1/4 throttle stumble

[Kapoor]

When does the back fire occur?

So i set the carbs back to 2.5 turns out and there's definitely a problem with something. I mean you can hear it in the idle. Here a video. listen for the popping.... and no i don't have a popcorn machine somewhere in my garage, haha... sigh

http://www.youtube.com/watch?v=SLqIKcgok6E

haha I just noticed the video is sideways, sorry about that. anyways it's just for the sound

So Is this a tuning problem in the exhaust cams? Honestly if i'm remembering right this began about a day before i monkeyed with the needles or increased the main jet size.

Also I bought 4 pilot jets that were one size up from the ones i got in there. I had 42.5's and i bought 45's however i don't know if it would be best to resolve the new-ish popping/backfiring problem first then increase the pilot jetting or if i should just do it all at once?

[chef1366]

Sounds like your missing. Like firing every other stroke.

[Kapoor]

Sounds like your missing. Like firing every other stroke.

So something electrical? I pulled the plugs and checked, they're all milk chocolate brown (that's the color we're looking for right?) I'll try to trace this backwards to the source. If it is electrical have a feeling it's probably going to be somewhere deep in my wiring harness. The thing is pretty ugly, and wrapped in electrical tape.

Where would be your first suggestion to look? I install all new plug caps, coils, and plugs like 400 miles ago.

[chef1366]

unplug each plug wire until the popping stops That'll peg the cylinder.

[Road_Clam]

So something electrical? I pulled the plugs and checked, they're all milk chocolate brown (that's the color we're looking for right?) I'll try to trace this backwards to the source. If it is electrical have a feeling it's probably going to be somewhere deep in my wiring harness. The thing is pretty ugly, and wrapped in electrical tape.

Where would be your first suggestion to look? I install all new plug caps, coils, and plugs like 400 miles ago.

Have you at any point done a compression test ?

[Kapoor]

Have you at any point done a compression test ?

Haven't yet, didn't have the gagues that I thought I did. I'll have to wait till this next pay check (6days and counting) and pick up some metric ones. Only set that are avalible to me are for chevy big blocks (a little big for this bike, haha).

Anyother measurments that I should be taking periodically anyways? How about special tools like the compression gagues? Also I don't know how many GSR members are in Spokane or even close by but if you had a set of gagues that I could borrow or rent that'd be awesome also!

[Road_Clam]

Haven't yet, didn't have the gagues that I thought I did. I'll have to wait till this next pay check (6days and counting) and pick up some metric ones. Only set that are avalible to me are for chevy big blocks (a little big for this bike, haha).

Anyother measurments that I should be taking periodically anyways? How about special tools like the compression gagues? Also I don't know how many GSR members are in Spokane or even close by but if you had a set of gagues that I could borrow or rent that'd be awesome also!

Your #1 diagnostic method should always be check compression first (or at least early in the chase). If you have a "bad hole" you are obviously chasing your tail as far as jetting changes.

[Kapoor]

So I did found a place with a set of gauges that fit my bike, and got some pressure readings. I did the test 3 different ways, not really knowing which was the correct way for a bike. but I figured it couldn't hurt to know what's going on anyways. So Here they are:

Engine Temp / Cyl. # 1 2 3 4
Cold 115psi 135 115 120
Warm 130 130 130 135
Warm/Wet 140 150 140 150

So the I did the cold test then read my Clymer book on proper procedure which it said to check it after warming the engine with all of the plugs out and the throttle wide open. So I don't know honestly if a cold compression test means anything, but there's my numbers. Anyways what the wet test was after adding ~0.25oz (7.5ml for those who prefer metric) of oil to the head then retesting the compression. What the book said about this was to determine if the valves are the problem or if the rings are the problem. A large gain after adding the oil would suggest poorly set rings. Also it said the pressures should be with a maximum difference of 28psi and between 128psi-178psi. So from what I read it would seem that the rings are (for the most part) fine and compression (even though not towards the top of the range) does fall within range. So What do you all think of these numbers? Does my logic check out or is there more to it than just that? Also if the numbers do check out should i then carry on and increase the pilot jet size from a 42.5 to the newly purchased 45's?

Oh also, I was on a bit of a trip the last few days, and found a radioshak with the assorted washer packs. Looks like a few of these will work perfect and there's 20 of each size so I'm set!

