Topic: Power Loss Trouble Shooting

[turkey4me]

I've just started having some strange power loss issues and was hoping for some help trying to trouble shoot.  76 CB750F with 836 top end rebuild a few years ago.  About 15-20 minutes into a ride at highway speeds I have a sudden loss of power that to me feels like two cylinders just stop firing.  Like running out of gas but I can keep going at 30mph and limp back home several miles.  First time it happened I continued riding and after about a mile full power instantly came back.  Week later happened again, I pulled over waited 15 minutes and the bike ran fine all the way home.  This has happened all of the last 3 rides. This past weekend I lost power again 15 minutes from home and never got it back.  Limped home at 30 mph on what felt like two cylinders.  For about 2 seconds I had full power then lost it for good.  Here is what I have checked.

Fresh Fuel in tank and in all four carb bowls. 
Air filter clean
Spark across all 4 plugs.  Visually all 4 plugs look rich.
Valve clearance good.
At idle there is some oil splashing out when tappet cover is removed from the cam cover.  Front Yes, Rear tappets a lot less.
Timing and Advance good.
Oil PSI 70 psi when I start riding then it drops to 55-60PSI after warm

I was thinking maybe the bike could be overheating so on the last two rides I put a probe in the oil tank.  About the time the power loss occurs the oil temp is 230 F.  Maybe it is overheating but this doesn't seem excessively hot?  Or is it? 

Let me know your thoughts, questions and where I should look next.





 

[scottly]

Plugged vent in the gas tank cap?

[bryanj]

Posible intermittent charging fault giving lower battery voltage.
Cause would be iffy bullet connectors or possible failing regulator

[rotortiller]

Dirty fuel filter?

[calj737]

Can you confirm whether you are points or electronic ignition? And if electronic, which?

[turkey4me]

Some good ideas I haven't considered.  I'm using a Dyna S ignition.  As for fuel.  High flow petcock with a internal fuel screen and no inline filter.  Also aftermarket tank and fuel cap.  I'll  check for vacuum lock and fuel blockage again.

Is is possible to check the ignition and regulator with a voltage meter while riding or when the power loss occurs?

[newday777]

A volt meter on the bike would indicate loss of volts.
How old is your battery? Is it an AGM? They have been known to get internal shorts and sudden loss of power, a voltmeter will show that. A Dyna S needs full volts to run.
How old is the Dyna S?

[calj737]

Some good ideas I haven't considered.  I'm using a Dyna S ignition.  As for fuel.  High flow petcock with a internal fuel screen and no inline filter.  Also aftermarket tank and fuel cap.  I'll  check for vacuum lock and fuel blockage again.

Is is possible to check the ignition and regulator with a voltage meter while riding or when the power loss occurs?

There have been several people who experienced an overheat of the ignition using the Dyna. A quick test would be to remove the points cover and ride. If it does not experience the issue, then the likely culprit is the ignition.

[scottly]

Is is possible to check the ignition and regulator with a voltage meter while riding or when the power loss occurs?

Try blowing the horn: if it beeps, the charging system isn't the problem.

[HondaMan]

Don't overlook the little vent hoses from the carbs: the 2 inside carbs have a small elbow on their upper half that should have 2 long hoses connected to them that run up, over, and behind the airbox. The carbs' float bowl levels need these to have still-quiet air feeding them or else they bowls fill quite unevenly, even worse in cross-winds.

[turkey4me]

Appreciate all the ideas now I have something to do this weekend:)  The battery is not AGM and 2 years old.  I'll put a volt meter on the battery tender lead and see if she is generating voltage at highway speeds.  Dyna unit is about 6 years old.  Great idea to remove the points cover and see if it helps.  I might throw an extra spark plug in my pocket to use for testing each cylinder plug next time the issue occurs.  Let you know what I find.

