Guys who have 50cc accelerator pumps
11-07-2010, 11:41 AM
#1
Guys who have 50cc accelerator pumps Hey guys I just installed a 50cc accelerator pump on my proform 750. Did you guys use the pump cams that come with the 50cc pump? It looks like a yellow and a brown one. Which one do i put on the front?
I installed the pump originally to try and cure my huge stumble off idle.
I installed the pump originally to try and cure my huge stumble off idle.
Last edited by tennerv8; 11-07-2010 at 11:48 AM.
11-07-2010, 09:25 PM
#7
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you have to dump the fuel to these motors becuase the heads are not designed to flow fuel and air, so the fuel just kind of chills out and it doesnt all make it to the combustion chamber when you smash the gas pedal.
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11-07-2010, 09:29 PM
#8
good to hear!! this info has to get out for people to try.with it being such an easy thing to try i dont know why more people havn't. this was the first carb i ever touched so ive been learning as i go so i never knew about the 50cc pump and with all the people i talked to no one ever even suggested it. or i should say only one did and i tried it, so thanks to him 
11-08-2010, 07:24 AM
#9
The stickys on carbs are full of the 50 cc pump info. In fact, there is only one guy I know of on this site who says it isn't needed.
I have a 50 cc primary pump on mine with I think the brown 50 cc cam. BUT I am starting to think it isn't needed. Mine would only stumble with slight throttle application. It did fine with a full pedal mash. Turns out I needed to reduce the idle air bleeds. I keep meaning to go back to the 30 cc pump and retune the accel circuits with the bleeds set properly. Unfortunately that test might have to wait until spring now.
The problem is it is hard to tell if the accel shot is too big. Putting in too big a shot will get rid of the bog but makes the engine "lazy" as one guy described it. The Holley manual says you need just enough shot to get rid of the stumble but no more. A lot of trial and error involved.
I have a 50 cc primary pump on mine with I think the brown 50 cc cam. BUT I am starting to think it isn't needed. Mine would only stumble with slight throttle application. It did fine with a full pedal mash. Turns out I needed to reduce the idle air bleeds. I keep meaning to go back to the 30 cc pump and retune the accel circuits with the bleeds set properly. Unfortunately that test might have to wait until spring now.
The problem is it is hard to tell if the accel shot is too big. Putting in too big a shot will get rid of the bog but makes the engine "lazy" as one guy described it. The Holley manual says you need just enough shot to get rid of the stumble but no more. A lot of trial and error involved.
Last edited by Pop N Wood; 11-08-2010 at 02:38 PM.
11-08-2010, 07:29 AM
#10
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Pop n wood does your carb have four corner idle mixture on it? If so you could have backed those screws out 1/4 of a turn and it probably would have cured the problem as well.
11-08-2010, 09:00 AM
#11
The stickys on carbs are full of the 50 cc pump info. In fact, there is probably only one guy who said it isn't needed.
I have a 50 cc primary pump on mine with I think the brown 50 cc cam. BUT I am starting to think it isn't needed. Mine would only stumble with slight throttle application. It did fine with a full pedal mash. Turns out I needed to reduce the idle air bleeds. I keep meaning to go back to the 30 cc pump and retune the accel circuits with the bleeds set properly. Unfortunately that test might have to wait until spring now.
The problem is it is hard to tell if the accel shot is too big. Putting in too big a shot will get rid of the bog but makes the engine "lazy" as one guy described it. The Holley manual says you need just enough shot to get rid of the stumble but no more. A lot of trial and error involved.
I have a 50 cc primary pump on mine with I think the brown 50 cc cam. BUT I am starting to think it isn't needed. Mine would only stumble with slight throttle application. It did fine with a full pedal mash. Turns out I needed to reduce the idle air bleeds. I keep meaning to go back to the 30 cc pump and retune the accel circuits with the bleeds set properly. Unfortunately that test might have to wait until spring now.
The problem is it is hard to tell if the accel shot is too big. Putting in too big a shot will get rid of the bog but makes the engine "lazy" as one guy described it. The Holley manual says you need just enough shot to get rid of the stumble but no more. A lot of trial and error involved.
11-08-2010, 02:35 PM
#12
This is on an LS2 crate motor. So LS6 cathedral port heads, stock LS2 cam (essentially an LS6 cam) and a Holley 750 cfm classic double pumper (model 4779) on a GMPP open plenum manifold.
The carb has four corner idle screws and is being checked with a wideband. Setting the idle mixture way rich definitely helped reduce the stumble. I did that for a while. I also experimented with hollow nozzle screws, #41 nozzles and upside down cams to get the shot to come on faster. But running with a rich idle mix still caused the cruise mixture to go lean and sooted up the exhaust if idling too long. Restricting the idle air bleeds made the engine run a whole lot better under 2500 RPM and was unquestionably the right fix. Interestingly my idle screws are under half a turn right now. I originally thought I needed to make the idle feed restrictors smaller to get the screws out around 1 ½ turns. But I don’t think that matters anymore. My idle AFR is near perfect so why worry about how many turns the screws are?
