Backfire/pops in muffler

[kellymont]

Hi over there,
I am having problems with my 81`Midnight Special, it backfires in the mufflers and sometimes a flame can be seen. The carbs have been cleaned and set,
I have checked the ignition timing with a strobe light, checked the carb rubbers, set all the carbs with a 4 x vac guage and they are all the same. It does not happen when it is still on choke or when the engine is under load but as soon as the engine warms it happens, I have done a compression test and all 4 check out good the situation improved when i fitted new plugs BP8ES instead of the BP6ES but it still backfires and pops in the mufflers. I have noticed that numbers 2 and 4 exhaust from the engine are hottest and 1 and 3 are coolest.
Do you think it could be carb- mixture settings?

Anyone with any ideas please

Steve in England

[John]

Popping in the exhaust can sometimes be caused by air entering the exhaust via a leak. That can allow enough oxygen to promote combustion of the hot exhaust gasses. Check to see if the header pipe to head gaskets are leaking and to make sure the nuts securing the exhaust pipe flanges are tight. Check for other leaks at the junction of #2 & 3 header pipe and the mufflers.

[eleven_special]

Is your exhaust system 2 into 2 or 4 into one?If 2 into 2 are they factory original with cross over pipe? On my 81SH I had to mig in a cross over pipe between two slip ons to give it balance to keep it from popping.I always wanted a midnight by the way.:-)

[kellymont]

Hi again,
I have sealed the exhausts at all of the joints but it has not helped, it is a 2 into 2 system but not the original mufflers and it has no balance pipe.
When I brought the bike over a year ago it did not have this problem, but I did not ride it at all because it needed some minor work doing and it was winter so I decided to strip the carbs down clean and set them up, I am sure that the carb floats are set correctly as I have had them off and checked them twice now.
I have tried another set of ignition coils but it behaved exactly the same and I do not think it is an ignition fault, I think it must be mixture setting somewhere in the carbs. It stops me from riding the bike because of the noise.
Is there someone on this site who knows how to set up the carb mixtures correctly.

Thanks Steve in England

[randy]

Have you done a valve adjustment. Playing with the mixture and synch won't help much if the valves are out.

[TopCatGr58]

Originally posted by kellymont
Hi again,
Is there someone on this site who knows how to set up the carb mixtures correctly.

Thanks Steve in England

Well Gosh Steve,

Let's see.....your bike, an 81LH(MNS) didn't "pop/backfire" when you got it. THEN you tore down the carbs to clean them. Now it does pop/backfire.

WHEN you tore them down, did you soak the carbs bodies in carb cleaner? IF so, you may have damaged the butterfly seals causing a lean condition due to a vacuum leak?

The 81 carbs come with the pilot "Screws" sealed under brass caps on the top front of the carbs. Did you drill these out, and remove the screws for a THOROUGH cleaning of the pilot circuit?
Also, without these caps removed, you can't adjust the pilot screws to set the proper mixtures!!

You're running aftermarket 2-2 pipes, which breath better than the stock ones, which can also contribute to a lean condition.

You have set the floats, and have synched the carbs, but IF that pilot circuit isn't clean it'll behave poorly. You've described cooler pipes on Non-linked cylinders, coils not in common, fuel supply not in common, so it comes back to the carbs themselves.

Also, have you checked the Intake Manifolds for leaks? Is your vacuum advance hose connected to the correct port, the metal one on the carb body, NOT the port on the intake manifold?

Have you done the cam chain tensioner adjustment?

There are a lot of variables that can contribute to the settings you need for your specific bike/engine, it's not as simple as just turn this screw 2 turns out, and viola!

[kellymont]

Hi Randy,
Yes done a valve check they all had the correct gaps, and I have done a compression test on all 4 pots and they were all above 145psi.

What can it be?

Thanks Steve

[kellymont]

Hi TC,
I did not clean the carbs in any kind of carb cleaner, the carbs were clean inside so I gave them a check around and set the floats.

The butterflys are brass and seal against the aluminium bore of the carb body (or am I missing something?) these were all in great shape and I am sure I have not damaged anything.

There were no brass caps over the pilot screws, these must have been removed by a previous owner.

But everything still points to something wrong with the carbs!

Thanks Steve

[GNEPIG]

.. TC has some very good ideas, have you tried spraying break cleaner around the intake manifolds, while it idles? and around the butterfly shafts?. if the engine revs up or wants to die, then you have an intake leak.. you have said something about relocating the bike from somewhere else, is there a big difference in the altitude of where you live now? you may need to rejet.
.. if you bike back pops heaviley on decelleration, then you are running lean.
.. how do your plugs look?

[KJK]

Steve, Here is my two cents. When I drive my 81SH on short trips at low speed it also backfires as yours. I found that a good hard ride down the highway makes the bike sound better.

Kevin

[sleazybear]

I had an xs400 that I got a ticket for because it backfired when I shifted and passed a cop, what the yamaha shop found was a whole in the exhaust that was there to let water drain out, might not fit your aplication, but sinces its a backfire thread thought I might post it, because its just a little round hole in the pipe.

[prometheus578]

"Sounds like fuel to me."

I did not clean the carbs in any kind of carb cleaner, the carbs were clean inside so I gave them a check around and set the floats.

What does this mean? Removed the float bowls, looked fairly good, so set the floats and were done with it? get a can o' spray carb cleaner and have at it, again. Remove the pilot screws and make sure everything is clean through and through. All channels and passages clear. Pull the jets, etc and blow them out.

The butterflys are brass and seal against the aluminium bore of the carb body (or am I missing something?)

What T.C. was referring to was not the butterfly's themselves, but the rubber seals on the butterfly shafts where the shafts go through the carb bodies to the linkages. A lot of people soak their carbs in a bucket of harsh cleaner and that screws up those rubber seals and they leak. You stated that you didn't soak the carbs, so they should be OK.