86 750 Engine Clunking After Carb Reinstall - Suzuki GSX-R Motorcycle Forums Gixxer.com
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post #1 of 38 (permalink) Old 06-12-2019, 04:11 PM Thread Starter
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86 750 Engine Clunking After Carb Reinstall

Hey All,

Bike was running great, let it sit for about a week or so, and then swapped the jets on the carbs. Started it up and it was making this really strange clunking noise. Swapped the jets back to the original which seemed to resolve it temporarily, but now it's back more than ever. Also really doesn't want to idle.

Any advice? Starting points for where to look? Air filters all seem solidly on there, and the boots are all tight to the carbs.

Video attached:
https://photos.app.goo.gl/tGfQNuUySNA7xeYk8

Let me know if that format works.
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post #2 of 38 (permalink) Old 06-13-2019, 09:37 AM Thread Starter
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Re: 86 750 Engine Clunking After Carb Reinstall

Bumping! Local mechanic said he thought it might just be that one of the carbs has a clogged pilot jet, and I will say that at idle one of the headers is stayin pretty cold.
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post #3 of 38 (permalink) Old 06-14-2019, 09:51 AM
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Re: 86 750 Engine Clunking After Carb Reinstall

yeah one of the cylinders probably isnt firing, check for compression, spark, and if both check good then it isnt getting fuel
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post #4 of 38 (permalink) Old 06-14-2019, 09:07 PM Thread Starter
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Re: 86 750 Engine Clunking After Carb Reinstall

After taking the carbs off and apart 3 times yesterday, and cleaning the crap outta them, still no luck. The far left cylinder definitely looked more fouled than the rest - possibly isn't firing, but not sure what the heck could have caused it.

Only thing is that during one of the early re-assemblies of the carbs, I damaged the float gasket. Can't imagine that would affect it like this, though.
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post #5 of 38 (permalink) Old 06-16-2019, 07:46 AM
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Re: 86 750 Engine Clunking After Carb Reinstall

It is certainly not running on all 4 cylinders...that is what is causing all the banging and stalling. I would suspect you got something wrong with your carb work.
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post #6 of 38 (permalink) Old 06-17-2019, 09:19 AM Thread Starter
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Re: 86 750 Engine Clunking After Carb Reinstall

Ahhh yeah. Couldn't be caused by the float gasket could it?
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post #7 of 38 (permalink) Old 06-17-2019, 09:42 AM
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Re: 86 750 Engine Clunking After Carb Reinstall

Not likely the issue unless you see fuel pooling out the gasket.

Follow the cold header route, it is informing you something is up. First check is good spark on the sparkplug. If it was running great before likely not a compression issue, but no harm in checking with the plug out. Plug caps were not removed were they? Back at the right place if so?

If all that checks out likely something in the carbs not going well. And while it is less plausible that in the time playing with carbs something else went wrong it is not entirely impossible (coil, CDI, fuel petcock, etc).
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post #8 of 38 (permalink) Old 06-17-2019, 01:24 PM Thread Starter
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Re: 86 750 Engine Clunking After Carb Reinstall

Cool! It's at the shop now - I gave up after taking it apart so many times and cleaning it so many times. I'm sure it's something obvious but I'm at a loss. Didn't check for spark, but I can't imagine that would have gone away after a carb clean.
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post #9 of 38 (permalink) Old 06-25-2019, 06:24 PM Thread Starter
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Re: 86 750 Engine Clunking After Carb Reinstall

OKAY! Ended up grabbing the bike back from the shop 'cause they were slammed. I'm going to dig into it more today, but I started it and got a reading on the temps at the exhaust headers:
1: 196
2: 390
3: 510
4: 600

This is with the bike on the side stand. I'm going to test again tomorrow with it on a service stand, but these seem suuuper odd readings. Any thoughts?
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post #10 of 38 (permalink) Old 06-26-2019, 05:20 PM Thread Starter
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Re: 86 750 Engine Clunking After Carb Reinstall

After investigating today, I found the following:


Compression test results:
Carbs Off:
1: 145
2: 140
3: 125
4: 150


Carbs On, Idle:
1: 60
2: 90
3: 120
4: 140

Carbs On, WOT:
1: 125
2: /
3: /
4: 150


Temps at the exhaust headers:
1: 196
2: 390
3: 510
4: 600



I cleaned the crap out of the pilot jets and had zero luck.

