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Discussion Starter #1
My 1982 GS750E does not want to rev while under load (on the street with rider). It starts, idles and revs to 8K+ RPM on the bench or if I pull in the clutch while in motion. But while riding, it is extremely underpowered and will not rev past 3 - 4K RPM and has no power. See list of what has been done below, educated suggestions are welcomed:
  • Stock exhaust, airbox and new air filter
  • Stock carb settings, recently cleaned and rebuilt (replaced all jets, etc.)
  • Brand new Dyna ignition, coils, plug wires and coil relay mod. Huge blue spark all cylinders.
  • Ignition advance works freely
  • 12.6V to the coils at 3K+ RPM (I know, could be higher but this should be plenty)
  • Fuel flow is fine, not a plugged vent on the tank and vacuum line from carb to petcock is fine
  • 160+ PSI compression for all cylinders
  • Does not surge at idle or backfire on decel
  • Plugs appear to be slightly on the lean side, all four look identical.
  • New battery and spark plugs
  • Timing marks on cams align, timing is set, cam chain nice and snug (tensioner works freely) no noise or rattling from top end.
  • Valves are all adjusted properly.
Need to recheck float bowl levels. Could that make it run fine with no load but like crap when there is a load?
 

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What jet size are in there? I’m assuming CV carbs, yes? Do the needles have any adjustment? When you cleaned the carbs, did you verify all the passages are clear and clean? Also make sure the diaphragms on top have no pin holes and the vacuum passages that lead to the top of the diaphragms are clear.
 

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Check the kit needles are actually the same as the oem. I usually clean the originals and put them back in.
 

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Discussion Starter #4
Thanks 8Ball;
Yes, CV carbs. I must admit that I didn't check the jet size as I assumed that it would be correct from the K&L rebuild kit for the '82 750E (maybe a dumb assumption). I was very careful to blow out all passages and assorted holes. No pin holes in the diaphragms and all are properly aligned/seated. I do believe there is one adjustment possible on the needle for a slightly more rich setting. This is my first play at CVs, not liking them too much right now.
 

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.... This is my first play at CVs, not liking them too much right now.
Horrible, Horrible things. CV carbs are the epitome of carburetor over-complexity.

The top half of those carbs is all air and vacuum controls, that sh*t don't go bad unless your rubber parts are rotted away, all the nasty problems will be fuel related and in the lower half of the carburetors.

:unsure: are you certain you rear brake is not dragging?
Because if it is not that, then you are not firing on all cylinders when under power.
"will not rev past 3 - 4K RPM and has no power." = running on half your cylinders.

Check your ignition coil outputs / 1 bad coil out of 2 etc. Run it and pull the spark plugs to inspect, I am guessing 2 of them look very different then the other 2.
" Plugs appear to be slightly on the lean side, all four look identical." Oh, I missed that on first read, are you sure you ran them enough?
 

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Discussion Starter #8
Hey wait! Your bike has breaker points and condensers! check those, that could do it.
Thanks TR! I did a plug chop and all 4 looked the same. Exhaust temp is very close on all four (at least on the bench). Brand new Dyna ignition, coils and wires. Beautiful blue spark on all cylinders (on the bench of course). Rear brake not dragging (coasts freely when I pull the clutch in), no clutch slip (all new plates and springs).
The bike seems to run hot so I am thinking this is a lean problem caused by jetting or low fuel level in the float bowls. It is just weird to me that it can run so well with no load but like crap under load.

Oh, one other question on CVs... if this is a lean issue, wouldn't it improve if I gave it some choke while under acceleration? Choking during the ride has absolutely no impact on how it runs, but choking is required to start it when cold, so I know it is functioning.
 

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Thanks TR! I did a plug chop and all 4 looked the same. Exhaust temp is very close on all four (at least on the bench). Brand new Dyna ignition, coils and wires. Beautiful blue spark on all cylinders (on the bench of course). Rear brake not dragging (coasts freely when I pull the clutch in), no clutch slip (all new plates and springs).
The bike seems to run hot so I am thinking this is a lean problem caused by jetting or low fuel level in the float bowls. It is just weird to me that it can run so well with no load but like crap under load.

