gs750 hard to start but once started runs good

[OldSoul67]

I'm looking for some carb tuning help. I have a Gs750 that the cylinders were damaged pretty good along with the pistons, well I ended up not being able to find pistons so I went with 850 cylinders and pistons. Ever since that I have not been able to kickstart or use the starter to start the bike without either starting fluid or trying for a couple hours until it finally lights off enough to run. After it gets started though it runs pretty good, still need more tuning there,  but it's hard to go back and forth tuning it if you can't start it. I do have pod filters which I know these bikes aren't to fond of most of the time and the exhaust is.... minimal, its just four individual pipes. Any recommendations for other carbs would be great as well, I've definitely grown to dislike the vm26's.

[wraith]

Is it the old slide carb 850 head you have or just using the gs750 carbs?

 

[OldSoul67]

4 hours ago, wraith said:

Is it the old slide carb 850 head you have or just using the gs750 carbs?

 

I'm using the gs750 carbs

[wraith]

As most of the gs850 had CV carbs (and you don't like the 750 carbs) look out for some gs850 carbs.

I take it you're using the gs850 carb rubbers? 

If so, there's a big difference between the 750 and 850 bore size of carb and you maybe getting air leeks on the carb rudders and that would/could be the hard starting problem.

Have you altered the getting and/or air screws at all?

[Blubber]

On one of my first backyard engine rebuilds - (as a spotty 17 year old) i managed to cock up the cam timing: it was off by one 1 tooth on both cams. 

It eventually would start.

 

Based on that experience: is your cam timing is correct ?

[OldSoul67]

13 hours ago, wraith said:

As most of the gs850 had CV carbs (and you don't like the 750 carbs) look out for some gs850 carbs.

I take it you're using the gs850 carb rubbers? 

If so, there's a big difference between the 750 and 850 bore size of carb and you maybe getting air leeks on the carb rudders and that would/could be the hard starting problem.

Have you altered the getting and/or air screws at all?

I'm still using the 750 heads with the original 750 carbs new boots wouldn't be a bad idea. the carbs were already messed with when I had gotten the bike so I found what the stock setting were for the 850 and set the carbs to that as a starting point. From there I've just taken some time here and there to try and figure out the hard starting

[OldSoul67]

11 hours ago, Blubber said:

On one of my first backyard engine rebuilds - (as a spotty 17 year old) i managed to cock up the cam timing: it was off by one 1 tooth on both cams. 

It eventually would start.

 

Based on that experience: is your cam timing is correct ?

It looked good when I had put it together, but of course it may still be off especially since I'm sure the timing chain is still the original and I didn't break the cases to replace it. If I can get some time tomorrow morning I'll pull the cover off and take a look to be sure

[Dezza]

Is the cold start mechanism on the carbs working and adjusted correctly?

[PanzerWomble]

Have you checked the compression , I'm assuming you bought a second hand set of pistons and barrel block , they might be shite ?  

 

Also - spark on the nail for timing ? 

 

GS750 ...the slide mikunis have just a manual choke flap , should be easy to check if it's working on or off.  

 

If choke is on, compression is ok ( and a test might also hightlight carb leaks to a point),  and still not starting you may want to play with the pilot jet setting and or sizes . More airscrew out to 2.5 turns max ( half a turn and try, then another half a turn etc ) and if still not playing increase the pilot jet one size and start at one turn out again . 

 

Pods and no exhaust as I'm sure you're aware = running very weak , does it get really hot when it runs? It'll probs want a step up in mains , needles rasing as well . 

Stock settings as picture 

If you want a link to download a copy of  the suzi manual pm me 

 

 

Edited November 12, 2021 by PanzerWomble

[OldSoul67]

23 hours ago, Dezza said:

Is the cold start mechanism on the carbs working and adjusted correctly?

The choke is working and adjusted, I made sure that was all right when I had went through the carbs

[OldSoul67]

6 hours ago, PanzerWomble said:

Have you checked the compression , I'm assuming you bought a second hand set of pistons and barrel block , they might be shite ?  

 

Also - spark on the nail for timing ? 

 

GS750 ...the slide mikunis have just a manual choke flap , should be easy to check if it's working on or off.  

 

If choke is on, compression is ok ( and a test might also hightlight carb leaks to a point),  and still not starting you may want to play with the pilot jet setting and or sizes . More airscrew out to 2.5 turns max ( half a turn and try, then another half a turn etc ) and if still not playing increase the pilot jet one size and start at one turn out again . 

 

Pods and no exhaust as I'm sure you're aware = running very weak , does it get really hot when it runs? It'll probs want a step up in mains , needles rasing as well . 

