Holley carb cutting out at half throttle
01-05-16, 09:57 PM
#1
Holley carb cutting out at half throttle
Hey guys, I just bought a 1984 Mazda B2000 with a 12a swap in it. I've owned 3 FC Rx7's but never anything carburated so this is new to me. The truck starts right up and idles great, but if I accelerate at half throttle it will just cut out hard and sound like its bouncing off a rev limiter around 3k RPMS. It seems if I drive it hard I can pull it all the way through the gears. I just took it on a 4 hour trip so its drivable just really aggravating. The guy I bought it from said he put an off the shelf holley carb on it which I've read wont work great, so I was just wondering if anyone can confirm that is the problem or any suggestions would help. I've attached some pictures of the carb, Thanks!
01-06-16, 05:15 AM
#2
carb whisperer
Thats a holley 600.
Thats why.
Do not listen to the morons on this forum that will claim that you can jet that to run properly on a 12A. It is far too large for the application. It idles and starts nicely because with the throttles closed it manages a bit of vacuum signal. Open them up, and unless its getting a pump shot, theres no signal to pull fuel through the carb.
Buy my 465 lol that will fix it.
Also, id be willing to bet that even if you did get that to run halfway decent, the secondaries would never open.
Thats why.
Do not listen to the morons on this forum that will claim that you can jet that to run properly on a 12A. It is far too large for the application. It idles and starts nicely because with the throttles closed it manages a bit of vacuum signal. Open them up, and unless its getting a pump shot, theres no signal to pull fuel through the carb.
Buy my 465 lol that will fix it.
Also, id be willing to bet that even if you did get that to run halfway decent, the secondaries would never open.
01-06-16, 09:41 AM
#3
Waffles - hmmm good
iTrader: (1)
Get a nikki and intake manifold and rebuild it. That will solve the problem completely. Also you
may want to hog out that nikki while your are at it to give that little 12A a chance at hauling that
REPU around.
may want to hog out that nikki while your are at it to give that little 12A a chance at hauling that
REPU around.
01-06-16, 01:20 PM
#4
I concur with what they're saying and have experienced something similar myself.
Let me relate a short story. I have a genuine 74 REPU with a 13B and a genuine RB Holley 600 that I tried in it and compared with a hogged out Nikki. Now as you might guess, this is a somewhat more ideal situation than yours, having as 13B with this size of carb. As such, I can tell you how the two carbs compare to one another. Basically, the Holley does have some wow factor in other vehicles, which I've come to find out is quite important to me. In other words, if it falls flat on me or simply can't "wow" me, I keep looking. The Holley didn't wow me in the REPU. I'll get to the Nikki experience in a moment, but I can tell you it's been much nicer. More wow factor.
Because the original REPUs from the 70s are fairly heavy, and the exhaust I was using at the time with the Holley was fairly small, only consisting of an RB header with its standard 2" collector leading into a full 2" exhaust, the flow at high RPM was not high enough to get any real power. However the flow at low RPM lacked low end torque as well. What's going on?
Before I get to that, I also tested a stock 74 Hitachi carb; known for being actually the most powerful stock carbs ever made, which also failed to wow me. Turns out the inlet strainers were clogged and I didn't know it. Moving on.
I also tested a Camden 5" and 7" in this REPU, all still on the crappy 2" exhaust. Failed to wow me.
Up next I built a brand new exhaust based on RB's long primary, using their road race header with dual pipes and dual presilencers under the cab, finally collecting into 2.5" as it goes up over the axle. Now this exhaust has some wow factor regardless of the carb! I tested it on a stock Nikki and wow! I'd never had so much low end torque before!
So obviously the next test was on a Camden and I have to say it did quite well. Felt like it had little V8 in it or something. I was very pleased but I was dabbling in turbos and wanted more out of the truck. So I smartened myself up on the subject and did some extensive Nikki mods and testing in the brown car with just a stock S5 turbo. Nothing fancy. Full 3" exhaust, then later added a manual boost controller and played around with venturi sizes and shapes. The whole setup is now a little scary in such a light chassis. So I'm going to repeat the process in the truck. I figure a little extra weight will tame it nicely. All I'm looking for is to double the stock HP rating of 110 to 220, which is quite doable with minimal work on an S5 turbo as they were rated for 205HP stock. Another 15 is child's play.