Oh, one other question on CVs... if this is a lean issue, wouldn't it improve if I gave it some choke while under acceleration? Choking during the ride has absolutely no impact on how it runs, but choking is required to start it when cold, so I know it is functioning.
I take it the Dyna ignition obsolete the points and condensers, I only recognized Dyna as being a manufacturer.
The symptom you described about the bike revving up under no load ok and then having no power under load, is very typical of what I have experienced with twins and fours when half the cylinders are out for some reason.

With CV carbs they have several fuel circuits (as many as 5), "choke" on a CV carb is usually a fuel enrichment circuit that dumps raw fuel into the intake path. CV carbs all run lean, that is the entire purpose of the carburetors design, it is a pollution control and fuel saving design, not a performance design. Practically obsolete now we have fuel injection that works so well.
 

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I had the same problem on a 1980 GS750E and here are a few of things to check.

Carb rubbers, Centrifical advance, Fuel Line, Petcock, Carb Admustments, Clean Carbs, Clean Ignition Contacts and Intermitting Cylinder Firing.

Hope that helps.

2-Stroke
 

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Discussion Starter #11
Thanks 2-Stroke! I have new carb rubbers, fuel line/petcock flow is strong, carbs are clean, advance is def not sticky, no ignition contacts with DynaTech ignition. I have the head off as I discovered it had a head gasket leak. Will replace apparently leaky valve oil seals and all gaskets as well as check float bowl levels while the carbs are off.
 

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You said new plug wires. Did you retain the plug caps. They have a built in resistance. Is that resistance value comparable with your ignition system.
 

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Just thought of something else. Make sure you battery is good and charging system is putting out like it should. I think 17 volts is correct from the stator. The regulator could cause issues as well. If you need information on how to check it I can send you a link.
 

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I did replace the plug caps and have had no issues.
The caps on the sparkplug wire that connects to the sparkplug has a built in resistance. Each of the Japanese manufacturers has a different resistance value for their ignition systems. Was the bike running ok since you replaced the wires? I learned this with my old Suzuki 550. Replaced plug wires with 8mm yellow wires with plug ends already on them. These had no resistance built in. Bike started, idled and revel just fine but would not pull any rpm under load. Turns out that ignition system required 5,000 ohms resistance in the secondary. Put the original plug caps on the new wire and all was right again.
 

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Discussion Starter #17
The caps on the sparkplug wire that connects to the sparkplug has a built in resistance. Each of the Japanese manufacturers has a different resistance value for their ignition systems. Was the bike running ok since you replaced the wires? I learned this with my old Suzuki 550. Replaced plug wires with 8mm yellow wires with plug ends already on them. These had no resistance built in. Bike started, idled and revel just fine but would not pull any rpm under load. Turns out that ignition system required 5,000 ohms resistance in the secondary. Put the original plug caps on the new wire and all was right again.
I replaced the plug wires and caps with Dynatek 8mm Suppression Plug Wire Set DW-800 which should be a match for the new Dyna ignition and coils.
 

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Discussion Starter #18
Just thought of something else. Make sure you battery is good and charging system is putting out like it should. I think 17 volts is correct from the stator. The regulator could cause issues as well. If you need information on how to check it I can send you a link.
Yes, please do send the link. I believe that there could be a problem with the charging system but the battery is new and will be putting out 12V or more, the spark is strong,at least on the bench.
 

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Your Service manual should tell you how to test the alternator.
Typical setup is a 3-phase alternator, that means there are going to be 3 coils inside the thing and 3 wires coming out of the motor that are all the same colour, usually white or yellow. Each of those circuits typically has the same resistance (a couple of ohms) and same alternating current output at approximately 17 volt peaks. All or at least 2 of those 3 wires are on their way to the rectifier and voltage regulator. Ground is the motor itself.

... more modern fuel injected bikes have higher power requirements, older bikes sometimes dedicated one coil to ignition and utilized only a 2-phase charging system.
 
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