Difficulty Starting Bike after Rebuild

[Gowron650]

I am having trouble starting my '83 XS650 Heritage Special which I recently took apart and put back together. I removed the electric starter and so it is kick start only. This is my first time doing this so it's very possible I messed something up.

I had it running a few weeks ago, and then spent some time getting the wiring harness all back together, and doing a few other things, and now I can't get it started at all. I can kick it over and I get spark, but it doesn't fire at all. It won't even fire when I spray starter fluid in the carb throat.

I have noticed a few issues that I suspect are related to the startup difficulty:

• There doesn't seem to be any fuel reaching the cylinder, as the spark plugs are bone dry.
• Compression is only around 75psi in each cylinder.

Because it's not even trying to start, I thought maybe it's a timing issue and tried reversing the spark plug wires, and it did actually fire very briefly! I don't know how I could have reversed the timing. I checked twice and I'm sure I have the valves on each cylinder adjusted to the TDC compression stroke.

What could possibly be going on here? Among the things I have done since I had it running:

• Installed oem wiring harness
• Identified TDC using degree wheel and piston stop
• Tightened cam chain tensioner
• Disassembled and cleaned carbs, replaced choke plungers and dust seals, adjusted float heights
• Adjusted valve clearance

 

[Jan_P]

Is it stock ?
If the plugs are dry ..full choke ignition off ...kick it over a while and see if you can get plugs wet
What petcock
Pri position tested ?

 

[Gowron650]

Is it stock ?
If the plugs are dry ..full choke ignition off ...kick it over a while and see if you can get plugs wet
What petcock
Pri position tested ?

Yes it is stock.

It has the vacuum petcock and I tried the prime position and still no fuel getting to the cylinder. I tried an auxiliary tank and still nothing. There is fuel in the carb float bowls and it seems to be at the right level.

Would the lower compression I’m getting (~75psi) mess up the operation of the carbs?

 

[Niels B]

If it ran before you started to fiddle with it should start again, as long as all adjustments are correct. Do you have point ignition ? Did you mess with them ? You say you have spark but no fuel to the cylinder ! If there are fuel in the carb bowl you should get air fuel mixture to the cyl. - when that’s said 75 psi seem on the low side - 80-100 should be enough to get it running but reality is it should be around 125-150 psi.
Have you tried spraying a little oil in the cyl and then measure psi ?
As Jan say, try to prime a couple of times with full throttle and no ignition, check plugs to see if they are wet. If they are wet try again prime 3-4 times full throttle no ignition - the turn on ignition and see if you can get it to fire.

 

[Jan_P]

Yes it is stock.

It has the vacuum petcock and I tried the prime position and still no fuel getting to the cylinder. I tried an auxiliary tank and still nothing. There is fuel in the carb float bowls and it seems to be at the right level.

Would the lower compression I’m getting (~75psi) mess up the operation of the carbs?

No the low compression wont disturb the carburetors ..but it can make ignition difficult but it is not straight forward measure It should happen with open throttle.

The dry plugs is a problem Possibly sticking float valves.
I would start tapping with a screwdriver handle on the outside
And if that does not help have the bike petcock off not running and then open the float bowl
( it depends how it looks ..there can be a drain plug or so )
from the closed position move to Pri see if there is petrol coming through be careful so it dont catch fire

Is there any difference left and right .. Any cylinder behaving better ?

 

[Its been a long time]

A stock motor 83 running TCI in wasted spark, it'll make zero difference swapping the HT leads over. Both plugs spark at every ignition event.

 

[Gowron650]

No the low compression wont disturb the carburetors ..but it can make ignition difficult but it is not straight forward measure It should happen with open throttle.

The dry plugs is a problem Possibly sticking float valves.
I would start tapping with a screwdriver handle on the outside
And if that does not help have the bike petcock off not running and then open the float bowl
( it depends how it looks ..there can be a drain plug or so )
from the closed position move to Pri see if there is petrol coming through be careful so it dont catch fire

Is there any difference left and right .. Any cylinder behaving better ?

I tried priming by kicking with the ignition off and full throttle, and still no fuel reaching the cylinders.

Tapping on the float bowls didn't do anything, so I pulled the carbs and examined the float needles and they don't seem to be sticking and they move freely up and down. What would be a way to test if they're sticking?

There is fuel getting to the float bowls. I attached a clear fuel hose to the drain nipple on the float bowls and the fuel is up to just ~2mm below the top of the float bowl.

 

[Niels B]

Have you check that all your jets are clear? What carbs are you running bs34?

 

[Gowron650]

Have you check that all your jets are clear? What carbs are you running bs34?

Yes I am running the bs34 and the jets are clear. I cleaned thoroughly using carb cleaner and compressed air. Also to answer a previous question above I am having the same issue in both cylinders. No gas getting to either cylinder.

 

[Niels B]

It’s hard to pinpoint but if you are sure the carbs get fuels the below are most common reason for not getting air/fuel mixture to the cylinder/ chamber

1) Clogged Jets: While a clean carburetor might look good, small jets can still be clogged with debris, preventing proper fuel flow.
2) Worn Metering Needle/Throttle Shaft: Over time, the metering needle or throttle shaft can wear, affecting the air-fuel mixture.
3) Leaking Float Valve: A leaky float valve can cause the fuel level in the bowl to be too high or too low, affecting the air-fuel mixture

That’s what I can think of based on your previous post. You could possible get false air at the throttle shaft - did you replace the little o-rings ?0

 

[GLJ]

Compression is only around 75psi in each cylinder.

