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Closing in on the Problem

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Mickey Richaud

Mickey Richaud

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OK - Here's the deal:

Everything on the rebuild looks really good. I can get the engine to fire, but it won't continue running more than a couple of seconds. Sounds great, too! But definitely not getting fuel. Since the wiring is not complete, I'm trying to get this thing going without the alternator. Got power to the coil and hooked the fuel pump up to my battery charger (10 amp setting). Question: after the fuel pump has satisfied the bowls in the carbs, what is the interval between "clicks" of the pump? This one seems to be around 10 seconds. I'm wondering if my pump (brand new) isn't working properly.

With the piston removed, gas barely seeps out of the jet, and only after pressure builds up.

What do you think - fuel pump or float/jet problem?

Mickey
 
Hi Mickey,

Sounds to me like you have a bad fuel pump. It is my understanding that once pressure is built up in the system, the pump won't click until the pressure drops off due to fuel use. You could also have a clogged jet because you are only getting seepage after pressure builds up. The gas should shoot out with much force if it is indeed under pressure.
 
Hey Darwin -

Thanks for the input. Last night I did get the beast to run, but it still has some fuel delivery issues. I'm not sure it's the pump - may have some trash in the filter (brand new also!) When the pump first starts up, it clicks away, then settles in to a click every ten to fifteen seconds (engine not running).

When it did run, it sounded fantastic! Didn't spend enough time to get it to idle well, but as the post title says, I am closing in on it.

Mickey
 
[ QUOTE ]
OK - Here's the deal:
With the piston removed, gas barely seeps out of the jet, and only after pressure builds up.
What do you think - fuel pump or float/jet problem?
Mickey

[/ QUOTE ]
Hi Mickey,
With a correctly working carb, the fuel level should be well below the jet top. Even with the chokes pulled. There should NEVER be fuel flowing out of the jet when the engine isn't running. I think either the float level is set too high or the float needle & seat are leaking, which causes a way too high float level. Fuel pressure higher than 3 psi could also cause the floats to not shut off.

You may have other problems, but this is definitely "one" of them.
D
 
[/ QUOTE ]
Even with the chokes pulled. There should NEVER be fuel flowing out of the jet when the engine isn't running.D

[/ QUOTE ]

OK, now I'm confused again (imagine that!). In a response on the other thread, Jeff (Bugeye58)said:

Did you replace the jets? I've seen them where the little 'O' ring gets dislodged, and blocks fuel from entering the jet. (Rare) Quick way to check that is pull the suction chamber and piston, turn the pump on, and see if you can see fuel rising in the jet itself.

At any rate, as I said, it's running now - just need to get it to idle properly, and figure out the delivery issue. (And this is something we do for fun, right?) /ubbthreads/images/graemlins/crazyeyes.gif /ubbthreads/images/graemlins/crazyeyes.gif

Mickey
 
yea...great fun isn't it?

You should be able to see the fuel in the jets, but if it is coming out, the float level is too high...I agree with Dave there and suspect to much fuel pressure is flooding it, that the floats are too high OR both.

Try disconnecting the fuel line at the filter, leave the pump off and gravity feeding the carbs through the fuel line. It should fire and run for about a minute until it runs out of fuel in the fuel line and bowls. If so, work your way back to find the problem.

Bruce
 
Thanks, Bruce -

The fuel that is present at the top of the jet is just barely seeping out. I would think that the needle would shut that off when the piston is in place, so that may not be the problem. Will give the gravity thing a try next chance I get.

Mickey
 
Mickey,
The needles when on their largest step are usually several thousandths (.010") smaller than the jets. There are only a couple of exceptions in the entire available needle range. Fuel "can" & does flow with the piston all of the way down. But only if the engine is running. If it didn't flow, the engine wouldn't idle. There is also a small spring loaded bumper on the bottom of the piston that prevents the piston from completely closing.
D
 
The piston will completely close if the engine is not running. It will be pulled up by vaccuum at idle but not much. There is no spring loaded bumper that arrests the piston (dash pot) from closing. From the gist of what you said, I suspect that you have too high a float level or a leaking needle valve that is causing the carbs to run rich. The fuel pump operation sounds OK and SU fuel pupmps usually cannot generate more the their rating (3psi). The jet should not allow gas to get into the throat when there is no vacuum at in the intake manifold. I hope this helps.
 
