850 Ultra XP lean bogs when secondaries open

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joes68340s

joes68340s

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Just bought this carb as soon as i try to open secondaries it lean bogs. Played with jetting running a plug in the rear now throwing jets at it still bogs 408 W2s 12-1 650 solid roller. Indy 360 intake. It idles fine just wont transition . stock jetting 84-84 with 4.5 power valves Im at 88 primary with valve and going to try 98 rear. It just seems excessive.
 
The last time I started jetting to extra large secondary jets it turned out I had a fuel delivery issue. Just my experience. Is it possible it's running out of fuel down track?
 
fuel pressure is good. I can rev it in nuetral and it bogs as soon as the secondaries start to open.
 
Have you stepped up to larger discharge nozzles?
 
Start with accelerator pump adjustors, accelerator cams, squirter nozzles. These are the easy.
 
Pumps are adjusted fine tried 35 squirters no help. Primary side works fine when seperated from secondaries. Discharge in squirters is fine through the intire shot.
 
joes68340s said:
Pumps are adjusted fine tried 35 squirters no help. Primary side works fine when seperated from secondaries. Discharge in squirters is fine through the intire shot.
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are you sure it's a lean stumble? Perhaps it's flash flood stumble ?????
 
I thought that at first as it did it out of the box. Plugs look good. It seems to bog when the secondaries crack open.
 
35 squirter seems small. That was the point where I finally got a rich bog in the PRIMARY side of Vixen's 450. Just to give some comparison......poor as it is. lol
 
those ship with extra secondary throttle arms, with various bends, that change the ratio of when the secondary starts to open.

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I typically go 1:1, and square jet my "big" motors... but i understand for street use that may be excessive.
Regardless, throw the 1:1 linkage arm on it, and see if its REALLY stumbling when the secondary's start to open )and it gets a bunch of air) , or not. at least you'll have data to compare against if you throw that 1:1 in there and no change, vs it helps / hurts. That should take 5 mins.
 
90% of the time, when you have a lean bog like that, on a fully built engine with big ports, big single plane intake, huge cam etc. the issue is insufficient pump shot. I The first thing I would do is to get a 50CC accelerator pump kit, and an accelerator pump tuning kit. The pump tuning kit is expensive, but if you work with Holley carbs, you will be using that kit for the rest of your life. Start with the 50cc kit in the back of the carb and see how that works, the 50cc may cure the lean bog by itself, but you will probably need to work with the cams and bigger squirters to get it to run exactly where you want it.
 
Put some 16 hole annular boosters in it.
2 50cc pumps.
That will fix your lean and distribute fuel better with less main jet.
What are you doing for fuel return to tank ?
 
joes68340s said:
Just bought this carb as soon as i try to open secondaries it lean bogs. Played with jetting running a plug in the rear now throwing jets at it still bogs 408 W2s 12-1 650 solid roller. Indy 360 intake. It idles fine just wont transition . stock jetting 84-84 with 4.5 power valves Im at 88 primary with valve and going to try 98 rear. It just seems excessive.
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The primary purpose of the pump shot is to cover for when the throttle is opened suddenly and vacuum drops to zero. If you disconnect the secondary pump shot and the then open the secondaries slowly it should rev and pick up RPM's without a problem. If it doesn't do that then there is an issue with the transfer fueling.

Pull it apart and make sure nothing is blocked.
 
The best method I've found is to hold your phone over the carb and video it while someone mashes the throttle. You will see/hear what happens. Is the pump shot late? Too short? Too long?? (Another member on here was doing this video method and I picked up on it and found it quite helpful.)

I've found that the pump shot is usually too short and the bigger 50cc pump with a smaller rear nozzle (33-35) makes for a longer shot that carries through the transition. My 408 has an 850 Holley-style with a Super Victor so that's how I solved the riddle. Has 2 50cc pumps and 37 shooters in front and 35 in back (both are tibes shooters).

