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Discussion Starter #1
Typically how much bigger do you go with your jets when you put an exhaust on your bike? I put a 2 into 1 from Dennis Kirk on my '78 Yamaha XS400. Stock size jets are 132.5 on the Mikunis. First attempt I drilled them out, it ran ok untill about 7k and it would break up, ok so too much fuel. I then bought new jets, 145 and 147.5, have the 145s in right now and its all screwed up, barely idles, won't rev, stalls if I touch the gas. I'm guessing I need to go way bigger? What size jets would you suggest? TIA!
 

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well how did it actually run with the stock jets

did you simply take the stock pipes off and replace them with an open 2 into 1?

if not, what is on the end of the header?

if you went from stock system to straight pipes

you'd should find the main jet would need to be smaller



the disconnect in the rest of what you type is the suggestion that your bike will barely idle, won't rev, and stalls if you touch the gas merely as a result of changing the main jets

solder up a set and drill them slightly under stock size if you are now running straight exhaust

but that still isn't going to fix your low speed problems

also cough up more pertinent info and details so nobody has to read your mind
 

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did you put uni pod filters on it at the same time too?

just wondering, coreyj just did and all he seems worried about is ignition timing
 

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my bike runs great w/ the mac 2 into one, and uni's
i am at a main jet of 142. i havn't messed w/ anything else. plugs looked fine for a while, but i think recently my carbs became out of sync or my float is sticking, the right cylinder is running rich while the left is running ideal.
the bike runs fine, i was just seeing if advancing the timing would add some hp.
 

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the main jet also doesn't effect idle (at least thats what i understand.) you may have an air leak somewhere, either on the exhaust side, or the intake side. both have happened to me on bikes and they do that sort of thing.
 

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actually i am at a #150 jet. (i have a book that has all that sort of stuff in it and missed the second jet up) but along the way i didn't have any running issues, just lean readings on the plugs.
 

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Discussion Starter #7
quote:Originally posted by HackAsaw

well how did it actually run with the stock jets

did you simply take the stock pipes off and replace them with an open 2 into 1?

if not, what is on the end of the header?

if you went from stock system to straight pipes

you'd should find the main jet would need to be smaller



the disconnect in the rest of what you type is the suggestion that your bike will barely idle, won't rev, and stalls if you touch the gas merely as a result of changing the main jets

solder up a set and drill them slightly under stock size if you are now running straight exhaust

but that still isn't going to fix your low speed problems

also cough up more pertinent info and details so nobody has to read your mind
The pipe does have a baffle in it, though I don't think theres really any sort packing inside the megaphone.

With stock size (132.5) jets, it did the same thing (wouldn't rev, stall, etc) I drilled them out, they were really big (way bigger than the new 145 and 147.5s I have) It would actually idle, and rev, but at 7k it would just sputter (redlines is 9), I'm thinking that was a rich condition.

I think I do have more problems then just the fuel now though. Over the winter I replaced the points also, they are set to the proper gap, but the bike is not timed. My friend keeps telling me thats not the problem but I think timing it properly can only help, right? [|)] We set it pretty much exactly where I had it before I replaced them (eyeballed the position, having taking a timing light to it yet). I think it might also help to adjust the valves, seeing as how I haven't done that since I bought the bike (put 1,000 miles on it before winter).

Another thing I've figured out is that its losing the left cylinder at idle, very intermittent spark if at all. Strong when revving, but pretty splochy at an idle. I changed the plugs and put on another coil, but that didn't seem to change anything.

Cory, I still have the stock airboxes.
 

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Discussion Starter #8
quote:Originally posted by coreyjdl

the main jet also doesn't effect idle (at least thats what i understand.) you may have an air leak somewhere, either on the exhaust side, or the intake side. both have happened to me on bikes and they do that sort of thing.
What do you mean exactly? The intake boots might not be sealing the carbs up all the way? They're brand new so I know they're not cracked. I also did use new gaskets when I put the exhaust on, and I can't hear an audible exhaust leak.
 

