So I have a 74 850 commando that will not start unless gas is squirted into the carb. The carbs are completely clean and gas is getting to them. The bike will start when it is pulled however it will not idle and when you open the throttle a mushroom of gas comes out the back of the carbs. So it is forcing the gas out of the cylinders and basically starving itself of gas. I have also tried to back out the intake valve screws and it made no difference. I have just rebuilt the motor including a re-bore and new pistons and rings where fitted. The cylinders have 80-90 psi. I did a leak down test and found that the valves where not leaking at all. The valve timing is correct to the book, it has been checked by 4 people. The last guy that looked at it watched when the valves were opening and he thought that it was not opening at the right time but he also checked the valve timing according to the book and found no problems. So I am at a loss of why it is blowing gas out of the carbs, any help would be appreciated, thanks.
Blowing gas out of the carbs
- Thread starter zcarman
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I'll go first. What kind of carbs? Going to assume Amals since you said plural.
Almost sounds like the crankshaft is turing the wrong way. :mrgreen:
Almost sounds like the crankshaft is turing the wrong way. :mrgreen:
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zcarman
Have you made sure that the timing of the cam chain is correct? You might need to verify that you have 6 links or 10 pins of the chain between the marks on the sprockets along with the marks on the crankshaft pinion and the intermediate gear. These marks all need to lined up. Did you use a Work Shop Manual? I am assuming that you are using Amals? When you tickle the carbs will it at least burp a bit as if it wants to start? Undo the Idle screw completely and then Use a guitar string First string “E” or a #72 drill to check if the idle pilot jet is clear. Blow out with air. Re-install and turn in completely then back out 1 1/2 turns. This should get you running if it’s the idle circuit.
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Have you made sure that the timing of the cam chain is correct? You might need to verify that you have 6 links or 10 pins of the chain between the marks on the sprockets along with the marks on the crankshaft pinion and the intermediate gear. These marks all need to lined up. Did you use a Work Shop Manual? I am assuming that you are using Amals? When you tickle the carbs will it at least burp a bit as if it wants to start? Undo the Idle screw completely and then Use a guitar string First string “E” or a #72 drill to check if the idle pilot jet is clear. Blow out with air. Re-install and turn in completely then back out 1 1/2 turns. This should get you running if it’s the idle circuit.
CNN
It does sound a little cam timey. You say you had 10 rollers on the cam chain between the mark on the gears but you didn't say that the crank pinion and intermedaet gear marks were on.
Does this mushroom of vapors blow out of both carbs?
I think i recall gas blowing out the back of carbs = lean and smoke out the pipes = rich.
Timing retarded?
Or maybe it's what Dave said, your crank is in backwards. :mrgreen:
Does this mushroom of vapors blow out of both carbs?
I think i recall gas blowing out the back of carbs = lean and smoke out the pipes = rich.
Timing retarded?
Or maybe it's what Dave said, your crank is in backwards. :mrgreen:
There is ten rollers between the sprockets and the little idler gear and the crank pinion gear are lined up properly. Yes the gas comes out of both of the carbs in what seems like equal amounts of vapor. When the bike was running it always was running rich, which was the strange part, because it would go through a set of plugs in a day or two.
If it starts and runs there can not be too much wrong with it. The motor has to suck the fuel and air in before it can blow a 'mushroom of gas out of the carbs. It will only do that if the intake valve is open while the piston is rising. You can verify the timing pretty easily. When the piston comes up does the exhaust valve close ? As it goes down does the intalke open ? As it comes up to tdc compression are both valves closed... And make sure the white wire from the points is on the right. Did you fix the rich running before you rebuilt the motor ? If not, you might have two peoblems to solve. If the idle screws are not making any difference I'd look at cleaning and rebuilding the carbs with standard jets, good floats, new float needles, and a new needle and jet and go from there.
Greg
Greg
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The thing to check is the overlap . Plugs out , rocker covers off .
As the exhausts are closeing & intakes opening . simultaeneously.
Find the point rotateing crank where there both Just Open (same amout )
Now find if youre before or after T.D.C. . Keyways arnt a reliable ' EXACT '
positionng . Could be a rouge setting .
Possibly ign out. possibly float level out.
Mk II Amals without chokes wont start cold without blocking intake to prime.
Theres some two stroke floats , so can be confuseing .
As the exhausts are closeing & intakes opening . simultaeneously.
Find the point rotateing crank where there both Just Open (same amout )
Now find if youre before or after T.D.C. . Keyways arnt a reliable ' EXACT '
positionng . Could be a rouge setting .
Possibly ign out. possibly float level out.
Mk II Amals without chokes wont start cold without blocking intake to prime.
Theres some two stroke floats , so can be confuseing .
The floats cannot be adjusted as far as I can see because they are plastic and the tab will not bend unless I break it and as far as the carb is concerned before I had it running I bought all new needles and jets with the hope of the jets being worn out. So the carb should be good in itself. I did not ever get the valves ground when I had the head out but they do seal so they should be good. The thing that I was going to try next was to try to advance the valve timing little by little to see what kind of results I get.
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zcarman said:
Two ways to adjust the floats, one is you move the valve seat (not that easy to get it right) or buy the new adjustable floats from Amal.
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It does sound like an intake valve problem.
Low compression and backpressure through the carbs.
I am wondering if you don't have a pair of tight valve guides. ie, sticky valves.
You did say that you pulled (towed) the bike, is it possible you have slightly bent the valves.?
Low compression and backpressure through the carbs.
I am wondering if you don't have a pair of tight valve guides. ie, sticky valves.
You did say that you pulled (towed) the bike, is it possible you have slightly bent the valves.?
That very thing happened to me with a sticky guide, right after a rebuild. After driving in the guides the head set up around them and became undersize. Reamed the guides, replaced the valve, cylinder psi went up to 150 a side. Down the road I went.AussieCombat said:
I took out the plugs and valve covers and turned over the engine slowly watching where the piston was to when the valve was opening. I found that the intake valve starts to open about halfway down the intake stroke and does not fully close until halfway through the compression stroke. I just re-checked the valve timing and it is spot on. Has anyone ever heard of factory timing being incorrect? I am going to try to advance the cam and see if I can get the bike to run.
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Sounds like valve timing is way off...............was it checked properly with a degree disc, and dial gauge? Rough check is whether or not pistons are at TDC when inlet and exhaust valves are "on the rock", basically both open approximately the same amount.
At TDC the exhaust valve will be coming closed and the intake will be slightly more open relative to the exhaust at the moment. You can see this by looking into the head from the exhaust cover removed. Of course the view will depend on which cylinder is starting the compression stroke.
You need to get a piston stop and a degree wheel in place and verify you position. I made a stop out of a spark plug. Break off the porcelain and punch out the center. Drill and tap to 3/8(i think), round off a bolt(protect the piston), screw it in with a stop nut.
You need to get a piston stop and a degree wheel in place and verify you position. I made a stop out of a spark plug. Break off the porcelain and punch out the center. Drill and tap to 3/8(i think), round off a bolt(protect the piston), screw it in with a stop nut.
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You have to be very careful with this because one of the valves can jam against the piston stop tool mentioned below. If you remove the spark plug and watch the valves when you rotate the crank you can see the valve move into the area that the stop tool would occupy. You can use a short stop tool but you must avoid turning the crank too far.
pvisseriii said:
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