gs750 hard to start but once started runs good

[OldSoul67]

2 hours ago, cnap504 said:

“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

that's what I've been doing. I've been taking it to work for the past week since i finally got the title. I do need to get some seafoam or b-12 to help clean it out. Tried to fire it today without any starting fluid and it got close, but just couldn't get it. If it'll let me I might take it on a little trip this weekend and see how much it helps

[johnr]

just out of interest, if its a 750 does it still run on points or is it a cdi model? and in either case, have you checked the automatic advance retard unit on the ignition, if the unit has corrosion or the bobweights are seized it could be stuck in either full advance or full retard, and if its points, then you need to make sure the points are clean and gapped and the condensors are new. worth going through the obvious basics first, before diving into the carbs. if it was a bit rough before and now its not playing ball it could be something as simple as the ignition timing being off.

[OldSoul67]

On 11/25/2021 at 4:26 PM, johnr said:

just out of interest, if its a 750 does it still run on points or is it a cdi model? and in either case, have you checked the automatic advance retard unit on the ignition, if the unit has corrosion or the bobweights are seized it could be stuck in either full advance or full retard, and if its points, then you need to make sure the points are clean and gapped and the condensors are new. worth going through the obvious basics first, before diving into the carbs. if it was a bit rough before and now its not playing ball it could be something as simple as the ignition timing being off.

It's still running on points. The points are still original from when I bought the bike, both condensers however are new. The weights were a little sticky but still moved, I took off the weights and cleaned everything, I'm still having trouble. I finally got new pilot jets in so hopefully I can get them put in and get everything adjusted to see if it helps.

[BigT]

I'd pull off the ignition cover and try to start it in a dark shop. See if the points are sparking. Easy to get bad condensors these days.

Also, I'd check voltage to the coils, especially when cranking

[OldSoul67]

On 12/1/2021 at 1:07 PM, BigT said:

I'd pull off the ignition cover and try to start it in a dark shop. See if the points are sparking. Easy to get bad condensors these days.

Also, I'd check voltage to the coils, especially when cranking

The Points are working good from what I can see, I do need new coils, not because they're bad but the wires have definitely seen better days. I ended up re-jetting the pilot I was a little lean, after that I couldn't get it to start again. so back through adjusting the carbs, I did like you had said in an earlier response starting at one turn on the fuel screw and one and a half on the air. I couldn't get the bike to start until 4 1/2 turns out on the fuel, even then it wouldn't idle the air screws did nothing. i eventually got to about 6 turns out on the fuel and the air screws were picking up the idle and now it will finally run without the choke. so as of now to get it running the fuel is at 6 turns and the air is at 3. I'm not sure why I need such extreme adjustments to get it running

[BigT]

Based on those results, your idle circuits are clogged

I didn't look back, did you strip, dip* and new O ring the carbs?

Or, ultrasound 

[BigT]

http://members.dslextreme.com/users/bikecliff/images/vm_carb_rebuild.pdf

This is how to clean VM carbs

If you short cut this, you're taking the long road

Cleaning the carb body is the most important part, followed by making sure the pilot fuel screw tips aren't broken off and still in the body 

[OldSoul67]

4 hours ago, BigT said:

Based on those results, your idle circuits are clogged

I didn't look back, did you strip, dip* and new O ring the carbs?

Or, ultrasound 

They were stripped and dipped, then sprayed carb cleaner through all the passages I could find and had all new rebuild kits put in, new o rings, gaskets, jets.

[OldSoul67]

3 hours ago, BigT said:

http://members.dslextreme.com/users/bikecliff/images/vm_carb_rebuild.pdf

This is how to clean VM carbs

If you short cut this, you're taking the long road

Cleaning the carb body is the most important part, followed by making sure the pilot fuel screw tips aren't broken off and still in the body 

I'll definitely go through them again when I get some time off, it'll probably be a couple weeks until I can get to it, but once I do I'll jump back on and tell every one how it worked out 

