Carbed FC cuts out at high RPM - Possible jetting issue?
 

Here's the situation.

1986 RX7 GX race car, NA 13B converted to Weber 45DCOE side draft carb, very aggressive Bridgeporting, Race engine with 285 hp on the engine dyno. It was built on this dyno, and worked fine on it, but only ran 1/2 a lap of the track in the car before the previous owner blew the clutch, then parked the car for 10 years until we bought it (although he drove it around the yard a bit, and winterized it, drained the fuel and the carb, etc).

Engine has turbo rotors and carbon apex seals. Runs on leaded 101 octane race fuel (VP Racing M105L) with 80:1 Amsoil premix oil (the OMP has been removed due to a previous leak causing an engine to fail).

Here's the jetting in the carb (this may matter later in this post)

mains: 210
air: 170
Emulsion tubes: F16
accel pump jets: 65
Idle jets: 65f9
Venturis: 45 (I think)

Here are the symptoms:

In neutral, the car will rev right through to 8,000rpm. When driving on the track, it will rev to 8,000rpm in 2nd gear, but in third gear, when it reaches 6,000-6,500rpm, it sounds like a rev limiter (but there is none on the car), it stumbles and cuts out and goes crazy until you let off and let the rpm come down. In 4th gear, it's even worse, only gets to 5,000-5,500rpm.

Also, in second gear, it will only go that high on gradual throttle, if you step on it really quickly, it won't get up that high.

If you sort of slow down and stop, the spark plugs are pitch black. Also, it was fouling the plug in the front chamber so badly, it was dropping rotor #1 at a couple times. If you go flat out on the track and shut down the engine as soon as it starts to act up and check the plugs, they have a slight white cast on them, and the bowl in the carb is almost empty. Also, it may be unrelated, but the racing beat intake manifold piece is getting so cold when you're at idle and playing with the throttle that it gets condensation and you could chill drinks on it, but the carb isn't cold, it's luke warm.

So here is what we've done already:

-new ignition coils
-new plugs
-new ignitors
-better ground on distributor
-cleaned pick up in distributor
-new cap/rotor button
-new plug wires
-played with timing a little bit, but it didn't help, so we put it back to where it was

-new Holley fuel pump (Holley Blue, 110 gph volume)
-new Holley fuel regulator, set at 3.5psi of pressure
-new 3/8" fuel hose from the pump to the carb
-new fuel filter (clear, allowing us to see fuel flowing through inside)
-Took carb apart and cleaned everything up, set floats to spec, checked all jets and needle seat, etc.


Here are some other jet combinations we tried on the weekend to no avail:

Main: 185
Air: 170
Emul tube: F16
Accel pump jet: 50

Main: 210
Air: 185
Emul tube: F16
Accel pump jet: 65


Does anyone have any ideas for this? Do you think it's a jetting issue or something else?

Ok so I thought for a minute it may be the 5th and 6th ports not opening, but here is what I found when I looked on the engine:

http://i62.photobucket.com/albums/h1...5926Medium.jpg
http://i62.photobucket.com/albums/h1...5927Medium.jpg

What size needle valve are you running..Should be a 300 for that kind of hp.
Also try bumping the fuel pressure to 4 before you play with the jetting.

And the biggest venturis for a 45 DCOE are 40mm.

Ok, then they are 40's that are in it. I'll check the needle valve, but 300 sounds familiar, and I'll bump up the pressure.

well, my only comments are these:

you may be feeling the effects of the manifold design. my experience with that setup proved that it was great on the street (low-end and midrange), but for real high RPM use it left a lot to be desired - and i had stock ports on a 48.

also, what exactly do you know about the motor itself? which apex seals used? new parts or used? have you actually taken the manifold off and checked things out for yourself? i ask because i don't see why someone would leave the 6-port sleeves in there after bridgeporting - but you are right in your assessment though, if the sleeves are in, then they have been rendered non-functional.

finally, i would probably turn up the fuel pressure a bit and play around with the air jets. what are the largest air jets available to you? aside from all of that, i'm not convinced that you have enough carbie for a bridge with a 45, i would think you'd need to start with a 48.

What puzzles me is that the engine was built on an engine dyno and worked great then! Maybe the additional load of an actual car being attached to the engine is what is making it rear its ugly head as a problem.

Carbon apex seals, they were new, most of it is made of new parts. They combined a turbo engine and a NA engine when they did it. Turbo rotors (supposedly stronger?), and a NA casing, bridgeported like crazy, and converted to carb. I'm going to take off the manifold and see what's in there and if the sleeves are still there.

I'm going to turn up the pressure to 4psi, the max I can put in that carb. And I have lots of sizes of air jets, I think right up to 220 or 230.

Ok, so the aux. port valves are completely gone, no sleeves or anything left.

And the needle/seat is a 225. Maybe go bigger with that?

I'm going to agree with diabolical1, a 45mm carb is just not big enough for a full bridgeport. I would think a 48mm at a minimum and most likely a 51mm. The longer intake for the side draft hurts the upper rpm range that the bridgeport needs to really shine as well, unless you can find one of the old LakeCity short side draft intakes.

Of course it could be a fuel delivery problem, think volume not pressure.

Well, we put on the high volume pump (110 GPH), with the regulator at 3.5psi, and 3/8" line all the way to the front, so the only other thing in fuel delivery is the question of enough of it getting INTO the carb, then into the engine, correct?

I wonder if a bigger needle valve will help. Perhaps the 225 that's on it is too small, and the 300 is a better choice?

