Thread:

vac secondary bog and backfire

I got a used holley marine 750 vacuum secondary, rebuilt it and just got it on the water today for the first time. around 3500 rpm, if I ease into the throttle, it will stumble and 1 time out of 4 it will have a small backfire before the rpms increase. So it seems like it is going lean as the secondarys are opening. I am going to try some other secondary springs, but should I also richen the primary jets a step or two? The bog I can handle with springs, but the lean backfire has me concerned about the overall a/f ratio on the primaries.

Put the light spring in it and try again. That will most likely fix it. Only do one thing at a time when tuning. So you can see the change and know what to do next.

I had to put on the 50cc acc. pump on the 750 vs. I ran on my old boat. I changed the springs, pump cam and squirters and could make it better, but not fix it. I put on the bigger pump and never had another problem.

tp

50cc pump and lighter spring. Let me be the first to say it...... get a double pumper.

To answer some of the q's, the power valve is a 10.5. I looked up the holley factory specs for a new R9015 (R9015-1) and it has 10.5 power valve, 76 primary, 37 plate secondary (equivalent to 80 jet according to a chart I found), -25 stock squirter, white acc pump cam. Mine had 74 jets in it so I just put those back in, and kept the -37 secondary plate. I dont know what secondary spring is in it now, I am ordering the spring assortment today.
I thought about the pump cam, size, and squirter deal, but actually when I hammer it, the bog isnt near as bad and it will not backfire. Its only when I ease into it from 3000 rpm up that it will bog worse and pop. Again its the backfire that really concerns me, the bog hopefully will be taken care of by the right spring. Im thinking I should jet up first then swap springs. I cant read plugs worth a damn so im thinking hard about an o2 sensor setup.
The engine is a 455 olds with mild cam (19" vacuum at 850 rpm idle). The holley runs much better than the q-jet that I have been screwing with all damn summer. I picked up 400 rpm over the q-jet, although the q-jet never was right. the boat is still damn slow though. Fuel pressure holds steady 6 psi regardless of rpm. I paid close attention to that, because when I had the q-jet problems I thought for sure I was running out of fuel.

Originally Posted by HammerDown

Hey Slim > did you totally go through the metering blocks and carb body > confirm no blockages?
I can check my 715 for how it's set-up and give you some suggestions > it works GREAT!
It's ALWAYS best to check vacuum at cruising RPM then pic a PV 1-2" below that

Yeah went through it all, soaked and sprayed with carb cleaner, then blow out with air compressor. So should be good. plus it was pretty clean to start with, only minor staining from fuel.
I would love to hear another setup. Mine runs way better than the q-jet but some reference numbers for a well dialed carb would be appreciated.
Good point on the PV tuning. this was first time out on the new carb and I forgot to take a vacuum gauge. I figured 10.5 was "safe" to start with then go from there.