Stock settings as picture 

If you want a link to download a copy of  the suzi manual pm me 

 

 

The barrels are the only thing i bought used, the pistons I had ordered were the  wiseco big bore pistons for the 750 but sized for the 850 bore. I had the barrels bored to match the pistons. Compression came out good this morning, timing is right on, i did have to do a little adjustment and its made it better however it's still hard to start. Ive looked into finding a jet kit because trying to tune it before it has gotten close. At the moment I'm not too far off of the original carb  settings. Thank you for the offer on the manual, I picked up a physical copy shortly after realizing it needed a rebuild

[OldSoul67]

On 11/11/2021 at 4:22 PM, OldSoul67 said:

It looked good when I had put it together, but of course it may still be off especially since I'm sure the timing chain is still the original and I didn't break the cases to replace it. If I can get some time tomorrow morning I'll pull the cover off and take a look to be sure

Update: cam timing is still good and while I was there I re-checked clearances to make sure I had the right shims and everything looked good

[TonyGee]

try tapeing up half the pod's and see if that makes a difference, if it does it confirmes its running to lean. 

[PanzerWomble]

Thanks for confirming

If you have compression , and a good spark, then I'd look at the pilot circuit in the carbs . Quick check using tape  as TonyGee suggested is a good idea . You've upped displacement by 13% , removed the airbox so it will be running lean by quite a bit . 

Pilot air jet / mixture screws control the zone from 0-25% of throttle , main jet and needle control after that to max - so if it revs once its started  started pilot circuit's yer problem fella . 

 

You might get a Dynorod kit for this , but it'll be as easy just to play around yourself as these are simple carbs. 

Edited November 13, 2021 by PanzerWomble

[OldSoul67]

On 11/12/2021 at 5:36 PM, TonyGee said:

try tapeing up half the pod's and see if that makes a difference, if it does it confirmes its running to lean. 

Taped up the pods this morning, but I didn't have time to mess with the air/fuel so I don't know if it helped yet. will try again tomorrow if I can get some more time to mess with it

[OldSoul67]

11 hours ago, PanzerWomble said:

Thanks for confirming

If you have compression , and a good spark, then I'd look at the pilot circuit in the carbs . Quick check using tape  as TonyGee suggested is a good idea . You've upped displacement by 13% , removed the airbox so it will be running lean by quite a bit . 

Pilot air jet / mixture screws control the zone from 0-25% of throttle , main jet and needle control after that to max - so if it revs once its started  started pilot circuit's yer problem fella . 

 

You might get a Dynorod kit for this , but it'll be as easy just to play around yourself as these are simple carbs. 

it definitely revs once its started. I got a chance to tape up the pods, however I didn't get any time to try and adjust the fuel/air so if I can get more time tomorrow or soon after can confirm if it helped or not and go from there

[BigT]

What  are the pilot air and pilot fuel screws set at?

No one's mentioned the pilot fuel screws. Pull the carbs and remove the pilot fuel screws. Are the holes open in the carb bodies, or are the tips broken off in the carb bodies?

Reinstall the pilot fuel screws and set to one turn out. Set pilot air screws to 1 1/2 turns

Check the pods and make sure they're not blocking the air jets in the carb mouth

[OldSoul67]

20 hours ago, BigT said:

What  are the pilot air and pilot fuel screws set at?

No one's mentioned the pilot fuel screws. Pull the carbs and remove the pilot fuel screws. Are the holes open in the carb bodies, or are the tips broken off in the carb bodies?

Reinstall the pilot fuel screws and set to one turn out. Set pilot air screws to 1 1/2 turns

Check the pods and make sure they're not blocking the air jets in the carb mouth

They were at 3 turns out and 2 turns on the air screws , but I've adjusted them 1/4 turn at a time from just a half turn all the way to where the screws are about to fall out. Each time I'd get to 2 1/2 turns on the air screw I'd go another 1/4 out on the fuel and try again starting at 1 1/2 turns on the air screw each time. The tips of the screws aren't broken off, I checked that when I replaced the original ones. The pods aren't blocking the air passage at the mouth of the carb, I still had the same problem running no filters at all whenever I got it running after the rebuild

[BigT]

On 11/17/2021 at 3:52 PM, OldSoul67 said:

They were at 3 turns out and 2 turns on the air screws , but I've adjusted them 1/4 turn at a time from just a half turn all the way to where the screws are about to fall out. Each time I'd get to 2 1/2 turns on the air screw I'd go another 1/4 out on the fuel and try again starting at 1 1/2 turns on the air screw each time. The tips of the screws aren't broken off, I checked that when I replaced the original ones. The pods aren't blocking the air passage at the mouth of the carb, I still had the same problem running no filters at all whenever I got it running after the rebuild

You're going about it all wrong.

