1991 ZX7 Troubleshooting - Vacuum leak symptoms with no leak? - ZX Forums
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post #1 of 26 (permalink) Old 11-12-2019, 11:56 PM Thread Starter
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1991 ZX7 Troubleshooting - Vacuum leak symptoms with no leak?

Hiya,

Earlier this year I picked up a 1991 ZX-750 J Model that was in need of some care.

The tank was FULL of rust and obviously the carburetors needed to be serviced. I de-rusted the tank, replaced all the fuel lines, and then had a pinhole in the tank welded at a radiator shop. I fought this bike, tooth and nail, over constant fuel delivery issues and odd-running. I also cleaned the carburetors up as best as possible and replaced the float bowl gaskets. I left the float seat o-rings and a/f orings alone. I replaced ONE bad OEM spark plug boot and all of the plug wires with NGK wires and installed new NGK plugs in every cylinder.

I have a lot of bikes so it kind of got put on the backburner. I got the bike to run ok enough to test ride it, but as soon as I gave it a good bit of throttle I would get backfiring out of the carburetors and lose power, but the bike would continue to run on 4 cylinders (I know as it was running on 3 at one point and would barely run.) The idle was unpredictable and would constantly raise the longer you rode. You could start at 1200 and once the bike got warm it would get up to 2000+. Backing the idle screw off fixed this . . . until you took off and stopped at the next red light or stop sign . . . then it was back up to 2,000. Rinse and repeat forever.

I recently pulled it into my work garage and started working on it today. The bike is running stock/oem jet sizes, stock needles, etc. I replaced an "intake pipe" today, that was broken, and dremeled the gasket out of all of my rubber intake boots and replaced them with O-Rings. I adjusted the float heights to 13mm and installed some factory pro needles that came with the bike. I set the needle clip to the 4th position (1 step richer due to my carb popping) and set all A/F screws to 2.25 turns. I verified the fuel pump is working. I also performed a compression check and got readings of around "120" psi in all cylinder dry and much higher with some oil. I don't think 120 is a cause for concern - im sure the gauge is inaccurate.

I gave the bike a test spin and it had the symptoms it had in the past plus symptoms of running rich - hesitation. The engine has 14k miles on it. While riding I hear a constant metallic sound from the top end - not the valves. Is it possible this is normal noise from the cam chain or just some vibration that I'm perceiving to come from the engine? I also hear "popping" but I can't determine if its the carburetors or not. I brought it home, pulled the plugs and they were sooted up so I wiped them off and reinstalled them. I ran the bike and nuked the fuck out of the reed valve/hoses and the intake boots and the idle did not move a hair.

I performed a carb sync (again) and got them as close as possible and adjusted the a/f screws to 1.75 turns and moved the needle clip to the center position. I ran the bike only in the garage and pulled one plug which looked like this:



I didn't run the bike terribly long . . .15-20 minutes. Long enough for the radiator fan to come in and blipped it some while syncing the carbs. Is this now set too lean or should I give it a proper rip and then determine?

The bike seems to rev well as it always has. The RPMs drop at a decent rate of speed, but if I "blip" the throttle they will hang around 2k for a moment. Is this because I'm too lean on my a/f screw and don't have enough overlap as a result? Is there somewhere else I should be looking?
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post #2 of 26 (permalink) Old 11-13-2019, 02:13 AM
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Hi. Just a thougt. Have ju check your coils,leads ect?
I helped a friend with his bike, he had similar problems. He had broken leads on two plug caps.
Bike ran ok cold and started backfire and missing when hot.
Something to think about

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post #3 of 26 (permalink) Old 11-13-2019, 08:03 AM Thread Starter
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Thanks for the response!

I have, many times, including yesterday :(. In the past I had one broken spark plug boot/lead and that was replaced with a new OEM.

The coils are slightly out of spec if you perform an ohm reading. I tried some new/cheap aftermarket and they kept going bad on me so I went back to my OEM ones.
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post #4 of 26 (permalink) Old 11-13-2019, 08:42 AM
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Ok hmm.
First, the engine on these bike sounds alot. Like racebikes...
However dont take my word for it.
I heard somebody say"if you dont feel the noise in your handlebar, buy a quite helmet and ride"
About the lean situation.
I had problems on a gpz900 years ago. Acted lean, ran better cold, popping.what you described is your pilot system down to idle. I had these problems. Tried the same, boots, sprayed startgas and so on.
What was the problem was that the CVK had badly worn choke circuit.
Sucked air thru it. I determend this problem by simply replacing the gaskets and the brass rods.
Did a quick carb sync.and the bike ran like hell again.
Most problems on carbs are pilot circuit, but... Sometimes its not and then it can be a royal pain!


