Ok heres the scoop, 12a half-bridge-45dcoe carb-open race exhaust-red holley fuel pump. I rebuilt this motor myself and it fired right up and sounds great. But when the engine is under load I get some "popping/backfiring" noises coming threw the carb. Im not to sure where to set the timing either. The noise sounds like detonation but im not really sure. One more thing the engine makes alot of power untill the popping starts then it just seams to lose poweruntill I back off the gas.
so answer these q's.
-where to set timing?
-what fuel pressure should I be running?
-Is this noise detonation?
-can this carb really not handle this motor?

 

sounds like timing...

wish I could help.

 

If it sounds like a rattling sound while under load, you likely have the timing set one notch too far. I wouldn't keep loading the engine when you hear that sound, its very bad...

 

Have you actually checked the timing yet? It could be the that or maybe your carb jetting is just way off. What are the specs? BTW, fuel pressure should be about 4.5psi.

 

runt det. sounds like popcorn in a motor. g how do i know

 

So what should the timing be set at?. Remember its a small half bridge. I am going to re check the fuel pressure today. Iam not to sure about carbs but I pulled one of the gas jets out and it had a 65 on it, Is this a gas jet or what?.

 

I'd recommend no more than 20* total advance on the leading with a 10* split for the trailing. But even the stock settings will be fine to get things started.

The jet you're looking at is probably the idle jet.

When you open the little cover on top of the carby you can see four "tubes" that screw into the body of the carb.

The two larger ones are the emulsion tubes. The main fuel jets press into the bottom of the emulsion tubes and the main air bleeds press into the top.

The smaller tubes are the idle fuel jet holders. Obviously they hold the idle fuel jet.

Hope that made sense.

 

Thanks Revhed ill play with this thing more tommorow any idea of what size jets i should start with?. Those jets that said 65 on them where the idle jets?. Any way i enlarged them to 72 at my buddies machine shop thinking they where the reg gas jets.

 

By pass the lower jets and go for the secondaries.....those are the ones that you are using under load anyway. You should be around a 176 but it really depends on Density Altitude.

Do two things first....pull your plugs and tell me what they look like. If they look like you spilled baby powerder on them that you REALLY need to add more jet. Fuel pressure should be at 2.75 - 3lbs. Timing on a 1/2 bridge port???...humm...I am running a race prep non ported engine and my timing is set at 28 and 20. But I am using 110 octane race fuel too. Try using Premium Fuel as well...VERY IMPORTANT...it runs a Lot cooler and prevents detonation.

Let me know how this works and BE CAREFUL!!! Also...do a run with the car and if it does the same thing then try this..

Put in BRAND NEW plugs....start the car up and run it right away...let it come up to temp with a MILD load on it but dont putter around....then run the car and if it starts to detonate the SHUT IT OFF RIGHT AWAY...and pull the plugs. Sometimes you think your mixture (air fuel) is right on and because you run the car at lower RPM with more fuel being delivered. Try my method and then inspect the plugs....that will tell you what your fuel system is REALLY doing.

Balz
#67 IT-7
www.balz.myip.org

I live my live one apex (SEAL) at a time"

 

Double post... what the hell's wrong with this forum.

 

I don't quite understand or agree with a few things you said.

Firstly, the Weber has no primary or secondary jets as it is a two barrel carb. It has an idle jet and a main fuel jet.

Also, the general consensus among most people including Mazda factory race teams is to use low octane fuel on n/a rotaries. The shape of the combustion chamber means rotaries are quite resistant to detonation so there is no need for high octance fuels. You can in fact make more power with the lower octane stuff.

 

Ok I had never pulled that top cover off the carb until you mentioned it, it has the wing nut on it and its round right?. Any way can you explain these 4 jet looking things under there a little better to me?. I took them all out to look at them but like I said earlier I dont understand carbs. Which ones are the main gas jets?, the ones on the air horn side or the ones on the manifold side?. Also which jets should I start with?.

 

i think he was talking about the idle and mains id bet its the jetting, last i knew he (piston pounder) had a STOCK setup in his weber so what ever r.b. or mazdatrix had in it is whats in it now. runt go to a v.w. tunner and get jets id do this to start, look at the larger jets (main fuel) and say its a 165 go up in 5's 170 175 180, this will give you a good idea.... you will need to fine tune it with say a 176 but the 5 rule is a good starting point, if you dont mess them up i bet you can get the $$ back just say you didnt need them.

 

Missed the fact that is was a weber...let me run back and check with my FM buddy and find out what he is running from a jet standpoint.

As for the fuel....Humm...what is that old saying "opinions are like...."

Don't think fuel makes a difference...well, that is "your opinion."

And I have seen a LOT of opinions on what works and what doesn't.

Try the higher octance fuel...if it doesn't work, no harm no "foul" and potentially no detonation.

BTW..."What do you race again?"

Balz

 

he has a 85? gs? with a half bridge (primarry ports) 12a he just got it in a few days ago and started. the intake is a r.b or m.t. weber dcoe? 45mm side draft. full mindtrain stage 5 exh. electric fan.

 

Ok tried the higher octane fuel and it didnt get better. Im pretty sure the jets are too small. Anyone know what jets I should start with?.

 

This isn't just "my opinion"... it is also the opinion of the likes of Jim Downing, Mazda factory race teams and most if not all serious racers I've talked to. If you can show me cold hard facts that state otherwise then I'll listen.

You must understand that the octane rating of a fuel is only a measure of it's ability to resist detonation. Any more octane than your engine needs is simply put to waste and will probably hurt power because of it's less volatile slow-burning nature.

If your engine was detonating with those timing settings then backing the timing off and switching to a lower octane would probably net you better results.

 

Can anyone tell me why my Rx-7 makes more power, has better throttle response, and better gas mileage when I use high octane 91 fuel?

 

What sort of Rx-7 do you have?

 

81 rx7
i noticed octane helped with the stock port and street port 12a

 

I've noticed the exact opposite, on the street and on the track, with my stock and ported motors. I'm a low octane fan, however, I've only tried pump gas and octane boosters, so I'm not going to jump to any conclusions about fuel that's $10 a gallon (what's the actual price of some 110 leaded these days?)

Timing for low octane:
I run 24 leading, 16 trailing advance on the 87 octane with no problems yet.

 

High Octane Fans out there? Have you tried Avgas (aviation fuel)? I use it exclusively on my heavily modified Sea-Doo because of the high compression.

It's not 110 Race Gas, but it IS 103 octane at $1.98 a gallon (beat that VP Race Fuels!). I've been using it for years, and just fill up (trailer and all) at the small private airport nearby.

 

Yeah, "Low lead" RULES. Keep it around for the 2 stroke dirt bike and occasionally mix it up for the other toys, when I'm not caring much for the oxygen sensor.

R

 

nimrodtt, last i checked 101 and 110 gas were in the $5.50 range, Union76 race gas, at the track. Last year they were in the $4.75 range.

This is not the higher octane gas I used and saw more power. That gas was Mobil 91 ocatne pump gas. From the advice of many people I now use 89 octane. They say it's better quality than 87. And when I'm done premixing it brings the octane down some too.

Brad