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Topic-icon '74 R90/6: left cylinder after or back firing above 3k rpm

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3 months 3 weeks ago #6224 by 10278
Hello all
Hope everyone is well
My bike has 37k miles and I have been fighting with this after firing issue since last year. I have been physically unable to ride the bike for a couple of years which makes road testing a problem and I'm hoping to get it running right before putting it up for sale. It starts up fine from cold and runs clean on "choke" but when warm and off enrichment it afterfires on the left side when run up to 3k or more. When the issue first appeared I was convinced my 45 plus year old coils were breaking down so I replaced them with a dyna coil. While i was at it I replaced the entire ignition system from the plugs to the condenser. The timing has been set and checked at least a couple of times. There is ghosting but it appears to be within bounds. The plug leads have been swapped to eliminate problems there. The heads have been retorqued and the valve clearances checked. The carbs have been gone through and cleaned with the rubber parts being inspected and replaced where suspect. At one point I believed I had a bad float, so I replaced and set up new ones on both. Basic carb setup appears to be the same for both sides with idle mixture screws ending up in about the same position and my 5 buck manometer tells me the vaccuums are good. Out of frustration I have even swapped the carbs around a couple of times and it works out better than you might think. Once the idles are set, the enrichment is managed manually, and the running speeds are set with a couple of sets of feeler guages. Sadly, the issue remained on the left side. My infrared temp gun tells me the left header runs 150 degrees F hotter than the right when warmed up. A leakdown test done both cold and slightly warm didn't show any real difference between left and right with any apparent leakage going past the rings. My current theory is that the exhaust valve is not sealing once the head has warmed up. Pulling the head is the last resort and I know I may lose the exhaust nuts as they haven't been off in years. Is there something I'm missing?

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3 months 3 weeks ago #6225 by Wobbly
I can think of 2 things....

• The most common is an air leak in the exhaust. On run-down so much raw fuel is thrown into the exhaust that all it takes is a little oxygen and it ignites. All it takes is a little pin hole. Look around the cross-over pipe and see if you see any blackened areas (usually about the size of a pencil eraser). If it's big enough for exhaust to come out, then air can go in.

• If the bike has been stored with fuel in it, then it will be necessary to look at the low speed jet and idle passages due to Ethanol clogging.

Hope this helps.

Don't hide 'em, Ride 'em !!
#15150

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3 months 3 weeks ago #6228 by 8166
Have you run compression checks, both cold and hot? The hot test would tell if you've got valve sealing issues on either side.

The temp difference tells me that the left side is running richer. Are you sure you got the needles set in the correct grooves on both sides, and that the throttle cables are properly synchronized?

But before you do all that, make sure you don't have any leaks in the exhaust system, as that would explain the popping on the overrun, especially if the left side is running rich. You can hear a bad leak, and feel with your face any small ones.

Good luck, and please let us know what you find.

8166 Scot Marburger, This email address is being protected from spambots. You need JavaScript enabled to view it.

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3 months 3 weeks ago #6229 by 10278
I will check the exhaust system over for leaks. I did do a leakdown test and it showed little difference from left to right. It's hard to get a good hot test as it takes time to set up the test and the engine cools down. I would have to borrow a compression tester to do that test but it should be quicker on a hot engine. The jet needle positions are a match but I failed to mention that there seems to be a difference of opinion on the correct position. The owners manual and the Clymers say position 1 and the Bing manual says position 2. the needles are currently in position 2. I have had them in position 1 with no real difference in running. I have sprayed Seafoam through the idle circuits so I think they should be clear. I'll check over the exhaust and get back to you
Thanks guys for the quick response.

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3 months 2 weeks ago - 2 months 3 weeks ago #6243 by Wobbly
Ideas....

• If you have run the bike within the last week, then the rings should be seated well enough to do a compression test cold. At least good enough for our purposes. The only requirement I would have is than both carbs be popped out of their rubber inlet hoses during the test. Using the starter motor, just turn the engine over until the pressure stops climbing. This may happen after 8-12 compression strokes. Readings should range between 130psi and 150psi, and be nearly identical on both sides.

• Since the Bing CV slide won't allow you to see the needle clip position directly, when you say they are in X clip position, we are assuming that you verified they were both equal with some sort of height gauge rather than simply counting the "clicks" during insertion.

Hope this helps.

Don't hide 'em, Ride 'em !!
#15150
Last edit: 2 months 3 weeks ago by Wobbly.

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3 months 2 weeks ago #6245 by 10278
I checked the exhaust system yesterday and couldn't find any holes. I have yet to aquire a compression tester but I had thought that the carbs would need to be removed for a good reading. I will check the needle length. I think a micrometer should work
Will get back to you.
Thanks again

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