One cylinder far off on vacuum test and misfire above 5K RPM


Information and questions on GL1100 Goldwings (1980-1983)
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ymmot04
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Location: Dixon, IL
Motorcycle: 1980 GL1100 Interstate

One cylinder far off on vacuum test and misfire above 5K RPM

Postby ymmot04 » Mon Jul 21, 2014 10:38 pm



Before I start spending more money on parts, I was hoping you guys could help me narrow down what I'm dealing with here. Freshly rebuild carbs and I see this with the vacuum gauges hooked up. The gauges are hooked to the cylinders in order 1,2,3,4.

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When completely rolling off the throttle, number 2 drops way down compared to the others. At 3-4K they all run about the same. Above 5K it starts missing from the 2,4 side it seems.



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virgilmobile
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Re: One cylinder far off on vacuum test and misfire above 5K

Postby virgilmobile » Tue Jul 22, 2014 7:33 am

Number 2 carb throttle valve is further closed than the other 3 thus the higher vacuum...
You might do 2 more things...
1st..connect all 4 vacuum meters together and hook them all at the same time to just one vacuum port...Check that the meters read exactly the same...If there different,mark on each one how far off they are..Use 10" of vacuum as a base line...
2nd..add a fish tank valve near each vacuum meter...squeeze it down till the needle has just a little bounce when its idling at 1000 rpm..
Keep the engine in the 1000 rpm range during each adjustment...this is where the sync is most important...
Always balance the right cylinders first...then the left...then left to right...All at 1000 rpm...

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virgilmobile
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Re: One cylinder far off on vacuum test and misfire above 5K

Postby virgilmobile » Tue Jul 22, 2014 7:34 am

Then see what happens about the misfire...One step at a time... :D

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virgilmobile
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Re: One cylinder far off on vacuum test and misfire above 5K

Postby virgilmobile » Tue Jul 22, 2014 7:42 am

One more thing to bug you about....The slides....
By chance,when you worked on the carbs,were you anal about how the slides operate.?
They absolutely must slide butter smooth in the caps...I usually use a shop vac and test each one before i put the carbs back on....
If one is sticking,you can end up with a misfire..
You can use a mirror in the bottom of the plenum box...looking at the slides...pop the throttle and watch each one operate...I did find one of mine sticking right as it tried to lift...It also ran poorly.

ymmot04
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Joined: Thu Apr 01, 2010 10:50 pm
Location: Dixon, IL
Motorcycle: 1980 GL1100 Interstate

Re: One cylinder far off on vacuum test and misfire above 5K

Postby ymmot04 » Tue Jul 22, 2014 11:39 am

Thanks for all of the tips!

I set the gauges to match when disconnected, but didn't think to verify they still match with vacuum being pulled. I'll test them all on one of the carbs to make sure it reads the same across the board. I'll also pick up a set of valves.

The one sync I tried to do which was fairly difficult without the valves and with the cylinders being so sporadic, I was doing them at about 4K RPM. I'll focus on cleaning things up a bit and then trying at 1K.

I did not do any testing on the slides other than verifying they moved freely by hand. Also.. when I was disassembling I lost track of which slide went to which carb. There is the possibility that a couple aren't perfectly matched so I guess I will pull the whole thing out and try to measure everything to make sure they are best fit. Aside from that, is there anything else I can do to help them move easier? Special lubricants or anything I should be aware of for those slides? I cleaned them out well and then hit them with a tiny amount of silicone spray when I put them together the first time.

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virgilmobile
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Re: One cylinder far off on vacuum test and misfire above 5K

Postby virgilmobile » Tue Jul 22, 2014 12:46 pm

Never lube the slides..dust and debre sticks to it...tolerances are very tight and a speck of dirt can jam a slide.
Here's a bit about mine..viewtopic.php?f=4&t=8928
I used 0000 steel wool to polish my slides and caps then cleaned with brake cleaner....
Test the slide in its cap without the spring...it should turn free,and float top to bottom with no catching...I also double check them after there put back on by attaching a shop vac to the down pipe and operating the throttle...They usually slide about half way up...again very smooth and in sequence with the throttle movement.
I did have one sticking on mine during the bench test after i serviced them.I rotated the cap 180* to clear it up...
The fish tank air valves are attached close to the engine..(I think I spoke wrong before) and adjusted till the needle just bounces a tiny bit...It lets the hose act like a reserve chamber to stabilize the vacuum.
This makes it easier to get a accurate reading...It does slow the response of the needle quite a bit...
Do not be concerned with the numbers...it's not the amount of vacuum on each cylinder...it's the balance of vacuum between the cylinders....
Your looking for each cylinder to draw within 1" to 2" of vacuum compared to any other one...
The closer... the better the balance of the engine at 1000 RPM

While the carbs are "on the bench".....do a Pre-sync of the throttle plates...
Pull the rubber loose and look at the gap at the bottom of every throttle...I'll bet #2 is closed off and the other 3 are opened...
You can adjust them right there closer and faster than on the bike...
When your done,as you turn the idle screw,you should see all 4 throttle plates open the same amount.
I have a lamp I shine down mine from the plenum to see the gap.

I bought a set of gauges once....All but one are in the trash can...Each one read different...I set one at 10" and the other 3 varied by at least 5" either way...I deemed them junk...
I now use a different method...viewtopic.php?f=16&t=8994

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virgilmobile
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Joined: Sun Sep 19, 2010 5:39 pm
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Re: One cylinder far off on vacuum test and misfire above 5K

Postby virgilmobile » Tue Jul 22, 2014 12:57 pm

By the way.the "sync" is pretty much negated above 2000 rpm.
1000-1500 is where you need the smooth thumping of all 4 cylinders to be the same.




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