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Full throttle and not idling

  • Thread starter Thread starter Swim_fan02
  • Start date Start date
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Swim_fan02

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Hey guys. So I reshimmed the valves and went to start it up and it fired right up. Let it run for a few minutes then it stalled out. Now I can't get it started and stay running even with the throttle held wide open. I'm 100% sure the timing is on. I pulled the plugs out and the left cylinder was clean and just a bit wet and the right was dry black and sooty. The carbs are clean as a whistle, all intake boots and o-rings are sealed with no leaks and in excellent condition because they were just replaced. The PO put high flow baffled slip on mufflers and header wrap. The main jets are 115 and I'm not sure what the pilots were. I believe they were 17.5. But that is just a guess. I can't remember exactly when I had them apart. It has the stock airbox on. It's like it's not getting enough fuel. Maybe the pilot is too small and wasn't jetted right for the slip on mufflers and wrap? Btw it's an 81 450l. Any help would be greatly appreciated. Thanks in advance.
 
How much gas is in the tank? Is anything flowing into the carbs (check by unscrewing a bowl drain screw)?
 
DO NOT twist the throttle..it will NEVER fire if you do so. How much fuel is in the tank? Has it sat a while..if so set it to PRIme for a few seconds after it starts to be sure the bowls get full..then swing back to ON.

Just choke it and hit the button if its got at least a gallon of fuel in it. If your unsure how much is in it, then choke it, set to PRIme and start it.
 
Once its running and warmed up well, set the idle at around 1200 with the big idle knob in the middle of the rack.
 
Which is why I clearly stated to swing it back to the ON position after a few seconds once it starts. Dont want to flood the crankcase with gas.
 
Yea it's got fuel in it. It was running yesterday before I did the shimming. There was only 1 valve that was out of spec. You think the petcock just decided to randomly take a dump?
 
What about the jetting? Should I have to rejet with the slip ons and header wrap?
 
Are they more or less restrictive than whatever it was tuned for before? Are the 115 main jets the ones it came with or have they been changed?
 
tkent02 said:
Are they more or less restrictive than whatever it was tuned for before? Are the 115 main jets the ones it came with or have they been changed?
Click to expand...

The 115's are what came in it when I got it. I never touched the jetting at all.
 
Anybody wanna chime in? She starts after sitting for a little bit and idles for a minute then stalls out and won't start again. Plugs are both wet but no backfiring when she starts. It has the stock airbox on with a clean filter. I tried running it without the airbox on and will stay running as long as I keep blipping the throttle and holding it steady. I'm kinda thinking I need to bump up the pilots. The PO gave me some pods to put on but I never did because I didn't want to mess with the whole jetting thing. I'm starting to lean towards putting the pods on. Any ideas guys?
 
Sounds like its a fuel starvation problem first and foremost..starts and runs the bowls dry and the bowls arent refilling until it has sat for a while and fuel has SLOWLY trickled back in and refilled them.

I suggest you take the hoses off the petcock and run a long one from the fuel nipple into a pop bottle and set the petcock in the ON position. Use some vac line and suck on the vacuum nipple like the engine normally would and observe how well ( if any ) fuel flow that petcock is putting out. Should be a good strong stream at least the diameter of the fuel line itself. If its just trickling, then remove the petcock and see if the internal filter screen is gunked up. If its a decently strong flow, the problem is in the floats.

Check the floats arent hitting any of the bowl gaskets and hanging up.
Visually verify the float needles moves up and down and doesnt hang up as you move the floats.
Lift the floats and let go of them and see if they drop to the open position freely when you let go.

Do a flow test with water when the bowls are off. Rig up a feed bottle and hook it up. Observe how fast the water will flow from each float needle..should be a decent amount from each one. If the water doesnt flow kinda strong then youve probably got a clog somewhere in the fuel tubes or the carb bodies themselves.

Remember that 2 feeds 1 and 3 feeds 4. So if your doing the water test, you would raise float 2 to make water flow out of 1. and do the same for 3 and 4...follow me? Iwill PM you my number if you wanna talk.
 
If you still have the stock airbox installed, KEEP IT.

Also, even if you put on pods and a header, you will not need to change your pilot jets. You need to add to the main jets because of less restriction (which allows more air flow) when you switch to pods and a pipe. At idle throttle settings, there is NO difference in restriction, so virtually no jetting changes are necessary.

.
 
Just realised its a 450..so two carbs..but you can still do a water test of the floats and needles. I use this as a test because its not gas for 1 thing, and it still checks to see the flow rate and if the needles are shutting off and opening up at the least.
 
Among everything else suggested ,might want to check the carb bowl vents, and the fuel tank venting, (atmospheric) Crack open the gas cap, not sure where the 450's carb bowl vent tubes are located or how the fuel tank vents, You stated that it was running properly before shimming and since you had the tank off shimming the valves may be poss. that you pinched a vent hose, would have the same effect, (starvation) run a little while, slowly refill, restart then repeat itself. :confused:
 
Fuel cap is vented via a small tunnel milled into the base metal below the chromed cover. the best way to be sure it is clean is to remove the black plastic cap inside, then the brown kidney bean shaped thing with the big ball bearing comes out...be carefull because theres another smaller BB underneath. Pay attention to the order they go in too.

Once its all apart youll see a hole there..blow some carb spray and compressed air in there to clear the vent passage way.
 
So I went through all the topics you guys suggested and to no avail. She will start after been sitting and will stay running as long as I keep the throttle at about 3,000 rpms. If I let it idle back down it will stall out and not start again. Like it's getting flooded. I did quite a bit of research on the internet about the mikuni bs34 and found that there should be a rubber plug over the pilot jets. Is this correct because I don't have any rubber plugs in my carbs.
 
Swim_fan02 said:
Anybody wanna chime in? She starts after sitting for a little bit and idles for a minute then stalls out and won't start again. Plugs are both wet but no backfiring when she starts.
Click to expand...
Chiming in.

Sounds like she's flooding out.

Have you taken the carbs apart yet and cleaned them?

Swim_fan02 said:
So I went through all the topics you guys suggested and to no avail. She will start after been sitting and will stay running as long as I keep the throttle at about 3,000 rpms. If I let it idle back down it will stall out and not start again. Like it's getting flooded.
Click to expand...
 
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