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BN6 runs rough on restart after running great

bighealeysource

Jedi Knight
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Hey all,
This is probably a carb issue but thought I would ask -
Healey ran beautifully for 90 minutes, stopped for about
30, then when restarted, ran rough for about 5 minutes until
fully heated back up. This was on a very hot day and would
not think it would even have to be choked on restart but I
did to try to smooth out the running. This only seems to
happen after a short time stopped, not if it has fully cooled
down. Timing has been checked and believe it is spot on.
Know it is running a bit rich but since running so well otherwise, have left carbs alone. Any ideas ?
Thanks,
Mike
 

red57

Jedi Warrior
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Mike, Fuel Percolation, aka vapor lock. When it's running you are using fuel and getting cool fuel from the tank (and have lots of air movement in the engine bay). When you shut it off there is no air movement to speak of and no cool fuel coming in from the tank so the fuel in the float bowls gets too hot and is trying to vaporize. Carburetors cannot make the correct mixture if the fuel is vaporized so it runs roughly until the air movement cools the carbs a bit and cool fuel reaches them from the tank. Once the carbs cool down and cool fuel reaches them, it will run great again.

One solution is to wrap insulating material around the float bowls and fuel lines in the engine bay - have not tried it but the logic is there.

Datsun 240z used a recirculating system with a return line to the tank to insure cool fuel at the carbs at all times - haven't tried that one either, but again the logic is there.

I have found there is a 'window of time' where it is most pronounced. I try to make either short stops (like getting gas at the station) or long enough stops for the carbs to cool down. BTW, lifting the hood will help some because of letting the hot air out and getting a breeze over the engine.

One thing I have found when it happens - light acceleration seems to run better than hard acceleration - don't know why but if I try to give full throttle, it runs rougher but if I give light throttle it seems to be less pronounced.

Bottom line, be patient - it will go away soon enough.

Maybe someone else has a solution to suggest?

DAve
 
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bighealeysource

bighealeysource

Jedi Knight
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Hey Dave,
Thanks so much for your description as it fits my situation exactly.
Short stops, no problem, 20 - 30+ minutes, stumbles upon starting.
I have read about percolation problems but didn't put 2 and 2 together
but makes perfect sense to me. When I stop after a long run, the engine heat
will spike up over 230 degrees so it is one hot engine compartment in my Healey.
I would leave my electric pusher fan on but then that is a problem with the Pertronix
igniter staying on if the engine is not running. I'll see what some other BCF's have to say too.
Many thanks,
Mike
 
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When I was autocrossing my gray car, I rigged up a relay timer to run the electric fan for a predetermined number of minutes (dip-switch settable in increments of 8, 16, 32, etc. minutes).

I put the momentary push-button under the hood, as I raised it after each run.

Quality Kits QK152: https://store.qkits.com/moreinfo.cfm/QK152

Ron Stygar's pictures of the kit are in this thread: https://www.bimmerfest.com/forums/showthread.php?s=&threadid=12901

You'd still need a power relay to handle the fan motor's current draw, but this timer can easily be rigged to pull in the fan relay when you turn off the ignition, and then run for some pre-determined time. you could use an under dash toggle switch to disable the timer in cooler weather.

BTW, the timer kit was very easy to assemble; you only have to know how to solder__not be an electrical engineer.

If you look through all the kits that they offer, I'm sure you could dream up a multitude of projects! Maybe use an interval timer instead...?
 

glemon

Yoda
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I saw a healey 100 at the all british show in Kansas City a few years ago that had a small fan, like the type used in cooling computers, pointed at the carb float bowls....
I assume as a bandaid for this type of problem, not advocating it, just saying.........
 

Johnny

Darth Vader
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I have a clear fuel filter mounted inline on the hose feeding both carbs. Having a similar problem I opened the hood and noticed the gas in the clear filter actualy "percolating" from all the heat. The hose was routed from below where it exits the frame up over the manifold and into the carbs. I re-routed the hose to lead back away from the manifold closer to the LH fender then back to the carbs, no problems since.
 
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bighealeysource

bighealeysource

Jedi Knight
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Thanks y'all for the recommendations and insight on what the problem might be.
I'm going to use a combo of Randy's and Johnny's ideas to try to solve the problem.
Regards,
Mike
 
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My BJ8 developed the same problem roughly coincident with the widespread addition of ethanol to gasoline. Can't prove a connection, but the timing is suspicious.

At the next rebuild, I'll get the exhaust manifold ceramic coated--figure it couldn't hurt.
 
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