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post #5 of 26 (permalink) Old 11-13-2019, 09:14 AM
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The floats are also very touchey on these bikes and generally need to be set with fuel level not float height. I have been fighting carb issues on the H2 we restored. My J1 on the other hand is running amazing with a simply carb clean and proper jetting. Sucks for the H2.
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post #6 of 26 (permalink) Old 11-13-2019, 01:37 PM Thread Starter
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^^^ I ALMOST set mine using fuel level, but decided to go with the floats themselves. Fuel level should come up to the lip of the bowl correct?

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Originally Posted by ZxNystrom View Post
Ok hmm.
First, the engine on these bike sounds alot. Like racebikes...
However dont take my word for it.
I heard somebody say"if you dont feel the noise in your handlebar, buy a quite helmet and ride"
About the lean situation.
I had problems on a gpz900 years ago. Acted lean, ran better cold, popping.what you described is your pilot system down to idle. I had these problems. Tried the same, boots, sprayed startgas and so on.
What was the problem was that the CVK had badly worn choke circuit.
Sucked air thru it. I determend this problem by simply replacing the gaskets and the brass rods.
Did a quick carb sync.and the bike ran like hell again.
Most problems on carbs are pilot circuit, but... Sometimes its not and then it can be a royal pain!


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This might be my issue. The choke is EXTREMELY finnicky like . . . when the bike is fully cold its completely dead or running at 6k rpm with very little choke. I have to hold it in a very very precise position to keep the bike from dying or going wide open throttle. Once it runs for a few seconds it will idle on its own.

I guess a full rebuild may have actually been needed on this bike.

Are these parts still available? I'm scared, but looking into partsfish now :o
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post #7 of 26 (permalink) Old 11-13-2019, 01:55 PM
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Should be pretty easy to find.
I think you have to add the parts for the choke and also orings for the fuel pipes between the carbs, and dont buy cheap parts. I have done and failed.
And yes when you set the Bowl height ju should run the hose from the drain to below the gasket face. I think keihin have a mark on the side.
I think the kawasaki manual have the right way to do it:)

Just finished a pair of CVK for a gpx600.
Helping out a friend. Bike has been sitting for 12 years with a rusty petroltank.
Was not pretty...

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post #8 of 26 (permalink) Old 11-13-2019, 06:54 PM Thread Starter
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Thanks!

The facebook guys all claim their bikes do EXACTLY what mine do, but I've found multiple videos of ZX7s . . .all model years . . . with with I perceive to be normal choke operation. They act just like my other bikes - they start at a low rpm when fully cold then rev up a tad . . . then a lot once they warm up a bit more.

The ZX7 is the only bike I've owned that exhibits "WoT" when the choke is in use even when fully cold. The closest bike I have for comparions is a 1988 zx10, but they are quite different bikes I believe.
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post #9 of 26 (permalink) Old 11-13-2019, 07:17 PM
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If it has a vacuum leak, the idle will hang up when you blip the throttle. Check that the diaphragms are not pinched or sitting wrong. If they are compromised, you will have issues.
As for checking fuel level, it is the correct way to set floats. Four different floats can move differently in the bowls. If you set fuel level, the manual float level check might show differences.
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post #10 of 26 (permalink) Old 11-13-2019, 08:51 PM Thread Starter
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^^^^^
The diaphragms are in excellent shape. I checked them many times yesterday and I'm very careful about making sure they seat properly when putting the caps back on.

Also thank you! And thanks to everyone helping thus far :)

Ok so . . . I took a 20-30 min test ride using the settings mention above and below:

13mm float heights
A/F screws 1.75 turns
Needle clip (Factory pro) in Center ("Stock") position
Jets -> ALL STOCK

Prior to riding I blasted all four choke plungers with carburetor/choke cleaner and observed NO change in idle speed.

The symptoms, when warm, are as follows:

- Hesitation when giving lots of gas quickly between 1000rpm and ~6000 rpm. This is more pronounced in the 5000-6000 range where almost any amount of throttle results in hesitation before the power kicks in

- 6000-12k (redline) seems OK! If I give max throttle quickly the bike will reach the rev limiter without issue and the rpm gauge doesn't even get a chance to hit 12k. If i quickly, but gradually give gas it will reach 12k and pull the entire time - once it gets over any hesitation hump from the initial throttle.

- Completely unpredictable idle. It started out around 1200 and worked its way up to ~2000. I adjusted it back to ~1200 then it worked its way . . . all over the place. Before I rode home I stopped in a parking lot and it was idling around 3000. Blipping the throttle would sometimes set it straight and sometimes would drop it under the set idle. When I got home it was idling around 1,000 rpm. Sometimes it flutters at idle and other times it seems pretty ok.

