Low compression

[Christopher]

Ok wow I really appreciate all the replies I got with so much solid information to consider. To give an update and further information. I will retest the compression with all plugs removed to get a more accurate reading. When I cleaned the carbs what I did was just the basic with my ability and that was to unblock and clean the pilot and main jets. Also I sprayed carb cleaner through all the passages and checked that it flowed through and came out the other end to ensure they werent blocked. Thats all I did but I realize it might need further work. It has the slide carbs GN model. Im pretty sure I bought a new oem fuel tap after messing about with cheap faulty after market ones so thats now ok. The airbox snorkel is missing (I just realized) but I see a few available on ebay so I will get one. The bike has a boyer bransden ignition which I believe is working. I dont know why only header 1 and 2 a little are getting hot so I might have another go at the carbs. I will definitely check the valve clearances next although im a little intimidated by the difficulty of this job but also keen to get into it. Thanks again for all the help so far and I will update as I progress with it.

[Christopher]

Any advice or links regarding valve clearances would be greatly appreciated? Do I need any special tools? Thanks Lads

[KiwiAlfa156]

Checking, rather than 'setting' valve clearances is what you want to do. Setting needs a special tool (valve bucket depresser) and at this stage, unless you your shim to cam clearance is zero or less than zero, it isn't stopping your engine from running properly, Perfect valve clearance in low priority in trouble shooting this. Although this would eliminate one potential issue.

The coils are paired with one firing 1 and 4, and the other firing 2 and 3. Just make sure that this is how they are connected. Indeed you want to check all the connections to the coils and the leads. Usually any ignition issue related to a single coil will effect the cylinders they service. So cylinder 3 and 4 being cold is, if everything is connected right unusual for an ignition fault. You could try swapping the leads between 2 and 3 to see if the problem shifts with the spark lead. Again check everything against the wiring diagram. I don't know anything about the Boyer ignition, someone else will pipe up about this, no doubt.

Some folk suggest poking wire through the jets, I'm not a fan. I use nylon fishing line as nylon is softer than brass and you can find stuff stiff enough to do the job. Wire can scratch the inside bore of the jet -which is the precision feature of the jet that actually meters the fuel. And if you are cutting wire with a pair of snips, the cut end will be shaped like a chisel.

If you can get carb cleaner to squirt though the various passages, this is good sign. If you remove the pilot jets, make sure all the holes are clear. There is one in the tip and a series of holes on the sides. You might even replace them as they aren't had to find. If you squirt aerosol carb cleaner (with the small tube that fits in the aerosol button) up the pilot circuit where the jet was, you will get carb cleaner coming out one of the small apertures in the bell mouth, but more importantly you should see the carb cleaner coming our of some small holes on the floor of the bore of the carb on the engine side of the needle jet. These tiny holes are the pilot and by-pass orifices and it is through these that the idle fuel/air mix and just off idle (throttle cracked) fuel air mix passes.

The brass screw at the bottom of the carb adjacent to the float bowl is the mixture screw. It had a very fine point at its tip and sits directly underneath the pilot orifice. The tip fits into the pilot orifice to give mixture adjustment. This screw tip is very easily damaged so has to be handled with care. If you remove it you should be able to see the underside of the pilot orifice and squirt carb cleaner though it. If mishandled the the tip can break off in the orifice and block it and stop the cylinder receiving any mixture at idle speed.....

When replacing the the mixture screw, it should be screwed in until LIGHTLY seated. The turned out the appropriate number of turns to get things running. These are then adjusted as part of the fine tuning process.

The choke system is completely separate to the idle/pilot system, hence the previous question of choke on/shut. As this help determine ignition vs. fuelling. I'll look for some photos of what I'm banging on about.

[KiwiAlfa156]

Here's a diagram of the Mikuni VM idle/pilot circuit

[Rich82GS750TZ]

I don’t recall the OP stating the year of his bike. So not sure if CV or VM carbs?

