1979 650 BACKFIRES!!!!!!

[Anonymous]

I know rhis is a Suzuki GS furum, but you all have been so helpful and informative about my GS I am restoring, I hoped you could also help out on my 1979 Honda CB650. I have narrowed the backfire down to cylinders 3 or 4. I have dual exhaust and the backfire is in my right pipe, if I was sitting on the bike. I have replaced the spark plugs, put in a brand new air filter and cleaned the carbs. The spark plugs are getting oily and the head gasket leaks just a little. I have put on brand new spark plug wires. The back fire is still there and pretty constant. In the dark I can see the the backfire is in the tail pipe, actually in the dark, I can see a blue flame while looking in the tail pipe. The bike is also very, very hard to start. I have to crank, and crank to finally get it to run when it has been sitting a while. Once I have warmed it up, It will start right up. Also the bike runs pretty slow. It tops out at about 65 MPH and the charging system does not work. I have narrowed the charging problem down to the regulator/rectifier, and do not know it that could be a problem with the backfiring. I also have not yet checked the timing, this may also be the problem. I was just hoping you might have some ideas. I figue the CB's and the GS's are pretty similar, as they are both Jap bikes. I have submitted this problem on a CB forum without a lot of help, and ya'll have been so helpful to me in the past.

Thanks in advance.

[earlfor]

Re: 1979 650 BACKFIRES!!!!!!

Whenever you have a problem and you think it is the carbs, the first thing you must do is make sure it isnt anything else. Low system voltage can disrupt the ignition system and cause backfiring. It can also cause a weak spark and oiled/wet plugs. Your batterY should show at least 12.5 volts and preferably about 12.7. Ignition timing should be checked with a continuity light or a timing light. Induction tubes between the carbs and cylinder must be checked to insure they are tight. Induction tubes between the carb and airbox must also be tight. The airbox should be sealed, The air filter should be clean. Wiring harnesses drop voltage at the end of their runs. You need to check battery voltage at the terminals and then check voltage at the coil primary wires with the ignition on. It should be the same. If everything above is as it should be, you can check float levels in the carbs. After that, check vacuum levels in the carbs (synchronization) When an engine missfires or backfires, the last thing you touch is the carbs because frequently, it is not the carbs causing the problem. Unless everything else is working properly, you will find yourself adjusting carbs until you retire and the problem will still be there. :-)

Earl

I know rhis is a Suzuki GS furum, but you all have been so helpful and informative about my GS I am restoring, I hoped you could also help out on my 1979 Honda CB650. I have narrowed the backfire down to cylinders 3 or 4. I have dual exhaust and the backfire is in my right pipe, if I was sitting on the bike. I have replaced the spark plugs, put in a brand new air filter and cleaned the carbs. The spark plugs are getting oily and the head gasket leaks just a little. I have put on brand new spark plug wires. The back fire is still there and pretty constant. In the dark I can see the the backfire is in the tail pipe, actually in the dark, I can see a blue flame while looking in the tail pipe. The bike is also very, very hard to start. I have to crank, and crank to finally get it to run when it has been sitting a while. Once I have warmed it up, It will start right up. Also the bike runs pretty slow. It tops out at about 65 MPH and the charging system does not work. I have narrowed the charging problem down to the regulator/rectifier, and do not know it that could be a problem with the backfiring. I also have not yet checked the timing, this may also be the problem. I was just hoping you might have some ideas. I figue the CB's and the GS's are pretty similar, as they are both Jap bikes. I have submitted this problem on a CB forum without a lot of help, and ya'll have been so helpful to me in the past.

Thanks in advance.

[SqDancerLynn1]

Timing is pretty fool proof, I would suspect a bad coil causing a weak spark or bad compression from the blown gasket. Have you done a compression test???? If two adjacent cyls are not working it would be hard to start

[Anonymous]

Have not checked compression.

I do not have a compression tester, but have read about putting your thumb over the spark plug opening. If you cannot hold your thumb over it no matter how hard you try you have good compression. Does this sound right, or should I invest in a compression tester?

[earlfor]

Re: Have not checked compression.

Errr ahhh, get a compression tester. They are not expensive. A thumb is not a a reliable test instrument. :-)

Earl

I do not have a compression tester, but have read about putting your thumb over the spark plug opening. If you cannot hold your thumb over it no matter how hard you try you have good compression. Does this sound right, or should I invest in a compression tester?

