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Discussion Starter #1
I'm looking for some carburetor tuning assistance. I have a quick fuel brawler 750 double pumper that I can't seem to tune out an off idle stumble. This carb is on a 460 in a 77 f150 4wd manual trans truck. Engine setup is a 72 block zero deck kb flat top bottom end. Heads are stock 429 PI. Intake is a stealth. Cam is a solid lift from the mad porter. 237/[email protected], 600 lift. L and L fender well headers with 3" glass packs. Distributor is a duraspark with 6A box 20 initial 36 total with 12 degree vacuum can. The engine runs great except for an annoying off idle stumble that happens at low rpm. For example I can pull off in 2nd, accelerate to about 1800 rpm,shift to 3rd rpm drops to about 1200, start easing down on the gas and the engine stumbles then pulls on fine. I'm fairly sure it's a lean stumble. I have and old edelbrock narrow band in the cab and it goes full lean during the stumble and the vacuum gauge drops down to about 5 or 6 inches. I have been able to improve it some with a larger shooter 0.039, orange accel pump cam, and a 9.5 power valve. But the stumble is still there. Better on warm days. Worse on cooler days. Worse until the engine is fully warmed up. Does anyone have experience with these carbs? I need some advice on what I should try next. If I accelerate harder the stumble doesn't show up. It's only when I'm easing around town or trying to save gas. So it's at low rpm with a decent size cam and big intake. I like the cam. Would a different intake help?
 

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I've had that same issue with a Brawler 650 on a 5.0 in the '31. I've wound up turning the secondary idle adjustment all the way in and fattening the primaries 1/8 inch clockwise (fat). This has eliminated the bog but it does run fat at idle. I have never been able to adjust a true 4 corner idle and have it run right.
 

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Float levels correct?
Vacuum advance canister hooked up?



SJ
 

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Discussion Starter #4
I've tried float levels all over the place low, high, middle of the sight glass, etc. The middle of the sight glass is supposed to be correct. Higher float level does seem to offer a slight improvement.
The engine idles the best with the 4 mix screws out 3/4 turn. 11-12 inches at 900 rpm idle. Further out improves the stumble but then the idle is rich.
I've got a couple of vacuum cans. One is 8 crank degrees and the other I've limited to 12. I've tried them both on full and ported vacuum. Seems to do better on ported. I can't tell any difference in the 8 or 12 deg cans.
I'm starting to think the stumble is issue is related to my combo of parts. I had a carb tuner tell me that it's an air speed issue at low rpm. Kinda makes sense with the this big intake and/or 3" exhaust. He has loaned me a 650 vac secondary carb to try and it runs much better as far as the stumble. When fully warmed up I can see my afr meter go lean after gear changes but this carb doesn't stumble 9 out of 10 times. Sometimes I can feel an ever so slight dip but that's it. MPG improved to 10 with the 650 vs 6 to 7 with the brawler. But the top end doesn't pull as hard and I can't seem to lean down the idle. Mix screws are only 1/2 turn out.
I hope all this is making sense and I appreciate the feedback.
 

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You may be running out of pump shot during mid pump lever travel considering you have a .039" shooter and orange cam. Maybe try a .031-.035" with a blue cam or a possible upsize to 50cc accelerator pump volume on primary side. Not familiar with the Brawler, what type of secondary linkage does it have, 1 to 1 or progressive, with 1 to 1 you may actually have too much pump shot(s) and the gauge reading is false. You can also install a 10.5" PV to get fuel a bit quicker but the PV has no real relevance until the main system starts flowing. Too much transfer slot exposure, either primary or secondary side, will tend to give a lean stumble very similar to what you are experiencing. Generally set transfer slots to .020-.030" exposure on primary side, a little less on secondary side if possible. If more idle air is needed/required, you can get a PCV valve that has just an orifice hole in it and enlarge it until you get the desired idle rpm without uncovering too much of the secondary transfer slot. Another option is drilling the primary blades to allow the extra air/idle speed. Hope this helps.
 

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As has been alluded to previously, the big plenum volume of the stealth will require a big pump shot. The fact it seems run better when you richen the idle mixture is another clue. If you are setting the idle mixture using the highest vacuum method you're too lean. Also try closing the secondary butterflies until they are just barely open.
 

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Discussion Starter #7
I tried the fixed orifice pcv. Ended up drilling it out to 7/32. This is working much better with the 650 carb. Front transfer slot is in spec and I had to crack open the secondaries 1/4 turn to get an 850 rpm idle. I also opened the mix screws to 1 turn out. Much cleaner idle and no stumble on the test drive. I’ll give this feedback to my carb tuner and hopefully he can put a good recipe in the brawler.
Does 850-900 rpm sound correct for this cam?
Is the brawler 750 sized correctly for my engine combo? One option is to work out a trade with my tuner for the loaner 650 but I’m thinking it’s too small?
Thanks for the help.
 

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I tried the fixed orifice pcv. Ended up drilling it out to 7/32. This is working much better with the 650 carb. Front transfer slot is in spec and I had to crack open the secondaries 1/4 turn to get an 850 rpm idle. I also opened the mix screws to 1 turn out. Much cleaner idle and no stumble on the test drive. I’ll give this feedback to my carb tuner and hopefully he can put a good recipe in the brawler.
Does 850-900 rpm sound correct for this cam?
Is the brawler 750 sized correctly for my engine combo? One option is to work out a trade with my tuner for the loaner 650 but I’m thinking it’s too small?
Thanks for the help.

The 750 is about the right size.

I'm wondering if maybe the idle feed restrictors need to be a bit bigger or the air bleeds a tic smaller to richen up the transition.

Do you have pump shot immediately when any movement of the throttle occurs?

The large plenum area of the stealth will require a good pump shot and richer transition.



SJ
used 2b RHP



:D
 
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