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Discussion Starter · #1 · (Edited)
Hello, guys,


As some may have noticed, I've been fixing a few issues with my bike - fixing whose knowledge I have to thank, to great extent, to the help of this forum.


My bike has lack of power on the right cylinder - I feel it while riding it, as if it's struggling to keep up with the throttle acceleration, and I feel it when I individually twist the throttle lever on each carb: the left one has loads of power, the right one sounds and feels a lot weaker.

The context:
- Did ignition timing (after valve clearance and cam chain adjst) - it's spot on;
- Cleaned carbs a few months ago. Floats have the right height (20mm);
- Switched main jets today from 145 to 155 (have UNI air filter pds) - noticed a small difference ONLY on the left carb;
- Tried adjusting pilot/mixture screws - have no response in RPM on either side. Idle screws do have an effect in RPM, though not drastic;
- Right side plug kept fouling until I switched from B8ES (original) to B7ES (hotter). Left spark plug only fouled once;
- Plugs look very dark, with loads of carbon. Right one is almost always wet. Indicates something concerning the mixture - have tried adjusting it, no difference. This is my pilot/mixture screw:
20180224_194519.jpg parafuso mistura ar gasolina.jpg
It seems to be the right one for my bike/carbs (723A);
- When the bike hasn't ran in a few days, darker smoke comes out of the right cyl;
- Have sprayed carb cleaner into all the carb's holes - no joy;
- Took air filters off - same issue;
- Previously, when idling afte the engine was hot, the RPMs went up to 3000. Tightened the mechanical advancer springs (which were loose) and the issue was gone;
- When I hit the starter button with the spark plugs out, by the frame/ground, I get, with the same plug, the exact same spark on both sides (same color, intensity and passage between electrode/tip);
- Do not know cylinder compression;
- Bike does not have trouble starting (either starts right up or at the 4th attempt at most).

Have bought this carb rebuild kit. Will give it a try after cleaning the carbs again.


Given the above, does anyone have any ideas as to what to try or what these symptoms may be about?

Thank you very much!!!!
 

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Discussion Starter · #3 ·
I would do a compression test next, just to see what you are dealing with mechanically.
Thank you very much, as always, Thomas! I will do that!

One question - if it does concern compression (or even if it regards the carb/jetting), is it ok for me to ride the bike for about 20 miles to a mechanic, given the fact that, supposedly, the left cylinder is making a bigger effort to move the bike (compensating for the lack of power of the right one)? Or is that too damaging for the engine?
 

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Has this developed recently, as in somewhat suddenly? Or was it a gradual loss of power over time on the weaker cylinder? If it runs obviously more poorly on one side that it significantly reduces power, then I wouldn't ride it that far.
 

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Your problem here stems from using an after market rebuild kit. The jets and needles from those kits do not fit correctly and will cause all kinds of issues.

The fact that turning the air/fuel mixture screws does nothing to the idle RPM means these screws are no good.

Why did you replace the OEM carb parts? Were they damaged? IF not then all that was needed was for the jets and needles to be cleaned and simply replace the gaskets and o-rings.
If you still have the OEM jets and needles, screws, I would put them back and see how the bike responds.
 

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Discussion Starter · #7 ·
Your problem here stems from using an after market rebuild kit. The jets and needles from those kits do not fit correctly and will cause all kinds of issues.

The fact that turning the air/fuel mixture screws does nothing to the idle RPM means these screws are no good.

Why did you replace the OEM carb parts? Were they damaged? IF not then all that was needed was for the jets and needles to be cleaned and simply replace the gaskets and o-rings.
If you still have the OEM jets and needles, screws, I would put them back and see how the bike responds.
As I wrote, I ordered an aftermarket kit for the carbs which I'm still waiting to arrive. The carbs were all stock (the issue was there, still) and I changed the main jet from 145 to 155 - to no difference. I had already thoroughly cleaned the carbs and this issue has been there all along.
 

