95' Idles Great, but Won't Rev - Suzuki ATV Forum
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post #1 of 18 Old 05-02-2019, 11:08 AM Thread Starter
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95' Idles Great, but Won't Rev

Just installed:
New Timing Chain
New Stator/Pickup
New Plug

Got it to fire right up, no issues.

Went to rev it... and it wouldn't rev.

Remove the air-box... same result, only I could see the vacuum butterfly fluttering like crazy when giving it gas.. and manually holding it open did nothing.

I had the carb off previously, and did a thorough cleaning, set the mix screw out 1.75 turns.. yadda-yadda. I figured maybe there was some junk in the lines that made it's way into the bowl or something. (I did clean out the fuel pump and petcock as well).

Get to tearing into the carb again and I notice that the needle on the mixture screw is a bit wonky (dented), and doesn't have the "Mikuni" logo on it (the double box logo). I also notice the main jet and pilot jet don't have the logo either, however the emulsion tube does, as does the air jet under the CV diaphragm in the top of the carb. I also notice that the washer under the main jet is missing.

So, I ordered some genuine parts from Bike Bandit... new O-rings, springs, washers, pilot, main, and mixture screws.. hopefully this will get rid of the bogging issue...

Any other ideas? I had checked my coil, CDI and R/R on my good-running KingQuad 300 with no issues... fired up, revv'd up, etc... so I am ruling them out as culprits.

While running, the battery voltage is pretty solid at 13.65V DC at idle. Spark is FAT and blue.

My only other guess would be that maybe I installed the cam 1 tooth off when I changed the timing chain... I will diagnose that soon enough, as I am guessing it's not the issue.

Finally, as it was idling, I did a "spray and pray" check around the carb, and boots with some carb cleaner... 0 affect to the engine.

My guess is a lean condition via the mixture screw being incorrect and sucking air through the emulsion tube base...

Here's hoping at least.

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post #2 of 18 Old 05-03-2019, 07:13 AM
Tim
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If your timing chain is off, it would rev up but sputter.

Are you sure you are getting adequate fuel supply?

Former: 09 LTA 450 AXI KQ, 93LT-F4WDX KQ, 93LT-F4WDX KQ, 94LT-F4WDX KQ, 95LT-F4WDX KQ, 87 LT230 QR, 89 LT80 QS
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post #3 of 18 Old 05-03-2019, 08:29 AM Thread Starter
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Quote:
Originally Posted by Tim View Post
If your timing chain is off, it would rev up but sputter.

Are you sure you are getting adequate fuel supply?
Hah, of course you say that and I already cracked the valve cover loose... I haven't taken it off yet, but I was letting it drain over night before making a huge mess.

I had to prime the fuel pump by taking the vacuum line and suck/blowing on it a lot until it started to pump through, then hooked up the line to the carb and cracked the bowl train, and continued to suck/blow until the bowl trickled fuel. Then shut the valve on the bowl, and continued to suck/blow for a bit until I had pressure.

Then I turned the petcock to "RES" from "PRIME", and fired it up and it burped right to life...

I think, potentially, I am sucking air from the needle valve because it is so damaged, and even worse, probably creating an air bubble in the bowl by not having a washer under the main jet to cap off the gap around the emulsion tube.

I could still have an air-bubble in the fuel pump. Is there any other way to prime it?

Also, I checked the bowl height by laying it horizontal on the bench and just letting the bowl rest against the spring pin on the shut off needle... and it measures about 1/2 an inch from the face of the bowl surface... seems too high? Manual says it should be .90-1.0"... is there any way to verify other than just to trial and error installation after installation?

Thanks!

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post #4 of 18 Old 05-03-2019, 04:42 PM Thread Starter
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Yup... Timing is 100% spot-on. Woof. Time to put all of that back together again.

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post #5 of 18 Old 05-04-2019, 09:10 AM
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Good news on timing. No real way to prime the pump other than suck on the vacuum tube like you did.

Is the throttle working correctly? Did you rebuild the carb?

Former: 09 LTA 450 AXI KQ, 93LT-F4WDX KQ, 93LT-F4WDX KQ, 94LT-F4WDX KQ, 95LT-F4WDX KQ, 87 LT230 QR, 89 LT80 QS
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post #6 of 18 Old 05-04-2019, 02:22 PM Thread Starter
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Yup, rebuilt the carb... but ran into a bunch of knock off components inside, a missing washer under the main jet and a really damaged needle jet that wont seat fully.

