OutdoorKing Repair Forum Forums Repairs and Maintenance Questions on Engines Questions On Briggs & Stratton Engines Briggs No Prime No Choke 675 Series

Hi Guys,
I recently picked up a 675 Series Briggs No Prime No Choke Mower which is in near brand new condition. It was very cheap on our local classifieds site so I really couldn�t refuse and see what was going on with it.
I bought it home and it didn�t start after 3 pulls. On the fourth pull it finally started.
I decided to strip the engine and give it a good service. I put the engine back together after giving the carb a good clean and giving it a new air filter and it now starts first pull and runs very nicely. I am little concerned it is running a little quick but before I pass judgement I want to give it a run out of the shed to see how it sounds.
I would like to understand a little more about how the No Prime No Choke system works. It doesn�t seem that complicated but I wanted to be sure about the mechanics of the system.
Any pointers I need to take into consideration compared to the standard briggs systems would be greatly appreciated.
Note: the plug was quite gummed up when I picked the mower up which made me think it might have been running with the choke partially closed. I could be wrong though.
The mower doesn�t blow any smoke.
Cheers
Jaffa

Hi Pete, i've only seen a couple of these myself but the setup is pretty straight forward. When it's in cold start mode the system self chokes and once the engine fires up the air pushes the air vane and disengages the choke. When it's in "hot start" mode there's a thermal coiled sprind insibe the inlet end of the exhaust that once heated pushes the lever on the exhaust side of the engine and disengages the choke thus starting without choke.
Did you get all that....good, now explain it back to me..lol
As for full throttle RPM the tab holding the spring does look like it's pulled forward a little too far and would probably be over-revving. You're aiming at about 2800rpm with these motors.

Could you post the model, type, code numbers please Pete, i'd like to see the IPL for this motor.

Hi Theo, thanks for the explanation. Makes sense.
The codes are as follows:
DOM Dec 2011 126T02 0480 H1 - 11120159
I gave it a good test today and adjusted the revs and its running very nicely. It just needed a slight adjustment like you mentioned.

When thinking about this system today and it seems pretty smart.
1) You avoid people over priming and flooding the engine - common problem on junkyard mowers
2) When the system is functioning properly you can�t run the engine with the choke on, like lots of people do with Chonda�s . So many just need a good spark plug clean and they fire back to life.
To me this system seems fairly fool proof however I suppose someone would find a way to stuff the thing up.
It surprises me that other engines haven�t adapted this principle. Well they may have just not from what I have seen. More than happy to be told otherwise if I have missed something.
No doubt Grumpy will chime in with his thoughts on the design.
In my opinion I like it.

That sounds a lot like the 'electric choke' system that was used on a lot of US 4-barrel car carbies.

There was a bimetallic spring, as you describe, in a sealed Bakelite chamber that also held a 12V heating element, wired to operate when the ignition was switched on. So as the chamber heated up after the engine started, the bimetallic spring would pull the choke off. As the chamber cooled down slowly, the choke would only re-engage when the engine had cooled down too. The main failure mode of these units was the open-wire heating element would fail. The Briggs setup would avoid that problem.

It was traditional (for a couple of decades anyway) for most cars (including all Australian-made ones) to have automatic chokes that just used exhaust heat rather than electric heating. Occasionally on old cars the heat pipe orifice at the carburetor end would clog up and the choke would stay on until someone cleaned it out. I seem to recall a few used electric heating instead, but AFAIK the reliability was worse, not better, in the long term. The main problem the auto chokes had was that some tenant would rotate the movable part of the housing slightly, which adjusted the choke setting. The setting was critical to a fraction of a notch on the outside of the housing, so usually home-adjusted ones worked badly thereafter. It was very rare for there to be any legitimate reason to make an adjustment.

Achieving reliability is mainly a matter of controlling the maximum temperature of the bimetallic coil. If it is exposed to too high a temperature, it is ruined quite quickly. Hence the tube with a small aperture at the choke end, used on the car auto chokes. The housing around the coil was also a heavy zinc casting, to provide the necessary thermal inertia so the choke would not be applied until the engine had cooled completely (which takes a very long time for a water-cooled car engine).

Of course for the past thirty years very few cars have had carburetors, and mixture enrichment for cold starting is controlled by an electronic temperature sensor.

Thanks for your assistance on this one guys. I was curious on the reliability of the thermal coiled sprind insibe the inlet end of the exhaust? Is this something that could easily fail moving foward?

The only problem I've found the these new no choke/ no prime is that if you start them,run for a minute or two, stop the engine then attempt a restart .....they are a bastard to get started as the choke is not engaged but the motor is cool enough to need the enriched mixture. Start them, get them hot then no problem

Deiselboy thats a good point you mention. I guess I was more interestered in the longevity of this setup and its reliability moving forward. I guess only time will tell regarding the systems ongoing reliability unless of course others have opinions of this new concept.

Personally I found the engine easy to start and smooth running once it was fully serviced. Other's will no doubt have different opinions.

I imagine Briggs have done their usual homework and come up with a design which will keep the average punter mowing suburban block for 8-10 years.... Lots of plastic on these new models, little disappointing but you have to understand intended market for these units.....

Getting an automatic choke thermal sensor to heat and cool at just the right rate is difficult. The ones used on cars chose rather expensive solutions, but achieved good results. The Briggs one should be inexpensive to make, but offers the development engineer very few control parameters to get it working well. If the sensor makes too good a contact with the exhaust port it may heat too slowly, thus keeping the mixture rich too long after a cold start. If it makes too poor a contact with the port it may heat too quickly, so it will not re-apply the choke if the engine is cold-started then immediately stopped and restarted. Perhaps Briggs concluded that cold restarts are hardly ever needed in normal use, or perhaps the extent of the contact with the port is difficult to keep consistent when making thousands of engines.

Thanks Grumpy that makes sense. If I come across another one of this type of setup I will be sure to take your notes into consideration.

Appreciate the assistance as always.