The vent hose should have a hose clamp on each end. It should be as easy as un-clamping the hose, pulling it off the tank and cleaning it out with something. Then after you get it cleaned out hook it back up to the tank and see what happens.

 

So it's a beautiful day down here in NE Georgia, about 80 degrees, bright blue skies with puffy white clouds floating by, very low humidity and a gentle breeze.

So I went out to check on this slow fill problem and discovered some interesting things.

Yes Jamie, I found those clamps, I could just get my hands on them by reaching up around the tank and sheet metal from underneath (for the lower one) and by taking out the rear interior trim and reaching down around the sheet metal for the upper one. I was even able to squeeze the hose over most of its length and it sure doesn't seem to have any major occlusions or crap in there.

But forget about getting a tool to either of those clamps to squeeze them and get the line out, the tank needs to be dropped if you want to service that hose. Of course on this beautiful day the tank happens to be full to the top, so it's not coming out this afternoon. Maybe never actually, hanging out at the gas station dribbling gas into the tank once in awhile is probably preferable to pulling and servicing the entire fuel system.

So at that point I'm thinking that 9595's idea of sticking a speedo cable down from the topside (and yes dislodging the crud into the tank or a filter or whatever is down stream) was making a lot of sense. Hey, what's a little more crap in the fuel tank if it fixes the vent problem?

However, the 84/85 Mazda engineers decided they didn't want us to do that so they placed a metal shield in front of the vent opening inside the fill tube where it comes in at the top - you can't get a piece of thin wire in there. Apparently they only did this for the 84/85 models, I have a tank from an 83 that doesn't have this "protection" (Damn It).

So I'm back to the science of troubleshooting to figure out exactly which component in the fuel system is causing the pressure to build in the tank while filling, causing my gas station visits to be longer than planned.

I checked the FSM and the Haines manuals and of course they provide all kinds of information about everything except this problem. I guess the Mazda engineers never had this particular problem, even though a bunch of us guys who drive their cars do.

Hopefully the wizened old crew here at RX7forum will provide the missing information, but in the mean time I do have what might be a clue. The problem began shortly after I discovered that the check valve in the fuel return line to the carb wasn't working, so I replaced it with one that was.

Could adding a functioning check valve to the fuel return line to the carb have revealed another problem in the system, say another malfunctioning valve somewhere that wasn't a problem when the fuel return check was letting fuel pass both ways?

 

I had this happen to mine and it turned out the Check and Cut Valve was installed backwards.

 

This happens to me too!! I also premix the seven. Does anyone use a funnel to add premix,or do u just add it right into the tank? Maybe the premix is actually running through the vent hose n causing some sort of restriction?

If I have time, I'll try n drop the tank n check the Vent hose

 

I think I got that check valve on the return line right. When you blow it goes, when you suck it doesn't, right?

I just pour my premix oil into the throat of the filler tube, flush it with the gas. Figure it must get down to the tank somehow, I never see any on the pavement.

 

I noticed something similar on mine. May shed some light on the whole situation.

Started premixing and first time added oil first (straight to the filler), and it gurgled and cutoff a bunch. Literally took 20 minutes to get 12 gallons, at which point I gave up. The second time I filled it up, I went ahead and started pumping gas, pumped perfectly fine up to 13 gallons or so, then I added the premix, started pumping gas and viola, the gurgling resumed and it wouldn't accept much gas.

Next time I'm going to try a funnel I have (somewhere :-/) that has a foot long clear hose (1/2" diameter or so) coming off the bottom. I'll thread the hose into the neck, most likely all the way into the tank, add the premix there, then pump the gas. I'm thinking that the thicker oil partially blocks that vent tube, so the air trying to come out of the tank is forced up the filler neck, causing all this trouble.

 

Hmmm, that does fit my situation exactly, the problem started when I started pouring the 16 oz. of premix into the filler tube. Since I haven't been using a funnel to make sure it's delivered passed the spring flap thing I suppose a good amount of the premix could be backing up and ending up in the vent tube.

But I still don't see how that would foul things up. Wouldn't the premix just drain down the vent tube back into the tank? The premix oil isn't that much more viscous than gas, I don't see how it could cause any channel meant to flow to clog up.

 

Similar story here in a reeeeealy old thread I happened across today. https://www.rx7club.com/1st-generation-specific-1979-1985-18/problems-pump-119085/

Again kind of pointing toward premix. If the weather cooperates this week I'll make a few commutes in the rex so I can fill her up again, using the long hose funnel for the oil this time.

 

Nice find Newbie, that's a perfect description of what I've been seeing since adding the premix. I think we are getting someplace - it might be how you add the premix.

In fact I tanked up today and tried something new - I used a long screw driver to open the spring loaded flap in the filler neck, then carefully poured the premix in so that at least most of it went down the filler tube.

It still had some problems but the tank was much easier to fill. I had to put the pump notch on low speed as usual, but this time it stayed on all the way till I reached 13 gallons (it was clicking off at 10-12 before, sometimes even earlier). Then I was able to top off the last gallon and a half without the usual dribble dribble dribble.

A bit early to tell but I'm going to get one of those pointy funnels before the next fill up later this week.

Thanks Newb!

 

OK, I think the problem is solved. Today I filled up again, but this time did a good job of getting all the premix oil delivered down the filler tube using my new high tech premix apparatus:



And what do you know, I put the pump setting on high and it filled the tank without a hitch, right up to 14 gallons. Then I topped off the last half gallon and the tank was full to the top. Just like old times, before I started adding the premix to the tank.

Three or four of the folks who posted on this thread with the same slow filling problem (including me) are all premixing their oil. So my guess is even a little oil down the wrong tube and the vent stops working properly, causing the problem.

Given this seems to be a common problem, it might be a good idea to archive this thread!

 

weird, the premix must be thicker enough to really foul things up!

explains why i didn't have this problem on my FC too, i had a funnel

 

We had the same problem in 1974 RX-4 wagon. Mazda had a recall. It was the hose from the filler tube to the gas tank. That solve the problem, before that is was hell after a half of tank and that was after we waited in line for 2 hrs, for gas.

 

Thanks Ray. I should have read this earlier today. I was getting the evil eye from this guy waiting for a pump.

 

It sure does make a difference. I filled up again last night using the funnel and once again, with the pump handle set to high the tank filled right up to 14.5 gallons before clicking off, right where it should.

I agree with J9 though, it's weird that a little premix oil down the wrong tube could have such a big effect. I'm just glad I didn't pull the tank and rip into the fuel delivery system before figuring this out.

Thanks to wankelnewbie for leading the way!