Backfiring through carbs

[Firewall]

Hello all,
I recently completed the following on my 77 GS750:

Valve adjustment
Carb cleaning (tear down, soak in berrymans for 24hrs, new orings)
New boots and O-rings between carb and engine
New boots between carbs and airbox
New K&N factory replacement filter
Sealed airbox
New spark plugs

My petcock seems to be fine, it only flows gas when on prime or when vacuum is applied.

I have not yet synced the carbs with a tool, however they are bench synced.

It fired right up the first time I hit the starter and idled pretty nicely. However, it dies with out the choke on and I'm getting some back firing through the carbs.

When I took everything apart, it was evident the backfiring had been happening before I got the bike. One carb had burn marks on the sliding piston and one boot going to the airbox was slightly melted. I got a little turned around but I believe both of these issues were on the same cylinder, #2.

I haven't yet adjusted the air/fuel screws. Right now the fuel is 1 turn out and the air is 2.5, I believe that's the recommended starting point around here. I recorded the numbers when I took the carbs apart, but now I think the screws were gummed up and not actually seated properly when I started counting so I figured I'd just start over. One reason I think this is the wide range of numbers I got for the fuel screws, going from .5 all the way up to 2.5

As I understand it, carb back firing is usually due to being lean. Is this something that will be fixed when I sync the carbs or do I need to take it apart again and start looking for something else?

The bike has what I believe to be a MAC exhaust. The previous owner (a friend of mine, who's father in law is the original owner) said the exhaust was installed at the dealership when he bought it. My friend spent a ridiculous amount of money having the carbs rebuilt (he was not familiar with this forum) 2-3 years ago but I have no idea if the carbs are properly jetted for this bike.

Here's a picture with the exhaust,

[terrylee]

would appear you are right, that is a Mac., I had to up my main jets one size to get mine to not run so lean. I'm no expert on the carbs. But those setting on the screws sounds a bit high. I would check on Basscliff site, should have a section on rebuilding VM carbs. and I believe there is a starting setting on those screws near the end of that. terrylee

[Firewall]

I think you're right. I see that document suggests 1 for the fuel and 1.5 for the air. I won't have time to work on it today but I'll give it a try tomorrow. Thanks!

[Big T]

You're definitely running lean, with backfiring and those colored head pipes

I'd go

Fuel screw - 1 turn
Air screw - 2 1/2 turns
Main Jet -up 2-3 sizes
Pilot jet- stay stock
Needle - up one notch

and , sync them soon

[Nessism]

Air screw should be roughly double fuel screw. I'd try 7/8 turn on the fuel and 1-3/4 on the air.

You need to vacuum sync. No substitute.

Needles should be raised, not sure how much. I'd try two notches and then see how it runs.

Lastly, did you check to make sure the fuel screw tips weren't broken off in the carb bodies when you rebuilt the carbs? It's important that all of these screws have a nice sharp point.

Good luck

[TheCafeKid]

You're definitely running lean, with backfiring and those colored head pipes

I'd go

Fuel screw - 1 turn
Air screw - 2 1/2 turns
Main Jet -up 2-3 sizes
Pilot jet- stay stock
Needle - up one notch

and , sync them soon

Maybe its just my monitor but... What colour?? They're barely gold.. Single wall pipes are going to colour. Theres no two ways about it. If they're not, you're wasting fuel running it too cold.


I dont mean to argue with anyone, but I have always been told, and have adjusted accordingly, that spitting through the carbs is a sign of being too rich, not too lean. When you start your bike with the enrichener (choke) what does it do until the cylinders warm a bit? It coughs a bit, it farts a bit, it spits through the carbs occasionally. Its getting too much gasoline dumped into it. In this instance, its required, you're trying to heat up the cylinders, the gas is not atomizing well, trying to condense on the cold cylinder walls, etc. But, in thinking about it, I guess it could go either way... What you need to do is figure out what the bike is telling you.. Based on what you've stated here, *I* think you're a bit rich on the idle/pilot circuit...

Im not saying you're not lean elsewhere, but, If your bike is starting and running dead cold with no choke, you're too rich on the pilot circuit. Set your pilot fuels as Ed said, 7/8 turn. Air at 1.75, maybe two but id start at 1.75. You MAY have to bump the mains with this system, but I cannot see more than one, one and a half sizes, and the needle, thats the wild card. You/We need more info. How does it progress through the rev range? Does it break up anywhere...fall on its face? Is it running great aside from down low?? Have you done plug chops?


