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Hi everyone, i've got a really odd problem. with my RX7 1980 Mk1, it has a Nikki carb on the engine. When I start car using the Manual choke the car idles smoothly at about 1000 rpm. Once the engine warms up to operating temperature shutting of the choke the car struggles to idle. When the choke shuts off the idle drops to 800rpm and slowly decreases 500rpm and then stalls, with the rev counter needle jumping all over the idle as it switch to volt meter. To stop the car from stalling i need to have my foot on the throttle peddle. This can make driving in traffic really difficult when you can't take your foot off the throttle.
However, here is the strange thing. if the outside temperature in above 7 degrees C the engine will idle smoothly all day long at 800rpm. Here is the stranger thing. If the temperature rises above 7 degrees C the engine will be at 500rpm trying to stall then the it will spring up to 800rpm and will be fine for the rest of the day.
I've looked at lots of posts here on the forum, but nothing that describes exactly the problem I'm having. Can anyone please help?
Sounds like you need to adjust the idle on your Nikki! Have you looked at the screws around the carb? They control the A/F mix for idling and also the idle RPM's. My carb chokes down in the cold and usually runs better when the air is warm.
similar problem. Does your engine reach operating temp before the choke shuts off (should be the 2nd line on the temp gauge if its like US RX7s)? My choke stays (magnetically) engaged about 30 sec then shuts off. But my engine has not reached op temps so it idles very poorly (or stalls if I don't nurse it). I drive it off anyway (gently), and in about 3 mi of driving (warmed up, in other words) it idles fine.
This is only on cold starts (ie first of the day) . Starts and runs fine rest of the day. Does this even on warm mornings. Wondering it the choke switch, which is screwed into the rear of the water pump, is the culprit?
Check the flapper valve in the horn of the air filter housing and the hose that goes from it down
to the top of the shielding on the thermal reactor. The valve should stay open when the engine
and air is cool to supply warmer air into the carburetor. I think this is to prevent icing when the
air temps are low in higher humidity (its england right ).
I agree, I'd check the flap out. And I never immediately drive mine away when it says its warmed up, I let it sit a minute or two more just to give it time to get toasty.
I've adjusted the idle screw Ricardo. It hasn't made a difference . My choke turns off when the engine fully warmed up (when the temp needle has stopped moving). The strange thing is, if the weather is warm and in turn of the choke early and the idle drop to 500rpm it will idle smoothly. As the engine warms up the idle increases to normal. The thermal reactor has been removed from my car t_g_Farrell. It just seems like a valve switches at a higher temperature.
Have you checked what connections are left from when the reactor was removed? could be a disconnected vacuum hose or such. also, what is it trying to idle at once it's at its maximum? Because it should get up to about 800 RPM.
The thermal reactor has been removed from my car t_g_Farrell.
There you go...
You need a source of heat for your carb heat stack.
Carbs don't have a way to adjust to a richer mixture when the air is cold and more dense (besides the choke), so you actually have to heat the intake air (like stock) when the air temps are too low.
Check the flapper valve in the horn of the air filter housing and the hose that goes from it down
to the top of the shielding on the thermal reactor. The valve should stay open when the engine
and air is cool to supply warmer air into the carburetor. I think this is to prevent icing when the
air temps are low in higher humidity (its england right ).
Stu, this may be your issue as well.
Too obvious! will check
I know hose-to-reactor cover is OK, but ahhh - the flapper....
thx t_g_
I have set the idle to 800rpm Ricardio I have checked the blacked of pipes but I may have missed one. How many connections should there be?
Could that be it, Blue? It needs a source of heat... Are rotary engine sensitive to intake air temp? Coz I have not had this problem with other carbs, but they were on a piston engine. Could the cold air be affecting the vacuum valve in the air box? It just seems like a sudden change at a specific temperature.
Thank you for your help guys. I've been struggling with this problem all winter. It make it un-drivable when weather is cold.
Could that be it, Blue? It needs a source of heat... Are rotary engine sensitive to intake air temp? Coz I have not had this problem with other carbs, but they were on a piston engine. Could the cold air be affecting the vacuum valve in the air box? It just seems like a sudden change at a specific temperature.