That brings us up to now. I've tested one of my early hogged out boost prepped Nikkis in the truck and it did ok -- about the same as the Camden actually. But having compared that early Nikki to my later ones, I've upped the low end torque and gotten the tuning right since then. This latest Nikki will probably be the most powerful NA setup I've ever done in it. And yes I'd like to do a turbo in it this year if I can. But NA testing comes first.
As for the Camden, I did a back to back comparison vs my turbo setup in two 84-85 chassis. I can tell you the Camden is a slow gutless turd compared to the turbo. I did everything right but realize I'm at the mercy of the preexisting free-flowing exhaust the owner had installed. I say free-flowing a bit dubiously because he was honestly a little misguided at what he thought would flow well vs actual reality. I know for a fact the RB full long primary flows quite well due to its length, size, design and all the research and testing that went into it 30 years ago. And you'll find positive things said about it on this forum, from peejay who drives a bridgeport as just one example. A BP doesn't run very well on a more restrictive exhaust, so if an RB long primary can handle a bridgie, you know it can handle a Camden on a basic 74 spec port job.
Once again my REPU's exhaust is based on the RB long primary, and it is quiet and flows good enough to wow me, but the guy's exhaust who got the Camden, isn't. It's a totally separate exhaust with no collector, and some sections enlarge to 2.25 or 2.5" after the header primaries of only 2". WTF? He will just have to learn the hard way, I guess. Some folks are hard-headed that way.
I hope that was a relatable story. Please don't fall into the trap of thinking if some if good, more is better. Your 600 Holley is not only too big but off the shelf/out of the box so was not designed for the unique strong rotary reversion pulse. It will never run very well for you, even if you had a 13B with massive porting (again, more of something isn't always better, and you want to keep the ports about 74 spec for a rotary powered truck, to retain some low end).
As for your possible Nikki swap, just know the weakest link is the intake manifold. They need work to flow well. Mazda's early ones from the '70s are best but all we have these days are the crappy 81-85 version. They're a dime a dozen so finding one is cheap and easy. Next you need to mod it. There is info on the forum about that. Be sure to get the phenolic spacer too.
The Nikki itself needs to be modded to get any real power out of it. I've found that just hogging out the primaries is good enough for increased flow because I don't want to mess with the secondaries and introduce a weaker vacuum signal under boost, where vacuum is a bit difficult to come by as you might guess. However for NA, perhaps hogging the secondaries is ok. Don't know first hand because I've never done it.
Well that turned out longer than I thought. Hopefully it was helpful.
Let me relate a short story. I have a genuine 74 REPU with a 13B and a genuine RB Holley 600 that I tried in it and compared with a hogged out Nikki. Now as you might guess, this is a somewhat more ideal situation than yours, having as 13B with this size of carb. As such, I can tell you how the two carbs compare to one another. Basically, the Holley does have some wow factor in other vehicles, which I've come to find out is quite important to me. In other words, if it falls flat on me or simply can't "wow" me, I keep looking. The Holley didn't wow me in the REPU. I'll get to the Nikki experience in a moment, but I can tell you it's been much nicer. More wow factor.
Because the original REPUs from the 70s are fairly heavy, and the exhaust I was using at the time with the Holley was fairly small, only consisting of an RB header with its standard 2" collector leading into a full 2" exhaust, the flow at high RPM was not high enough to get any real power. However the flow at low RPM lacked low end torque as well. What's going on?
Before I get to that, I also tested a stock 74 Hitachi carb; known for being actually the most powerful stock carbs ever made, which also failed to wow me. Turns out the inlet strainers were clogged and I didn't know it. Moving on.
