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Old 11-15-2010, 02:23 AM   #1 (permalink)
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Default Carburetor Gurus. Won't rev.

I’ve got a problem on my ‘90 ZX7 project bike. I’m about sure it is a carburetion problem. The bike has been sitting for several years. I pulled the carbs apart and cleaned them very carefully according to a “How to” I found on a forum. Everything was checked. I was very thorough. I measured float levels, set the mixture screws and took apart and cleaned everything I could. Put them back on the bike, got them to prime and was finally able to fire the bike on it’s own gas. The bike cranks great and once warm enough to get off of the choke it idles good and smooth at about 13-1500. The problem is that it won’t take any gas.

From idle I can smoothly rev it up and it acts like it runs out of gas by about 4000 or I can blip the throttle and hold it and it may or may not make it to about 4500. Either way if I try to hold the higher rpm the bike dies. It cranks right back up though. The first time I blip the throttle and release it the revs go to about 4000 and fall quickly back down to idle. If I blip it again right after idle settles then it will only go to about 3000. Blip it again only to about 2200. The next blip kills the engine again. .If I let it idle a minute between blips it is okay and seems normal.

If it was an electrical problem I think it would happen at a certain rpm and not all over the place so it has got to be a fuel problem, right? The carbs seem to work okay on the pilot jets but it's my guess the main jets are not taking over. What could cause this? If the carbs just needed synching then I could see it rev roughly but stay running. This acts just like running out of gas, immediately after you touch the throttle. The bike does not smoke and does not smell rich.
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Old 11-15-2010, 03:34 AM   #2 (permalink)
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too bad you're not closer..

sure theres no gas in the bowls?
will it rev on choke?

make sure you don't have any vac leaks or anything as that'll piss the bike off.. you may have to go back thru the carbs tho if they were really bad. i spent a solid week once on an ST1100.. went thru those fuckers 4 times before getting the stumble gone and everything perfect.
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Old 11-15-2010, 09:49 AM   #3 (permalink)
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There is fuel in the bowls. Crack the drains and I get a good flow. The fuel pump seems to be working fine too. I have a couple of canister filters before the fuel select and it will empty those out quickly if I tilt the tank and uncover the supply lines. It will rev with the choke after it warms up a bit but that is with using the choke lever. Will not rev with the throttle on or off choke.

Took some carb cleaner and sprayed it around. I replaced one vac. cap but found no other leaks. Didn't make a difference. I even added a deliberate leak that just barely sped up the idle but didn't change the throttle problem.

Last edited by corsair231; 11-15-2010 at 09:52 AM.
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Old 11-15-2010, 09:52 AM   #4 (permalink)
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old bike.... did you check float needle tips? rubber carb diaphragms for splitting? did you take carbs completely apart and spray all orifices w/ Chem Tool.... under pressure? Synch the carbs? Check carb boots for splitting or leaks? make sure the air box and air delivery system is well sealed? Blip the throttle w/ air box off and make sure all carb pistons are reacting uniformly? check spark plug color for uniformity... replace plugs? Replace fuel filter?

Last edited by Old No. 7; 11-15-2010 at 09:55 AM.
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Old 11-15-2010, 01:05 PM   #5 (permalink)
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Filter clogged? Fuel lines kinked?
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Old 11-15-2010, 02:59 PM   #6 (permalink)
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Originally Posted by 57x View Post
Filter clogged? Fuel lines kinked?


With the symptoms you're describing that's where I'd check.Also,did you check your fuel pump pressure output?
I once had a fuel pump that was working but the bike still hesitated,I checked my fuel pump pressure output and it was less than 1psi.A new pump took care of the problem.

I'm with 57x though,if you checked over the carbs,vacuum lines,etc and you're sure you got it,start working your way back.
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Old 11-16-2010, 12:22 AM   #7 (permalink)
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is this with the air box on or off...? if the air box isn't on there, it won't rev.
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Old 11-16-2010, 08:43 AM   #8 (permalink)
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old bike.... did you check?
Yep, did everything but synch the carbs but I really don't think that is the problem. This thing does not sputter or stumble. It just dies. Like hitting the kill switch so what ever it is doing it seems to be uniform across all 4 carbs.

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Filter clogged? Fuel lines kinked?
New filters and lines, no kinks. The pump will empty the filters out if I tilt the tank to uncover the pickup tubes

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is this with the air box on or off...? if the air box isn't on there, it won't rev.
I never thought about this. I will have to put the box back on and see if it makes a difference. Like I said, what ever is happening seems like it is uniform. This makes more sense than all four of the carbs having the exact same individual problem. I have to keep remembering, "This is not a car, this is not a car".
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Old 11-24-2010, 02:23 PM   #9 (permalink)
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did you put the carbs back on and start it while the rubber diaphragms were still wet? that will cause them to tear pretty quickly. i know its an old thread but no update saying he fix the issue.
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Old 11-27-2010, 12:09 AM   #10 (permalink)
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Nope, the problem wasn't the airbox. Put it back on and it still dies. I went back thru the carbs and cleaned them again but haven't had a chance to see if that fixed it. I did buy another set of carbs to try but they haven't gotten here yet. That or out of the eight carbs maybe I'll have enough parts to get 4 good ones.
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