77 650D blowing carbs off when engine is hot/idle dropping at stoplights

[cjv2001]

Like the title says, today was my second time having the carbs blow off. First time it snapped my fuel filter and I was stranded but luckily close to my house. Today, unluckily for me I didn't have a screwdriver and I picked my girlfriend up from work so we were both stranded till I had a buddy bring me a screwdriver to loosen the carb boots to pop back in LOL, quite an adventure.

What has happened both times now is I am sitting at the light idling fine, and all of a sudden my RPMs will drop from 1300 to 700-800 for a few seconds, and return to normal. Occasionally, it will go down to 700-800, then sputter and backfire hard, causing the carbs to pop off. Today it was the left side.

I am concerned this may be a coil problem especially since it runs rougher when hot, but please let me know what you think and if there are any tests I should run. I have not had the chance to test my coil with a meter yet.

Some other symptoms along with this problem I continue to have are deceleration popping (most noticeable in 2nd gear), and it is hard to start when the engine is hot. I have not messed with electronics too much other than the points for timing.

Things I have fixed and am 99% sure they are in proper order:

- Cam chain tightness (1-2mm movement)
- Valve clearance (intake .003 - exhaust .006)
- Floats are set to 24mm
- Timing (tuned slightly retarded with a tad bit higher idle speed at 1300-1400
- BS38 air mixture screw turned 1 3/4 turns out (jetting 130 and 27.5 pilot), running slightly rich
- Jet needle is in the stock position
- Greased advance rod

Stock exhaust.

 

[Highpower]

I suspect that you are running pods? I have found pods to be a pain to properly tune at all RPM ranges. Plus, if you had the stock air boxes, the carbs cant blow off because they are contained between the air box and engine---just sayin'.

As to your running issues, do you have an aftermarket electronic ignition? It sounds to me that you may have an issue where the timing is jumping around at idle or the control unit could be failing. The high speed miss could be coils or maybe it is connected to your idle issue. You may have a bad ground or other electrical connection.

If you are still running points, you may have a condenser that is checking out. If you are running points, the mechanical advance could be sticking and that would also cause a jumpy idle, although I don't think that is causing your particular problem.

 

[cjv2001]

I suspect that you are running pods? I have found pods to be a pain to properly tune at all RPM ranges. Plus, if you had the stock air boxes, the carbs cant blow off because they are contained between the air box and engine---just sayin'.

As to your running issues, do you have an aftermarket electronic ignition? It sounds to me that you may have an issue where the timing is jumping around at idle or the control unit could be failing. The high speed miss could be coils or maybe it is connected to your idle issue. You may have a bad ground or other electrical connection.

If you are still running points, you may have a condenser that is checking out. If you are running points, the mechanical advance could be sticking and that would also cause a jumpy idle, although I don't think that is causing your particular problem.

You are right I am running Uni pods, the shorter ones and they do seem harder to tune. having small breakup in 4-5k rpm range as well so that may be the cause to that problem. I just like the convenience of the pods haha.

As far as my main problem in the post, I am still running points. No upgrades. I saw on a post to check condensors you should run the bike with the points cover off and see if there is any arching between the points so I may give that a go. Do you think it could be a combination of condensor and coils that could be going bad? Thanks for the reply.

 

[Tiggertoo]

See: https://www.xs650.com/threads/backfiring-through-carburetor.27454/ post #8
Also you are running standard jetting with short pods, this should be making it run very lean (hence the decel popping).

 

[AUTOMAN]

How many condensers?

 

[cjv2001]

See: https://www.xs650.com/threads/backfiring-through-carburetor.27454/ post #8
Also you are running standard jetting with short pods, this should be making it run very lean (hence the decel popping).

Thank you for linking that. Will look into it more, but from reading that I am guessing I could just clip the springs a tad bit shorter to make them stronger to fix that issue? let me know if that sounds correct.

For the pods, I guess I have it backwards I thought since they were smaller they would restrict air flow and bigger one would allow more?

 

[5twins]

You are correct, smaller pods flow less air. The ones you have really don't flow much more (if any) than the stock airbox. I see you've increased your jet sizes. That may not be needed if your exhaust is still stock.

 

[cjv2001]

You are correct, smaller pods flow less air. The ones you have really don't flow much more (if any) than the stock airbox. I see you've increased your jet sizes. That may not be needed if your exhaust is still stock.

Got it. My bike is running decently well (besides the stalling at lights and decel popping) and I have sprayed around carb boots and they are replaced with the JBM industries boots, as well as have the air mix screw turned to spec leaning slightly on the rich side. Do you think it could be butterfly/throttle shaft seals? I don't really know the symptoms of that problem. Or do you think it could be more coil going bad? Thanks.

 

[5twins]

Well, when coils go bad, they normally do so when hot, but the bike usually just dies, it doesn't run rough. Have you replaced your spark plug wires and caps? They may be bad, either bad connections at the coil or plug cap, or the caps are going bad. When the caps go bad, their resistance climbs and eventually starts choking off the spark. The wire connections may be all corroded so you should re-do them. Clip about 1/4" off the end of the wire to get to some "fresh" wire then strip about another 1/8" off the wire and fan the wire strands out in a radial pattern like so .....

This will insure a good connection. For replacement caps, NGKs are the standard of the industry. You can use 5K ohm resistor caps ......

I prefer the "F" type, the kind that fits on the terminal stud, as opposed to the "E" type that fit on the terminal nut. I think the "F" type grips the plug better. But, being that you still run points, you can use a non-resistor type cap and I recommend you do. NGK makes a nice one, the LZFH ......

But yes, the butterfly shaft seals may be bad, especially if they're original. When they go bad, they don't seal and air can get sucked in. That may be the cause of your popping on decel and maybe even your random idle speed changes. Spray around where the shafts enter the carb body to test the seals. If the idle speed changes or the bike up and dies, you're sucking air through the seals.