I also tested a Camden 5" and 7" in this REPU, all still on the crappy 2" exhaust. Failed to wow me.
Up next I built a brand new exhaust based on RB's long primary, using their road race header with dual pipes and dual presilencers under the cab, finally collecting into 2.5" as it goes up over the axle. Now this exhaust has some wow factor regardless of the carb! I tested it on a stock Nikki and wow! I'd never had so much low end torque before!
So obviously the next test was on a Camden and I have to say it did quite well. Felt like it had little V8 in it or something.
I was very pleased but I was dabbling in turbos and wanted more out of the truck. So I smartened myself up on the subject and did some extensive Nikki mods and testing in the brown car with just a stock S5 turbo. Nothing fancy. Full 3" exhaust, then later added a manual boost controller and played around with venturi sizes and shapes. The whole setup is now a little scary in such a light chassis. So I'm going to repeat the process in the truck. I figure a little extra weight will tame it nicely. All I'm looking for is to double the stock HP rating of 110 to 220, which is quite doable with minimal work on an S5 turbo as they were rated for 205HP stock. Another 15 is child's play.That brings us up to now. I've tested one of my early hogged out boost prepped Nikkis in the truck and it did ok -- about the same as the Camden actually. But having compared that early Nikki to my later ones, I've upped the low end torque and gotten the tuning right since then. This latest Nikki will probably be the most powerful NA setup I've ever done in it.
And yes I'd like to do a turbo in it this year if I can. But NA testing comes first.As for the Camden, I did a back to back comparison vs my turbo setup in two 84-85 chassis. I can tell you the Camden is a slow gutless turd compared to the turbo. I did everything right but realize I'm at the mercy of the preexisting free-flowing exhaust the owner had installed. I say free-flowing a bit dubiously because he was honestly a little misguided at what he thought would flow well vs actual reality. I know for a fact the RB full long primary flows quite well due to its length, size, design and all the research and testing that went into it 30 years ago. And you'll find positive things said about it on this forum, from peejay who drives a bridgeport as just one example. A BP doesn't run very well on a more restrictive exhaust, so if an RB long primary can handle a bridgie, you know it can handle a Camden on a basic 74 spec port job.
Once again my REPU's exhaust is based on the RB long primary, and it is quiet and flows good enough to wow me, but the guy's exhaust who got the Camden, isn't. It's a totally separate exhaust with no collector, and some sections enlarge to 2.25 or 2.5" after the header primaries of only 2". WTF? He will just have to learn the hard way, I guess. Some folks are hard-headed that way.
I hope that was a relatable story. Please don't fall into the trap of thinking if some if good, more is better. Your 600 Holley is not only too big but off the shelf/out of the box so was not designed for the unique strong rotary reversion pulse. It will never run very well for you, even if you had a 13B with massive porting (again, more of something isn't always better, and you want to keep the ports about 74 spec for a rotary powered truck, to retain some low end).
As for your possible Nikki swap, just know the weakest link is the intake manifold. They need work to flow well. Mazda's early ones from the '70s are best but all we have these days are the crappy 81-85 version. They're a dime a dozen so finding one is cheap and easy. Next you need to mod it. There is info on the forum about that. Be sure to get the phenolic spacer too.
The Nikki itself needs to be modded to get any real power out of it. I've found that just hogging out the primaries is good enough for increased flow because I don't want to mess with the secondaries and introduce a weaker vacuum signal under boost, where vacuum is a bit difficult to come by as you might guess. However for NA, perhaps hogging the secondaries is ok. Don't know first hand because I've never done it.
Well that turned out longer than I thought. Hopefully it was helpful.
Last edited by Jeff20B; 01-06-16 at 01:37 PM.
01-06-16, 01:50 PM
#5
Thanks for all the information everyone, I'm thinking I will just buy the racing beat 465cfm Holley carb and hope for the best. I'm not looking to get any power out of the truck I just want it to run smooth lol, I will be daily driving it
01-06-16, 02:29 PM
#6
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