[Anonymous]

I know it's not the coils

I checked the coils tonight. I swapped the coils, and the problem didn't swap with them. The right side is still giving me problems. I have not yetgot a compression tester, but I went ahead and tried the thumb technique. When I pulled the spark plug from cylinder 1 I noticed that it was extremely oily. I put mu thumb over the opening for the plug on cylinder 1 and the end of my thumb was covered in oil. I think my problem may be that it is time for and engine rebuild. The spark plug in cylinder 2 had a slight amount of oil on it, but very little compared to cylinder 1. One funny thing, when I had the spark plug pulled out of cylinder 1 and I tried cranked on the bike, it fired up and rand actually pretty good. I did not even hear it backfire. I only let it run for a second like this. I also pulled plug two, with plug 1 still out and it still cranked over and ran, on just 2 cylinders. I though that was good, but don't know.

I have a haynes manual, and have gotten pretty mechanical. If I do most of the work myself, is an engine rebuild, or overhaul very expensive?????

[earlfor]

Re: I know it's not the coils

I would still do a compression test to find out if the problem is rings or valves. Once you know the problem, you can determine cost.

Earl

I have a haynes manual, and have gotten pretty mechanical. If I do most of the work myself, is an engine rebuild, or overhaul very expensive?????

[Anonymous]

Compression Test Done

I did a compression test. I ahve about 120 psi in each cylinder. They each started about 40 psi on the first crank and went up to a peak of 120 psi. The only cylinder that has a problem with oil is cylinder 1. Does it sound like a valve problem, or a ring problem. I need to change the head gasket as it is leeaking anyways, so could this be something that might be an easy fix. Open for ideas and suggestions.

[Anonymous]

Re: Compression Test Done

I did a compression test. I ahve about 120 psi in each cylinder. They each started about 40 psi on the first crank and went up to a peak of 120 psi. The only cylinder that has a problem with oil is cylinder 1. Does it sound like a valve problem, or a ring problem. I need to change the head gasket as it is leeaking anyways, so could this be something that might be an easy fix. Open for ideas and suggestions.

Are you sure it is oil on the plug and not unburned fuel? Somtimes the fouled plug carbon mixed with fuel can look like oil.

If you did a dry compression test and all four cyl are right at 120. The fact that there is little difference is a good one. I don't know the compression numbers for your bike. I seem to recall for my GS850L
120>170 range is serviceable.

[earlfor]

Re: Compression Test Done

120 is on the low end of the scale, but the bike would still run on all four.
try putting about a teaspoon of oil into each spark plug hole, reinstalling the compression testor and doing another compression test. If the problem is rings, the compression will increase with the oil sealing the rings. If it is valves, there will be no change. If you know your head gasket is leaking, there isnt much you can do about that other than to replace it.

Earl

I did a compression test. I ahve about 120 psi in each cylinder. They each started about 40 psi on the first crank and went up to a peak of 120 psi. The only cylinder that has a problem with oil is cylinder 1. Does it sound like a valve problem, or a ring problem. I need to change the head gasket as it is leeaking anyways, so could this be something that might be an easy fix. Open for ideas and suggestions.

[Anonymous]

I may have done the compression test wrong. I followed the directions in the kit. I warmed up the engine and took out all 4 spark plugs. I then tested 1 cylinder ata a time, with no spark plugs installed. Is this correct, or shoult the other spark plugs been installed, or since each is a seperate cylinder does it not matter.

[earlfor]

No, you were correct and I was an idiot? Instead of telling you to reinstall the spark plug, I mean to say re-install the compression testor.
Whe you add the teaspoon of oil, and recheck compression, be sure to do this with the throttle twisted open.
Leave the spark plugs out, that is fine. What PSI do you now get?

Earl

I may have done the compression test wrong. I followed the directions in the kit. I warmed up the engine and took out all 4 spark plugs. I then tested 1 cylinder ata a time, with no spark plugs installed. Is this correct, or shoult the other spark plugs been installed, or since each is a seperate cylinder does it not ma
tter.

[Anonymous]

did compression test with oil

Ok, I did the compression test again. This time after warming up the engine, I removed all the spark plugs and added 1 tsp of oil to each cylinder before testing. I came up with 180 psi in each cylinder. Without the oil, only 120 psi. So I guess the rings are pretty well shot. I figure with would cause the backfire problem. I also noticed when I first got the bike and began fixing everything, it would only go 70 mph. Now it is topping out at 60 mph. Things are getting worse. I have parked it for now, and really want to get it rebuilt before it warms up. Luckily I got my dad's GS 750 running tonight, so I will at least have something to ride, while I am working on my bike...

[Anonymous]

GOOD RINGS

Does anyone know a good supplier for piston rings for vintage jap bikes. I have been looking on the internet with limited success. Do some of the dealers you guys use also supply fo honda motorcycles?