally correctly) would point me to verifying fuel bowl levels are high enough & accelerator pump passages & pumps are clear and working properly, and then making sure none of the air bleeds are plugged up (dirt or whatever).

My twocents (which are usually only worth about $.0125) is to tear it down completely and get (buy/borrower) the proper tools to measure all the circuits & orifices so that you KNOW what the current tune is. Post 'em, and get some feedback on what makes sense to change from there.

This is still a pretty basic carb... it can't be THAT effed up.
Posted by: Thumperdart

Re: Another Carb question... - 12/06/17 11:08 AM

I agree w/Brad.......miss matched parts and not "ideal" however, I've modded a few of those w/excellent results and one friend picked up 500 rpm's and mph in a v-drive boat and the rest complain of traction issues........ thumbs If you're handy w/drillin n tapping I will help ya out for a cold beer............
Posted by: Benforsp

Re: Another Carb question... - 12/06/17 11:35 AM

Originally Posted By BradH
Hmmm...
-> 440-ish cubes with "big" heads that work better on 500+"
-> moderate CR
-> 260+ at .050" street roller
-> limited stall speed
-> street-tuned(?) carb w/ 1.56 venturi & downleg boosters
...

Starting to look to me like a mismatch of parts, of which the carb is likely one of the "What's wrong with this picture?" items, along w/ the converter being too tight. The carb may be workable, but I'll take a s.w.a.g. that the way it's set up right now is... uh... "wrong".

FWIW, the backfire upon WOT acceleration (IIRC what you posted originally correctly) would point me to verifying fuel bowl levels are high enough & accelerator pump passages & pumps are clear and working properly, and then making sure none of the air bleeds are plugged up (dirt or whatever).

My twocents (which are usually only worth about $.0125) is to tear it down completely and get (buy/borrower) the proper tools to measure all the circuits & orifices so that you KNOW what the current tune is. Post 'em, and get some feedback on what makes sense to change from there.

This is still a pretty basic carb... it can't be THAT effed up.


I had the carb apart and physically went through everything. I did not go to the extent of measuring all of the orifices though...I've never actually done carbs to that point.
Posted by: Thumperdart

Re: Another Carb question... - 12/06/17 11:53 AM

You have to to in order to have a base tune and to know what needs to be changed to bring it around to your cars needs...........no other way........ thumbs
Posted by: Benforsp

Re: Another Carb question... - 12/07/17 04:17 PM

Originally Posted By Thumperdart
You have to to in order to have a base tune and to know what needs to be changed to bring it around to your cars needs...........no other way........ thumbs


Thumper,

What all is involved in this and what specialty tools are needed?
Posted by: Thumperdart

Re: Another Carb question... - 12/07/17 05:04 PM

We need to talk, too much to type..........
Posted by: BradH

Re: Another Carb question... - 12/07/17 05:32 PM

Originally Posted By Thumperdart
We need to talk, too much to type.......... with one finger

haha
Posted by: fast68plymouth

Re: Another Carb question... - 12/07/17 07:16 PM

Quote:

-> 440-ish cubes with "big" heads that work better on 500+"
-> moderate CR
-> 260+ at .050" street roller
-> limited stall speed
-> street-tuned(?) carb w/ 1.56 venturi & downleg boosters


Those are my thoughts as well.

1.56 venturi downleg carbs can be a real challenge to "tune the bog out of".

IMO, I think a main body swap to something with a smaller venturi would ultimately yield better results(I'd probably use a 1.400 x 1.750)....... But if I were trying to work around that combo, I would expect to have to run fairly large squirters, and probably a 50cc pump/cam on the secondary side, and I would absolutely ditch the 2.5 pv.
You want to get a head start on the main circuit enrichment when going to wot...... It would get a 6.5 if I were messing with it.

There is a lot of plenum and runner volume in that combo, and not a lot of cubes generating signal.......... It's going to need a lot of help(pump shot) to give the motor something to work with until the main circuit is activated.

Not what you asked, but there is some performance left on the table with that cam, in that combo.
Posted by: Dadodgekid

Re: Another Carb question... - 12/07/17 07:34 PM

whats the fuel pressure set to...
Posted by: Thumperdart

Re: Another Carb question... - 12/07/17 07:52 PM

Originally Posted By BradH
Originally Posted By Thumperdart
We need to talk, too much to type.......... with one finger

haha


You know me too well...............lol
Posted by: Benforsp

Re: Another Carb question... - 12/08/17 09:19 AM

Originally Posted By fast68plymouth
Quote:

-> 440-ish cubes with "big" heads that work better on 500+"
-> moderate CR
-> 260+ at .050" street roller
-> limited stall speed
-> street-tuned(?) carb w/ 1.56 venturi & downleg boosters


Those are my thoughts as well.

1.56 venturi downleg carbs can be a real challenge to "tune the bog out of".

IMO, I think a main body swap to something with a smaller venturi would ultimately yield better results(I'd probably use a 1.400 x 1.750)....... But if I were trying to work around that combo, I would expect to have to run fairly large squirters, and probably a 50cc pump/cam on the secondary side, and I would absolutely ditch the 2.5 pv.
You want to get a head start on the main circuit enrichment when going to wot...... It would get a 6.5 if I were messing with it.

There is a lot of plenum and runner volume in that combo, and not a lot of cubes generating signal.......... It's going to need a lot of help(pump shot) to give the motor something to work with until the main circuit is activated.

Not what you asked, but there is some performance left on the table with that cam, in that combo.