[OldSoul67]

well, I got time to put some work in during the holidays. I pulled the carbs apart and re-cleaned them using chem dip and got all new needles and jets to replace some of the last original stuff that had definitely seen better days. Got everything put back together and bench synced and put back on the bike. once everything got adjusted to run I let it warm up and started adjusting jetting for partial, full throttle, and finally mid range. now having said that it is still hard to start and I have some ideas why. since I ended up going with a big bore kit for the 750 it also increased the compression to 10:1 along with raising displacement to 850. now when I originally put the bike together I was planning on it being a kick only bike, well with the starting issues i finally decided to put in a relay and rewire the starter. well, with the higher compression the starter is pulling more amps than it definitely should be and when the bike does try to fire up it pushes hard enough to even blow a 40 amp fuse, but it does try to start once it can get a few revolutions. Back to one of my ideas as to why its hard to start is because of the new compression, just kicking it over I don't think I can get enough dynamic compression and fuel at the same time to make it happy. I went through about 12 sets of plugs tuning it to make sure every major point of fueling that I can measure, from idle to going a little faster than I should down the highway was as good as I can possibly get it. yet I can still only get it to sputter without using starter fluid. I'm not really too pressed about the issue since I don't really daily the bike either, it just would have been nice to not have to carry around a can or two of fluid. I figure just with the higher volatility of the starter fluid that's why i need it just initially, maybe ill change out to some carbs later with some accelerator pumps to help get more fuel on start up. Thank you for all the suggestions, it's definitely not only helped me understand my problem but, learn a little more about how my bike is supposed to work which will only help me in the future.

[Dezza]

Did you clean the float bowls in the same way as the rest of the carbs and are any of the float bowls carb specific, by which I mean the bowls have differences in the internal casting so they have to go on the 'correct' carb? I have not worked on these ancient carbs before but past experience with another non-Suzuki bike makes me wonder if they have cold start drillings in the float bowls and the bowls have become accidentally put back oin the wrong carb a cold-start nightmare can be the result.

[Blubber]

Thing I noticed on my EFE back in the day:

I was running K&N pods and the shed smelled of fuel every time after I parked it.

The fuel in the float bowls evaporated very quickly in that setup.

If you combine empty float bowls with a vacuum fuel tap, it will take multiple revolutions to fill and eventualy start.

 

Just a theory

 

[BigT]

The high compression is not causing your starting issue

My 78 1000 has the WISECO 1085 kit with 10.25:1 compression and it starts just fine

As Blubber says above, maybe the carb bowls aren't filling?  The VM carbs are notorious for evaporating the float bowls empty.

Have you tried setting the fuel tap on Prime for 30 seconds prior to starting?

[johnr]

when you say the adv ret weights are moving, does the advance retard unit rotate by a few degrees when you try to twist it and in doing so pop the bobweights out? it should all move freely with no stiction or seizure.  if your ignition isnt advancing and retarding correctly, you can fit a million sets of carbs and it still wont run.

 

[OldSoul67]

16 hours ago, Dezza said:

Did you clean the float bowls in the same way as the rest of the carbs and are any of the float bowls carb specific, by which I mean the bowls have differences in the internal casting so they have to go on the 'correct' carb? I have not worked on these ancient carbs before but past experience with another non-Suzuki bike makes me wonder if they have cold start drillings in the float bowls and the bowls have become accidentally put back oin the wrong carb a cold-start nightmare can be the result.

the float bowls are carb specific from left to right, the left two are the same from what I can tell and the right two are the same, I can pull them off so see if there are are sort of drillings like what you're talking about but if i remember right I didn't see anything in the diagram for them

[OldSoul67]

15 hours ago, Blubber said:

Thing I noticed on my EFE back in the day:

I was running K&N pods and the shed smelled of fuel every time after I parked it.

The fuel in the float bowls evaporated very quickly in that setup.

If you combine empty float bowls with a vacuum fuel tap, it will take multiple revolutions to fill and eventualy start.

 

Just a theory

 

oh definitely if I don't shut off the fuel you can smell it for sure, however the petcock isn't vacuum anymore it's just gravity feed, most of the bike was in rough shape when I got it and that included the tank so it now has a tank with a vented cap and it's just gravity fed now, I made sure that it was above the carbs which isn't too hard to do with this frame design and increased the size of the petcock outlet and fuel line to help more with fueling. the original vacuum port is capped off.

[OldSoul67]

9 hours ago, BigT said:

The high compression is not causing your starting issue

My 78 1000 has the WISECO 1085 kit with 10.25:1 compression and it starts just fine

As Blubber says above, maybe the carb bowls aren't filling?  The VM carbs are notorious for evaporating the float bowls empty.

Have you tried setting the fuel tap on Prime for 30 seconds prior to starting?

I usually turn it on for a few minutes before I take it out, it's definitely not a daily so I plan for rides and let everything fill up for a while before I actually leave. One thing I've noticed is once it is warmed up I have no problems starting it, but I've also rebuilt everything in the carbs including the choke plungers, I don't run ethanol fuel anyways but I used seals that are safe with it as well.

