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Mystery of the missing combustion!

  • Thread starter Thread starter Anonymous
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Anonymous

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First let me say that this site is invaluable. So thanks a lot!

I have an 83 GS650G that's in great shape save for a recent problem. I noticed the power wasn't as it had been, so the next time I started the engine (after it had cooled) I checked the temp of the headers to see if everything was warming up equally. I noticed that the 2 header pipes on the left side of the engine warmed up to operating temp quickly, but the 2 headers on the right stayed cool. The are cool for 5-10 minutes at idle and even then the don't warm that much. Same with the engine block, hot on the left, cool on the right. Even the exhaust followed the same pattern. I pulled and replaced the plugs (the plugs were black and smelled of gas but otherwise ok - thought replacing them was prudent since I've only had the bike for 6 months and had no history on the previous plug change). I checked for spark by grounding the plug against the engine (on the casing) while engaging the starter and they sparked although it wasn't the huge robust spark that the Clymer manual mentions but I have limited experience with this stuff to judge. The bike starts and idles ok (although a little rough now that its winter).

I'm thinking that I should start looking into fuel supply to the carbs. Any ideas as to how this could have happen all at once and how to fix it?

Thanks

Dan
 
Don't fall into the trap of thinking it can only be the carbs, and neglect to check the ignition as well. How do the plugs look on the dead cylinders?
 
Check the plug caps, but the fact that it is #3&4 cylinders say that it is probably a carb problem. One coil feeds 1&4 the other feeds 2&3
 
Re: Mystery of the missing combustion!

OK 3 and 4 weak are not firing. Thats not an ignition system division.
Ignition is paired 1,4 and 2,3 as Lynn said. Its rare for a carb problem to hit two carbs at exactly the same time in the same way. Could happen, but not likely. I dont think you have a single problem. I think you have a group of small misadjustments that cumulatively result in the 3,4 plugs fouling out and the cylinder not firing. I would make these checks.

1. check battery voltage at the terminals. Bike not running it should be a minimum of 12.5 volts.
2. Check input voltage to the coils with the 12 v + leads disconnected from the coils (usually orange/white wire) It should be within 1/4 volt of the battery terminal voltage.
3. check resistance on the coil primary. (between the clips on the end of the coil) It should be between 2 and 5 ohms.
4. check secondary resistance at between the plug caps on both coils, (between the 1,4 caps and then between the 2,3 caps.) It should be between 30k and 50k ohms resistance.
5. Check the float height levels on the 3,4 float bowls. I dont know the height setting for a 650 G, but if 1,2 carbs are running well, you can get the measurement from one of them.
6. I would check compression on all cylinders. If 3,4 are low, I would check valve clearances.

Come to think of it, you might want to do step 6 first. :-)

Earl


daniel12371 said:
First let me say that this site is invaluable. So thanks a lot!

I have an 83 GS650G that's in great shape save for a recent problem. I noticed the power wasn't as it had been, so the next time I started the engine (after it had cooled) I checked the temp of the headers to see if everything was warming up equally. I noticed that the 2 header pipes on the left side of the engine warmed up to operating temp quickly, but the 2 headers on the right stayed cool. The are cool for 5-10 minutes at idle and even then the don't warm that much. Same with the engine block, hot on the left, cool on the right. Even the exhaust followed the same pattern. I pulled and replaced the plugs (the plugs were black and smelled of gas but otherwise ok - thought replacing them was prudent since I've only had the bike for 6 months and had no history on the previous plug change). I checked for spark by grounding the plug against the engine (on the casing) while engaging the starter and they sparked although it wasn't the huge robust spark that the Clymer manual mentions but I have limited experience with this stuff to judge. The bike starts and idles ok (although a little rough now that its winter).

I'm thinking that I should start looking into fuel supply to the carbs. Any ideas as to how this could have happen all at once and how to fix it?

Thanks

Dan
Click to expand...
 
Mystery of the missing combustion!

Thanks for all of your suggestions. I'll start with the compression test. But if the cylinder has no combustion (since no heat is generated to the header) wouldn't the compression be way off anyway? The plugs are new and the spark intensity of all cylinders looks the same, but I'll take Earlfor's suggestions and start looking at fuel intake and work my way toward the electrical system.