[Nessism]

Any advice or links regarding valve clearances would be greatly appreciated? Do I need any special tools? Thanks Lads

https://gsarchive.bwringer.com/

Download the factory service manual, and read. It's all quite clear.

[KiwiAlfa156]

I don’t recall the OP stating the year of his bike. So not sure if CV or VM carbs?

In the OP he described it as a GS850GN which was the first model in 79. VMs and a kick-starter. Confirms slide carbs above.

[KiwiAlfa156]

You'll need a tool like this to swap valve shims without pulling the camshafts

https://www.mandp.co.uk/products/suzuki-valve-shim-tool-500596

[uk gs nut]

A cable or zip tie down plug hole works to do shim changes. Look for zip tie method in search bar. Carb O rings etc can be supplied by Brian McCall at Centurion motorcycles in Hexham, he has an Ebay shop.

[Rich82GS750TZ]

In the OP he described it as a GS850GN which was the first model in 79. VMs and a kick-starter. Confirms slide carbs above.

It was right there and I missed it. Carry on.

[Christopher]

Reply to Kiwialfa156 Thanks for the pilot system info (where is this from?) I didnt remove the pilot screw when cleaning carbs. What do you use to plug the threaded hole and the pilot jet as per the instructions? Can you just use your finger? Ill have to check again if header is hot just while idling on choke or also when giving some throttle. I think it was both. I think I will again clean carbs with this additional info as my next task aswell as recheck the sparks.Then go onto valve clearances. I previously had weak sparks to all cylinders before I replaced the coils leads caps RR and stator but didnt recheck after this work. Atleast it was charging which it hadnt been before? Do you have any further instructions for cleaning all the mikuni vm carbs? Thanks alot Chris

[Rich82GS750TZ]

There is a link to a VM carb cleaning tutorial on BikeCliff's website. under the the Wiring diagrams, right column, third link down. Or just click on THIS.

[KiwiAlfa156]

It was right there and I missed it. Carry on.

All good, my friend.

[KiwiAlfa156]

+1 on the BikeCliff VM tutorial. It's excellent.

[KiwiAlfa156]

Reply to Kiwialfa156 Thanks for the pilot system info (where is this from?) I didnt remove the pilot screw when cleaning carbs. What do you use to plug the threaded hole and the pilot jet as per the instructions? Can you just use your finger? Ill have to check again if header is hot just while idling on choke or also when giving some throttle. I think it was both. I think I will again clean carbs with this additional info as my next task aswell as recheck the sparks.Then go onto valve clearances. I previously had weak sparks to all cylinders before I replaced the coils leads caps RR and stator but didnt recheck after this work. Atleast it was charging which it hadnt been before? Do you have any further instructions for cleaning all the mikuni vm carbs? Thanks alot Chris

Hi Chris, using your finger to plug thing is fine. Whatever works for you. Each of carb circuits (choke, idle/pilot, needle/main), mix air and fuel and THEN let this mix into the carb bore through their outlets to mix with the air coming through the bore. This froth or emulsion forms smaller, more easily combustible, droplets when entering the main air flow. The holes on the side of the pilot and on the emulsion tube (which is either part of, or feeds the needle jet) is there to do this 'pre-mixing'. All the holes in the jets, all the outlet orifices all the internal passages in the carb need to be clean and clear for the carburettor to work as intended. So generally a full strip down and scrupulous cleaning is the only way to guarantee this.



Having a plastic organiser box is an easy way to keep the pieces sorted during disassembly, cleaning and reassembly.



You can clean the bits manually, taking care to to scratch the interaction. I used plastic cleaning implements.

You can can also soak the parts if you have access to carb dip. Or use a ultrasonic cleaner, if you have access to one of those. A YouTube search will show you what these things are.

If stripping carbs you might take the opportunity to replace the nitrile orings that seal the various parts of the carb. They're 40+ years old now, so liable to leak after being disturbed. One of the guys on the forum sells kits.