JGM BMC 1.5 questions

[SimonA]

I have a BMC 1.5 engine marinised by JGM and with a Bowman heat exchanger and a calorifier.

I can't find the thermostat on the engine which I am told should be somewhere near the top hose. I can't see anything that looks like a removable cover under which the thermostat might be. Any ideas?

Also how high should this engine rev? Mine won't go above 2000 RPM which gives about 6 to 7 knots top speed (27 foot boat, about 2.5 tons). The idle speed is about 1100 RPM which also seems a little high?

 

[dickh]

Hi, I also have the BMC 1.5. The thermostat is under the dome shaped housing which has the 'top' hose going to the heatexchanger(just checked it this weekend!) should be an 80ºC. Secured with 3 nuts, one immediately under the top hose.
Idle speed from memory should be about 6oo RPM(but I will check tonight in the handbook).
Regarding top revs, you sound as though you are going fast enough at 2000 RPM doing 6-7 knots on a 27' boat, assuming displacement vessel. The engine should certainly rev higher, probably up to 3000 RPM. What ratio gearbox do you have? Mine has a 1:1 gearbox and I cannot go above 2500 without prop cavitation apparently(my tacho's not working so not sure of exact revs).
If you get black smoke if you try going above 2000 RPM, you are overpropped or overloading the engine in some way.
Mine is difficult to start, see my earlier post on PBO forum.

 

[Piddy]

Hi. I have the 35 Hp version that is (If I remember correctly) the 1.5. With a 2:1 Hurth gearbox I can reach 2950 after a scrub and about 2850 at the end of the season. Oil and water consumption is negligable and without wanting to bring bad luck on myself seems to run like a peach.
The only mods I have done is to add a dedicated starting battery with nice short leads so that it spins like an egg whisk when starting. I never have to use any pre starting aids such as heat, although after not running for a few weeks, I hold the stop button out 'till the oil pressure comes up before letting it start.

 

[squidge]

"I hold the stop button out 'till the oil pressure comes up before letting it start."
Good idear. Mine usually takes a few turns to start after a week or so but it usefull to remember that i can crank it over without it firing up.

 

[squidge]

Simon , sounds like your power is fine if she pushes along as 6-7 kn.
When you look at reducing the tickover check the throttle cable mounting on the engine. The bracket on mine had been bent shortening the cable so that no amount of adjustment would reduce revs.Once the bracket was bent back end of problem. There is an adjustment screw on the high pressure pump which stops the throttle cable from returning , this will reduce your tickover (bracket aside).

 

[nedmin]

Used to have this engine the thermostat is under the top hose casting that is fastened to the cylinder head by 2 bolts.remove the casting and you will see the thermostat,you should be able to pull it out,or carefully prise.Dont know how high they rev.6/7kts is about right for your hull length.The engine is based on the B series that used to power the Morris Oxford,lovely quiet & reliable .

 

[Joe]

Hi
If the tickover does not reduce via the low speed idle nut and bolt adjacent to the high speed one (Which should be sealed via a lead squashy thing and a bit of wire or shroud to stop over revving. This is a factory set setting and should not be touched! ) Then it may be that the injection pump may need re-conditioning.
A raised tickover is sometimes a telling factor to this problem.
Good idea to do the injectors at the same time.
Joe

 

[Micky]

Always like to get interested in these topics concerning the 1.5 BMC engines, as i have a 35HP in my boat.

However, mine will run at 4000 RPM, but it performs best at 3000 RPM.

Idlel speed is 800 RPM.

No need to interfere with the seals on the main pump, these are factory set.
The tickover screw, to raise or lower the RPM is via an 8mm nut/bolt, found on the main fuel pump near to where the throttle cable is fitted. Slacken off the lock nut, adjust and retighten.

Good solid working engines.

 

[oldharry]

You are unlikely to get more than 7 knots tops out of your boat under power anyway, so I wouldnt worry. 2000rpm is about as fast as you want to rotate your prop (assuming you have a 1:1 ratio gearbox) as slip and cavitation at the prop will be absorbing more power than you can put in. 2200rpm is reckoned to be the highest viable speed for a displacement hull. From memory the BMC is developing around 20 hp at 2k rpm, and this will push a 22- 24 foot waterline boat at its maximum speed anyway. Any more power will simply make bigger waves, and add very little speed.

Tick over should be 800rpm adjustable as others described.