That is pretty low. You need to find out why.
Start by rechecking valve adjustment and cam timing.

 

[AUTOMAN]

Might as well check intake vacuum while cranking at zero throttle.
Hook up vac. gage to line for petcock.
As glj said, check valve lash.

Carbs are full so no lack of fuel supply, demand is another story(valves)

 

[Gowron650]

It’s hard to pinpoint but if you are sure the carbs get fuels the below are most common reason for not getting air/fuel mixture to the cylinder/ chamber

1) Clogged Jets: While a clean carburetor might look good, small jets can still be clogged with debris, preventing proper fuel flow.
2) Worn Metering Needle/Throttle Shaft: Over time, the metering needle or throttle shaft can wear, affecting the air-fuel mixture.
3) Leaking Float Valve: A leaky float valve can cause the fuel level in the bowl to be too high or too low, affecting the air-fuel mixture

That’s what I can think of based on your previous post. You could possible get false air at the throttle shaft - did you replace the little o-rings ?0

Thank you for the ideas. I cleaned the carbs once more, using carb cleaner and compressed air on all jets, and this time I also replaced the float needles with replacements from a MikesXS rebuild kit.

I am actually able to start the bike with starting fluid, but it won't stay running. I think it's just burning the starting fluid and dying. It still doesn't seem like there is fuel getting to the cylinders. When I try to prime the engine and check the plugs they are dry.

For your other ideas - worn metering needle/throttle shaft - I will look at these things but would they cause there to be no fuel at all getting to the plugs?

 

[Pat D]

Thank you for the ideas. I cleaned the carbs once more, using carb cleaner and compressed air on all jets, and this time I also replaced the float needles with replacements from a MikesXS rebuild kit.

I am actually able to start the bike with starting fluid, but it won't stay running. I think it's just burning the starting fluid and dying. It still doesn't seem like there is fuel getting to the cylinders. When I try to prime the engine and check the plugs they are dry.

For your other ideas - worn metering needle/throttle shaft - I will look at these things but would they cause there to be no fuel at all getting to the plugs?

Here's the thing- the engine itself is an air pump. Air is drawn into the cylinders via low pressure created when the intake valve is open and the piston is going down. Given your dodgy compression readings, it may be that the "pump" isn't generating enough low pressure/isn't efficient enough to draw fuel into the carbs. That could be rings not sealing, or valves not sealing. Kinda like trying to drink a drink through a straw when there is a hole in the straw- you get air instead of drink, or, at best, a frothy mix of air and drink. AUTOMAN's suggestion of checking vacuum while cranking sounds spot-on to me- I suspect it's low. Then you get to track down the source of the air.

 

[Gowron650]

Here's the thing- the engine itself is an air pump. Air is drawn into the cylinders via low pressure created when the intake valve is open and the piston is going down. Given your dodgy compression readings, it may be that the "pump" isn't generating enough low pressure/isn't efficient enough to draw fuel into the carbs. That could be rings not sealing, or valves not sealing. Kinda like trying to drink a drink through a straw when there is a hole in the straw- you get air instead of drink, or, at best, a frothy mix of air and drink. AUTOMAN's suggestion of checking vacuum while cranking sounds spot-on to me- I suspect it's low. Then you get to track down the source of the air.

Thank you that makes a lot of sense. I will get my hands on a vacuum tester and check the vacuum.

I tried doing a leakdown test based on the low compression readings but my untrained ears weren't able to identify the source of the leak.

Is the vacuum test going to test different components/point to different issues than the compression test? For example I know that my carb intake manifold boots are stock and are looking pretty gnarly. Would they contribute to low vacuum pressure in the carbs?

 

[AUTOMAN]

No need if L-D was done and failed.
What were the numbers?

Really pointless at this time.

 

[Gowron650]

No need if L-D was done and failed.
What were the numbers?

Really pointless at this time.

I ran the leak down test at around 90psi and both sides showed around 11% leakage, and there was definitely air leaking from the spark plug holes. I couldn't get the o-rings on the leak down tester to completely seal. Should I take that as a good sign that there isn't major leakage?

Also I think I could hear air escaping near the carb intake manifold boot on the right side.

 

[Gowron650]

As the carb boots are worn and cracked, I have tried to buy replacements, but I can't find ones that fit. And MikesXS is sold out of the replacements for the '83 XS650.

The OEM carb boots have a lip that is about 4mm from the edge, which matches the lip around the outlet of the BS34 carb. The two aftermarket pairs I have purchased so far have a lip that is ~8mm from the inside edge, which doesn't fit. Even using a little grease and a heat gun and hulking them on there, they just pop right back off.

I have attached pictures for reference.

Does anyone know a reliable supplier of replacement boots?

 

[jetmechmarty]

GTS “XS650 carb holder”. $14 to $80. Lots of them.

yamahaxs650.com has them for $40. He’s a fast responder.