rebuilt the 351 following the oil problems in my other post. Have a few more questions:

1) while tweaking the carb, the temp light came on and fans kicked on. Noticed I can hear and feel gurgling/agitation in the coolant hose near the thermostat. Making me wonder if coolant isn’t circulating. Maybe either bad thermo or improper thermo install, or maybe the water pump (I had to take the right side cover off last week so removed the impeller in the process)?

2) the throttle isn’t springing back rapidly like it should. Closes slowly. I lubed the throttle tube area where it meets the housing. No help. I’ve moved the adjustments at both ends trying to get the free play right - maybe I don’t have it adjusted right? I don’t fully understand the interplay between the adjustments on either end of the cable

3) is there a method to dialing the main screw and idle screw (this is on bills mikuni)? I can’t get the idle steady, and for that matter it’s really hard to start. I didn’t have this problem last go round. Bill had me increase the main jet one size as he thought I was too lean.

 

Sounds right off the bat lean now. If you had the incorrect rings to start off with, oil would be combusting also throwing off your jetting. <br /><br />The fans lights shouldn't come on quickly upon idling imo. Stock the bikes fan/lights would turn on after idling for a bit n the header glowed red. I bumped up the pilot one size and starting &amp; that hot from running heat are improved. <br /><br />I always run run premium or race fuel for break in to keep it cooler.<br /><br />hope you have great success with your new cyl/piston upgrade. I have had the same sluggish throttle return symptoms, try to relax your throttle cables routing as it was before. I have the keihin fcr35 so I can't say but raising the needle/increasing the pilot will help leanness best of luck to you

 

Is there a way to check the thermostat? I’ve never before been able to hear the coolant in the pipe like that. I don’t think it’s getting thru the thermo. Or either the pump isn’t working.

 

May not have had enough coolant - it drew some from the reservoir and I topped it off. We’ll see.

I do see a bend bend in the throttle cable where it comes out the sleeve and to the carb bracket - may be enough to add friction will try tomorrow.

 

Yeah, Sounds like running lean, therefore hot. The gurgling could be coolant boiling, or air in the system. Did you keep checking the coolant level after running a few times? Sometimes you can have air pockets trapped in the system. With bike cool, and radiator cap off, try squeezing all the hoses in different spots and see if coolant overflows or goes lower. Sometimes you can find and work trapped air out this way. Just a few things you can try, you may get lucky, can’t hurt. If the light is going on and fan runs, most likely thats all working properly. As far as maybe thermo switching on to soon, maybe try an Infrared thermometer/temp gun. You can see if the water temp is hot enough to kick the fan on. I would think your engine is running hot to quick due to lean jetting or you have air in coolant system. Temp gun will be the quickest way to find out. Only way to test the thermo switch is remove it and do the torch bench test. That will only tell you if it works or not. I would bet its air in your coolant system. Check level at radiator cap, not reservoir. Once the level stays full to top at rad cap, then fill level in reservoir.

 

On this mikuni which direction of the screw leans vs enriches?

im a little baffled - with the smaller jet I had before and the screw three turns out it would start instantly with no choke. Now I have to choke and coax.

Wondering if air air is getting in somewhere - like around the boot. Need to check the accel pump as well

 

U need to 'tune' the fuel screw, don't worry which way is lean or rich. If you go over 4 turns out you need a bigger jet on that circuit.

 

[QUOTE=sphen;538909]rebuilt the 351 following the oil problems in my other post. Have a few more questions:

1) while tweaking the carb, the temp light came on and fans kicked on. Noticed I can hear and feel gurgling/agitation in the coolant hose near the thermostat. Making me wonder if coolant isn’t circulating. Maybe either bad thermo or improper thermo install, or maybe the water pump (I had to take the right side cover off last week so removed the impeller in the process)?




Make sure you have the two coolant hoses routed correctly. They can be very easily reversed and still look correct.

 

Turn out (counterclockwise) to richen, turn in to lean out.

Idle mix screw on the head end is usually adding fuel as it is turned out (counterclockwise), most four strokes have this type. Idle mix screw at the air box end is usually air mix, turned out is adding air, most two strokes have this type.

I will say the old VM Mikunis were designed for two strokes but sometimes used on four strokes, like the SR500, XS650, and Brit bikes when changing out carbs.

 

The thermostat should be closed when cold; throw it in a small pot and heat, or heat a little water in microwave to boiling and add the stat. If it does not open in 212F water it is stuck closed. It should start to open at whatever the stat's set point is; around 160F in my copy of the service manual. I have a 90C stat from my thermobob kit and that does not get my fan running. I did reverse the coolant hoses at my water pump and rode the first 20 mile break-in procedure that way and did not kill the bike. I switched the hoses and the power increased as miles accrued. Right now I have a couple thousand miles on my BB351 getting 62mpg, good power, and no oil use that I notice,
Addressing the fuel:air mixture there is a general misunderstanding that leaner is hotter. It can be if your ratio is on the rich side of stoichiometric. If you are already leaner than stoichiometric the adiabatic flame temperature will decrease with even leaner mixtures. Stoichiometric mix gives the maximum adiabatic flame temperature and adding either extra fuel or extra air will result in lower flame temperature due to the extra (over stoichiometric) reactants absorbing some of the heat of combustion cooling it down. Slight variation in real engines is likely as nearly perfect lab test conditions give slightly different results than found in practice but are pretty close.