Hitachi 340 accelerator pump

[LenRobertson]

Got the 510 running pretty good (stock L20b with 340 Hitachi carb), except when giving it gas coming off low idle, it bogs really bad. Unless the gas is kind of feathered, it will die completely. Once it is above idle and given throttle, it responds fine, which doesn't quite seem like accelerator pump problems I've had with previous vehicles. With the engine off, when I operate the throttle by hand, I'm not getting a really strong stream of gas out of the pump nozzle.

 

So I poked around at the accel. pump. Plunger rubber is good, check ball is free in the bottom of the cylinder, spring installed so the "bar" end of the spring is down on the ball. Passage back to the float bowl seems clear. Sprayed carb cleaner everywhere for good measure.

 

Then I took the top off the carb and removed the plug above the accel. pump discharge valve. The ball is there and moves free, and there is a long spring, but no weight as shown in some of the carb exploded views. I took the discharge apart on two other Hitachis I had around here. One had two check balls and no spring or weight (no idea how if this carb was being used like this or maybe someone robbed parts from it). The other carb had one ball, a shorter spring and a brass weight, like the books more commonly show. I sprayed more carb cleaner and think the passage to the discharge nozzle is clean. When I leave the accel. pump arm off and push down on the pump plunger, I get a good stream into the carb throat.

 

So I'm wondering if it is worth putting the brass weight into the carb, that the problem is the discharge valve is not working right and letting air back into the system. So that when the accel pump works, it is pumping air as much as gas.

 

Or am I totally barking up the wrong tree? It would be fun and educational to experiment with different parts in the discharge valve, except it is getting pretty cold to work on the thing outside now. If I can just get rid of the carb bog, I'll start doing serious road testing (and find out the next thing that is wrong I'm sure).

 

Len

[LenRobertson]

I should have mentioned, my bog coming off idle happens with a warmed up engine and the auto choke correctly working and open. ggzilla mentioned in another thread on a 210 bogging out when cold that a vacuum leak is one possible thing to look for. I don't think I have any vacuum leaks, but haven't done a carb cleaner spray around the carb base or anything like that. Could a vacuum leak cause my problem?

 

Len

[ggzilla]

I would try it.

[LenRobertson]

I would try it.

Okay, thanks! Sure easier to try that than take the top off the carb again in 34F weather. Of course the fix if there is a vacuum leak may not be quick and easy, but at least I'll know what the problem is.

 

Len

[ggzilla]

Vacuum leaks at the carb base is not the most common type (just easy to check). They are usually at the hose ends or in the various diaphragms- vacuum advance can leak being one of the most common.

[LenRobertson]

Temperature got up to 40 degrees today, so I checked for vacuum leaks. Couldn't find anything there. Then I yanked the top off the carb again and looked at the accel. pump discharge valve. It turns out there is a brass weight in there under the spring. I don't know how I missed that before, although it was cloudy today so I had to rig up a decent trouble light. Got the weight out (push down on the accel pump plunger to shoot the weight up where it is reachable. Took me awhile to figure that one out). I made sure the ball in the bottom was loose. Sprayed carb cleaner inside everything, including my eye. Always lean way back from the carb when spraying cleaner down any holes, or you may get a face full of cleaner. Put it back together. Then I changed the rod that comes up from the throttle linkage to the accel pump arm. The old one was shaped so it hit the anti-dieseling solenoid in every hole except the very end one. I thought it might be worth playing with the different adjustment holes, so I put on a different rod. I was feeling pretty good that all that work might solve my problem, but now I have NO output from the accel pump nozzle that I can see. Oddly the engine isn't really acting much differently. Still boggy coming off idle, but maybe slightly better. So tomorrow I'll mess with trying the different adjustment holes on the pump arm. Except snow is predicted. I think it is time to clear a spot in the junk filled old garage enough to get the front of the 510 in there.

 

And I did bring one of the old dead 32/36 Webers to the house to look over. I hate to admit defeat on the Hitachi, but if I'm going to put a kit in a carb I may go ahead and do the Weber. Maybe I'm NOT smart enough to fix a Hitachi.

 

Len

[JamesE]

Any luck Len?  Your problem sounds quite similar to mine...

 

http://community.ratsun.net/topic/51630-l16-stumbles-at-wot-with-hitachi-dch340-with-video-suggestions-please/