I have the carb apart again, double and triple checked all passages... and am soaking it again in Berrymans for 24 hours while waiting on some genuine Suzuki/Mikuni parts to arrive.

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post #7 of 18 Old 05-05-2019, 08:07 AM
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It should help.

Former: 09 LTA 450 AXI KQ, 93LT-F4WDX KQ, 93LT-F4WDX KQ, 94LT-F4WDX KQ, 95LT-F4WDX KQ, 87 LT230 QR, 89 LT80 QS
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post #8 of 18 Old 05-12-2019, 05:44 AM Thread Starter
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Ok, so the new carb parts helped a little... but it still stopped revving around 1/4 throttle.

I took my CDI off and swapped it for a new Caltric one and it was able to over-come 1/4 throttle of I kept the throttle pegged, and it eventually hit full throttle.

I took the carb back off got the air screw to 2 7/8 turns out and got the float to about 1/2 inch... and it revved through 1/4 throttle much faster... but not perfect. Plug was clean, spark was perfect, no smoke!

Then, after holding the throttle for a second or two and letting go... or after blipping... a HUGE yellow flame out of the exhaust... and the header began glowing.

I let it cool down. Fired it up and it would rev up better... still stumble through 1/4 throttle, and pop through the airbox and backfire HUGE out of the exhaust.

It's almost as if it's too lean at idle, but too rich when transitioning to the main jet.

I've tried two coils as well, and 2 rectifiers (both new and known good). No change.

I am narrowing it down to a fuel/air issue.

I have no fuel in any of my vacuum lines. I've sprayed carb cleaner all around the carb at idle and intake and airbox boot with no change in how it runs.

I think the carb is physically worn out. Either the diaphragm is sucking air, or the slide is too loose, as it rattles around a lot in the carb body.

*shrug*

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post #9 of 18 Old 05-13-2019, 01:35 PM
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Damn CV carbs are really tough to get to run just perfect. Then to top that off, they put the air screw in a spot where you cannot get to it with a normal screw driver.

I took a small bolt soldered it to a washer. Ground the tip into a slotted screwdriver tip and use that to adjust the air screw with the carb ON the engine while it is running.


Couple of things:

1. The air diaphragm is often really tough to not twist (even a little will effect it) when putting the cover back on. This is important as the valve will open quicker if it is twisted.

2. The spring does wear out. Worn spring will result in lean burn above 1/4 throttle as the slide opens quicker and allows more air.

3. Float level is very critical. It must be at the spec.

4. Alternative: remove the CV carb and put an older non-CV carb on it. Gains: easier to adjust and get perfect, Faster throttle response.


Note that a non-moving 250 will get hot and possibly over heat. Needs air flow.

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post #10 of 18 Old 05-13-2019, 07:47 PM Thread Starter
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Quote:
Originally Posted by TheBearAK View Post
Damn CV carbs are really tough to get to run just perfect. Then to top that off, they put the air screw in a spot where you cannot get to it with a normal screw driver.

I took a small bolt soldered it to a washer. Ground the tip into a slotted screwdriver tip and use that to adjust the air screw with the carb ON the engine while it is running.


Couple of things:

1. The air diaphragm is often really tough to not twist (even a little will effect it) when putting the cover back on. This is important as the valve will open quicker if it is twisted.

2. The spring does wear out. Worn spring will result in lean burn above 1/4 throttle as the slide opens quicker and allows more air.

3. Float level is very critical. It must be at the spec.

4. Alternative: remove the CV carb and put an older non-CV carb on it. Gains: easier to adjust and get perfect, Faster throttle response.


Note that a non-moving 250 will get hot and possibly over heat. Needs air flow.
Mine didnt over heat... but I had the shield over the exhaust off and melted my airbox a little... lol.

What would you reccomend for a non-CV carb? Will an 87-89 LT250 or LT4WD model work? Are they 32mm?

Seems round slide Mikunis are fairly cheap.

I plan on running a gravity feed tank eventually, so extra vacuum ports wont be an issue.

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