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Topic-icon 1973 R75/5 right carb running far too rich

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9 months 3 weeks ago - 9 months 3 weeks ago #4090 by Wobbly
Replied by Wobbly on topic 1973 R75/5 right carb running far too rich
All it takes for an engine to run is: Spark (at the spark plug), Fuel, and Compression. The "Big 3". Test and verify, don't guess. If your bike passes all those tests, then one of the 3 is not happening at the correct Time.

cynispin wrote: I'll put in an order for carb rebuild kits, wires, caps, and plugs. In the meantime, I should be able to check compression this weekend. Don't have the tools to do a leakdown check, but I can still check for air leaks around the hoses.

In my professional opinion, a "leak down test" is simply a tool to sell more top end jobs. In other words, if it's not a SCAM I'd be hard pressed to tell you what is. Think about it, the complete firing cycle happens in a fraction of a second, but a mechanic is telling you that your cylinder needs to hold pressure for, what, 1 minute ? :S

Try this instead... With the carbs pulled off the rubber intake tubes, place both plugs on top of the cylinder in their caps, place your thumb over one empty plug hole at a time, and spin the electric starter. If the air feels hot and will blow your thumb off the plug hole, then your compression is high enough to start the engine.

cynispin wrote: Will probably pull those wires this weekend and check them with a multimeter as well, but regardless it's probably time to change them out. At least I can know whether or not it's part of the problem!

The metal-core wires simply corrode due to water over the years, which degrades their capacity. Not much there to measure with a meter. However, the OEM plug caps are known to degrade AND then ultimately fail. You might see that with a meter, but since new NGK 5K Ohm caps are $2 each it's simply easier to replace. And of course, anytime you deal with ethanol fuels you can permanently foul a plug in a heartbeat, so spares (NGK BP7ES) are a must.


All the best.

Don't hide 'em, Ride 'em !!
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Last edit: 9 months 3 weeks ago by Wobbly.

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9 months 3 weeks ago #4095 by cynispin
Replied by cynispin on topic 1973 R75/5 right carb running far too rich
Didn't read this post until after I'd started on the work, but it definitely helped! And I didn't unnecessarily pay for a leak-down test.

First, pulled the carbs and did a compression test with a gauge (cold). Left side was pretty normal at 120, but right side was around 130. Just to see, I cranked for a few more revolutions and was able to get it up to 140. Not sure what that means, but I figure it should be included here, since I think that is closer to normal-operating-temperature compression. Anyway, compression seems to be good!

I noticed when I pulled the right side carb that there was quite a bit of oil in the air tubes. I know the crankcase breather vents into it, so when I saw a little oil on the bottom of the snorkel I didn't think anything of it. Checked inside, and there was quite a bit of oil around the aperture that the snorkel fits onto (I mean COATED), and a light film all through the snorkel, and even a little on the filter. I cleaned everything out and reassembled.

I pulled the spark plug wires and cleaned all the corrosion off the contacts. I then tested for resistance, which read a little over 4 kΩ. Considering it's usually around 5, that seemed pretty good to me. I applied the slightest amount of dielectric grease to the contact for good measure. (I will be replacing the plug caps per Wobbly's advice regardless.) For comparative purposes, the plug wires were switched (right-left).

I went for a ride, and actually got out on the highway for about 20+ miles. Tried to keep the RPMs up for as much of the journey as I could. Performance was better than anytime in recent months! Got back home and did carb balancing, and brought the idle to about 1,000 RPMs. However, I still need to turn the idle mixture screw all the way in to get it running properly...

Went out for another ride, and things are still great. Put a total of 50 miles on her. Now, I at least have a theory as to what was happening. I believe that the shop overfilled my oil (when I checked, it was a little high, but I assumed this must have been related to old bikes "eating" oil). When the engine heated up and pressure increased, it was probably blowing all of that junk into the right carb, fouling the plugs. Because I had been tinkering so much with the bike, riding around the neighborhood, worrying about bogging down on the highway, I prevented the engine from actually burning off all the junk on the right side. Meanwhile, gas is just flooding the right side as well. I know this is common sense to everyone reading this, and now I know! Get the thing out and RUN IT.

In any event, I can actually run my bike again and enjoy it! Going forward, I'm replacing all the things Wobbly mentioned as I get them over the next week or two. With any luck, one of them will address this weird issue with the mixture screw. That's my update, and thanks Wobbly for all the advice!

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9 months 3 weeks ago #4096 by dcb_wvu
Replied by dcb_wvu on topic 1973 R75/5 right carb running far too rich
thanks for the update. Awesome that you are learning too! and Wobbly's advice is always top notch! thanks, Wobbly.

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9 months 2 weeks ago - 9 months 2 weeks ago #4109 by Wobbly
Replied by Wobbly on topic 1973 R75/5 right carb running far too rich

cynispin wrote: Went out for another ride, and things are still great. Put a total of 50 miles on her. Now, I at least have a theory as to what was happening. I believe that the shop overfilled my oil (when I checked, it was a little high, but I assumed this must have been related to old bikes "eating" oil). When the engine heated up and pressure increased, it was probably blowing all of that junk into the right carb, fouling the plugs. Because I had been tinkering so much with the bike, riding around the neighborhood, worrying about bogging down on the highway, I prevented the engine from actually burning off all the junk on the right side. Meanwhile, gas is just flooding the right side as well. I know this is common sense to everyone reading this, and now I know! Get the thing out and RUN IT.


Great news !! Congrats.

• Your assumptions on oil in the intake may need some modification. There will be normal blow-by until the piston rings re-seat... which they are now doing. However, excessive oil in the intake can be caused by 2 common things.... 1) plain old dirty air filter. Replace it !! 2) Next to the starter motor, under the starter cover is a small, rectangular plate held on with 3 screws. Remove that cover and there should be a "well" to collect oil vapor and put it back into the engine as liquid. If that "well" is full of liquid oil, the the drain hole into the engine may be clogged. Probing the bottom of the "well" with a guitar string may find and clear that drain hole. (Snowbum has a nice write-up on this with photos.)

• Your valves will need re-adjusting (or at least checking) now that the engine has been run.

• Fuel should Never Ever overflow out of the carb. Replace (don't simply "check") the float needle, the float, and check the float level with the new parts.

• The lack of idle is either the carb flooding, or blocked idle passage.

• Smooth, balanced idle at 1000 RPM is important, but equally important is balanced running at 1800 RPM with good throttle cable adjustment.


Hope this helps.

Don't hide 'em, Ride 'em !!
#15150
Last edit: 9 months 2 weeks ago by Wobbly.

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