nerd racing

My 85 base 12a rx7 bogs almost like a misfire or flooding, but only sometimes. When I shut it off and turn it back on its back to normal. Stock Nikki carb stock ignition with blaster coils and new plugs. Need to pump it twice to restart it after bog. Only mod is hot start actuator removed. Still has rats nest can't find a vacuum leak anywhere.

GSLSEforme

Fuel filter recently changed? Try that first,doesn't cost much. No change? check fuel pressure,2.5 psi good,check flow next,remove line from carb and put into a can/glass and crank it up and see how much fuel comes out,should fill 8 ounce container in 30 seconds or so. Better done with 2 people. If you don't get that volume of fuel,look at the pump/fuel supply from tank(crap in tank/partially plugged lines). If that all checks out,and you know you have good plugs/spark timing set correctly,distributor advance mechanisms operating correctly,then its time to look at the carburetor itself...

nerd racing

Thank you for the response. I will pick up a fuel filter on the way home from work today. That was next on my list to check. Thank you for the pressure and volumes to check fuel flow. Maybe I can get my roommate to help me with that.

I'll keep you guys posted. I'm trying to get it going more consistent for the CNY Mazda meet.

t_g_farrell

iTrader: (1)

Also could be gunk in your tank that gathers around the pickup over time and slows the flow
enough to cause you to lean out.

nerd racing

I swapped out the fuel filter last night. It was already a plastic one so I went with one of those. Some nasty stuff came out of it so I figure changing it didn't hurt anything. The problem is still present but the engine seemed happier.

I sprayed around with carb and choke cleaner again to look for vacuum leaks and didn't find anything notable.

Discovered my idle is set by the throttle cable, which is fan freaking tastic. I loosened the cable and it's not idling with the proper slack on the throttle cable. All signs point to a vac. leak but I can't seem to find it. Do the backfire valves leak when the diaphragm is ripped?

GSLSEforme

Bought a Rex a lot of years ago had some of same symptoms,you mentioning idle speed with cable pulled tight jogged my memory,could be simple fix. On my car,diaphragm was blown in shutter valve causing it to not fully close and would not idle. With engine running at best idle you can get(did you try to adjust base idle speed/mixture?),take off air filter lid and listen for a sucking sound from one of large tube openings in base of air cleaner,or put your thumb over the opening in the tube. If the idle speed changes/improves/smooths out,likely bad shutter valve. You can temporarily plug the opening with something so you can adjust idle speed/mixture and drive car and see if it's better. I used a plastic cap off an a/c line,(used to install a lot of air conditioners) that fit perfectly over the opening and drove it that way for couple months til i changed the valve. The valve opens on decel and lets filtered air in to lean out mixture to prevent backfires when throttle is closed-if stuck open partially=vacuum leak. Happy hunting,post back with your findings.

nerd racing

Thank you for your description and diagnosis. Also the ways to check and test it too. The problem only seems to crop up under hard deceleration and then persists until I cut power and re-start. I have not messed with base idle, but I did adjust the mixture, went rich with it until it wandered, then turned it back until it was smooth. I will attempt to plug the shutter valve and see if that lets me release the throttle cable and still idle. At that point I can then check my timing and re-set that too.

nerd racing

Thank You for your help, I found a vacuum plug and plugged the hole in the air filter plate. That did the trick, no more vacuum leak from the shutter valve. I've got to replace it now, but it is good to know what was going on. I got it to idle around 1000 RPM but still with the throttle cable. Still have some work to do to get it set right, but I'm on the right track.

GSLSEforme

Yur welcome,try adjusting base idle speed screw in while running(with throttle cable the way it is) til it picks up the idle speed,then release tension on throttle cable and start to back idle speed down. 800rpm is what you're looking for,may have to adjust idle speed/mixture adjustments alternately to get best idle quality,engine warm,not hot. Post back with your results. Looks like you're closer to driving it to the CNY meet. Happy rotoring....

nerd racing

Thank You for the tip! I released all of the tension yesterday, I'll set it up hand tight and set it just warm enough to release the choke fully when I adjust the idle speed screw. It's not easy to see on the 85's. Still haven't spotted it yet. Hopefully after work I will get the time to wrap it up.

Thank you again! I'd definitely prefer to take the RX7 to the meet, otherwise it will be my Euro spec 90 323 Diesel.

nerd racing

Ended up not taking it to the meet because of a rear differential leak. It was a 2.5 hour drive one way and I didn't want to risk it.

When I got back I fired the car up and went to go get some fuel. I didn't let it warm up all the way before I left and I noticed the car ran like a bat out of hell with the choke still partially engaged. In the higher rpm you could tell it was choked, but the low range was perfect. Should I be setting my idle mixture more rich to compensate for this? I got the car to idle with the idle screw but I'm still around 1200 RPM otherwise it dies. There has to be another vacuum leak somewhere I imagine.

GSLSEforme

Suggest you revisit your idle adjustment procedure to make sure it's right. With your idle speed set where it is,adjust mixture screw in til rpm drops off,then go back the other way til no more rpm gain,then a little more. At this point begin turning down idle speed,if/when engine begins to stumble go back to mixture screw and richen it some more,go back to lowering idle speed,back to mixture screw to balance air/fuel as you lower idle speed to about 800 rpm. If everything else is ok with carb you should have a solid idle. If not you'll need to look closer at carb.

nerd racing

Will do, just as long as it stopped raining. The Miata Roof project is taking up my working bay right now.

nerd racing

I've come to the conclusion that I still have a vacuum leak, but it doesn't show up until the car is fully warmed up and the choke is released. What vacuum items activate once the car is fully warm and unchoked?

GSLSEforme

Were you ever able to get it to idle without throttle cable tension? With engine idling lowest/best idle speed possible,use an unlit propane torch with valve just cracked open to where you can hear gas coming out and sniff all around carburetor(with air cleaner installed),solenoid rack,vacuum hoses including brake booster and its hose. If there's a vacuum leak,engine will pull in propane and burn it. Idle speed may increase,engine smooth out or stall out completely-if so restart it and return torch to that area,if same thing happens,concentrate your search in that area for a leak. One other thing you can try,pinch off hose going to brake booster. If this causes a noticeable change in speed/idle quality you may have a leaking diaphragm in booster. Several posts back you mentioned the fuel filter was dirty when you changed it-go back and look at the filter again and see what comes out. Quite likely you don't have a vacuum leak,but a partially plugged carburetor.....with the same stuff you saw come out of that filter. Two things here: you're going to have to take the carb off/apart to clean it(properly)& you have to verify you have a clean(dirt,rust free) supply of fuel to the pump an to the carb. Believe i mentioned early on about possible contamination in fuel tank. As far as the carb cleaning/overhaul goes,there a numerous threads on the site pertaing to this. Look for posts by Jeff20B,pay particular attention to what he says about NOT using the replacement needles&seats supplied with carb repair kits,also note what he says about not using supplied baseplate gasket-use it as a template to cut out a thicker one from gasket material. Post back with your results.

nerd racing

I did get it to idle with the screw but it will not idle below 1200 RPM no matter how much I fiddle with the idle air adjuster and the speed screw.

I'll give the carb rebuild a go, I've read about 10 different posts about what to and not to do with the carbs, Jeff's article is the one I'm going to follow. I met up with another FB owner that had a couple nikkis that they offered me. I'll see if I can get one off of them too.