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check for loose jets then, maybe one of the new ones you screwed in has fallen out and that will indeed allow the main circuit to affect low speed

while you make sure you have no float problems or trash in your fuel

vacuum leaks are easy to check for

WD40 or even brake clean at possible leak spots will affect the engine speed

you really don't give enough info about what it is doing at idle for anyone to do anything but guess if it's fuel, ignition, or compression


make sure none of your plugs are fouled and don't be afraid to simply replace them if questionable but be sure and blow the chambers out if the plugs are wet

make sure your ignition is correct and you have good compression on all cylinders...... make sure valves are correctly clearanced




but the message you should be getting is that if you haven't changed the stock airbox and have simply gone to straight pipes

your main jets may actually need to be leaner than stock

I don't really know what exhaust system you installed and if it is capped or wide open
 

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check for loose jets then, maybe one of the new ones you screwed in has fallen out and that will indeed allow the main circuit to affect low speed

while you make sure you have no float problems or trash in your fuel

vacuum leaks are easy to check for

WD40 or even brake clean at possible leak spots will affect the engine speed

you really don't give enough info about what it is doing at idle for anyone to do anything but guess if it's fuel, ignition, or compression


make sure none of your plugs are fouled and don't be afraid to simply replace them if questionable but be sure and blow the chambers out if the plugs are wet

make sure your ignition is correct and you have good compression on all cylinders...... make sure valves are correctly clearanced




but the message you should be getting is that if you haven't changed the stock airbox and have simply gone to straight pipes

your main jets may actually need to be leaner than stock

I don't really know what exhaust system you installed and if it is capped or wide open
 

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Discussion Starter #11
quote:Originally posted by HackAsaw

check for loose jets then, maybe one of the new ones you screwed in has fallen out and that will indeed allow the main circuit to affect low speed

while you make sure you have no float problems or trash in your fuel

vacuum leaks are easy to check for

WD40 or even brake clean at possible leak spots will affect the engine speed

you really don't give enough info about what it is doing at idle for anyone to do anything but guess if it's fuel, ignition, or compression


make sure none of your plugs are fouled and don't be afraid to simply replace them if questionable but be sure and blow the chambers out if the plugs are wet

make sure your ignition is correct and you have good compression on all cylinders...... make sure valves are correctly clearanced




but the message you should be getting is that if you haven't changed the stock airbox and have simply gone to straight pipes

your main jets may actually need to be leaner than stock

I don't really know what exhaust system you installed and if it is capped or wide open
k I'll check that stuff..

heres the exhaust..it is MAC, but wasn't labeled as such when I bought it. http://denniskirk.com/jsp/product_c...atId=4&productId=p25640&leafCatId=&mmyId=6143
 

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Discussion Starter #12
quote:Originally posted by HackAsaw

check for loose jets then, maybe one of the new ones you screwed in has fallen out and that will indeed allow the main circuit to affect low speed

while you make sure you have no float problems or trash in your fuel

vacuum leaks are easy to check for

WD40 or even brake clean at possible leak spots will affect the engine speed

you really don't give enough info about what it is doing at idle for anyone to do anything but guess if it's fuel, ignition, or compression


make sure none of your plugs are fouled and don't be afraid to simply replace them if questionable but be sure and blow the chambers out if the plugs are wet

make sure your ignition is correct and you have good compression on all cylinders...... make sure valves are correctly clearanced




but the message you should be getting is that if you haven't changed the stock airbox and have simply gone to straight pipes

your main jets may actually need to be leaner than stock

I don't really know what exhaust system you installed and if it is capped or wide open
k I'll check that stuff..

heres the exhaust..it is MAC, but wasn't labeled as such when I bought it. http://denniskirk.com/jsp/product_c...atId=4&productId=p25640&leafCatId=&mmyId=6143
 

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quote:Over the winter I replaced the points also, they are set to the proper gap, but the bike is not timed. My friend keeps telling me thats not the problem but I think timing it properly can only help, right? [|)] We set it pretty much exactly where I had it before I replaced them (eyeballed the position, having taking a timing light to it yet).
If you had electronic ignition, you could basically get away with this. But you don't. Suppose the old points were gapped differently - within spec, but off by a few hairs in the spec range - the position of the points plate would have to change to compensate. Several degrees worth. And, because you're running a four-stroke, that crank of yours turns over twice for every rotation of the cam/timing lobe, so the difference in degrees on the crank doubles. I'd place the odds at less than 50% that you're within ten degrees of the timing marks.

You don't need a timing light to get it fairly close. Before I got a light, here's how I'd time my bike. Open up the stator and points covers and turn my bike switch on (don't start it). Pull both plug wires off the plugs and put a fresh plug on the side I'm timing. Ground the plug against the engine block while I very slowly turn the crank over with a wrench. Plug should spark right when the timing mark lines up. It's easier done in the evening (you can see the spark), but if you listen closely you don't need to see it - you can hear the spark jump.

Alex
 

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Discussion Starter #14
well I figured out my main problem of it not wanting to idle or rev. I simply replaced the old, cloth insulated wires that go to the points because they felt very flimsy. Started on the first kick and was idling perfect! I'm probably going to get some pod filters too here soon.

Thanks for the tip on timing Xander I'll try that out!
 
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