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Discussion Starter · #8 ·
Has this developed recently, as in somewhat suddenly? Or was it a gradual loss of power over time on the weaker cylinder? If it runs obviously more poorly on one side that it significantly reduces power, then I wouldn't ride it that far.
No, I've always had trouble with the right cyl. I haven't ridden the bike much as I was still getting my license. Now that I have it and as soon I got each of the parts through time, I realised that the issue musn't have been with the carb - I thought it was but, after having gone through everything I previously mentioned, I now have some doubts. I always felt the bike didn't have enough power and that the right carb wasn't tuned properly. Very scarred that it actually wasn't the carb but something else in the cyl.

Will give the carb one last cleaning (I use straight fuel lines with no gasoline filters) and check if it all stays the same. If it really does, off to the mechanic.
 

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Discussion Starter · #9 ·
Has this developed recently, as in somewhat suddenly? Or was it a gradual loss of power over time on the weaker cylinder? If it runs obviously more poorly on one side that it significantly reduces power, then I wouldn't ride it that far.
One thing I forgot, I actually get a very good idle and the pressure is almost the same on both cylinders. Not sure that makes any difference.
 

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It sounds to me like the difference isn't as bad as you might seem to sense - if it idles decently and runs okay under riding load, I'm not sure I'd be in a hurry to break into the engine (especially if you can't do it yourself and you'll be paying a mechanic to do it). You'll know when it's time to truly do a rebuild... the performance will decline further and the engine will start showing visible signs of decline (blue smoke, regular oil drips from the crankcase breather tube, more decrease in power, maybe even oil-fouling plugs). However, a compression test is easy and it may tell you more definitively what is going on. and BTW - lifting one throttle lever independently of another at idle isn't necessarily an accurate test of separate cylinder condition
 

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Discussion Starter · #11 ·
It sounds to me like the difference isn't as bad as you might seem to sense - if it idles decently and runs okay under riding load, I'm not sure I'd be in a hurry to break into the engine (especially if you can't do it yourself and you'll be paying a mechanic to do it). You'll know when it's time to truly do a rebuild... the performance will decline further and the engine will start showing visible signs of decline (blue smoke, regular oil drips from the crankcase breather tube, more decrease in power, maybe even oil-fouling plugs). However, a compression test is easy and it may tell you more definitively what is going on. and BTW - lifting one throttle lever independently of another at idle isn't necessarily an accurate test of separate cylinder condition
Thank you very much. Very well. Definitely will do the compression test ASAP!!
 

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Verifying the Compression is a critical step in trouble shooting a problem like this one.

If there is a significant difference more than 5% between cylinders you will begin to get poor performance.
As the difference increases it may come to the point where one cylinder is doing all the work and the other is just being dragged along with it.
10% or more you have a problem that needs to be addressed.

CARBS: - Maybe / Maybe NOT !
If one Carb has a significant Float Height issue you could have a problem like this one where it is either flooding or starving.
That would also show up as the idle mixture doing nothing when adjusted.

If your Compression is good I would try checking the timing with a STROBE Light.

If you find the Right Side has intermittent flashes from the strobe you may have a POINTS issue.
A failing Condenser can cause sever pitting on the point contacts which will create a probelm just like yours.
Poor Contact at the one of the points can related to a weak charge at the coil and weak spark.

Also try a NEW Plug.
I have seen PLUGS that Spark perectly well when tested outside of the cylinder fail to spark under compression.
A New PLUG is a VERY cheap test.
 
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Have you checked to see how the sparks from the right cylinder are as compared to the left? It may be that your right coil or spark plug wire are no good and producing inconsistent sparks.
It the carbs are OEM presently and turning the mixture screws does nothing then you probably have blocked passages somewhere or possibly a vacuum leak. Have you checked for vacuum leaks around the carb boots? Your plugs being black and sooty with carbon mean you are running very rich.

Also as Tom mentioned previously, take a compression reading of both cylinders and compare, they should read about the same.
 

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You say the timing is spot on. Did you individually check the timing for the left and right cylinder?
 

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Discussion Starter · #15 ·
Verifying the Compression is a critical step in trouble shooting a problem like this one.

If there is a significant difference more than 5% between cylinders you will begin to get poor performance.
As the difference increases it may come to the point where one cylinder is doing all the work and the other is just being dragged along with it.
10% or more you have a problem that needs to be addressed.