Well, this is my gut feeling.
The heat riser thermal switch is working, but instead of a source of heat from the thermal reactor heat shielding it is now sucking cold air off the lower section of the engine compartment (closer to the cold ground), which is colder than the normal intake path of the upper engine bay compartment behind the radiator.
I would first just try disabling the heat riser thermal switch and see if it can fun off the normal top intake inlet when cold.
I have to say, starting my RX in the winter is a struggle. It seems like these rotary engines are way more sensitive to temp. You may want to try getting it way up in temp first and seeing if it still wants to come down.
I stay out of threads like this because it's all stock Nikki talk. I don't do stock Nikkis. All of my carbs receive a recalibrated idle circuit which is simply a set of short slow air bleeds drilled to 118 (1.18mm). This reduces most of the unwanted air and prevents cold start difficulties you are all experiencing. The stock size on SA carbs is 150 and most FB carbs have 170 but I've seen the entire range from 150 to 190 in 10 number increments.
The reason your stock carbs behave the way they do is due to the way the factory calibrated the idle circuit to work with a choke flap and all the other associate bulky clunky cluttery crap that allows a semi-automatic choke to work. When it's all stock, it works relatively well for the most part, but as parts age, things don't go according to plan anymore, as you all are experiencing.
My fix is not really a fix. It is a side step maneuver. It works every time but I don't know how it affects a carb that must retain emissions controls and all the bulky clutter.
I can recommend a set of air bleeds already drilled to the right size (more or less) that anyone can order from eBay or other places online. They simply swap right in and can give you some of your sanity back.
Many sizes are available in 5 number increments but you could start out at say 120 or so. Just don't go smaller because then it fattens things up too much and no amount of mixture screw adjustment will fix it for you.
This project requires you to remove your air horn (carb's top). While an easy project as far as Nikki work goes, it is not for the faint of heart because there are a lot of screws you can lose or misplace, and some of these screws will be at Thor levels of tightness so it's best to have an impact screwdriver with a flat tip (slotted) handy because you don't want to try to free them up with a phillips and I'm sure just about no one has a proper JIS phillips kicking around anyway. Once freed up, then you can use your north american spec phillips to spin them out.
Take lots of pictures and clear a whole day for this project.
Thanks Blue. I will try to sort out something.
I know what you mean about stock Nikki carbs. And those needle jets would be worth a try. However, I have never had cold, or hot, start problems. It starts first time, within 4 turns of the engine.
The idle problem still happens when I've been driving for over 20 minutes at 70mph. By that time I've hit traffic and really need the car to idle.
I was wondering. Could it be condensation in the dizzy cap? It not just been cold but also a high humidity in the mornings. Maybe that's stopping a spark from firing. I listened to it yesterday (when trying to stall) and it sounded like it was missing even when I touched the throttle. Push the throttle more and it sounds fine.
After runing for 20 minutes it should be more than warm enough to not care about the air temp.
Your guess about the spark is probably not for off. You could be experiencing the classic
symptoms of an igniter failing or a coil failing.
Well I have replace the spark plugs, dizzy cap and rotor arm. Still have the same problem. What I did find is that (i'm not sure which one) either the coasting richer solenoid or slow idle speed solenoid is not working. I disconnected it it should have shut off the engine but it continued running.
Just try plugging/taping off with aluminum flue tape the old unused heat riser that used to come off the thermal reactor heat shield and disable the flapper valve there to the carb to see if that helps.
Icing is a possibility and was initially what I thought the issue was seeing as you are in the UK
with persistent dampnes at low temps. Usually icing goes away after a few minutes when the
intake warms up.
To fix the icing problem I had to contain the heat from the exhaust manifold and stop the fan from cooling it down. So I designed this:
It protects the hot air from the fan keeping the exhaust manifold hot. The connection on top is to fit an aluminium flexible pipe pipe to the air box.
I had to wire lock the nuts to give a positive fixing.
I have my own engineering design company if anyone is interested in this part or something that needs designing let me know.
). 