- The longer the bike sits at idle the lower the rpms drop when you blip the throttle

- Example 1: Bike is idling at 1200 rpm for 1 second. Blip the throttle any number of times. The rpm will rise then drop down to ~1200
- Example 2: Bike is idling at 1200 rpm for 15 seconds. Blip the throttle and the rpm will drop low enough to kill or almost kill the bike. Immediately after you can blip the throttle over and over and the bike seems ok

- The bike sounds like its running on 3 cylinders INTERMITTENTLY during the ride. If I let the bike sit for a while like 30 seconds or so then take off the bike sounds smooth as butter. If I ride it for a few seconds I get a constant fluttering thud and the bike loses some power. The odd thing is that the idle doesn't seem to change at all. This is most noticeable when taking off from a dead stop as the bike feels like its about to bog and you have to give it more throttle. I know its not running on 3 cylinders CONSISTENTLY because it was doing that in the past and I had to keep the revs around 5000-6000 to get home. Taking off was nearly impossible.

- When coming off of WoT I get a series of pops coming from under the tank so I assume this is the carburetors backfiring. Maybe 4-10 pops.

- Temperature gauge runs at normal ~half way point

- Fuel pump (aftermarket) is running and seems to be maintaining fuel

So it seems I have a low/mid-range lean condition. Last time I was running at 2.25 turns on the A/F screw and the 4th (1 step richer) position on the needles and it had similar symptoms, but the plugs were black as soot.

Is it possible I'm also chasing an ignition related issue considering it feels like im running on 3 cylinders intermittently? The fluttering noise sounds EXACTLY like a cylinder dropping out, just not as loud as on that's 100% dead. At second glance I'm running one OEM coil and one aftermarket cheapo coil. The cheapo is brand new and hasn't seen much use at all . . . though I have had a few of these die on me. The OEM one is SLIGHTLY out of spec, but was used to replace the aftermarket ones that went bad on me. This is an issue the bike has had since I got it . . . I want to say it did the same thing on OEM coils, aftermarket coils, and my current combo. The spark plug wires are brand new NGK wires. The boots are the OEM boots. Boot #3 is brand new and was ordered to replace a broken boot. I quadruple checked to make sure all plug wires were well seated in their boots.

Can the igniter cause issues like this? The reason I ask is because I recently fixed a 1992 EX500 that sat for 18 years. When I got the engine to start I couldn't rev it past ~5k. It sounded like a rev limiter was being hit (the ex500 doesn't have one!) so it seemed like something was breaking down under load. The issue eventually turned into intermittent spark on both cylinders. I replaced the igniter and the bike revved normally without issue.

Considering the needle clip position supposedly affects 1/8 throttle up to 3/4, while tapering off, should I try adjusting my needle clip position and leave the a/f screws at 1.75 turns?

FWIW I am in the Denver area which is why I backed them out 1.75 instead of 2.0. I know the elevation doesn't matter that much since these are CV carbs . . .


^ Around 0:50-1:00 to see the blip that almost kills it.

^ Idle in parking lot . . . I adjusted it down towards the end and give some blips

^ This was as soon as I got home

Last edited by mh4ult; 11-13-2019 at 09:17 PM.
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post #11 of 26 (permalink) Old 11-14-2019, 06:49 AM
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Do you have a fresh inline fuel filter?
If you can run bike with tank off, in the dark, might see high voltage arc on plug wires.
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post #12 of 26 (permalink) Old 11-14-2019, 07:47 AM Thread Starter
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Quote:
Originally Posted by GpzRider View Post
Do you have a fresh inline fuel filter?
If you can run bike with tank off, in the dark, might see high voltage arc on plug wires.
No arcs. I also haven't been zapped yet :P. Those plug wires are brand new. One boot is brand new and the other 3 are in the same condition as the new one. If you put them together you wouldn't know which was which.

I'm not running an in-line filter. Only the OEM tank filters which are also brand new. I tried multiple inline filters when I got the bike and they all caused an air lock at the filter for some reason.
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post #13 of 26 (permalink) Old 11-14-2019, 01:09 PM
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What about the ignition timing sensor, wiring to/from ignitor?
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post #14 of 26 (permalink) Old 11-14-2019, 02:13 PM
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Check the float needles - they can rust under the rubber tips.
Caused me all sorts of mid rev-range grief.
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post #15 of 26 (permalink) Old 11-14-2019, 06:41 PM Thread Starter
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@GpzRider - I have not tested any of that yet. The wiring is visibly in good shape. I need to test the pickup.

