I'm getting 10mpg in my 85 GSL. I did strip the nikki and put mech. secondaries on. I stay away from full throttle but this thing drains gas. I have the mixture screw turned out 2 1/4 turns. I get maybe 200 miles to a tank....Mixed driving. (city, highway, spirited) Is this normal?

 

No it's not normal. So first question is: What was your mileage before the carb mods. If it was better before, go back to vacuum secondaries and see if mileage improves. Your tank holds 16.4 gallons so your mileage is more than 10 mpg. Keep an accurate track of mileage, don't guess.

 

i face a similar issue. dunno the difference between s4 and s5 but my full gas tank takes up 17.5 gal and it lasts me around 170-180 miles. which seems to be around 10 mpg too.

 

I had the same experience when I took out the rat's nest, stripped the Nikki and wired the secondaries - big drop in gas mileage (and the throttle progression was terrible, very herky jerky and a real pain for daily driving).

After a few months of trying to get it right I ended up putting the rat's nest back in and reconnecting up the vacuum advance. Gas mileage returned to the usual 23-24 mpg and the throttle response is much smoother.

 

Ah, I really didn't want to hear about having to go back to the rats nest What about a different carb? I love the nikki but I also enjoy some sort of fuel milage.

 

It's not the rat's nest but rather the vacuum advance that probably helped (and perhaps there was some undetected vacuum leak). When I ran my streetported 12A with a stripped Nikki and no rat's nest I was getting the same mileage and same driveability as I was with the rat's nest. Only difference was that it was a little louder and a little faster because I took out the cat.

 

1/2lb of fuel per horsepower per hour is the hard floor... no avoiding it.

Much of the complexity of the stock Nikki and ratsnest were added by Mazda engineers to try to get the car efficient with regard to fuel use (and lower emissions, which are often at crossed purposes). Tricky to do. Especially with a device as non-linear in response as a carb. Took a lot of compromises.

 

so what do you suggest I do? This isn't my only vehicle. I am just driving every day because it's snowing very often here in VA which is quite fun with RWD. Power is more important than the fuel ecomony on this car. I was just curious if this is normal. Should I start checking my timing? I am in the process of installing my wideband gauge. I had it hooked up over the weekend and at cold idle it was at 7.4 a/f

 

My monster port with stripped/modded nikki gets close to 30.

 

Man, I can hit 20+ MPG in my carbed SE, even driving it rather spirited. That 600 CFM edelbrock can feed quite a large volume of fuel, when requested, but I bet it doesn't drop much below 10 MPG.
My 12A car got 11 MPG at the track, 26 MPG on the way to the track......

 

Another thing that probably doesn't help my situation is that my car doesn't even get to it's full operating temperature from my house to my work. This is a 4-5 mile trip and about 10 - 15minutes of driving. 4 traffic lights and about 25 - 40 degree weather. It get's to the second line on the temp. gauge. I don't even have heat. It just get's hot enough to idle without the choke. The car does eventually get to it's full operating temp on longer trips. I'm thinking the thermostat is stuck open.

 

I get about 200 miles on 15 gallons, 15 MPG. It has dropped steadily with no real change but who knows. People who get more than 20 are very lucky, because I've never gotten more than 16 in any of my cars. I only drive spirited sometimes, too.

 

i was getting mid 20's on mixed hwy and city with some NGK cold plugs. and high 20's to low 30's on only hwy driving. anyways i switched to regular autolite and it dropped to low 20's on mixed city/hwy i think best i'd seen is 23mpg. i haven't drove it much this winter so my calculations are only from this past summer.

mods are about the same, stripped nikki, no rat nest, mech secondaries rb header. and cut air snorkel for more air (i think this is what hurt me more than the plug difference as far as mpg in the summer, didn't do much calculation to figure out problem tho).

 

10 MPG is most certainly NOT normal!
look for vacuum leaks and any other anomalies. also, is your exhaust completely stock?

 

Could a vacuum leak cause that much of a decrease in milage? The exhaust is stock except I don't run a muffler at the end. There's defiantly a hissing sound on top of the motor. I read somewhere when I was rebuilding my carb that the gaskets on the plastic spacer inbetween the manifold and carb has gaskets on it that you should not remove. I did not remove those instead I placed new gaskets on top of the old ones. Maybe I will try removing those and I will also run a vacuum test.

 

Right, the phenolic spacer does not take fresh gaskets, nor are the old ones removed. What was you mpg before the mods and carb rebuild. A vacuum leak may decrease mpg some but not substantially, i.e. from 15 mpg to 10 mpg. You probably do have a vacuum leak but I suspect the mechanical secondaries mod is causing most of it.

