Lack of power

[Swede]

Hi there. I have a problem. My initial job was to replace the water hoses. In order to do this I removed the carburetors. While they were off, I removed the float bowls and cleaned them out and checked the vacuum diaphragms. Then found I'd lost all electrics to the dash and traced it to the wiring connectors by the steering head, cut them all out and fitted new terminals, also checked and silicone greased any that were exposed. Refitted all up and found lack of power. No misfire, starts and idles as normal, just lacks power and torque. On a hill that it would accelerate up, in top, now I have to change down. I've checked the carbs, air filter, replaced fuel filter, checked the tank can breathe, (getting desperate) I checked valve timing and ignition timing and advance. I had a motorbike mechanic have a run on it and look at the carbs. I've tried it on an independent fuel source, put fuel stabiliser in it and filled it up with new fuel on top of a half tank. The fault has not changed. I'm a car mechanic of over 50 years, an ex bike racer and mechanic and I'm baffled. Any ideas... Please?..... Keith.

 

[Uncle Phil]

First thing I would look for a vacuum leak somewhere.
Then I would pull the plugs and see if they are the right colour.
Next would be the checking the coil output and making sure the spark plug leads are good and connected.
Next thing I would do is remove the vacuum fuel cutoff and see if that makes a difference (easy to do and undo).
Next I would check the output of the fuel pump to see what it looks like.
Next I would probably sync the carbs just to eliminate that issue.
Hope that gives you some ideas.

 

[Upt' North]

Check the plug colour as per UP.
Are the brakes dragging badly? Check the disc temp after a short ride.
Good luck.
Upt'North.

 

[Swede]

Ah, you got me thinking there! More than once I've been cought out by thinking a fault was weak fuel when it was weak spark, very similar symptoms. The plug color is normal to Rich and then I remembered, on test, giving it choke or partially blocking the intake didn't help at all so sparks? I found, on cranking, the right side had a weak, stringy spark and the left had None. Coils test out as good, tested primary feed to coils good, ht leads good. I'm getting worried now, getting towards ecu! Next, when I get to get my led cercuit tester from the m/c workshop, I'll test to see if there's any pulses coming from the ecu to the coils, also, swap the coils over and see if the missing spark changes sides on the engine. If it does it could still be coils. Last will be check related cable cercuits to the ecu then, if nothing comes to light, send the ecu off for testing.. Anything I might have missed out that anyone can think of would be appreciated. The suggestions this morning certainly fired up the little grey cells and many thanks for that.

 

[Swede]

Sorry for the spelling mistakes, should have been 'caught' not cought. It's getting late!

 

[SMSW]

You sound like you have a very good handle on things. I admit I am surprised at your, "I'm getting worried". It's just a mechanical beast, and you have to keep plugging away until you find the problem. Then, it will all seem so easy! Keep up the good work!

 

[dwalby]

Regarding Uncle Phil's question about vacuum leaks, when you had the carbs off you replaced all 4 carb boots before re-installing them, right? If not, shoot some carb cleaner on them and see if the idle goes up, you may have a vacuum leak in one or more boots if you re-used the old ones.

 

[Swede]

You sound like you have a very good handle on things. I admit I am surprised at your, "I'm getting worried". It's just a mechanical beast, and you have to keep plugging away until you find the problem. Then, it will all seem so easy! Keep up the good work!

 

[Swede]

My worry is of the cost, honda parts are so expensive!

 

[Swede]

Oh yes! They sure are.

One of the ways you can tell if the carb clamps are leaking vacuum. Is to hook up your vacuum guages.
And apply a firm pressure to each corner of the plenum. If your vacuum increases when applying pressure to one of the corners, then you may have a vacuum leak.

When I replaced my rubber boots, I went from 32mpg to about 47mpg immediately!
I found carb 1 had a big leak.

I find that the wd40 test is also a good way to test. Squirt WD40 on the boots one by one with the engine running and listen for an RPM increase.

I've found the fault on the missing and weak spark. It was the three contact, white connector on the LH frame rail just level with the front of the dummy tank. The wires are blue/yellow--yellow /blue - - black /white. The w/y are from the ecu to the coils, the blk is the common feed wire to the coils. I've cut it out and fitted individual connectors and have good, healthy sparks on all cyls. Let's hope it cures the original problem;when I've refitted everything I took off to enable me to find it! I

 

[Swede]

It's running and revs up to 8000rpm in about 1sec instead of 6000rpm in about 3sec. Horray, fault found and cured (I'm fairly confident) I've got to put all the panels back on now and road test. Got to come off it now though, till the weekend probably.