Carb tuning on a rebuilt 787

[DaddyLoafin]

I’ve got a rebuilt 787 that’s .5 over in my 98 Spx and with the added oil in the fuel for break in it’s not running well. On the trailer it’ll back fire and if you try to rev it a little the rpm’s hang and sometimes it seems like it’s trying to run away. The water is still too cold here to do any testing there but if it’s running like this on the trailer I can only assume it’ll be worse in the water.

What I should set the screws to for break in?

Also what should I set them at once it’s broken in?

 

[1of500]

Factory settings. Did you rebuild the carbs? Are you 1000% sure your rotary valve is timed right?

 

[etemplet]

Shouldn't backfire at all. I'd recheck the rotary valve as well.
The carb issue could be an air leak. I clamp off the fuel lines near the baffle and remove the feed line from the carb. I pull a vacuum first on the whole system switching the fuel selector to make sure that isn't leaking. If I have a problem I pressure up to 10psi and use a soap mixture with water so leaks will bubble up. Generally you'll find a leak on the water separater or fuel selector. Carb base gaskets could be leaking or fuel pump not properly assembled. Did you pressure test the carbs? What pop-off pressure are you running?? Stick with it. You'll get it.

 

[DaddyLoafin]

The carbs were fine, I took them apart while I was swapping the engine and everything was set to stock specs. Pressure tested and everything. I had them set to factory settings and it was running but if you removed it sometimes it would stay or trying to run but very slowly. I figured this was because the pistons are slightly bigger and I have the fuel premixed at 50:1 for break in so I turned the low speed screws out like a 1/4 turn or somthing like that and that’s when it back fired.

I don’t know about the roatart valve, I didn’t build the engine. I got it from a local guy who runs an exchange service. So I’d like to think it’s alright but not sure.

 

[etemplet]

Silly question but is the ski connected to the hose while you are testing? An engine can backfire if it gets too hot and the fuel ignites before the spark is triggered.

The engine should run smoothly I got an engine back from SES and I tested the timing like I always do. Trust nothing or no one. It was within spec but on the very margine. I was able to flip the rotary valve and get the timing spot on in the middle of the tolerance. I called about this and of course was told my gauge is probably not as accurate as theirs. That may be true at times but it wasn't in this case. I don't argue just notify out of courtesy. The timing is correct and is always my main concern.

In your case I'd also check compression. Verify everything.

 

[DaddyLoafin]

No it wasn’t on a hose. I was only running it for a short period just to see if it was behaving properly. It’s also filled with anti freeze currently

 

[DaddyLoafin]

Just out of curiosity, how much of a pita is it to check the rotary valve timing while the engine is still in on the x4 hauls?

 

[etemplet]

It don't hurt too bad. Get one of those Rotary Degree wheels for $12.00. I'm a bitoverboard but I'm a machinist by trade. LOL

 

[DaddyLoafin]

So I finally got around to looking at the rotary valve timing and it was at like 155. I flipped it around and got it to like 144 or 145. Would that really cause it to try and run away?
It’s currently not turning over. Hoping it’s just the battery

 

[mikidymac]

An air leak will cause the runaway, not typically rotary timing.

 

[DaddyLoafin]

That’s what I figured. The starter solenoid went out so I’m just waiting on that so I can do more testing

 

[DaddyLoafin]

Just as an update, starter died so I replaced that with a used unit off a boat and it started up and appears to be running fine and revving up good and not trying to run away. The rpm’s seem the hang slightly but it’s hard to tell when I’m only letting the engine run for 40 seconds at a time lol.
Time for a water test.

Appreciate the help as always

 

[etemplet]

Fatten up the low speed 1/4 to 1/2 turn and see what happens with the hanging.

 

[POPPS]

I’m with miki,,,either fuel starvation, or air leak.

 

[DaddyLoafin]

I’m with miki,,,either fuel starvation, or air leak.

When I took the carbs off the check the rotary valve timing one of the gaskets for the carbs looked like it wasn’t making a great seal. I forget where I got those gaskets from but I did not like them.