Fast,

The carb had 6.5's in it and I can put them back in no sweat. I knew the cam was pretty big but had talked to some folks and they suggested it would be suitable, but I would consider other options, what might you advise?
Posted by: Benforsp

Re: Another Carb question... - 12/08/17 09:24 AM

Originally Posted By Dadodgekid
whats the fuel pressure set to...


Kid,

The fuel pressure is 6.5-7 PSI. I run an electric pump with a return style fuel system with -8 lines.
Posted by: fast68plymouth

Re: Another Carb question... - 12/08/17 10:48 AM

It would require a conversation before I would recommend any other cam, but I think getting rid of some seat timing would be a step in the right direction.

You said "pv's"....... If you're still running one in the secondary side, remove it and replace with a plug....... And jet up accordingly(around 8 sizes).
Posted by: Thumperdart

Re: Another Carb question... - 12/08/17 11:30 AM

Originally Posted By Benforsp
Originally Posted By Dadodgekid
whats the fuel pressure set to...


Kid,

The fuel pressure is 6.5-7 PSI. I run an electric pump with a return style fuel system with -8 lines.


Still waiting for a call...........lower the pressure to 5.5 max IF you run .120 needle n seats which you should....... callme
Posted by: Cab_Burge

Re: Another Carb question... - 12/08/17 12:53 PM

Originally Posted By Benforsp
Originally Posted By BradH
You still haven't given any details of the cam or the carb's current tune. That doesn't make it any easier to try and help.

The 0-82951 is the "Street HP", right? Check the actual venturi diameter and see if it's 1.38" or 1.56" I've seen conflicting specs, depending upon when the carb was made I guess. The tune would likely vary quite a bit depending upon which size it is.

I have no first-hand experience w/ the "Street HP" stuff. I'd hope that they're "flexible" enough to be tuned to work w/ your combination.


http://www.compcams.com/Company/CC/cam-specs/Details.aspx?csid=739&sb=1

Installed straight up with 1.6 ratio rockers to bring the lift up.
Cam Detail....

Did you degree the cam or did you install it with the dots "straight up" ? If not degreed you should do that now twocents
I would advance it so the intake lobes are from 107 to 105 max lift after top dead center up
I've ran a very similar cam (260@.050 int., 266@.050 ex. ground on a 108 LC installed two degrees advanced on the intake lobes)) Comp solid roller with 1.6 ratio rockers, no bog even with a six pack set up after I had to sort it out.
I had another bracket motor with a Holley 80496 950 HP on a TM7 intake with ported 906 heads and a bigger Crane solid roller and it never bogged ever, that cam was advanced 4 degrees work
Posted by: Benforsp

Re: Another Carb question... - 12/08/17 01:46 PM

Originally Posted By Cab_Burge
Originally Posted By Benforsp
Originally Posted By BradH
You still haven't given any details of the cam or the carb's current tune. That doesn't make it any easier to try and help.

The 0-82951 is the "Street HP", right? Check the actual venturi diameter and see if it's 1.38" or 1.56" I've seen conflicting specs, depending upon when the carb was made I guess. The tune would likely vary quite a bit depending upon which size it is.

I have no first-hand experience w/ the "Street HP" stuff. I'd hope that they're "flexible" enough to be tuned to work w/ your combination.


http://www.compcams.com/Company/CC/cam-specs/Details.aspx?csid=739&sb=1

Installed straight up with 1.6 ratio rockers to bring the lift up.
Cam Detail....

Did you degree the cam or did you install it with the dots "straight up" ? If not degreed you should do that now twocents
I would advance it so the intake lobes are from 107 to 105 max lift after top dead center up
I've ran a very similar cam (260@.050 int., 266@.050 ex. ground on a 108 LC installed two degrees advanced on the intake lobes)) Comp solid roller with 1.6 ratio rockers, no bog even with a six pack set up after I had to sort it out.
I had another bracket motor with a Holley 80496 950 HP on a TM7 intake with ported 906 heads and a bigger Crane solid roller and it never bogged ever, that cam was advanced 4 degrees work


Cab,

I had thought about installing it advanced and regret not doing so at this point. The cam was degreed.
Posted by: PorkyPig

Re: Another Carb question... - 12/08/17 01:54 PM

Originally Posted By Benforsp
Originally Posted By Thumperdart
You have to to in order to have a base tune and to know what needs to be changed to bring it around to your cars needs...........no other way........ thumbs


Thumper,

What all is involved in this and what specialty tools are needed?

How much carb tuning experience do you really have? It sounds like you may be in over your head.
Posted by: Benforsp

Re: Another Carb question... - 12/08/17 01:57 PM

Originally Posted By Thumperdart
Originally Posted By Benforsp
Originally Posted By Dadodgekid
whats the fuel pressure set to...


Kid,

The fuel pressure is 6.5-7 PSI. I run an electric pump with a return style fuel system with -8 lines.


Still waiting for a call...........lower the pressure to 5.5 max IF you run .120 needle n seats which you should....... callme


I tried! No Answer.
Posted by: fast68plymouth

Re: Another Carb question... - 12/08/17 02:47 PM

Quote:
I had thought about installing it advanced and regret not doing so at this point. The cam was degreed.


If the cam was installed as per the cam card in the link you posted, then it's installed 4deg advanced.
The advance is ground into the cam.

Advancing it "more" isn't going to cure your problem.
Posted by: Tommy D

Re: Another Carb question... - 12/09/17 08:15 AM

It was mentioned earlier to check the float levels. I did not see a response where that was checked. Where do you run the float level?