CARBS: - Maybe / Maybe NOT !
If one Carb has a significant Float Height issue you could have a problem like this one where it is either flooding or starving.
That would also show up as the idle mixture doing nothing when adjusted.

If your Compression is good I would try checking the timing with a STROBE Light.

If you find the Right Side has intermittent flashes from the strobe you may have a POINTS issue.
A failing Condenser can cause sever pitting on the point contacts which will create a probelm just like yours.
Poor Contact at the one of the points can related to a weak charge at the coil and weak spark.

Also try a NEW Plug.
I have seen PLUGS that Spark perectly well when tested outside of the cylinder fail to spark under compression.
A New PLUG is a VERY cheap test.

Hello, Yendor. Thank you for your detailed advice and knowledge.

Will indeed do a compression test, as soon as I possibly can.

I have checked float height thoroughly on both carbs with a credit card cutaway with the correct height - both identical (20mm). The mixture/pilot screw does not change RPM in any of the carbs - hence why I ordered a carb rebuild kit.

I have checked with a strobe light (prior to using a test light) and I indeed had intermittent flashes (something which I think happened on both sides - will test again) which led me to think the strobe was faulty and to do the timing with the test light instead. If the compression is within spec, I will check the condenser next. Will also use even fresher plugs (these were very, very recent).
 

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Discussion Starter · #16 ·
Have you checked to see how the sparks from the right cylinder are as compared to the left? It may be that your right coil or spark plug wire are no good and producing inconsistent sparks.
It the carbs are OEM presently and turning the mixture screws does nothing then you probably have blocked passages somewhere or possibly a vacuum leak. Have you checked for vacuum leaks around the carb boots? Your plugs being black and sooty with carbon mean you are running very rich.

Also as Tom mentioned previously, take a compression reading of both cylinders and compare, they should read about the same.
Hello, 76. Thank you again for your reply.

I have checked each side with the same plug and got the same spark on both sides - same color, intensity and route.

I would indeed think I had blocked passages if I hadn't cleaned them throughly a while back (at the time, I still noticed no changes in RPM regarding the pilot screws. But I have not checked if the carbs had air leaks - will do!

As will do the compression ASAP.
 

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Discussion Starter · #17 ·
You say the timing is spot on. Did you individually check the timing for the left and right cylinder?
Yes, checked the timing on both the LF and F marks at the flywheel, respectively connecting the test light to the left and right contacts. The light comes on on the compression stroke of each cyl, exactly when the LF and F hit the mark at the alternator cover.
 

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Yes, checked the timing on both the LF and F marks at the flywheel, respectively connecting the test light to the left and right contacts. The light comes on on the compression stroke of each cyl, exactly when the LF and F hit the mark at the alternator cover.
Valves are properly adjusted, You may consider swapping the coils left to right and see if the problem transfers. Original 40+ year old coils can be a challenge.
 

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Discussion Starter · #19 ·
Valves are properly adjusted, You may consider swapping the coils left to right and see if the problem transfers. Original 40+ year old coils can be a challenge.
Great ideia! Will do it if compression is ok and situation maintains after the carbs have been cleaned again.
 

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Discussion Starter · #20 ·
Hello, guys.

Been a few months and it's unbelievable what happened in the meantime.

I took the bike to a "mechanic" recommended by a friend of my dad's because compression test kits are not cheap (at least, around here) - and I'd only be using it once - and I thought that guy could figure the problem and check for compression, etc. Result: the bike came even worse from the workshop.. Kept stuttering all the way home..

In order to get it back to an at least better condition, I decided to take the carbs out again to get some new parts in from a kit I ordered from Taiwan to check for improval. When I (for the first time, shame on me) checked the floats with my ear to understand if they were leaking, I realised there was gas inside!!!!
My question is, if a float is leaking, could it result in the problems I'm having (right carb stuttering when I pull the throttle, loss of power, spark plugs always fouling, pilot screw not having any effect on RPM, etc.)??

I have ordered new floats and also already ordered a kit to measure compression (finally, shame on me). I hope for all that is sacred to me that this problem was simply caused by leaking floats.
 
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