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Hs4 Float Bowl Failure

dklawson

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This post does not directly involve a Spridget but it does involve SU HS series carbs. I want to ask the question here to see if anyone has ever seen what I found yesterday.

My sons drive a '72 Triumph Spitfire. The car was originally outfitted with a single Zenith Stromberg 150-CD2 carb. I don't like those carbs so I replaced it with a single SU HS4 from a Mini. The SU HS4 swap is fairly common for the later Midgets that use the Triumph engine. The float bowl for the HS4 carb is also the same as used on the HS2 carbs used on earlier A-series Spridgets.

Yesterday I moved the car and smelled a lot of gas. Opening the bonnet I found fuel all over the engine bay. I ran the electric pump and watched for leaks. Fuel was pouring out of the bottom of the carb. I thought it was the rubber seal between the float bowl and jet tubing.

My diagnosis was VERY wrong. It was not the jet tubing. It was not the float valve. It was the float bowl itself. The float bowl had cracked from bottom to top on both sides of the bowl's mounting boss allowing huge gaps to open in the walls of the bowl. I had a junk carb to get a replacement bowl from. I noticed the replacement bowl was from an older carb and the casting was much thicker and heavier than the later bowl that failed.

The broken float bowl was never bumped, knocked, or dropped by me. I did buy this used so I don’t know the history behind this carb. However, I have never seen such a failure. The purpose of this thread is to ask... have any of you seen this before on your Midgets or on any other car equipped with HS2 or HS4 SUs? I'm just thankful my sons were not cruising around somewhere when the float bowl let go. It could have been a very unhappy father's day for me.
 
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Had someone smoothed/polished it?
 
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dklawson

dklawson

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Nope. This bowl still has the standard SU as-cast finish. I took a picture of it during lunch. I am never good at posting picture links so I hope this works.

Broken SU Float Bowl
BrokenFloatBowl.jpg
 

drooartz

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Well, that's your problem right there.
grin.gif


Never seen that, but my experience is quite limited. Fair to call that a catastrophic failure.
 

Billm

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I have never seen/heard of that issue-
Doug, you now have a first!! (and hopefully "last")
BillM
 
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dklawson

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I have now installed a 1/8 NPT pipe nipple in the bottom of the bowl so it will stand up, so my son has his pen cup for his college desk. Mom doesn't want it in the house yet as it still smells a bit like gasoline. At the end of a day working on cars I smell a bit like gasoline also.

In spite of this failure I still really like this carb conversion on the Spitfire. I find the HS4 so much easier to work on.
 
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I've seen it one time before on the internet, broke in the same exact place, I never seen it in real life. I have theory why it happen though, over time the aluminum threads stretch in the float bowl where the the retaining bolt threads into, this when using the normal repalcment parts in rebuild, the rubber grommets, steel washer, on the bolt head side and special shaped steel washer and rubber angle grommet on the float side, do not let the float bowl be properly tightened to carb body. Most HS series carbs you can walk up on them grab the flaot bowl and they wiggle, this was not how they were designed to be from new, most professiona SU rebuilders have learned about this and add a 5/16 flat washer to the bolt head side, this take the flat bowl wobble out of play, bacily what ahppen is the retianign bolt is bottoming out before the flaot bowl is securley held to the carb body. I beleive the flaot bowl wobble is causing additional stress to this area of the flaot bowl casting and the additional washer elminates the wobble, also the wobble will help loosen the retaining bolt over time. While I never had the float bowl crack like this, I did have a flaot bowl retaining bolt back out enough on the race car to let the float bowl just be dangling by the jet tube.
 

weewillie

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dklawson said:
At the end of a day working on cars I smell a bit like gasoline also.

Well Doug at least gasoline smells better than gas :yesnod:
 
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dklawson

dklawson

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Thanks Hap. I'll make sure the bowl is secured and not free to vibrate. On the HS2s I have worked on there was always a rubber alignment grommet between the bowl and carb body. This was the first place I had come across a metal bowl alignment grommet/bushing.

Willie, she complains about that smell also.
 

Timmy1959

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Doug, I agree with the others. The cause of the failure is metal fatigue. If you look closely, the cracks have patterns that are caused from vibration. Any time metal is allowed to bend beyond it's normal designed limits, it will fatigue and crack. Great photo!
 
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Yep, the angle grommet should be rubber, I get my carbs form Joe Curto , www.joecurto.com, he has what you need. I've only seen metal angle grommets one time, in the Volvo HS6s, and for the life of me , I can't understand why they were ever made in metal.
 
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