1/4 throttle stumble

[Kapoor]

Not a PDF, per-say, but I think you'll find the following links rather useful:

http://members.dslextreme.com/users/bikecliff/
http://members.dslextreme.com/users/bikecliff/images/carb_sync_notes.html
http://www.iwt.com.au/mikunicarb.htm
http://members.dslextreme.com/users/bikecliff/images/carbspec_float_height.html

That's a pretty good primer in carb theory and tuning. Also, when you rebuilt them, you rebuilt them using this guide, right?

http://www.mtsac.edu/~cliff/storage/gs/mc_maint/GS_CV_Carb_Cleaning_Series.pdf

I know it's a no brainer, but I'd also wager that you replaced all those thirty year old o-rings with pieces from Mr. Robert Barr, correct?

http://www.cycleorings.com/#BS_Series_O-ring_Kit

Buy a bunch of spark plugs for your chops. Use a new set for each circuit, if you can swing it. That's a lot of plugs to buy, but you should be able to reuse the plugs after you get the tuning dialed, thus lowering your maintenance cost. Nice new plugs make it real easy to read the A/F ratio. A Colortune is also an option. Also, make sure you sync the carbs before trying to tune them.

Thanks again! As for using the guide I did not, however after flipping through it I seems I had preformed many if not all of the same steps just in a different order. Also about the O-Rings the kit came with all of the rings but the fuel line T-adapter o-rings, but a quick trip to lowes and I was good to go! I guess the only ones i didn't get to was the choke o-rings!

On to the next topic plugs! So $5-6 each sound about right or really expensive? picked up my last set at a local shop which usually means more expensive, but the shipping online (and the wait! ) didn't make it seem worth it. Also after reading BassCliffs plug chop method it appears i need 3 NEW sets of plugs then? So we talking about $60 in plugs?! lol

As for the guides I seemed to have figured out how to sync the carbs fairly efficiently using a homebrew rig made from nylon tubing, some motionpro carb sync adabtors, a meter stick, and some ATF. I also had stumbled upon the carb theory 101 guide about 4 days ago and have read it multiple times, and will probably have to read it a few more! Haha, only thing I haven't done is the plug choping which is something I've begun to look into as my next mini project. Thanks again for the great resources!:dancing:

 

[Steve]

On to the next topic plugs! So $5-6 each sound about right or really expensive? picked up my last set at a local shop which usually means more expensive, but the shipping online (and the wait! ) didn't make it seem worth it. Also after reading BassCliffs plug chop method it appears i need 3 NEW sets of plugs then? So we talking about $60 in plugs?! lol

Sounds a bit high on the price there, but I am not home to check my receipts.

No, you only need ONE new set of plugs. You don't actually CHOP the plugs, as in cutting them up.

You simply install new plugs so there is no color on them at all. I start with the pilot circuit/low-speed operation first. Actually, you will want to warm up the engine on the old plugs, then install the new ones for the tests.

Idle around the neighborhood, keeping the throttle as closed as you can. You might need to to this for a full minute or two. Use higher gears. Yes, ou will be doing 20 mph in fifth gear, but that's what keeps the throttle opening very small. Hit the kill switch and pull the clutch, coast to a stop, hopefully in a safe area. Pull the plugs, read and record their colors. Repeat this at 1/4 throttle. You won't have to go quite as long, maybe just one minute. Helps if you have a small hill to climb to keep your speed down at 1/4 throttle. Again, CHOP THE THROTTLE, hit the kill switch, pull the clutch, coast to a stop, read the plugs. Repeat at 1/2 throttle, then full throttle. Fortunately, the full-throttle run will color the plugs quickly, you will only need one good run to about 70 or 80 mph, then kill the engine and coast to a stop. Keep in mind that if you let the engine idle as you coast to a stop, that test is WORTHLESS.

Now you can either keep the new plugs in the bike or go home, put the old plugs back in and keep the new ones for spares.
No sparkplugs are destroyed in the process.

.

 

[Turtleface]

Must picked up two sets of plugs at Autozone, each plug was just under $3 bucks. NGK D8EAs.