Would love any advice at all you guys could provide. I'm completely at a loss. Obviously something is blocking airflow, but I have no clue where.
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post #11 of 38 (permalink) Old 06-27-2019, 03:19 AM
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Re: 86 750 Engine Clunking After Carb Reinstall

Spark plug is the first place to look imo. I put a new set of iridium plugs in my bike and after 100kms one cylinder not firing. I checked everything over and over but not the plugs thinking what are the chances. Well 1 of the almost new plugs had clapped it. I had a WS model also and after tinkering on the bike, one cylinder stopped firing, yep , spark plug. Shit happens randomly
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post #12 of 38 (permalink) Old 06-27-2019, 09:23 AM Thread Starter
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Re: 86 750 Engine Clunking After Carb Reinstall

I checked all of the plugs and no luck. Unless I'm completely mistaken, the compression test readings would point to something restricting the flow, correct?
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post #13 of 38 (permalink) Old 06-27-2019, 03:45 PM Thread Starter
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Re: 86 750 Engine Clunking After Carb Reinstall

Bumping! I feel like I have narrowed it down to a flow issue on the carbs - particularly at idle, but I can't figure out what parts would cause it. It is almost definitely not the pilot jet.
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post #14 of 38 (permalink) Old 06-28-2019, 12:17 PM Thread Starter
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Re: 86 750 Engine Clunking After Carb Reinstall

Checking in again - I'm curious what else could be blocking flow at idle besides the pilot jet. I haven't been able to find a solid diagram of how the air flows through the carb, and I don't see a pilot air jet.

Photos of the carb are here:
https://photos.app.goo.gl/NVceiFRUV1p8L3QA7
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post #15 of 38 (permalink) Old 06-28-2019, 05:25 PM
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Re: 86 750 Engine Clunking After Carb Reinstall

I've been following your adventure, and this is getting weird...there is more than enough air passing through the carb throat and into the intake port, to fill your cylinders, regardless of any jetting issues.

I imagine you're frustrated by now. Did you try again, after getting zero on 2 & 3?

Do you have a quality compression tester? I've seen the Schrader valves in them get sticky and cause faulty readings.
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post #16 of 38 (permalink) Old 06-28-2019, 05:48 PM Thread Starter
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Sorry the '/' on 2 and 3 are not a reading of 0 but me not taking a reading. I can go through the process again but for all cylinders
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post #17 of 38 (permalink) Old 06-28-2019, 06:03 PM
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Re: 86 750 Engine Clunking After Carb Reinstall

I'm having a hard time following the plot here.

Start the engine, warm it up to operating temperature...perform a compression test with the throttle wide open. If you have a low cylinder, squirt a bit of oil down it and see if you're numbers improve.
A leak down tester will tell you more...worn rings or leaky valves, but a compression test is a good start.

I agree with George, that your cold cylinder is likely spark plug or ignition related.

Carbs that are out of synchronization, will cause an engine to make all kinds of extra noise too.
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post #18 of 38 (permalink) Old 06-28-2019, 08:00 PM Thread Starter
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Re: 86 750 Engine Clunking After Carb Reinstall

Hey, thanks all, and sorry for the total chaos. I'm pretty good at carbs, but these are finicky.

The plot:
1) carbs ran great
2) took carbs off, rejetted, reinstalled, carbs ran like crap - seemed mostly to run like crap at idle
3) jetted back to original, carbs still ran like crap
4) did temp test, and it seemed like carbs 1 & 2 were the culprits - 1 being the worst
5) did compression test (cold) with carbs on and carbs 1&2 were significantly lower. Numbers corresponding pretty closely to the temp differences.
6) did compression test with carbs off, and all were pretty even
7) did compression test with carbs ON, with throttle open (not connected, so maybe 50%-80% open), and carbs were much closer to even, though 1 was a bit lower. I stopped there because 4 was the same as carbs off, and 1 was lower, but better.
8) I took this to mean that there was an issue with flow with the problematic carbs - probably with the pilot circuit
9) cleaned the pilot jets, reinstalled the carbs, got same results for the compression test.

Would love next steps! I will absolutely run through anything you recommend.

I am definitely new to this kind of problem - I'm used to dual carbs. I'm pretty close to taking it into a shop, but I would need to take it in with the carbs installed (so they could diagnose), and that's about 4 hours of labor.
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post #19 of 38 (permalink) Old 06-29-2019, 11:07 AM
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Re: 86 750 Engine Clunking After Carb Reinstall

Do your compression numbers come up if you fog the bad cylinders with oil?

As for the carbs, at this point, I'd give them a full rebuild. That means removing and cleaning all removable parts and cleaning the carb bodies. Either clean them ultrasonically or in a carb cleaner like Berryman's. Verify that all of the jets are the same and are undamaged. Change ALL of the o-rings with replacements SOURCED THROUGH A SUZUKI DEALER. Make sure the float needles and valves are perfect. Make sure the float heights are set to within .5mm, THE CORRECT WAY. There's a wrong way to do this. Make sure all of the slide diaphrams and springs are undamaged and use a vacuum gauge to check the slide operation for each slide. They should all work the same. Do a mechanical synch of the throttle valves on the bench, before re-installing them. Verify that all of the small internal passages are clean.

I'd probably go ahead and change the o-rings on the intake boots, the plugs and do a valve adjustment.

When you get them back on, set the idle speed and synch them with a vacuum gauge.
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post #20 of 38 (permalink) Old 06-29-2019, 11:42 AM Thread Starter
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Thank you for the reply! They were just rebuilt before this all happened, which is why I'm thinking some small thing is just fouling some part of the pilot flow. I'm definitely down to do the full rebuild if it needs to happen, but would love if there are any carb parts to check first, in the hope that I can clear out whatever is causing this.

When you say fog the head with oil, how do you do that? I've never tried doing this before. Just put oil in through the spark plug hole before compression testing?
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