Question

[jlaird]

Thought I best start a new thread.

When I put the unisync to one of the carbs it covers the(1/4") hole in the body that goes inside and causes the engine to jump maybe 1K rpm. The hole is next to the large hole in the body inside which is the butterfly. I always thought it was an air balance thing.

What is that hole for and why would it now be doing this. You can also put your thumb on the hole and raise the rpm.

 

[startech47]

On my TR3 when you look at the air filter face of the carb there are 5 holes. The large one is the carb throat. There are 2 holes to mount the air cleaners. There are 2 vent holes that provide filtered air venting to the bottom of the air valve. The 2 holes in the bottom of the air valve that provide the depressed pressure to lift the air valve are to be orientated towards the engine on the down stream side of the carb bridge in the carb body.

Does the air valve rise or drop when you cover the hole?

Is the air valve keyed to the body preventing rotation as it moves up and down?

I am assuming all parts of the air cleaners are off when you are making these adjustments?

 

[jlaird]

Yes all parts off.

Only one hole for venting on these. Think the piston drops. No valve.

 

[startech47]

Air valve is another term for piston.

Just tried blocking the piston vent holes on my TR3 and only had a barely noticeable drop in RPM.

The key in my 64 Sprite was a brass tee piece that fit into a hole in the carb body. If it is not in the piston can rotate in relation to the carb body and cause all kinds of strange things.

Since the carbs appear to be slightly different I would measure the diameter of the pistons of the part that blocks the passage of air into the engine and the bores that they fit into and determine if the clearance is the same for both carbs. The relation of the ratio of the clearances between the air chamber and the largest diameter of the piston and the clearance I just asked you to measure could change the reaction of the carbs when you block the hole.

Where are you on the mixture adjustment?

What is the history of the carbs?

Is this a new condition for the engine or has it been operating this way since you built it?

Does blocking the hole have the same reaction for both carbs?

 

[jlaird]

Carbs were junk two years ago.

This is a new condiction, they were working ok with a couple of small problems in alignment so I made sure the needles were centered, throtles were square on so there would be no binding, lubed the throtle cable, etc.

Yes blocking hole has the same reaction on both carbs, it has not done this before, have always used a unisync. Causes rpm to rise tonite when I cover that hole.

Pistons are the same size, as are dash pots. Drop rate is the same swaping around between bodys and domes. All seems well. Only difference I could find between the two carbs, and I measured was the back mounting plate was 1/8 thiner on the rear, shimed it out with two gaskets.

Keys are steel in these H1s, both are there, tight and smooth so pistons drops freely with a clunk, no hang ups.

Both look like the one below, no leaks inside or out, and no air leaks as well. New throttle rods, and buterflys installed by Joe.

Will give it another go in the morning. Wish me luck.

Forgot to say, have about 650 miles on these.

 

[Biff]

Jack,

The 'extra' hole is for air to enter the bottom of the piston assembly so that the vacuum pressure can raise the piston easily. Block it and air cannot get to the bottom of the piston. Try removing the damper assembly when using the Uni-synch. This should remove the vacuum on the piston and eliminate the need for the air hole. Swapping pistons and dome covers on SU's is a no-no. Those two pieces were matched at the factory to eliminate manufacture differences as best as possible.

 

[jlaird]

Yes I understand no swicheeee. I just did so for testing purposes.

 

[jlaird]

finaly, I think I got it.

Found a dampner hanging up, we shall see.

Can only get idle down to about 1K. Think new carbs are in my future, HS2s.