Exhaust gaskets and back pressure

brasco18 · Posted: June 19, 2007, 10:49 am Post subject: Exhaust gaskets and back pressure

This may be a stupid question but I recently got some exhaust gaskets from mikes xs and I don't know where they go. I thought they went between the header and the exhaust port but apparently not. Are they suppose to go between the head and muffler? If so that sux since the threads on my mufflers are rusted away and can no longer properly seal them. My other question is if I have a serious exhaust leak between the header and muffler (which I do, on both pipes) what does this do to performance? I ask because I am having a hard time getting the mixture to thin out. My gas mileage seems to be at about 30mpg which seems low.

Xumi

Here's a parts diagram.. should help you figure out your gaskets.
www.biker.net/77-79_65...0main.html

An exhaust leak will always lean out an engine. Lose the backpressure, and the engine breathes more (pulls more air = lean).

As far as tuning goes.. fix your exhaust leak and things will be a little clearer.. But in general - what do your plugs say? Are they black, tan or white (rich, good, lean)?

Are the jets stock? What about your intake? Need more info.

xsjohn

Maybe you could re-thread the head exhaust bolts a tiny bit smaller size...??? Is it the bolts or the nuts that are bunged....Not sure you can get the header bolts out without trouble...Maybe someone else has done it.......

Should always use antiseize on the bolts and spark plugs......As a matter of fact I use it everywhere......Yes even there......

John

brasco18

Xumi wrote:

Here's a parts diagram.. should help you figure out your gaskets.
www.biker.net/77-79_65...0main.html

An exhaust leak will always lean out an engine. Lose the backpressure, and the engine breathes more (pulls more air = lean).

As far as tuning goes.. fix your exhaust leak and things will be a little clearer.. But in general - what do your plugs say? Are they black, tan or white (rich, good, lean)?

Are the jets stock? What about your intake? Need more info.

Leans it out you say... I can't imagine how rich it is going to be then when I do seal those pipes. My plugs are telling me it is running rich. I keep adding air via the air screw but it doesn't seem to lean out. I have stock jetting I believe. I got new jets recently from Mikesxs in a carb rebuild kit. My intakes are stock as well. I have stock oil/foam air filters and air box.

Xumi

brasco18 wrote:

Xumi wrote:

Here's a parts diagram.. should help you figure out your gaskets.
www.biker.net/77-79_65...0main.html

An exhaust leak will always lean out an engine. Lose the backpressure, and the engine breathes more (pulls more air = lean).

As far as tuning goes.. fix your exhaust leak and things will be a little clearer.. But in general - what do your plugs say? Are they black, tan or white (rich, good, lean)?

Are the jets stock? What about your intake? Need more info.

Leans it out you say... I can't imagine how rich it is going to be then when I do seal those pipes. My plugs are telling me it is running rich. I keep adding air via the air screw but it doesn't seem to lean out. I have stock jetting I believe. I got new jets recently from Mikesxs in a carb rebuild kit. My intakes are stock as well. I have stock oil/foam air filters and air box.

Double check your jet sizes to make sure they are stock - perhaps the wrong sizes were included in the kit?

Also some of these carbs have a rubber boot over the pilot.. if that is missing it will not meter the pilot circuit correctly.

And - make sure that your jets are seated fully, and that your float height is correct. Any of those will cause a rich condition.

brasco18

I'm pretty sure my float height is off. The book I have says 23mm but other specs say 25mm. Either way, I set the height with the weight of the floats on the needle valve. I have heard this is the wrong way to do it. It should be done with the floats just barely touching the needle valve, I have heard. If the floats are floating too high, this means too much gas is in the bowl, right? If this happens it spills over and drains onto the ground, correct? Because that would explain why I have to always turn my petcocks off whenever I let the bike sit 'cause otherwise gas leaks. And you say this could cause a rich mixture? I know have a 122.5 pilot or main, which ever that makes sense for. The other I am not sure. I will have to check. My carb doesn't have the rubber boot over the pilot. My jets are seated fully, I am sure. When I get a chance I will check my plugs again. I have fiddled with it a bit and haven't checked it since then. It still sounds like it is running rich though.

Xumi

Weeeelll - that sounds like it is probably your problem in this case.

Basically, the float controls how much gas is available to the jets... Too low, and you starve for fuel. If too high, you will overflow the bowl, and raw (unmetered, unmixed) gas goes into your carb body - when the engine is running, it gets sucked into the engine along with the mixed fuel, probably causing rough running, and definitely causing rich running. When the engine is off, it just drips out of the carb, typically into your air filters, or possibly into the engine (oil will smell like gas, oil level high.. this is bad. If this happens, change the oil).

Do a search for float height here and you'll find some good techniques for setting them - I believe xsjohn has an excellent writeup on the subject.

brasco18

thanks xumi. I will have to spend some time and do these things. I know this isn't the topic of exhaust but, correct me if I'm wrong, the air screw on a '77 carb... turn out = more air? I just caught wind of another topic and someone mentioned the opposite of this.

Xumi

No problem...

I read your other post about the air fuel mix...

In every bike carb I've seen so far, the pilot screw meters fuel to the pilot (idle) circuit, not air... so as you open the screw, you are letting more fuel into circuit, making it more rich. As you close (tighten) the screw, it puts less fuel into the circuit, making it more lean.

Something to remember: When your carbs are pretty far out of whack (technical term there) or something is malfunctioning (like floats...), symptoms can be pretty strange. Stick to your plug colors, and keep jumping into them - eventually you will get it tuned. It takes patience, and more patience. Tear it apart, change something, test. Repeat.

The worst I ever had was my Vulcan when it tore a diaphragm.. That was easy to diagnose. While I was waiting for parts though, I moved some stuff around on my workbench, so when i put them together, I put the wrong needle in the wrong carb.. what I didn't know then was that the needle taper is different on the rear cylinder than the front on that bike. When I got the new diaphragm and put it all back together, it would idle perfect, but hit the throttle and it would backfire, carb fart, and die... Like it was horribly fuel starved. I could watch the slides work - but really erratic, like one of them was still holed... I tore those carbs apart, cleaned, balanced at idle, adjusted the fuel mix screws until it was kinda running better.. like if I'd just tweak it a little more perfect it would be fine.. but no.. it just would not run right. Took 'em apart again.. clean again.. inspect everything again.. Put them back together... same ... Take 'em apart AGAIN... Tore everything apart top to bottom, soak the carbs... And that time I did something different.. I was in such a bad mood that I didn't put the parts into separate containers.. When I started putting it back together I started thinking "oh crap.. what if one of the parts is different between the carbs?" - look in the parts diagrams.. Sure enough. Needles were different. Put it back together right and WOOHOO! It worked.

So.... Be patient. Verify that everything works. Clean them thoroughly, then clean them again. Keep the rubber bits out of the cleaner. Set your float height.. just keep playing with it. You will get it, and then you'll know your carbs better than you ever did.

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