Of course pump cams and hole position of those cams also matters. I find that hole position 2 is useful if you stage the car with the throttle partly open (like I do holding it at about 2200-2400 rpm). Position 2 usually gives you more of a given cam whereas position 1 is already partly "used up" at that throttle position.

As for jets, I'm at 81 frt & 82 rear. Picked up 1/2 mph when I leaned the front out so gonna lean it out again and see what happens. Plugs still look dark so we shall see....
 
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I run 37/39 nozzles 50cc pumps, blue cams primary #2, secondary #1 hole on my stock stroke 340. This tune gives me the best 60' time with progressive linkage and .020 t-slot, 6 psi fuel pressure, 1/2 site glass up front, slightly above 1/2 secondary bowl. I also make it a habit of purging any air prior to the first pass of the day.
 
mderoy340 said:
I run 37/39 nozzles 50cc pumps, blue cams primary #2, secondary #1 hole on my stock stroke 340. This tune gives me the best 60' time with progressive linkage and .020 t-slot, 6 psi fuel pressure, 1/2 site glass up front, slightly above 1/2 secondary bowl. I also make it a habit of purging any air prior to the first pass of the day.
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Good info! For the OP, what intake are you trying to fuel? And maybe my big ole Super Victor could use more shooter size!!
 
Demonx2 said:
The best method I've found is to hold your phone over the carb and video it while someone mashes the throttle. You will see/hear what happens. Is the pump shot late? Too short? Too long?? (Another member on here was doing this video method and I picked up on it and found it quite helpful.)

I've found that the pump shot is usually too short and the bigger 50cc pump with a smaller rear nozzle (33-35) makes for a longer shot that carries through the transition. My 408 has an 850 Holley-style with a Super Victor so that's how I solved the riddle. Has 2 50cc pumps and 37 shooters in front and 35 in back (both are tibes shooters).

Of course pump cams and hole position of those cams also matters. I find that hole position 2 is useful if you stage the car with the throttle partly open (like I do holding it at about 2200-2400 rpm). Position 2 usually gives you more of a given cam whereas position 1 is already partly "used up" at that throttle position.

As for jets, I'm at 81 frt & 82 rear. Picked up 1/2 mph when I leaned the front out so gonna lean it out again and see what happens. Plugs still look dark so we shall see....
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This is a video of my 850 demon carb, that shows the front squirt time is to short.
I had the same problem, although to a much smaller degree, with my 850 ProForm.
In the end i fixed the stumble on my ProForm by going from a 37 squirter in the front to a 28. Seams counterproductive but it worked. Would my 60 time improved more if would have went to a 50 cc squirter..........Probably. But this fix was cheep.

Go to the 2 minute mark, I did a slow mo if it. this was in gear with my foot on the brake.

 
Cudafever said:
This is a video of my 850 demon carb, that shows the front squirt time is to short.
I had the same problem, although to a much smaller degree, with my 850 ProForm.
In the end i fixed the stumble on my ProForm by going from a 37 squirter in the front to a 28. Seams counterproductive but it worked. Would my 60 time improved more if would have went to a 50 cc squirter..........Probably. But this fix was cheep.

Go to the 2 minute mark, I did a slow mo if it. this was in gear with my foot on the brake.

Click to expand...

Now I recall - you taught me that video trick!! It helped me when I was working to solve my stumble.
Your fix is not counterintuitive. You lengthened the pump shot with the smaller shooters to cover the lean hole until the main circuit could catch up. Pretty straightforward theory but takes a lot of trial and effort to get it right!
 
joes68340s said:
Thanks Demonx2 I will look into that.
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I know it seems obvious, but not mentioned, are the fuel levels on the nuggets? If the level is even little low on a combo prone to a weak low-speed signal, the mains will be too slow to start discharging. Just a thought, I've rarely found one correct frt & rear after being shipped, if it hasn't been outright damaged..
 
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