In all honesty once I got the engine running right I just wanted to drive the thing rather than mess with the accel circuits again. I had struggled with a couple of gasket leaks and a bad power valve that sent me in circles for a while. For some reason the hollow nozzle screw caused the gasket under the nozzle to have a slow leak. That caused a delay in the shot if the throttle hadn’t moved in a while. Man, finding that one caused me fits. So further tuning got put on hold for some tire roasting.
Once the weather warms up I will probably feel like playing with accel pump circuits again. All I am saying is I personally need to do some more tuning before I will be 100% convinced the 50 cc pump is needed.
The carb has four corner idle screws and is being checked with a wideband. Setting the idle mixture way rich definitely helped reduce the stumble. I did that for a while. I also experimented with hollow nozzle screws, #41 nozzles and upside down cams to get the shot to come on faster. But running with a rich idle mix still caused the cruise mixture to go lean and sooted up the exhaust if idling too long. Restricting the idle air bleeds made the engine run a whole lot better under 2500 RPM and was unquestionably the right fix. Interestingly my idle screws are under half a turn right now. I originally thought I needed to make the idle feed restrictors smaller to get the screws out around 1 ½ turns. But I don’t think that matters anymore. My idle AFR is near perfect so why worry about how many turns the screws are?
In all honesty once I got the engine running right I just wanted to drive the thing rather than mess with the accel circuits again. I had struggled with a couple of gasket leaks and a bad power valve that sent me in circles for a while. For some reason the hollow nozzle screw caused the gasket under the nozzle to have a slow leak. That caused a delay in the shot if the throttle hadn’t moved in a while. Man, finding that one caused me fits. So further tuning got put on hold for some tire roasting.
Once the weather warms up I will probably feel like playing with accel pump circuits again. All I am saying is I personally need to do some more tuning before I will be 100% convinced the 50 cc pump is needed.
11-09-2010, 02:27 PM
#13
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I also had the off idle stumble and I swapped to the 50cc pump and it went away. Can't remember what color cam I put in it though
11-09-2010, 04:48 PM
#14
I'm running 50cc fronts with 30cc rears, 40 squirters up front and 37 rears, no stumble and all and it's a beast. I will be eventually changing out idle air bleeds to get the screws 1.5 turns out, right now they're only 7/8 out, not sure how that effects the fuel curve yet, need to talk to some experts since the tech guys didn't know.
04-25-2011, 11:54 AM
#15
I ran last year with an E85 carb from CSU, it had a very slight stumble but ran great otherwise (the carb was set up for a totally different engine and was perfect on it), I never messed with the stumble.
I put my gasoline carb on the engine (LQ4) and had a huge stumble. It was 30cc, blue cam and #31 squirters in the front. It stumbled so bad it would almost kill the engine, I thought it was so bad that my carb had to be screwed up.
I put the 50cc pump in, the brown cam it came with, and #40 squirters, stumble is totally fixed. What a magical experience. Amazing these little 6.0 engines take that much pump shot to not stumble!
I put my gasoline carb on the engine (LQ4) and had a huge stumble. It was 30cc, blue cam and #31 squirters in the front. It stumbled so bad it would almost kill the engine, I thought it was so bad that my carb had to be screwed up.
I put the 50cc pump in, the brown cam it came with, and #40 squirters, stumble is totally fixed. What a magical experience. Amazing these little 6.0 engines take that much pump shot to not stumble!
04-25-2011, 03:23 PM
#16
Interesting timing for this thread to come back up. I took off the 50 cc pump and went back to the 30 cc just this weekend. Still have instant throttle response from low RPM. The thing that fixed mine was necking down the primary idle air bleeds two steps.
I have pretty much convinced myself that smaller air bleeds is the correct fix for the off idle bog, not just dumping more fuel on a throttle transition. The wideband readings support this. With the oversized accel shot there was no stumble when accelerating from low RPM. But what I noticed cruising at say 1500 to 2000 RPM the mixture would be unsteady and lean. Changing the bleeds stabilized the AFR in this region. That leads me to believe that while a big squirt helps you get through the idle circuits and into the mains, it is probably masking the idle circuit problems.
Took the car on a 2 hour drive Sunday. Was able to pull from 1200 RPM. Got on a winding road and where the throttle response with the new set up really made a difference. Car is point and shoot going through the corners now. Big fun.
So with the stock air bleeds I had to run a 50 cc pump, #35 nozzle and hollow nozzle screw on the primary side to avoid a bog. With the smaller bleeds I am back to a solid screw and the #31 nozzle the carb came with. I am still using the brown cam that came with the 50 cc pump, but I am about to pull that off too. It is almost certainly dumping too much gas when I hit it with a full throttle. Still a bit more tuning to do.
I have pretty much convinced myself that smaller air bleeds is the correct fix for the off idle bog, not just dumping more fuel on a throttle transition. The wideband readings support this. With the oversized accel shot there was no stumble when accelerating from low RPM. But what I noticed cruising at say 1500 to 2000 RPM the mixture would be unsteady and lean. Changing the bleeds stabilized the AFR in this region. That leads me to believe that while a big squirt helps you get through the idle circuits and into the mains, it is probably masking the idle circuit problems.
