Hot Rod Forum banner

1 - 12 of 12 Posts

·
Registered
Joined
·
5 Posts
Discussion Starter #1
Hi guys any help with this would be greatly appreciated. I have a sbc 355 in a 1977 chevy c10 with a built turbo 350 and 2800-3200 stall. The problem is I can't get the edelbrock 750 performer series to run rich enough at wide open throttle. I have changed primary and secondary jets and metering rods and springs and fuel pump and still can't get it to run rich enough any advice would be greatly appreciated.
 

·
Registered
Joined
·
1,542 Posts
Need more info.
2WD? Built motor? Stock?
Cam? Gears?
Symptoms?
AFR Meter?
Read Plugs?
Be prepared to answer even more questions, your problem is very vague.
There is tons of knowledge here (not me, LOL!) they need more background though.
 

·
Registered
Joined
·
5 Posts
Discussion Starter #3
The motor is a 355 with10:5:1 compression, 305 vortec heads ported and polished, elgin cam lifts is 450 intake 460 exhaust duration @50 is 224 intake and 224 exhaust on a 114 lobe center line, speed pro forged flat top pistons 4 valve reliefs, hei distributor, headers. I am reading it with a innovative motorsports afr gauge. It's 2wd. 3.07 gears. The problem I'm having is I can't get it to run rich enough at wide open throttle. I have a 116 primary jet and a 119 secondary jet and I went the richest on the chart for metering rods on primaries. At cruise it runs perfect good afr and it has a lean stumble off and on but at wide open it's reading anywhere between 14:8 to 15:2
 

·
Registered
Joined
·
1,211 Posts
The charts may not get you where you need to be.
  • EDELBROCK CARB TUNING NOTES;
1 - .003 jet size change (with no rod change) will equal 2 stages >/< (richer/leaner)

1 - .002 rod size change (with no jet change) will equal 1 stage >/< (richer/leaner)


 

·
Registered
Joined
·
5 Posts
Discussion Starter #5
Okay, so that's a 120 jet that's on the link that's attached to your post. So should I go richer on the primaries as well I have another pair of 119 jets?
 

·
Registered
Joined
·
1,542 Posts
Okay, so that's a 120 jet that's on the link that's attached to your post. So should I go richer on the primaries as well I have another pair of 119 jets?
I'm no expert, but if your cruise is good just change the secodaries first and see where that goes.
Sounds like you are very close already.
 

·
Registered
Joined
·
5 Posts
Discussion Starter #7
I'm no expert, but if your cruise is good just change the secodaries first and see where that goes.
Sounds like you are very close already.
I have the biggest jet in the secondaries already, but the weird thing I noticed is that I had a 113 in the secondaries before and when I went to a 119 it got leaner so that's why I'm at a lost here
 

·
More for Less Racer
Joined
·
20,926 Posts
Search for and read up on "emulsion tubes in secondary of Edelbrock 750".

The emulsion tube becomes a restriction when big jets are used.
 

·
Registered
Joined
·
15,298 Posts
Sounds like a reversion issue where the cam and heads are working against the intake and the heads. This is a not all that an uncommon problem, it can be frequently seen in effect on an engine dyno where there are odd places where power drops and the AFR goes wonky.

I’d play with different thicknesses of carb spacers to see if changing plenum volume has an effect on carb signal. Same kind of dinking around on the exhaust side as well in lengths and diameters of pipe to see if this is a wave frequency issue. There is a lot of chatter between the intake and exhaust side during overlap. One complaint of straight timed cams and open exhausts is over scavenging the exhaust, this shows as a lean drop for which adding jet size doesn’t fix.

If you have some wood working tools you can save some cash by making your own intake spacers from lumber, since you will want some as 4 holders others as 2 primary holes with an open secondary and others just open and all in different thicknesses. These are just for test purposes, if or when you stumble on an improvement you can then consider a permanent operational solution.

Another thought is your fuel pump may not be keeping up with top end consumption, watching fuel pressure at the carb should give a hint about this. My preferred design of street carb fuel systems is a high capacity low pressure TBI pump with a bypass regulator and return line. This provides more capacity than WOT fuel burn so the pressure at the carb is a steady 5.5 PSI no matter the load and RPM on the engine. Another trick I do on the Carter/Edelbrock is to use thin aluminum from a soda can to make a splash guard over the secondary jet’s. This is a an L shape with a second lip bent on the bottom to provide support so the section over the jet can not flutter. I install these with the bulkhead piece that comes on these carbs as the soda can aluminum is thin enough to make a tight fit with these parts. This prevents the inflow fuel from spraying directly onto the jets which in my so called mind may alter the secondary fuel ratios since this disturbance on the jets may not net the same flow as a calm fuel level above these jets. No matter how you slice it a carburetor is just a hap-hazard device hoping to throw a reasonable average AFR at the motor in any dynamic moment. Willy’s Racing Carburetors has a few tricks to smooth these issues but the cost is beyond the average guy‘s VISA card limits.

