What does the bubbleback do?

[MakoNY]

I have had engines (V4) both with and without the bubbleback. What exactly does the bubbleback do? I know it is something with the exhaust, but just don't exactly know what it does or why it's there.

All I know is that it makes it's next to impossible for me to do my T-stats. I have once stuck open, the starboard side, so that head runs cooler. I have been told not to worry about it, but I would change it if I could do it easily, which I can't because of the bubbleback.


Thanks.

 

[Bob_VT]

Re: What does the bubbleback do?

The bubbleback tunes the exhaust for the 2 stroke. It is two pieces and really not too bad to remove to get to your thermostats. The 85 usually does not gave a BB plate but the 115-140 usually do.

 

[MakoNY]

Re: What does the bubbleback do?

Bob, what exactly does, "tunes the exhaust mean"? I knew it had something to do with the exhaust, but what exactly does it tune?

When you say it's two pieces to remove, do you mean the bubbleback, or the T-stat housing, or do you take the bubbleback off to get to the T-stat housing? I don't see a way to get into the T-stat housing with any wrenches with the bubbleback there.

I have seen it mentioned on here that guys sometimes cut holes in the lower cowling to get a socket into the T-stat housing. I don't want to do that, but is there a way the lower cowling or pan or whatever it's called can come off, so that there will be more room to get into the T-stat housing?

It's a '98 115 by the way. I have been told that the T-stat housing bolts seize up a lot and if you break one there is no room to drill it out or do anything.

Any tricks to doing T-stats on a bubbleback?

Thanks.

 

[F_R]

Re: What does the bubbleback do?

Bob, what exactly does, "tunes the exhaust mean"? I knew it had something to do with the exhaust, but what exactly does it tune?

When you say it's two pieces to remove, do you mean the bubbleback, or the T-stat housing, or do you take the bubbleback off to get to the T-stat housing? I don't see a way to get into the T-stat housing with any wrenches with the bubbleback there.

I have seen it mentioned on here that guys sometimes cut holes in the lower cowling to get a socket into the T-stat housing. I don't want to do that, but is there a way the lower cowling or pan or whatever it's called can come off, so that there will be more room to get into the T-stat housing?

It's a '98 115 by the way. I have been told that the T-stat housing bolts seize up a lot and if you break one there is no room to drill it out or do anything.

Any tricks to doing T-stats on a bubbleback?

Thanks.

Exhaust tuning means that they take advantage of the sound waves to scavenge the exhaust out of the cylinders. It also uses the sound waves to pack the otherwise wasted fuel/air charge back into the cylinders at the last instant before the ports close. It's all very complicated, relating to rpm, speed of sound, back pressure and whatever. Let's just say it's engineer stuff and it is effective, one of the reasons they are able to get the large horsepower out of a small engine.

 

[imported_Jimbo]

Re: What does the bubbleback do?

Hello,

If you take a look at motorcycle using 2-strokes you can see that the first part of the exhaust-system is quite "swollen", its called an expansion-chamber. This chamber is basically the same thing as the bubble-back. In any 2-stroke a certain portion of the air/fuel mixture passes out through the exhaust-ports and the expansin chamber "turns" some of the exhaust-fumes back towards the exhaust-port which forces some of the air/fuel mixture back inte the cylinder. There is actually quite complex physics involved as F_R said, but basically it gives you more power and lower fuel consumption.

The first (to my knowledge) person to implement this principle was a East-German engineer named Walter Kaaden, he tuned a 125cc (7.6ci) MZ race-bike from 8hp in 1955 to 25hp in 1961, mainly due to the introduction of the expansion-chamber. Actually the bike was stolen by the race-driver Ernst Degner who defected to west in 1961 and sold the bike and drawings to Suzuki, but thats another story...

BR Jimbo

 

[MakoNY]

Re: What does the bubbleback do?

Wow!! Thanks for the explanations, sounds very complicated as you said, but at least now I know it's general purpose.

Was a bubbleback ever used on any other OMC engines, any of the 2 or 3 cylinders or V6's?


Anybody have any hints or tips on changing t-stats with a bubbleback in the way?

 

[F_R]

Re: What does the bubbleback do?

Wow!! Thanks for the explanations, sounds very complicated as you said, but at least now I know it's general purpose.

Was a bubbleback ever used on any other OMC engines, any of the 2 or 3 cylinders or V6's?


Anybody have any hints or tips on changing t-stats with a bubbleback in the way?

Many of the OMC motors over about 10hp and after 1968-ish have some sort of exhaust tuning. Usually it is all out of sight though.

 

[emdsapmgr]

Re: What does the bubbleback do?

I'd encourage you to replace the stat that is stuck open. A cool-running cylinder does not have the efficiency of one that runs at normal temps. Long term it can promote carbon (coking) around the rings which can lead to other problems like hard starting and cracked ringsets. The factory flat rate on the thermostat repl is something like 3 hours-enough time to pull the powerhead. Most folks don't, but struggle with clearance issues and skinned knuckles on this job. On some of the older models, you can loosen the lower cowling enough to get better clearance for your fingers. On these newer ones, the cowlings dont' move much when loosened up. A tough job for sure.