High speed rpm start problem

[RJO]

2000 GTX 951 model

I just rebuilt the carbs according to the sticky, found out it was a lot more than that, I had no compression in one cylinder, the piston was shot.

Installed a new SBT engine and everything went together fine. Carbs are adjusted 0 on the idle screw and the low end needles are at 1.25 turns open.
Once I got the fuel lines and oil lines filled I tried to start it.

It started and ran itself up to high rpm. Pulled the key off quickly and it didn’t shut down, then the plug wires and it was still running (like a diesel or glow engine). . It finally died. Tried it again and the same thing happened.

It seems to be acting like it gets a surge of fuel and leans out, creating the high RPM and finally starves itself.

 

[JJinSC]

You have an air leak some where. You need to find the leak.

FYI, if it ever runs away again like that pull and hold the choke. That will usually stop it.

 

[mikidymac]

Most common leak is the carb base Gaskets.

Did you pressure test the engine before dropping it in? You should have.

 

[RJO]

I didn’t pressure test the whole engine. I can’t find info on that, only compression tests and leak test.
I do have a small water leak on the exhaust manifold gasket and the block I am fixing.
Carbs were installed after the engine was installed.

 

[JJinSC]

Download the service manual for your model. It will contain the procedure for pressure testing the motor and parts needed.

Like Mikidymac said, most common leak is carb gaskets. That is fairly easy to check. Pull them back off and make sure all is well and the mating surfaces are clean.

 

[mikidymac]

Also make sure the throttle plates are closing all the way. Easy to not have enough slack in the throttle cable.

 

[RJO]

Found it in the manual. I don’t have a leak test kit.

Would a leak in the exhaust manifold at the block cause the high RPM problem?

Is the rubber on the reed valve sufficient to seal the intake manifold or should gasket sealer be used? The rubber is in good shape.

Throttle cable is good, it is not pilling the carbs open at idle.

 

[POPPS]

No sealer required. That carb install is not easy, start from scratch, drop the carbs, make sure the throttle cable is relaxed, butterflies closed, and that the carb base gaskets are intack, not torn, and not at the bottom of the hull. Also check that the carb plates are sync'd, if one is open too much more, that could cause a lean situation. You have a new SBT engine, and I am sure it has been assembled without seal leaks. Also check that the hose at the pulse nipple is not cracked and secure.

 

[mikidymac]

You are also checking the intake gaskets and exhaust gaskets for air leaks, not just the seals on a new engine and the SBT engines don't come with the intake or exhaust on them.

 

[RJO]

I ordered from them from SBT and put on new intake and exhaust gaskets.
Everything is sealed and tight.
Started it again. With just the flame arrestor on and the rpms creep up.
I can control the rpm by blocking the air silencer inputs. It is sucking hard through the intakes on the air silencer.

 

[POPPS]

Perhaps you have fuel starvation outside the carbs. So,starting at the pickup, ( clear the screen at the bottom), then,at the petcock, (bypass it) and run straight to the carb. Perhaps there is a a leak at the fuel filter( is the bowl full?, is that O ring pinched)? Is there a kink in the fuel line going to the fuel bowl, (have seen it)? I assume you have no gray fuel lines. And finally, is the in line to the carb on securly, (not leaking).

 

[mikidymac]

You should not be running it without the air box, that can cause your lean running issues.

Also if you did not pressure test it they you don't know if there is an air leak even with new gaskets it's not uncommon.

 

[RJO]

I changed the fuel lines and selector valve when I put the engine in. I was surprised the carb filters and lines were clean, I replaced them anyway.
There are no kinks anywhere.
I have the air box on, but without the snorkels. Should that make a difference.
The air box doesn’t seem to create much of an air restriction.

On the carbs the high speed needle valves only have an adjustment of about 1/4 total. They were centered when I took the carbs apart. Everything I read says to start them closed. This would seem to be too lean.

 

[RJO]

Got it all fixed. Now I have to break it in and tune it on the water. Not exactly sure what the exact problem was, but I reseated the carbs with new gaskets and sealer, put the snorkels on the air box and bypassed the fuel selector, pulled the carbs apart and reseated everything. Now fuel goes from the tank to the filter, then the carbs.
The new fuel selector doesn’t have positive stops on it. Once it hits the indentation on the knob, it passes just a little further. I might hook it up in the future, but don’t see a need for it. Just don’t run out of gas alone.



Thank you everyone for all your input!!

 

[mikidymac]

I would spend the $25 and buy an oem selector.

 

[RJO]

I looked and it was discontinued

 

[mikidymac]

I have had really good luck with the WSM #006-604 fuel valves.