@Zed Kable I appreciate the idea. The float needles and seats have been removed/inspected. I'm not really concerned with the hesitation symptoms as I've had those ironed out in the past. I'm really worried about treating the major issue and then tuning the bike properly after the fact. The "intermittent 3 cylinders" exists no matter how the carbs are tuned.
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post #16 of 26 (permalink) Old 11-15-2019, 12:37 AM Thread Starter
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Can someone point me in the direction of performing a pickup coil test?

I took my crappy aftermarket coil off and installed the OEM one on it, but didn't get to test ride. I seriously loathe working on this bike due to having to drain the damned tank all the time. Its impossible to get all the fuel out so the residual always pisses all over the bike during reassembly. . .

Another idea for you guys, but I don't think this is the problem as the fuel pump stays full . . . but my petcock is set to the "off" position I believe but fuel is flowing freely out one of the lines - the pump stays completely full and pumps new gas no problem. When I got the bike the petcock was so stiff you couldn't really turn it so I disassembled it and cleaned it up. It now turns very easily, but I'm not sure why it operates in the "off" position unless I'm just looking at it incorrectly. I have verified that the fuel flows freely from both the Reserve and ON outlets so this is probably barking up the wrong tree.

Another thing I noticed is that I have 6.5v to the gray wire (testing from gray wire on CDI to battery positive post) when the key is on. No starting issues.

The coils have ~10.5 v at their plugs, but I can only get a reading from plug terminal to a ground EG: Positive lead on GREEN terminal wire on coil, Negative wire to any ground = 10.5V reading. . . . Positive lead on RED terminal wire on coil, Negative wore to any ground = 10.5v reading. Not sure if this is a cause for concern or not . . .

When I gather the motivation and time to reassemble it and test ride it I'll post my findings now that I'm running 2 OEM coils again.
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post #17 of 26 (permalink) Old 11-15-2019, 10:05 AM
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I had an issue that behaved like this that left a friend and me scratching our heads all day and he is a full time bike mechanic. What it ultimately was the linkage between carb 2 and 3 had some play in it because it was not stamped on properly at the factory. We used a punch to mushroom the part out and took all the play out. Just like that all the problems were gone. It had the off throttle idle problem you have along with random over revving.

People have probably been fighting this bike since day one.
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post #18 of 26 (permalink) Old 11-15-2019, 11:36 AM
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Read what you wrote on the coils. I am not so good on electrics, i know some, just enough to be dangerous.
So i am not gonna bother you with my shitty advice.
However, have you looked at the airvent on the tank?
I heard about guys having wierd symptoms with idle and up to 3/4 throttle.
Easy to check if you have not yet.



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post #19 of 26 (permalink) Old 11-15-2019, 06:52 PM Thread Starter
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@liquidxen0n The linkage in the center IS very sensitive . . to the point where a touch with a screwdriver will raise the rpm. I'm not sure if there is any play in it, though. I thought this was fine since the plate that the cables attach to returns to home 100% of the time.

@ZxNystrom I have not looked at the tank air vent. What should I be looking for specifically? The tank HAS been fully derusted using pure evaporust so the same should be said about the vent since its submerged in that part. I'm pretty certain the tank is venting properly. I never hear any suction or whine from it. I always get evaporust around the lip of the tanks I'm derusting and it comes straight out the vent without issue.

Thanks guys :)
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post #20 of 26 (permalink) Old 11-15-2019, 09:07 PM Thread Starter
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I still haven't ridden the bike yet, but I did idle it in the garage a bit. Gotta put it back together. Tomorrow is my "friday" so I'll have some time this weekend :).

@liquidxen0n

If I barely put ANY pressure on the sync screw/linkage between 1 and 2 or 2 and 3 the bike revs up significantly. If I pull on them slightly the bike revs down significantly - to the point where it will die. This definitely could explain my idle. I'd say there isn't even 1mm of play in the linkage . . .it doesn't feel like it moves much if at all, but the bike definitely revs up/down depending on which way pressure is applied.

Can you provide any insight as to how I may be able to resolve the issue? Do I need to tighten my linkage screw/springs or is a punch necessary? Any advice as to where to apply it?

Video below . . . the first 80-90% of the video is me pulling on the linkage. Ignore the weird noise sometimes, I kept bumping the emissions junk loose.

Towards the end I press the linkages forward . . . not much change between 3/4, but a noticeable difference between 1,2 and 2,3. Note that ONLY the linkage is moving, i'm not actually pressing hard enough to simulate turning the throttle.


As you can see in the video . . . no thumping around when its running in the garage.
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