BEFORE you should start messing with the jetting, you need to assess WHICH circuits you're having an issue in. As educated about this subject as a lot of people on here may be, the BEST anyone can do is give you a ball park setting... Unless of course one of them was there with you. EVERY bike is a little different, every one will require slight changes, and without listening/reading what the bike is saying it needs, you're getting shots in the dark with a flashlight at best from us.

Ed is also right in that a dynamic (vacuum) sync will go a long way, but its pretty pointless until you're sure youre close, as to change the needle position in the VM carbs requires removal of the slides, and the throttle arm, which will cause you to have to resync when you put them back together...

[Firewall]

Thanks for the help everyone.

I got some time to work on it after all and took the carbs off again.

Currently, the main jets are stamped "102" is that stock? I just want to make sure this hasn't already been adjusted for this exhaust

Adjusting the needles may be an issue, some of the screws to take the sliders apart are stripping on me :-\

[cowboyup3371]

According to this page...http://members.dslextreme.com/users/...ight.html...it looks like you should have 105s for main jets.

[Firewall]

Interesting, I wonder why someone would put smaller jets in then.

So should I go up 2-3 sizes from stock, or from what is in there?

Fuel screws appeared fine

Btw, definitely was not planning to skip the vacuum sync if thats how it came across, I just didn't want to do it then have to tear back into the carbs and do it all over again

I can't believe I left this out, but the back firing happens when you give it gas, not at idle. I did all my work in the basement and we were trying to ride it around to the drive way so I could do the carb sync.

[Firewall]

Oh, and since it was mentioned here's a better pic of the pipes. Of course who knows when something happened that made them color. This is from the day I got it

[Big T]

Oh, and since it was mentioned here's a better pic of the pipes. Of course who knows when something happened that made them color. This is from the day I got it

What I notice there is that #3 is really lean, with all the blue. Hopefully, that difference was cured by the new O rings and such. There may also have been an air leak at the vacuum port, so be sure to replace your vacuum hose from the carb to the fuel petcock

I'd go to a 110 main jet and make the other adjustments

[Firewall]

Thanks, I ordered the 110's and will keep on trying to get to where I can change the needles

[Firewall]

You're definitely running lean, with backfiring and those colored head pipes

I'd go

Fuel screw - 1 turn
Air screw - 2 1/2 turns
Main Jet -up 2-3 sizes
Pilot jet- stay stock
Needle - up one notch

and , sync them soon

Could you clarify for me, do you mean move the clip up one notch (which I think would make the needle sit lower) or move the clip down one notch which would raise the needle?

And I'm guessing there isn't, but is there any other way to get the same results? I'm having some real issues with the screws getting to the needle clips

[TheCafeKid]

The 77 750 had slightly different carbs than the 78/79 model. It had different mains. 102.5 I believe were in fact stock. The needle also wasn't spring loaded like that of the later models. Couple other differences as well, float height spec, etc. But essentially they should be tuned the same. Now that you've taken a better pic of the pipes I do see the colour. But do bare in mind that those aftermarket pipes will colour some, no matter what, because they're a single wall pipe. Look at Harley, Triumph etc pipes. All single wall from the factory, and all colour out of the head.
So, with that in mind what you really need to look at are the plugs. It's evident that your pipes aren't heating equally, so that's a clue. But the plugs will tell you more. Unfortunately with the ethanol used in today's fuel in most states, plug chops aren't quite as easy to read as they used to be. But they're still very helpful in tuning the individual circuits of your carbs.
Even with your aftermarket pipe, you pilot sizes should be stock, and I'm fairly confident that the mixture screws should be fairly close to where the stock setting should be. Maybe an 1/8 more on the fuel, a 1/8-1/4 less on the air.
With 102.5 as your mains I would shoot for a 107.5 to start with, also remove the needle, and move the needle clip down one slot (towards the point of the needle) this effectively raises the needle, speeding up the time in which it takes to add specific amounts of fuel to the needle circuit (allowing more fuel earlier in the rev band). Again the draw back is every time you pull those slides to change the needle, you'll have to re-sync. So I'd suggest going 107.5 on the main, leaving the needle alone at first. If it still stumbles, then look at WHERE it's stumbling (1/8 throttle? 1/4 throttle? 1/2? So on) as that will clue you in to which circuit needs attention. Once you have it running pretty well, plug chops in the different circuits will show you what circuit needs fine tuning.
Frankly I find the VM carbs to be easier to tune, and more tune-able than the CV type used later on.

[Firewall]

I've already ordered the 110s, they came in today.

Would going with 110 by any chance make up for not changing the needle?

It's not that I don't want to change the needle, but short of trying to drill out these screws that's my only option. I've already let them soak several days with penetrating oil and hit the head with a soldering iron with no luck