Board index Flathead Power-Technical Questions, Answers, and Suggestions 45 Flatties compression


Post Mon Nov 18, 2013 7:24 am

Posts: 35
does anybody know what the compression PSI should be in a healty, stock 45?

Post Wed Feb 19, 2014 8:39 am

Posts: 35
took the trike out yesterday. took the wife for a drink to end our week out of town. it ran fine, but there is something wrong. hopefully somebody will "hear" my description and can maybe shed a clue on my problem?

it ran just fine. had been sitting for at least 2 weeks, but started rite up like it wanted to go somewhere.
we got rolling. then i began to notice a foreign sound in the mix. something like hitting a wood block. it seemed to be in time with the power pulse(Combustion, if u will). it slowly got more pronounced as the ride progressed. it also had a tendency to stall more easily leaving a stop.
on the way home it was the same sound. after a bit, the "wood block" backed off and things sounded like normal. by the time we got home, it was coming back into the mix again.

i am wondering if it could still be a timing issue, but i'm afraid of it being a rod knock. i'm not familiar with what a rod knock sounds like in a 45.

any ideas?

Post Thu Feb 20, 2014 9:26 am

Posts: 309
Location: Ohio
Just a shot in the dark, but a sticking valve could cause the problem you’re describing. How do the plugs look? I can’t tell you what the compression should be but anything above 45/50 I would consider ok as long as they are both within 10% of each other. Pull both plugs and put you’re thumb over one of the plug holes and give it a couple kicks. That should give you a definite pop when it blows you’re finger away.

Post Sat Feb 22, 2014 9:24 pm

Posts: 35
last time i checked i had a pretty well balanced 60 PSI in each cylinder. i do have electric start so it's pretty easy to run a test. if it could be a sticky valve i will suspect the #2 exhaust. it seems to have enuff pressure to blow oil past the valve cover seal. . .so?

Post Sat Mar 01, 2014 9:23 am

Posts: 309
Location: Ohio
I just reread your original post and I just noticed that you had a passenger when you heard the sound. Does it make that noise if you ride it solo? If it doesn’t than very likely the sound you heard was just the engine “lugging” which can sound like something hitting a piece of wood. These 45’s actually enjoy being ridden at high rpm’s. You may not have any problems at all. Just keep those rpm’s up.

Post Sat Mar 01, 2014 7:52 pm

Posts: 767
Location: Pa. , USA
Your exhaust valves are #1 and 4 and they will normally stick before the intakes, as Ohio rider said check the spark plugs first, then I would check the compression while hot, may have a leaky head gasket, don't think a loose rod will cause it to stall on pull out but a fouling plug will, is your breather timed correctly ? Also check primary chain tension .. IMO , Tim
Vintage roadracing, Class C, AHRMA # 335

Post Sat Mar 01, 2014 11:01 pm

Posts: 1654
For historic reasons, people tend to think that side-valve engines possess unlimited low-end torque, but this isn't the case. When ohv engines were introduced, they tended to be a bit more expensive, and a bit faster because of the more advanced breathing, but more fragile because of the then-new metallurgy. Hence the fallacy of the "slogging sidevalve", born of natural conservatism, lack of development and economy ( although the higher fuel consumption and valve/cylinder wear of the sidevalve rapidly devour any savings in initial cost, several times over )

They are also resistant to harm, in that a stuck valve in an ohv engine is a disaster but a stuck valve in an sv engine isn't, and they are generally so robust and under-tuned that no real harm comes to them, whatever you do.

45s need to be worked quite hard to get anything out of them, and fast side-valves are almost like two-strokes. That's why the 4-speed conversion for the 45 is so successful, by enabling yoiu to keep the revs higher and more constant.
Shoot, a man could have a good weekend in Dallas with all that stuff...

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