So recently bought a 1980 gs750l to have some fun with. So when the bike is in neutral you can snap rev it and gradually rev it above 3k with no problems. But when I decided to take it out for a ride under a load in 2nd through 5th it will just spit and sputter when it's in 3500 to 4k anything below is fine. Is it to small of a main jet drive bad vaccum needle? Also does adjusting the fuel screw mess with how much fuel the main jet gets or only during idle to 20 percent range?
1980 suzuki gs750l won't go above 3500 rpm
1 - 20 of 25 Posts
Joined
·
6,468 Posts
Are you running vacuum carburetors with no air box?
This will explain carburetor adjustment and balancing, unfortunately if you are running vacuum carbs there is a Lot more complexity to it but the basic principles are the same for jetting changes.
Mikuni Tuning and Jetting Guide
This will explain carburetor adjustment and balancing, unfortunately if you are running vacuum carbs there is a Lot more complexity to it but the basic principles are the same for jetting changes.
Mikuni Tuning and Jetting Guide
I just recently installed some pod filters so I began to wonder with stock jets it's probably the cause
Joined
·
6,468 Posts
Jets are likely fine, your problem is the bank of pod filters on a bank of CV carbs.
Paper pods or oiled felt?
... if they are paper pods, throw them in the garbage and put a plenum on the intake or your carbs will never operate in synchronization with one another.
States they are woven cloth so probably paper. Will do trash can they go and I guess I'll be putting the stock airbox back on tonight
Joined
·
6,468 Posts
Very wise, although it would normally be described as treated paper. Woven cloth sounds like something was lost in the Chinese to English translation.
... If your air box is actually restrictive with a good filter installed, you can possibly enlarge the opening where the air enters the box, that will give you the extra breathing if you need it.
I very much appreciate your help and cannot wait to get it back on the road since it hasn't since 2014
Joined
·
6,468 Posts
Straight header pipes is not helping your performance either, but it's not the source of your intake problem.
Definitely I know that I'll be ordering the new exhaust here soon I got the bike for 250 bucks and compression checked it at 120 to 125 per cylinder clutch will have to be replaced here soon which is when I plan to do exhaust. Just fixing alot of jerry rigged parts and electrical.
Joined
·
6,468 Posts
What's the compression suppose to be? That sounds low, compression on my trials bike is 188.5psi, check your service manual, check your valve clearances and test it again.
... trying to remember when I last wore out a clutch, pretty sure it was never.
... trying to remember when I last wore out a clutch, pretty sure it was never.
Service manual normal is 128 to 171. Limit is 100. I also did it on a cold motor.
Joined
·
6,468 Posts
I can tell you this; if my trials bike compression drops to ~160 the bike becomes impossible to start until I adjust the valves and then it runs like new again. Check your valve clearances 
they might be responsible for the difference between the cylinders and consider this:
Your carburetors need to balance during operation, how can you balance a bank of 4 carburetors that operate off vacuum, if your compression pressures are low and different between the 4 cylinders?
they might be responsible for the difference between the cylinders and consider this:Your carburetors need to balance during operation, how can you balance a bank of 4 carburetors that operate off vacuum, if your compression pressures are low and different between the 4 cylinders?
I'll rip it all apart tonight and see what all the seats and clearances tonight I really appreciate the help.I can tell you this; if my trials bike compression drops to ~160 the bike becomes impossible to start until I adjust the valves and then it runs like new again. Check your valve clearances
Your carburetors need to balance during operation, how can you balance a bank of 4 carburetors that operate off vacuum, if your compression pressures are low and different between the 4 cylinders?
Joined
·
6,468 Posts
Hope that turns out to be all it is I'm going riding on studded tires tomorrow, weatherman is calling for freezing rain.
... I'll be riding at the motorcycle scrap yard again if anybody needs me to look for something interesting that's always fun. Never rode through it in winter before.
I'm going riding on studded tires tomorrow, weatherman is calling for freezing rain.... I'll be riding at the motorcycle scrap yard again if anybody needs me to look for something interesting
that's always fun. Never rode through it in winter before.Couple of points:
Woven cloth is not paper, it's what K&N typically use. It's oiled cotton. Most OEM filters I believe are paper and UNI and a few others use oiled foam.
