backfiring and reduced power - 75 Mark III

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Sep 26, 2007
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A few weeks ago, I had a valve job done at Rabers in San Jose. the initial problem was a bad seal on left side intake valve, causing lots of smoking. When the head was examined, decided to do valves, guides, springs, etc - to bring everything up to spec. (bike has 15,000 miles, and other than the smoking problem, was running perfectly) (Boyer, single Mikuni 34 mm)

I did a few short rides (20 to 80 miles) at light throttle after putting the head back on. No smoking, and running smoothly - I was in heaven.

But last Thurs, 25 miles after the valve job, about 12 miles into a ride, the bike started to backfire and spit. Then there was a drastic reduction in power. Limped home and checked everything (valve clearances OK, plugs nice and clean, plug wires new, fuel flowing fine).

Started the bike next morning - did a short 3-4 mile run - all normal!

Went out on Sat, and after a few miles, got the missing/spitting/ reduced power thing again.

So it's happening when the engine gets warmed up, and seems ok before that.

My guess is that a guide is loose or a valve is sticking from the rebuild, and it only manifests itself when the head gets hot enough. But there's no noise coming from the head....

Anybody got a suggestion on what i can test? Will talk to Rabers about it tomorrow, after they re-open from the weekend. But if i ship them the head, will they be able to reproduce or verify the issue with just the head itself?

Anybody got an idea here?

Keith Kelly, Encinitas, CA
Is the bike backfiring violently ? To be honest my first suspicion would be over the ignition.

In order to remove / replace the head, you will obviously have had the coil cluster off and possibly disturbed the main harness as well.

Do you have the standard Mk111 harness with all the multi connectors ? A dry contact here or an internally failing cable either in the feed to the ignition switch or the coils will give the symptoms you describe - absence of spark and timing all over the place.

The intermittent factor can be caused by any number of things - vibration, temperature, day of the week, what you had for breakfast, anything

A sticking valve will cause a misfire but not usually one that comes and goes in my experience.
I agree with 79 X 100. Start looking at the ignition wiring. Make sure that connections did not get pinched when the tank was re-fitted. Also check the trigger wires carefully, both at the box and at the trigger.

The symptoms you describe sound exactly like the problems I had with the Boyer electronic ignition on my '69 S-Type.

So, I bet your problem is ignition (Boyer or related wiring) and not valve or valve guide related. As Ron and 79x100 suggested, inspect all ignition components and connectors for damage or looseness.
problem still there

Thanks for the suggestions about checking wiring around the coil cluster/boyer control unit.

That was the first thing i checked when the problem came up last week.

And checked again when it happened again on the following day.

Today i pulled the tank off to really get my nose in on those wires and connectors - all ok.

Took the bike out for another test run - it started missing, spitting, and hesitating after about 15 minutes, but would at least pull reasonably well with the revs up. And i didn't experience the dramatic power loss i felt the other day.

But still something wrong.

Electrical - battery is strong, plugs and wires fine. steady current to light and horn.

Fuel - i will check to see if something got into the carb from the tank.

Mechanical - hate to think it's a problem with the recent valve job. It ran so nicely for the first 250 miles.

Could also be a bad ignition switch. The stock location on a Mk III isn't ideal.

The wires at the Boyer pick up plate can fracture at the soldered connection. Look closely, sometimes this is hard to see.

Take nothing for granted..recheck all connections (including grounds). Load test the battery and check alternator output.
The wires at the Boyer pick up plate can fracture at the soldered connection. Look closely, sometimes this is hard to see.

Those wires can and will fracture inside the insulation upstream of the soldered connection where it is impossible to see. I've had this happen twice. It will produce some stange symptoms. My bike would start easily, run great up to about 3,000 rpm. Then it would then start missing. If you got above 3500 rpm it would sort of clear out. You immediately start thinking carburetion, but like they say, 95% of carb problems are electrical. The fix is to solder threaded studs to the board and then fasten the wires to the studs with ring lugs, crimped, not soldered. When the wire is soldered the solder travels up the wire for some distance. Vibration then does a fine job of breaking the stiffened wire.
my guess is if it is temputer sensitive would be to look at coils or boyer box. loose connections should do it no matter cold or hot.

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