Throttling problems

[MadsKrabbe]

Hi GSR!

I have '78 GS750 with aftermarket airbox and 4-in-1 aftermarket exhaust.

I have recently cleaned my VM carbs (disassembled, cleaned with carb spray and dipped in soapwater and ultrasonic cleaning. Unfortunately I ordered the wrong float bowl gaskets, so I did have NOT have the float bowls off and have all jets out and "poked" through with needle. Rookie mistake I know, and know I hate that I didn't wait for the right gaskets to arrive before I put it all back together)

My situation is right now:

- Requires choker to start from cold. Idles rather fine.
- After getting warm it can idle well at low rpms.
- When I give it throttle it have problems keeping up and start coughing and setting out.
- I can "slowly" give throttle and it follows.
- If I release throttle abruptly, it does not follow immediately (should it?)

I have nice blue spark on all plugs.

I have noticed that #3 cylinders exhaust pipe is not as hot as the other three, so I dont think it fires properly.

I have just replaced old rubber air intake boots, and I have replaced intake O-rings.

PO said, that he had valves adjusted, and I also got the bike with old shims and a valve tool. (How big of a task is it to check valve clearances?)

From what I have read so far, the problem could be clogged main jet or maybe "faulty" jet needle?

What do you think? I will try to put up a video link so you can hear the throttling issue when I get the chance.

Thanks in advance to this great forum!

Best regards,
Mads

 

[tom203]

What's an aftermarket airbox? pic?
Does #3 carb have a vacuum conection to petcock?
Don't believe that PO checked the valve clearances correctly..... check them yourself and be sure!
It's hard to clog a main jet... did you note its size? Some PO's can be creative.

 

[MadsKrabbe]

It looks like this. (I have recently replaced the filter material)

i don't see any hoses from the petcock to the carb except for the fuel line. Have I completely missed something?

i will check valve clearances when I get the chance. (This is a rather complex operation or?)

Unfortunately, I didn't note the jet sizes, but I can check them again.

 

[MadsKrabbe]

The below picture is appearantly very small. (At least on my phone

I had to make my own air intake boots from silicon tubing because the old ones were done and I couldn't get my hands on a new set

 

[tom203]

I see your custom airbox and to me it seems very small compared to stock unit- but I'm not familiar with the vm setup, so wait for someone else to respond. Did you get this bike without any airbox?
Valve checking is simple and a must to do! don't rely on any promises. You'll need a camcover gasket,gauges,and a few tools. Have a look here, this is for 8 valve suzukis,no matter whether vm or cv carbs

 

[MadsKrabbe]

I think it is slightly smaller as well, but I think the overall "filter area" is at least comparable. I got the bike with this aftermarket airbox.

I think the link is missing for the guide.

And thank so much for answering. I'll try to get around to checking the valve clearances asap.

But you don't think it is related to any jets being clogged?

 

[Blue Falcon]

thats the stock air filter with a metal shroud around it.... looks home made. It's like they slapped a top and bottom on it and rolled...

 

[Guest]

IDK, it looks to be a big air filter that covers all four carbs to me, much like you'd put on a bank of Webbers on a car. Either way, I would treat it the same as individual pods and increase the main jets accordingly if it still has the stock sizes. You are surly getting a greater air flow over the opening from a stock air box, which is a small slit, well at least the L models.

 

[MadsKrabbe]

According to the manual, the stock sizes for mains are 100, do you have a suggestion for what size to increase it to? (I still haven't had the chance to check what is actually in there, and whether it has been rejetted for the new airbox and exhaust)

 

[Guest]

I can't give you an exact number or size as there are many factors that come into play like your elevation and condition of your engine and I really don't have the experience tinkering with those carbs. I would think a minimum of four sizes up over stock as a start. A friend once told me 5 sizes up for pods, but it may need the sizes either side of that and a possible adjustment on the jet needle. I suggest if you can afford it to get multiple sizes like 4, 5 and 6 over stock and see what works best. So if you have 100 stock mains, I'd order maybe 120, 122.5 and 125, if they were pods thinking that the 122.5 is 5 sizes up and each size up or down. You might also contact your source for jets and talk to them, tell them what your trying to do, to see what they suggest and if there may be a possibility of returning the ones you don't need after you figure it out. You never know, they might do that for a minimal loss in restocking cost as long as you don't damage them. Depending on the price and possible future bikes, it may be worth holding on to them instead of returning them.

 

[MadsKrabbe]

Tank you guys for all the feedback. I have had some time to look at the bike again this weeekend. I've tested coils and tried to clean the points. All this looks alright from what I could gather. I then had the carbs off again for thorough cleaning and new o-rings. Mains are 110 so I think PO have changed to accommodate for the new air box.

after reassembling it ran a lot better. However it still doesn't run on cylinder 2. There is fuel in the float bowl and it does spark.

the only obvious thing I haven't checked yet is valve clearance. Could this be next?

thanks a lot guys!