Engine lag/stall: HELP - ZX6R Forum
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post #1 of 11 Old 08-04-2019, 12:34 PM Thread Starter
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Engine lag/stall: HELP

Hey guys, big updates to come, I decided not to sell the bike and subsequently tore the engine completely down, put a new upper and lower crankcase on and all new gaskets, and timing back to factory. Replaced everything out of tolerance and after a carb sync I thought she was going to run beautifully...

Important Specs:
-1996 zx6r-f
-D&D exhaust
-smog block-off plates and all extra vacuum lines closed off
-K&N air filter (brand new... problem?)

Problem:
She idles and revs fine when on the bike stand. Took her out and pulls well in 1st and 2nd throughout the entirety of the rev range, although a slight hiccup on the low rev at first. Once in 3rd (and up), she bogs a bit and will not accel much, and stops just short of 9k rpm no matter what (WOT). I found that if I downshifted and got her up past 11k, she would take off hard up to redline. So... fuel is good, spark is good, air problem? I have had this happen a few time in the past, but she would ALWAYS backfire hard and take off and run great up until the next time I tore her down. Side note, dont continue to ride with an problem like this at high revs, I melted the fiber casing on my exhaust completely off and she was running hard enough to piss oil out of a loosely torqued banjo bolt. Temp was fine though and had plenty of oil when that shit went down. Anyways, Iíve been testing different vacuum hose configurations with no luck and want to see if maybe im having some sort of back-pressure issue. May try and swap my old air filter back in to see if that is the issue to. Let me know what you think.
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Last edited by Brandon35; 08-04-2019 at 12:38 PM.
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post #2 of 11 Old 08-05-2019, 01:03 AM
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I'd expect that the carbs are clogged in some of the passages. To melt the can, exhaust temps have to be extremely high..... That only happens with dangerously lean conditions. Lack of fuel in high demand scenarios seems like the probable cause.
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post #3 of 11 Old 08-05-2019, 01:05 AM
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Ya I would say this is probably carb related as well.
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post #4 of 11 Old 08-05-2019, 09:03 AM Thread Starter
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Okay running lean makes a lot of sense, i’ll dive into the carbs soon and update when I can. At least she’s purring at idle.

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post #5 of 11 Old 08-05-2019, 12:43 PM
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If you have a fresh air leak, that could also alter the mix. Old boots can split, or simply not seal well.

WD-40 sprayed around the boots one at a time, in moderation can show a vacuum leak.... When you spray the area while the engine is running, and the exhaust smokes heavily..... That's a point of entry due to a leak.

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post #6 of 11 Old 08-07-2019, 07:39 PM Thread Starter
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Quote:
Originally Posted by RJ2112 View Post
If you have a fresh air leak, that could also alter the mix. Old boots can split, or simply not seal well.

WD-40 sprayed around the boots one at a time, in moderation can show a vacuum leak.... When you spray the area while the engine is running, and the exhaust smokes heavily..... That's a point of entry due to a leak.
I checked the carb jets and cleaned everything. No change there and no boot leaks. Really not sure what’s going on - first gear rips, second bogs down low for a sec and takes off, and third on up falls flat on its face.

I ordered a new fuel pump to install just to eliminate another possibility

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post #7 of 11 Old 08-07-2019, 07:40 PM Thread Starter
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Only thing i adjusted on the carb was that I did a carb sync, she purrs pretty nicely on the stands. Another weird thing is that with full choke she wants to cut off but comes back alive if i back if off a tic

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post #8 of 11 Old 08-12-2019, 09:16 AM Thread Starter
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Checked my fuel pump relay and wasn't getting any resistance indications per the manual so I ordered a new one and have a new pump to install as well.

I ran with the new air filter, then tried the old with no change.

I reinstalled the old vacuum lines for the old smog system (only up to the airbox) with no change

I adjusted all mixture screws one turn out from factory (most were close to four turns out but this was a stock exhaust so I split the difference for now) with no effect (yes I know that would make it "leaner" but I'm looking to make everything equal... carb sync was done)

So now I wait till the weekend to dive back in with the new relay and pump. Crossing my fingers.

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post #9 of 11 Old 08-12-2019, 09:20 AM Thread Starter
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Also, I found a nice replacement exhaust. Since I ruined the old one and decided that buying a cheap used one would yield shitty results, I bought a new one. I went with the Dominator HP1 exhaust that bolts right on and it will be here in a few weeks I believe. I have never seen an F model with this kind of exhaust, and even though it was pricier than the standard can, I'm excited to see and hear it. Has a 5 year warranty, and if you bust it up you can get a replacement for half off. Not a bad deal and sure to be unique

https://www.dominator-exhausts.nl/do...1995-1997.html

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post #10 of 11 Old 08-17-2019, 09:35 PM Thread Starter
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Figured out my issue. I thought the vacuum valve that connected to the carb and the airbox was something that could be nixed from the system to clean things up, thinking it was solely in charge of airflow for the smog system. I was WAY wrong; that valve is critical to allow excess air at speed to simply recirculate into the airbox instead of over-pressuring the carb and leaning out the mixture. The vacuum hoses are back on the valve and all other hoses connected, took for a quick spin and this mf moves.

I could not find the operation of the valve in the manual but remembered a critical component that works in a similar way on turbine engines - essentially the excess intake air (RAM air effect with forward airspeed) continues to circulate back through the intake until the demand picks up and the air is required.

Just waiting on the exhaust now


... Did I mention how accurate my signature block is?
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Last edited by Brandon35; 08-17-2019 at 09:42 PM.
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post #11 of 11 Old 08-18-2019, 02:18 AM
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