Blocked pilot jet - any tips?

[Argee]

Ladies and Gents,

I'm just in the stages of tuning my GS1100GL following a top-end rebuild, and have been using a color-tune to set the idle mixture.

What I have discovered is that the No 3 carb has a blocked pilot jet. I expect that I know the answer.....but I wondered if you have any tips for clearing the blockage without doing a complete carb overhaul (which I did a couple of years ago)?

Thanks in advance,

 

[chuck hahn]

unscrew it and poke the holes out with a wire and rinse with carb spray and air. Poke the wire into the end as well to be sure the hole down the center is open. AND you mayn have to clean the tank to stop crap from migrating to the carbs in the first place. At least try and find out where or how crud is making its way to the carbs.

 

[chuck hahn]

Lots of carb spray and air into the carb body passages wont hurt either.

 

[Argee]

Thanks Chuck. I'll give that a go.

 

[aclaytonb]

Dudes over at the CB750C site used to swear you should never use carb cleaner on these old bikes. They said always used nonchlorinated brake cleaner - easier on the rubber bits, orings, etc. I always use brake cleaner to good effect.

 

[Steve]

..., and have been using a color-tune to set the idle mixture.

How's that ColorTune working for you? :-k

Many of us have tried it on the CV-type carbs with very mixed results. Some say it worked for them, but more of us have found that the color never changed. The frequency of the flame became more erratic, then finally disappeared when the screws were turned IN. When they were turned back OUT, the flame started popping back into existence, finally getting regular again.

Dudes over at the CB750C site used to swear you should never use carb cleaner on these old bikes. They said always used nonchlorinated brake cleaner - easier on the rubber bits, orings, etc. I always use brake cleaner to good effect.

Not sure what difference that would make on all the brass pieces. Unless you use a strong acid, nothing is going to be on them long enough to do any damage, and they will be rinsed and dried before being re-assembled, so the cleaner that was used won't really matter. As far as spraying any rubber bits, it still doesn't matter, but only a quick spray, followed by wiping the crud off. The solvents in the two sprays are designed to remove two different types of build-up. Carb spray happens to dissolve carb-related deposits a bit better, I will continue to use it (sparingly, of course).

.

 

[850 Combat]

The pilot circuit has an air jet in the bell mouth. I once had a GS1100G carb with a blockage there,which I did get clean with carb spray.

 

[Argee]

Hi Steve,
I've used the colortune before and it seemed to be pretty consistent.
The trouble I had this time was that the engine was not idling well but was pretty good at about 2000rpm. Having recently seen a diagram that showed that other jets take over more as the throttle butterfly opens, I realised that I wasn't getting an accurate assessment of the pilot jet. Obviously, backing off the rpm to idle then made it run a little rough. At least the colortune let me see that I was getting a spark but no fuel into cylinder 3 at low rpm.

 

[Steve]

Always a good thing. :encouragement:

Even if your idle is at 2000, you are still on the pilot circuit. In fact, if you hold a steady 5000, you are likely still on the pilot jet. That chart is pretty accurate, but remember that it is related to throttle opening, not engine speed. If you open the throttle all the way, while at 2000 rpm in fifth gear, you are trying to run on the main jet. Fortunately, your CV-type carbs keep the slides down to keep the velocity of the air over the jets high enough to work.

.

 

[Argee]

That makes sense Steve, thanks!

I've been 'around the houses' with my diagnosing today, and come back to the opinion that I have a blocked pilot jet on the number 3 carb.

As I'd had the carbs off and apart while I was doing my top end rebuild, I wondered if the carb balance had gone out of whack. So I hooked up my vacuum gauges and got this:

View attachment 54366

Although 1,2 and 4 were fairly close, no.3 cylinder had a really low value. The value would stay low even when I revved the engine.(The vacuum in 1,2 and 4 would increase with the revs.)

I wondered if there was some sort of trouble with the throttle butterfly so I took off the carb assembly to have look. Everything looked fine. I then used a couple of 2.5mm drill bits as spacers to set the throttle sync and gave everything a good soaking in carb cleaner before putting it back together.
When I started it up, I still had the same low vacuum in no.3 but 1,2 and 4 were very close together in value. I wondered if I'd got the exhaust shim wrong and that I was losing compression - so I tested every cylinder and they were all pretty even, so discounted that as the issue.
I then wondered (which I believe is the case) if getting a good burn in the cylinder and with it escaping through the exhaust port made a big difference to the vacuum level at the inlet.
I put the colortune back on cylinder 3 and found that I still had a good spark, but no ignition. When I wound the pilot needle all the way out, I found that I could get the occasional igntion (phutt and bang noises.)
I reset the pilot needle to 2.5 turns open from fully closed and went for a little ride in the hope that a little heat, the carb cleaner and some of the stuff they put in fuel these days would help to clean it out.
I can report that with the throttle open a decent amount, she runs smoothly and accelerates very well - thank you very much! .....but at low throttle settings she does not sound as good with the occasional misfire.
I think I'm going to have to take the no.3 carb apart and clean out the holes in the pilot jet.... :-/

 

[chuck hahn]

I would resoak that carb in the dip for another full 24 hrs AND reclean all the jets. Some passage may also need that extra soaking to shed any blockages.