Took the car on a 2 hour drive Sunday. Was able to pull from 1200 RPM. Got on a winding road and where the throttle response with the new set up really made a difference. Car is point and shoot going through the corners now. Big fun.
So with the stock air bleeds I had to run a 50 cc pump, #35 nozzle and hollow nozzle screw on the primary side to avoid a bog. With the smaller bleeds I am back to a solid screw and the #31 nozzle the carb came with. I am still using the brown cam that came with the 50 cc pump, but I am about to pull that off too. It is almost certainly dumping too much gas when I hit it with a full throttle. Still a bit more tuning to do.
04-25-2011, 06:50 PM
#17
Interesting timing for this thread to come back up. I took off the 50 cc pump and went back to the 30 cc just this weekend. Still have instant throttle response from low RPM. The thing that fixed mine was necking down the primary idle air bleeds two steps.
I have pretty much convinced myself that smaller air bleeds is the correct fix for the off idle bog, not just dumping more fuel on a throttle transition. The wideband readings support this. With the oversized accel shot there was no stumble when accelerating from low RPM. But what I noticed cruising at say 1500 to 2000 RPM the mixture would be unsteady and lean. Changing the bleeds stabilized the AFR in this region. That leads me to believe that while a big squirt helps you get through the idle circuits and into the mains, it is probably masking the idle circuit problems.
Took the car on a 2 hour drive Sunday. Was able to pull from 1200 RPM. Got on a winding road and where the throttle response with the new set up really made a difference. Car is point and shoot going through the corners now. Big fun.
So with the stock air bleeds I had to run a 50 cc pump, #35 nozzle and hollow nozzle screw on the primary side to avoid a bog. With the smaller bleeds I am back to a solid screw and the #31 nozzle the carb came with. I am still using the brown cam that came with the 50 cc pump, but I am about to pull that off too. It is almost certainly dumping too much gas when I hit it with a full throttle. Still a bit more tuning to do.
I have pretty much convinced myself that smaller air bleeds is the correct fix for the off idle bog, not just dumping more fuel on a throttle transition. The wideband readings support this. With the oversized accel shot there was no stumble when accelerating from low RPM. But what I noticed cruising at say 1500 to 2000 RPM the mixture would be unsteady and lean. Changing the bleeds stabilized the AFR in this region. That leads me to believe that while a big squirt helps you get through the idle circuits and into the mains, it is probably masking the idle circuit problems.
Took the car on a 2 hour drive Sunday. Was able to pull from 1200 RPM. Got on a winding road and where the throttle response with the new set up really made a difference. Car is point and shoot going through the corners now. Big fun.
So with the stock air bleeds I had to run a 50 cc pump, #35 nozzle and hollow nozzle screw on the primary side to avoid a bog. With the smaller bleeds I am back to a solid screw and the #31 nozzle the carb came with. I am still using the brown cam that came with the 50 cc pump, but I am about to pull that off too. It is almost certainly dumping too much gas when I hit it with a full throttle. Still a bit more tuning to do.
04-26-2011, 02:28 PM
#18
Heres another way to fix the dreaded stumble put a 1:1 secondary throttle link on there I put new throttle shafts on mine with a quick fuel kit and it came with 3 dif links for the secondarys after I went 1:1 I went back to the stock 31 squirters and 218 pump cams front and back you want to talk about responce and power right off of idle holy crap
04-26-2011, 09:44 PM
#20
On The Tree
The pump shot is for sudden and big throttle changes. When you are rolling easily into the throttle the air bleeds and idle feed restrictor are the cure.Try drilling the holes .002" at a time. Turning the idle mixture screws out makes for a very rich idle mixture and this covers up the lean transition period.
You can also try a bigger # power valve to bring on the enrichment sooner.
Drilling out the power valve restriction holes in the metering block. Go .005" at a time.
The idle air bleeds only help until you are reving high enough to be on the main circuit. Usually 2,500 rpm and up gets you onto the main jets.
Try driving along @ 3,000rpm, then roll into the throttle gently to go 3200. If there is a slight hesitation you need to look elsewhere as you are not in the idle circuit.
Idle mixture screws are for idle. Idle air bleeds are for fast idle 1500-2000.
As many know these carbed intakes have very large plenums and runners so the carb signal is not very good at low speeds. They need the fuel to flow sooner and easier that's why the smaller air bleeds,larger idle restrictors are needed.
Aleck
You can also try a bigger # power valve to bring on the enrichment sooner.
Drilling out the power valve restriction holes in the metering block. Go .005" at a time.
The idle air bleeds only help until you are reving high enough to be on the main circuit. Usually 2,500 rpm and up gets you onto the main jets.
Try driving along @ 3,000rpm, then roll into the throttle gently to go 3200. If there is a slight hesitation you need to look elsewhere as you are not in the idle circuit.
Idle mixture screws are for idle. Idle air bleeds are for fast idle 1500-2000.
As many know these carbed intakes have very large plenums and runners so the carb signal is not very good at low speeds. They need the fuel to flow sooner and easier that's why the smaller air bleeds,larger idle restrictors are needed.
Aleck