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Carb (?) problems

Old 07-04-2008, 04:16 PM
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Carb (?) problems

Since i got the motor rebuilt (last years lowside) I have been having issues with it surging, with choke on and riding. and it just wont run at WOT. I know my chain has a couple of suspect spots, but that obviously wont have anything to do with the surging when it is on choke (never surged on the choke before)

It was running like a raped ape, just screaming before I wrecked. I rebuilt and the only new item was the JE drop in pistons and the 4 deg advancer. (KN, Leo Vince, and moriwaki stg 1 cams with dynojet kit with the recommended 190/195 mains and needle at 5th position were in prior)

Now it will surge when the choke is on and it sounds like a rod knock when the surge happens. It also sound like a knock when taking off in first and second gear. No sounds in higher gears. I know it cant be a rod as it would do it all the time if it was. I have had the covers off 3 different times trying to push/pull/pry etc to see if I can find something, anything loose, but I cant find anything. Everything plastiguaged within specs when I put it together

I have the valve covers off again rechecking the cam timing. (yes the intakes are on the intake and exhaust on the exhaust side) I tried loosening/tightening the APE CCT's, but that does not change the knock sound.

It runs strong at 1/4, 1/2, 3/4 throttle, but as soon as I twist it to WOT it just bogs down and dies out. Doesnt matter what gear. I took it to a track day 2 weekends ago and 110 ~ 115 was as fast as I could get it. (before the speedo could hit ~140+) it ran great and pulls hard at 3/4 throttle, but at WOT it just wont go.

I pulled the carbs off again this weekend, and resprayed carb cleaner and looked at the floats (I dont have a float guage but they both drop the same amount) I have also been putting in a ounce or 2 of carb cleaner in the tank every time I fill it up, so if there was varnish, it should have been cleaned out a long time ago.

I tried covering 1/2 of the airfllter with tape to see what would happen, but it died at WOT, I then took the air filter off and it still bogged/died at WOT. The slow and mid speed circuits of the carbs seem to be working quite well, its just WOT that wont work.

I had the 4 deg advancer on it, but took it off to see if that made a difference, but it didnt.

I still have my trailer park flex pipe on the left mid pipe where it was crunched. I could see where that would be a issue at a specific rpm. But, the bike will bog at any rpm that I twist it to WOT. ie: if I punch it in first, it will be 5500rpm when it dies. If I get it up to 7000rpm at 3/4 throttle, then WOT, 7000rpm is where it dies at WOT. So I cant really see it being the exhaust doing it at a range of rpms. but it is possible I guess. (I received the wrong replacment pipe and now am waiting for the new one to come from Italy)

So, has anyone put in JE drop in pistons and experienced carb issues? or experienced any issues like mine? Id pull my hair out, but I dont have any left on top
.
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Old 07-05-2008, 03:07 AM
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1. The high compression pistons will necessitate premium fuel. Do you have 93 octane gas in the tank? If not , this could account for the "knocking" sounds you hear. This would be very BAD.
2. As for the full throttle bogging out, I would imagine that with the new pistons sucking in more air through the same carburetor with each intake stroke, thus causing an increase in air velocity, that a SMALLER MAIN JET will be needed. Other members have gone your route before, and will surely give suggestions for you. I remember reading something about mains going smaller than stock sizes? At any rate, you seem have a massive rich condition on the main jet circuit.
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Old 07-05-2008, 06:03 AM
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Some plug reading would help here, a pic of the plugs after a WOT pull would tell more about your problem.
You may just run pig rich as said above, but can't explain the knock sound.
You may enter heavy detonation when you go WOT or worse pr ignition. Detonation will not bog down the engine as much as pr ignition and will generate a hell of a rattle trying to compress an already burning mixture.
Remember that the Hawk fires twice per cycle, the wasted spark 30BTDC should not ignite any fuel, but with different cams phasing, I'll check the intake opening point to make sure it's still closed by the time the plug fires it's wasted spark.
Why at WOT, gases velocity pull more air at overlap, it sould concentrate around torque peak (highest VE)
As for the choke it make no sense with pr ignition, it may be a different problem, that make it so hard to troubleshoot.
Good luck, feel free to ask more questions I like engine dynamics.

Question, is the WOT problem worst with the choke on once you past the initial bogging ??