[turkey4me]

I finally got out for a test ride with the points cover removed to expose the Dyna S ignition to some cool wind.  Problem with the power loss is gone so it must be an issue with overheating the Dyna unit.  FWIW I am running an aftermarket frankenbike points cover.  Going back to traditional points if I can find one that isn't aftermarket.

Thanks for helping put a smile back on my face!

[turkey4me]

Im going back to points and my spark advancer is missing the center bushing the points ride on.  Does anyone have a spare?

[bryanj]

It only comes with the advancer

[HondaMan]

You'll need a new spark advancer: the little pin that locates the points cam on the inner post was removed when the Dyna magnet was installed. The points cam needs that little post to hold it in proper location.

You should be able to pick up an advancer on eBay. You'll need one from either the 1069-1975 versions of the "K" bike or one from the F0-F1 like yours. These usually have a "300" or "AS125" or "TEC" stamped on the baseplate.

The advancers sold aftermarket all have incorrect springs. I've sometimes bought one of those, then used the old springs on the new advancer. The new springs are much too strong on the [several] aftermarket advancers I've bought and used. That said, the OEM springs you have now are heat-softened from years of use, and you should clip off at least 1/2 turn from both springs when installing them on the new advancer to return the advance rate to something a bit slower (more like when new) and more compatible with today's gasolines. All gas today burns far slower than it did in the 1970s!

[turkey4me]

Bummer!  Normally I save parts I remove.  Ill go shopping on ebay. Thanks Mark and Bryanji for the knowledge. 
Could the springs be heated to non magnetic, quenched in oil then tempered at 550 F to "restore" them to original condition?

[turkey4me]

Good info here

http://forums.sohc4.net/index.php/topic,113338.0.html

[bluezboy]

Going back to points is a smart move, I had a Dyna on my bike and it was nothing but expensive towing bills! Points rock man!😀

[turkey4me]

Found a 300 TEC advancer on ebay and today I went back to new OEM TEC points from Yamiya. Set the gap and static timing.  Went for a quick ride and Wow! Honestly felt like a 10% HP gain over the Dyna unit.  I see snow on hill tops.  Probably my last ride til March

[madmtnmotors]

You'll need a new spark advancer: the little pin that locates the points cam on the inner post was removed when the Dyna magnet was installed. The points cam needs that little post to hold it in proper location.

The only "pin" that I'm aware of is on the back of the advancer plate that engages the crankshaft. AFAIK the points cam is held in place by the little ears on the swing weights. The Dyna trigger magnet only requires sliding off the points cam and sliding the Dyna trigger in its place also engaging the little ears on the swing weights. Both the Dyna trigger or the original points cam are then further held in place by the outer slotted nut and the little 6mm nut that holds the slotted nut and advancer assembly on the advancer shaft. I'm sure there are some timing differences between early and late points cams but I believe just about any points cam will fit just about any advancer unit, no? 

[scottly]

You'll need a new spark advancer: the little pin that locates the points cam on the inner post was removed when the Dyna magnet was installed. The points cam needs that little post to hold it in proper location.

The only "pin" that I'm aware of is on the back of the advancer plate that engages the crankshaft. AFAIK the points cam is held in place by the little ears on the swing weights. The Dyna trigger magnet only requires sliding off the points cam and sliding the Dyna trigger in its place also engaging the little ears on the swing weights. Both the Dyna trigger or the original points cam are then further held in place by the outer slotted nut and the little 6mm nut that holds the slotted nut and advancer assembly on the advancer shaft. I'm sure there are some timing differences between early and late points cams but I believe just about any points cam will fit just about any advancer unit, no? 

IIRC, the Hitachi advance had the pin on the inner shaft, and a groove on the inside of the points cam to retain the cam. The pin had to be removed or pressed flush to fit an electronic ignition, but wasn't necessary once installed, just like the cams with the flange that fit under the weights. 

[Deltarider]

Going back to points is a smart move, I had a Dyna on my bike and it was nothing but expensive towing bills! Points rock man!😀

Yeah, points are for men, nothing beats struggling with them! EI's are for sissies.