9hp Subaru Robin horizontal, stalling issue

I am attempting to complete what has turned into a rather extensive project. What started out as a redneck bar stool racer has turned into a very large 11' long custom trike. It is running on a 9hp Subaru Robin horizontal shaft engine with a torque converter and electric start. The engine has a new carb. I removed all of the governor linkage. I am running a straight pipe exhaust that's about 2' long total with two 90 degree bends. I removed the stock fuel tank and I'm using two round tanks, the fuel lines connect at a "T" before heading to the carb. Each tank has a separate fuel cutoff and fuel filter, all new stuff. I have only fueled up and used one tank.

Last fall I went on a couple of short test runs and everything worked well mechanically. I parked it over the winter and just finished a few things on the trike. I've now taken it out twice in the past week and I have experienced the same problem. It cranks great and idles great as long as I want it to. The first ride I went about a quarter of a mile on a straightaway, running about 30 mph. It ran great. I stopped at a stop sign and when I gave it throttle to go it bogged down. It just sputtered when I would give it throttle and eventually it cut off and would not restart. I pulled it home and it sat about 10 minutes then I cranked it and it throttled and idled fine.

Today I took it out and rode it about 100 yards, cranked and ran fine. When I came to a stop and then went to throttle it to go it bogged down again. Sounded like it was choked off but that was not the issue. It eventually died and would not restart. Again, got it home, sat a few minutes and it restarted and ran fine.

I'm hoping I'm overlooking something very simple. I have a little experience with small engines, rebuilt several. I have very little experience with custom fuel tanks, lines and custom exhaust.

Any suggestions and advice would be greatly appreciated.

From your description I suspect you are getting a lean mixture, once your engine gets hot. You are going to have to open the jets to get more fuel into your engine. Post your engine numbers so I can see the type of carb you have. Neat project, but don't know if we can get you more fuel, but still allow it to idle. Hopefully you have an adjustable carb.

I hope so, the stinking carb for that engine was about $120. You may be correct though, I put that new carb on but didn't make any adjustments to it. The engine is an EX27, I can get all the numbers tomorrow. I really appreciate your help. Built this thing from scrap and I'm so close now.

Engine Code:
EX270DS50205769

Here are all the numbers on the carb, I'm not having any luck identifying the carb. I contacted Subaru Robin directly and this is the carb they told me I needed for this engine for this application. On the side of the Mikuni carb is stamped "W3672" and under that "Z341". On the opposite side, on the fuel cutoff switch beside the "Mikuni" stamping is "1344" and then on the top of the carb the number "3" above the numbers "23"

I only see one adjustment screw, on top of the carb, it's held in position by a metal tab that is screwed in place with a Phillips head screw. I removed the tab and the jet has a groove for a flathead screwdriver. The jet turns freely left or right and does not "seat" or move in or out to adjust the jet so I'm not sure what's going on there. I don't see any other adjustment screws.

Could the backpressure from the straightpipe be off and causing this problem? How would I know apart from putting the original exhaust back on, which I can't do due to space issues without some alteration.

I have attached two manuals for your engine, a service and a parts manual. They may help you understand what is going on. I would start by opening the idle mixture screw about 1/8 turn and as you give it throttle when warm, does it seem to be better. I have a feeling if we can get that bogging and sputtering problem under control, we're almost there. You may have to increase your idle speed also. That long exhaust pipe also has me wondering, but I'm unsure how to address it. We'll look into it later. I would also contact Subaru/Robin and see if one of the two main jets is larger, may need to rejet. This is very hard for me, as I wish I was there to see what is happening. Would make this much easier. I'm sure someone else will chime in.



http://www.smallenginesuppliers.com/html/engine-specs/robin/EX27_part.pdf

http://www.smallenginesuppliers.com/html/engine-specs/robin/EX13_EX17_EX21_EX27_service.pdf

I really appreciate your help Rivet. I already had those manuals, that's what got me thinking about the backpressure/straight pipe issue. I looked closer at that carb, the main jet has a slotted screw head but it is not adjustable. It is not threaded to move in and out. The needle drops down into the jet port and it has an O ring to prevent fuel leakage and it is held in place by the slotted metal and screw I mentioned. I have attached pictures of the jet (removed), in place and with the slotted metal and screw holding it in place. So, it appears that carb is not adjustable, unless I have missed something.

I think I'm going to rule out the backpressure issue first. I have an original muffler. It sits on top of the engine and won't fit in the space I have so I'll have to do some adjusting and welding on the manifold to slightly reposition the muffler. I attached another picture with the straight pipes removed just to give you an idea of what I'm working with.

If I can rule out the backpressure issue, I'll revisit the carb fuel/air mixture. Just curious, could I have thrown anything out of whack by removing the governor linkages?

I need to figure out which carb you have. Does it look like the one in the service manual or the one in the parts manual? Did the jet you have in your hand come out of the top of the carb? If you can, when you refer to parts, would you look at the parts diagram and tell me what they are called there. This way I will understand exactly what you are talking about. Eliminates confusion.

I don't know if you will understand what I am about to talk about, but if you do it will make it easier to work this out.

Today's carbs have 4 circuits, choke, idle, off idle and power. The choke circuit provides a rich mixture for starting and cold operation. When you close the choke and open the throotle on the engine, fuel will enter the air horn through the main jet and both idle and off idle ports. Lots of fuel, little air. During the idle circuit the throttle is closed and the choke is open. Fuel enters the air horn thru the idle port, which is located on the side of the air horn just past the closed throttle plate. Small amount of air and fuel. The off idle circuit is used to transition from idle to full power. As you open the throttle plate, you are allowing an increasing amount of air into the air horn. To keep the engine running smoothly you must keep increasing the fuel. This fuel comes thru the idle port and off idle port (which is located just inside the throttle plate in the air horn), increasing as more air enters the air horn as the throttle opens. This continues until fuel starts flowing out the main port in the air horn. During the power circuit fuel only enters thru the main port. No fuel coming out the idle or off idle ports during the power circuit.

I feel that the problem you are having is either your carb cannot allow enough fuel in during the off idle circuit. Or without a governor to control how smoothly the throttle opens, it is just popping open, not give the off idle circuit time to do it's job. I must add that some of the smaller engines do not have an off idle circuit and I am hoping that this is not the case with your engine.

Sorry I am falling back into teacher mode, but was hoping if you understood better how the carb works, it will help the cause.

As for right now I would reinstall the governor system, so we can diagnosis the problem with everything in place. We can always remove or adjust it later, when the engine is running properly.

I will be n and out for the next three days, but will try to checkin when I have time.

I am following you completely and what you are saying makes perfect sense and it does sound like I need to go back to factory setup and work from there. I hacked the governor linkage, springs and throttle connection plate so I'll have to piece it back together or get a whole new setup. Might take a few days so I can give you a break from my problems! Thanks again, your experience is much needed!