As for pulling the spark plugs and checking to see which one causes the popping to stop, i haven't done this yet and realized it once the tank was off

[tkent02]

So I did found a place with a set of gauges that fit my bike, and got some pressure readings. I did the test 3 different ways, not really knowing which was the correct way for a bike. but I figured it couldn't hurt to know what's going on anyways. So Here they are:

Engine Temp / Cyl. # 1 2 3 4
Cold 115psi 135 115 120
Warm 130 130 130 135
Warm/Wet 140 150 140 150

So the I did the cold test then read my Clymer book on proper procedure which it said to check it after warming the engine with all of the plugs out and the throttle wide open. So I don't know honestly if a cold compression test means anything, but there's my numbers. Anyways what the wet test was after adding ~0.25oz (7.5ml for those who prefer metric) of oil to the head then retesting the compression. What the book said about this was to determine if the valves are the problem or if the rings are the problem. A large gain after adding the oil would suggest poorly set rings. Also it said the pressures should be with a maximum difference of 28psi and between 128psi-178psi. So from what I read it would seem that the rings are (for the most part) fine and compression (even though not towards the top of the range) does fall within range. So What do you all think of these numbers? Does my logic check out or is there more to it than just that? Also if the numbers do check out should i then carry on and increase the pilot jet size from a 42.5 to the newly purchased 45's?
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Nothing at all wrong with your compression, those are good numbers.

[Kapoor]

Alright, Well that's a relief. I figured since it's my first engine build that I probably forgot to do some crucial little step and buggered myself from the get go. I'm gonna play with the carbs some more and see if anything gets better or worse, then once they're stable-ish I'll take a peak at the valve adjustments again. Maybe I missed something when I went through and adjusted everything the last time i was in there. Once again thanks for the quit reply this really expedites my build!

[Kapoor]

Thank you thank you thank you. Tonight I threw in those pilot jet 45's replacing the old pilot jets which were apparently size 42 (even smaller than I originally thought), and now all my throttle problems are gone! I guess the combination of increasing the main jets, and the pilot jets, as well as some fine tuning and everything is good to go...

Except for the backfiring issues. So tomorrow I'm going to check the valve clearances and see what's going on. Currently the mixture screws are 2.5 turns out with the needle shimmed from ~2.56mm (stock) to ~2.07mm, with 117.5 main jets, and 45 pilot jets, and stock everything else. Any ideas where to start to look other than the valve clearances? Also should I start a new thread about this new problem? And could it be from fouled up plugs?

[chef1366]

How many shim washers on the needle? Was I close?
Glad you got it sorted
Could you have an exhaust leak?

[Kapoor]

How many shim washers on the needle? Was I close?

It took 4 of the radioshack washers. I think they're ~0.5mm in height each. So just a little more gas than stock.

Glad you got it sorted
Could you have an exhaust leak?

You know that's totally possible, when I got the bike the exhaust was missing a clamp where the 2 went into 1. So I'll give that a good look over as well. That'd be nice if that was the only problem. Thanks for the idea. Wouldn't have thought of it otherwise.

[Road_Clam]

Could you have an exhaust leak?

Exactly what I was thinking. Those copper crush gaskets are prone to air leaks and poping especially if re-used ! Also, a rust hole underneath near the pipe welds and slip joints is another location prone to leaking. Water condensation sits inside the exhaust pipes causing them to rust.

[Kapoor]

Exactly what I was thinking. Those copper crush gaskets are prone to air leaks and poping espacially if re-used ! Also, a rust hole underneath near the pipe welds is another location prone to leaking. Water condensation sits inside the exhaust pipes causing them to rust.

Well when I rebuilt the engine I used all new exhaust gaskets so I wouldn't think it'd be because of them. Also this morning I realized that the sound does sound like it's coming from that faulty spot in the exhaust. I think I need to replace the whole thing since they're pretty roughed up from the last owner (let to rust, lots of missing chrome on the headers, dents all over the underside, and was on the bike when he laid it down) So this problem just might accelerate me buying a new exhaust, I want a 4-1 system but which performs the best and sounds the best? Honestly I'm not sure if I care if it's legal (loud is alright, keeps me from being hit on the road; but real deep like a ducati would be amazing). What do you suggest? Also if I get a new exhaust do I probably need to rejet again to compensate for the difference in pressure?