 

I don't think you are supposed to put those new gaskets that come with the rebuild kit on top of the phenolic spacer, which has a permanent gasket built into it. As I recall the FSM is pretty clear about not putting those gaskets in for some reason.

Concerning gas mileage, I track mileage on my daily driver pretty closely and because the commute is long and consistent, I always expect to get the same result (23-24 mpg, with the rat's nest). Every once in awhile I'll see that start to drop all of a sudden - the first thing I do is look for the vacuum leak, which is almost always the cause (I use starter fluid, it's a good way to find those little ones in the rat's nest).

There's one tube in particular that can cause a massive leak that not only kills mileage, it significantly cuts into power as well. It's a wide "U" shaped rubber tube, about a half inch inside diameter, that runs from the base of the carb near the passenger's side fire wall down to the exhaust manifold. It takes a beating because of the exhaust heat, if it hasn't been replaced in the last few years it probably is getting weak around the connection to the manifold and even a small crack can cause a very large (and audible) vacuum leak.

Also I'm not sure what not having a muffler will do (other than make it intolerably loud) but not having that back pressure might be another factor.

Trochoid probably has it pegged, you have a combination of factors contributing to the problem - wired secondaries, vacuum leak(s), maybe some other things like carb adjustments (you can't do these correctly if you have a preexisiting vacuum leak), decent spark plugs, dizzy rotor and cap, plug wires etc. Doing a lot of driving on an engine that is not fully warmed up certainly doesn't help. Even low tire pressures can take a few miles off the mpg's, add it all up and you get 10 mpg.

 

besides all the carb suggestions ....... check that your tire pressure it up. You might be surpised how much low tire prerssure in the winter can reduce fuel economy

 

Alright it's making for sense. I will give it a complete over look instead of trying to put the blame on one thing. The issue is that I can't say what the previous milage was because I bought the car and it wasn't able to idle. So I took the carb off and rebuilt it. The problem was still there. I took the rats nest off and now it idles. It will only idle when the motor is warm. If the motor is cold then it wont idle. I am still confused about why it takes so long to warm up? Sorry for this useless thread guys I just couldn't take running in circles anymore. Atleast I have a direction now.

 

Sounds like a combination of issues, yep.

T-stat may be stuck open or even removed by PO. Many times people think removing it makes for 'better cooling.' Instead it robs the engine of the ability to reach an efficient temperature.

The carb spacer has permanent gaskets glued on, and you should not use other gaskets in addition to them - - I did once way back when, and had horrid results. The resilient permanent gaskets can get stiff with age, but the carb will still settle into them. I've occasionally used a VERY THIN (like transparent) film of silicone over them, but it didn't seem to make any difference other than being a pain to get off later.

Make sure your secondary mod hasn't prevented the secondaries from closing completely at idle or part throttle.

You might even try running with the secondaries disconnected (will be gutless, but will still run the car at reasonable speeds) and seeing if your mileage goes way up. That would tell you if the issue is related to your secondaries - - provided they close properly.

 

Wait I modded the secondaries to close all the way. My primaries open all the way then my secondaires start to open. I've checked my float levels. They are good. I'll remove those gaskets and check all my vacuum lines.

 

One of the reasons to not use gaskets on the spacer, is they tend to block the vacuum passages.

 

First part right second part wrong. The secondaries should be fully open when the primaries are fully open. The secondaries open at a faster rate so they begin to open when the primaries are partially open. Don't recall at what point they start opening, someone should chime in on that one.

 

When the vacuum tells them too? That's probably why the car came with vacuum activated secondaries instead of wires. Those Mazda engineers were pretty clever.

Might even affect gas mileage, since the 4 barrel Nikki is well known as just about the most fuel efficient carb option for the SA/FB (especially when coupled to a functioning rat's nest).

But hey, we all love to "mod" our cars, I even tried this one myself. But it's probably a good idea to keep all that stuff you tear out, just in case.

 

I got a chance to change the thermostat today and check out the gaskets on the carb. Turns out the thermostat was done and so was it's gasket. I changed it and now the car warms up like it should, it actually overheated? The thermostat is in the correct way as it only goes one way. Turned the heat on and it was fine. For the gaskets around the plastic piece, well they were blocking a bunch of vacuum holes so I removed the "new" gaskets and left the old OEM ones on there. Car started right up which it has never done and idled with the choke on. I was very happy . I think gas milage will be better now. Thank you Rx7Club