Last edited by gboezio; 07-05-2008 at 06:06 AM.
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Old 07-05-2008, 02:09 PM
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one more thought

It is possible that you have the valve timing out of phase? The crankshaft turns 720 degrees in order to completely cycle the cams. The front cylinder should follow the rear by 450 degrees, but it is very possible to set the front to follow at 90 degrees crank rotation. This has been discussed more than once here in the past.
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Old 07-06-2008, 07:56 AM
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Originally Posted by BeerHunter View Post
It is possible that you have the valve timing out of phase? The crankshaft turns 720 degrees in order to completely cycle the cams. The front cylinder should follow the rear by 450 degrees, but it is very possible to set the front to follow at 90 degrees crank rotation. This has been discussed more than once here in the past.
Got to sound good
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Old 07-06-2008, 01:19 PM
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Not mentioned in any post above is the TPS. I wonder..........
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Old 07-06-2008, 01:55 PM
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Thanks for the replies.
I use premium gas all the time. I wondered about the cam timing, but IIRC, if it is out, it runs like a 1 cyl ?? It runs great untill I hit the WOT circuit on the carbs, thats when everything goes to hell in a handbasket.

I wish I had the gastester (on order) and my midpipe(on order)

I will try to go down on the main jets 1 size and see what happens.

Going WOT the shutting down and checking a plug sounds like agood idea.
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Old 07-07-2008, 02:33 AM
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I have the same problem with WOT and still struggling to find whats wrong. I bought the bike this winter so I had no possibility to take it for a test drive and now I am not sure if the problem came with the rebuild to Street fighter or if it was there before. I have heard that the cct:s were replaced after break down by the earlier owner, and thinking that the cam timing could be wrong.

I will follow this thread closely hoping to get some good ideas!
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Old 07-08-2008, 04:40 PM
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Well, I went down from 190/195 to 185/190. That helped alot, so maybe this weekend I will go down to 180/185. I will probably try to go down on the pilot jet to. I went to the dealer, and of course, there are 2 possible styles

So progress is slowly getting made
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Old 07-08-2008, 06:19 PM
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Originally Posted by BeerHunter View Post
...I would imagine that with the new pistons sucking in more air through the same carburetor with each intake stroke, thus causing an increase in air velocity...
I had to chuckle at this one. It's still 1000cc's so it's drawing in the exact same amount of air as before. It's just squishing it into a smaller space on the compression stroke.

Back on topic - I had a similar thing happen one time when my cams were out 360 of crankshaft rotation. You need to check the cam timing on both heads at the same or you will never find this. I learned that the hard way!
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Old 07-08-2008, 07:34 PM
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Originally Posted by JamieDaugherty View Post
I had to chuckle at this one. It's still 1000cc's so it's drawing in the exact same amount of air as before. It's just squishing it into a smaller space on the compression stroke.
I seem like it is, but the difference is in the Volumetric Efficiency, a literbike don't pull a full liter, let's say that the engine is working at 80 % VE at torque peak 1000cc x 80 % = 800 real cc

If you increase VE by putting an exhaust to 85 % for example, each cycle will draw an extra 50 cc of air, specially at higher rpm where the VE drops rapidly.



AS for the knocking sound, there were no harm then, a pig rich engine will not knock or pre ignite, mostly crap the plugs with carbon


Edit : I just read this thing again and I realized that it was a piston question, the pistons only don't affect the VE, induction and exhaust do.

Last edited by gboezio; 07-09-2008 at 06:42 PM.
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Old 07-09-2008, 07:59 AM
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I'm with Jamie on this. Only changing to a high compression piston won't increase the charge velocity. If anything, it would reduce it. Most high compression pistons reduce the displacement slighlty, so a 996 might be a 995. Changing the intake runners and maybe a different cam profile would be the direction I'd go.
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Old 07-10-2008, 06:10 PM
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My Gunson Gastester will be delivered on the 16th

I might just wait for that before experimenting *****-nilly.
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Old 07-15-2008, 04:39 PM
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Gastester arrived today

Here its warmingup and calibrated to 2%



at idle:



~2500rpm



~3000rpm



~4000rpm

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Old 07-19-2008, 06:04 PM
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Well, put 42 slow jets in. At idle, it was 2.2% co, but at 2500 it went down to 1.1%. so I took carbs off ~again~ and put the clips of the needle on the 6th slot (they were on the 4th) the co went to around 4% at 2500 rpm, so I left it there and went for a ride

nice power now

It still has a knock like sound when first starting out in 1rst and 2nd gear, unless I really let the clutch slip.?
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