BS series carbs can work fine with good pod filters, but that said, most cheap pod filters are not what anyone would describe as "good" many have poor airflow and poor filtration and some partially obscure the air inlets in the carb and some make it impossible for the slides to lift at all.
That's why it is usually better to get a bike running well on a stock airbox with a clean OEM filter inside. It eliminates one variable. People will tell you that a CV carb cannot be jetted with pods or that they don't work on two strokes. Those are vague generalities that are not always true but are a good indication that maybe don't go down that road just yet.
120-125psi of compression hot, isn't spectacular but the bike should run well enough at those numbers. Old bikes never came with very high compression, but it's always a good idea to check valve clearances anyway.
Short open pipes without mufflers will screw up the jetting and exhaust resonance, so fitting a decent exhaust is always a good place to start.
And that brings us to other things that need to be checked. Start with giving it a complete service. Fresh oil, new filter, fresh spark plugs. Check the HT leads and caps because they break down with age. Testing resistance is easy. Ask if you are not sure how to and get a Factory Service Manual. They are available for download.
If the bike has sat since 2014 there is about a 99% probability that the fuel system will need to be cleaned. If the gas is old, dump it and flush the tank out. Clean out the fuel tap and filter. And then clean out the carbs. It is quite likely that some of the jets and passageways will be partially or completely clogged. That sounds like a daunting task but it's not as hard as it appears. Just read the FSM a few times and approach it methodically and ask questions as you go. If the prior owner emptied the tank and drained the carbs, you might get lucky with fresh gas and a can of Seafoam, but don't bet on it.
This link http://pinkpossum.com/GT750/carb/BS40carbs.htm is for a GT750 Suzuki two stroke, but the carb design is the same as yours. The jets may be different but it gives you some idea as to what is what.
Woven cloth is not paper, it's what K&N typically use. It's oiled cotton. Most OEM filters I believe are paper and UNI and a few others use oiled foam.
BS series carbs can work fine with good pod filters, but that said, most cheap pod filters are not what anyone would describe as "good" many have poor airflow and poor filtration and some partially obscure the air inlets in the carb and some make it impossible for the slides to lift at all.
That's why it is usually better to get a bike running well on a stock airbox with a clean OEM filter inside. It eliminates one variable. People will tell you that a CV carb cannot be jetted with pods or that they don't work on two strokes. Those are vague generalities that are not always true but are a good indication that maybe don't go down that road just yet.
120-125psi of compression hot, isn't spectacular but the bike should run well enough at those numbers. Old bikes never came with very high compression, but it's always a good idea to check valve clearances anyway.
Short open pipes without mufflers will screw up the jetting and exhaust resonance, so fitting a decent exhaust is always a good place to start.
And that brings us to other things that need to be checked. Start with giving it a complete service. Fresh oil, new filter, fresh spark plugs. Check the HT leads and caps because they break down with age. Testing resistance is easy. Ask if you are not sure how to and get a Factory Service Manual. They are available for download.
If the bike has sat since 2014 there is about a 99% probability that the fuel system will need to be cleaned. If the gas is old, dump it and flush the tank out. Clean out the fuel tap and filter. And then clean out the carbs. It is quite likely that some of the jets and passageways will be partially or completely clogged. That sounds like a daunting task but it's not as hard as it appears. Just read the FSM a few times and approach it methodically and ask questions as you go. If the prior owner emptied the tank and drained the carbs, you might get lucky with fresh gas and a can of Seafoam, but don't bet on it.
This link http://pinkpossum.com/GT750/carb/BS40carbs.htm is for a GT750 Suzuki two stroke, but the carb design is the same as yours. The jets may be different but it gives you some idea as to what is what.
Next question I've got a terrible slip at about 5k to 6k I took the springs and caliper read 38.8 (38.5mm) max service wear on the forward springs within limits my clutch pads are 2.7mm (2.4mm max service) pulled off what I would guess is a flywheel style plate with springs inside but yet I see no measurement or inspection of it. But I can wiggle some of them back and forward
![104038]()
Joined
·
6,468 Posts
Were you running synthetic motor oil containing friction reducers at the time?
and did you check the thickness of your clutch friction plates.
1 - 20 of 25 Posts
Join the discussion
Join now to ask and comment!