What puzzles me is that they ran the engine with this carb on it on the engine dyno and got HUGE numbers out of it. If all else fails though, I have a 48IDA kicking around at home, but I need to rebuild it and buy the intake manifold piece for it. Racing beat has them, but they're over $200.

that was my first thought, but if you look closely he listed a Holley Blue (110 gph) and the appropriate regulator.

EDIT: he beat me to it.

she ;)

opinion ... and nothing else, so get your grain of salt. :)

sell that DCOE setup, use the money to get an IDA manifold and whatever you need to tune your IDA, get an EGT gauge ... and beer or whatever is left over. :) then tune that engine with the IDA. 285 at the flywheel is what an average tuned bridge should put out - bare minimum! it's as old as the RB catalog. i'm not saying that to shit on your engine. i'm saying it because you keep referring to the dyno-tune and i get the feeling it was a huge selling point that the previous owners used to sell it to you. if i am wrong - i apologize for speaking out of turn. at any rate, i think you should be thinking more 2000's as opposed to 1980s. not sure where that 6-port bridge is going to max out, but at the very least, you should get it to rev cleanly to redline in every gear.

LOL! many apologies. :D

We must remember, they built the engine in 1996. :)

I actually have an early' 90's RB catalogue at home that came with it haha

Yeah I ordered a rebuild kit for the IDA today...I'll see what I can do.

Any jetting suggestions to start with on it?

Also, could it be a timing issue at all?

I ask that because would we be getting the amount of flame we are on shifting if it had just run itself lean?

okay, you've shed some light on a few possible issues. i'll take it in order.

so the engine is 11 years old on this current setup. what is your terminal RPM? how many racing miles are on it? it could just be time for a refreshening - probably not the only issue, but it could be complicating your fuel issue. something to think about ...

as far as jetting recommendations go for the IDA. i have no IDA experience for myself, but if you PM the user wackyracer, he should be more than capable of getting you started or referring you to someone that can.

could it be timing? for sure it could be, but i'm thinking it wouldn't be the only thing causing what you describe. where is it at 5500 RPM? the flames are probably excess fuel getting spit out the exhaust and igniting. it will do that even with the timing being dead-on as long as there is enough excess fuel.

If you floor it in 2nd, you'll get 7,000rpm, but if you use gradual throttle, you'll get 8,000. Then if you shift to 3rd, you'll get 6500rpm max, no matter what you do, and in 4th, you'll get 5500rpm.

There was about 0.75 racing miles on it. Our track is about 1.6 miles, and it did about a 1/2 lap before we got it (they blew the clutch after 1/2 lap with the new engine, and never took it to the track again after replacing the clutch). lol

Thanks, I'll look into that!

I have no idea what the timing is at right now, but I know that the guy who built the engine says he left it at stock ignition timing settings, but it could have changed somehow, been loose, or something crazy. I'll have to time it and see what it's currently at.

And I can understand one loud pop with about 6" of flame on shifting, but excessive pop-pop-pop-POP with 16"-20" of flame seems like a little much.

8000 RPM ????? wow, something is truly wrong!

any decent-sized hole in the exhaust would cause the incessant popping you describe - and just for the record, what exhaust setup are you running on that thing - i'm interested in size, configuration and the muffler designs. i know your signature states RB header and HKS exhaust, but i question it's abilities of the HKS section on a bridge.

to be honest, at this point, i question a few of the choices the engine builder made (the T2 rotors, the 6-port, etc.) for building a N/A bridgey.

what you can do is start by setting the timing for yourself. the RB catalog should give you some recommendations. if you have earplugs (or you're a masochist, like me) take the rear section off the exhaust and run an open header (maybe just bitch up a quick section to divert it out the side or downwards), see how it behaves for a hot lap or two and report back.

We're running a RB header that takes 2 pipes into one, then connects up to the rest of the exhaust, and there is a leak at that flange. It's 2 1/2 inch on the single pipe part of the header. Then it goes into an HKS exhaust off a turbo 2, with the Cat deleted through the addition of a straight pipe in its place, and then a "Y" pipe and 2 mufflers. I'm assuming 2 1/2" as well.

Hehe I'm sure the track officials would LOVE us if we took off the rear section, but I'll see what we can do. I anticipate black flags flying.

The header is like this:

http://www.racingbeat.com/photos/16010.jpg

I saw that but it never hurts to check and very easy to do. Un-plug it from the carb, stick the end in a bucket and make sure that really is flowing well. I worked on a supercharged car over the winter that had "all new fuel parts" on it, it wouldn't rev at all, it ended up being the fuel pressure regulator wouldn't let enough fuel threw to run the car up high. Just trying to help.

Edit: But I still think it needs a bigger carb and here are stock jets for a 51 IDA on a full bridge.


51MM IDa
45mm Venturis
#235 fuel Jet
#165 air jet
f11 emulsion tube
#300 Needle valve
#50 Pump Jet

i didn't mean to sound dismissive. you're absolutely right, you can never be too thorough and yes, i do believe that 45 is choking it off. however, there seems to be other things compounding the lack of lungs.

i only read first 10 posts or so. i know shit about carb/jetting etc. i deal with EMS. but the symptoms you're describing sounds like you're running out of fuel

can you hookup wideband o2 and check that?

Update: I am ordering a Racing Beat 51 IDA today, and ordering this jetting set-up to try, plus a few extra jets on either side of the range. Rotary Reliability and Racing suggested a little different jetting set up for our build and track, so I may try that too.

hey bro this is off topic and i dont mean to jack your thread but what does your ignition system look like? are you runnin first gen dizzy? if so how did you wire it up to ur fc?