So I buried a few ideas in here you’ll have to tease out and test.

Bogie
 

·
Registered
Joined
·
5 Posts
Discussion Starter #10
Sounds like a reversion issue where the cam and heads are working against the intake and the heads. This is a not all that an uncommon problem, it can be frequently seen in effect on an engine dyno where there are odd places where power drops and the AFR goes wonky.

I’d play with different thicknesses of carb spacers to see if changing plenum volume has an effect on carb signal. Same kind of dinking around on the exhaust side as well in lengths and diameters of pipe to see if this is a wave frequency issue. There is a lot of chatter between the intake and exhaust side during overlap. One complaint of straight timed cams and open exhausts is over scavenging the exhaust, this shows as a lean drop for which adding jet size doesn’t fix.

If you have some wood working tools you can save some cash by making your own intake spacers from lumber, since you will want some as 4 holders others as 2 primary holes with an open secondary and others just open and all in different thicknesses. These are just for test purposes, if or when you stumble on an improvement you can then consider a permanent operational solution.

Another thought is your fuel pump may not be keeping up with top end consumption, watching fuel pressure at the carb should give a hint about this. My preferred design of street carb fuel systems is a high capacity low pressure TBI pump with a bypass regulator and return line. This provides more capacity than WOT fuel burn so the pressure at the carb is a steady 5.5 PSI no matter the load and RPM on the engine. Another trick I do on the Carter/Edelbrock is to use thin aluminum from a soda can to make a splash guard over the secondary jet’s. This is a an L shape with a second lip bent on the bottom to provide support so the section over the jet can not flutter. I install these with the bulkhead piece that comes on these carbs as the soda can aluminum is thin enough to make a tight fit with these parts. This prevents the inflow fuel from spraying directly onto the jets which in my so called mind may alter the secondary fuel ratios since this disturbance on the jets may not net the same flow as a calm fuel level above these jets. No matter how you slice it a carburetor is just a hap-hazard device hoping to throw a reasonable average AFR at the motor in any dynamic moment. Willy’s Racing Carburetors has a few tricks to smooth these issues but the cost is beyond the average guy‘s VISA card limits.

So I buried a few ideas in here you’ll have to tease out and test.

Bogie
I already have a 4 hole phenolic spacer with a thin metal plate on the bottom of the spacer. I have 1 and 5/8 long tube headers with 2.5 exhaust as straight as you can get it all the way back. And with the pop can idea your talking about is to bend it in a l shape over the original divider blocking fuel from coming straight down on the secondary jets am I understanding that correctly??.
 

·
Registered
Joined
·
15,298 Posts
First you may need to play with different types and sizes of spacer, we’re trying to find if a change to the volume of the plenum changes the carb response.

The same goes for headers and exhaust, we’re trying to see if changing exhaust velocity, volume, and tuning or frequency lengths changes the carb response.

These systems talk to each other a lot, the evidence of this is when one cylinder blows a piston and those pieces show up in other cylinders. The more overlap the cam has the more this happens.

So what I’m trying to do is run some low-buck experiments to see if there is a pulse frequency or a synchronous flow displacement that is upsetting the carb. An engine is not like a flow bench where air streams in the in to out direction. The intake and exhaust system is dynamic in both directions. Changing volumes and lengths of the major components will vary these effects.

Typically carbs go lean up on the top end, where attempts to riches the mixture tends to mess up everything, so you gotta work through this stuff one at a time then maybe combinations if you see a trend, the Smokey method of cut and try.

My AFB/AVS gadget is to make a shield over the secondary jets, unfortunately the only example I have at the moment is sitting in a carb on a running engine. In cross section it is an L shape with a lip facing down at the end of the lower leg it’s just there to keep the lower leg from obstructing the jet. The upper leg is trimmed to fit in the slot with the factory divider. So my gadget is trimmed to fit that slot which has some taper. It is assembled into the slot with the factory divider it is a tight fit but the thin stiff aluminum of the gadget is not so thick as to obstruct insertion to the slot of both items. The whole purpose is to prevent the fuel inflow directly above the secondary jets from impinging directly onto the jet thusly keeping the jet pulling liquid fuel not a mousse of air and fuel.

Bogie
 

·
Google "Tunnel Ram 406"
Joined
·
1,137 Posts
Check your fuel pump or fuel line. You may be restricting the fuel pressure at high rpm.
 
1 - 12 of 12 Posts
Top