[OldSoul67]

5 hours ago, johnr said:

when you say the adv ret weights are moving, does the advance retard unit rotate by a few degrees when you try to twist it and in doing so pop the bobweights out? it should all move freely with no stiction or seizure.  if your ignition isnt advancing and retarding correctly, you can fit a million sets of carbs and it still wont run.

 

I ended up pulling all of the ignition components out and cleaning everything so it all moves freely now, before it was a little sticky and I figured it was based off of the original throttle response and even after re setting the timing since the top end was apart

[OldSoul67]

Well after getting a little frustrated with it and letting the bike site for a couple weeks, I've found the problem, but I haven't been able to find the solution. I pulled the cam cover to see if anything changed as well as the head to replace the head gasket. First thing I noticed was when resetting the timing that I was 22 pins apart rather than the 20 I was supposed to have, easy enough fix. I checked my valve lash and was definitely too tight, so I re-shimmed the valves and now have .04-.05 clearence on the valves. The valves shims were the only thing I hadn't messed with during the original rebuild so I figured I had found it. Did a compression test again and had about 130psi on each cylinder, checked the book and said I needed a minimum of 100 so it looked good. Well half an hour of kicking later I decided to do another compression test and had 70psi on three of the four cylinders and 65 on cylinder three. I tried to add  some oil to the cylinders to see if it would help and it did a bit, I got up to 80psi on each cylinder. I figured it would since the ring gaps are a little wide but I was planning to mount a small turbo later after I got the bike running well so it actually worked out where I needed it. Now I just don't know where I'm losing the compression at especially since my first test was good

[Gixer1460]

Its possible, if timing was out a couple of teeth, that you've bent some valves which can mess with valve clearances and compression tests - overly wide ring gaps certainly aren't going to help that either. If you know the engine is worn / weak, it'll never run or tune right! Fix all the problems before even contemplating 'power adders'!

[TonyC]

I once fired up a GS750 engine with the timing 1 tooth out - managed to to get away with 1 bent exhaust valve - must've had tighter clearance than the others!

Much later in it's life the compression had dropped below 100psi and I remember it was a bitch to start but then would run OK - got through loads of oil as well.  Solved the issue with a GSX1100 lump!

Good luck with it.

 

TonyC

 

[cnap504]

You had 130lbs across them it went down nearly 50%, check the cams again for correct timing, and also if your motor has a cam chain tensioner that requires loosening of the lock bolt so the spring can pop in to take up chain slack.  And that adjustment has to be done at a specific crank position, iirc there’s a “T” mark on the timing plate.  When it was out 2 pins there is a chance of a bent valve.  

[OldSoul67]

17 hours ago, Gixer1460 said:

Its possible, if timing was out a couple of teeth, that you've bent some valves which can mess with valve clearances and compression tests - overly wide ring gaps certainly aren't going to help that either. If you know the engine is worn / weak, it'll never run or tune right! Fix all the problems before even contemplating 'power adders'!

Yeah I wanted to make sure it would run well before running any power adders. The rings came out actually to what I was looking for to do about 10psi so I ran with it knowing it will lower the compression a bit, I was just planning for the future once it was running well so I didn't have to pull it back apart. But it looks like I am either way

[OldSoul67]

17 hours ago, TonyC said:

I once fired up a GS750 engine with the timing 1 tooth out - managed to to get away with 1 bent exhaust valve - must've had tighter clearance than the others!

Much later in it's life the compression had dropped below 100psi and I remember it was a bitch to start but then would run OK - got through loads of oil as well.  Solved the issue with a GSX1100 lump!

Good luck with it.

 

TonyC

 

Thank you, I might need it. To me the bike was running good, but to be fair this is the first bike like this I've had so I don't really have a comparison

[OldSoul67]

15 hours ago, cnap504 said:

You had 130lbs across them it went down nearly 50%, check the cams again for correct timing, and also if your motor has a cam chain tensioner that requires loosening of the lock bolt so the spring can pop in to take up chain slack.  And that adjustment has to be done at a specific crank position, iirc there’s a “T” mark on the timing plate.  When it was out 2 pins there is a chance of a bent valve.  

I rechecked the timing and had to fight it skipping a tooth every time I tried to tighten down the idler pulley in between the cams. It is the correct 20 pins now lined up at the timing mark on the advanced side of the 1-4 mark. Looks like I'll be pulling the head off this weekend to see what I have in regards to bent valves. Hopefully I got away with it and just have to lap them back in. If not it looks like a new set and valve job might be in order