 

[Kapoor]

Hey thanks about the autozone tip, and letting me know I didn't need a set of plugs for each chop. Lol and I figured I wasn't actually 'chopping' them, just didn't know if it was best for each curcuit to have it's own set.

So currently I have the new 117.5 main jets in there, and I went back to the 2 washers. Before I went for a ride the air fuel mixture was way off and it took a bit to get her to start. So I referedto Mr. Basscliffs page for the best method of adjusting the mixture screws, ended up using the highest-idle method. Got that squared away and took her for a quick spin.

Even though it's a small change I like the new tiny bit of power the jets added, or if they didn't add any tangable amount of power I like a better running bike that's for sure! Everything felt great, there's still however a tiny tiny blip in the acceleration, so I think I'm going to find a washer that is just slightly smaller than the ones in my stack and try splitting the difference between 2 and 1 washers. I think I got some somewhere. Oh and another quick question. I recently have been getting a backfire from the engine while I ride is this an adjustment that needs to be done to the intake and exhaust cams? And when I do adjust them where in the range of factory spec should I adjust them to? Like the tightest, or middle of the range?

 

[KDS111]

Can someone go into detail please the procedure on shimming the needle.
This can be done with the carbs on the bike?

 

[Guest]

Can someone go into detail please the procedure on shimming the needle.
This can be done with the carbs on the bike?

Take the needle out of the slide, remove the plastic spacer and replace it with a lesser heigth of washers. This raises the needle and richens the needle circuit.
Look for the small assorted washers at Radio Shack.

 

[Guest]

When does the back fire occur?
Try Radio Shack for the correct washers.

 

[Kapoor]

Well RadioShack out by me doesn't carry very many washers at all , so i went to Lowe's and home-depot but they didn't have anything small enough either. I'll try a few more RadioShack's in the area and maybe I'll get lucky.

The backfire occurs after rolling off of the throttle and letting the engine slow you down. I've known this to happen on a few trucks, but never really a bike (then again I really don't have the experience to speak for either). So is this something to do with the lifters on the exhaust cam getting stuck or are out of tune?

Also currently there's another problem. After installing the 117.5 mains the bike had been running rough, and after adjusting the mixture screws is was still running really rough. So I tried to remove one of the washers, and it runs horrible. I'd say as bad as it did just before I started to shim the needles. But honestly it sort of reminds me of when the carbs were first out of sync, so I'll have to do that tomorrow when I get off work. I guess my question is: After jetting your carbs should you have to resync them?

 

[Road_Clam]

I don't really favor shimming the needles as an only adjustment. I found that bone stock GS's respond very nicely by bumping the pilot jets one size richer. Pilots primarily help from idle to 1/4 throttle, but what most people don't know is pilots continue to feed fuel througout the entire carb range. If you are truly at 1/4 throttle you might be just at or beyond a pilot jet change, but still something to think about.

Here's a trick to tell exactly what throttle position you are at without "guessing". Wrap a piece of masking tape around your Throttle switch housing. Stick a sewing pin with a red knob in your grip flange up high where you can see this. Mark the tape with a line corresponding to the pin. This is "idle". Now twist the throttle WO, and mark the tape again "WO" . Now divide the remaining incraments to 1/4, 1/2 and 3/4 between your baseline established throttle positions. Now as you are road testing your jetting you can look at the throttle position and precisely tell what carb circut is not performing correctly

 

[Kapoor]

Alright so I see a problem in my descriptions now. First off I left out that I did indeed do the thing with the tape before posting this thread (and I swear it's at exactly 1/4 throttle) and so my last description of how it was running was poor. It's running really rough as in stumbles at 1/4 throttle again, the idle is not as smooth as it once was, and just just sounds off. I'll look around for the next step in pilot jet size, and maybe pick them up before I get off of work. I don't see this being a problem because I believe once I get a new exhaust I'll be jetting the carbs again anyways. However I think I'll attempt to sync the carbs again (or at least check them) before I install the new pilot jets.

 

[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?

 

[Guest]

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.

 

[Guest]

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?
|

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!