AQ170 motor overheating very quickly

[StewBeans]

Hello All,
I am currently the not-so proud owner of a 1969 Silverline rambler. I took out the original B20 motor to find that it had rotted from the inside out. I managed to find a B30 to stick in there instead. It bolted up to the existing outdrive with no problem. When I cleaned everything up and started the motor in the boat, I found that the engine was overheating quite quickly. On the muffs it would go from cold to ~200 in about 10 minutes. After reading the forums on here I went thorough all the usual suspects, oil cooler, impeller wheel, vacuum leaks before the raw water pump, changed the belt, got a new circulation pump, took the thermostat out, put the thermostat back in, tried new thermostat. When I disconnect the water pickup line and put hose into a 5 gallon bucket, the motor drains it in about 3-4 minutes, so I know it is sucking water well. I am pretty sure the carbs are tuned well, but it is pretty hard to get them really dialed in when I have to keep an eye on the temp. Timing is set to 30 degrees at 4K RPM like listed on the top of the motor. Anyone have any ideas about what to do next?
Thanks!

 

[Bondo]

When I disconnect the water pickup line and put hose into a 5 gallon bucket, the motor drains it in about 3-4 minutes, so I know it is sucking water well.

Ayuh,..... Welcome Aboard,.... Sounds like the problem is 'tween the raw water pump, 'n the water pick-up,....

Have ya back-flushed it,..??

Isn't there an aluminum fittin' where the water hose comes through the transom that corrodes badly on those ole Volvo's,..??

 

[Scott Danforth]

that 5 gallon bucket should be drained in about 20 seconds, not 3-4 minutes. you do not have enough water flow PERIOD.

you did replace the water inlet casting on the outdrive, correct? they do corrode and need to be replace ever time the impeller is replaced.

 

[StewBeans]

Hey Bondo, back flushed and back flushed again, pumped water both ways through the whole system, seems to be coming out clean. I have heard about the problems with this fitting, but have not yet replaced it. It is on the to do list but was trying to tick off any problems upstream first.

Scott: 20 seconds?! definitely not getting the flow I need. I did not replace the casting on the out drive yet, it was on my list of things to do after I solved the overheat with the cooling system using the bucket problem... 20 seconds is fast, I cannot seem to find any vacuum leaks and my impeller looks good, impeller wheel is new. I have not done the shaving cream trick yet though. Any thoughts on what could slow the flow down this much?

 

[Scott Danforth]

Your raw water pump pushes about 19 rpm at about 20 psi. Should be closer to 16 seconds, however you get the point.

Change the inlet casting

 

[StewBeans]

sounds good, thanks for the advice scott, i'll change it out, take some pictures and let you know how it goes.

 

[StewBeans]

hey scott and bondo, i got the flow rate problem solved by removing the strainer, the rubber on it was corroded and cracked, then i put a 1" hose on there and stuck that hose in a trashcan full of water. it now is draining at about 18 gallons a minute, but it is still overheating pretty quickly ~5 minutes from cold to over 175. i am running it without a thermostat right now, was hoping i could get some over cooling to happen. i did redo the gasket on the outdrive casting you mentioned, but as i am picking up from a bucket, nothing is running through it. Any ideas of what to try next?