Sorry for the crosspost, just thought you lot here might be more experienced with PPs, feel free to delete if necessary

Hi all,

Recently finished a 13b PP build for my first gen. Engine has RX8 rotors with full depth RX7 ceramic apex seals. Running stock ignition for now with fresh leads and BR8EIX plugs leading and trailing. Mixture delivered by a 51IDA carb with 42mm chokes, F2 emulsion tubes, 245 mains and 195 air corrector.

The problem I'm running into is that the engine wont rev above ~5k rpm, and at WOT at any rpm the engine sputters rapidly and accelerates very slowly. Took it out for a quick test drive with a lambda sensor hooked up, idles around 11:1 and at WOT it stays around 12.5 (not optimal, but shouldnt be causing the issues I'm having)

I'm quite stumped with this as I originally thought it may be a fuelling issue, does anyone have any ideas?

 

I'm assuming this is running factory ignition of some sort? Those chokes are far too small, I also don't tend to use F2s in a periph either. F8s have been my go-to and 46+ on the venturis. I'd at the very least give that a shot and see if the lambda sensor is lying to you - periphs can do some whacky things with overlap. I'd also read the plugs if you're seeing 12.5 - they should be dark chocolate-y if the lambda sensor is accurate. If they're black you're not actually seeing real information.

 

You have a few settings wrong 42 chokes good F2 em tubes ok Mains are to big should be around 220 to 230 and air 120 Also timing should be 15 deg advanced maximum . Also make sure fuel pump has good volume And you need B9eg spark plugs

 

Mazda's setups have the venturi the same size as the intake runner, so that is probably an easy way to be in the ballpark

 

Find me a way to fit a 52mm venturi in a 51IDA and I'll do it

In all seriousness, this is quite a few people who have pointed to the venturi size as a possible issue, but I still don't see how it would cause the misfiring/stuttering. I can definitely see it causing a big drop in power at the top end, but enough of a drop for it to not want to rev out at all? The only reason I put the 42s in and jetted it accordingly was to keep it a bit more streetable while I work out all the kinks, maybe I went overboard with it?

Could try going with RacingBeat's recommendations for 51 jetting, seems well proven enough and a good starting point

 

I don't think changing to RBs specs - which are very close to what I've come to use as well - is going to be a magic fix, but it would certainly help to remove any uncertainty to further diagnose.

 

Yeah me neither, but I think it's worth changing them out anyway. I was going to go with those jets originally but I've done a lot to the car recently, so wanted to get it sorted on the street first, then more track focused (hence the pretty small venturis)

Working under the assumption that the lambda sensor is reading true (it's a brand new gauge kit from AEM, but I'll do a free air calibration just in case), I have a feeling it's ignition related. Before this, I pulled apart the dizzy to lock it and give it a fresh lick of paint, and in doing so was a massive idiot and forgot to mark where everything was originally positioned, so it's possible that something is out. Plus, I've been having some issues getting the timing right. According to my timing gun, it's jumping around at idle (no vac or mech advance) from what looks to be 0 - 25 degrees. That very well might be the gun though, it's 20ish years old and sometimes won't pick up the ignition signal at all (thanks SnapOn)

 

It's been a while but I've only just gotten back the motivation to work on the car . I stripped the dizzy down again and actually welded the weights this time instead of just leaving the springs out, I also checked everything was in the right place when compared to some photos from online.

I advanced the timing by half the slot in the dizzy and it allowed the engine to rev up to around 5.5k when it originally stopped at 4. I will advance it more later, but apparently my timing light is giving incorrect readings, so I don't want to go in blind.

Anyway, if anyone has the same issue in the future *check your timing and always verify your timing light reads good*

 

Alright, I hold my hands up, I'm an idiot and I deserve to be flogged in the streets. I've had the car down at my garage and it's on the lift now. It's only just occured to me that the exhaust is waaaaay to restrictive. I knew I would have to change it eventually but thought I could stick with the original just for breaking the engine in. DUMB DUMB. Turns out, having measured it, after the RacingBeat 2 into 1 header, it's 2 inch OD pipe all the way to the back. I was under the impression it was 2.5 inch; still not great but would be a lot better. DUMB DUMB.

I'm going to assume this is what my problem is, no way 2 inches can flow enough for a PP at full throttle. I also noticed at WOT in neutral a lot of pressure seems to come OUT of the carb, I'm guessing this is exhaust gas that's forcing its way out due to the high backpressure.

Does anyone concur with my findings or should I keep looking? (I'm changing the exhaust anyway, stupid 2 inch pipe)

 

i'm not sure if that is going to fix that specific problem, but you will want a bigger exhaust.

if you have the header off, you will want to have a look at the header inlet vs the exhaust port. an FC/FD exhaust port is ~48mm and the header inlet is 44mm
depending on the header you have, the collector looks small, although i've never tried any different ones.

 

They're FC Turbo housings with the 48mm port. Off the top of my head I'm pretty sure the manifold inlet was ~50mm, but I'll have a look at it. This whole problem is a bunch of "that probably wont fix your issue" , so I'm beginning to tear my hair out. Put a different dizzy in as I thought I might have messed up somewhere while locking it, no fix. A set of known good and recently tested igniters, no good. New MSD coils, no fix. Noticed fuel pressure would drop a bit at full throttle from 4.5 to 3 psi, so bumped it up to 6psi (any higher leaked past the needle), no fix.

I've got a compression tester on the way as it's nice to have one anyway, but I don't for a second believe it's an engine problem as it cold and hot starts perfectly, even after only 10 or so miles. It's such a simple setup and yet the solution is evading me. Pulled the plugs and they were pitch black, so I think the lambda sensor may also be lying, though it's been idling for quite a while and idle circuit is pretty rich.

 

If you are running a crazy exhaust to quiet it down that could be your problem. I was running a restrictor in mine and it would struggle to rev past 6k. It caused so much back pressure on track that it separated my slip fit headers and when that happened it picked up 50hp easily and reved great all the way to redline.

 

For the 5th? time now, I think I know what it is. It's almost certainly a rich bog caused by an improperly tuned carb and restrictive exhaust. Pulled the plugs and they're pitch plack, plus it starts and idles way too easily in the cold.

The carb is jetted to RacingBeat specs, but they're assuming an open exhaust, so my tiny exhaust would most definitely cause a rich condition. I was throwing away most of my piston engine knowledge for this, but it very much sounds like a rich bog/stutter to me now. You can also rev it out fully at part throttle, but as you get higher in the revs, the window where it doesn't bog gets smaller. Plus at WOT it stutters straight away and doesn't recover.

I'm getting a new exhaust fabbed up anyway, so fingers crossed that will fix it