Carb problem on 5.7 hemi

[heminut]

I've got a Holley 750 vac. sec. carb with a Proform body on the hemi. Right now I have the heaviest spring in the secondary vacuum pod and a #40 accelerator pump squirter in the carb. This carb has always had a bog when floored from just off idle on this engine. When I had the carb on my 392 hemi it had the lightest vacuum secondary spring and a #31 accelerator pump squirter and never had a bog of any kind. Changing the squirter and spring has helped some, but it still has a slight bog and will pop through the carb if floored below 2000 RPM. Any body have any ideas about whats causing this and how to fix it? It's got me stumped!

[Chryco Psycho]

I assume you have checked your float height  to be sure it is not low front or rear you may need more squirter , you could try a 45 with a hollow screw , it sounds like it is going lean if it is popping when the throttle is opened so for what ever reason it seems to want a lot of fuel ,
I doubt changing the secondary spring will have any effect on instant throttle opening   

[heminut]

Neil I didn't think the spring would have much effect, but put the heaviest one in so I would know for sure that the secondary opening was not part of the problem. Once I get the bog problem solved I'll tune the secondaries. What baffles me is that I had no problems with this carb on the 392 with the #31 squirter in it. I'm thinking the 5.7 cam with 114 deg. LSA has a lot less overlap than the 392 cam had with 106 deg. LSA and maybe this is why this engine wants more fuel on the bottom end? This thing is a beast over 3000 RPM!

[Chryco Psycho]

the cam will have an effect for sure but what you need is more timing at low RPM so if you can change the timing in the computer to give more initial timing without having too much timing higher up

[heminut]

But it's already popping through the intake at lower RPMs, wouldn't more initial timing aggravate the problem?

[Chryco Psycho]

no , the popping is lean not advanced timing

[Changin Gears]

A lean idle mixture can contribute to this problem.

[heminut]

I understand that the popping is a lean mixture problem, I just don't understand how advancing the initial timing can help the problem.

the cam will have an effect for sure but what you need is more timing at low RPM so if you can change the timing in the computer to give more initial timing without having too much timing higher up

[Chryco Psycho]

with long duration cams engine response is lost so advancing the timing helps to overcome the lack of responsiveness at low RPM , I typically run 14-18* initial timing in engines with longer durattion cams

[heminut]

Okay, I see what your saying now. Other than the bog, the bottom end responsiveness is pretty snappy. If I roll into the throttle to avoid the bog instead of just flooring it the power is there. Like I said, I haven't checked the curve with a laptop, and need to reload the program into my PC to check what type of curve MSD put in there. I don't think the duration on the cam is all that long though, most of the F.I. roller cams have a relatively short duration with a higher lift compared to a flat tappet cam. The cam I have is the factory SRT 6.1 cam and if I remember right the duration was only around 217 deg. @ .050 but the lift was in the .530 range. I'll try your suggestion on the 45 squirter and hollow screw, going from a 31 to a 40 seemed to help the problem but didn't eliminate it. I've thought all along that it was a pump shot problem, but when I jumped from a 31 to a 40 and it didn't cure the problem and the carb worked great with the 31 on the old hemi I thought there might be something else involved besides a lean pump shot. BTW Neil, the float levels are good. Thanks for the input, it seems I'm on the right track, just need to go a little farther with it.

[MEK-Dangerfield]

For what it's worth guys, I'm enjoying this thread.   

It's hard to believe you need a accelerator pump squirter larger than a 40, but I guess it's possible. I just keep going back to the popping thing. Have you checked your spark plugs to see if you are running too lean? If your jets are too small, going to a larger squirter will only compensate that somewhat.

[heminut]

Mike, I haven't checked the plugs and it's hard to read them anyway with the unleaded crap we get nowadays. I don't think that's an issue, as the carb was fine on the other engine and the problem is basically a WOT problem just off of idle. I've never ran much over a 35 pump squirter on anything before, but then again I've never tuned an engine with this type of cam or timing either!

[71chally416]

I've never ran much over a 35 pump squirter on anything before, but then again I've never tuned an engine with this type of cam or timing either!

Or with as much air flow through the heads. I'll bet those 5.7 heads will eat up the old Hemi heads on a flow bench 

[MEK-Dangerfield]

I'll say it again, I love this thread.   

The fact that this carb worked on another engine doesn't mean it will play nice with your new engine and combo.

I agree, it sounds like an accelerator pump problem. The bog would explain this, but the popping, not so much. That is why I asked about what your plugs looked like.

[heminut]

Or with as much air flow through the heads. I'll bet those 5.7 heads will eat up the old Hemi heads on a flow bench 

I wouldn't take that bet! This engine is a lot stronger than the 392 was, and the 392 had 47 cubic inches and a more radical cam than the 5.7.