Set the fuel screw at 1 turn. Then, starting at 1 1/2 on the air, open it up a 1/4 turn at a time to achieve high idle on each carb. Remember, these only control idle and just off idle.

Your issue appears to be fueling on start up. VMs are known to evaporate fuel faster than other designs. Have you tried putting the fuel cock on Prime for about 15-20 seconds prior to starting? 

[OldSoul67]

1 hour ago, BigT said:

You're going about it all wrong.

Set the fuel screw at 1 turn. Then, starting at 1 1/2 on the air, open it up a 1/4 turn at a time to achieve high idle on each carb. Remember, these only control idle and just off idle.

Your issue appears to be fueling on start up. VMs are known to evaporate fuel faster than other designs. Have you tried putting the fuel cock on Prime for about 15-20 seconds prior to starting? 

I have, I figured it was an initial fueling issue and after sitting for a couple days I leave the petcock open for a few minutes to make sure the bowls on all the carbs would be filled. 

[cnap504]

How did it run before you had piston/cylinder issues?  

 

 

[OldSoul67]

1 hour ago, cnap504 said:

How did it run before you had piston/cylinder issues?  

 

 

It didn't run great, however I didn't get to ride it either, the wiring was cut up so I had to rewire the bike to get it started. I just followed the factory wiring diagram to redo it. After I got it started is when the carbs were rebuilt and I didn't see much of an improvement so that's when I checked compression and the top end rebuild was done

[Bongo_H]

Assuming it has Mikuni CV-carbs:

If the bike SAT for any significant length of time (years) make sure the "enrichment circuit" passages in the bottom of all carbs are clear.

I have a GS1150 that sat for 30 years and had similar issues with starting, but ran "OK", once it was running.

After dismantling and cleaning the carbs multiple times, I discovered the enrichment-circuit ("choke / cold-start") passages were blocked.

- the tiny brass tube that protrudes into in the float bowl chamber is part of this circuit:  Need a TINY, stiff wire to clean this out.

-- And this brass tube goes into...:

- A small passage in the BODY of the float-bowl.  This passage often has a sharp, right-angle cross-drilled passage that is a bugger to clean.

 

I believe this 'enrichment circuit' also comes into play when the CHOKE is OFF, for normal running

Getting this circuit cleared on my old bike made ALL the difference for starting the smooth running after running.

 

And as mentioned above, the 750cc carbs may not be up to the task of feeding the 850cc cylinders with a rich-enough mix.

What COLOR are you getting on the plugs??

Larger pilots and/or main-jets may help, but the main air-bore of the carbs may not be large enough to flow enough air.

The 750 and 850 engines MAY (or may-not) have used the same bore carbs.  I don't know...

[OldSoul67]

5 hours ago, Bongo_H said:

Assuming it has Mikuni CV-carbs:

If the bike SAT for any significant length of time (years) make sure the "enrichment circuit" passages in the bottom of all carbs are clear.

I have a GS1150 that sat for 30 years and had similar issues with starting, but ran "OK", once it was running.

After dismantling and cleaning the carbs multiple times, I discovered the enrichment-circuit ("choke / cold-start") passages were blocked.

- the tiny brass tube that protrudes into in the float bowl chamber is part of this circuit:  Need a TINY, stiff wire to clean this out.

-- And this brass tube goes into...:

- A small passage in the BODY of the float-bowl.  This passage often has a sharp, right-angle cross-drilled passage that is a bugger to clean.

 

I believe this 'enrichment circuit' also comes into play when the CHOKE is OFF, for normal running

Getting this circuit cleared on my old bike made ALL the difference for starting the smooth running after running.

 

And as mentioned above, the 750cc carbs may not be up to the task of feeding the 850cc cylinders with a rich-enough mix.

What COLOR are you getting on the plugs??

Larger pilots and/or main-jets may help, but the main air-bore of the carbs may not be large enough to flow enough air.

The 750 and 850 engines MAY (or may-not) have used the same bore carbs.  I don't know...

I've cleaned the carbs a few times now, but I'll see if there's anything like that on these ones. These are the vm26 carbs and from what I found before the early 850's used the same carbs just with different jetting. I've cut a couple sets of plugs now, the latest ones are a little rich on idle, but I've got the idle circuit adjusted pretty far out right now. It's trying to start now but still not quite enough to get it going yet

[cnap504]

“once it’s running it runs pretty good”.   If it has brakes and decent tires get it fired and take it for a ride.  A little mileage does wonders for these bikes.  Maybe add some fuel system cleaner to the gas