1978yamaha400 Posted November 2, 2009 Share Posted November 2, 2009 In the process of fixing 400, mounted rebuilt carb with "factory" settings. Right ignition point, was not opening, set with difficulty as outlined in Hayes. After lots of kicks and priming the cylinder with choke cleaner, engine started, but immediately ran up to red-line. After 5 or 6 starts it keeps doing the same thing.... does not seem to be flooding, any thoughts? Link to comment Share on other sites More sharing options...
turrican64 Posted November 2, 2009 Share Posted November 2, 2009 In the process of fixing 400, mounted rebuilt carb with "factory" settings. Right ignition point, was not opening, set with difficulty as outlined in Hayes. After lots of kicks and priming the cylinder with choke cleaner, engine started, but immediately ran up to red-line. After 5 or 6 starts it keeps doing the same thing.... does not seem to be flooding, any thoughts? What 400 is this? If it's a two stroke RD400, and you got it started with straight gas, you have an air leak or your carb slide is backwards. My guess would be your crank seals (tranny side would result in lots of white smoke, mag side the condition you are describing), but could also be the cylinder head gasket, base gasket, or the reed valve gasket. Link to comment Share on other sites More sharing options...
Runningdog Posted November 2, 2009 Share Posted November 2, 2009 Is the throttle cable running free? Do the trottles come right back down to the stops on the carbs? Have you recently added a different handlebar? (check routing of cable.) Next step, pull carbs and make sure the piston springs are not to weak....engine vacuum pulls the piston up, which increases vacuum....check to see if pistons are stuck in up position. It's likely something very minor, but pick through the whole system a bit at a time...good luck. Link to comment Share on other sites More sharing options...
Moderator DirtyDT Posted November 2, 2009 Moderator Share Posted November 2, 2009 I would agree with some of the earlier answers. I would check that the carb sliders are not sticking in the open position. even if the throttle cable seems OK the slider may still be stuck fully open (maybe a bent jet needle). I would also use a little fuel in the plug holes as carb cleaner may be too harsh to "coax" the engine into life. Link to comment Share on other sites More sharing options...
1978yamaha400 Posted November 2, 2009 Author Share Posted November 2, 2009 Is the throttle cable running free? Do the trottles come right back down to the stops on the carbs? Have you recently added a different handlebar? (check routing of cable.) Next step, pull carbs and make sure the piston springs are not to weak....engine vacuum pulls the piston up, which increases vacuum....check to see if pistons are stuck in up position. It's likely something very minor, but pick through the whole system a bit at a time...good luck. Link to comment Share on other sites More sharing options...
1978yamaha400 Posted November 2, 2009 Author Share Posted November 2, 2009 Thanks for the info. The throttle cable and assembly seem to travel normally. But while I checked the floats in both chambers, I did not check the upper assemblies with the diaphrams and springs. Another bit I read online said something about control needles breaking on Yamaha dual carbs, effectively making the throttle action useless, is this so? Link to comment Share on other sites More sharing options...
1978yamaha400 Posted November 2, 2009 Author Share Posted November 2, 2009 PS, this is a 4stroke and we did throw some raw gas in the cylinders before the choke cleaner. Thanks again. Link to comment Share on other sites More sharing options...
Moderator drewpy Posted November 3, 2009 Moderator Share Posted November 3, 2009 PS, this is a 4stroke and we did throw some raw gas in the cylinders before the choke cleaner. Thanks again. probably all the fuel and choke cleaner you put in the engine. has it settled down yet or still reving? could also indicate a very weak(lean) mixture!! Air getting in somewhere ( use easy start or similar to check for leaks) drewps Link to comment Share on other sites More sharing options...
1978yamaha400 Posted November 3, 2009 Author Share Posted November 3, 2009 Each of times we started it, it took about thirty - forty seconds of run time from idle till we killed the switch around 5000RPM. We did this 5 or 6 times. So we really only had 3 or 4 minutes of total run time. After each run, I pulled a plug to see it was soaked, it never was. Will try the starting fluid next time. I did notice the fuel vapor return line on the right intake throat was off and put it back on, I also backed out the nylon? half inch turn screw at the rear of the carb (not the two slotted mixture screws towards front of the carb) one full turn. After that we could not start it again. Day had grown short, and we were hoping to find some pointers like you all have given before starting again. Have not gotten back to it yet, but pretty sure the fuel and carb cleaner should have burned off with the runs. Will take the carb off again, and this time inspect the vacuum chamber springs and diaphrams to see if one or both is stuck open. Thanks Link to comment Share on other sites More sharing options...
Moderator Airhead Posted November 3, 2009 Moderator Share Posted November 3, 2009 is the petcock a vacuum type and if so is the vac line connected propery to the manifold, as has been said...sounds like an air leak.... Link to comment Share on other sites More sharing options...
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