Hello RX7 world,

My name is Jason, I am a Mazda Tech in North Carolina. A customer brought a mint 79 rx7 5 speed, all original 50k miles to our shop with driveability concerns. My problem is the car is leaning out at high rpm trying to accelerate, and with our little knowledge of 79 carbs, are lost as to why. The point right before when secondaries come in seems to be the worst, like its struggling for fuel. This is how a lean condition was determined. Also, when the choke is applied at this RPM/load range of concern, the problem seems to clear up slightly. Around 3800 rpms while maintaining a speed of 70+-, a light fishbiting sensation is felt, and is only present with the higher rpms and steady driving. Secondaries feel like they are working properly with no fishbiting or bogging, or any other symptoms. The fuel pressure is consistant at 4psi, float levels are set, the idle mix and speed are adjusted, it idles beautifully at 800 rpms, and the pump shot is working as described in your articles. The thought of having to rejet or modify a stock carb for a stock 12a to make it drive like a stock rx7 seems a stretch. What are we missing? All help will be appreciative.

Does anyone know how to reach the sterlingmetalworks.com guy? I see he was on here years ago, but his hotmail address isnt valid.

Thanks,
Jason

 

Shoot him a PM (user name Sterling). He's been on here a few times over the last week, so you might get lucky.

Based on the description, have you replaced the fuel filter? If not, start there. They barely flow enough when brand new, and even a slightly dirty filter will cause starvation issues as you've described.

Another possibility (based on the fact that things improve with the choke on) would be clogged primary jets (located in the bottom of the float bowls). Fairly simple to remove those for inspection/cleaning.

A basic check of the ignition system would also be a good idea, simply to rule it out. A lot of times a carb issue turns out to be an ignition issue with these cars.

Best of luck...



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I would try a fuel volume test. It only take a minute and will tell you a lot. Get an empty gallon jug and stick the fuel feed line in it. Get the pump running and time for 1 minute. See what you get.

What can happen is a system that isn't putting out enough fuel (clogged filter, bad pump, etc.) can show the correct fuel pressure at idle, but when you go to drive it and put a load on the engine, the fuel doesn't keep up and the fuel pressure drops like a rock and you'll starve for fuel.

If you get enough fuel, then you know the problem is related to the carb (if it is really fuel related).

 

Ok, so ive removed the fuel filter completely and put just a straight thru connector, didnt change the problem. It has new points (set to .018"), plugs, and wires. The carb jets are clean, as well as the air bleeds under the air horn. Im going to attach a fuel pressure gauge and drive it to see if psi is constant, but, in my thinking, wouldnt if the fuel pressure was a concern, when the secondaries open it should really bog hard, and possibly stall due to the increase in fuel volume leaving the bowls? Im still going to try it and make sure its good, running low on options here. Thanks for the help guys!

 

Gotta have 3 posts to pm people (sterling)...heres 3.

 

You won't get an immediate effect, because it can still run off the fuel in the float bowls. Once those run dry, then you bog and sputter...

 

its in the carb for sure u can be certain about that clean EVERYTHING then clean it again im almost eighteen and I've worked on more then my fair share of carberated engines (mostly small single or twin cylinder engines) and this is one of the most common problems i've seen even if it looks clean as can be something is probably plugged one of the best things u can do if u dont know exactly whats plugged is compressed air through every hole usually will the the obstruction out also if u take everything rubber plastic or anything that can be eaten by carb cleaner out you can soak it in a container of carb cleaner.

 

Okay, so the car runs out of fuel at high rpm. Right?

First step is to determine if it is a fuel delivery problem (to the carb), or an internal carb blockage. This, fortunately, can be easily determined. Here's how:

1. Drive the car up to high RPMs until it begins to die.
2. As soon as you experience the issue, kill the ignition and pull off the road.
3. Pop the hood and check the float bowl windows.

Reading the Evidence:

A1. If they still have the correct level of fuel in the bowls (halfway full), then the problem is internal to the carb (blockage).

A2. If the bowls are empty, then you have a blockage before the carb (filter, pump, line, internal tank issue, etc.).

Try this, it should reduce the number of systems that you have to check through.





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I am the original owner of a 79 GSr (52k miles) which is exhibiting a "Hesitation Problem".

Starts fine and Idles at 800 solid - choke goes off when heated up - when I am sitting still in neutral, clutch in, all is totally smooth as I accelerate the engine through the entire range of RPM. Get on the road and there is marked hesitation under normal pedal pressure starting at about 2,800 thru about 4,000 rpms - in each gear. As I am driving along, if get to an upgrade (and we have lots of hills here in the Shendoah Valley) it starts balking/hesitating- HOWEVER, if I punch the pedal while hesitation is present, power kicks in and off we go... In the "trouble" range of RPM, the car chugs and seeems to be strugglijng to get fuel. It feels like it has no power, until I punch it and then it feels good. It is almost like a miss in a piston engine, although I don't suspect an electrical problem. The problem isn't subtle; like hitting a brick wall in power unless you floor it for a for seconds or down shift to get higher RPMS.

My Car: 1st GEN - Stock 79 GS - 5 speed manual - (Has new aluminum radiator) Carb taken apart and "reconditioned" (I dont know much about carbs; the mechanic kept talking about taking apart the "top" of the carb several times over the past few weeks so I am assuming that he "rebuilt" it.) - fuel accelerator diaphragm replaced - new fuel filters - new fuel pump - vac lines to carb inspected and replaced where needed - generally looked at all vac lines - air filter good - seems like accelerator cable is working smoothly - car does not smoke at all

More info: The car had ten year old gas in it, until about two years ago when I siphoned it out. I then put a full tank of fresh gas with sta-bil in it and changed the fuel filters. Filters have been changed since. Could it possibly still be a partially clogged pick up in the tank? Do I need to boil the tank out?

Thanks for any ideas you may have.