Any motorcycle mechanics here?

[Michael Weber]

I have a Honda 2004 Shadow Aero 750 with a problem. It may be a VERY bad vacuum leak (or not) but isn't around any typical hoses/connections/inlet manifold I can find. Is there somewhere else on the engine I should be looking? Is it possible something broken in the engine would cause a bad vacuum problem even with 180 PSI compression in both cylinders? I'm pretty convinced it's not the carb. It's been dismantled and cleaned and all the jets and other passageways are open. Only way to start the bike and keep it running is to constantly operate the throttle. Essentially the engine is running off the injector pump. Any ideas cause I'm pulling my hair out here

[Pat Barry]

Its hard to say without more info so I'll just throw a few thoughts out for you. Do you hear a vacuum leak when its running or what is making you think vacuum leak? What do you need to do to start it? When you hold the throttle open does it still die or just when you back off the throttle? Is it just a cold situation like maybe you need more choke to get it warmed up? When was the last time it was running right? Did it just start acting up out of no where? Have you ruled out an electrical issue, for example, the timing may be a bit retarded and when you open the throttle it advances to a better timing to make it run. When its running does it smell like its running rich or does it run hot like its too lean? Is this a single carb or one for each cylinder?

[Michael Weber]

Hi Pat. I can't hear a vacuum leak, nor been able to find one. That's why I was wondering if a vacuum leak could be occurring somewhere past the intake manifold. What makes me think it would be a vacuum leak is that it just doesn't seem to be getting fuel delivery. The float level is correct. To start it, I must operate the throttle over and over as I crank the engine. Choke in or out it won't start without doing the throttle thing and with the engine cold or hot. The second I stop operating the throttle, it dies unless the rpm's are very high (racing). It's a single carb. I know the timing on this particular bike is not adjustable. I can't say if it's rich or lean but assume lean since it needs the extra gas from the accelerator pump to run. Unfortunately, the bike just found it's way to me and I don't really know much about it's history. I can tell you, it only ran (badly) when I got it because the petcock vacuum diaphragm was torn and raw gas was being supplied to the cylinders via the vacuum lines. If the petcock was not shut off when the engine was off gas would find it's way into the cylinders. Took care of that, changed the oil, rebuilt the carb and that's where we are today. I've changed the air/fuel jet adjustment and that makes no difference.

[Stephen Cherry]

I know that you rebuilt the carburetor, but this sounds like an ethanol issue to me. That would be first up. Have you changed the gas?

[Graham Wintersgill]

With the revs as low as the engine will run spray WD40 around the intake manifolds(s). If they are leaking it will suck in the WD40 which will then be ignited in the engine and the revs will rise. HTH.

Regards

[Tim Boger]

I have a Honda 2004 Shadow Aero 750 with a problem. It may be a VERY bad vacuum leak (or not) but isn't around any typical hoses/connections/inlet manifold I can find. Is there somewhere else on the engine I should be looking? Is it possible something broken in the engine would cause a bad vacuum problem even with 180 PSI compression in both cylinders? I'm pretty convinced it's not the carb. It's been dismantled and cleaned and all the jets and other passageways are open. Only way to start the bike and keep it running is to constantly operate the throttle. Essentially the engine is running off the injector pump. Any ideas cause I'm pulling my hair out here

Chances are pretty good that the Pilot Jet is not allowing fuel to flow, easier to replace it than try and get it clean .... sometimes they even look clean but still cause problems.

Tim