Dan
 
But if the cylinder has no combustion (since no heat is generated to the header) wouldn't the compression be way off anyway?
Click to expand...

No. The compression has nothing to do with combustion, just how well the cylinder cavity is sealing. Without a good seal between the rings and both intake and exhaust valves, combustion would be poor- maybe even impossible.

If you mean that the readings won't be accurate becuase the cylinders don't get hot, then you might be right, but it might shed a little light on things. I'd do a cold compression test on all cylinders then try and warm the block up by running it like it does now on only two cylinders. Then perform a hot (or somewhat hot) test and post the results. Do a wet test both cold and hot also.

If it is a carb problem, I'd put my money on the float hights or maybe sticking float valves. Maybe it's flooding because you say the plugs get wet?

As far as ignition, I'll defer to Earl, the electrical wizard. I don't know squat about the electrics, hopefully that will change after this years restoration of my 81.
 
Re: Mystery of the missing combustion!

Combustion and compression arent the same things. Whenever you take a compression reading, you couldnt possibly have combustion because the spark plug is removed and the compression guage fitting is in the hole. :-)

When you do the compression test, do it with the throttle held wide open.
Disconnect all spark plugs. You dont want the engine going to redline. :-) :-)

Earl



quote="daniel12371"]Thanks for all of your suggestions. I'll start with the compression test. But if the cylinder has no combustion (since no heat is generated to the header) wouldn't the compression be way off anyway? The plugs are new and the spark intensity of all cylinders looks the same, but I'll take Earlfor's suggestions and start looking at fuel intake and work my way toward the electrical system.

Dan[/quote]
 
Mystery of the missing combustion!

When I was inspecting the carburetor I noticed that fuel was seeping out of the #2 drain plug of the #2 float bowl. Its making me think that I have a plug in the fuel hose that connects carb #2 with carb #3 such that carb's 3 and 4 are fuel starved. Does this make sense to anyone?

Thanks,

Dan
 
When I was inspecting the carburetor I noticed that fuel was seeping out of the #2 drain plug of the #2 float bowl. Its making me think that I have a plug in the fuel hose that connects carb #2 with carb #3 such that carb's 3 and 4 are fuel starved. Does this make sense to anyone?
Click to expand...

Maybe. I would definitely pull the carbs and check all the passages and float heights.
 
Re: Mystery of the missing combustion!

Nope, not a bit. :-) 3 and 4 are not firing and they are black. Black suggests firing weakly or they are way rich. The fuel inlet Tee left side feeds carbs 1 and 3 and right side feeds 3 and 4 carbs. The number 2 float bowl overflowing would have nothing to do with the fuel supply going to 3 and 4.
Different sides of the same Tee supply 1,2 and 3,4. You have either an incorrectly set float height on the #2 carb or you have a leaking float needle assembly on that carb. That needs to be fixed.
Plugs 3 and 4 are black. There are a few things that can cause that. NO spark will not cause that because no fuel is being burned. A weak spark will cause blackened plugs. Too rich a mixture will cause black plugs.
That can be too much fuel, or it can be not enough air. Float levels wrong, float needle valves leaking, or a blocked aircleaner element can be the cause, It could be a little of each of those things. Valves out of adjustment can also cause blackened and consequent fouled plugs and loss of ignition. Low compression on those cylinders can be the cause also. Again, it could be partially lowered compression complicated by leaking float needles, weak spark and incorrect float height.

Earl



daniel12371 said:
When I was inspecting the carburetor I noticed that fuel was seeping out of the #2 drain plug of the #2 float bowl. Its making me think that I have a plug in the fuel hose that connects carb #2 with carb #3 such that carb's 3 and 4 are fuel starved. Does this make sense to anyone?

Thanks,

Dan
Click to expand...
 
Are your plug leads connected correctly???
Coil No 1 (LH viewed from the seat should go to plugs 1&4, coil No2 should go to plugs 2&3
And check float height, and needle valve of no2 carb.
Dink
 
Another think to think about Since 3&4 share the same vent IF the needle is leaking on either carb it could possibly flood both cylinders. IS the vent tube pointing up or down???
 
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