Mickey, I just went out and checked the time between clicks of the pump on the Midget, and after the initial series, I sat there with the stopwatch, and finally quit after 30 seconds with no noise. That's with a year old SU pump, and the car runs just fine.
Jeff
 
OK - thanks Jeff, John, and Dave. This is all very helpful. I do know that I adjusted the float level according to the book, even though it is difficult to get a feeler gauge in the space it calls for. The float is set right at .040" and I put in new needle valves. I may order Grose jets to replace them - that's what I have in the TR3, and haven't had this problem with it. I'm going to work on it some more over the weekend and see what develops.

Mickey
 
[ QUOTE ]
There is no spring loaded bumper that arrests the piston (dash pot) from closing.

[/ QUOTE ]
Jack,
I attached a pic of a SU carb piston bumper to an email that I sent to you. It is the small brass button in the photo. It holds the piston about .015" above the bridge & requires about 48 oz. of pressure to retract it. With the usual 16 to 24 oz. return springs used, this will certainly hold the slide up even with the throttle plate fully closed. Let me know what you think.
D
 
[ QUOTE ]
I do know that I adjusted the float level according to the book, even though it is difficult to get a feeler gauge in the space it calls for. The float is set right at .040" and I put in new needle valves.
Mickey

[/ QUOTE ]
Mickey,
I can't find anything in my SU service manual that calls for a .040" gap between the float & the lid. The info that I have shows .125" to .187" gap. This with the needle plunger spring NOT compressed. Maybe this is the problem?
D
 
WELLLLL... That is interesting. My MGB shop manual says to set the float .040" (1.0mm) with a straight edge across the face of the carb body, measuring from the straight edge to the center of the concave part of the float. That's why I said it was difficult to get a feeler gauge in there. If your number is correct, it would be quite a change.

Mickey
 
Mickey,
I'm sorry. I gave you the wrong info. The book says .040" + or - .020" from the gasket surface to the lower step on the float. Still, fuel should not be seeping out of the jet, so something is wrong with the setting or the valve & seat.
D
 
OK - This is getting ridiculous! I've set the float EXACTLY at .040" and it would start, but wouldn't run. Then I set it just over .040" (maybe .050") and it started, but wouldn't idle down. The throats were wet with gas, so I took the pistons and chambers off, and turned on the fuel pump, and fuel squirted up 3-4 inches. Set the float below .040" (maybe .030"), and nothing. Reset the float barely over .040" and again with the squirting already. Somebody's bound to have the answer out there...

Mickey
 
Hi Mickey,

It's starting to sound like a float bowl venting or hose connection problem. There should be two hose connections, one above the other. The top connection is a float bowl vent & should be open to the atmosphere. The lower connection is the fuel inlet.

If the fuel pump were connected to the top fitting it would bypass the float valve completely.

If the fuel pump is connected to the lower fitting, as it should be, but the top vent fitting is somehow blocked, incomming fuel will be forced directly to the jet without having a chance to raise the float for shutoff.
D
 
Dave -

Thanks, I really appreciate your help. The inlet is inline with the connection to the rear carb. These HIF4's actually have three connections on the forward carb. Here's a picture of the carb - I've got the fuel hooked to the one on the lower right, near the bottom of the bowl.

https://mossmotors.com/Shop/ViewProducts.aspx?PlateIndexID=28997

I've got all the vents open, and they're clear. The only thing I note from the picture in the Moss blowup is a gasket or washer on the needle and seat. These carbs didn't have one, and the rebuild kit didn't include one, either. If it's supposed to be there, is it a fiber washer, aluminum, o-ring, or what? Does it make any difference, regarding the problem?


Gonna take time off from all this and go to the show in Nashville tomorrow (even in the rain!) with the TR3. Maybe I'll have a vision!

Mickey
 
Yes Mickey, the washer could make a very big difference, as could a seat that isn't fully tightened. It is usually a small red fiber washer.
D
 
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